Roy L. Clough Jr. - Svenskt Modellflyg

Model air car
skims the ground
By
ROY
L.
CLOUGH,
JR.
Working model of a ground-effect vehicle
rides on a cushion of air from a model-airplane
engine
Tethered to a stake, the car will skim half an
inch or so off the ground, around and around
until it runs out of fuel WITH A hollow whistling
note audible over the whine of its tiny engine,
this advanced working model of a ground-effect
vehicle skims across the floor supported on a
cushion of air. What makes it go?
Air is supplied by a prop to a peripheral slot,
which produces a high-speed wall of air around
the edge of the model to retain the lift. A
separate propulsion-system tube bleeds off air
for reactive propulsion---from the blower section,
not the skirt. Supporting pressure is not reduced
--- a major fault of ground-effect vehicles which
propel by dumping air pressure and lifting the
skirt on the opposite side from the desired
direction of travel. Stabilizers on each side act
somewhat like the dihedralled wings of an
airplane— if the model tilts to either side, air
pressure escaping from the skirt builds up under
the vane and returns it to even keel. The result
is a model, which can buzz along at a good clip
on any level surface with a minimum of sideslip
due to minor irregularities on the surface.
Attached to a tether it will whiz merrily around in
a circle until the fuel runs out. It rides a half-inch
or so off the floor even when running free. Any
small airplane engine can be used to power it. If
you use the engine installed in the original
model, which is supplied with a three-blade
prop, you won't have to make a prop of sheet
metal, a pattern of which is given. If the engine
is new, break it in by running on a test stand for
15 or 20 minutes.
Top and rear views above and below show the
engine mounted in the intake duct and the propulsion
tube outlet at the rear. Note the wire hook on the door
for tethering.
A squeeze bulb feeds fuel through the plastic fuel line
connected to the tank. The dry cell plugs into the
phono jack
First study the cutaway drawings given to
become familiar with the various parts. Then
begin construction by making up the base, top
and diaphragm plate from edge-glued 3/32-in.
balsa sheet. Use stiffeners where shown and
allow to dry on a flat surface. Make up the 3 1/2in. intake duct from art paper and use this as the
first structural member to hold the top and base
together. When dry, add the 3/16-in. uprights,
which form the supports for the side covering.
Next install the paper propulsion tube. Note the
vane to direct airflow within it.
The skirt is vertical grained 3/32-in. stock
glued around the bottom edge of the base. Use
basswood or balsawood and there will be no
difficulty in making the bends. Cover the
framework of the car body with art paper, one
section at a time, beginning at the rear. Add
dummy headlights and fin, stabilizing vanes and
handrails. Windows may be glazed with sheet
acetate or left open. Finally turn the model over
and install stator vanes. Coat the interior with at
least two coats of hot fuel proof dope (clear).
Dope the exterior in your favorite color. The
original was painted light blue outside, fire red
inside.
Make up the engine pedestal, mount the engine
and cement the pedestal to the wires to the glow
plug. Cut out a 3 1/4-in. disk of cardboard and
use this as a guide in centering the engine.
Actual installation is made by cementing the
diaphragm plate to the stators. Use a slow
drying cement to allow time to even up the slot
around the skirt and center the engine shaft.
Run out the fuel filler tubes and engine lead
out wires and make up a needle valve extension
shaft. The glow-plug wires lead to a phonograph
jack---a great convenience in starting. A recoil
spring starter is a must and is installed before
the prop.
A length of 1/8-in. cord is cemented around
the skirt as a buffer. (A round shoestring works
very nicely.) Suspend the model by the engine
shaft and balance it so that it hangs evenly.
Small bits of solder, coated with cement and
dropped inside the body on the light side or end
will do the trick. If tethered operation is desired,
cement a wire hook through the covering to the
upright on the centerline.
Scanned from:
Popular Mechanics Do it yourself Encyclopedia
MODEL CAR RACING AT ITS SIMPLEST
Deceptively simple in appearance, every line on his
car has been carefully planned to make it easy to
build and maintain and fun to race
By ROY L. CLOUGH, JR.
WIND
• Here is the perfect project to absorb your old
three-port .19-.29 engine that still has a few kicks
left in it despite being outmoded for flying use by
newer and hotter engines.
We wanted to find out if a really good air-drive
car could be built to look like a real race car and
not a runaway fuselage. The results were
surprisingly good. With an old O&R side-port .19
it hit a zooming 62.8 mph. Even a beaten-up .14
diesel engine kicked it up to 40 mph. This
compares very favorably with wheel-drive cars,
and Wind Wagon is a lot simpler to operate and
cheaper to build.
The plan should be followed fairly closely.
Making an air-drive race car is not just a matter
of sticking a motor and prop on a wheeled
platform; balance and dynamic loadings are
important. There are a couple of angles designed
into this one which are essential to good
performance. For example, the wide underchassis is not a matter of taste in style, but is
made that way to create an airflow pattern to
prevent fine abrasive dust, present on concrete
surfaces, from being thrown up to the engine
intake; the washer arrangement which holds the
model plane type wheels came about because
other arrangements wore out aluminum hubs in
WAGON
nothing flat.
Begin construction with the chassis. This is
carved from white pine or equivalent. The wheel
axles are 1/8" steel landing gear wire. It is not
difficult to drill holes in the chassis for these by
hand if care is used, but a drill press makes the
job a cinch. Don't try to drill all the way through
from one side; drill in from each side so the holes
meet in the middle. Note how the axles are held
by soldered washers. It is not essential that the
axles be held so firmly they cannot rotate, but
there should be no end-play. Do not put the
wheels on yet.
The body is carved from soft pine. Some
originality of line is permissible here, but don't
depart too far from the plan and be sure the
thrust line comes as shown. The motor mount is
1/2" plywood, cut to fit your particular engine.
Chassis, body and mount are assembled with 11/2" flat head wood screws and good wood
glue—do not use model cement. The cockpit
details add realism. The driver is a ping pong
ball, painted to look like a helmeted head and the
rim of the pit is padded with heavy twine
cemented in place. The fuel tank can be anything
handy that suits the engine. Several types have
been used on the original and all worked well
provided the usual U-control fuel feed to the
outside was observed. Be sure it is anchored
firmly.
Next bore a ballast hole, put in the screweyes and bridle and make a trial fit of the engine
and tank and check the balance. The model
should hang the slightest trifle nose-down when
suspended by the bridle, so pour in melted lead
or old bearing metal to balance. Pull the engine
and tank and finish the model with fuel-proof
dope or enamel—incidentally, it seems easier to
clock a bright-colored car.
When the paint job suits you install the
wheels, using the washer arrangement shown to
minimize wear on the hubs. Install the motor and
tank and check it over to make sure the wheels
spin freely and track and foot well. Put a little #50
oil on the axles every other run and clean them
off whenever they seem to be picking up dirt.
Because the engine is pushing instead of
pulling you must now check the shaft end play to
make sure the push doesn't move the shaft in so
far that it puts a strain on the connecting rod or
allows the crank throw to strike the back of the
crankcase. Usually on such engines as the O&R
.19 and .23 side-port jobs the hard steel propeller
back plate running against the end of the bronze
main bearing is all the thrust bearing required. If,
however, there is considerable end play, or gap,
fill this in with thin brass or surface ground steel
shim washers until the play is just perceptible. Do
not run the shaft tight and be sure to oil the
shims well before the first run—see page will
take care of lubrication after that.
39
We found that maximum speeds came from
careful selection of propellers and by setting the
car off a bit rich so that it would start to lean out
by about the tenth lap—presumably because the
rich start prevented the pusher engine from
overheating before the car got going fast enough
for the slipstream to provide sufficient cooling.
While this vehicle will never set the world on
fire from either the appearance or performance
standpoint, it does bring auto racing within the
reach of the average miniature engine owner
who may not have access to a lathe or the other
specialized equipment so vital to AMRCA
followers.
By limiting a group of racing enthusiasts to
same size or maximum power plants any hobby
shop or model club can come up with a fleet of
cars in jig-time for some informal, strictly-for-fun
racing. Almost any smoothly surfaced parking lot
will suffice (school yards frequently provide the
type of running surface most desirable). A
portable center post sandbagged in place and
mounting a skate wheel well bolted to a sturdy
timber or piece of piping is about all that's
needed for "Sunday" drivers. A steel wire
connects from the pivoting center piece to the car
bridle; be super sure that your lines are all of
sufficient size and free of nicks.
As with all projects presented in this
publication, the editors welcome your comments
and photos of completed models. Let your fellow
hobby fans see your handiwork; send along your
model pictures
Air Trails HOBBIES for Young Men
By ROY L CLOUGH JR.
Unique gyrocopter-kite design
launches itself without towing,
costs less than $1 to build
there's not enough room
WHEN
to run with a kite or too
Amt. Req.
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Misc.
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MATERIALS LIST—WHIRLYBIRD KITE
Size and Description
Use
1/8 x 3 x 36" sheet balsa
rotors
1/4 x 3 x 20" sheet balsa
fuselage
1/16 x 3 x 20" sheet balsa
vane, stabilizer
1/8 x 3/4 x 5" pine or basswood
landing gear support
1 1/4" Dia wooden wheels
main wheels
3/4" Dia wooden wheel
nose wheel
10" x .030 wire
axles
9" length coat hanger wire
rotor mast
#6-32 brass hex nuts
rotor spacers
3/32" id brass washer
thread washer
3/32" id x 3/4" brass tube
rotor bearings
2 x 4" tin can stock
rotor hubs
3/16" id brass eyelets
tether eye
model airplane cement, red, black, and silver dope; and kite string
much wind to fly model
airplanes, it's a perfect time and
place to test fly this newly
designed rotary-wing kite. All materials you'll need are available at
your local hobby shop and in one
evening's time you can have your
'copter-kite ready to fly.
First, layout the fuselage, stabilizer, rotors, and vanes (Fig. 3) on
balsa stock and cut them out with
a sharp hobby knife or razor
blade. To shape the rotors and
stabilizer, first crack them along
scored lines as in Fig. 3A, then fill
the crack with cement and prop
up the end of the part until the
cement dries. Note that the rotors
are not identical, but are a pair,
having opposite pitch for counterrotation and are oppositely coned.
Now cement the stabilizer and
landing gear support (Fig. SB) to
the fuselage, reinforcing the joints
with a fillet cut from the 1/4-in.
balsa stock.
Next cut the rotor mast from
coat hanger wire and cement this
to the right side of the fuselage,
12 3/4 in. from the aft end. Reinforce by sewing it to the balsa
with a needle and heavy thread,
then coat the threads" with more
cement. After installing the eyelet
to which the tether attaches, sand
the assembly with fine sandpaper
and paint as in Fig. 3C, with model
airplane dope. Do not apply dope
to the rotors or vane.
The rotor bearings and hubs
(Fig. 3D) are made of 3/4-in.
lengths of tubing soldered to strips
of tin can metal. Make two of
these, form them over the blades,
and cement them to the rotors as
in Fig. 3E.
SCIENCE and MECHANICS
by ROY L. CLOUGH, Jr.
ONE evening of fairly intensive work
will put this little whirligig-type
helicopter aloft.
The construction is quite simple and
offers no problems to the builder who is
handy with a soldering iron. Good balance
without excessive weight is the primary
consideration, and care along this line will
result in good flight performance.
A Campus Bee was used for the original and proved to be very reliable. A bit
34
of cellulose tape which can be flipped over
the filler hole after charging, to keep dirt
out, is good performance insurance because of the exposed nature of the model.
Begin construction by unscrewing the
filler valve. Put this in some safe place
until again needed, taking care not to lose
the tiny rubber plug which serves as a
check valve. Unsolder the feed line at the
tank end. A 2-56 or 3-48 stud, an inch
long, is tapped, and then soldered in the
hole.
Layout the motor mounting holes on
a 3/4" disk of tin can stock, punch or drill
them and solder the disk to the lower end
of the tank.
Next mount the engine with common
pins, which are bent over and soldered
securely in place. While doing this, carefully check the alignment of the crankshaft with the tank and the stud. A bit
of 1/16" rod is soldered to the tin disk
opposite the cylinder, the end of this being wrapped with a few turns of tinned
copper wire to serve as a counterweight.
The counter-weighting may be done very
exactly, as follows: wrap the wire on
fairly tight until it seems correct; then
slide the wrapping a bit one way or the
other for fine balance. When you find it,
heat the end of the rod briefly and the
wire will "tin on" and stay put.
Drill a new hole for the feed line near
the top of the tank and on the opposite
side from the filler valve. Solder the line
in place carefully, to prevent leakage.
The big rotor is made from a good
springy variety of balsa, which may be
finger-doped for added toughness. It is
of left-hand pitch to permit the interchange of various easy-to-obtain righthand propellers on the motor end. We
have used the Hillcrest adjustable plasticblade prop and also Monogram kit props
with fairly good results but top performance will come from a wood propeller
carved to fit the individual machine. The
fairly small total area of the rotor system
results in a rather rapid descent, but is
necessary because the engine must turn
up quite rapidly in order to develop
enough power to fly the machine. There
is, of course, a very favorable heat-exchange setup because of the rapid motion
of the cylinder through the air as the
tank rotates, but set the motor at near
maximum output to get the best results
altitude-wise. One word of caution: be
absolutely certain the motor is running
in the right direction when released!
MODEL
AIRPLANE
NEWS
• April.
1950
WHAT'S THE SCORE ON
helicopters?
By ROY L. CLOUGH, JR.
• Easily the most fascinating thing that
flies, the helicopter is making a name for
itself in peace and war as the marvelous
machine that can land or take off
anywhere, hover over one spot or
tuck its nose down and scoot away in
any direction the pilot chooses.
It is only natural then that model
builders have been attracted to the
type, for here is a flying machine which
does its stuff close in where it may be
observed and enjoyed, does not require
huge tracts over which to fly, and which
should give more solid hours of model
fun than anything ever invented.
Or so it would seem at first glance.
However, as many a model builder
who has tried it will testify, it isn't
quite that simple.
To some who have tried it, at first it
appears a rather straightforward
proposition—merely arrange a prop to
pull upward, provide some method of
torque nullification, presto, there is a
helicopter.
And
several
bitter
disappointments later, the model
builder sweeps up the shattered balsa
wood, lays aside his tools and tries to
dig up some information on the type.
In many cases a study of the
problems involved makes the whole
thing
look
so
impossibly
complex that there seems to be
little point in trying it at all. The
envisioning
of
complicated
controls, pushrods, flap hinges,
dampers, complicated power
transmissions,
hairline
adjustments and impossibly
complex and delicate structures
places the designing and
building of a successful free
flying model helicopter on the
level of a major engineering
feat—so the modeler puts the
whole thing aside and starts
sketching a new pylon job.
The truth of the matter, as in
most cases, lies somewhere
between the two extremes of
utter simplicity and impossible
complexity. A fair statement of
the case is that a good model
helicopter is no more difficult to
build and fly than any other type
of fairly advanced model aircraft.
What may appear to be
difficult at first becomes at
second
glance
merely
different. This is because there is
very little in the way of carry-over
analogy from fixed-wing models.
Roughly speaking, the differences are
of the same order as those between
building a hot engine into a speed job
and building the same engine into a
boat. The principles are approximately
the same, but the factors are
different.
Helicopter
information,
in
the
empirical form in which it is most
useful to model builders, has
unfortunately been neither complete
nor widely available. Therefore the
design picture has been a bit clouded.
Any up-to-date aviation fan, if he
scratches his memory a bit, will recall
having read that "articulation" is a
good thing, that helicopters are
inherently "unstable," that there are a
great number of things like "cyclic"
pitch and gyroscopic precessive
forces to be dealt with. All of which
contains elements of truth, yet just
how these things apply to sitting
down and actually building a flying
model helicopter has been obscure.
The writer scored considerable
success in rubber-powered helicopter
models with his development of the
cage drive co-axial system, which
allowed two rotors to revolve in
opposite directions about a common
center, thus canceling out both
torque and gyroscopic effects. With this
system, which first appeared in Air
Trails some time ago, incorporated into
the model, stable power-on free
flight was possible for the first time
without the use of complex control
arrangements. Extremely steady in
flight, the machine and a later
variation of it were capable of
rather surprising duration when
winder-wound.
However, these flights were very
largely vertical; it was not possible to
secure any marked forward flight with
the machine, barring the use of
extreme nose-heavy trim.
Later a further development, a kit
design
manufactured
in
limited
numbers, featured a stabilizing or
"damper" fin which permitted a fair
degree of forward flight, with great
steadiness and every indication of
stability. So consistent was this type
that by counting the number of turns
stored in the motor the model could
be flown from one table-top to
another, time after time alighting
within an inch or two of the desired
spot. Several further variations of this
machine were built, performing well.
Now, note that no blade articulation
of any sort was used in these models.
There was no provision for cyclic, pitch
changes and gyroscopic action did not
enter the picture. Yet they flew very
well under rubber power. Keeping
that empirical fact in mind, let us
further examine the co-axial system.
This system—meaning two rotors
revolving in opposite directions,
about a common center—is one of
the oldest designers' answers to the
question of what to do about torque
reaction. Further, it appears to be
ideal in many respects because it
would seem that any disturbances,
either aerodynamic or gyroscopic,
which occur in one rotor would be
balanced immediately by the reverse
of that reaction in the other. Now
then, why, if this system is so
theoretically ideal, with equalized thrust,
cycling and gyroscopic moments, has it
not appeared in any machine which has
demonstrated itself to be of practical
commercial importance?
The answer lies in the fact that coaxial
systems have two major
requirements, and, from a practical
standpoint, these requirements tend to
be mutually exclusive.
First, it is desirable that the two rotors
be mounted close together. The reason
for this is that widely spaced rotors
introduce great stresses in the mast and
rotor system when the blades are cycled
for forward flight, that is, when the blade
angle is increased at the rear and
reduced at the front each revolution of
the rotor. The uppermost blade will
require a greater pitch change than the
lower blade in order to equalize the
couple between the points of applied
force and the center of gravity. Thus, if
we wish to avoid excessive stressing of
the rotor and mast system, the rotors
must be quite close together.
Second, the rotor blades of a co-axial
machine must be spaced with a large
gap from one set to the other because
otherwise they may clash together due
to the flexibility of the rotors. Blade
deflection due to gusts or even normal
cycling pitch changes is great enough,
ordinarily speaking, to make any
spacing of the rotors less than one
third rotor diameter apart definitely
hazardous. Thus, the rotors must be far
apart.
From these two requirements it can
be seen that the only way out of the
problem is not actually an "out" at all, for
it will mean building the rotor systems
impossibly heavy and rugged and thus
losing many, if not all, of the
advantages of the co-axial system. In
the practical sense, then, this means
that co-axial systems are limited in use
to such cases as where it may be
possible to stress the system all out of
proportion to its size—models and tiny
man-carrying jobs.
Now, what is this business about
cyclic pitch? Did we not just describe
rubber models, which flew well without it?
Why not merely eliminate cycling pitch
from the co-axial system and fly forward
by shifting the C.G. ahead?
The rubber models did fly forward by
means of the stabilizing fin and C.G.
shift. But don't overlook the fact that a
rubber model flies with continuously
diminishing power after the rotors come
up to speed. This is very important
because it meant the writer could
eliminate cycling controls because of a
characteristic of the power plant. Just
why this was so may be explained as
follows:
Picture a purely hypothetical co-axial
model helicopter in which the thrust
does not vary, which has stiff, fixed-pitch
rotors, and which is trimmed nose
heavy to make it fly forward. All set?
The machine, because of the unequal
loading of the disk (area covered by the
rotors) begins to slide forward. It will not
tip to either side since the advancing
blade of one rotor, creating more lift as
it encounters a relative wind due to
forward motion has a balancing
counterpart in the blade, which is
rotating forward on the opposite side.
This, incidentally, is a major advantage
of the co-axial configuration.
Now, as the machine gains speed
we find that the front edge of the rotor
disk is entering the wind while the rear
edge is leaving it. This means that the
rotor is lifting more at the front than at
the back, and it will tilt upward, moving
the machine into what is actually a
stall—one of the few analogies which
occur between fixed and rotating wing
craft. Now this will happen no matter
how much weight is placed in the nose,
consistent of course with the ability of
the machine to lift it.
Now, when this stall occurs, the
helicopter
will
slide
backward
increasing the lift at the rear of the rotor
disk until the model stalls tail up,
whereupon it repeats the trick,
oscillating back and forth with
increasing amplitude and violence until
it finally crashes.
The reason the rubber-powered
models flew steadily ahead under C.G.
shift is that the power gradually
decreased as the machine flew forward
and the damping fin provided a
stabilizing surface, which served to
maintain the proper angle. Thus it might
be said with considerable accuracy
that the rubber models actually made
use of the oscillating tendency of stiff
rotors to secure forward flight and that
they were successful in doing this
because the oscillation was damped at
proper time by the motor running out.
Or, to put it a bit differently, the coaxial rubber model helicopters with
damper fins as developed by the writer
were simply highly modified co-axially
driven planes flown vertically. Have you
ever seen Jim Walker do the Saber
dance? The principle is the same.
Thus, to fly forward in a co-axial
machine, as in any other, requires that
the pitch of the blades decrease in front
and increase at the rear of the rotor
disk. And, since we must be able to
control the pitch of the rotor blade
around the circumference of its sweep,
we may as well abandon the co-axial
configuration and its power transmission
problems and go over to something
simpler, the single rotor and torque
prop. Either that, or power the rotor
itself by means of tip jets or motors
which eliminate torque effects entirely.
For most model work, and at this
stage of the game, we will find the
torque prop type more practical, since
this gives us a heading and trim control
and the boom helps to round out the
design by balancing the weight of the
power plant, which will be forward in
most cases.
Which means we promptly dive head
first into the rotor head business—but
don't let it scare you.
The earliest successful helicopters
used a refinement of what is known as
the flapping, drag link rotor originated by
Juan de Cierva, the autogiro inventor.
To Cierva's basic invention were added
mechanisms to secure collective and
cyclic pitch control and with such an
arrangement the first man-carrying
machines flew.
This system (Fig. 2) makes use of a
hinge between the blade root and rotor
mast. This allows the blade to flap up
and down relative to the plane of
rotation and was originally introduced in
the autogiro to permit the machine to fly
forward without tipping over due to
relative wind differentials.
Soon after this was invented it was
found necessary to add another hinge
which would allow the blade to swing
fore and aft, since, as the blade coned
upward on the advancing side it would
tend to lag behind the retreating blade,
thus setting up terrific stresses in the
flap hinge. This drag hinge then had to
be fitted with a dynamic damper—a
gadget similar to the device which closes
doors without slamming —to absorb the
jolts and shocks that such a system was
heir to and to assure a correct nominal
circumferential position of the blade.
This system is in wide use among fullscale helicopter makers but it has some
limitations in that it is not truly stable,
although quite flyable with a competent
pilot.
As far as model helicoptering is
concerned the system is of interest
chiefly as background material. In
practice it proves tricky to build, fragile
and easily misaligned, and it falls down
on a most important model requirement
in that it will not fly "hands off."
As a matter of fact, it is not even a
satisfactory
free
flight
autogiro
arrangement as the writer discovered
some years ago, and this lack of
satisfactory flight characteristics started
a series of experiments, which led into
contact with a type of rotor arrangement
that would perform satisfactorily on an
autogiro.
Initial experiments with articulated
rotors on rubber model autogiros
resulted consistently in dismal crackups at the end of erratic flights of a few
feet. Apparently the fault was somehow
connected with the articulation that was
supposed to promote flight stability. So
we committed heresy. We stopped
articulating the rotor blades.* This was
better, but still not good, and then came
the inspiration—why not articulate the
blades in some other fashion, so that
the effect would be the same, but the
big bounce would be taken out?
Why not, instead of allowing the blade
to flap upward, allow it to rotate span
wise— that is, instead of letting the tip
fly up, let the trailing edge flap upward?
A new rotor was built with the blades
spring-loaded against their span wise
axis at zero pitch, and since the rotor
was stiff, tip-to-tip, the mast, which held
the rotor, was jointed, or articulated to
the
fuselage
to
prevent
the
transmission of disturbances back and
forth. Aerodynamic pressures would
result in the slight negative pitch angle
necessary for autorotation, and a sort
of automatic cycling pitch would be
obtained as the advancing blade
flattened to the relative wind, which
would flip the trailing edge down as the
blade passed in front and started back
dh the downwind side.
It worked.
*Louis Garami built a successful nonarticulated, non-feathering giro at about
the same time which flew by using
torque of the motor to counteract the
progressive tipping of his stiff four-bladed
rotor. This appeared in Air Trails.—RLC.
(To be continued in subsequent issue.)
AIR TRAILS SEPTEMBER 1952
(Continued from previous issue)
This initial model was soon followed
by others, simplified greatly by
eliminating the springs and mast
rubbers, and using thin wood blades
attached to a stiff spar and a flexible
wire mast. Models built here and in
England proved the soundness of the
system. Fig. A.
The articulated rotor mast was fine for
autogiros, with due restrictions on the
articulation to prevent clipping the prop
or the tail, because it permitted the
initial tilting, which starts the gyroscopic
wobble, to* damp out without running
through a cycle of self-excitation back
and forth through the mast. But, for
model helicopter work a flexible mast
seems impractical.
If rotation is supplied by the rotor, as
in the case of the autogiro, it will run
concentric and true if in balance. But, if
one tries to feed power into the rotor
through a flexible mast the whole thing
will jump out of line and wobble violently
with the whole system acting as an
eccentric.
In the practical sense, then, this
means that a flexible mast cannot be
used on model helicopters except,
possibly, where the propulsive force
originates in the rotor, as for example,
tip-jet propulsion.
At this point the writer became
acquainted with the work done by
Arthur Young, who designed the Bell
helicopters, and this takes us into a
discussion of the so-called "feathering
rotor" system in which the blades neither
cone upward nor drag back, and in
which, for the first time, we begin to see
inherent stability introduced into rotary
wing flight.
In
order
to
understand
the
differences
between
the
fully
articulated and feathering systems it is
necessary to look a bit more closely at
gyroscopics, the seemingly puzzling, but
actually
rather
simple
set
of
shenanigans which take place when a
rotating system is disturbed.
Any rotating system, and this
applies with particular emphasis to
helicopter rotors, is a gyroscope, and
jointing, hinging or springing it will not
make it anything else; it will still
observe the natural laws which govern
gyroscopic action. This is probably the
most puzzling and least understood
factor in rotary wing flight, and the
reader will find it of value if at this, point
he takes the trouble to make up a small
"understanding tool" and follow along
with the text.
Cut out a six-inch circle of heavy
cardboard and mount it with a pin
through the center in the end of a short
length of 1/4" stock which has its sides
numbered consecutively: 1, 2, 3, 4.
This represents a rotor—and a
gyroscope. If you wish, mark the disk
with an arrow showing left hand rotation
(most model helicopter rotors will be left
hand in rotation because of mechanical
considerations which will develop later
on).
First, flick the disk with the fingers to
make it spin rapidly, then poke gently
under the rim of the spinning disk on
the side of the stick marked 1. . What
happens? The edge of the disk rises, not
where you poked at it, but at 90
degrees, the side, marked 4. Repeat
the experiment, only this time - poke it at
3. It rises at 2. What is happening?
Simply this: the deflecting force is
modified by a factor of 90 degrees.
This is the first rule: that a
gyroscope, if deflected, reacts with a
displacement
proportional
to
the
deflection at 90 degrees in the direction
of rotation from the impingement of the
deflecting force. (Of course it also
deflects an equal amount in an opposite
direction 90 degrees back from point of
impingement, but we're trying to keep
this thing simple.)
With the understanding tool at hand
this becomes quite easy to understand,
or think of it this way: if a rotor blade is
deflected at one point in its
circumferential travel, it will tend to pop
up 90 degrees further along in the
direction of rotation.
Now, spin the disk again and gently
tilt the stick. Note that while the
gyroscope initially tends to remain in
its original plane, it will now gradually
readjust itself until it is spinning with its
plane perpendicular to its axis (the
stick). If the disk were mounted on a
universal joint and driven by rotation of
the stick this effect would be even more
pronounced.
This occurs because it is a function of
centrifugal force to' cause the rim of the
disk to recede as far as possible from
the axis, and this condition is satisfied
at. 90 degrees.
These two related phenomena are of
extreme importance to helicopter
designers: first, a deflection reacts at 90
degrees; second, a gyro seeks rotation
perpendicular to its axis.
Now remove the stick and take the
disk outside and scale it through the air
with a quick spinning motion. Note how it
tips over toward the advancing side.
Two factors are responsible for this
behavior. The first is related to the
Magnus effect and is due to differing
pressure gradients on the surface of the
spinning disk, and the second, which is
our primary interest helicopter-wise, is
that the air pressure at the entering
edge of the disk is producing a
gyroscopic resultant at 90 degrees, thus
turning the disk over.
Bear in mind these vital functions as
we consider the requirements of a
stable helicopter rotor. One thing should
be evident, that forward flight produces
a problem in tip speed differentials, the
advancing blade in meeting a faster
relative wind than the receding blade
tends to tip the machine toward the
retreating
blade
side
due
to,
aerodynamic
forces,
but
these
aerodynamic forces are balanced to a
large extent by gyroscopic forces,
provided the blades do no flap upward
or drag back circumference-wise and
are free to rotate, within limits, about
their span-wise axis.
And, if the rotor head is mounted in a
gimbal, rotor deflections will not be
transmitted back and forth between
rotor and fuselage through the mast,
which means that the helicopter will be
stable. By the addition of a stabilizing
fly-bar between mast and fuselage
connected by linkages, which can alter
its plane of rotation, the rotor can be
made to take up any position desired by
the pilot.
In other words, then, if we use a
flapping blade with drag links we must
also include a device to alter the pitch
of the blades cyclically, mechanically;
but, if we eliminate the flap hinges and
drag links and allow the blades to
"feather" as it rotates, cyclic pitch is
automatic in accordance with the gyrodynamics of the system.
This basic discovery about the
helicopter rotor was made by Arthur D.
Young of Bell helicopter fame. In this
writer's opinion, based on literally
scores of experiments with flying
models, this discovery is one of the great
aerodynamic advances of the century.
Not only for the mechanism which
Young worked out, which is remarkably
effective, but for the principles his work
outlines, which it is not extravagant to
state mark the greatest single
contribution to rotary wing flight.
From these principles we can arrive at
a stable rotor for model helicopter work.
Fig. B illustrates such a rotor. The
simplicity of the thing is its beauty. With
a little study of the sketch no reader will
have trouble duplicating it. Note that the
fly-bar is moved up level with the blades.
This is simply a length of wire with a
few turns of solder on the ends to
provide
operating
momentum—not
much weight is needed.
Note that the function of the fly-bar is
to steer the rotor blades back into the
proper position with respect to the mast
through a damping interval, hence
cyclic control may be had by arranging
the fly-bar in such fashion that it can be
deflected into a new plane of rotation—
but, very important, the fly-bar must not
be rigidly attached to the mast because
this will cause rapid following; that is, the
mast will tend to swing immediately the
rotor is displaced, because the
independent nature of the system will
have been destroyed.
In Fig. C we see a system developed
by the writer, which uses the
independent rotor principle together with
control "trailers" similar to the Kaman
system. The advantages of this system
for model work is that it provides
automatic-rotation in addition to the
stability of the Young system, plus the
fact that cyclic control by trailer
deflection (only one trailer need be
connected) is very easy to arrange with
a line up through a hollow mast and out
to a "loaded" trailer.
The other end of the line being
attached to a cam ring which may be set
to produce the cyclic deflection at any
desired point. The drag of this system
is higher than others and it must be
designed carefully if maximum lift is to
be obtained, yet its many advantages
make it attractive and it is certainly
capable of much greater development,
particularly since it is possible, with
trailers, to make use of high lift sections
with less regard for pitching moments.
This system is particularly attractive
for jet work, and it should be noted that
by placing the line of thrust of the jet
motor a bit below the center of the
span-wise hinge the blade pitch will be
controlled by the thrust output of the
engine. However, this factor must be
used judiciously because too great a
moment here will result in the blade
mushing around slowly at a very high
pitch, which will produce no lift at all.
Remember too that trailers, in
operation, should drop down into the
rotor plane— don't seek to drive them
either above or below the plane, since
this puts them at a great, mechanical
disadvantage and may result in their
control over blade pitch becoming
ineffectual and the drag of the system
very high.
Note, of course, that it is possible to
use H series stable airfoils and
eliminate the trailers entirely; however,
if this is done the rotor system may
show a tendency to tip back in forward
flight, and cyclic control installation may
be more difficult. But for straight
hovering or vertical flight this system is
unmatched for stability.
I have been asked, "How does the
Jetex kit helicopter work?" This model is
a good performer with a very fair
duration for this type of craft and in flight
exhibits stability. If we examine the
machine we can find the answer. Two
blades are used which are articulated to
a supporting beam. These depend, as
does the Cierva system, upon
centrifugal force to keep them at the
proper angle in flight, without a rigid
connector.
Underneath the rotor is a thrust beam
to which are mounted two Jetex engines
and the machine is driven by the
reaction of these units.
Now, please note that the mass of the
jet motors is quite high in-regard to the
total mass of the ship; that is, it has a
sort
of
high-momentum
flywheel
beneath an articulated rotor with the
mass of the pod, or fuselage being
quite negligible.
Thus we see here a combination of
the articulated blade system with the flybar stabilizer, which operates well if we
confine activity to vertical flight. In
essence this machine is dynamically
similar to the writer's little Infant job of a
couple years back, but being jet
powered it requires no damping fins or
fuselage-rotor-mast brake to retain
proper heading.
However, in its present form
forward, flight by changing the C.G.
trim will be found unreliable; the
machine isn't designed for that sort of
thing. It should prove a very interesting
experiment to move the power beam
up level with the rotor blades, and
rebuild the rotor along the lines shown in
Fig. B. This should produce a model
capable of forward flight, as well as
vertical, but the problem of autorotational letdown will have to be solved
with some new mechanism to alter
blade pitch. Also it may prove
necessary to introduce a small amount
of friction between the motor-fly-bar and
its gimbal connection to the mast in
order that adding weight to the nose will
make it go forward and not just alter the
angle of dangle of the fuselage from the
rotor.
On the basis of experiment the writer
suggests that configurations for first
experiments be kept as simple as
possible. The first helicopter should be
rubber powered, something along the
lines of a simple stick model, probably
direct drive. It won't fly for very long with
this limited power, but it can be fooled
with indoors at great length and the
rotor set-up ironed out. After you get the
"feel" of the rotor set-up and a bit of
experience in handling rotary wings, try
something in gas or jet power.
The best bet for gas engines is the
single rotor, or tandem rotor type—like
the Piasecki jobs with a rotor at each end
of a long fuselage. Coaxials, or rather
composite rotor jobs, such as the
writer's little Infant machine, are very
simple and okay for direct vertical
flight, but they won't do well in forward
flight and will return to earth with quite a
thump unless some free-wheeling
arrangement, other than spin-in-reverse
is used.
Better than freewheeling, which will
mean pitch change too, is engine twospeed control. Have a timer rigged to
drop a partial obstruction into the
venturi after a certain interval. This will
bring the model down under power, quite
gently. The really ambitious may use
spark ignition in conjunction with timer
control—but watch the weight. It takes
power to fly helicopters.
It is impossible of course, in a 2-part
article to cover all phases of helicopter
design. However, it is quite possible and
the writer hopes he has been successful
in this respect, to outline some primary
essentials. In answer to the question,
then: "What's the score on the
helicopters?" the reply is this: you can
build them and fly them successfully if
you understand the basic dynamics.
In this report are shown two stable
rotor designs. They work, they have
been flown, they do not hop and jump
madly about and there is no element of
luck about it; they can be duplicated by
any average modeler who will take the
time to understand just what forces are
involved and how they react. If, in times
past, the reader has tried unsuccessfully
to design his own, or has purchased kits
which did not perform as well as
expected, he should re-examine his
experience in the light of the material he
has just read, make a couple more
experiments and decide for himself that
practical model helicopters are not
only possible but downright fascinating.
In general, then, we may sum it up as
follows: It is better to use a shallow pitch
and run the rotor at a good clip; make
the blades of high aspect ratio—and thin
section. Be sure the pitch is equal on
both blades; mount the torque prop as
far aft as practical—it will absorb less
power, and keep the design of the model
as simple as possible and the bearings
as near perfect as you can get them.
There are several things to watch out
for. For one thing, the rotor blades
should be quite stiff. Balsa wood if hard
is pretty fair, but pine or even birch is
better. Because of the thin sections,
which must be used, and the high rotor
speeds, an overly flexible rotor blade
may develop "whip" due to resonance.
This will make the blades run out of
track, absorb a terrific amount of
power and may result in tearing the
model apart, particularly if a whipping
blade connects with the drive string to
the torque rotor.
Some of the writer's early models
used exceptionally flexible blades
without articulation or feathering
features, yet flew. How? By adding
quite a bit of weight to the tips. Thus as
the rotor was spinning up for take-off as
the model sat on its wheels a
gyroscopic plane or reference was
established which held the model fairly
steady for the brief duration of the
rubber power.
This won't work well if power is
continuous, as for a gas motor; the
model will gradually tip over a bit to
one side and then jump violently at 90
degrees. The reason for this behavior is
that a sort of cycling occurs due to
blade flexibility, at the first small tilt, and
this condition is rapidly excited back and
forth
until
something
disastrous
happens.
With single rotor and torque prop
models a change of heading may be
noticeable immediately after jump-off.
This may be only a few degrees or a
quarter circle, even if the balance of
thrust is correct. The way to minimize
this is to hold the model for a second or
two to allow the rotors to come up to
speed. For gas jobs use old friend
"stooge" to hold the model down until
ready.
Theoretically the axis of the torque
prop should lie in the plane of rotation
of the rotor. Practically it must be
somewhat below this plane. This is due
to fuselage effects, aerodynamically
speaking, and to the mechanical
consideration that we must keep the
prop drive string (or shaft) well clear of
the rotor tips. Empirically it works out
rather
well,
because
design
considerations necessary for power
location, landing gear, lateral areas,
etc., work out so as to make it possible
to locate the torque prop axis well down
out of the way.
Assuming a left hand rotor, it will be
noted that if the axis of the torque
prop is too high the machine will fly
sidewise to the left, and if the axis is too
low it will fly to the right. Properly
located, the model shows no tendency
to slide off in either direction.
This is a very happy circumstance
because it means that by combining a
few factors, such as level of the torque
prop-axis with C.G. we may make a
model of very simple design fly in any
desired direction without a separate
cyclic control mechanism.
This statement reads rather simply,
but go back and look it over again, for it
contains the essential elements 'of
practical model helicopter flying in that
we see how that adjustment of the
model helicopter, and the mechanism
to make that adjustment possible, are
not a complicated mess of pushrods,
cams and levers, but are in fact no
more complicated than the adjustment
of an ordinary free flight model of the
conventional type, and in many ways
simpler.
And,
incidentally
this
is
no
condemnation of cyclic controls, as
such, for they have much to offer the
purist and the researcher. However, and
analogically, the writer must point out
that we do not find it necessary to build
operating ailerons, rudder, elevator and
flaps into a conventional free flight
model
in
order
to
enjoy
it
tremendously.
Power transmission may scare the
uninitiated at first, particularly the bevel
gear drive shown in one of the
pictures. It shouldn't. It does present a
new problem, true, yet a simple drive of
this nature is but the work of a few
minutes to produce, requiring far less
effort than the control system of, say, a
team racer.
The gears are readily obtainable in
any toy store; for 20 cents two toy
eggbeaters yield six gears, four small
and two large ones. You can't even buy
a good bell-crank for that. There are
dozens of ready sources of small gears,
toys, old alarm clocks, etc. Fuel line
(brass) and landing gear wire takes
care of the shaft and bearing problems
nicely and adequately.
To drive the torque prop use balsa
wood pulleys faced with #1 sandpaper
and a string belt. About 2½-to 3-1 is a
practical ratio and slippage with this
system is nil.
Gas power requires heavy reductions
in speed and some sort of shock clutch
or take-up may be necessary if any
considerable weight or power is to be
used. We hope to cover this phase
more fully at a later date and show
some simple gas motor hook-ups,
which require no machining or difficult
work.
One pitfall to avoid is the idea that a
large, high pitch, slow moving rotor will
produce longer and better flights. It
won't. Power requirements will be
tremendous, which means a thicker
motor will be required, meaning more
weight, meaning it can't be wound as
many turns, meaning a great deal more
power will be needed for torque
correction—a vicious circle that adds
up fast.
Rotor blades should be of fairly high
density, thin in section and operated
fairly fast at low angles of attack. For
the present, helical pitch should be
shelved in the interests of simplicity; use
flat blades of uniform pitch until you
build up a little experience at helicopter
flying.
The
sketches
show
general
proportions and sound design practice.
Stick pretty closely to these layouts, at
least for your first machine, then strike
out on your own.
ROY L. CLOUGH, JR.
AIR TRAILS
SEPTEMBER and NOVEMBER, 1952
Water Bug
By ROY L. CLOUGH, JR.
INSPIRED BY those little aquatic insects
called "water striders," this unusual model boat
flits along the surface of the water on three
flipper shaped planing feet mounted at the
ends of long legs.
Although it travels fastest on calm water,
Water Bug can run through 4 to 5 in. ripples—
the scale equivalent of 5-ft.waves---with no
trouble. The struts simply slice through the
wavelets and keep going. The boat is very
stable and can be run with a guideline or turned
loose in small ponds where recovery is easy.
The original model showed no tendency to trip
or tip over, even with the rudder bent sharply for
free-running turns.
The hull is a simple box structure of 1/8-in.
sheet balsa. It should offer no problems, but
remember to soak the sheeting, which covers
the front section in hot water before bending.
Finish the boat with a couple of coats of sanding
sealer and one of dope, or cover the bare wood
with a layer of lightweight model tissue laid on
with heavy dope.
The motor mount is a disc of plywood pinned
and cemented to the rear leg. Drill for the engine
mounting bolts, set the engine in place and build
up the fairing on the rear of the bulkhead with
scrap balsa left over from the hull planking.
Don't worry about access to the rear of the
bulkhead to tighten up the nuts. If built-in blind
this way, they'll stay put. The thrust line of the
motor should be almost parallel to the bottom of
the boat, but pointed slightly downward.
Cut the planing feet out of .019 sheet metal,
then make up the holders and rudder and solder
them to the feet at the angles shown. Attach the
feet to the struts by lashing and cementing
securely.
Finish off the model with scale radio masts,
running lights and foghorn. A couple of screw
eyes are used for the restraining bridle. If you
use a tether, attach it in such a fashion that the
boat dangles level when suspended by it.
If you don't have a model boat basin with
tether post, you can run Water Bug off a
spinning rod from a rowboat.
To launch the boat hold it by the motor
mount and give a gentle push. It should climb
out of the water in about 6 ft. with an immediate
increase in speed. If it doesn't, turn up the front
edges of the forward planing feet slightly.
Scanned from:
Popular Mechanics Do it yourself Encyclopedia
'TYPHOON-STEAM OR AIR EXPANSION ENGINE
By Roy L. Clough Jr.
Of all the gadgets a hobbyist
can make, few offer greater kick
than a homemade steam engine
that really hums along smoothly.
Usually, however, making even a
simple oscillating type engine
entails considerable machine
work, lots of know-how and
experience—which keeps many
from trying it.
This little expansion engine,
which runs beautifully on air or
steam pressure, was designed
especially for the hobbyist who
would like to try this phase of
model building with good
assurance of success, without a
well-equipped machine shop at
his disposal.
All difficult parts are adapted
from readily obtainable things;
the rest are odds and ends that
everybody has kicking around.
For instance the crankcase is
made from a couple of catsup or
chili-sauce bottle caps, the piston
and cylinder from an old fish-rod
ferrule, bearings from odd bits
of brass tubing. A little
rummaging around in the scrap
box will probably turn up most
of the rest in nearly finished
shape. You don't have to stick
very closely to the dimensions
either;
just
watch
the
proportions so you arrive at
enough clearance between the con
rod and cylinder sides at quarterstroke.
The completed engine fills the
gap between gas engines and
clockwork drive for model boats,
or if you're really ambitious and
want to see what it was like back
in the compressed-air flying model
days, make up an air tank and
fly a plane with it.
Despite its extreme simplicity
the engine is very efficient and
powerful, due chiefly to unique
valve system worked out by the
author after much experimenting
with rotary, ball valve and
oscillating cylinder engines. This
system has all the simplicity of the
latter and none of its drawbacks.
Cylinder is stationary and
valving is taken care of by
simple flat valve rocked back and
forth by means of slotted drive
over crankpin.
Feed line is flexible neoprene
tubing, which moves with valve.
Hunt up, or buy, a mating
pair of 9/16" fish-rod ferrules,
cut to proper length and lap with
fine pumice and oil to smooth slide
fit. (Alternatively you may use
9/16" tubing and turn a piston
from aluminum or brass.) Next
locate a couple of bottle caps of
right size and cut out for
cylinder, which is then
soldered in place in one.
Other cap is cut back on
rim for about 1/4" to provide
valve clearance and two 4-40
bolts soldered in place to act
as mounting studs. Put this
aside.
Crankshaft is built up from
length of steel rod, threaded
into heavy brass washer and
soldered.
Bearing is any handy bit of
brass or copper tubing, which
fits or can be reamed to size.
Cut hole in front case and
solder it in. Make up wristpin bracket from scrap of
.024 stock and set aside.
Find two bits of brass tubing
for con rod bearings, one to fit
machine screw (around 6-32
size), which forms crankpin.
On
smooth
board,
measure off con rod length
and with appropriately sized
nails hold bearings to board.
Then bend length of 1/16"
brass or soft iron rod around
two bearings in shape shown
on plan and solder in place.
Melt a little solder into inside
of piston, heavily tin con-rod
bracket, put rod in bracket,
push pin through, then dangle
bracket into piston, apply heat
to piston until solder fuses.
Drop piston into cylinder;
poke crank pin into crankpin
bearing and screw lightly in
Typhoon Engine
(Continued from page 26)
place. Rotate shaft to check
for sticks and binds. Now
remove piston, shaft, etc.
Make up a valve plate from
scrap of sheet brass, threading
and soldering valve rocker
stud in place and filing off
even so as not to project
against cylinder. Drill inlet
hole.
Now wire valve plate to
cylinder in a couple of places
with enameled copper wire
(which won't stick), line it
up carefully and run solder
along both sides to hold in
place. Remove wire tie-down
and drill through cylinder wall
through valve plate inlet
hole. (Doing it in this order
prevents
distorting
thin
cylinder tubing.) Carefully
scrape off rough edges.
Now reassemble shaft
and piston workings and
recheck for sticks or binds.
Make up valve—but do not
drill intake hole yet, just
rocker shaft hole. Put valve
in
place,
holding
with
spring, washer and nut, and
rotate shaft to quarter-stroke
in direction you wish engine
to run. (Note valve can be turned
over at this point for left-hand
rotation.)
With a scribe mark valve for
drilling through intake hole in
cylinder. Then turn shaft 180°
and make certain inlet hole is now
open, the edge of the valve having
slid past it. Remove valve, drill
marked spot, then with wood
peg in hole solder on short length
of brass tubing for feed line.
Polish valve face by rubbing
lightly on fine file, oil up and
reassemble the works. The
cylinder head, a disk of tin can
metal or .024 brass, should fit quite
well with a push fit. Then rotate
shaft so piston is at bottom stroke
and carefully solder head in
place.
Attach 5" length of black
neoprene tubing to intake line
(clear
tubing
is
not
satisfactory—steam hardens it
and it works too stiffly for air) and
put on flywheel or prop and apply
a little pressure. A flip is
necessary to start it, or else
simply rotate to valve "on"
position for self-starting. If
everything works right, and it
should, you may now solder rear
case in place. Use lots of oil for
first few runs; when run in a bit, a
few drops on valve plate and
crankshaft bearing are sufficient.
Air pressure up to 100 p.s.i.
may be used, steam to 60-70 p.s.i.
American Modeler — September I960
Try TURBINE
It's not too hard to convert your regular glow
plug engine into a passable jet power plant!
By ROY L. CLOUGH, JR.
COMING ATTRACTION: scheduled for next "AT"
is Roy's "Blow Bug" pressure jet basic F/F
The increasing importance of jet and rocket aircraft has focused
interest on power plants which can be used to propel their scale
model counterparts without the ear-splitting racket of a pulse jet,
and with longer duration than is provided by current rocket motors.
In an effort to achieve this, a number of builders have been
experimenting with various arrangements whereby a conventional
model engine is used to produce a jet blast. Although less thrust
can be produced in this fashion with any given engine than that
same engine would put out with a propeller, the results can often
be satisfactory if the weight of the model is kept low due to the
terrific power to weight ratio of small glow, plug engines.
There are at least three types of possible piston-engine operated
jet motors. These are 1) ducted fan, 2) pressure jet, 3) jets operated
by means of a positive displacement, or Rootes-type blower.
Since initial experimentation has been along the lines of the
ducted fan system this type is best known. Basically the ducted fan
is simply a model engine and propeller placed inside a tube. For
optimum efficiency the tube should be so shaped that the ambient
pressure is about the same on both sides of the propeller, or fan.
This means that the ducted fan should produce a great increase in
velocity of air flowing through it without producing a corresponding great increase in pressure. This is accomplished by proportioning the tail pipe and the intake pipe so that their difference in size
matches the pressure/velocity of the air flow within each to the
other, and by locating the fan in the necked-down portion between
the two sections. In other words, the airflow behind the fan will
be much faster than the flow ahead of the fan, so the tail pipe
diameter will be proportionately less than the intake diameter,
and the fan will be located at the transition point, or, where the
air is speeded up. The sketch illustrates this.
The reason for this proportioning is that pressures within the
tube are functions of air velocity, and, the faster the air is moving,
the less pressure it exerts on the walls. If we tried to build a ducted
fan with the same diameter from intake to exhaust end we would
find it very inefficient because the intake end would be operating
at a much higher pressure than the exhaust end, the engine would
tend to starve, race, and possibly overheat, and the thrust from
the efflux would be small.
The ducted fan obtains reactive propulsion by accelerating the
column of air within it; it is the inertia of this column of air to
being set in motion, which provides the thrust. Thus anything that
impedes the flow of air within the system tends to reduce thrust.
It is better, then, to have tail pipe a bit on the small side than
too large, and, from the empirical approach, which is generally the
best m model sizes, a variable area exhaust nozzle can be used to
advantage. A very simple instrument which can be made from a
two foot length of transparent fuel line and an oil bubble is helpful
in determining pressures at various points within the duct. This
is called a manometer. To work it simply suck up a slug of light
oil an inch or two into the line and hold the other end first at the
intake, then at the exhaust end. By comparing the behavior of the
bubble in both positions a good idea can be gained of the pressure
pattern. Even better, tap into the duct in several places and
compare pressures.
The tail pipe pressure should be higher than the intake pressure,
but not a great amount higher, for ducted fan designs. Too great
a pressure difference means that the fan is working against back
pressure and is not handling all the air it can; which is in direct
opposition to the basic philosophy of the ducted fan to move the
greatest mass of air possible in the shortest possible tune.
Simplicity is a great advantage of ducted fans, and of course,
the tube can be used as the keel or central member of the airframe
AIR TRAILS
26
JETS for your Models
which supports the other assemblies. The tube may be constructed of steamed sheet balsa or stiff hard-finished paper.
The writer has found that aluminum foil makes the best
interior finish; being smooth and nonabsorbent it is easily
wiped clean of oil from the engine. Stick it in place with hot
fuel proof dope which has been allowed to thicken a bit The
necked down portion of the tube could mean making a
layout for each new motor built, but this can be avoided by
building this portion using a dime store funnel for a form
and trimming the section where it fits. The angle is just
about right and the bother of laying out a conic section is bypassed with plenty of trim allowance.
Standard practice with ducted fans is to angle the motor
supports to act as stator vanes to straighten out the airflow
and reduce, or eliminate torque effects. In some cases it may
be necessary to add a vane or two to do the job. These
should be adjustable to permit interchange of fans without
losing torque trim.
Small sheet metal "flower petal" fans are generally used
in ducted jobs, but try carving small wide-bladed props of
correct helical pitch for added thrust A carefully carved fourblader will move more air than metal fans A spinner is
another aid to accelerated flow Actually, the ideal ducted fan
would be a wheel-type plastic job with a well-shaped rim
to prevent tip losses and to provide a good flywheel
effect. Perhaps some manufacturer will read this and produce one
Generally speaking, to get the most out of a ducted fan
use as large a diameter as possible for any given model,
cutting down on the number of blades and pitch where
necessary to maintain correct engine speed. Be sure to
streamline the backside of the engine mount and lead-out
glow plug wires and needle valve extension for convenience
in operation. If the engine can be fueled and started without
using a removable section in the tube, so much the better.
An external fuel supply, or filler line, depending upon engine
used will simplify this part of operation, and a starting rod,
JULY. 1953
which can be poked down the intake to flip the engine over
will eliminate fiddling with strings or risking chopped
fingertips to get the thing going.
The ducted fan motor works best in scale-type models
flown at moderate speeds and may be used singly, as a
fuselage member, or mounted in external pods. If pod mounts
are used, keep them as close together as possible. Some
adaptation to helicopter use is possible, but the bulk and
clumsiness of this type of model jet makes it less practical
for rotary wing craft than our next consideration, the
pressure jet
The pressure jet system has not been widely employed in
conjunction with model engines. This is quite a different
affair from a ducted fan The pressure jet consists essentially
of an impeller, or blower, which keeps a reservoir or plenum
chamber stuffed with compressed air, and from this reservoir
lines are bled off to feed jet reaction nozzles which may be
located some distance away from, or at least angles to, the
blower.
Unlike the ducted fan, the idea is not to provide a large
mass of air with velocity, but to supply a smaller mass of
air at a relatively high pressure, to high velocity discharge
nozzles (or afterburners).
Pressure jet systems may use either an axial flow (propeller)
fan, or a centrifugal blower, depending upon the general
layout and designer's discretion. They could also use
positive displacement blowers of the Rootes type, as
mentioned above. The chief objection to use of this type
blower is that there is none light enough for model use, and
in any event they operate at a slower shaft speed than
model engines provide, which seems to suggest infinite
complication, hence impracticality for our purpose.
Pressure jets offer intriguing possibilities for buried installations, experiments with multiple jets, boundary layer
control, helicopter propulsion, etc. We may use a simple box
with a tightly fitting rotor for the plenum chamber and bleed
off jet tubes wherever we wish. (Continued on page 69)
27
Turbine Jets
Since high internal velocities are of less concern,
internal streamlining of the system is relatively
unimportant within reasonable limits. We may simply
seal up a fuselage, or wing," and use it as the plenum
chamber, locating the jets at handy points and placing
the air pickup, or intake, either at the top (at or slightly
behind the CG) the front, or split to the sides. Whether
we use axial or centrifugal blowers will depend upon
various considerations A centrifugal blower is good
where the air intake is "dead," that is, open to the
inside of a ventilated fuselage, it may also leak less air
under high pressures. It can, however, be used in podtype jet motors as shown in the sketch
It is a characteristic of a centrifugal blower that air
rammed into it puts a load on the engine, just the
opposite of what happens with an axial or fan blower,
which speeds up when rammed This means that in
applications where a centrifugal blower is to be used in
conjunction with ram air, the motor should run at its
optimum rpm under ram, which means it may race when
the model is not in motion. Since it is usually better to
locate the engine within the plenum chamber to take
advantage of its heat and exhaust gases this means that
it will operate in a supercharged condition
The way a glow motor will wind up under such
conditions may surprise the builder the first time he tries
it. Operating well above atmospheric pressure, the
engine really stuffs itself with air and water
condensation and it can get very hot in the process.
For this reason it is a very good idea to use cold
plugs and fuel in engines put to this use.
Pressure jets work to best advantage where
mechanical advantage can be taken of their unique
characteristics, one of the best applications being driving
helicopter rotors, where a stream of high speed air
emerging from a blade tip can produce considerable
rotational speed and lift. For scale type models of
fighters they are not quite as good as ducted fans, except
where highly streamlined and fast jobs are to be used,
or where used in conjunction with a thrust augmenter,
which we'll discuss in a moment
From the above material the reader should be able
to arrive at workable jet propulsion with a standard
engine for his jet project As best thrust will not be as
high with a propeller, so the weight must be kept down.
But nevertheless, workable scale type jet models can be
built and flown if a little attention is given to detail, and
surprisingly "hot" models can be built by anybody
willing to take a little extra care with internal finish and
experiment a bit with intake and exhaust diameters
Our interest in model jets is largely limited to such
uses as scale model power-methods of the big ships, and
for special model uses, such as helicopters, where, by
sacrificing some mechanical efficiency we can do away
with a great deal of mechanical complexity
(.Continued from page 27)
This does not mean that extremely good performance
lies beyond the reach of our engine driven jets.
Attention to the factors involved, minor adjustments,
care in detailing and improved operational experience
can result in greatly improved thrust figures over what
we are able to obtain at present. Such angles as variable
tail cones (in flight?), improved ram scoops and fans of
improved design should be exploited more fully, and
possibly the use of afterburners and the many problems
they present can be licked
A good rule for the experimenter to remember is that
the propulsive efficiency of any system is highest when
the exhaust efflux velocity most nearly matches the
forward speed That is, the air should be blown
backward about as fast as the model moves forward
Practically speaking, this means it is more efficient to
move a large mass of air at a moderate speed than it is
to move a small mass of air at a very high speed,
because moving the larger mass gives us the advantage
of inertia.
In the concrete application we can improve propulsion
efficiency in those systems where the exhaust velocity is
several times higher than the forward speed of the model
by using augmenters, that is, devices which will
introduce more air into the efflux, increasing its reaction
mass and slowing it down.
A tail pipe shroud, as shown in one of the
sketches, is a good way to do this. Another method,
useful in ducted fan systems suspected of having low
tail pipe pressures is the simple clapper valve—a reed
of tough paper, which admits more air to the tail pipe if
its internal pressure drops below atmospheric
The essential thing to remember in designing engineoperated jets is that the basic problem is to put a gas
(air plus combustion products) in motion, in order to
get the opposite reaction (propulsion) produced by
doing work upon that selfsame gas.
A good rule of thumb for empirical model jet design is
"The greater the gob of gas you can grab, the less 'give'
it's got and the more got you'll get "
TRIAD...A Radial-Wing Flying Model
By ROY L. CLOUGH, Jr.
An interesting experiment in radial-wing ships,
this model is incapable of stalling out of a climbing
turn.
DESIGNED
with radially mounted wings as
found on some robot bombs, this novel model has
brought favorable comment wherever it has been
flown.
Its surprising stability is due directly to the
vertical wing. In straight and level flight, the two
lower wings provide lift. Should the model tend
to bank in either direction, the vertical wing will
exert lift and return the ship to an even keel. In
156 POPULAR SCIENCE
fact, turning and banking can be controlled entirely by warping the single aileron on this vertical
wing.
Construction is extremely simple. The fuselage is
built up of three sheets of 1/32" medium-hard
balsa, all cut to the same shape. Stiffeners of
1/16" square balsa are cemented to the inner faces
of these sheets. The nose end is reinforced with
plates of 1/16" balsa, cemented on after
assembly.
Cut three identical wing frames from 1/16"
medium soft balsa and install the 15 ribs, trimming
a little from the trailing edges of the outer ribs to
make them fit. Cover with regular model tissue
and water-shrink, but do not dope.
Wing design makes for speedy construction. The
aileron shown on the plan is fitted to the vertical
wing only. Other wings have landing wheels at
their tips.
A shallow groove cut chord-wise in the butt
edge or root of each wing will make a stronger
joint. Use cement liberally to attach the wings to
the fuselage. The two lower wings, which take up
the shock of landing, are reinforced with wire
crosspieces as shown on the plans.
A notch in the elevator, reinforced by an
additional thickness of stock, retains the rear rubber
hook. Mount a wide-bladed prop of fairly high
pitch and make test flights over tall grass on a
calm day. The sturdy fuselage permits use of a
powerful enough rubber motor to give the ship a
skyrocket climb.
A nose wheel helps preserve the prop from
damage. Performing much like a high-pylon design,
the plane climbs to the right and glides to the left.
SEPTEMBER 1946
WHIRLIGIG
By Roy L. Clough Jr.
Two views of the whirligig in actual flight. Note the flexing of the rotors under power.
EXPERIMENTING CAN BE FUN WITH THIS VERY SIMPLE HELICOPTER PROJECT
THIS direct-lift model utilizes a simple
and foolproof counter-rotational system
based on the familiar principle of the
contest "whirligig."
Due to the nature of the rubber motor
hook-up the two rotors are constantly
and automatically in balance with each
other, effectually neutralizing the torque
element without gearing of any kind.
This model is the 73d helicopter built by
the writer and experience gleaned from the
first 72 is incorporated in its construction.
Counter-rotation is considered the best
approach to the torque problem, due to
the fact that as long as some power must
be used to offset rotor torque that power
might just as well be turned to
additional lift. The length of a rubber
motor is in any case limited (barring
the use of gearing) in a model of this
type and it is therefore essential that the
best possible use be made of the power
that is available.
Two-bladed rotors were used on the
original because they may be placed in
line with the fuselage making the model a
rather "flat package" and easily portable, a
somewhat important factor in cramped city
quarters. Three-bladed rotors would
probably be okay if you wish to try
them. More power would be needed and
slightly less pitch would doubtless
increase the soaring qualities of the
model.
Start construction with the coaxial
unit. Study the plans until there is no
doubt as to the action of the unit. The
motor tube is 1 1/4" in diameter and is
constructed of either balsa sheet or stiff
drawing paper. The thrust bearing is
located at the bottom of the tube and
consists
of
a
pin-washer-bead
arrangement. This bearing should rotate
freely but should have very little
"play." Strive to get the bearing as close
to the dead center of former "C" as
possible. The pin-shaft is bent at this
point to keep it from falling inside the
tube, but it is attached to its crosspiece
only after the fuselage is completed.
Cement former "B" into the other end
of the tube leaving a 1/8" rim to
accommodate "A." If you have chosen
drawing paper for the construction of
the motor tube, dope it now for
additional strength. Allow plenty of
time for the dope to dry before cutting
holes in it for access to the rubber motor.
The shaft, which turns the lower rotor, is
made from either thin-wall brass or
aluminum tubing. 1/8" O.D. The tubing
is split two ways for about 1/2" and the
split ends flattened at right angles to the
tube. Now "soak" the tubing for at least
five minutes in dope thinner to remove all
grease and/or dirt. If this is done no
difficulty will be experienced in making
cement stick to it. Push the tube
through the hole in former "A" (it
should fit snugly), coat liberally with
cement and push the whole assembly
down into the "cup" formed by former
"B" and the end of the motor tube.
Allow a few minutes for the glue to set
a little, then true up the shaft with the
tube. That is, make certain the motor
tube and the hollow shaft rotate in the
same plane. The upper bearing is an odd
piece of aluminum tubing, 1/4" long,
slipped over the hollow shaft and mounted
in the 1/4" by 3/8" crosspiece in the top of
the fuselage. It simplifies matters to put
the crosspiece with the bearing in it on at
this point, because it is impossible to
slide it on after the tube-shaft is flared to
take a bead bearing.
The lower rotor hub is mounted next.
Drill the shaft hole a little undersize and
force the hub on the tube. If it feels a
bit loose, wedge it tightly with short
lengths of toothpick. Apply plenty of
cement, let the whole thing dry for a
couple of hours, and go over it with
cement again. Hundreds of flights on
the original model failed to loose a hub
attached in this manner.
After the rear prop hub is well set,
flare the end of the tube slightly to hold a
glass bead, the thrust bearing for the
upper rotor. Insert the propshaft
through one of the access holes cut in the
motor tube (tweezers are a great help)
and slide a bead and washer and the upper
rotor hub over the shaft. Use your favorite
free-wheeler.
The rotor blades are cut from 1/20"
sheet and sanded over a bottle to produce
a slight camber. Note that the angle of
the lower rotor blades is slightly more
than that of the upper. This is done
because the upper blade "bites" into dead
air and the air forced downward by the
lower blade is already in motion. A
higher blade angle on the lower blade,
in theory at least, prevents compression
between the rotors, and consequent instability.
The fuselage of the helicopter model is
built up of 1/16" strip, and is quite
conventional, except for the sharp curves
in the forepart of the longerons. If the
longerons are soaked thoroughly in hot
water before any attempt is made to
bend them into place, no difficulty will
be encountered. Because of the light nature
of the construction it will be necessary to
install internal cross-braces at the points
marked on the plans. Where the landing
gear legs are attached on the bottom of
the fuselage it is a good idea to cover in a
couple of sections with 1/32" sheet
balsa. Tail fin is of 1/32" sheet. The
cabin section is cellophaned in before the
balance of the fuselage is covered. The
original was covered with orange tissue
and water-shrunk, not doped. Weight is an
important factor in a model of this type;
try to keep it down. The top section of
the fuselage should be left uncovered until
the coaxial unit is mounted. The bottom of
the tube is anchored to a 1/8" crosspiece
and the upper crosspiece is mounted
directly into the fuselage. Check and
make certain that no part of the rotating
tube binds on the inside of the fuselage.
AIR TRAILS
Power will vary according to the weight
of the model. Start with a double loop of
1/8" flat brown (four strands) and build
it up from there. The original model
flew well on this power until stepped upon
and re-built, after which another loop had
to be added to take care of increased
weight. The tailskid, which is not shown in
the photograph, was added later to avoid
the necessity of holding the tail up in
position in ROG flying. The model
should balance 1/4" ahead of the rotor
axis for straight vertical flight. Hand
launching is accomplished by letting the
ship take off from the hand with the
nose pointed slightly downward. This
model can be made to fly forward as it
climbs by adding a small weight to the
nose. The free-wheeler is to let it down
easy.
A few pertinent facts on model
helicopter design should be mentioned at
this point.
The first and holiest commandment of
model helicopter design is: Rotor blades
absolutely must balance. Lack of balance
induces vibration and loss of power.
Vibration induces instability and the
average helicopter is, by reason of the
principles involved, not overly stable to
begin with. Rotor blades should have a
certain amount of "flex" to them.
Rigid blades and instability are, in
models of this type at least, synonymous.
The center of lateral area should be high,
otherwise the flat fuselage sides will act
as a fin, causing the model to spill over
at the top of its flight and descend
upside down. The center of gravity
should be as low as possible for the same
reason. About 75 percent of lateral area
should be behind the axis of the rotors;
otherwise the model may fly tail-first in
"forward" flight.
Stub-wings, elevators, flaps, and whatnot
are just so much junk on helicopters. Any
unnecessary object sticking out into the
slipstream of the rotors is a good bid for
instability.
Do not expect a model helicopter to turn
in performance comparable to a
conventional model of similar weight. Don't
forget rubber length is limited and that the
lift of the model is produced entirely by
rotating vanes. Good performance,
however, can be had as a result of careful
construction and half-minute flights should
not be uncommon with this model. If the
free-wheeler works smoothly, a certain
amount of soaring ability will be noticeable
if the ship gets caught in a thermal. Use
fresh rubber, as the model is hand-wound,
or go us one better and figure out a way to
use a winder on this ship. Making the
bottom end of the coaxial tube and the
bottom section of the fuselage under it,
removable, might do this.
OCTOBER, 1946
THE MODEL 'COPTER
By ROY L. CLOUGH, JR.
Roy's Little Infant job (AT Sept. '52) in flight. Damper fins have been removed and rudder
turned over. Model will fly steadily forward, descends via auto-rotation pitch changes.
Roy's continued research into the whirlybirds has
produced some fine models including the first truly
successful stable co-axial type. This is fascinating stuff,
In our previous series of articles,
("What's the Score on Helicopters?")
the writer tried to present a simple basic
understanding of the major forces
involved in a rotary wing flying
machine.
We saw that the problem of flight
stability largely resolves into integrating
the natural gyroscopic forces of a
rotating system with its aerodynamic
characteristics in such fashion that a
reaction by either tends to maintain the
positional integrity of the system with
respect to the rotor mast.
This, in the practical application,
requires a certain amount of
independence between mast and rotor in
order
to
prevent
immediate
displacements from setting up a chain
reaction of self-aggravated wobbling,
and
a
certain
amount
of
interdependence in order that control
may be effected, or imposed upon the
rotor, and that the mast shall serve as a
28
reference point, ruling the me of
rotation of the rotor.
Thus it becomes quite simple to
design a vertical-lofting or hovering
model, by simply arranging the rotor to
feather along its longitudinal, or span
wise axis and by hanging the hub in a
gimbal which permits a seesawing
action, and positioning the blades by
means of a flyweight or paddle bar so
they will not roll over or develop flutter
in a chord-wise plane A rotor such as
this is said to be independent, for if all
bearings are free it will rotate in its own
optimum plane regardless of the
position of the fuselage, or mast.
This is fine for an indoor model where
gusts are not a factor, and when it is not
desired to obtain forward flight.
However, the completely independent
rotor is not desirable, even for model
work, because it has no reference point
from which control can be effected. (In
the practical sense it is well to point out
that a completely independent rotor
does not exist; there is always some
friction in the pivots and gimbals which
tends to position the blades at 90
degrees to the mast, but this residual
friction is seldom much in a model)
Therefore, we must build in a small
amount of friction, either by making the
gimbal fittings a bit stiff to begin with,
or by providing a drag of some sort,
which can be adjusted. When this is
done the model will fly forward by
simply changing the C.G slightly, since
the reaction of the rotor, in seeking to
justify its position with respect to the
angle of the mast and aerodynamic
pressures, will result in cycling pitch.
This is the simple way of doing it
and it works quite well for models. By
judicious use of a small weight arranged
to slide fore and aft, the model will
climb vertically or by forward at a fast
clip in satisfactory fashion. By reference
to the previous articles, note that
sidewise flight can be obtained by
raising or lowering the torque prop axis,
or alternatively the weight can be
attached to a wheel strut Keep this trick
in mind, later, when building gas
models, you may wish to position the
gas tank in such fashion that the attitude
of the model changes in flight; as for
example, take-off directly into forward
flight with the speed decreasing as fuel
is consumed and with let-down in
autorotation vertical.
Cyclic control of the rotor is a bit
more complicated, but not greatly so,
and undoubtedly it will eventually
replace C.G shift control except in the
simplest models. This is particularly
true when we consider the advantages
of such a system in contest flying.
A cyclic control system means
having a control which can be moved to
secure flight in any desired direction,
without changing ballast, by altering the
pitch of the rotor blades for a segment
of their sweep around the circumference
of the "disc "
The type of cyclic control we are
interested in for model work is the socalled "indirect" or reactive control, in
which the linkage is not directly
attached to the rotor, but to an
intermediary point from which the rotor
is controlled. If we tried to attach the
cyclic mechanism directly to the blade
roots, and connected the other end of it
to the fuselage, we would find that this
AIR TRAILS
would freeze the system, resulting in a
stiff rotor and destroying the stability
we gained by freeing the rotor from the
mast in the first place
Therefore we must control the rotor
from some point not rigidly attached to
the fuselage. With the Bell system, this
is the Young fly-bar control (see
sketch). To work this connect the flybar to the longitudinal pivot with a
jointed lever which can be cyclically
pulled inward at the joint, thus changing
the angle between the fly-bar and the
blades, for a segment of each revolution
The reaction of the blades to this
deflection gives cyclic control.
The Hiller Paddle system (see
"Rotor-matic" sketch) uses two short
wings set upon a cross-arm, which is
attached to the central pivot. The angle
of these wings or paddles may be
changed through a simple scissors type
linkage, which is attached to a swash
plate. When the swash plate is tilted the
angles of the paddles change in
rhythmic cyclic fashion with each
revolution, and the rotor blades' reaction
to this produces a longitudinal rolling of
the rotor, which results as cyclic pitch.
This system is very simple, as is the
Bell, but in both cases avoid any
considerable play in the linkages since
this may result in excessive wobbling
and erratic control. However, for most
small models this may not be critical
because of the strong damping effect
(scale effect) present in models.
As a footnote to these two systems
we can add that it isn't strictly necessary
to duplicate the control systems of the
originals in order to get satisfactory
performance. Here is a simple dodge,
which judiciously applied works most
effectively Build the rotor, with its
paddle beam or fly-bar in the simplest
fashion and simply stick a wire up from
the fuselage in such a way that the
cross-arm bumps it gently at the same
point every revolution Presto' The
reaction gives you cyclic control.
Remember, however, that this is
control by unstabling—the bumper wire
should be quite flexible or the fuselage
may sway excessively, and. while it
sounds very simple, and it is, it can get
out of hand by displacing the rotor too
far if the jolts are too heavy.
OCTOBER,
29
I953
Tandem rubber job. Rotors are Young fly-bar type; Power transmitted through bevel gears at
each end of long motor which equalizes thrust. Brake on one rotor permits forward flight.
Co-ax close up. Dimensions not critical, but if large version is built top rotor should be
hung In gimbal since tension of heavy rubber motor may not allow sufficient see-saw.
In an effort to develop a system,
which would lend particular emphasis
to the qualities desirable in model
helicopter work, we have designed a
two-part series of rotors, which we term
the "bungee-dynamic series."
These rotors cover a wide field of
application and include power delivered
at the hub and power applied at the tip,
which means the series covers rubber,
internal combustion, rocket, pressure jet
and ducted fan configurations In the
middle of the series is our special pride
and joy—a system which we believe to
be the first truly successful, inherently
stable, co-axial rotor arrangement,
which positively controls the ancient
problem of rotor clash
The basic rotor and its derivations
are
shown
in
the
drawings
accompanying this article We start with
a two-bladed rotor (see sketch "Basic
Rotor Design—Cyclic Control") which
is see-saw mounted in gimbals and free
to pivot, within limits in a span wise
fashion This rotor is of the first part of
the series, which we term "locked "
By this is meant that the pitch of the
rotor blades relative to each other is Axed
at all times (as in the Hiller) except as
subject to collective pitch control. The
blades are stabilized by means of dynamic
weights, which protrude tangentially,
about one chord length ahead of the
Blades' leading edge. There is no fly-bar
or paddle-beam; instead, we have a double
horn to which are affixed two snubbers, or
"bungees" which run to a swash plate
which may be tilted to secure cyclic
control. These elastic connectors replace
the inertial damping forces of the fly-bar,
or the aerodynamic damping of the paddle
beam, and are simpler to work with than
either of these.
Because of the concentration of mass
in the rotor tips we can use much lighter
blades successfully— meaning balsa
instead of birch or pine, and the corrective
force is balanced at all times, exerting a
positive, yet gentle steering action upon
the system. This is the cyclic control
version.
And if we desire extreme simplicity
we just move the bungee connections 90
degrees, that is, amx them to the span wise
pivot to exert a continuous corrective force
upon the seesaw axis, and the machine
becomes controllable through C.G. shift
30
From this point of departure we
proceed to multi-blade applications —we
may build a four bladed rotor simply by
doubling up what has just been described,
except that of course only one mast
attachment, or swash plate will be
required.
In order to build a stable co-axial
machine, using rubber power as an
example, we merely construct the lower
rotor attachment with span wise pivots and
attach it rigidly, that is, without see-saw
gimbal to the drive tube with a bent wire
"lead-around" interconnecting the blades.
The upper rotor must be free to see-saw
about ten degrees up and down, which
allows plenty of leeway against clashing
with moderate gap, and this may be
accomplished, in a rubber model, by
simply rounding the edges of the thrust
button—the tension of the rubber motor
being sufficient "bungee" action.
If we build a gas model, with shafting,
then of course the upper rotor must be
mounted in a gimbal and snubbed with
rubber bands, permitting damped motion
between stops. Now with such an
arrangement as this we secure forward
flight by trimming slightly nose-heavy. No
other cyclic control is required. For
heading control a simple fin, as shown,
corrects for the downwash, which may
tend to rotate the fuselage the same way as
the lower rotor. The sketch ("Basic
Coaxial 'Bungee' Dynamic"), incidentally,
is all the plan needed by any reasonably
able builder to turn out his own machine in
short order.
One note on adjustment: The down
wash’s tendency to rotate the fuselage is
corrected by bending the fin. However,
after doing so the machine may show a
tendency to drift sidewise. This is due to
the reaction of the air against the fin, so
just move the rotor mast a trifle off-center
to correct it, countering the side thrust with
a bit of off-center lift.
The second part of the series includes
the unlocked rotors. These rotors may be
built in any number of blades, from one,
with counterweight, through two, three,
four, five, or as many as desired. In the
unlocked blade series (final sketch) we run
into "dynamic pitch." By this is meant that
the blades have no particular fixed pitch
relative to each other or the mast, but seek
pitch angles individually according to the
speed of rotation. This is
AIR TRAILS
accomplished by positioning, the dynamic
balances well below and ahead of the
leading edge of the blades, which causes
them to ride up under the action of
centrifugal force until a balance is reached
between the force exerted by the up-thrust
of the counterweights and the aerodynamic
pressure on the blades.
It is important with this system to
locate the hinge line knowingly to obtain
high efficiency, but in the practical
application we find it works well even
with rough approximation of position. The
hub attachment of this system to the mast
may be quite varied, from a simple rubber
disc which functions as a universal joint,
to separate snubbers for each blade pivot.
This system gives us a built-in and fully
automatic cyclic and collective control.
Auto-rotational letdown is fully automatic
(with a simple ride-out dog release on the
mast) which solves a mechanical problem
that can be knotty, and cyclic control is
merely a matter of shifting the C.G.
There is just one precaution to be
observed with this system in securing
forward flight by C.G. shift. It is better to
have the snubbers a bit too limp than a bit
too tight, and don't overdo the noseheaviness. The reason for this is that a
condition of "over-cycling" will occur if
the snubbers are too tight, that is, the blade
pitch will adjust itself too rapidly,
accelerating the cyclic action, meaning the
model will nose down and dive into the
ground.
If the snubbers are too limp the worst
that will happen is that the rotor will tilt
backward and forward flight will proceed
at a snail's pace. This adjustment is by no
means highly critical—the admonition just
given is of the same order as instructing a
builder of conventional planes not to tilt
up the leading edge of the stabilizer too far
if he doesn't want the model to dive in.
Whatever system you elect to use, try
to make your selection knowingly, based
OCTOBER, 1953
This machine believed to be fist truly successful stable co-axial type to solve problem of
control by C. G. shift and eliminate problem of blade clash. Could be flown with bungee
cyclic control to lower rotor only. Model has been kept simple, no collective pitch used;
hence letdown must be under residual power. Dimensions: Disc: 24". Mean chord; 1½ ".
Tube: 7½ " x 1" dia. (1/32" sheet). Fuselage: 3/32" sq. stock 18" long. Weight: 2 oz.
Performance: Altitude: 20 ft. Forward flight: 25-30 ft. at 5-6 ft. altitude (Hand-wound). That's
high performance on three loops of 3/16" flat!
on what you want to do with it,
compared with its characteristics. For
example, the unlocked bungee-dynamic
rotor is perfect for jet power, quite good
for gas engine, and a complete bust for
rubber—because it wastes too many
revolutions in getting started. Rubber
is a special case anyway, since the number
of turns is always strictly limited by
dimension, which isn't at all true in the
case of jet or gas power.
For rubber models the best bet is to
skip autorotation and bring the model
down under residual power, or if you are
the ambitious type, fly with a locked rotor,
which unlocks and de-pitches itself when
the power is exhausted.
Once again, we strongly recommend
that the first model should be rubber
power—it will give you a wonderful
opportunity to get the "feel" of rotor wing
flying, without introducing a lot of
distracting complications.
(To be continued)
31
Once the reader has flown a rubber job
successfully and wants to build a model capable of
really big performance, it will be necessary to
switch to gas engine or jet power. Let's deal with
jets first. The Jetex motor is an excellent source of
power for model helicopters; generally speaking
two will be used, although it might prove practical
to use up to four, although this complicates the
problem or getting a number of motors ignited at
the same time in order that the charges bum evenly
to preserve the balance of the rotor. For that matter
a one-bladed rotor, with the blade balanced by the
motor, can be used very successfully—which I
know sounds a bit contradictory, but the practical
fact is that the burning charge getting out of balance
in a one-bladed system is considerably less critical
than, say, two or three charges consuming at an
uneven rate in a multi-blade system.
The reason for this seems to be that the thrust
output of the Jetex varies according to the amount
of fuel left at any given instant, and peaks at the last
few seconds. Thus in a multi-bladed system we
have several thrust peaks, and if they do not closely
approximate each other the thrust load on each
blade may vary widely, meaning considerable pitch
variations in a dynamic pitch rotor. In a one-bladed,
single-motor job, the thrust variation is inherently
"in gear" with the single rotor blade. Unbalanced
centrifugal loads due to fuel, charge consumption
result in a narrow period of oscillation of the rotor
mast, but since this vibration lies in a span wise
plane the practical effect is not serious—for a
model.
The jets replace the dynamic weights of the
unlocked type rotor, being mounted below and
ahead of the* rotor tips. The angle of thrust should
be slightly downward, and it may be necessary to
provide up-pitch limit stops to facilitate getting the
rotor going. The balance of the rotor blade on its
pivot should be slightly nose heavy with fuel charge
aboard. Note this, because of the position and forces
exerted in this type of rotor it is not necessary to use
stable blade sections— use the highest lift cambered
section you deem practical and don't worry about
pitching moments; the orbiting of the tip weight
clamps the blade firmly at whatever pitch the speed
and dynamic settings call for, and transition into
auto-rotation after burnout is smooth and easy with
a good let-down.
The adapting of a tiny internal combustion
engine that screams out its very high power rating at
speeds in excess of 10,000 rpm to rotors, which run
under 2,000 rpm, offers an interesting challenge.
This may be achieved in a number of ways.
The classical method is to reduce the speed and
increase the torque through reduction gears. These
should be of at least 5-1 ratio and there must be
some sort of clutching arrangement between the
gears and the rotor, otherwise it may prove to be
impossible to start the motor, or gear teeth will be
stripped by the high starting loads. A clutch
satisfactory for this purpose should en-
NOVEMBER, I953
With full size choppers more and more in the news this
informative series will get you started off on the right
foot in building your own model helicopter
CLOUGH’S CONCLUDING COMMENTS
CONCERING 'COPTERS
By ROY L. CLOUGH, JR.
25
gage smoothly and positively, and may be either of
the manual engaging type, in which the release of a
lever holding two faces apart permits them to be
forced together under spring compression, or of the
centrifugal type which engages automatically with
an increase in speed. See center sketch on this page.
Clutches require access to tools and a knowledge
of machining operations, but this should not deter a
really determined builder. The ideal thing, of course,
will 1% for some hep manufacturer to read this and
produce a small lightweight clutch-reduction-gear
unit at a reasonable price. Experience in the model
racecar field indicates this can be done.
Reduction can be had by means of pulleys and
belts, with a sliding engine mount serving as a
"clutch," but belting is not the most satisfactory
power transmission. I have flown a K&B .049 job
with belt reduction, briefly. The belt begins to slip
after a time and the model descends. For this job I
used a round belt running over wood pulleys at 4.5:1
with a heavy application of a good belt dressing. The
problem seems to be that the high speed of the
engine pulley soon glazes the belt, causing excessive
slippage. Howard G. McEntee has suggested using
small Vee belts. This might work a lot better due to
the better traction offered by such a belt, but
obtaining Vee belts and pulleys small enough for the
purpose has been a poser. When using belt drive
with gas engines, great precautions must be taken to
keep fuel spray 06 the belt and pulley. A baffle
between the shaft and intake tube and exhaust ports
is highly necessary, and frequent wiping of oozed oil
from the end of the main bearing is a must.
Another angle, which I’ve been experimenting
with lately, is to use a torque converter between
engine and rotor. A torque converter is simply a
specialized type of fluid clutch and operates without
any direct connection. I use a small high-speed rotor
connected directly to the engine shaft, running inside
a larger rotor, which is connected, to the helicopter
end. The casing is filled with castor oil. This device,
in bench tests, appears to transmit a fair amount of
power—with redesigning and a bit of finagling it
should be quite evident. However, I have had a lot of
trouble due to overheating, which causes some of
the oil to ooze out past the bearings, and that results
in lowered efficiency of power transmitted.
In any event power for the torque prop isn't hard
to arrange. Turn this about two to three times as fast
as the main rotor by means of a simple string belt
running over sandpaper-faced pulleys. Remember
that the torque prop should stop when the model
goes into autorotation in order that it won't swing the
tail around in a circle on the way down. Simply
attach the driving pulley between the clutch and the
ride-out dog of the rotor. We mentioned this before,
the rotor release. Whether or not you plan for
autorotation you must have a rotor release, which
permits the rotor to override when the power quits.
(Continued on page 66)
26
AIR TRAILS
Clough on 'Copters
(Continued from page 26)
Otherwise the great amount of
kinetic energy stored in the
spinning rotor may twist off a shaft
or strip gears or even shatter the
blades if the system suddenly
freezes when the motor stops. This
unit can be incorporated in the
function of the clutch or may be a
separate item in the rotor hub: I
prefer it to be separate since this
simplifies the operation of
stopping the torque prop when the
motor quits.
Now, how about really simple
gas motor hook-ups, requiring no
gears or clutches?
Sure, it is possible and
practical,
and
may
be
accomplished in several ways. One
way is to use torque reaction
drive—such as in the little Infant
powered job of the previous
article. However, don't use the
primitive semi-articulate rotor
system of that model, but build
your rotor along the lines discussed
in the previous issue for the rubber
co-axial job, except use unlocked
blades on the big rotor to get a
good auto-rotational descent, and
locked, but feathering blades on
the small rotor attached to the
engine shaft, say in a rubber
mount, to permit a small amount of
see-saw action. Because of the
strong downwash of the small rotor
a brake or fin is required to prevent
fuselage rotation, but for simplicity
this is hard to beat.
Propelling the rotors at the tips
by means of propellers has been
suggested many times by many
people. It seems simple, but it can
be very troublesome. The reason is
two-fold. First, the props act as
gyros running in a tight circle—
meaning the engine shaft tends to
twist upward or downward,
depending
upon
rotational
direction of the blade to which it is
AIR TRAILS
affixed; second, torque effects may
add a bit of complication. You can,
however,
make
such
an
arrangement work if you use my
rotor configuration, the unlocked
system, and play off torque and
gyro effects against centrifugal
loads. Use very light driving props
of as high a pitch and small a
diameter as possible, and place the
thrust line of the engine angled
toward, or away from, the chord
parallel, depending upon which
way you run the rotor; to help
compensate for gyroscopic twist.
In connection with this, note
that the props can be shrouded,
converted into ducted fans, with
stator vanes to eliminate torque
effects. This makes a neat looking
job, but auto-rotation suffers
heavily
from
any
bulky
excrescence at the rotor tips.
Another method of drive is to
use a pressure-jet configuration.
Mount the engine in the fuselage,
or in the rotor, as a blower
supplying air under pressure to jet
nozzle* in the tips of the blades.
This system isn’t terribly efficient,
but the great power-to-weight ratio
of modem small engines will let
you get away with it if you are
careful. This produces very dean
structures, smooth blades, and
excellent
auto-rotational
and
control characteristics—so in a
way it might be said that the
system is efficient after all.
There is one angle in designing
pressure jet jobs, which I am not
too happy about. From the
standpoint of efficiency it is a fine
thing to have the cylinder head and
exhaust opening inside the duct,
that is, behind the fan, in order that
the motor may cool better, run
faster from the supercharging
effect of air being rammed into the
intake, and the pressure augmented
by the heat of the cylinder and
exhaust gas efflux.
However, the oil sprayed by a
2-cycle engine tends to gum up the
works and mess up the blower
ducts. Tentatively I run my jobs
that way anyhow and clean the
ducts after each flight with a wad
of cotton tied on a string, which is
pulled through. Old-fashioned but
effective.
I have a grave suspicion,
nevertheless, that the drag of the
air against oil-coated tubes may
cancel out the gain produced by
exhaust heat-gases. Have you, for
example, ever noted the ripples
and ridges that develop on the
surface of an oil-coated wing
exposed to prop blast? That means
quite a bit of drag, and it seems
logical to assume that the same
condition obtains within the
pressure ducts of a blower jet
system. While it is true that a lot of
the oil is blown out with the air
and by centrifugal force, a lot of it
still sticks to the inside. And that
isn't so good. Probably, in the
interests of tidiness, it might be
well to advise piping the exhaust
into the open, and eschew the
theoretical benefits in favor of the
practical considerations.
I have tried to present as many
applications, suggestions, and
observations as space will allow, in
the belief that model builders win
find more of value in something
like this than they would in an
article which dealt with the
construction of one particular
model and consisted largely of
instructions to glue stick A to stick
B and so forth. This 2-part article,
together with the previous series
covers, I fondly believe, enough of
the basics of helicopter principles
to permit anybody to turn out a
very satisfactory job with a
minimum experiment. Try it and
see for yourself!
NOVEMBER 1953
36
Air Trails, HOBBIES For Young Men
"This is one of my all-time bests," says
the designer who has been acclaimed as
one of the country's most original men
of modeling. No fancy gimmicks here, no
frills, just a little easy construction, and
then lots of fascinating flying ahead
The
Control-Line
Gyro-Copter
By ROY L. CLOUGH, JR.
You'll stop the show when you put
this spectacular rotary-wing job aloft.
In flight it looks just like a big tandem
rotor helicopter with lines reminiscent
of the Piasecki and Bristol machines.
Although the appearance of the model
is very close to the double-ended helicopter types it is really more closely related to the gyro-dyne family—rotary
wing machines which may rise up vertically, like a helicopter, but which depend
upon a propeller for forward motion. In
this respect it is somewhat similar to an
autogyro.
To avoid mechanical complication our
model uses a short ground run instead of
vertical take-off. With this system it is
not necessary to power the rotors and
taking off with forward speed is more
practical in a controlled model because
it keep the lines tight.
Okay, it sound great, but how does it
handle? Is it hard to fly? How does it
behave in a breeze?
The answers are that this model is
actually easier to fly than the average
sport job. The control response is very
smooth and positive and it stays right
out at the end of 50 foot lines with any
good .19, as high as you'd care to fly any
non-stunt type model of this weight, and
the wind bothers it less than fixed wing
models. There is a barely perceptible
cyclic slap from the rotors, but, far
from being a nuisance, this gives the
"feel" of real rotary wing flying. You
do not require any particular knowledge
of rotary-wing craft to build and fly it
successfully.
Begin with the fuselage which consists
of two 3/32" x 3" x 36" sheet sides cut
to shape. The bulkheads are 1/8" sheet
and the two rotor mast carrying bulkheads should be cut from very hard
stock, or else substitute plywood. You
will note that the fuselage follows very
conventional construction lines for sheet
balsa building and requires little or no
explanation except at the front end.
This model differs from usual controlliners in that the elevating surfaces are
at the front end instead of the tail.
Therefore study the control hook-up and
be sure you understand it—the elevators
are depressed to raise the nose, and
lifted up to lower it, just the reverse of
SEPTEMBER. 1955
usual. The landing gear arrangement
should be followed; if you use a radial
mount engine, for example, put in a plywood floor to bolt the landing gear firmly
in place. Note the L.G. wire should
not be firmly attached to the elevator
cross-arm piece, but is held to it by rubber bands which act as shock absorbers.
The motor mount depends upon the engine. We used a McCoy Sportsman .19
with rear rotor valving. This is about
the top power which should be put in
this model—in fact, if you go down to 35foot lines, a good hot .09 engine might
prove quite adequate.
The rotors are very simple to build,
but a good touch with a soldering iron
is necessary. These rotors are not rigid
as they may appear at a glance, but semiflexible, which takes the cyclic jar and
shock out and greatly increases the
operating life. We mention this so you
will not substitute a heavier wire than
specified for the arms, or try to by-pass
the soldering job by gluing up a solid
wood rotor head. A glued-up rotor head
seems simple and easy, and it is, but the
catch is that if you equip the model
37
with rotors like this you can expect
cracked blades after the first flight and
somewhere along about the third flight
you will get an interesting shower of
broken balsa as the rotors shatter under
cyclic pounding.
Clean metal and acid-core solder and
an iron a bit hotter than necessary will
insure a good job. You do not have to
use bottle caps of the exact size shown,
for anything similar which will fit is
okay. Be sure to leave at least one inch
of wire between the blade root and the
hub for flexing. The cone angle should
be as shown; if no cone angle is used the
model will not fly well in level flight,
but will have to be nosed up, which is
sloppy. Be sure the rotor masts tilt at the
correct angle; the rear rotor operates at
a greater angle of attack than the front
to compensate for downwash effects. This
will not make the model nose in. When
you assemble the rotors to the mast make
sure they rotate freely; there should
not be any great difference in the ease
with which each bearing turns.
Flying the model is not much different
than flying any sport job. The four-wheel
gear produces exceptionally good ground
stability, but do not neglect the usual
down-wind take-off precaution—you have
two big rotors here, plus a propeller,
and if you flub a stall-off in a strong
wind and the model rolls up in the lines
it will take five years to untangle.
After a couple of normal take-offs
under normal conditions you will learn
the trick of yanking the nose up immediately after your helper releases the
model, and then letting it drop back.
This trick sets the rotors spinning very
quickly and reduces an otherwise 15-20
feet take-off run by half. Spinning the
rotors by hand before releasing the
model does not work well and should be
avoided. Near the end of the run, when
the motor starts to sputter, bring the
model down to five or six feet. When
the motor dies bring it in gradually.
Full-size plans for the Tan-Giro
are part of Group Plan # 9 5 5
Hobby Helpers, 770 Hunts Point
Ave., New York 59, N. Y. (50c).
A SURE FIRE
AUTOGIRO
Autogiros are usually tricky, but this one IS a sure
fire performer
by ROY L. CLOUGH JR.
A DISTINCTLY rare item—the free flight model autogiro—has the
reputation of being an extremely difficult thing to build and fly.
Demonstrating that this reputation is quite undeserved, this model
is simple enough to be knocked out in an evening and is no more difficult
to fly than a conventional ship. In fact, it is not even necessary to set the
rotor in motion before launching This model climbs at a steep angle and
when power is exhausted floats gently back to earth on its spinning rotor,
thus eliminating the chief cause of destruction of flying models—a headon glide into a solid object
Begin construction with the fuselage, which is built up from 1/32"
medium sheet balsa. Stiffened are used at appropriate intervals and the
thin covering is backed up with 1/16" sheet at the point where the
landing gear is attached
Nose and tail plug openings are reinforced with strips of 1/16" x 1/8"
stock. The bubble canopy is carved from a block of soft balsa
Tail surfaces are 1/16" sheet and are cemented in place with no
offsets of any kind Carve two end plugs; adapt one as a tail hook and the
other as a thrust bearing for the propeller The prop may be sanded down
from a purchased pre-sawed blank or built up as was the original. The
prop should be of medium low pitch, and diameter should not exceed 9
inches Freewheeling would be of no particular advantage in this model
Bend the landing gear from 1/20" steel wire and cement it to the
reinforced underside of the fuselage Wheels are 1 1/4" in diameter and
must be hardwood.
The rotor and rotor mast, while quite simple, must be made exactly
according to plan to obtain optimum performance. The mast is bent from a
length of 1/20" steel wire and is anchored to a plate of 3/32" hard balsa
which is cemented to -the top of the fuselage. A short length of drilled
hardwood dowel is slipped over the mast and cemented to the plate for
added strength.
The rotor is acted upon by highly complex forces in flight and must be
highly flexible to permit these forces to be damped out without upsetting
the model The hub is a piece of dowel which is drilled to permit a
loose fit on the mast The spars are 1/16" x 1/8" hardwood, pushed into
slots in the hub at the angle shown on the plan, and cemented. Two short
pieces of 1/16" x 1/8" balsa are cemented to the upper sides of the
spars next to the hub. Rotor ribs are simply toothpicks. Cover the blade
sections with a strip of smooth typing paper, 2 3/8" x 11" for each side.
Note that the rotor has 0 degrees incidence and will spin in the proper
direction regardless of the direction from which the relative wind comes.
This is very important!
Drill out two short pieces of dowel; slip one over the rotor mast, then
put on the rotor, using the other bit of dowel to hold it in place. The
proper height of the rotor above the fuselage is the shortest distance,
which will give good propeller clearance. The retainers are cemented in
place after testing.
To test fly: install an 8-strand motor and balance the model at the rotor
axis. Drop it from shoulder level a few times to make sure the rotor works
well, then try short powered flights in calm air The model should climb
without deviating right or left and is performing best when it gains a foot of
altitude for every foot of forward flight Whatever minor adjustments may
be required can be made by slightly bending the rotor mast
MODEL AIRPLANE NEWS
MARCH 1948
BEFORE YOU conclude that this is a model
of a helicopter, take another look. It's a model
plane with a spinning wing, or rotor, that
windmills in the slipstream of a conventional
propeller to provide the lift necessary for flight.
The rotor is self-spinning and that's where the
autogiro or gyroplane, as it is now called,
differs from the airplane and the helicopter in
appearance, in flying characteristics and also in
construction. And on the end of a control
line it is a new experience for model-plane
fans.
Control-line gyros have been built, of course,
and flown with fair success. But none could be
considered spectacular performers. Some showed
a persistent tendency to roll up in the control
lines and some that performed satisfactorily
otherwise developed an arm-shaking vibration.
This appears to have been largely due to use of a
rigid motor, which tended to develop a condition
of unbalance while in flight. The rocker-type, or
seesaw, rotor used in this model starts spinning
quickly and easily and the gyro lifts off and flies
smoothly without dipping, diving or rolling. It
pulls hard, but not too hard, on the control lines
and settles as lightly as an autumn leaf when the
motor fades.
The fuselage is simply an elongated balsa box
made mostly from 3/32-in. material except the
bulkheads, A, B and C, and the stabilizer which
Mount engine on the firewall with four small bolts,
using washers under left mounting to provide offset
Remember the
spinning-wing
autogiro?
By
ROY
L.
CLOUGH,
JR.
are 1/8-in. stock. Although the over-all size
of the bulkhead, C, is given, you may have to
do some fitting of this member to assure a true
fairing of the fuselage sides and top piece. The
fuselage is fitted with a conventional engine,
propeller, landing gear and a standard controlline elevator for controlling the gyro in flight.
The rotor mast centers 4-1/4 in. from the
forward end of the fuselage. In assembly it
passes through the bottom of the fuselage, the
bend at the end being seated and cemented in a
notch cut in the bottom of the fuselage. Note
that it also passes through the control strut and
the bell crank. Washers are soldered above
and below the crank, permitting the latter to
swing freely on the mast. After installing the
controls and the reinforcing strips at the forward
end of the fuselage, cement the fuselage top
pieces in place, then the pilot's head and the
fairing.
Study the rotor drawing closely. Note that
the blades operate at a negative pitch and that
the rotor, although stiff from tip to tip, is pivoted
POPULAR MECHANICS
at the center to permit a seesaw motion. Stops
on the hub pivot, limits vertical motion so that
the blades won't strike the tail. This type of
rotor mount allows the blades to rock without
transferring motion to the fuselage, yet keeps
the rotor tracking evenly.
Give the model a coat or two of sealer before
applying pigmented dope. Make certain that
everything runs freely, and that the center of
gravity is either right on, or just ahead of the
control crank or bell-crank axis. If the center of
gravity (CG) is behind the control-crank axis,
the model may not pull hard enough on the lines
to give good control.
After you get the feel of it, you can make jump
takeoffs by letting the model roll about 6 ft.,
giving it full up and dumping the-elevator quickly
to bring it into a normal flight attitude.
Incidentally, that machine-gun-like popping
you hear in flight is common to all rotorcraft.
It's caused by the rotor blades running into their
own tip vortices.
JULY, 1962
Torque Reaction Helicopter Models-further experiments
By: Roy L. Clough Jr.
Thus far in discussions of model helicopters most
reports have stuck pretty closely to single rotor
machines, or those in which a rotor, or pair of rotors
support the weight symmetrically.
However, for model work we find that duplicating
the rotor arrangements of the big craft is not very
practical except in rubber or jet power configurations,
both types being unfortunately of short-lived duration.
If we wish to use gas engine power, at this stage of the
art, we must find some method of using a power plant,
which grinds out several thousand rpm without getting
into too much complication. One method of doing this
is by designing our ships to the torque-reaction drive
specifics, the system whereby the engine torque spins
a large rotor in opposition to the rotation of a smaller
prop on its shaft. (This, incidentally, should not be
confused with true co-axial systems, which utilize
equal-sized rotors turning in opposite directions.)
Very good performance is possible with torquereaction drive although it has two major drawbacks: 1)
it is not very efficient because of low mechanical
advantage; 2) the system does not behave in classical
fashion—that is, we have a new and different set of
forces and reactions to deal with. Objection #2 is not
serious if we remember to keep the reactions of this
type isolated in our minds from the reactions of
standard types and not confuse them.
Torque-reaction drive helicopters are queer birds.
They are almost as removed from conventional
helicopters as, for example, an autogiro. The reason
for this is that torque-reaction drive helicopters split
flight duties between a large, slow-moving rotor and a
prop attached to the engine shaft. The engine shaft
prop is generally standard and it has one main
function-it provides 99% of the lift. In some cases it
may be used to contribute to stability as well, but the
primary function is to lift the machine. Its gyroscopic
effects are completely negated by the much larger
mass of the engine and big rotor whirling around in
the opposite direction underneath it. The function of
the large rotor is to provide a torque drag on the
engine, a device with which to secure stability and
control, and finally, to serve as a parachute to let the
machine down without damage when the power stops.
This division of labor produces an unusual
condition because, under power, the large rotor is
operating in a substantially unloaded condition. Its
blades are not damped by a strong aerodynamic
pressure as they would be if the weight of the machine
was being supported by them. As a result, control and
stabilizing reactions assume an altered aspect. The
builder who does not understand this may find his
model crashing repeatedly despite his efforts to re-rig
it for normal flight, because the control reactions, in
most flight regimes, are actually reversed. If we build
a very simple helicopter, with rigid, un-pivoted blades
without tip weights and adjust it to fly forward we
find that it starts to slide ahead, then noses upward
sharply, slides back and repeats the pattern with
increasing amplitude until it crashes. The reason for
this is that the advancing blade produces a high lift
force when it encounters the relative wind; this lift
processes 90 degrees forward tilting the nose up,
which kills forward speed, then the model slides back
with what was the retreating side of the rotor now
producing a lift which will move 90 degrees, or to the
tail, riding the tail up then sliding back, etc., etc.
So we now pivot the blades and fit them with
dynamic balances. Now when the model moves ahead
the air pressure on the front of the disc makes the
rotor blade twist its pitch angle upward, 90 degrees
ahead that is the retreating side, and downward, that is
90 degrees behind on the advancing side. When this
happens the change in pitch 90 degrees to the side
produces a force that is moved another 90 degrees, so
that the rotor disk tends to tilt up at the rear and down
at the front. When the model is in a state of balance
the forces cancel out and the machine flies forward
without riding up at the nose or going into a dive. This
is due to the upward force of the relative wind striking
the advancing blade being leveled out by the precessive pitch shift in the blades produced by the
pressure of the relative wind, which tends to nose the
rotor disk down. This is the way it should work, and
does work when the CG is properly located. However,
if the CG is improperly located trouble develops, and
this trouble is usually a dive. Why?
Suppose the builder flies his model once or twice
and it works quite well, moving forward steadily. He
then wishes to see it rise vertically. It would seem
reasonable then, to add a bit of ballast to the tail to kill
off the forward motion? Unfortunately it does seem
very reasonable. We have the past precedent of fixed
wing models; we have the precedent of single rotor
helicopters, which fly forward if the weight is moved
forward and back if the weight is moved back. Seems
reasonable. So weight is added to the tail, the model
rises up, starts forward faster than ever, noses down
and crashes. Why was this?
We just said that torque-reaction helicopters are a
special case because of the unloaded condition of the
rotor, which does the controlling. Here is what
happens: We have noted that air pressure on the rotor
results in a cyclic action which resolves to cancel the
nose up effects of that pressure, that is, relative wind
effects are self-nullifying. But, any force applied to
the rotor reacts in cyclic control, the blades shift in an
effort to nullify the applied force.
Now, when we add weight to the tail we are
placing a steady pressure on each pivot blade as it
passes over the tail-the CG has shifted—and by
reference to gyroscopic precession laws we see that
this force will result 90 degrees further on, or at the
side. Thus the blade advancing tilts down and the
blade which is retreating on the other side, tilts up.
This tends to twist the rotor laterally, but again
referring to gyroscopic rules we can see that this twist,
moving 90 degrees, resolves to push the nose down!
Thus if we take a helicopter which is adjusted to rise
vertically, and move the CG aft a little bit the model
will now fly forward—BUT if we move the CG too
far aft the model will overdo it and dive into the
ground. This is because we have two cyclic instigators
working, the CG imbalance, plus the normal cycling
produced by forward flight.
A model of this sort therefore is fairly sensitive to
CG location, too sensitive, as a matter of fact, so it is
customary to build in a safeguard which will allow a
wider altitude of CG travel before diving or tail
sliding occurs. A good example of this is the Berkeley
kits, which use two different methods of obtaining the
same result. In the D model we note that two of the
blades are fixed in pitch. Thus, as the model moves
forward the lift build, caused by increased relative
wind meeting the stiff blades tends to push the nose
up, while the cyclic action tends to push it down.
Since the up couple is a bit stronger we also have a
drogue on this job, which increases the cyclic reaction
of the pivot blades, and, secondarily slows the model
down. '
Thus, within allowable CG travel the tilt angle of
the machine is self-governing. If it slows down the
cycling action, which is fully automatic, tends to
speed it up, if it goes faster, the stiff blades bring the
nose up, slowing it down. This governing action is
pronounced enough to permit flight in surprisingly
high winds and gusts without getting into trouble.
However, if the CG is moved too far aft, the balance
of forces is upset and the model will dive in.
Ordinarily the model D gives no trouble—except
where the builder has put on several heavy coats of
dope and has not re-checked his CG afterward. The
CG position shown on the plan, incidentally, is for
absolute maximum top speed. To climb vertically it
must be moved ahead with ballast.
The other model, the TR, has four pivoted blades
and uses a swivel prop to provide a recovery couple.
When the tilt is to the right, changing the lift vector,
which puts a side load on the rotor which induces a
cyclic shift which pushes the nose upward. This
model has a tail rotor to control heading, and a few
words on this: A rudder will not make a helicopter
turn. A positive side thrust is required; hence a tail
prop is needed to push the tail around or to hold it
steady. A rudder will only crab the ship slightly while
it continues in substantially the same direction.
Another method of obtaining turn without a tail rotor
is to tilt the rotor mast toward the side toward which
turn is desired. Don't get confused on this, the model
does not slide that way, but the downwash rebounds
from the side of the fuselage at a different angle,
tending to roll the model over—but again by
gyroscopic reference, the roll is resolved at 90 degrees
into turn.
There are many ways in which a torque-reaction
helicopter can be set up. One thing, which is quite
important, is to respect the fact that the fuselage lies in
the downwash of the small prop, and exposed areas
should balance, or very nearly so or there may be
serious trouble. The use of small fins in the prop wash
to obtain turn, or to reflect the wash' backward for
reactive forward propulsion meets with some success
and one can use a twisted stabilizer which tends to put
torsion on the fuselage with increased forward speed,
to induce nose-up cycling, relative wind strikes the
prop edge it as a corrective force couple.
When designing originals it is a very good idea to
include always some governing factor on forward
speed. Rig it either with a swivel prop, a stiff,
alternate set of blades, or torsion fins. Speed of
forward flight will vary with the design and power
plant, and will not be as high as a fixed wing model—
a good fast walking pace is about right with present
designs.
—ROY L. CLOUGH, JR.
THE USE OF TORSION FINS
"A" must clear rotor disk. Fins may be either
horizontal or vertical. As forward speed builds up,
fins tend to roll fuselage to right, which induces
nose-up cycling moment.
FORCES ACTING ON TYPICAL HELICOPTER
Barred lines: drag of drogue tail produces down
pressure on balance weight of pivoted blades which
induces cycling action which tilts blade down at side
and pushes nose down. Dash lines: fixed pitch of this
blade encountering relative wind produces lift at side,
but since rotor functions as a gyro the reaction moves
90 degrees, producing nose-up tendency. Solid line:
wind pressure on pivot blades reacts at 90 degrees to
make pitch change shown to hold nose down. Thus, if
CG is too far aft it induces cycling in the pivoted
blades which reinforces down control of drogue tail
and wind impingement on pivoting blades and
overrides nose-up tendency of fixed blades and the model will dive in. Centered CG has no cyclic effect and model
will rise vertically. CG too far to front will cause model to back up or tail-slide. NOTE: CG is usually slightly aft
of mast CL in order to balance fuselage effect and promote forward flight.
FORCES ACTING UPON TYPICAL SEE-SAW PROP HELICOPTER
A) CG rigged tail heavy induces cycling in
pivoting blades which causes nose-down forward
motion (barred lines). B) With CG centered, model
rises vertically. C) Nose-heavy CG will cycle blades
into backward flight (dash lines). Wind pressure on
blades produces precessive pitch change at side
position preventing nosing-up (solid lines). Role of
the swivel prop: wind pressure on front of see-saw
prop, by gyro precession, causes it to tilt to model's
right, which angles lift vector to right. Side thrust on
rotor system produces cycling which makes nose of
model ride upward, thus limiting forward speed and
preventing dive (assuming CG is correct). Seesaw
prop must be mounted to rock freely for best results.
(Seesaw prop can be eliminated if two opposite
blades are fixed pitch with counterweights removed.
Air Trails Model Annual - 1956
Snapper
By Roy L. Clough Jr
Ever
since the first half-A engines
appeared we've thought it would be a riot
to stick one on a ten-inch wing and turn
the contraption loose. We finally pared
some wings down to mere stubs on an old
free flighter and tried it.
It was a riot . . . a short-lived one.
After we swept up the balsa wood
(never leave the pieces in a hayfield—it's
bad for cows) and cleaned the turf out of
the engine we came to the conclusion that
a free flight speed job would have to be a
carefully tailored affair. The wild
corkscrew gilhooley and painful thump
of the clipped down free flighter had
given us a lot to think about.
A model intended to reach high speeds
in free flight must have (1) a high degree
of directional stability,
(2) a sharply limited
motor run, (3) no
tendency to climb or dive
under slight variations in
power output, and (4) be
light and very rugged.
In order to obtain these
characteristics we must
(A) find some way of
managing a whale of a
lot of torque with a small
Author Roy L. Clough Jr. wants you to admire that ultra simple, surgefree, positive-duration fuel tank! Full size working
Drawings are on Hobby Helpers’ group plan #1162
American Modeler --November 1962
wingspan, (B) limiting the motor, run
reliably without a timer mechanism,
(C) making the model insensitive to
gusty weather, and (D) using the
lightest and most simple geometry
consistent with adequate strength.
"Snapper" is the answer. It is a
reliable little ship. It will fly in calm
or windy weather with anything from
a Cox .010 to an Atwood .049. With a
good hot .049 it will hit close to
100mph.
Snapper is rakish, but, except for
the cockpit, styling had nothing to do
with its appearance. It's designed that
way because that is the shape that
will best do the job.
Any built-in offsets in thrust line,
wings or control surfaces are out. It
doesn't take an engineering degree to
figure out the reason. The model is
light and it goes like a devil with his
tail on fire. You'd never get a chance
to balance off wing incidence by
varying down thrust or to use any of
the usual free flight adjustment
tricks. The wings, stabilizers, fin and
thrust line must be substantially
parallel to each other. Dihedral angle
raises the tips of the wings and this
raises the center of resistance. We
have to counter it by raising the
thrust line a little above the wing. We
require a special fuel tank and it has
(Continued on page 47)
29
to be on the bottom of the plane.
This pretty clearly calls for an
inverted engine mounting. So, we
start the motor with the plane held
upside down where we can check
the fuel flow.
Don't try to use a timer tank or
fuel shut-off. This little job will
chew off about 100 feet of airspace
a second when it gets rolling. That
means it could be well over a mile
away in less than a minute if you
forgot, or if something stuck.
Somebody is sure to ask why we
included a landing gear on a handlaunched speed job that lands in
the grass.
The answer is: it helps to keep
dirt out of the engine. But don't
expect any wheels-on landings.
47
All that torque and no rudder?
Just elevators? That's correct.
Because of the small span of the
model the elevators act as ailerons.
This means we correct for torque
effect by giving the elevators a
differential twist. The left one
slightly down, the right one
slightly up. The less one has to
monkey with on a high-speed rig
like Snapper the better.
Cut off the needle valve about a
sixteenth of an inch outside the
needle valve body, file a slot in it
and do your adjusting-with a
screwdriver. If necessary, jam the
needle valve body a little out of
round to get the necessary friction.
Needle valve settings (on the
Atwood) vary about half a turn
around the "ideal" position. Model
is started with a heavy prime and
the first tankful is run out to warm
it up. It doesn't seem to be practical
to connect the open end of the
coiled fuel line to a tank then
detach it before flight. This
changes the suction pattern and the
needle valve, would be in need of
adjustment before launching.
With the arrangement shown the
motor turns over for about 12
seconds. That gives you about 8
seconds to detach battery wires,
straighten up your aching back and
give the model a solid heave.
Plenty of time, once you get used
to it. The model will whizz for a
couple hundred feet before running
out of juice. When it does it will
slow up like it hit a brick wall and
roll to the right. Don't expect much
of a glide.
Pre-flight adjustments call for an
arrow-straight "glide" with a fair
amount of roll to the right. Make
your first test hops over deep grass
with low pitch props; step up the
pitch, and the speed as you –
become accustomed to the
adjustments. Snapper should fly 10
to 15 feet up,
No thermal worries here!
American Modeler -- November 1962
Skyhook...
By ROY L. CLOUGH, JR.
DRAWING boards and in secret test
ONhangars
of at least a dozen major
companies and scores of smaller ones the
helicopter of the near future is being evolved.
Rotating-wing craft have caught the
public fancy, and manufacturers are
hurrying to perfect their direct-lift wares
for the postwar market. Already at least
three
distinct
types
have
flown
successfully.
In the helicopter, torque effect is one of
the major problems. If a single rotor were
attached to a power source with no
provision for this effect the entire
fuselage would spin around and around.
Two rotors revolving in opposite directions
around a common center overcome this
tendency.
Counter rotation is employed in Skyhook,
a model that flies straight up like the real
thing and, when the power is expended,
descends in a spiral and lands on its wheels.
Skyhook is best flown indoors, but may be
sent aloft outdoors on calm days.
It will be noted that the motor tube, which
also serves as a shaft for the lower rotor,
is attached only by a simple bearing at the
148
lower end. The upper end rotates freely in
a hole in the top of the fuselage with 1/8"
clearance all around. Power absorption in
one rotor is constantly balanced by that of
the other and an unbalanced torque
condition cannot occur.
Elimination of torque is not enough,
however, to obtain satisfactory flights in
this type of model; hence the directional
vane pointing rearward from the top of
the fuselage.
As the downwash of air leaves the lower
rotor, it does so with a rotary motion and
tends to impart this motion to the fuselage
and cause it to revolve in the direction of
rotation of the lower set of blades. To
forestall this effect, the vane is attached at
approximately the same angle as the upper
rotor, and in this position acts to exert a
push in the opposite direction and keep the
fuselage pointed straight. The tips of the
lower rotor are bent downward at a 30-deg.
angle to give adequate air flow over the
directional vane.
A word of caution: A free-flight model
helicopter is not an easy thing to build
because of the number of "bugs" inherent
in the helicopter idea. In fact, it might be
said that if a model helicopter flies at all,
it's good! Therefore, it is recommended
that the plans be followed closely.
Try to keep the weight down. Do not
dope or add unnecessary detail to the model.
Excessive weight means more power will
be required to fly it, and more power means
more rubber, fewer winds, and more weight.
This can develop into a vicious circle.
Notice mounting strip for the front wheel between the
bottom longerons of this unfinished fuselage. A cross
brace midway at the rear adds strength.
A FLYING MODEL HELICOPTER
It is best to form the motor tube of
1/20" medium-soft sheet balsa, but if this
is not obtainable use 1/16" stock, well
sanded. To avoid splitting, soak the
wood in hot water before bending it
around a circular form and wrapping in
place with gauze. If a form of the size
indicated is not available, the diameter
of the tube may be safely altered, taut
remember to change the hole in the
motor guide plate accordingly to retain
the 1/8" clearance.
When the tube is thoroughly dry,
slip it from the form and cement the
edges together. Near one end of the tube
Two rotors revolving in opposite directions overcome torque.
cut two small holes in opposite sides for
a short length of 1/8" dowel to hold the
Build one side of the fuselage upon the other as in
end of the rubber motor. Run a bead of cement a conventional model, using 3/32" square medium
around the edges of these holes to prevent the wood balsa strip. Soak the upper longerons in hot water
from splitting.
before bending. Crosspieces are required at the points
From 1/16" medium stock cut three disks to fit marked "X" in the plan, and another may be placed
closely inside the tube. Cement one flush with the between the upper longerons near the rear for greater
upper end of the tube and another about 1/8" inside strength.
The
floor
the lower end. Keep the third for future use.
Motive parts include six rotor blades, motor tube, and top rotor hub. Note lower blades are
cut shorter.
plate shown in one photo is 3/32" by 2-1/4"
by 2-3/4". Center a 1/32" by 1/4" strip between the lower longerons from the floor plate
to the nose to support the front wheel.
Cover the windows with cellophane or light
celluloid and place dark-blue tissue on the rest
of the fuselage, except for the space occupied
by the motor guide plate on top and the floor
below. Water-shrink, but do not dope, the
covering.
Form the landing gear from 1/32" hard balsa
sheet, cement it to the lower longerons as
indicated, and mount 1-3/4" balsa or celluloid
wheels on its ends with pins. Attach the 11/2" front wheel with a wire strut.
Cut the six rotor blades from 1/20" balsa,
well sanded. Notice that the plan below
shows the top surface of the upper blades and
the bottom surface of the lower, since the
upper rotor moves counter-clockwise and the
lower clockwise when viewed from above. Also
notice the shape of the cross section. Do not
dope these blades.
Carve the upper rotor hub from soft balsa and
install the motor hook through a standard
thrust-plug and bearing as shown. The motor
ring may be cut from 1/8" balsa, but two 1/16"
sheets glued together with the grain crosswise
have greater strength. Cement the ring to the
motor tube about 1/2" from the upper end.
Attach the upper blades to the rotor hub at a
pitch of 25 deg., with a 2" dihedral under the
tips. Mount the lower blades on
the ring at 25 deg. but with no dihedral. Bend
150
down the tips as shown. Make certain that all
blades balance and track evenly.
Now bend a paper clip to the shape shown on
the plan so that it will stand upright. Thrust
the stem through the third disk cut for the
motor tube until the bent part rests flat against
one side. Put plenty of cement into the recess at
the lower end of the tube and push the disk with
wire attached inside.
The lower bearing is a 1/2" length of 1/16"
OD. brass tubing soldered into a 1/2" by 1/2"
piece of tin plate. Cement the latter to the center
of the floor plate.
Next cut the directional vane from 1/20"
sheet and cement it to the motor plate at the
point indicated on the plan. Wet the wood at
the thin part and twist until the vane has
approximately the same pitch in the same
direction as the upper rotor.
Now install the rubber motor. About four
loops, or eight strands, of 1//8" flat rubber are
required. Slip the tube through the motor guide
plate and lower it into the fuselage. Poke the
lower shaft through the bottom bearing.
Solder a washer to the end to hold it, or
bend over the end of the shaft, and Skyhook is
ready for a trial flight.
All take-offs should be from a level surface.
Hold the circular lower rotor hub stationary
and wind the upper blades. Set the model on
the ground with upper and lower rotors
between forked thumb and forefinger and
release both simultaneously.
Above: A shot of the full-scale Sikorsky S-51 helicopter in
flight. About the only real difference between the real one
and the model is that the aft rotor on the model does not
turn. Instead, it is replaced by a celluloid disc that supplies ample stability to keep the model flying straight.
Left: A still shot of the model hanging from a string. The
prop was held rigid for this shot so as not to tilt the model:
The working prop is mounted so that it will swivel on the prop
shaft, as shown on the plans. The ship is made of sheet balsa
sides cemented to sheet balsa formers, with a simulated cockpit.
SIKORSKY S-51
by Roy L Clough, Jr.
In answer to many requests for a realistic helicopter that
actually flies, we offer this semi-scale Sikorsky.
A good action shot showing the model in actual flight. Performance is realistic with no
danger of hard landings when the engine cuts as freewheeling vanes ease the model down.
FLYING MODELS
FEBRUARY 1953
• Probably the most famous design
of pioneer helicopter builder Igor Sikorsky, the S-51 helicopter has racked
up an impressive record in life-saving
and rescue operations, both at home
and abroad. It is the standard unit of
Los Angeles Airways, first airline with
scheduled helicopter operation, and
has been manufactured in England by
Westland under license from the parent
company. In addition to its performance record, it is generally agreed to
be the most beautiful rotary wing craft
ever built, with clean flowing lines that
adapt themselves readily to scale
model practice.
The big ship uses a three-bladed rotor
of the type known in the trade as the
"flapping blade" or articulated system.
This rotor type has an extreme degree
of maneuverability with rapid control
reactions and is quite stable in normal
cruising flight. The torque effect of the
(Please turn to Page 34)
15
SIKORSKY
(Continued from Page 16)
rotor is nullified by a small rotor attached to the tail
boom which produces a counteractive side thrust and
is used as a heading control.
This arrangement is fine for a full-scale craft, but
the unstable hovering characteristics of an
articulated rotor limits its usefulness to piloted craft—
it is not the best system to use on a model which
must fly independently and has no pilot aboard to
correct disturbances due to gusts or roughness in the
air which are present under even the most ideal
conditions. Therefore we must use a rotor, which has a
degree of automatic stability if we wish our model
to fly for more than a few seconds without skating
wildly in all directions and finally crashing.
Our first S-51 model, therefore, used a
dynamically stabilized rotor of the so-called
"feathering" type. That is, the blades did not flap up
and down, but rotated within limits in a span-wise
plane, allowing flight deflections to register as pitch
changes instead of flapping movements. The pitch
changes served to maintain the stability of the
machine by adjusting the lift of the rotor from sideto-side as required, since the blades were
independent of each other and, to a limited extent, of
the rotor shaft. Pitch was determined by rotational
speed (centrifugal force) and the model used a rear
torque correcting propeller.
The power was rubber and thrust was
transmitted through a bevel gear drive to the main
rotor and by pulley to the tail rotor. This model flew
very well, was stable and able to cope with rough air
without upsetting or going into a wild dance.
However, the duration was very limited and the
altitude attained was not very great, due to the
complexity of the machinery required and the rather
sharply limited output of twisted rubber bands.
We decided to scrap this design in favor of
something that could be powered by a ½A engine,
on the theory that builders would rather have a much
simpler model with greater performance, and would
be willing to sacrifice a bit of scale appearance to
get it.
By using the torque-reaction drive, we eliminated
gearing and clutches and the need for a torque
prop. The dynamic stabilizers, which govern
individual blade pitch, were retained, giving a good,
positive and fully automatic auto-rotational let-down
when the motor quit—an important factor in models
having any considerable weight and power. The
torque prop, an outstanding feature of the original,
was replaced with a clear disk of plastic, which
serves as a fin. The result is a model which is quite
realistic in flight, more rugged in construction, and
actually much simpler to build than the original
rubber-powered version.
Construction: The fuselage is a straightforward
semi-monocoque keel-type affair. Build one side over
the keel on a flat surface to insure good alignment,
then put on the half-bulkheads on the other side and
finish the job. Attachment of the landing gear and rotor
34
shaft should be quite clear from the plans. Note that the
landing gear struts have been omitted since we want a
springy gear to absorb landing jars. Window details are
best painted on with a contrasting color—plastics do not
take kindly to the spray of fuel and oil from the engine
(yes, you can use a 1/4" sheet profile fuselage if you want
extreme simplicity—but move the CG ahead about 3/8" to
allow for altered fuselage effects).
The rotor mechanism and engine hook-up is quite a
simple affair, but study the plans carefully and make
certain you know how it works before tackling it. The hub
is tin-can stock, the blade arms 1/16" steel wire soldered
carefully in place. All bearings and bushings may be
made of brass tubing, or simply rolled around a music
wire form with flat nose pliers, using thin brass or tin-can
stock material.
Make the rotor blades from a good tough variety of
3/32" sheet balsa and be sure the rotor blades' tip
weights (dynamic stabilizers) are securely cemented in
place and bent to the proper angle. The amount of weight
should be just enough on each blade so that the blade
will tip forward to maximum "down" pitch when at rest.
This weight will vary somewhat from model to model, but
is not critical within wide limits as long as only a
reasonable amount is used.
Understand how these weights operate—under
centrifugal force they ride up, increasing blade pitch, if the
model tilts they alter the blade pitch to provide a
corrective side thrust and, lastly, when the engine runs
out of fuel they govern collective pitch and throw the
rotor in auto-rotation to bring the machine down safely
and slowly.
The recommended engine for this model is the K&B
.049. It is recommended because of the ability of this
engine, which has a longer stroke than some, to carry a
very large prop without killing out—a very important
consideration in torque-reaction-drive helicopter models. It
also has a very handy mount and the gas tank comes in
the right place for this sort of model. Balance the cylinder
with a blob of solder on the blade holder arm opposite
the tank.
The finished weight is of much greater importance in
a helicopter than in any other type of model. This model
should weigh at least 4.5 ounces and less than 5.5
ounces for best scale-type performance. If it is too light it
will zoom up much too rapidly to be properly enjoyed,
and if it is too heavy it may be sluggish about rising—
particularly on hot days.
When the model is finished (up as far as the special
engine shaft rotor, and we're saving that to describe last
for a good reason), check it over for good blade tracking
and alignment.
This model flies not only vertical, but forward as
well. In order to get it to do this without incorporating a
cyclic control mechanism we took advantage of the
shape of the thing.
Forward flight is produced by the reaction of the
downwash from the engine prop against the flat front
of the rotor pylon which creates a force which tends to
tilt the nose down, and the reaction to this tendency to
tilt by the main rotor blades results in an automatic
shift of pitch which propels the model forward.
FLYING MODELS
FEBRUARY 1953
SIKORSKY
(Continued from Page 34)
Now, when the model is finished up as far as
the special see-saw engine rotor, it may be a
strong temptation to stick a prop on it and turn it
loose— just to see what will happen. If you try this
stunt be sure to have a dustpan with you
because you may need it. It it quite true that you
can fly the model this way, and if your balance is
good you may get away with it for half a dozen
flights in dead air. But, just about as you are
deciding that Clough is an overcautious old fuddyduddy, your model may tilt over on its nose and end
its career in one glorious full-throttle plunge into
the ground.
The reason for this happening (with a stiff
center prop) is that the main rotor blades
"feather" or adjust pitch angles relative to the air
pressure on them at any given rotational speed.
This is a condition of neutral stability which means
that the model will fly stably unless disturbed by
some external force—all things being equal.
Unfortunately all things are not equal in
actual practice. A gust may hit the model, or a
bubble in the feed line may make the motor kick -or
bump, which may set up a jiggle or tilt which will
cause the rotor to shift its position.
Study the rotor a bit. Note that it functions as a
gyroscope. If a condition of excessive forward speed
or sidewise skid occurs, we have a strong pressure
upon the tip of the lead blade (the blade which at
any given moment is perpendicular to airflow on the
upwind side), and this pressure produces a
resultant force 90° behind the point of
impingement, which is the characteristic gyroscopic
reaction.
This force resolves into altered blade pitch as
the weight seeks to justify its inertial forces, and
this pitch alteration causes another reaction back
to the front of the rotor disk which pushes the nose
of the model down into a dive. Since this condition
is self-propagating, there is no hope of recovery
from such an attitude barring a miraculous gust of
wind or engine stoppage which, by pulling power
from the rotor, kills off the precessive cycle
allowing the nose to ride up again as the blade
shifts into auto-rotation.
At this point somebody is sure to ask: why
not add little dihedral sections to the tip of the rotor
ahead of the pivot line, so that air pressure in
forward (or any directional) flight will impinge upon
the tip of the leading blade, forcing it to assume a
greater pitch and lifting it upward to return the
model to even keel?
It won't work that way. If we try it we find that
the model, instead of diving, now rolls over on its
44
side, because when the blade pitch changes at one
spot—in this case the front of the model —the reaction
comes 90° behind the alteration and the model tilts
sidewise and crashes (Factually there is a reverse
reaction 90° ahead of the deflection in addition, since if
one side tilts down the other must tilt up).
Thus the solution of this knotty little problem calls,
in this case, for an automatically regulating counterreaction that will at all times interact with the forces of
the main rotor to provide recovery couples.
We do this by installing on the engine shaft a special
type of rotor or propeller which is pivoted to "see-saw"
gimbals. This isn't very difficult to build. Simply carve out
a prop to the dimensions shown and, instead of
mounting it with a hole through the center, mount it in a
U-shaped pivot bearing, using a length of 1/16" steel
wire for a pivot shaft, which runs through the side of
the prop.
What have we here? Well, this prop has fixed
pitch, it does not feather as does the big one beneath
it, but it can see-saw on its trunnions. When the motor
is running, it "planes" and runs flat and true. When the
model starts to slide, this see-saw prop tilts back
opposite to the direction of sliding. This produces a
complex of forces, but essentially it serves as a drag
in that the faster the model goes the more it tilts back,
producing a counter-reaction and an opposing
gyroscopic deflection that limits the speed of the slide
to practically zero.
The model, which was neutrally stable with a stiff
center prop, now becomes inherently stable with a seesaw prop because a positive reaction is obtained which
produces automatic correction. This means the model
will simply hover in one spot unless some other factor
is added to make it fly forward, and the flat front
surface of the rotor pylon takes care of that. If you
want more speed in forward flight, gradually shift the
CG ahead a little at a time until you get what you want.
In our experience, few things offer the satisfaction
and enjoyment of a good model helicopter. This model
is a good one. Build it carefully, according to plan, and
it will give you performance you may have believed
impossible,
particularly
if
you
have
tried
unsuccessfully in times past to adapt model engines
to helicopter practice.
BILL OF MATERIALS (Balsa unless otherwise
specified)
3-3" x 1/32" x 36" (medium)………….Fuselage sides
1-3" x 1/16" x 36" (medium hard)..…..Fuselage keel
1-5/8" x 7" x 1/8" (hardwood)…………..Prop blank
1-4 ¼" dia. .012 thick (plastic)……………………….Fin
12" length of soft iron wire 1/16" O.D.; clean discarded tin can; loop of rosin core solder; 2, 1" dia.
rubber wheels; 1, ¾" dia. rubber wheel; leather or
fibre; small washers; dope, cement, fuel proofer and
decorations; K&B .049 engine.
FLYING MODELS
FEBRUARY 1953
This mini whirly-bird is a lot closer to scale configuration-wise than you'd think; big difference
is way rotors are constructed and how they work.
Semi-Scale
Sikorsky R-6
One of history's most significant 'copters
by one of modeling's most unorthodox
designers; it's a mighty happy combo!
In flight the model R-6 is a majestic sight. You'll really stop traffic
with this one. Easy to build and a real thrill to fly as a free flight job.
By ROY L. CLOUGH, JR.
•The Sikorsky R-6 helicopter is historically
important for several reasons. As a design it marked
the beginning of a trend away from unlovely steeltube box structures, demonstrating that a helicopter
could be beautiful as well as efficient. It was one of
the first to be operationally fitted out as a flying
ambulance— it could carry two external pods, each
holding two stretchers on either side—and it took
part in many early military experiments designed to
test the utility and application of rotary wing craft.
The model R-6 follows in the historical tradition
by introducing a new model helicopter rotor system,
the "multi-phase rotor" which permits use of true
mechanical cyclic control in addition to the automatic cyclic and collective pitch normally used in
models.
The "multi-phase" rotor system is based on the
idea that it is possible to build three distinct types of
rotors which occupy the same space at the same time,
so that the reactions of one rotor are modified by the
others, etc. This is an extension of the co-axial
system previously published in which one rotor was a
rigid-feathering affair with the other a see-sawfeathering type, the interaction resulting in an
automatically stable co-axial system. The multiphase system uses three rotor blades, each with a
different characteristic dynamic reaction. Thus we
include the desirable features of each blade type,
while suppressing the undesirable reactions in the
composite meld.
(The reader should be cautioned that simply tossing
together a number of different types of blades in one
rotor is no guarantee that the good features will
emerge and the bad features be suppressed—it can
come out the other way around.)
Air Trails Model Annual '55
For example: A rigid, non-feathering blade
produces a nose-up resultant in forward flight, it
does not autorotate; a Clough-type tip weighted
blade produces a nose-down resultant in forward
flight, and it autorotates beautifully; an angled hinge
blade will tend to adjust pitching motions, it
autorotates fairly well, but it is very critical in
adjustment. In the multi-phase system we use the
rigid blade to counter the nose-down tendency of
the feathering blade (as in the Berkeley model D).
What does the angled hinge blade do?
On this particular model we wanted to use a
manually set cyclic control, which flips the
feathering blade to produce forward flight by
mechanical cycling, as in full scale, rather than by
induced or C.G. shift cycling as is generally done in
models. But, if we use a fixed cyclic deflection on
the feathering blade, in conjunction with a fixed
pitch stabilizing blade, we discover that as the speed
of the model increases in forward flight we have two
cyclic forces: first, the fixed mechanical cycling
which will always be the same, and second, the
dynamic cycling caused by air pressure on the
entering edge of the rotor disk. This induced cycling
increases with forward speed at about the same rate
as the nose-up cycling effect produced by the fixed
pitch blade; therefore, if we add mechanical cycling
to it we find that the model will accelerate and go
into a dive. We could prevent this by using larger
fixed blades operating at greater pitch—but this
would spoil the autorotation.
So we add an angled hinge blade to the system. This
will autorotate, and as forward speed increases the
air pressure in front of the disk causes it to bend
downward and increase its pitch on the advancing
47
side, and bend upward and decrease its pitch on the
retreating side. Now we can pre-set the rotor system
with a fixed amount of cyclic deflection on the feathering blade and the model will accelerate up to the
point where the induced reverse cycling of the anglehinge blade cancels out both the pre-set and induced
cycling in the feathering blade and will go no faster.
If air conditions, gusts, tend to speed up the model
the rotor system increases it’s cycling momentarily to
kill off the speed, then resumes normal operations. If,
in calm air, we launch the model sharply nose down,
it slides forward, slows down, then finds its own optimum speed, and proceeds at that rate.
When the power stops the model descends in autorotation, the rate of descent being governed by the
fixed pitch blade, which acts as a governor upon the
two automatic blades. This incidentally does not produce a wobbling descent, provided any reasonable
autorotational speed is maintained—90 rpm or a
little better.
Construction
The model is a keel job; lay out and bulkhead in
the usual manner. Note how the landing gear struts
are cemented in position between blocks. Make sure
these are dry before covering. The rotor mast should
not be hard steel wire—use something fairly soft so
it can be adjusted easily. The nose block goes on after
covering. The covering may be 1/32" sheet balsa, or
fairly stiff double-calendered paper miking about
.008. If you use paper, start at the tail and work forward, lapping each joint 1/8" on the bulkheads. This
paper covering trick produces an extremely smooth
sheet-metal appearance and finishes with a minimum
of doping, but it is slightly heavier. Whichever cover
is used, note that the top area between bulkhead C
and B must be covered before the pylon is sheathed
in. This provides a working base to which to trim the
pylon covering. Also be sure to cut a slot for the rear
wheel strut.
The tail rotor is simulated by a plastic disk. Edge
it with a circle of rattan or reed to keep its shape. It
functions purely as keel surface. Cabin detail is
painted on in a contrasting color—silver blue makes
a good "glass"—and the front of the machine may be
doped as much as desired since additional ballast will
be needed in any event to balance the tail boom.
Before tangling with the rotor mechanism study
the plan carefully. Sheet metal parts may be cut from
tin can stock or secured from a Berkeley kit where
applicable. The big idea here is to have everything
tight that should be tight and freely-working on
pivots and hinges. The rotor mast bearing should be
very free, almost sloppy, but the pivots should work
easily without any play for best results.
The rotor blades are very simple, and while you're
at it make a couple of spares, just in case. Assemble
Sikorsky R-6 built in 1943 was the first military high-performance
helicopter. It would climb to 5000 ft. in 7 minutes, could carry bombs.
48
the works and check the blade balance next. It won't
ordinarily happen, but it is possible that the rotor
assembly may balance perfectly by accident due to
wood density variations. If this should happen, don't
fly the model this way because a little extra mass is
required on the tips of the fixed and angled hinge
blades—about equal to a dime. If in the balancing
operation more weight is required on the feathering
blade with the tip weight, add this extra weight on
the hinge line, not on the counterweight arm. Do not
put the cyclic mechanism on yet.
The original model used a Wasp with its fuel tank
modified as shown on the plans so it would operate
without throwing fuel out of the vents.
Let's fly it. Check the C.G. location by holding the
model sidewise by the engine shaft. If the tail dips
down it is tail heavy and may dive; the best trial
position is very slightly nose heavy or balanced on
the mast axis.
Fire up the engine and make sure it is delivering
full power before releasing the model—a ragged 2-4
cycling engine is poison. Release the model from a
level position and watch it! If you have followed directions- closely the machine will rise up steadily,
move forward very slowly and will probably circle
rather tightly to the left—probably a bit tighter than
you may desire, so bend the rotor mast slightly to the
model's right, which will make it fly straight, or turn
to the right, depending upon degree of bend. This, by
the way, is your combined rudder-aileron control on
this type model.
Fore and aft trim is accommodated by shifting the
ballast—in reverse fashion, that is, more weight forward to kill off a dive, more weight aft to stop tailsliding.
Once the trim settings are mastered the model can
be flown that way if desired, but the cyclic control is
more fun since it allows a choice of vertical ascent or
horizontal flight at the flip of a lever (or more accurately, the bending of a wire!). Trim the model to
rise vertically with a minimum of forward motion,
then install the cyclic control. Simply cement a length
of paper clip wire to the side of the pylon, as shown,
and solder a short length of springy steel wire to the
arm holding the feathering blade and blend it so the
blade is flipped gently each revolution. Vary the
position of the tripper by bending it up or down to
regulate the cyclic deflection—not much is needed.
Now, with this control, and by varying the rotor
tilt you can make it climb straight, fly forward,
cruise in circles or any combination desired. Nearhovering can be obtained by cutting the fuel with a
straight 3-1 mix of alky and castor oil.
. Guide to Helicopter Adjustment
Model helicopter flyers may find it easier to reGlidden Doman of Doman Helicopters, Inc., used an R-6 to experiment with his dynamically balanced rotor system (no vibration).
Air Trails Model Annual '55
The cabin ol the R-6 was built of molded plastic impregnated Fiberglass, probably the first use of this material in production aircraft.
member helicopter model adjustments by making an
analogy to fixed wing practice, thus:
Conventional Equivalent
Helicopter
Horizontal Stabilizer
Fixed pitch rotor blade or
See-saw engine prop or
Torsion tail surf ace (s)
Ailerons (wash-in—out) Lateral mast tilt
Rudder
Tail rotor, mast brake,
downwash fins under motor.
Thrust Line
Center of gravity location
moving aft increases D.Th.
Glide Angle
Speed in Vertical FPM in
Autorotation
Therefore we note that increasing the pitch of the fixed
blade (s) reacts similarly to increasing the angle (negative)
of a horizontal stabilizer, bringing the nose up under
power, or, decreasing the gliding speed (rate of descent,
power off). The rudder function, yaw, is obtained by a
tail rotor, by a brake on the spinning main rotor, or by
means of fins tending to promote or stop rotation of the
fuselage in the slipstream. Aileron control, or bank-turn is
by tilting the rotor laterally so the engine shaft inclines
toward the desired direction, and altering the dynamic
pressure on the feathering blades by moving the C.G. aft
(or forward) resolves out as down-thrust or up-thrust
respectively.
Air Trails Model Annual '55
Full-size plans for the Sikorsky R-6 are part of
Group Plan # 55A from Hobby Helpers, 770 Hunts
Point Ave., New York 59, N. Y. (50¢)
49
30
YOUNG MEN
FLY THE ALL-ALUMINUM PLANE!
SHEET METAL
SUSIE
By ROY L. CLOUGH, JR.
No hot fuel problems here; Susie's unique vibration
isolation assembly means a long-lasting model
• Suzie looks like a rather complex bit of
sheet metal work, but actually this gleaming
aluminum beauty can be yours at a cost of so
little time and effort that you won't believe it
until you try.
The trick is the use of simple basic geometry
which will distort naturally into the shape
we desire. We squeeze the ends of a cylinder
and there is our fuselage; we draw together the
sides of a right angle and there is our wing section.
The main disadvantage of sheet metal models
is the effect that high frequency engine vibration
has upon metal-to-metal junctures. This
punishing vibration will erode or fatigue the
toughest metal in a short time. Add to this the
rough shocks of repeated landings and it is easy
to understand why the operating life of
MARCH, 1956
metal models of the past has been brief.
Suzie was designed with the elimination of this weakness as a major point
of effort. Note: there are no direct metalto-metal component junctures; no points
where metal can chatter against metal,
and no points where heavy mass or
flight loads fall upon flat, or unsupported
sheet metal areas, in concentrated fashion. Engine vibration is isolated and
absorbed by a wood bulkhead, which
also takes landing shocks; the wings are
attached to a wooden spar, as are the tail
surfaces, the wood in turn being attached
to the fuselage. The result of this type
of construction is a model which will
still be flying years from the time you
build it—provided you don't run it into
stonewalls too often.
How about the weight?
We won't kid you. Suzie is fabricated
from .019 aluminum sheet and this stuff
isn't microfilm. She squats on the takeoff line with a full tank at 24 oz., and
uses up one-third to half a circle to get
airborne with a Cub .14—the smallest
engine you should use. Once aloft, however, she flies as good as any sport-type
model with an elevator response, climb
and dive, which belies its weight. When
the engine quits Suzie whistles into a
high-momentum glide as flat as a
tabletop and keeps pulling on the lines
until she stops rolling. You'll like her.
Construction:
Stop in at your local building material
outlet and ask for a couple feet of 24"
aluminum flashing. This shouldn't cost
over 75c. This stuff should mike about
.019; don't get the heavy-duty .024 grade.
Note that this material has a "grain,"
that is the long way and you'll get a
better and easier job by observing the
lay of the metal. Cut out a piece 18" x
10" and roll this into a cylinder, then
get your perimeter dimension by setting
the bulkhead in place in one end, mark
and then remove the bulkhead, line up
the cylinder, prick and drill and bolt the
edges together with ¼" round head 4-40
bolts and nuts. (Riveting is okay if you
have the equipment.)
Now observe the inside edge of the lap
joint; this must come at the bottom right
(outside) fuselage. With this in mind
gently squeeze the tube into shape to
receive the tailpiece, the wood rudder,
and bolt this in place. Run a #3 drill
through the sides, taking care to be
perpendicular to the rudder piece and
then carefully slit the fuselage as shown
and bend the resulting tab upward on
each side. Drill and bolt the hardwood
stabilizer to these tabs—a modicum of
bending is permissible if necessary for
good alignment.
Set the fuselage aside and make up the
engine - mount - bulkhead - tank - landing
gear assembly. This is a separate and independent unit and note that the engine
shaft will be off-center to the left; regardless of what engine you use, the
shaft position will be dictated by having
just the glow plug tip project beyond the
fuselage side. This permits use of the
popular "flat-opposed" type of engine
cowl. Next take the nose assembly and
stick it into the open fuselage end—line
your assemblies up simply by twisting
one way or the other, and when you are
satisfied with the alignment notch out
the fuselage end—line your assemblies
up simply by twisting one way or the
other, and when you are satisfied with
the alignment notch out the fuselage
metal around the landing gear legs—this
will maintain the arrangement. You may
wish to drill for the wood screws now
and put them in place temporarily.
With a straightedge draw pencil lines
down each side from the stabilizer center to the centering marks on the bulkhead. Balance the assembly between
fingertips and mark at this point. (This
will vary somewhat between the ultralight Cub .14 and something heavier,
like an O&R .23). Do not make any allowances for missing elevators or prop,
but cut for the spar at this point in
similar fashion as for the stabilizer-spar,
except that the tabs here are on the bottom. Poke the wing spar into place, drill
and bolt temporarily.
Only at this time do we cut out the
cockpit. Use a coping saw and leave the
edges rough, smear
with Pliobond cement, slit a 12½" length of heavy-wall
black neoprene tubing and push it in
place as moulding around the cockpit
rim.
(Continued on page 81)
Full-size plans for Sheet Metal Susie are
found on Group Plan #356 by Hobby
Helpers, 770 Hunts Point Ave., New
York 59, N. Y. (35c)
Sheet Metal Susie
(Continued from page 31)
Now for the wings. These
are simply cut out and bent to
shape with the trailing edges
bolted or riveted together—
there is a little trick that makes
it easier and more foolproof,
however. Clamp the metal
along the leading edge line
between a couple of stout
sticks and bend it to 90
degrees, then remove the sticks
and finish bending by hand to a
nice "natural" symmetrical
section. Then hold the trailing
edges together with the sticks
while you drill and fasten them
together—this prevents a
wavy edge from developing.
Remove the spar bolt from one
side, slip the wing over the
spar, spot the inboard hole and
the one near the end of the spar
and drill them. Dig up a couple
of "J" bolts and drill into the
underside of the wing and the
side of the fuselage and install
the "J" bolt with a fiber
washer between wing root and
fuselage as shown on the plan
view. Do not omit this!
Now do the other wing,
spotting the "J" bolthole so as
to provide a slight amount of
washout effect on the outboard
wing. Next put in the balsa
end caps, sticking them in place
with Plio-bond. The cowling is
simply a strip of aluminum
wrapped around the nose,
slotted for the landing gear legs
and the glow plug tip and held
together with a couple of bolts
or rivets at the bottom. You
will find it easy to bump a
nice edge on the cowl using
an inkbottle form (one of the
oval ones) and a plastic mallet.
(From my experience I suggest
you use an empty inkbottle!)
Add an extension to the
needle valve and lead this
through the cowling. Four nickel-plated wood screws hold
the whole front end together.
The elevators are next. Use
a stitched bias tape hinge and
roll an inter-connector of 1/16"
wire into the flippers as
shown—it is fairly easy, but
maybe you would like to
practice rolling a bit of scrap
with wide flat nose pliers
first. (This is a good trick to
know—once you learn it you'll
never be caught short for a bit
of tubing for any purpose!) The
elevator horn is simply an
integral tab on the left side.
Make up the tailskid and bolt
this in place. We put the control quadrant under the left
wing, pivoting on the inboard
bolt for simplicity, but you
can pick up a little speed by
putting it inside the fuselage,
on the spar, if you wish to
take the extra trouble. If an
external horn is used, make up a
wire line-guide and bolt this in
place with the spar tip bolt.
The deep tail and the skid
places the model in proper
take-off position, just let it
taxi—it’ll take off by itself as
the proper speed is reached.
Check all bolts for tightness
after the first flight and
occasionally after that until
you are sure she has snugged
down to business. Never wipe
sand or dirt away with a rag,
wash it off with kerosene to
prevent scratching the metal.
FULL SIZE PLANS
YOUNG MEN & AIR TRAILS MODEL ANNUAL
Group No. 256
50¢
ENCLOSE COIN OR MONEY ORDER
"BUNKER BOAT" by Cal Smith and Frank Lashelt. The long
awaited fishing boat model for electric power, can be radio
controlled Scaled 5/16" to foot, 35¼" long, 6¾" beam First
shown on June 1955 cover of ATHFYM
Group No. 356
35¢
ENCLOSE COIN OR MONEY ORDER
"SHEET METAL SUSIE" by Roy L Clough, Jr. Aluminum
covered control line model that's easy to make and a snap to fly.
Extremely durable model. Spans 27 Inches; overall length 21
inches Uses 14 to 23 power plants
FOR SPECIAL HANDLING( Add 9¢ per plan—1st class )
of Plans Only
( Add 18* per plan — Air mall
NEW 1956 CATALOG 12-5
28 PAGE, FULLY ILLUSTRATED. SEND 10¢ TO
COVER HANDLING .
HOBBY HELPERS
770 HUNTS POINT AVENUE NEW
YORK 59, N. Y
By ROY L. CLOUGH, JR.
Saturnian
SPACE SKIMMER
• This ultra-weird and flashy, U-control looks like it just
zoomed out of the pages of a science-fiction magazine. A startling eye-catcher, our vane-winged dreamboat has a lot to
recommend it besides its unusual looks It is easy to build, easy
to fly and never breaks a prop. Odd as it may seem, lift is
excellent despite the unusual fore and aft arrangement of the
lifting surfaces This permits a very good glide and nice handling
charactestics. The huge dorsal and ventral fins which flare
forth so rakishly are not just decorative, they're functional.
They provide lift to hold the model out when flown near the
vertical, an idea we may see adapted t o future stunt jobs The
construction, although specialized to fit the unusual geometry, is
quite ordinary in method. Start with the cabin or pod.
which is built up on the plywood cross beam. This may be
covered with 1/32" sheet balsa, or stiff tag stock Take
particular care with the engine installation since the thrust and
surface incident lines must be parallel. We used a Space Bug
Jr. running backwards. A left-hand Wasp would also do the
trick, or the ambitious could carve a left-hand prop for any
.049.
Regardless of the engine used, note that the tank, if integral,
must be arranged so the fuel will feed properly. Generally this
will mean running the engine on its side with plastic lines
attached to the filler and vent and brought above fuel level.
Make up the wing vanes, joined at the rear with the
stabilizer and elevator and stiffened with the long fins. Note
how the joint goes together between the cross beam and the
wing vanes, stiffened by the projection of the long fins. Use plenty
of cement and give it plenty of time to dry. Check the thrust line
against the wing plane and add the struts next.
Tie control system hardly requires explanation, except to
note that unlike most models, your Skimmer has no lead-out
wire. The control lines attach directly to the control quadrant.
A wheel landing gear may be added if desired, We built one
into the original in order to locate the correct placement for
those who wanted a wheel. However, there is a lot to be said
in favor of flying the model hand-launched or with a U-Reely,
and it looks even more "unearthly" without it.
Much of the charm of this model is i t s co l o r scheme,
the flasher, the better.
Modeling a rotor sailing ship
By
ROY
L.
Anton flettner's famous rotor ship Buckau
was propelled by spinning drums upon which
the wind acted to produce thrust. While it
required a small motor to spin these rotors, the
main source of power was the wind harnessed
through the Magnus effect—if a drum is set to
spinning in a wind, a thrust vector is produced
at 90 deg. from wind impingement.
Unlike the original rotor ship, this little
model uses an un-powered modified rotor
CLOUGH,
JR
which also provides rotary shaft power to spin
an underwater propeller. It's actually a
Magnus windmill that will spin regardless of
the wind direction.
The thrust produced by the rotor is in direct
proportion to the speed at which it turns. However, you will note that this rotor has been
loaded with a rather large propeller which
holds down its speed, since the propeller
thrust must be greater than the rotor thrust.
The reason for this is actually quite simple.
When the model has the wind on its port
beam, the thrust of the clockwise rotor is dead
ahead, but when the wind is on the starboard
beam, the rotor thrust is toward the rear of the
ship. In the first instance, the rotor thrust is
added to the propeller thrust, but in the second
it is subtracted. Thus, the greater prop thrust
makes it possible for the model to tack either
way.
The graceful little hull will go together as
easily as a box if you allow the cement to dry
well between assembly steps, Solder the
shaft-log bearing into a bit of tin-can stock and
cement it into the hull. The deep keel (pine)
should be cemented and nailed to the hail.
Drill an. undersized lead hole in this and drive
in the 1/8" wire mast. Ballast can be a length
of old shafting, solder, Babbitt or type metal,
soak the balsa.
To shape the vanes of the rotor, soak the
balsa until it's pliable, and then bind the two
sheets around a 2-1/4" mailing tube which you
have covered with wax paper. When dry, you
can make minor dimensional corrections by
springing them slightly during assembly. Next,
roil up a tube of stiff paper, soak it well in
model airplane dope and mount it between the
rotor end disks on locating plugs. Finally, glue
the curved vanes in place.
It's particularly important that the gearing
and shaft in the power tram work freely,
flattened egg-beater gears were used in the
model illustrated, but many similar gears
would have worked just as well. Be careful to
align the cross member which supports the
upper part of the propeller shaft. This should
mesh perfectly with the gears. You can adjust
the location of the rotor gear by slipping
washers or bits of tubing over the rotor mast
so that this gear engages the prop shaft gear
favorably. The shaft-log bearing may be very
loose and still not leak water, since a couple of
drops of lubricating oil will be enough to keep
the water from entering.
The two sides should be attached at the stem and
stern block, and then sprung apart so that the
bulkheads may be inserted. Pin the guide strip to the
keel so that different sections of the bottom planking
may be aligned easily
A rotor sailor makes the best speed when the wind is on
the port beam. At varying angles in front of the wind the
speed will vary with the thrust vector resultant
Gears which link the rotor to the propeller may be
salvaged from any clockwork mechanism. However, be
sure to mount them so they mesh perfectly and work
easily
Roy Clough PDF files
(Rock collection)
Air Car
Basic Design Problems of Model Helicopters
Blow-Bug
Build Your Own FLYING SAUCER
Channel Winger
Cloud-Copter TR
CLOUD-COPTER-D
CLOUD-COPTER-TR
Clough Helicopter From Toys and Games
Clough autobiography
Expansion Engines
Gyro Kite
Hoopskirt
Hover Bug
Hydro jet Powers Little Speedster
LITTLE DRAGON b
LITTLE DRAGON original
Martian Space Ship
More on Helicopters
New Model Helicopter
Original Channel Winger
Parasol Plane
Peter 0'Dactyl
Saturian
Sheet Metal Susie
Sikorsky R-6
Sikorsky S-51
Slat Wing
Snapper
Spinning Disc Saucer
Spinning-Wing
Sure Fire
Tan-Giro FULL
Tan-Giro Tiled
The Model Copter
The Whirligig
TRIAD
Try a Turbine
Typhoon Engine
Venusian Scout
Water Bug
What's the score on helicopters?
WhirliC02pter
Whirly Bird Kite b
Yankee Flea
Build it... and watch it soar 1,000 feet in the sky:
New Model Helicopter:
By Roy L. Clough Jr.
EVER since the first helicopter got off the
ground, model-makers have been trying to
design a miniature version that would do the
same. Here's one of the first model-helicopter
designs to succeed really well.
Its secret? Most early models were such
complicated contraptions that they sometimes
worked-but more often didn't. The new one is
ingeniously simple in construction, yet makes
"HERE'S a radically new free-flight
helicopter,” says noted model-airplane
authority Howard G. McEntee, shown at left flighttesting the model for POPULAR SCIENCE. "Its ingenious
engine-on-rotor-blade design is the first such I know
of. It gives the model a stable, soaring flight,
uncomplicated by the many problems that have plagued
other copter designers for so long."
Why It Flies
use of half a dozen complex principles of
flight. The result is a fascinating study of
aerodynamic problems that have plagued
designers of both real and model copters
for years.
The power plant is a glow-plug engine.
at the rim of a spinning gyroscope
1Poke
and it immediately tilts. But not
where you touch it—instead at point
90 degrees past where you touch it.
Scientists call this "precession." In the
PS helicopter, the whirling rotor acts
like a horizontal gyroscope and the
propeller blade a vertical gyroscope.
To keep a copter level, rotor blades must
Whirling blade-tip weights react 90 degrees
2decrease
3later
pitch on forward stroke to balance
to the force at the tail by twisting
reduced lift of rearward stroke—called "cyclic
pitch." This is done by weighting the model
tail-heavy so it pulls down on the blades at
the rear—like pushing on rim of a gyroscope.
blades down on copter's right side. This reduces lift as blades advance into the wind.
At same time, air pressure against the downtilted blades exerts a new force on the rotor.
tips, again acting as a gyroscope,
4Blade
react 90 degrees later to force of air on
spring back up on left side to take
5Blades
full bite of air. Since lift is less on rearward
the right side by tilting down in front. This
tips the nose down so the copter, while tailheavy, tilts forward for straight-ahead flight.
stroke away from the wind, the full-pitched
blades now balance reduced-pitch blades on
opposite side, and the copter flies level.
CONTINUED
147
How you can build the gas-model helicopter
Underside of rotor hub shows how spider forces two nonpowered blades to tilt at the same angle as powered
blade. Wire blade stems are bent at right angles to
engage holes in spider plate. Solder parts with hub held
flat, upside down, to insure uniform alignment of the
blades.
148 POPULAR SCIENCE
MAY 1962
like that used in most model planes. But instead
of being mounted inside the fuselage, the engine
is attached directly to one of the copter's three
rotor blades. In most conventional copters, both
real and model, the engine drives the rotor from a
central shaft. In turning the rotor forward, it
"braces its feet" against the fuselage and also
pushes backward. This backward torque keeps
attempting to spin the fuselage in the opposite
direction from the rotor.
In full-size copters, torque must be offset by a
separate stabilizing tail rotor or other special
devices to keep the craft flying straight. In the
model shown here, the blade-mounted engine
pulls the rotor around instead of pushing it. It
creates no torque and thus needs nothing to
counteract it.
The model is a whopper, too—nearly 4' across
the rotor tips. Yet, despite its size, it's so
efficient that it flies on a tiny .020 Cox engine—
one of the smallest made. Designed for free flight,
it has hit altitudes of 1,000 feet on two minutes
of fuel, giving it a rate of climb of 500 feet a
minute. Earlier models have required much bigger
engines to achieve the same lifting power.
How the model flies. The three rotor blades
are pivoted loosely at the hub, leaving them free
to tilt up or down like the elevator on an
airplane. The blades are also linked together at
the hub by a bell-crank mechanism so that
whatever one blade does, the other two do
likewise. Unlike a conventional helicopter,
however, no special controls are needed to tilt
the blades up or down for takeoff or landing, or
to provide complicated changes, known as cyclic
pitch, during flight. They're automatic.
The trick is based on the fact that the whirling
rotor and the spinning propeller
[Continued on page 186]
How the model climbs
Whirling propeller acts like a gyroscope in the same
way as the rotor, but this time in a vertical
plane. It reacts at 90 degrees to the sideward
twist on it by tilting upward. This tilts up the
rotor blade (and the other two blades linked to it),
giving the helicopter lift for climbing.
How the model lands
When the engine quits, the upward gyroscopic
twist on the rotor blades also stops, and they
pivot freely. Upward air pressure on the trailing
edges forces the blades to tilt down, and the
helicopter glides slowly to earth.
New Model Helicopter: Why It Flies
[Continued from page 149]
attached to the rotor both act like
gyroscopes. As the drawings show, a
gyroscope reacts to a force placed on it by
tilting at 90 degrees, or sideways, to the
original point of force. This is used in
several ways to provide stable flight.
The upward gyroscopic twist of the propeller tilts the rotor blades up to give the
copter lift for taking off and climbing. A
similar gyroscopic reaction is given to the
rotor by weighting the copter tail-heavy.
This causes weights fastened to the leading
edges of the blades to twist the blades
downward and reduce lift as they advance
into the air stream—just like the cyclicpitch mechanism on big copters.
An additional gyroscopic reaction in the
rotor forces it to tilt downward at the nose
to keep the model flying forward. Earlier
attempts to make a model copter fly
forward by simply weighting it nose-heavy
proved disastrous. The rotor, pulled down
at the front, reacted like a gyroscope and
flipped over, on its side, sending the craft
crashing to the ground upside down.
Earlier models had another fault: Blade
pitch was fixed at an upward angle for
climbing. To provide the downward pitch
for landing, the rotor had to come to a
stop, then reverse its direction. This time
lag caused the copter to drop a long
distance before the reversed rotor got up
enough speed to break its fall. In the new
design, the pivoted rotor blades continue
to turn in the same direction, but
automatically tilt downward when the
engine stops to let the model glide gently
to earth.
Only the rotor is tricky, the copter's
fuselage is a simple sheet-balsa job. But
the rotor, the heart of the craft, must be
carefully balanced to provide correct blade
pitch and avoid vibration. The blade-tip
weights are blobs of solder, each equal to
the weight of three nickels. They are used
only on the two non-powered blades as the
engine supplies the weight on the third
blade. After the weights are mounted,
gradually shave off bits of solder until the
rotor remains balanced in any position.
Weight the fuselage with clay until it
balances at a point 1/2" behind the rotor's
axis. This will make the ship slightly tailheavy as required for proper flight.
Blade pitch is controlled by a spidershaped plate on the underside of the hub.
This works like a three-way bell crank.
(86 POPULAR SCIENCE MAY 1962
When the plate is twisted by the stem on
the power blade, it in turn twists the stems
on the other two blades to a like angle.
The U-shaped stop bracket should limit
the plate's movement to provide a
maximum of 12 degrees upward blade pitch
for climbing. Downward or negative
pitch should be set as shallow as possible
for a slow, leisurely descent.
Note that the engine is mounted at an
angle on the rotor blade, rather than
straight-ahead. This puts its thrust line at
a tangent to its circle of rotation. If it
pointed straight ahead, it would exert a
side thrust on the rotor as it whirled around.
Note, too, that it is turned partially on its
side, with its cylinder tilted inward toward
the rotor hub. This puts its fuel reservoir
in line with centrifugal force so the gravity
feed will continue to operate even though
the engine is being slung around sideways
by the rotor.
The engine must also be tilted slightly
downward to minimize the force of its
slipstream. The slipstream tends to turn the
copter's fuselage to the right, but is offset
by the rotor's downwash and bearing
friction, which tend to swing the fuselage
to the left.
The engine can be mounted on the metal
bracket shown in the construction drawing
or, for a neater appearance, can be faired
into the rotor blade with a shaped balsa
block, as shown in the photos. If a larger
engine than the .020 Cox is used, it will
require additional counter weighting of the
rotor blades. In this case, add the extra
weight to the tips of the blades themselves,
not to the tip weights, which must remain
the same.
Flight-testing the copter. An ROG (riseoff-ground) takeoff is slower but safer at
the start since you can see what's
happening. When all adjustments are
perfect, you can go to the faster handlaunch.
Begin with a 6"-diarneter, 3"-pitch
plastic prop and trim it a little at a time
until the engine reaches maximum r.p.m.
Hold the ship by the tail until the rotor
gains speed, and duck out of the way. The
model should rise slowly, then tuck its nose
down and climb in a right-hand spiral of
20' to 30' in diameter. During trials and on
windy days, let some of the fuel flow/
through before letting go—or you may wish
the copter didn't fly so well.
•
....MORE ON
HELICOPTERS
by R. L. CLOUGH JR.
Twin spool drive of tail prop is used on this model
THE design of a helicopter poses
many problems which often cannot be
solved by analogy to fixed-wing
practice.
In "Basic Design Problems of the Model
Helicopter" (M.A.N. Sept. 1945), the
writer endeavored to present briefly, and
in general terms, several types of flying
model helicopter arrangements together
with their characteristics.
Since that time two new types of
helicopters have appeared, both of
which offer interesting possibilities to the
model experimenter. Also during this
interval, the writer found time to conduct
further investigation into the subject and
has reached a few conclusions, which
should be illuminating. Many incidental
mechanisms were tried, many discarded
and a few retained.
Fig. 1 represents the dual rotor intermeshing machines. The Kellett and
Flettner helicopters utilize this form of
torque nullification. On full scale
machines the rotors are meshed by
gearing, but in models it is possible to
construct the rotors in such fashion that
they mesh of themselves. Use of piano
wire sections near the hubs is a fairly
good substitute for gearing as well as
providing a desirable degree of blade
articulation. Both rubber motors
should be of the same tension, and
winding is best accomplished from the
underside. Rotors must be of equal
degree of pitch and very well balanced for
good results.
Fig. 2 is a single rotor helicopter with
torque effect compensation obtained by
means of an airfoil shaped tail vane,
which is provided with an adjustable flap.
This type is particularly well adapted
to models and will probably be the
favorite in future duration contests. It
has
the disadvantage of over-correcting for
torque when fully wound and undercorrecting when nearly exhausted, but is
MODEL AIRPLANE NEWS May, 1947
Coaxial rotors eliminate torque propeller
otherwise very satisfactory once the
proper relationship has been worked out.
Rotor speed should be high enough to
provide a good slipstream over the vane
and slow enough to permit a fair
duration. Theoretically the rotor
should be as small as possible and of
very low pitch. It is necessary to
incorporate a cyclic pitch mechanism if
forward flight is desired. The CG should
come slightly forward of the rotor axis
in order to balance the drag of the vane.
To
understand
thoroughly
the
difference between helicopters and
conventional
aircraft—and
this
understanding is the difference between
success and failure—it must be realized
that, unlike the airplane, the helicopter is a
machine of variables. By way of
illustration let us consider a conventional
model plane. The performance of such a
machine is fixed and does not vary; thus
at a certain speed a certain amount of lift
is
produced.
The
fundamental
performance is the same whether the
machine is under power or gliding; this
is because the machine is one distinct
mass in motion.
Now, in the helicopter we have the
rather paradoxical situation of part of
an airborne mass being exposed to a
relative wind (the rotors); and part of
the mass (fuselage) having no relative
wind or greatly varying degrees of
relative wind depending on the velocity
of the mechanism as a whole through
the air.
Let us repeat the above, substituting
"kinetic energy" for relative wind. The
kinetic energy of a flying machine is the
product of its speed times its mass, minus
air friction or drag. Thus a helicopter in
hovering flight, in calm air, has a relative
wind over the rotors, a much smaller
amount over the fuselage, and no kinetic
energy of the mass as a whole. When
hovering into a strong breeze the above
factors are the same, except that the
fuselage now possesses a stronger
relative wind; it is in effect flying into
the wind at the speed of the wind, but
with the important difference that no
momentum is produced as a result of this
forward flight. Therefore, it may be
seen that "groundspeed" is an important
factor
in
reckoning
performance
characteristics of helicopters.
If a helicopter is hovering into a stiff
breeze and that breeze suddenly ceases,
the machine is apt to drop to the ground
(such accidents have occurred) But, if
the machine is flying forward at a good
rate of speed into the wind and the wind
ceases, the momentum of the mass will
accelerate it as the air resistance drops
and "carry over" loss of altitude will be
negligible.
The effects of this differential of inertias
are quite marked in helicopters and in
some maneuvers can become very complex. In the practical application to our
problem of designing stable model
helicopters it serves to remind us that it
is not wise to go overboard on the
matter of "pendulum stability." Too
great a distance between rotors and
fuselage is likely to accentuate swinging
moments rather than minimize them.
Relative wind differentials must be
borne in mind as well when designing
rotary wing craft. When the fact that
the relative wind over the rotors and over
the fuselage have different effects, is
thoroughly digested, many seemingly
baffling problems are made clear at one
stroke.
Power, the degree of power that is, is
another important factor. A rubber band
motor's output varies with every
revolution of whatever mechanism it
drives. This humpbacked power curve of
rubber has considerable influence upon
the design of models so propelled.
Often it is
39
necessary to alter a design to a
considerable degree from what it
"should" be in order that stable flight
be obtained through all phases of the
power curve.
The over-powered model helicopter is
lifted rapidly and stops climbing very
suddenly—with the effect that inertia may
carry the machine a bit higher after lift
ceases, permitting the machine to fall
free until it attains sufficient velocity for
the fuselage area to act as fin surface and
invert the model.
The properly powered model rises more
slowly, perhaps to a slightly lower
altitude, and performs the transition
from climb to controlled descent without
the weight of the machine being
removed from the rotors and transferred
to the fuselage at any time.
It may be stated that the ideal power
loading for any given model helicopter
is the highest which can be achieved
without
sacrificing
desired
flight
characteristics. More simply: use the
smallest possible amount of rubber.
There are several types of flight
performance possible: (1) power climb,
power braked descent; (2) power climb,
free-fall descent; (3) power climb,
reverse free-wheeling descent; (4) power
climb, over-riding freewheeling descent:
(5) horizontal flight, landing being
accomplished while the machine is still
under power, (It is almost impossible to
secure hovering flight in rubber powered
helicopters without using a very tricky
automatic pitch rotor head.)
Of these types No. 4 is the most
desirable, and often No. 1 is the most
practical. Fig. 3 shows a method of
obtaining overriding free wheeling (and,
incidentally, automatic pitch). This
method can be adapted to either co-axial
or single rotor designs, and produces
very realistic nights.
If greater simplicity is desired, two
40
methods of obtaining freewheeling by
reversing direction of rotation are shown
in Fig. 4. Both these methods are
adaptable to various types of rotor
arrangements. Of the two, the coil spring
is the most positive, but in some situations
the friction disk setup is more practical.
Friction disks should be alternately inner
tube rubber and coarse emery cloth or
sandpaper.
In the previous helicopter article the
writer laid great stress on the necessity
for well-articulated rotor blades to
minimize forces set up by gyroscopic
action. At this point something called
"cyclic pitch" should be explored in
some detail.
Cyclic pitch means independent
control of the pitch of individual rotor
blades at various positions as the rotor
turns. By means of cyclic pitch
mechanisms the angle of any blade may be
increased or decreased as it passes
through any segment of the rotor disk.
Thus it is possible to increase the lift
produced by the rotor on one side and
decrease it on the other. This causes the
rotor to tilt toward the side on which lift
is decreased. Increasing the lift of each
blade as it passes through the rear of the
disk causes the machine to move forward;
increased lift at the front causes it to
move backward. Thus the force which
is applied to single rotor helicopters to
secure forward flight is not a constant
thrust as in the case of propeller driven
planes but is like a series of "jiggles"
which nudge the machine along. This
accounts for much of the vibration
incident to machines of this type.
Now it would seem that forward flight
could be attained as easily by shifting the
center of gravity in relation to the rotor
axis, as is done in many co-axial
machines, and avoid the vibration caused
by cyclic pitch. To a casual observer this
appears to be quite logical, but unfortu-
nately the solution is not that simple.
Picture a two-blade rotor spinning in a
horizontal plane. When hovering, the lift
of each blade is equal that is each blade
is moving a similar mass of air
downward. Let's assign an arbitrary tip
speed to the rotor of, say 100 mph. Now,
by shifting the CG of the rotor we
cause it to move forward at 50 mph.
What happens? The blade advancing
into the slipstream encounters a relative
wind of 150 mph, its own speed, plus the
speed of forward motion; the blade
receding from the slipstream encounters a
relative wind of 50 mph, its own speed,
minus forward speed. Such a difference
in lift makes the arrangement unflyable; it simply wouldn't remain right
side up.
Therefore, in order to make a single
rotor stable in forward flight it is necessary to incorporate a mechanism that
will decrease the pitch of the advancing
blade and increase the pitch of the
receding blade to an extent which will
equalize the all-over lift of the rotor disk.
A mechanism for producing cyclic
pitch in models is shown in Fig. 5. This
is quite simple and produces less friction
than some methods that have been
suggested. The rotor hub is thin brass
tubing through which is slid the blade
holder before the blades are affixed.
Cyclic control is obtained by means of an
eccentric mounted disk, which may be
shifted about and pinned to secure
horizontal flight in all directions. This
disk controls the movements of a small
wheel or roller, which in turn transmits its
movements to the rotor by means of a
simple linkage. It is desirable to have a
pin and hole setup in the plate upon
which the cam is mounted in order to
prevent it from shifting while the model
is in flight. A light coil spring between the
roller arm and rotor shaft is necessary to
prevent (Turn to page 64)
MODEL
AIRPLANE
NEWS
.
Moy,
1947
... More on Helicopters
(Continued from page 40)
"floating." Sometimes, depending on the
size of the rotor, static balance weights
ahead of each blade will give a smoother
performance.
The rotor head of the model with tail
prop illustrated herewith is an attempt to
secure fully automatic cycling. This
model is flown forward by adding a
small weight to the nose, the angle of the
blades on each side of the rotor being
controlled by air pressure on them.
This method shows promise, although
the-writer has had only indifferent
success with it to date. The crux of
the matter seems to be obtaining the
proper static balance.
Now the question is sure to be raised:
"Is it possible to obtain forward flight in
machines of this type without using
cyclic pitch?" The answer is "Yes," but
in a rather weak voice. If the rotor
speed is high enough and the forward
speed of the machine slow enough, a
fairly decent horizontal flight can be
obtained—but the rotor must be highly
articulated. If the speed increases above
a certain critical point, however, the
model will tilt and begin to fly sideways
at the same time, which condition will
soon resolve itself into the helicopter's
equivalent of a nose dive.
Several methods of transmitting power
to. the torque prop are shown in Fig. 6.
"A" is the common sandpaper-faced pulley and string belt arrangement, "B" is
the flexible shaft drive which uses aluminum and neoprene tubing and a friction roller at the rotor head; and "C" is
the wind-up, wind-down string and spool
system in which the string winds both
ways at once and rewinds itself automatically as the main rotor is turned. In
any case the blades of the torque prop
should be adjustable.
In dealing with the problems of control of co-axial model helicopters we
find there are several methods that give
good results. Because there are two
rotors turning in opposite directions in
co-axial machines, it is possible to
make these fly forward by adding
weight to the nose or by shifting the
angle of the rotor axis. In models there
are two good methods of obtaining
forward flight by CG shift: the simplest
is to add weights to a hook in the nose.
This works well on experimental
machines and is very convenient. A
model using this method should be
slightly, tail heavy without weights
making it possible to obtain reverse,
64
vertical and forward flight by flying
with or without various small weights.
A slightly more complicated method is
shown in Fig. 7. With this setup the
motor tube is moved fore and aft
along an arc by means of a movable
guide plate and pivoted thrust bearing.
A friction holder is required on the
movable plate to prevent shifting in
flight. When laying out a model of this
type, be sure to allow ample
clearance for the rotors at both
extremes of movement so they won't
hit fuselage or tail surfaces.
Slipstream controls may be used to
turn the nose of the model right or left
(or correct turning tendencies), secure
forward flight, or may be used in conjunction with CG shift to obtain extremely accurate trim. Several types of
slipstream controls are shown in Fig. 8.
Control surfaces of this type on a helicopter differ in function from that of a
conventional plane, and it is important
that this difference be understood.
In a conventional plane, control surfaces are used to set up or arrest
turning moments around one or
several axes. (Flaps and slots are no
exception; neither are they true
controls.) In the helicopter, only one
control surface effects a turning
moment, that is the heading surface
which points the machine in the desired
direction. The other surfaces are not as
much control surfaces as they are secondary propelling surfaces and act by
reaction to direct slipstream in a direction opposite to that in which forward
motion is desired. This is the function of
the "elevator" shown in Fig. 8, as should
be quite clear from the drawing.
It. must be noted that the resistance of
these control surfaces at a distance from
the rotor axis has a tendency to
produce a condition similar to that of
CG shift, and this effect must be taken
into consideration when designing this
type of model.
A
question,
not
covered
exhaustively in the previous article, is
that of "coning angle." Coning angle is
the helicopter's dihedral angle. It is_ the
angle of "dish," positive or negative, of a
spinning rotor. From the writer's
experience it seems difficult to lay
down any hard and fast rule regarding
it. Apparently it is wise to use positive
cone on single rotor and dual rotor
machines. With co-axial machines
negative cone seems to give the best
forward flight characteristics. At any
rate either one appears to be better than
a perfectly flat disk, and the writer
suggests that builders try both and decide for themselves. Much can be
said in favor of either.
Sometimes, after carefully designing
and building a helicopter of his own design, the experimenter finds that
despite all his efforts the machine will
not perform as intended. What is
wrong?
First, check the model for gyro effect.
The procedure is relative in nature and
must be learned but is not difficult.
Wind up the rotors and hold the
machine nose down. Releasing the
rotors but not the fuselage, move the
machine around in various directions
about its longitudinal axis. If there is
considerable resistance to such
movements, the chances are that the
blades are gyroscoping. This calls for
more sandpapering to add flexibility or
easing up of the articulation system if
the rotor is of that type.
If the machine rises but shows a tendency to oscillate, the trouble may be
caused by an unintentional pitch differential between two or more blades of the
same rotor. Check the pitch carefully,
using a template to insure uniformity.
Plain and simple wobbling is usually
caused by unbalanced blades or uneven
tracking. Another type of wobble difficult to correct is caused by "puffs" of air
striking the top of the fuselage aft of the
rotor axis and ahead -of it at different
times. This effect is most often
noticed in machines employing a
three-blade rotor. For this reason the
writer recommends that rotors having
two or four blades be used. This may
sound like a small thing, but in
helicopters it is often the small things
that count.
The helicopter experimenter should
never take it for granted that something
which is good practice in airplanes will
work equally well on rotary wing craft.
Usually it will not. However, if there is
ever any theoretical doubt as to
whether or not a thing will work, try it!
Some of the most important inventions
have been accidental discoveries. Such
discoveries are the rewards of
providence to an inquiring mind—and
certainly nothing to be ashamed of.
Good luck!
MODEL AIRPLANE NEWS .
May,
1947
Conventional model-plane power plant pushes little
craft over water, and standard model-plane tech-
niques are used to build the craft from balsa. The
nacelle accepts most .020-049 radial-mount engines.
MODEL HYDROPLANE Skims
Pusher prop spun by model-plane
engine gives high performance.
Construction is easy and fast
By ROY L. CLOUGH JR.
H
ydrofoils have been around for some
time, but even so, nothing on the boating
scene draws every eye like a hydroplane
lifting out of the water as it gains speed.
Even the U.S. Navy has been attracted
to foils, and has tested them on its fast
PT boats.
The PS model shown here can be completed in a couple of work sessions. Surface-piercing foils and air-prop drive give
it speed and stability with minimum complexity. Construction is far simpler than
you'd guess from the performance.
Basically, these craft deliver greater
speed because resistance against several
small areas (foils) is considerably lower
than against a complete, submerged hull.
Resistance declines as the craft rises.
Completely submerged foils are the
most efficient, but they require sensing
140
POPULAR SCIENCE
and control systems to keep them at
proper depth. Surface-piercing foils automatically adjust for depth—but they
also have a tendency to create air bubbles that reduce lift. The PS model uses
a foil design that minimizes this undesirable side effect.
Instabilities can develop in either type
of foil. This is particularly true of models. Simply put, the angle necessary to
make the foils "fly" at low speed can also
make them hop out of the water at high
speed and spill the boat. The model has
a designed-in, relatively steep foil incidence and a high thrust line to minimize
the possibility of this happening.
Building the model. Typical modelplane construction techniques are used.
But keep in mind two important construction hints: Cut all parts very accurately. Use ordinary pins to hold the
components while the glue dries.
Build the cabin first, complete with
tail boom and rear foils. Cover the lower
half of the cabin with lightweight model
tissue before doping, for a smooth and
watertight finish. The windows are simply clear plastic (I cut mine from bubbletype packaging).
Next, make up the front foils, floats,
the Water
and crossbeam as a unit,
and cut them into the cabin
floor at the correct angle.
Shape and fasten the motor-mount nacelle; the one
shown on the blueprint w i l l
accept most .020-.049 radial-mount engines. To be
safe, check your engine before shaping the part.
large lake or a broad river will serve as a suitable playground for
Mount the engine to the A
the model. Run it on a tether around your boat, or turn it loose for
plywood firewall during as- a "free flight" if there's enough water area to do so safely.
sembly; epoxy cement is
best here and a good-size
dab on the nuts holding the mounting control heading and raise or lower the
nose, much like the elevators on an airscrews is recommended.
The pusher engine. If you use a reed- plane.
For your shakedown cruise, bend the
valve type you can use a standard p r o p but be sure to put it on backward. If t r i m tabs up at the rear edge until the
your engine has a rotary valve, use a model rises up on its foils and scoots.
left-hand pusher prop of the type used To get maximum speed, bend the tabs
upward to the minimum that w i l l make
for air-drive model race cars.
Important: The model should balance the craft "fly." Direction is controlled by
when fingertip-held between the points differential bending of the tabs.
I flew the model on a large lake, chasshown on the blueprint. Though a little
tail heaviness is allowable, a nose-heavy ing it w i t h a boat. But you might also fly
it on a tether around a boat.
model puts you out of business.
Double-check all foil angles before
making a test run. The rear, inverted-V
Turn the page for PS lie-flat blueprint
foil is fitted with bendable tabs, which
MAY 1969
141
LITTLE
DRAGON
Part One
by ROY L CLOUGH, JR.
Little Dragon glow engine is a project any amateur
THE
machinist can tackle with full confidence of good results.
It does not require any special tools, special talents, or extreme
precision. A large part of the total time spent in developing the
design was devoted to eliminating awkward machining jobs,
delicate operations, and tricky assemblies. If the reader owns
a small lathe and can center a piece of stock with 1/64", he
need have no qualms about being able to turn out the job. On
the other hand, the skilled builder who has a good "touch" for
this sort of thing will discover he has an engine, which requires
absolutely no apologies on the score of being homemade.
The motor is a basic design, as old-engine hands will recognize by the drawings. It has great amounts of leeway at every
step of construction. This means there is plenty of room for
the correction of errors, which should appeal to the amateur,
and equally of importance, it allows the experienced motor
builder to "soup-up" the design as he sees fit. For example,
the weight of the original came out at 2 oz., complete with
plug and prop. Skillful shaving-down by experienced machinists can reduce this figure greatly, but this has nothing to do
with the operating characteristics. Port areas and valve timing
are laid out with an eye to obtaining maximum start ability
and a good rate of speed with average construction and
internal fits, but the experienced worker who is capable of
doing very good work will find it possible to increase the
porting and degree of valve opening to obtain an extremely
hot engine.
The original Little Dragon was turning the plastic prop
shown in the photo at 8,000 rpm, 5 minutes after it was
assembled. It did this on a break-in mix of 3 parts O & R
No. 2 and 1 part castor oil. This is the performance the
average builder can reasonably expect. For experts, and with
one of the hot Francisco Lab fuels, 10,000 rpm is a reasonably
conservative estimate.
The mounting of any engine uses up time and energy and
in too many cases is finicky and bothersome. We have tried to
get around this and come up with something that is simple,
quick, and practical. The two-stud mount is our answer.
Simply press the studs against a piece of plywood to mark it
for drilling, set the engine in place and run on a couple of
nuts. No muss, no fuss, and no bother.
A bore of approximately 7/16" with a 3/8" stroke fixes the
displacement at about .06. By selecting these dimensions, it
was possible to take advantage of material sizes which sim-
An ordinary book of matches dwarfs our tiny engine
MODEL
AIRPLANE
NEWS
•
October,
1950
Little Dragon all ready to roar!
This engine is really "on the square"
plified construction considerably and is one of the reasons you
will find dimensions indicated in 32nds of an inch instead of
thousandths. (Editors note: Cad drawing in thousandths.)
However, those who wish to build the engine to conform to
AMA 1/2A regulation can use a 1/2" O.D. cylinder liner instead
of the 9/16" specified on the plans. This will bring the displacement down to about .049, safely within the rules for
engines of .05 or less. This will require slight alterations in
the width of the con rod for adequate clearance, and of course
the liner hole, piston size and head are changed accordingly.
Conversely, a skillful builder can increase the displacement to
more than .070 if he desires.
To those who think an elaborate machine outlay is required
in order to build engines, a glance through the list of tools
used to make the original should prove refreshing. These were:
small lathe, hack saw, hand drill, two files, two taps, one die,
and a pocket scale graduated in 64ths. A micrometer was used
to check sizes, but actually could have been dispensed with.
And the big news, of course, is that no milling operations are
required.
The Little Dragon employs what is known as the "sleeve-inblock" style of construction. Instead of having separate cylinder
and crankcase, one blends into the other, eliminating cylinder
tie downs, heat dams, and two more places for errors to
accumulate. The engine block serves the same purpose as the
"keel" frequently used in model airplane construction, being a
basic member, which when laid out correctly serves as an
accurate basis for the remainder of the construction. The block
is the easiest part to make, in terms of tolerances, and serves
the amateur builder the purpose of getting his hand in as he
goes along. Once the block is made the rest of the engine falls
right into line.
Cut off about 1-7/8" of 3/4" sq. hard aluminum alloy bar
stock, center it accurately in the four-jaw and face off the
end. This gives a plane surface to set against the chuck face.
Remove the piece, re-center, and face it down to the proper
size. Next, outline the cylinder block. Here is a good rule to
remember: always keep as much stock between the end of the
piece and the chuck as possible; in other words, make the first
cuts out near the end to leave a maximum diameter of supporting metal. By this rule, we see that the fins are machined first.
If your lathe is a light one, use the back gears and feed the
finning tool in slowly, particularly at the start of each cut
where it is chopping at the square corners of the stock.
The first fin is extra heavy because this must carry the
screws, which will hold the head in place. If you have cut the
piece a bit short by accident, you may take the error out of
the first fin. For example, if the piece is 1/32" shorter than it
should be, the first fin would be 3/32" deep instead of 1/8".
This is all right, but don't make it any thinner. There are three
fins below this, each 1/32" deep with a 1/16" gap between all
fins. Since the lowest fin must come in exactly the right position because of the exhaust port, cut this one after you have
made the top fin. If your finning tool isn't quite the correct
width, split the difference to fix the location of the middle two
fins. Do not cut the fins too deeply and weaken the block. A
fin depth of 1/32" measured from the flat of the stock is
entirely adequate. Turn down the "barrel" and buff it with a
crocus cloth. A good shine here increases (Turn to page 48)
23
Little Dragon
(Continued from page 23) the eye appeal; rough tool
marks give the engine the appearance of having been
whittled out of a stove bolt with a cold chisel!
The block is drilled out to an I.D. of a shy 9/16" and
brought up to size with a boring tool, or reamer. Getting
the correct depth is important, not because it will prevent
the engine from running, but because you will have to go
over the whole thing changing other dimensions to make it
come out right. Note how a shoulder is left to support the
liner. This liner must fit closely in order to prevent blowby around the exhaust port and at the head. This does not
mean a piston-type fit by any means, but it should be
tight enough so that it is just about possible to pull the
piece out with the fingers. A dummy sleeve cut from the
steel tube stock is a great help here. If by some mischance
the hole is oversize, don't scrap the job, just tin the sleeve
and resize it to fit the hole. (But you won't be able to
harden the sleeve if you do this, and then you will have
to use an aluminum or cast iron piston.)
Remove the piece and re-chuck it in order to bore out
the front of the case. Open this up part way with a drill
and bring to final size with a boring tool. The inside rear
must be faced off smoothly because the rotor valve will
ride against it. The rotor pin hole is drilled out by holding
a 3/32" drill in the tailstock. At this point lay the piece
aside and make up the crankcase front section. Do a
good job here and no gasket will be needed; in any case a
short piece of thread wrapped around the plug portion of
this part will serve very well as a gasket. If the inside
end is turned first and fitted to the block, the job is
easier, as this leaves something to chuck with. Next,
reverse the piece and bore out to 3/16" I.D. and bring the
outside down to size. No bearing is used other than the
metal itself. If you want to be ritzy about it, the hole can
be bored oversize and bronze, or Oilite bearing material
pressed in. In practice, the writer has found aluminum to
serve very well, but the coefficient of friction of Oilite
material is undoubtedly more favorable if the end in view
is extreme performance. For that matter, ball bearings
small enough to be used in this engine can be obtained,
and we are indebted to Malcolm D. Whitman, Jr., of
Carmel, Calif., for that information. If you use ball
bearings, the crankshaft diameter must be reduced or the
outboard end of the bearing increased to accommodate the
ball race, should double ball suspension be desired.
The crankshaft belongs to a breed of cats that seems
able to scare a lot of people. Don't worry about it. Put
the three-jaw chuck on your lathe and insert the piece of
9/16" drill rod. Bring this down to size with light cuts and
power feed. Finish the journal with a fine file, crocus cloth
and common sense. If you happen to have a tool post
grinder, by all means use it. Custom fit the shaft to the
crankcase front section, double checking to be certain the
thrust washer clearance is adequate, then mark the spot
and turn down to the size you have selected for the
threaded portion. We call for an 8-32 thread, but this is a
matter of choice and whatever die you have handy. The
threads may be cut on the lathe, but some will find it less
trouble to back off the tail stock and use an ordinary die.
Be sure to start it straight; back the die off every half turn
to break the chips and insure a good thread. A few drops
of light oil makes the cut easier. Near the end of the cut it
is a good idea to reverse the die in order to cut the
threads up close to the journal. The thrust washer is
simply a threaded disc. The writer has used threaded
drive washers on a number of engines with good results
and why no commercial engine uses them is something of
a mystery since it is certainly easier than milling splines or
grinding flats.
Now, remove the piece and put on the four-jaw. Chuck
up the shaft by the journal and off-center the piece 3/16"
by adjusting the jaws. It is possible to hold the piece
adequately without marring the journal, but the cautious
may wish to push the shaft into a length of brass tubing
and squeeze the jaws down on this. If this is done, be sure
to use only a gentle tapping to put it in place, because the
journal must be knocked out again afterward. The whole
secret of turning off-centers is setting the lathe tool on
center, feeding in slowly, and using power feed to drag the
tool along the work. In addition, be willing to take a little
time to do the job. The pin should be brought to a good
surface finish. The crank disc may be ground or filed away
as indicated by the dotted lines on the plan for a sort of
counterbalance effect, but this is not critical.
The rotor comes next. Most people seem to have the
opinion that disc valves must be tricky since they come
in the more expensive engines, so we'll give a little
background on this. When the original Little Dragon was
being laid out, a great deal of consideration was given to
the induction method. It had to be very simple and very
effective. Three-port induction seemed simple, but it
meant tapping into the block and cutting another hole in
the liner. Besides this it did not allow much leeway for
error and would not produce the best power output. Shaft
rotary looked good at first glance, but this would mean
less than optimum strength for the crankshaft, chances for
errors in both the port hole and the hollow shaft, and the
added difficulty of setting the intake tube into the front
case. So that was out. Next we toyed with the idea of
flutter valve induction; these arrangements are simple, and
since they work on crankcase pressure, very effective.
However, a speck of dirt, or oil hardening in the valve
makes them inoperable with a vengeance. Further difficulty
was foreseen due to the great powers of fuel meniscus in these
small sizes. Fuel meniscus? That did it! So we used a disc
rotor and made it free-floating. Five minutes after assembly
we knew we had it. Only two smooth faces are needed, and
one of them is already inside the crankcase. Best of all the
intake hole can be spotted easily, and if missed, it can be
tried again with a new rotor. In addition to all these
advantages, the thickness of the rotor is not critical and this
gives the amateur machinist another place to pick up and
correct accumulated error.
Chuck up the 5/8" aluminum rod and turn down the
rotor shaft to fit the hole in the rear of the engine block.
This should not be a tight fit, but smooth running and a bit
loose if anything. Bring the O.D. of the rotor down to size
and face it with a gentle cut, making certain there is no
shoulder next to the shaft as this will prevent seating.
Cutting loose from the stock should be halted about halfway
through and all sharp edges broken, then complete the
severance. Locate and drill the drive hole. This should be a
bit oversize, but do not drill all the way through the disc,
just enough for good clearance. Mark the outline of the port
and file away the indicated area. The rotor may be held
between two thin bits of wood or fibre in a vise for this
operation. Break any edges that develop. Now, take up the
block and put the rotor shaft into the hole from the outside
rear of the case and scratch the outline of the round portion
as shown on the plan. Split the difference between this line
and the edge of the shaft hole and drill a 1/8" hole along the
diagonal. The intake tube is a 'length of 5/32" thin wall
brass tube. Taper one end slightly (this taper is emphasized
on the drawings) and press the tube firmly into the case. The
needle valve assembly may be from a Baby Spitfire or other
small engine.
The crankcase front section must now be drilled; set it in
the engine block and spot the holes for drilling. These
holes are drilled and tapped 2-56. The original engine used 348 screws throughout, but the larger size is not needed.
However, it may be comforting to know that if you ruin
the No. 2 hole you can always re-tap it for 3-48. Tapping is
easier if the holes are first filled with kerosene and the tap
backed off at frequent intervals. Tap in the mounting studs
next.
At this point, clean up all the parts and make a trial
assembly to be sure everything fits smoothly and turns freely.
If the rotor valve shows a tendency to creep forward on the
pin, don't worry about it—it won't when the engine is
running. The important thing is that it seats well and does
not bind at any point.
MODEL AIRPLANE NEWS • October, 1950
(Next month we'll bring you the concluding part of the
Little Dragon construction article, giving instructions for con
rod, cylinder head, and the amazingly simple sleeve and
piston arrangement which eliminates the need of
conventional milled or cast-in by-pass. Details of fuel,
starting and running will also be covered.)
BILL OF MATERIALS (for entire engine)
8" of 3/4" sq. hard aluminum alloy rod
2" of 5/8" round hard aluminum alloy rod
3" of 1/2" round 17ST rod
3" of 9/16" seamless steel tubing
3" of 9/16" drill rod
1" of 1/8" O.D. heavy wall brass tube
1" of 5/32" O.D. thin wall brass tube
Scrap of 1/8" thick dural sheet (hard) for con rod
2—3-48 studs 5/8" long
8—2-56 or 3-48 screws, fillister head, 3/8" long
2—No. 4 hole washers
2—3-48 nuts
l"-sq. thin gasket material
1 Baby Spitfire needle valve assembly
1 McCoy Hot Point Plug
49
little
dragon
part two
by ROY L. CLOUGH
JR.
Here are details for finishing testing, and troubleshooting; let
us know your results with this simple but efficient power plant
THE Little Dragon shapes up rapidly once the block and lower
assemblies are completed. The cylinder liner is made from a
length of 9/16" seamless steel tubing which has a wall thickness
of about 1/16". Use great care in cutting it to size. Hold it by
means of a leather strap in a vise and use a fine hack saw blade
(put in the frame backwards). Take light strokes. The final
sizing is done in the lathe three-jaw chuck. Once again
please note, here is an opportunity to adjust for previous error.
The internal finish of cold drawn tubing is quite good to begin
with. It can be brought to a fair running finish by means of
lapping. If the builder has access to a hone, so much the better;
if not, make up a brass lap and polish out the inside of the tube.
This can be done easily by holding the sleeve in the three-jaw
chuck, running the lathe slowly and working the lap back and
forth evenly. Always run the lap into the sleeve from the same
end; it is a good idea to daub a bit of red dope on this end to
keep track of it. This will be the lower end of the sleeve. When
the lapping has been finished, clean all traces of lapping compound from the liner.
At this stage of the construction it may be helpful to read
over either of the two previously published engine articles, the
Simplex 25 (M. A. N. March and April, 1947,) and "Build Your
Own Diesel" (M. A. N. May and June, 1948,) on the subject of
piston and cylinder fits. This ground has been covered very
thoroughly. The state of the fit, the material and degree of
hardness of the piston, have great bearing upon the life of your
motor, and to a lesser degree, its original performance. The best
piston as far as wearing qualities are concerned is hardened and
centerless-ground steel. The next best is cast iron ground on
centers. Both of these methods offer difficulties to the
homebuilder, but there is no question of their excellence. If
either of these two methods is elected, there is little, if
anything, the writer can add, as the Little Dragon piston design
is about as straightforward as they come, and requires no
special instructions.
If, on the other hand, the reader is anxious to get his motor
running and wants a method of fitting a piston that will produce
quick results, although it will not last as long, he can make up
a hard aluminum piston in very short order by means of the
"cold broach" method. If the internal liner surface is in good
condition to begin with, aluminum pistons sometimes last for a
surprisingly long time, and several commercially produced
engines have used aluminum pistons with good results.
Here is the method: chuck up a piece of 1/2" 17ST, about 3"
long in the three-jaw. Do not support the tail end. Using power
feed and very light cuts, bring the O.D. down so that the piece
will barely fit in the lower end of the liner. Because of the
length of the piece and its unsupported condition the finest
cuts that can be managed will still produce a slight taper. The
surface must be very bright and free of tool marks. Bore out the
piston, cut it loose, and drill the wrist-pin holes. We now have
the following condition: the skirt of the piston will go into the
lower end of the liner about 1/8". This is backward from the
way it will run. Set the liner on a block of wood, dip the piston
in castor oil, set it into the liner, and making certain it is square,
take a drift and drive it right through. After a couple of trips
through the cylinder liner backwards, the piston can be re30
versed and driven through the right way. Always drive it in
from the lower end of the liner and turn the piston slightly
from its last position. Use plenty of castor oil and keep the
piston wiped clean. After a time it will be found possible to
rotate the piston without a great deal of resistance and a fairly
good fit will have been established.
The conn rod is worked up from 1/8" hard dural stock. Check
with the plans against what you have built, before drilling the
rod holes. The piston skirt must not hit on the front case plug
at bottom stroke. If you require a longer rod due to small
errors, make it longer—you can take the difference out of the
head. The wrist pin is simply a short length of 1/8" heavy-wall
brass tube.
Assemble the engine, using light oil on all bearing surfaces,
and turn it over by hand. The piston will (or should) have
quite a bit of resistance, but no real "sticks" should develop
anywhere. If they do, correct them. Check the position of the
piston relative to the liner at top and bottom center. If these
positions are within 1/64" of the positions shown on the plan,
congratulate yourself on a job well done. If not, you will have
to make allowances for the liner ports.
The exhaust port and intake by-pass are cut into the liner
with a small, medium-fine file. Support the liner endwise between two thin pieces of wood in the jaws of a vise. This operation is very simple as can be seen by the plan, but care and
accuracy can return big dividends here. Carefully remove any
burrs that develop. The piston deflector is now filed on. Check
this against both the plan and the liner. The exhaust port must
open first, and leads the intake port by 1/32" of piston travel.
File through the barrel of the engine block below the last
fin, put the liner in place and check the alignment. The block
slot should be slightly larger than the liner slot, but no smaller.
Take a scribe and mark the portion of the liner supporting rim
below the by-pass slot, remove the liner and cut away this portion of the rim with a fine metal chisel or hand grinder. This
licks the by-pass problem.
The cylinder head is of the "plug" variety and also has a
slight gasket-retaining groove. The combination makes it leakproof even with very indifferent machining. Ideally, the plug
should be a smooth push fit into the liner. The plug portion is
extra long to facilitate final assembly. Tap a 1/4" x 32 hole into
the head for the glow plug. The head is mounted the same way
as the front case section, that is, drill the head holes first and
use these as guides for drilling the holes in the engine block.
These are tapped 2-56 all the way through the first fin.
Clean up everything, wipe castor oil on all contacting surfaces and assemble the engine except for the head. Put a prop
on the shaft and turn it over a few times. There will probably
be quite a bit of piston resistance, but this is all right. The thing
to watch out for is jamming, though this is not likely to occur
due to the general design of the engine. If it should happen,
take the engine apart and look for surfaces that appear unnaturally bright, or scored-looking. Fix them up. The internal
clearance between rotor, rod and crankshaft need be no more
than 1/32". If the clearance is as much as 1/16 the meniscus
effect will be lost on the rotor plate. This will not prevent
running, but it will make starting a bit harder.
MODEL
AIRPLANE
NEWS
•
November,
1950
a half dozen bursts, the parts will "find" and the motor will run
out the fuel. Don't hook it up to a tank and lean it out until it
has run off a few minutes of four-cycling.
The fuel you will use depends to a great extent upon the
piston fit. It is loose, more oil will be required, but if very
good it can be run on straight O & R No. 2. Don't jump to conclusions about the compression ratio if it doesn't run correctly.
Try altering the fuel mixture (oil ratio) because it may be a
case of poor piston fit. If you're certain the piston fit is good,
then increase the ratio by deepening the gasket groove. This
engine has quite a wide range of glow C.R. because of good
thermal characteristics and will operate well between 7- and
10-1, with 8 being about optimum. Once the engine is running
properly don't take it apart unless absolutely necessary, as
this disturbs the run-in. This is particularly true if the aluminum piston is used.
The Little Dragon is the result of about three months of
design consideration by the writer, in an effort to obtain a
layout in the ½A size, which could be, quite literally, all things
to all men. A basically simple construction which could return
good results to the beginner, yet give the old motor hand a
design which would permit him full exercise of his skill and
require no apologies for the fact of being homemade. If you
like it, let's hear about it. If you run into difficulties, don't
hesitate to write the author. Good luck.
Set the head in place (without glow plug) and turn the
piston up to top center, and see how much the head lifts off.
Then rechuck the head and face off the plug portion enough to
allow 1/32" clearance between it and the piston, at top dead
center. Don't break the sharp edge of the cut—it's too handy
to cut holes in gaskets with. Use it now to cut a gasket, and trim
around the edges enough to allow the head screws to go
through. Dip the gasket in castor oil and assemble the head to
the engine. Pull the screws down "cross corner" fashion, a little
at a time. They should be snug and fairly tight, but don't strip
the threads out of the block.
Mount the engine on a piece of wood, which can be screwed
down to something solid, and put on a four- or five-inch length
of fuel line. Turn the engine over by hand for a few minutes
and get used to its grunts and groans. Don't put the glow plug
in yet. Get to know the various wheezes and pops and what
they mean. Note the soft "tunk!" made by the intake port opening into the cylinder, the gurgle of the intake rotor. These
sounds are usually masked by the louder pop of released compression when the motor is flipped over.
Turn the motor over slowly several times and the intake
port noise may disappear. This means the rotor has ridden off
its seat. A quick flip backward reseats it. Repeat this trick with
the glow plug in place (McCoy Hot Point plug is recommended). When the rotor unseats there will seem to be a loss
of compression. Again flip the prop backwards and the "compression" reappears. Unless you are familiar with this stunt,
you may think the head gasket has blown, or there is dirt under
the rotor, and tear the motor down to find the "trouble."
Mix up a break-in mix of three parts O & R No. 2 and one
part castor oil. Fill the fuel line and squirt a couple of drops
into the exhaust port. Hook up the wires and give it a flip. After
MODEL
AIRPLANE
NEWS
•
November,
1950
GLOW PLUG DIAGNOSTICS
SYMPTOM: Starts readily, revs up well, rpm drops off when
plug wire is removed. Indication: Compression ratio is too
low.
Cure: Add oil and/or nitrate to fuel, deepen gasket groove to
decrease head space.
SYMPTOM: Starts very hard with much flashback, but runs
well once started.
Indication: Exhaust port not opening soon enough before intake
transfer.
Cure: Check liner slots against plan, file more "lead" into
exhaust.
SYMPTOM: Must be flooded to start and will run only on rich
mixture.
Indication: Cylinder head, or glow plug gasket is leaking.
Cure: Replace gaskets, check surfaces for damage.
SYMPTOM: Kicks violently, runs in short high speed bursts,
kicks off prop, stops suddenly. Indication: Compression ratio is
too high.
Cure: Reduce compression ratio by shaving down inserted
"plug" portion of cylinder head, or try high compression fuels
with a cold glow plug.
SYMPTOM: Starts easily, holds up speed when wire is removed, leans out well, then gradually dies out. Indication:
Motor is not yet broken-in, probably overheating. Cure: Use
heavy, low pitch prop and run motor as rich as it will take
for ten to fifteen minutes, then try it again. General: Watch
for the usual bugs, clogged fuel line or needle, tanks not vented
correctly, loose prop, bad plug, loose wires, insecure mounting,
evaporation weakened fuel. Use a port prime to start engine.
31
LITTLE
DRAGON
Part One
by ROY L CLOUGH, JR.
Little Dragon glow engine is a project any amateur
THE
machinist can tackle with full confidence of good results.
It does not require any special tools, special talents, or extreme
precision. A large part of the total time spent in developing the
design was devoted to eliminating awkward machining jobs,
delicate operations, and tricky assemblies. If the reader owns
a small lathe and can center a piece of stock with 1/64", he
need have no qualms about being able to turn out the job. On
the other hand, the skilled builder who has a good "touch" for
this sort of thing will discover he has an engine, which requires
absolutely no apologies on the score of being homemade.
The motor is a basic design, as old-engine hands will recognize by the drawings. It has great amounts of leeway at every
step of construction. This means there is plenty of room for
the correction of errors, which should appeal to the amateur,
and equally of importance, it allows the experienced motor
builder to "soup-up" the design as he sees fit. For example,
the weight of the original came out at 2 oz., complete with
plug and prop. Skillful shaving-down by experienced machinists can reduce this figure greatly, but this has nothing to do
with the operating characteristics. Port areas and valve timing
are laid out with an eye to obtaining maximum start ability
and a good rate of speed with average construction and
internal fits, but the experienced worker who is capable of
doing very good work will find it possible to increase the
porting and degree of valve opening to obtain an extremely
hot engine.
The original Little Dragon was turning the plastic prop
shown in the photo at 8,000 rpm, 5 minutes after it was
assembled. It did this on a break-in mix of 3 parts O & R
No. 2 and 1 part castor oil. This is the performance the
average builder can reasonably expect. For experts, and with
one of the hot Francisco Lab fuels, 10,000 rpm is a reasonably
conservative estimate.
The mounting of any engine uses up time and energy and
in too many cases is finicky and bothersome. We have tried to
get around this and come up with something that is simple,
quick, and practical. The two-stud mount is our answer.
Simply press the studs against a piece of plywood to mark it
for drilling, set the engine in place and run on a couple of
nuts. No muss, no fuss, and no bother.
A bore of approximately 7/16" with a 3/8" stroke fixes the
displacement at about .06. By selecting these dimensions, it
was possible to take advantage of material sizes which sim-
An ordinary book of matches dwarfs our tiny engine
MODEL
AIRPLANE
NEWS
•
October,
1950
Little Dragon all ready to roar!
This engine is really "on the square"
plified construction considerably and is one of the reasons you
will find dimensions indicated in 32nds of an inch instead of
thousandths. (Editors note: Cad drawing in thousandths.)
However, those who wish to build the engine to conform to
AMA 1/2A regulation can use a 1/2" O.D. cylinder liner instead
of the 9/16" specified on the plans. This will bring the displacement down to about .049, safely within the rules for
engines of .05 or less. This will require slight alterations in
the width of the con rod for adequate clearance, and of course
the liner hole, piston size and head are changed accordingly.
Conversely, a skillful builder can increase the displacement to
more than .070 if he desires.
To those who think an elaborate machine outlay is required
in order to build engines, a glance through the list of tools
used to make the original should prove refreshing. These were:
small lathe, hack saw, hand drill, two files, two taps, one die,
and a pocket scale graduated in 64ths. A micrometer was used
to check sizes, but actually could have been dispensed with.
And the big news, of course, is that no milling operations are
required.
The Little Dragon employs what is known as the "sleeve-inblock" style of construction. Instead of having separate cylinder
and crankcase, one blends into the other, eliminating cylinder
tie downs, heat dams, and two more places for errors to
accumulate. The engine block serves the same purpose as the
"keel" frequently used in model airplane construction, being a
basic member, which when laid out correctly serves as an
accurate basis for the remainder of the construction. The block
is the easiest part to make, in terms of tolerances, and serves
the amateur builder the purpose of getting his hand in as he
goes along. Once the block is made the rest of the engine falls
right into line.
Cut off about 1-7/8" of 3/4" sq. hard aluminum alloy bar
stock, center it accurately in the four-jaw and face off the
end. This gives a plane surface to set against the chuck face.
Remove the piece, re-center, and face it down to the proper
size. Next, outline the cylinder block. Here is a good rule to
remember: always keep as much stock between the end of the
piece and the chuck as possible; in other words, make the first
cuts out near the end to leave a maximum diameter of supporting metal. By this rule, we see that the fins are machined first.
If your lathe is a light one, use the back gears and feed the
finning tool in slowly, particularly at the start of each cut
where it is chopping at the square corners of the stock.
The first fin is extra heavy because this must carry the
screws, which will hold the head in place. If you have cut the
piece a bit short by accident, you may take the error out of
the first fin. For example, if the piece is 1/32" shorter than it
should be, the first fin would be 3/32" deep instead of 1/8".
This is all right, but don't make it any thinner. There are three
fins below this, each 1/32" deep with a 1/16" gap between all
fins. Since the lowest fin must come in exactly the right position because of the exhaust port, cut this one after you have
made the top fin. If your finning tool isn't quite the correct
width, split the difference to fix the location of the middle two
fins. Do not cut the fins too deeply and weaken the block. A
fin depth of 1/32" measured from the flat of the stock is
entirely adequate. Turn down the "barrel" and buff it with a
crocus cloth. A good shine here increases (Turn to page 48)
23
Little Dragon
(Continued from page 23) the eye appeal; rough tool
marks give the engine the appearance of having been
whittled out of a stove bolt with a cold chisel!
The block is drilled out to an I.D. of a shy 9/16" and
brought up to size with a boring tool, or reamer. Getting
the correct depth is important, not because it will prevent
the engine from running, but because you will have to go
over the whole thing changing other dimensions to make it
come out right. Note how a shoulder is left to support the
liner. This liner must fit closely in order to prevent blowby around the exhaust port and at the head. This does not
mean a piston-type fit by any means, but it should be
tight enough so that it is just about possible to pull the
piece out with the fingers. A dummy sleeve cut from the
steel tube stock is a great help here. If by some mischance
the hole is oversize, don't scrap the job, just tin the sleeve
and resize it to fit the hole. (But you won't be able to
harden the sleeve if you do this, and then you will have
to use an aluminum or cast iron piston.)
Remove the piece and re-chuck it in order to bore out
the front of the case. Open this up part way with a drill
and bring to final size with a boring tool. The inside rear
must be faced off smoothly because the rotor valve will
ride against it. The rotor pin hole is drilled out by holding
a 3/32" drill in the tailstock. At this point lay the piece
aside and make up the crankcase front section. Do a
good job here and no gasket will be needed; in any case a
short piece of thread wrapped around the plug portion of
this part will serve very well as a gasket. If the inside
end is turned first and fitted to the block, the job is
easier, as this leaves something to chuck with. Next,
reverse the piece and bore out to 3/16" I.D. and bring the
outside down to size. No bearing is used other than the
metal itself. If you want to be ritzy about it, the hole can
be bored oversize and bronze, or Oilite bearing material
pressed in. In practice, the writer has found aluminum to
serve very well, but the coefficient of friction of Oilite
material is undoubtedly more favorable if the end in view
is extreme performance. For that matter, ball bearings
small enough to be used in this engine can be obtained,
and we are indebted to Malcolm D. Whitman, Jr., of
Carmel, Calif., for that information. If you use ball
bearings, the crankshaft diameter must be reduced or the
outboard end of the bearing increased to accommodate the
ball race, should double ball suspension be desired.
The crankshaft belongs to a breed of cats that seems
able to scare a lot of people. Don't worry about it. Put
the three-jaw chuck on your lathe and insert the piece of
9/16" drill rod. Bring this down to size with light cuts and
power feed. Finish the journal with a fine file, crocus cloth
and common sense. If you happen to have a tool post
grinder, by all means use it. Custom fit the shaft to the
crankcase front section, double checking to be certain the
thrust washer clearance is adequate, then mark the spot
and turn down to the size you have selected for the
threaded portion. We call for an 8-32 thread, but this is a
matter of choice and whatever die you have handy. The
threads may be cut on the lathe, but some will find it less
trouble to back off the tail stock and use an ordinary die.
Be sure to start it straight; back the die off every half turn
to break the chips and insure a good thread. A few drops
of light oil makes the cut easier. Near the end of the cut it
is a good idea to reverse the die in order to cut the
threads up close to the journal. The thrust washer is
simply a threaded disc. The writer has used threaded
drive washers on a number of engines with good results
and why no commercial engine uses them is something of
a mystery since it is certainly easier than milling splines or
grinding flats.
Now, remove the piece and put on the four-jaw. Chuck
up the shaft by the journal and off-center the piece 3/16"
by adjusting the jaws. It is possible to hold the piece
adequately without marring the journal, but the cautious
may wish to push the shaft into a length of brass tubing
and squeeze the jaws down on this. If this is done, be sure
to use only a gentle tapping to put it in place, because the
journal must be knocked out again afterward. The whole
secret of turning off-centers is setting the lathe tool on
center, feeding in slowly, and using power feed to drag the
tool along the work. In addition, be willing to take a little
time to do the job. The pin should be brought to a good
surface finish. The crank disc may be ground or filed away
as indicated by the dotted lines on the plan for a sort of
counterbalance effect, but this is not critical.
The rotor comes next. Most people seem to have the
opinion that disc valves must be tricky since they come
in the more expensive engines, so we'll give a little
background on this. When the original Little Dragon was
being laid out, a great deal of consideration was given to
the induction method. It had to be very simple and very
effective. Three-port induction seemed simple, but it
meant tapping into the block and cutting another hole in
the liner. Besides this it did not allow much leeway for
error and would not produce the best power output. Shaft
rotary looked good at first glance, but this would mean
less than optimum strength for the crankshaft, chances for
errors in both the port hole and the hollow shaft, and the
added difficulty of setting the intake tube into the front
case. So that was out. Next we toyed with the idea of
flutter valve induction; these arrangements are simple, and
since they work on crankcase pressure, very effective.
However, a speck of dirt, or oil hardening in the valve
makes them inoperable with a vengeance. Further difficulty
was foreseen due to the great powers of fuel meniscus in these
small sizes. Fuel meniscus? That did it! So we used a disc
rotor and made it free-floating. Five minutes after assembly
we knew we had it. Only two smooth faces are needed, and
one of them is already inside the crankcase. Best of all the
intake hole can be spotted easily, and if missed, it can be
tried again with a new rotor. In addition to all these
advantages, the thickness of the rotor is not critical and this
gives the amateur machinist another place to pick up and
correct accumulated error.
Chuck up the 5/8" aluminum rod and turn down the
rotor shaft to fit the hole in the rear of the engine block.
This should not be a tight fit, but smooth running and a bit
loose if anything. Bring the O.D. of the rotor down to size
and face it with a gentle cut, making certain there is no
shoulder next to the shaft as this will prevent seating.
Cutting loose from the stock should be halted about halfway
through and all sharp edges broken, then complete the
severance. Locate and drill the drive hole. This should be a
bit oversize, but do not drill all the way through the disc,
just enough for good clearance. Mark the outline of the port
and file away the indicated area. The rotor may be held
between two thin bits of wood or fibre in a vise for this
operation. Break any edges that develop. Now, take up the
block and put the rotor shaft into the hole from the outside
rear of the case and scratch the outline of the round portion
as shown on the plan. Split the difference between this line
and the edge of the shaft hole and drill a 1/8" hole along the
diagonal. The intake tube is a 'length of 5/32" thin wall
brass tube. Taper one end slightly (this taper is emphasized
on the drawings) and press the tube firmly into the case. The
needle valve assembly may be from a Baby Spitfire or other
small engine.
The crankcase front section must now be drilled; set it in
the engine block and spot the holes for drilling. These
holes are drilled and tapped 2-56. The original engine used 348 screws throughout, but the larger size is not needed.
However, it may be comforting to know that if you ruin
the No. 2 hole you can always re-tap it for 3-48. Tapping is
easier if the holes are first filled with kerosene and the tap
backed off at frequent intervals. Tap in the mounting studs
next.
At this point, clean up all the parts and make a trial
assembly to be sure everything fits smoothly and turns freely.
If the rotor valve shows a tendency to creep forward on the
pin, don't worry about it—it won't when the engine is
running. The important thing is that it seats well and does
not bind at any point.
MODEL AIRPLANE NEWS • October, 1950
(Next month we'll bring you the concluding part of the
Little Dragon construction article, giving instructions for con
rod, cylinder head, and the amazingly simple sleeve and
piston arrangement which eliminates the need of
conventional milled or cast-in by-pass. Details of fuel,
starting and running will also be covered.)
BILL OF MATERIALS (for entire engine)
8" of 3/4" sq. hard aluminum alloy rod
2" of 5/8" round hard aluminum alloy rod
3" of 1/2" round 17ST rod
3" of 9/16" seamless steel tubing
3" of 9/16" drill rod
1" of 1/8" O.D. heavy wall brass tube
1" of 5/32" O.D. thin wall brass tube
Scrap of 1/8" thick dural sheet (hard) for con rod
2—3-48 studs 5/8" long
8—2-56 or 3-48 screws, fillister head, 3/8" long
2—No. 4 hole washers
2—3-48 nuts
l"-sq. thin gasket material
1 Baby Spitfire needle valve assembly
1 McCoy Hot Point Plug
49
little
dragon
part two
by ROY L. CLOUGH
JR.
Here are details for finishing testing, and troubleshooting; let
us know your results with this simple but efficient power plant
THE Little Dragon shapes up rapidly once the block and lower
assemblies are completed. The cylinder liner is made from a
length of 9/16" seamless steel tubing which has a wall thickness
of about 1/16". Use great care in cutting it to size. Hold it by
means of a leather strap in a vise and use a fine hack saw blade
(put in the frame backwards). Take light strokes. The final
sizing is done in the lathe three-jaw chuck. Once again
please note, here is an opportunity to adjust for previous error.
The internal finish of cold drawn tubing is quite good to begin
with. It can be brought to a fair running finish by means of
lapping. If the builder has access to a hone, so much the better;
if not, make up a brass lap and polish out the inside of the tube.
This can be done easily by holding the sleeve in the three-jaw
chuck, running the lathe slowly and working the lap back and
forth evenly. Always run the lap into the sleeve from the same
end; it is a good idea to daub a bit of red dope on this end to
keep track of it. This will be the lower end of the sleeve. When
the lapping has been finished, clean all traces of lapping compound from the liner.
At this stage of the construction it may be helpful to read
over either of the two previously published engine articles, the
Simplex 25 (M. A. N. March and April, 1947,) and "Build Your
Own Diesel" (M. A. N. May and June, 1948,) on the subject of
piston and cylinder fits. This ground has been covered very
thoroughly. The state of the fit, the material and degree of
hardness of the piston, have great bearing upon the life of your
motor, and to a lesser degree, its original performance. The best
piston as far as wearing qualities are concerned is hardened and
centerless-ground steel. The next best is cast iron ground on
centers. Both of these methods offer difficulties to the
homebuilder, but there is no question of their excellence. If
either of these two methods is elected, there is little, if
anything, the writer can add, as the Little Dragon piston design
is about as straightforward as they come, and requires no
special instructions.
If, on the other hand, the reader is anxious to get his motor
running and wants a method of fitting a piston that will produce
quick results, although it will not last as long, he can make up
a hard aluminum piston in very short order by means of the
"cold broach" method. If the internal liner surface is in good
condition to begin with, aluminum pistons sometimes last for a
surprisingly long time, and several commercially produced
engines have used aluminum pistons with good results.
Here is the method: chuck up a piece of 1/2" 17ST, about 3"
long in the three-jaw. Do not support the tail end. Using power
feed and very light cuts, bring the O.D. down so that the piece
will barely fit in the lower end of the liner. Because of the
length of the piece and its unsupported condition the finest
cuts that can be managed will still produce a slight taper. The
surface must be very bright and free of tool marks. Bore out the
piston, cut it loose, and drill the wrist-pin holes. We now have
the following condition: the skirt of the piston will go into the
lower end of the liner about 1/8". This is backward from the
way it will run. Set the liner on a block of wood, dip the piston
in castor oil, set it into the liner, and making certain it is square,
take a drift and drive it right through. After a couple of trips
through the cylinder liner backwards, the piston can be re30
versed and driven through the right way. Always drive it in
from the lower end of the liner and turn the piston slightly
from its last position. Use plenty of castor oil and keep the
piston wiped clean. After a time it will be found possible to
rotate the piston without a great deal of resistance and a fairly
good fit will have been established.
The conn rod is worked up from 1/8" hard dural stock. Check
with the plans against what you have built, before drilling the
rod holes. The piston skirt must not hit on the front case plug
at bottom stroke. If you require a longer rod due to small
errors, make it longer—you can take the difference out of the
head. The wrist pin is simply a short length of 1/8" heavy-wall
brass tube.
Assemble the engine, using light oil on all bearing surfaces,
and turn it over by hand. The piston will (or should) have
quite a bit of resistance, but no real "sticks" should develop
anywhere. If they do, correct them. Check the position of the
piston relative to the liner at top and bottom center. If these
positions are within 1/64" of the positions shown on the plan,
congratulate yourself on a job well done. If not, you will have
to make allowances for the liner ports.
The exhaust port and intake by-pass are cut into the liner
with a small, medium-fine file. Support the liner endwise between two thin pieces of wood in the jaws of a vise. This operation is very simple as can be seen by the plan, but care and
accuracy can return big dividends here. Carefully remove any
burrs that develop. The piston deflector is now filed on. Check
this against both the plan and the liner. The exhaust port must
open first, and leads the intake port by 1/32" of piston travel.
File through the barrel of the engine block below the last
fin, put the liner in place and check the alignment. The block
slot should be slightly larger than the liner slot, but no smaller.
Take a scribe and mark the portion of the liner supporting rim
below the by-pass slot, remove the liner and cut away this portion of the rim with a fine metal chisel or hand grinder. This
licks the by-pass problem.
The cylinder head is of the "plug" variety and also has a
slight gasket-retaining groove. The combination makes it leakproof even with very indifferent machining. Ideally, the plug
should be a smooth push fit into the liner. The plug portion is
extra long to facilitate final assembly. Tap a 1/4" x 32 hole into
the head for the glow plug. The head is mounted the same way
as the front case section, that is, drill the head holes first and
use these as guides for drilling the holes in the engine block.
These are tapped 2-56 all the way through the first fin.
Clean up everything, wipe castor oil on all contacting surfaces and assemble the engine except for the head. Put a prop
on the shaft and turn it over a few times. There will probably
be quite a bit of piston resistance, but this is all right. The thing
to watch out for is jamming, though this is not likely to occur
due to the general design of the engine. If it should happen,
take the engine apart and look for surfaces that appear unnaturally bright, or scored-looking. Fix them up. The internal
clearance between rotor, rod and crankshaft need be no more
than 1/32". If the clearance is as much as 1/16 the meniscus
effect will be lost on the rotor plate. This will not prevent
running, but it will make starting a bit harder.
MODEL
AIRPLANE
NEWS
•
November,
1950
a half dozen bursts, the parts will "find" and the motor will run
out the fuel. Don't hook it up to a tank and lean it out until it
has run off a few minutes of four-cycling.
The fuel you will use depends to a great extent upon the
piston fit. It is loose, more oil will be required, but if very
good it can be run on straight O & R No. 2. Don't jump to conclusions about the compression ratio if it doesn't run correctly.
Try altering the fuel mixture (oil ratio) because it may be a
case of poor piston fit. If you're certain the piston fit is good,
then increase the ratio by deepening the gasket groove. This
engine has quite a wide range of glow C.R. because of good
thermal characteristics and will operate well between 7- and
10-1, with 8 being about optimum. Once the engine is running
properly don't take it apart unless absolutely necessary, as
this disturbs the run-in. This is particularly true if the aluminum piston is used.
The Little Dragon is the result of about three months of
design consideration by the writer, in an effort to obtain a
layout in the ½A size, which could be, quite literally, all things
to all men. A basically simple construction which could return
good results to the beginner, yet give the old motor hand a
design which would permit him full exercise of his skill and
require no apologies for the fact of being homemade. If you
like it, let's hear about it. If you run into difficulties, don't
hesitate to write the author. Good luck.
Set the head in place (without glow plug) and turn the
piston up to top center, and see how much the head lifts off.
Then rechuck the head and face off the plug portion enough to
allow 1/32" clearance between it and the piston, at top dead
center. Don't break the sharp edge of the cut—it's too handy
to cut holes in gaskets with. Use it now to cut a gasket, and trim
around the edges enough to allow the head screws to go
through. Dip the gasket in castor oil and assemble the head to
the engine. Pull the screws down "cross corner" fashion, a little
at a time. They should be snug and fairly tight, but don't strip
the threads out of the block.
Mount the engine on a piece of wood, which can be screwed
down to something solid, and put on a four- or five-inch length
of fuel line. Turn the engine over by hand for a few minutes
and get used to its grunts and groans. Don't put the glow plug
in yet. Get to know the various wheezes and pops and what
they mean. Note the soft "tunk!" made by the intake port opening into the cylinder, the gurgle of the intake rotor. These
sounds are usually masked by the louder pop of released compression when the motor is flipped over.
Turn the motor over slowly several times and the intake
port noise may disappear. This means the rotor has ridden off
its seat. A quick flip backward reseats it. Repeat this trick with
the glow plug in place (McCoy Hot Point plug is recommended). When the rotor unseats there will seem to be a loss
of compression. Again flip the prop backwards and the "compression" reappears. Unless you are familiar with this stunt,
you may think the head gasket has blown, or there is dirt under
the rotor, and tear the motor down to find the "trouble."
Mix up a break-in mix of three parts O & R No. 2 and one
part castor oil. Fill the fuel line and squirt a couple of drops
into the exhaust port. Hook up the wires and give it a flip. After
MODEL
AIRPLANE
NEWS
•
November,
1950
GLOW PLUG DIAGNOSTICS
SYMPTOM: Starts readily, revs up well, rpm drops off when
plug wire is removed. Indication: Compression ratio is too
low.
Cure: Add oil and/or nitrate to fuel, deepen gasket groove to
decrease head space.
SYMPTOM: Starts very hard with much flashback, but runs
well once started.
Indication: Exhaust port not opening soon enough before intake
transfer.
Cure: Check liner slots against plan, file more "lead" into
exhaust.
SYMPTOM: Must be flooded to start and will run only on rich
mixture.
Indication: Cylinder head, or glow plug gasket is leaking.
Cure: Replace gaskets, check surfaces for damage.
SYMPTOM: Kicks violently, runs in short high speed bursts,
kicks off prop, stops suddenly. Indication: Compression ratio is
too high.
Cure: Reduce compression ratio by shaving down inserted
"plug" portion of cylinder head, or try high compression fuels
with a cold glow plug.
SYMPTOM: Starts easily, holds up speed when wire is removed, leans out well, then gradually dies out. Indication:
Motor is not yet broken-in, probably overheating. Cure: Use
heavy, low pitch prop and run motor as rich as it will take
for ten to fifteen minutes, then try it again. General: Watch
for the usual bugs, clogged fuel line or needle, tanks not vented
correctly, loose prop, bad plug, loose wires, insecure mounting,
evaporation weakened fuel. Use a port prime to start engine.
31
Hydrojet Powers Little
Newest thing in nautical propulsion, this clever midget boat
has no weed-snagging paddles or prop—won't nip unwary fingers.
By Roy Clough
a little boat that skims over the
HERE'S
water like a speedy sea sled, riding
high on two rocket-like chine planes. Yet
when you pick it up you'll find no fingernipping propeller underneath. The only clue
to its hidden power is the intake port on the
bottom, and an exhaust port directly behind
170 POPULAR SCIENCE
it at the stern. Housed in a casing between
them is a rotary pump, which draws in
water and kicks it backward at high speed.
Reaction to this stream drives the boat.
The hull. Use 1/8" sheet balsa for the
sides, bulkhead, transom (stern), pilot'shead support and chine planes. Thinner
balsa serves for the bottom, deck and gunwales. Before decking in the bow be sure to
Speedster
cement a couple of ounces of ballast to the
front of the bulkhead. A ping-pong ball
forms the pilot's head, and the windshield
is a scrap of acetate sheeting edged with
tin. To finish off the hull, sand down the
assembly and cover it with model tissue,
then paint with hot-fuel-proof dope.
Pump and housing. Before building an
engine mount, the driving unit should be
OFF TO A FLYING START, the hydrojet speedster is already climbing. Top and side ele vations on opposite page are half size for
a boat powered by an .047 displacement
engine. The upper detail drawing at left
indicates the planing angle; the exploded
section below it, the pump-housing pattern and impeller-and-shaft assembly.
AUGUST 1954 171
OPENING AT BOTTOM OF PUMP HOUSING takes
in water, which is kicked back through rectangular port at the stern. The planing angle
of the hull eliminates need for a water scoop.
assembled and placed in the hull. The
pump housing is made from a single piece of
tin-can stock (see detail drawing). Dotted
lines are right-angle bends, and the tab
extension between the pear-shaped bottom
and top sections is curved around the rotor
end to form a continuation of the sides.
Butting edges are soldered, starting at the
nozzle to insure good alignment. Make sure
that the bearing hole is centered with the
larger water-intake hole. A short piece of
tubing should be soldered over the bearing
hole to keep the impeller shaft aligned.
This impeller shaft has a slot sawed in one
end to receive the blade, which is a strip of
thin brass stock curved in the form of a
shallow S.
172 POPULAR SCIENCE
Before mounting the blade in the housing, solder a short arm, or "dog," to the top
of the impeller shaft. Then tin both the
slotted end of the shaft and the blade,
scraping off just enough solder to let the
notch slip firmly over the center of the
blade. To mount, turn the housing upside
down and press the shaft through the bearing from below. Drop a small washer
through the water-intake hole and over the
notched shaft end. Slip the blade through
the nozzle with tweezers and press it onto
the shaft. Rotate the shaft to make sure the
blade doesn't scrape against the housing,
then solder it in position.
Engine. The power plant is a miniature
gas engine, suspended in a U frame of /s"
sheet balsa directly above the impeller shaft.
For the slightly angled coupling between
the engine and shaft, a pin on a flywheel
engages the shaft dog. The flywheel is a
large iron washer backed up by a small Vgroove pulley turned from hardwood.
Mount the fuel tank on one side of the
cockpit floor and connect it to the engine
with plastic hose.
Operation. The hydrojet boat is started
by winding a number of turns of string
around the grooved pulley and then hauling
the twine sharply back through the engine
frame.
If you don't want to chase the craft
with a rowboat, tether it to a line from five
to six feet long. One end of the line is
attached to an upright post, the other to the
side of the bull nearest the fuel tank. Otherwise centrifugal force would starve the
engine.
END
by ROY L. CLOUGH. JR.
This stable model gives true helicopter performance
DESIGNED to fly vertically, forward or back,
this rubber powered helicopter is easy to
build and certain to give good results.
Instead of individual rotor blade articulation,
which is usually necessary to secure steady
flight, the entire rotor mechanism of this
machine is permitted relatively free motion about
its point of attachment. This motion must be
limited in order to obtain forward flight;
therefore there is only 1/8" clearance between the
rotor tube and the fore and aft cross-members.
Side motion is permitted up to the width of the
fuselage, about 1/2 in each direction.
A long fuselage is used on this model to
spread its mass over a large area, thus
minimizing disturbing effects, which may occur
in the rotor. A high tail fin performs the function
of maintaining proper heading and brings the
CLA into a favorable position relative to CG.
Despite appearances, the vertical control surface
employed on this model does not create an
untoward amount of drag in forward flight.
This is because the actual relative wind is
largely downward in the immediate vicinity of
the ship. As shown in the photographs this
surface is cambered, but further experiments
made after the pictures were taken indicate
that it is more effective if built flat; therefore it
is recommended that it be made this way. A
simple push-rod linkage is used to hold it in
any desired position.
A word of caution: It is frequently desirable
to enlarge plans of conventional model planes
above the size recommended by the designer
and this is often done with good results. But
this procedure or any other alteration of the plans
must be discouraged by the writer as regards to
Hoverbug because, to do so, may result in an unflyable machine. This is because weight
distribution, articulation problems, and power
requirements may be greatly modified by a size
increase.
Begin construction with the fuselage, which is
built on the plans (presented full size). The
structure is strictly conventional except that it
becomes tri-angular aft of the rotor tube
location.
The fin may be integral or built separately.
Use 1/16" hard balsa strip for all members.
The rotor tube mounting plate is a bit of sheet
balsa to which a reinforcing washer has been
centered and cemented. Cover windows with
cellophane, and balance of the fuselage with
tissue. Water shrink but do not dope.
The control surface is next. For the lower
spar, which is also the wheel axle, use a length
of 1/8" sq. basswood or very hard balsa, as this
piece must sustain landing shocks. Tissue
cover and pin flat when shrinking. A small
hardwood block is fastened to the center of this
piece with a liberal quantity of cement. (See
detail sketches.) This block is pierced with a pin
and linked to a similar block cemented to a length
of 1/16" hardwood dowel. Attach the control
surface to the rotor base plate with cloth hinges
as shown. A small block of soft balsa is drilled
to fit the 1/16" dowel tightly; it is then slid
over the dowel and cemented to the fuselage.
The control is adjusted by sliding the dowel
fore or aft and the control surface should depart
45° from the vertical in either direction.
Use pin axles to attach two hardwood
wheels to the ends of the control vane.
Cementing together two more wheels of the same
size and attaching them to the nose block by
means of a wire yoke make the front wheel.
The rotor tube is formed from a 6" length of
1/16" medium soft balsa sheet, soaked in hot
water, wrapped around a dowel, and held in
place with gauze. Permit it to dry thoroughly
before removing and cementing up the seam.
The upper end of the tube is plugged with a disk
of 1/8" hard balsa drilled to accommodate a
standard hardwood thrust button. Cut out and
reinforce two notches in the lower end of the
tube to hold the lower rubber anchor, which is
a short length of hardwood dowel. Center a pin
or piece of wire in another disk of 1/8" hard
balsa, cement firmly in place and attach the disk
to the bottom of the tube.
Next cut four rotor blades from 1/16"
medium sheet and sand them over a bottle to
produce a slight camber. The lower rotor
blades are cemented directly to the motor tube
at a pitch of 30° and with a slight negative
coning (or dihedral) angle. Don't spare the
cement on this assembly. The unit just
assembled is tested for balance separately.
Build up the top rotor by cementing the
two blades over the 1/4" sq. hard balsa hub
piece at an angle of 35°. A wire hook, washer
and thrust plug complete the assembly. Six
strands of 1/8" flat rubber comprise the motor.
Drop the completed motor tube into place in
the fuselage, poking the pin-axle of the tube
through the reinforcing washer in the
mounting plate and bending it over to hold in
place.
Since there is no freewheeling device, this
model is flown under power at all times, using
residual power to brake its descent. It is best
flown indoors and first hops should be of short
duration. Balance should come just ahead of the
rotor tube axis, but to secure maximum distance
in forward flight it may be necessary to
make the machine slightly nose-heavy. For
forward flight slant the control surface rearward,
just the reverse to fly backward. Experiment
with varying degrees of power for best results.
For outdoor flying greater duration can be
obtained by using considerably more rubber and
incorporating one of the reversing free-wheelers
described in the writer's previous helicopter
article in the May issue.
MODEL AIRPLANE NEWS
September. 1947
airplane models
For a real eyestopper, build
"Hoopskirt"
By
ROY
L.
CLOUGH,
JR.
Flying barrels have been in the air since
Bleriot, but this model proves they can
still turn in a top performance
TROT THIS MODEL out on the field at your next meet
and watch the eyes bug. If anybody snickers, put 'em in
their place by reminding them that the annular wing is
a very old aeronautical principle. Then launch your
Hoopskirt. If its tradition hasn't impressed them, its performance is certain to!
At least a half-dozen full-scale planes (plus innumerable
kites and gliders) have been built on the "flying barrel"
design. One of the initial aircraft made by Ellehammer—
the first Dane to fly—took this form. Louis Bleriot, the
daring Frenchman who was the first to fly the English
Channel, perched one on floats and tried, with indifferent
success, to get it off the water. The French are still at it;
their latest attempt at annular-winged aircraft is a tail
sitting jet.
One of the big advantages of this design is its propulsive
efficiency. Efficiency in a flying system is highest when
the velocity of the discharged air is almost as great as
the forward speed of the plane. This means that it's
better to
46
move a lot of air relatively slowly than a small amount at
high speed. (It's rather like matching impedances.) The
annular wing with a propeller ahead of it functions as an
effective aspirator to increase the amount of air thrust
backward.
Such a wing has more lift than you might think. The
closed-circuit nature of the airfoil eliminates wing-tip
vortices. Theoretically, a hoop-wing plane shouldn't have
to bank in order to turn. This model does, however,
because of the vertical stabilizing fin at the top of the
wing. This was added to produce an effect comparable to
dihedral.
The Hoopskirt is an extremely stable flying machine. It'll
teach you a lot about this offbeat configuration. Don't let
the circular wing scare you—it's quite easy to build. Any
cylinder with a diameter of about 10 in. (a half inch
either way won't hurt) can serve as a mold for the two
spars. I used a straight-sided layer-cake pan. The
spars can be of any lightwood that bends easily when
soaked in hot water. Bind these
around the mold with a strip of rag. When dry, trim the
ends in long, matching bevels to form the lap shown in
the sketch; cement and bind with sewing thread.
You can trace the wing-rib pattern directly onto your
balsa, stacking blanks to cut as many at once as you
can manage. The slots in each end are 3/32 in. wide and
1/4 in. deep. The width should provide a snug fit over the
spars. When these hoops are seated in the notches,
their outer edges will protrude 1/16 in. for rounding off.
An easy way to space the ribs accurately is to set the
spar-mold cylinder on a piece of cardboard and scribe
around it to produce a circle the same diameter as the
spars. Mark off sixteen rib positions by means of radius
lines and assemble the wing vertically over this pattern.
Cover the frame one section at a time with light
model-plane tissue. Sections into which the strut, fin
or booms will pass can be left uncovered until assembly
is completed—or you can cover the entire wing and
then slit the paper
of these sections when you install parts that must be
cemented to the ribs. Water-shrink the paper; when dry,
give it a coat of clear dope.
Careful alignment of all balsa parts pays off in good
performance. Don't diminish the strength of the rockhard-balsa booms by sanding off the corners—leave
them square.
The tail plane has a deeply notched trailing edge, backed
up with parallel pieces of soft wire cemented to the wood.
These wires—which can be snipped from a paper clip—
will hold any flight-adjustment bends you may give the
two elevator sections after trial runs. An annular wing
operates at zero incidence, so you'll have to bend the
elevators up two or three degrees to get an angle of
attack for climb. Bending one elevator up more than the
other makes the model turn in that direction. The rudders
47
have no adjustments, and are simply cemented to the
sides of the booms after the tail plane is in place.
The engine-pilot nacelle is given a coat of pigmented
dope after the motor is fastened on its plywood mount.
The color scheme of the model shown is: red nacelle,
rudders and fin: natural white wing; silver booms, strut
and tail plane—a highly visible combination against a
blue sky.
For best performance, be sure the model balances at a
point about 1-1/4-in. ahead of the trailing edge of the
wing. An easy way to balance the plane is to stick straight
pins into both booms 1-1/4-in. ahead of the trailing
edges. Support the plane on these pins between two stacks
of books, and add weight—in the form of bits of clay,
small pieces of lead, etc.—to either the nose or the tail
until the plane is suspended between the books in a level
flight position.
Hand launch the model over tall grass until, by bending
the elevators up a little at a time, you get a flat glide. As a
check on these adjustments try a flight with the motor
running rich, then lean it out and watch your model
zoom.
This is a free-flying model, and has not been adapted for
control-line operation. It is a stable flyer, and when out of
fuel, it will glide gracefully to a landing if you balanced
it carefully.
If you're flying it in a limited space, it's a good idea to
burn off some of the fuel before turning it loose,
because the model travels at a good clip.
In any event, you'll draw a good many curious glances—
and perhaps a few snorts of derision —when you take
Hoopskirt out for its first flight. Any snickers in your
direction, though, will quickly change to whistles of
admiration when onlookers see the stability of the "flying
barrel," one of the earliest of all aircraft designs.
POPULAR MECHANICS
APRIL, 1963
Fig. 1 V-4 motor with gas generator was made in Japan
Fig. 2 Compressed air plant offered clean, quiet power but took lots of pumping
Expansion Engines
By ROY L. CLOUGH JR.
The author feels these power units have been
neglected--get busy, experimenters!
Fig. 3 Business end of successful steam-powered controlliner
THE re-introduction and acceptance of expansion engines as model airplane
power plants must bring a definite "I told you so" grin to the faces of oldtimers. For here is a category of prime mover sadly neglected up to now by
builders and manufacturers alike, yet which is in many ways more suited to
free and controlled flight than the presently popular gas and diesel engines.
At this writing there are 2 expansion engines on the market, both CO2
powered. Their quiet operation, nonexistent starting troubles, reliability and
cleanliness are appealing to many.
Expansion engines are those in which the gases which drive the piston are
brought in from an outside source instead of being generated in the cylinder.
Engines of this type were the first dynamic power plants used in model planes.
They fall into 2 main categories: reservoir and generator engines. Reservoir
engines operate from a tank or cartridge of compressed gas; generator engines
from a generator or boiler, which produces the energizing gas.
The small CO2 engines available today are reservoir engines. The
performance of this type is similar to a rubber, or spring motor, the greatest
thrust being exerted as soon as the propeller comes up to speed, with output
continuously dropping off as the energy (temperature and pressure) of the gas
decreases.
There are at present no generator engines on the market. An olden example of
this type is shown in Fig. 1—the Imp Tornado, offered by International Models
during the 30's. It was produced in 2 models, of 2 and 4 cylinders. One of the
most powerful pre-gasoline power plants, the Imp gave many a good flight to
those who could find a convenient source of the dry ice propellant.
The engine in the picture is the 4-cylinder model. The cylinders are arranged
in "Vee," 2 cylinders to a bank, with a slide valve for each bank, which
operates from a throw at the rear of the crankshaft. The engine is very lightly
built of soft brass and light sheet steel stampings, soft soldered and bolted
together. The pistons are a good fit and the crankshaft is a very neat job of
precision bending. Provision is made to oil the slide valves through screw caps
on each bank and the crankcase holds 1/2 oz. of 3 in 1 or mineral oil for
lubrication.
The gas generator at the right is an ingenious mechanism. It requires small
lumps of dry ice and carbide plus water to operate. It is designed to be
removed from the plane for loading and cleaning and the feed line detaches
from the tank for this purpose. Here there is some leakage in evidence. The
inside of the tank is compartmented to hold the 3 requisite fuels. Water is
used to heat up the dry ice to
Fig. 4 The author's version of a "hot" compressed air engine.
MODEL
AIRPLANE
NEWS.
June.
1948
13
cause it to evolve into gas, and the heat developed by the carbide
added to the water prevents the water from, freezing in the process. A
dumping lever is used to bring the components together, after which the
gas is produced very rapidly.
Frankly, we never start this thing up without a bit of fear and trembling
because we have heard of similar mechanisms exploding violently when
overloaded. However, it does have ample power to fly a 6-foot model and is
reliable and consistent in operation when one can obtain the all-essential
dry ice.
The compressed air unit shown in Fig. 2 is typical of the engines of
this type offered during the late 20's and early 30's. The tank is
(apparently) phosphor bronze about .005" thick, and wrapped with .010"
steel wire for extra strength. The ends are closed with .012" spun brass
caps and a standard tire valve is used for inflation.
The motor, of the rotary valve type, is mounted to the tank with soft
solder. This part of the unit is rather poorly constructed; a three-cylinder
affairs, the crankcase is of spun brass, cylinders of brass tubing, slotted
aluminum pistons with leather compression rings and the connecting rods
are merely hard copper. The crankshaft is a 2-piece assembly with a
machine screw crankpin. The fit between crankshaft and main bearing,
which forms the rotary valve intake and exhaust arrangement, is very poor
and leaks badly. Soft solder is used as an assembly medium throughout.
The tank, we discovered accidentally, will hold 100 Ibs. pressure
safely—how much more is problematical. At this pressure the motor will
swing a 12D 10P Pawlownia prop for 45 sec. The first burst of power
when the valve is opened is rather surprising, but within 15 sec. the
thrust begins to fall off rapidly. After 30 sec. the thrust is negligible. The
efficiency of this engine is very low due to excessive leakage of the
rotary valve; with the prop held still and the admission valve open a full
tank of air will leak out in just under a minute!
In order to evaluate properly the worth of compressed air in an efficient
engine we built a single cylinder, poppet-valve motor of .20 cu. in.
displacement and with the above tank obtained runs of over a minute and a
half with power output equal to the 3 cylinder engine originally supplied
with the tank. With this arrangement mounted in a 48" span free flight job
we obtained flights of 1000 feet and up, with 20 to 30 strokes of a tire
pump. A relatively constant pressure supply to the engine was established
by use of an Austin flight timer connected to gradually open the feed valve
as the tank pressure lessened. We found best results came when the engine
was made to carry as much propeller as possible, smaller props at high
speed resulting only in a needless waste of pressure.
One of the most interesting expansion engine experiments made by the
writer is shown in Fig. 3, a steam engine control line plane. The engine,
boiler and burner are built as an integral unit, which bolts to the front of
the plane. About 4 min. is required to get up steam after which the burner
is doused, refueled and relit for flight. This engine has never been checked
with a Strobotac but the best estimate of its rpm would not be over 2000 on
the ground. Thus the trick is to get it airborne, which requires a smooth
takeoff surface and a bit of leading. Once in the air, however, the engine
picks up and puts out enough power to fly the ship at about 40 mph on 20
ft. lines, without whipping. This is because of the small size of the burner
and boiler, which requires a considerable blast of air into the intake scoop to
build up a good pressure. With naptha fuel the engine has quite a bit more
pep, but since this soots up badly in the burner employed we have had to
stick to denatured alcohol, which burns cleanly.
The engine is an inverted oscillating cylinder type with a 1/2" bore and
3/4" stroke. It swings an 11 dia. 9" pitch balsa, or an 8-8 Pawlownia
prop with about equal thrust output. A fairly heavy counterweight is used
which makes operation nearly vibration-free. Weight (fueled and watered)
is a shade over 8 oz. The boiler, incidentally, is stuffed with copper wool to
prevent sloshing and improve thermal efficiency. A molded asbestos cap,
removed for the picture, keeps the slipstream from hitting the cylinder in
flight; otherwise condensation of the steam would lower efficiency.
Fig. 4 illustrates an interesting but rather impractical experiment—a "hot"
expansion motor, sort of a "McCoy" among compressed air engines. This
develops more power than any of the other engines shown. It has 1/2"
bore, 7/16" stroke and is constructed of hard brass tubing, except for the
crankshaft, which is steel. The piston is hand-lapped to fit and connects to
the connecting rod with a ball and socket joint. Full counter- (Turn to page
38)
Fig. 5 Below are shown six different valve mechanisms, applicable to various types of expansion engines
14
MODEL
AIRPLANE
NEWS
• June 1948
Expansion Engines
(Continued from page 14)
weighting is used and a splash oiler with
breather vent provides lubrication.
At bottom, of the stroke, air is exhausted
not only through the very large port but also
through a pressure release slot in the rotary
valve. This motor turns up 10,000 rpm with a
2 oz. flywheel at 90 Ibs. pressure, but
consumption of air is terrific. We loaned it to a
friend who tried it at 200 Ibs. pressure on a
factory airline, and he gave a speed estimate of
20,000 rpm, which seems a bit overenthusiastic.
After about 2 hours running time (much
of it spent in getting it synchronized with an
electric motor—so we could put a revolution
counter on the motor and see what speed we
were getting), the motor shows considerable
wear of piston and cylinder and the rotary
valve no longer fits as snugly as it should.
However, at lower speeds and particularly with
steam we have found brass to be quite a
satisfactory material.
All expansion engines have 2 main requirements: a good smooth valve action, and a
source of pressure. There are many types of
valve gear, which give good results, and a few,
with appropriate comments included here, are
shown in Fig. 5.
(A) The first is the oscillating cylinder valve,
which is adaptable to steam, compressed air or
other pressure. This is about the simplest and
most positive valve gear since it requires no
cams, cranks or levers and wear tends to
improve the seal. Since much of the operating
stress of the engine comes upon the cylinder
stud it is essential that this part be well
anchored. This engine may be reversed by
simply switching the intake line from one
tube to the other.
(B) is the rotary valve. This, too, is a very
simple form of valve and operates with
minimum drag. However, a rotary valve is
subject to leakage with wear, is limited to
relatively low pressures and is not particularly
suited to steam. If carefully made it is a fair
compressed air valve, but its efficiency is not
as high as the more positively sealing types.
(C) the piston valve is very positive in
action but has the disadvantage of requiring a
separate linkage to operate, and it does put
some extra load on the engine since the valve
closes against line pressure. It is however
practically leak-proof if well made, and this
feature alone makes it worthwhile.
(D) the slide valve is perhaps the most
widely employed expansion engine gear. It
wears well, seats without leakage and offers a
comparatively minor drag on the engine. It is
equally good for steam, air, or CO2, but should
have some provision for lubrication if the latter
2 "dry" gases are used. Leakage is apt to occur
around the gland where the activating rod
enters the pressure chest.
(E) poppet valves offer fast action with
minimum leakage and good wearing
characteristics. Excellent for steam or
compressed air, they may tend to stick if
high-pressure CO2 is used, due to the great
refrigerating quality of this gas.
(F) the ball valve requires no external
drive, seats well and gives little trouble.
However, this type of admission works well at
high speeds and pressures only. Because of the
great amount of "lead" (gas enters the cylinder
before the piston reaches top dead-center) a
considerable amount of flywheel effect is
necessary. If this is not supplied the engine may
refuse to start, or may oscillate back and forth
without turning a full revolution. This is
probably the best type for high pressure CO2
work and both present day CO2 engines use
this principle, but we do not recommend it
for low pressure or steam engines.
Efficient exhaust porting, of expansion
engines offers a special problem. Usually it is
not enough merely to cut a hole in the
cylinder as is done in 2 cycle engines where
the piston itself acts as an exhaust valve*
This is because the cylinder will exhaust only
down to atmospheric pressure, the portion of
gases
remaining
offering
considerable
resistance to the piston on the way up again.
Drag of this sort can absorb a high proportion of
the potential power of a low-pressure engine.
*Although this practice is permissible
where very high pressures are employed,
such as in the popular CO2 engines with their
four large ports, and in some types of annular
ported steam engines where the sudden drop
in pressure causes "condensation vacuum."
To obtain optimum efficiency the exhaust
valve should operate from the head of the
cylinder and remain open until the piston is
nearly at top of the stroke, closing before or if
possible at the same instant the inlet valve is
opened.
Thus far we have not mentioned double
acting engines; a word about them is in order.
A double acting engine is one in which the
operating gases push the piston both ways by
means of a duplication of valves at lower end of
the cylinder. This necessitates some sort of
packing gland around the connecting rod, a
heavier structure and complication of the valve
gear. It is, in effect, a 2-cylinder engine in 1
cylinder. On the basis of our experiments we do
not recommend this type for aircraft use,
although it may be advantageous for model
automotive and marine installations. Instead we
suggest the use of multiple cylinders, 3 being
ideal, since there is no "dead center." In radial
"ex-pension engines, any number of cylinders
may be employed, odd or even, with the power
output becoming smoother as the number of
cylinders is increased.
Now, how can we obtain pressure? There
are several methods of storing gas under
pressure to drive an expansion engine. The
simplest, oldest and in many ways most
satisfactory method is to compress air in a
light tank by means of a and pump. For those
who dislike pumping operations the tank may
be filled from a seltzer cartridge by means of a
little gadget used to secure emergency inflation
of bike tires. We once used an old high-pressure
truck tire, inflated to 80 Ibs. at a local service
station; rolled to the flying site it provided, by
means of a detachable air-chuck, about a dozen
flights. (Surplus oxygen tanks now available
quite cheaply are light, strong, and would serve
this purpose well. —Ed.)
The gas generator engine offers literally
dozen of methods of securing pressure. Dry
ice can be heated in a tank by means of a tiny,
well shielded alcohol flame; CO2-evolving
chemicals can be mixed with a small quantity
of water, and a small quantity of air and
gasoline exploded by an electric spark into a
larger amount of dry air will produce instantaneous pressure. However, do not attempt any
of these methods without using a reliable
safety-valve, and in particular do not try the
gasoline exploding stunt without first
calculating very carefully the proportions of
the generator. If you don't know how, don't
try it!
Steam is probably the safest thing to
generate and the easiest to handle. The boiler
should be strongly constructed of non-corroding
metals, and baffles of some sort are needed to
prevent sloshing around in flight. The steam
line should be attached to the tank in such
fashion that it cannot pick up water if the plane
should bank sharply. The burner is best fueled
with alcohol. The safest type we have found is
simply a pad of asbestos wicking soaked in
fuel. This removes practically all fire hazards
since the fuel stays put and there are no feed
lines to overheat, fracture, or otherwise cause
the model to ignite. Air scoops can be used to
increase the heat of the burner and, by strategic
placement, carry away all heat, which might
be transmitted to the plane's framework.
A carefully designed steam plane is
perfectly safe for free flight since it will not
descend until the burner goes out. Smooth
operation makes it possible to use lighter
construction to improve soaring qualities.
The possibilities of steam have never been
fully realized. In view of the results obtained
with comparatively crude constructions we
venture to suggest that if somebody with the
time, money and ambition put as much
engineering effort into developing a good
airplane model steam engine, as has been put
into development of present day gasoline engines, the steam engine would give internal^
combustion a good run for its money.
There are many sound reasons for this
premise. First, there is absolutely no
question of whether or not it will run. If
you can scratch a match you can start a steam
engine. Second, there are no electrical
problems, no wiring, and no switches. Third,
steam is quieter, cleaner, and generally easier to
handle than internal combustion with its special
fuel mixtures, variable compression (in the
case of diesels) point clearances, cowling
problems, and oil-throwing disposition.
But is steam capable of offering as much
power?
We think it is, and with weights comparing
favorably with internal combustion. The
reasons are:
Although the steam power plant is not as
efficient—interims of B.T.U. converted to
mechanical
energy—as
the
internal
combustion engine, it can quantitively convert
more fuel into mechanical effort for any given
displacement.
Thus, if a gasoline engine of .299 cu. in.
displacement, obtains a thermal efficiency of
25%, and a steam engine of the same size has an
efficiency of only 10%, this does not
necessarily mean the gasoline engine is more
powerful even though it is 2-1/2 times as
efficient. We can burn 5 times as much fuel
in the steam engine and raise its power
output to twice that of its rival. The amount
of fuel, which can be burned in an internal
combustion engine, is strictly limited by its
displacement. Therefore, for each revolution it
is practical to convert to heat only as much fuel
as will give the greatest expansion of gases;
additional fuel will not give additional power
but will simply be wasted. The maximum gas
pressure over the piston for any given stroke is
limited strictly to the maximum pressure it is
possible to obtain by burning a given quantity
of gasoline and air.
This does not hold true for steam. Here, the
pressure over the piston is limited only by
how much heat is being applied to the bottom of
the boiler. We can raise this limit by raising
the burner temperature; that is, by forcing
more air and fuel into the firebox.
Another angle: in order to get any sort of
power output from internal combustion engines
they must turn over at very high speeds, speeds
which it has been demonstrated are not the
most efficient propeller wise. To permit high
engine speeds and lower and more efficient
propeller speeds, the use of some sort of
reduction gearing is mandatory, bringing added
weight and frictional losses. However, with the
steam engine it is possible to hold the speed
down with a larger propeller, of higher pitch,
and let the increase in boiler pressure carry the
load. What should we be able to do with
props of 20" pitch, 3 to 4 times the area of
today's racing toothpicks?
And—just in passing, because we have done
but a sketchy amount of work on the
subject—what are the possibilities of using tiny
impact turbines and driving the prop at a
lower speed through a magnetic slip clutch?
To sum it up, it is the writer's conclusion that
there has never been a fair test of basic worth
between the expansion engine' and gasoline
power plants for model plane use. There is,
today, a need for a good reliable compressed
air motor for free flight which can be filled with
a few strokes of a tire pump. The whole thing
should not be over 24" in length and should
be manufactured to tolerances comparing with
those used for gasoline engines. In steam, there
are boundless opportunities for both control
and free flight. The application of cartridge
gas (CO 2 etc.) to model prime movers has only
been briefly exploited.
In short, the expansion engine field is one,
which has been but lightly and sketchily
touched, with no real effort to extract the
utmost performance from the basic idea.
Now, who is going to do something about it?
MODEL AIRPLANE NEWS • June 1948
I Fly
a Control-Line
Trainer
It looks like a refugee from a
Venetian-blind factory, but it
flies fine—and it won't stall.
By James Webb
The prop is spun . . . the engine roars .
. . must remember . . . pull back on
stick. Then ease off . . . in the air now . . .
everything's going by so fast . . . trees
. . . ski . . . water . . . trees . . . a blur . . .
beginning to feel dizzy . . . nose down
again . . . must pull stick back . . . there's
the sun . . . dazzling . . . can't see very
well. . . she's stalling . . . Crash!
I'd cracked up on my first solo flight.
COURSE I could walk away from
OFit—you
always can; that's a big
advantage in flying control-line models
over the real thing. But the crash
discouraged me from getting a model
for the kids. Now, here was Roy Clough
Jr., whose model I'd cracked up, with
another plane, a funny-looking one. The
wing was a series of slats with air spaces
between. And there was a big cardboard
disk in front of the prop.
"What have you got there?" I asked.
Roy had been mighty nice about his
plane—explained that lots of beginners
cracked up control-line jobs because they
are so fast and so easy to stall if you
freeze on the controls. Then he grinned.
"This," he explained, "is a control-line
model designed especially for beginners
—a basic trainer. She's slow and she won't
stall. Swell for kids to learn with. Why,
AUGUST 1955
189
After a flight checkout ............away she goes
I bet even you will be able to fly her."
I was in no position to resent that. Instead, I asked for a demonstration.
The little plane took off slow and easy
and went around the circle with a lazy
lope like a tired hound-dog. Then Roy
stuck her nose up and held it there. Instead of stalling and crashing, she kept
mushing around, nose in the air, even
slower—walking speed.
Roy wasn't kidding. This little job
was as different from the model I'd flown
before as a cub trainer is from a Shooting
Star. "What's the secret?" I asked.
He pointed to the slat wing and the
prop disk. "The disk spoils the air stream
from the prop so that the plane travels
slowly. And she gets the lift to keep going at low speeds from this fancy wing.
WING SLATS set at varying angles, ranging
from negative (downward) angle at leading
edge of wing to positive (upward) angle at
trailing edge, give lift at low speeds. The
tail surfaees are hinged with cloth tabs.
Air compressed under the wing by the
forward motion is squirted through the
spaces and over the slats to provide the
lift.
"And notice the low angle of that front
strip," he went on. "That's what keeps
her from stalling even when the nose
is way up."
I took the controls, and Roy was right.
Even I could fly this job, and I could
turn her over to small fry with little risk
of a discouraging crack-up on the first
try. What was equally important, I found
I could build a trainer model for the kids
myself. So can you. Here's how to go
about it.
Fuselage. Trace the fuselage outline
on a sheet of 1/4" balsa and cut it out
with a razor or a modeler's knife. Cement
a 1-3/8"-diameter disk of 1/4" plywood
to the nose for the engine mount. Then
add two half-round blocks of balsa,
faired from the disk to the fuselage, to
reinforce the engine mount and hold the
mounting nuts and landing gear in place.
Cement the cloth-hinged tail surfaces
in place and brace them with two wooden
toothpicks. Bend a tailskid from soft
wire, press it into the fuselage, and
secure it with cement.
Wing. Cut the slats for the Venetianblind wing from 1/8" balsa and cement
them to tip racks made from scraps of
1/4" balsa left over from the fuselage.
These tip racks must be stepped or
190
POPULAR SCIENCE
climbing high .......................................... into the sky
notched exactly like the fuselage for good
alignment. Add 1/16" tip plates to the
wing and cement to the fuselage. Coat
the model with fuel proof dope to protect
it from the engine fuel.
Controls. Cut a T-shaped bell crank
from tin can stock and pivot it on a nail
pressed into the fuselage. Link the control crank to the elevator horn with a
length of 1/32" music wire. Support the
wire at its center by a piece of plasticsoda straw cemented to the side of the
fuselage. Tie and cement a 25' length of
light fishing line to each leg of the bell
crank and pass the lines through the wire
loop on the wing tip. Trim the free ends
to exactly the same length and attach
them to the control handle.
Engine. Almost any small half-A engine can be fitted to the mounting disk.
Just be sure the engine shaft is mounted
at a slight downward angle for good lowspeed behavior. Fit it with a 6" propeller
and kill its efficiency by fastening a 3"
disk of cardboard in front of it.
Operation. As soon as the trainer was
finished, the kids and I took her out to
the empty lot next door. With Roy along
to give us some expert advice, we quickly
got the hang of it and could walk her
around the circle without any trouble.
Engine tune-up. After you've been flying a while, you may find your engine
getting cranky and hard to start. This
probably means that a gummy deposit
What a show!
from the fuel is clogging the tank and
feed line. They can be cleaned with lacquer thinner. But a word of caution.
Even a tiny half-A engine has nasty exhaust fumes; spend the evening in your
shop turning her over and you'll end up
with a bad headache. Have plenty of
ventilation, or better still, do your tuningup outdoors.
Fueling. Your fuel comes ready-mixed
—a typical formula has castor oil plus
methanol plus nitro-paraffins—in halfpint cans that cost about 55 cents. This
may seem pretty high on a gallon basis—
$8.80-but you'll find it's only slightly
over a penny a flight. You can fill the tiny
tank with an eyedropper, but it's a lot
easier and safer to use a miniature pump
that any model-supply store carries.
Starting. Prime the engine first until
it slops over—don't try to be neat if yon
AUGUST 1955
191
want easy starting. Then hook on the batteries—a couple of 1-1/2 volt jobs wired in
parallel. These supply the juice to heat
the glow plug that ignites the fuel inside
the cylinder. One contact goes to the
glow plug, the other to the engine frame.
You can make your own connecting
wires, but again it's easier to use a readymade wire with a neat double tip that
fits exactly and costs only 35 cents. Of
course, you disconnect the batteries as
soon as the engine catches.
Before spinning the prop, set the fuelair-mix screw at the point recommended
by the manufacturer—usually three to
five turns open. If the engine catches
readily but then dies, your mixture is too
lean. If it does this even with a rich mixture, your fuel line is probably clogged.
Once the engine catches, the engine
will sputter and spit, so lean the mixture
until it roars smoothly (it will really roar,
too).
Controls. Be careful when you unwind
the reel not to cross the control lines.
Operation is simple. Hold the reel vertically, with the end upward that is connected to the up action of the elevator.
(Mark this end with a red dot on the
reel.) Then you pull the top of the reel
back to make the plane rise; the bottom
to make it dip.
Flying. It takes two people to handle
any control-line plane: one to spin the
prop, one at the controls. If there is no
bare earth or pavement handy for the
take-off run, the prop handler will have
to hold the plane in the air and give her
a little toss forward; she won't take off
from grass. The take-off run should last
10 or 15 feet. Level off after a slow climb.
Gentle the controls; the plane responds
readily as she picks up speed.
If you freeze onto the controls with
the nose up, the trainer will just mush
around instead of stalling. But don't get
in the habit if you plan to fly conventional models.
You may think the fuel supply is very
small; later you can add auxiliary tanks
for longer flights. Actually, however,
you'll find at first you can get pretty
dizzy even with a short flight.
Props. You can vary the speed considerably with different combinations of
props and spoiler disks. Those recommended for the plane fly her about as
slowly as she'll go and still take off.
You'll soon find props are expendable,
so take along half a dozen spares.
Beware the wind. Because the trainer
flies so slowly, she develops very little
centrifugal force to keep her out at the
end of the control lines. Even a slight
breeze may blow her toward you as she
flies crosswind—and if the lines slacken,
your control is lost. If necessary, keep
lines taut by running downwind. - END
A TIN CRANK transmits your pull on control
lines to elevator. Lines are attached to a
control stick hand-held in vertical position.
The plane climbs when you tilt the stick back,
dives when you ease it forward. Cabin windows
are painted on the fuselage.
164
TOYS AND GAMES
Try a Real Challenge-Build
Free-Flying Powered
HELICOPTER
High-flying helicopter takes off vertically, and solves control problems
of gyroscopic forces and torque
By ROY L CLOUGH JR.
Taking off vertically and flying at reduced speed
forward allows you to experiment with the helicopter
in reduced flying areas without it disappearing.
Craft Print Project No. 202
little chance you'll lose this helicopTHERE'S
ter in a free-flight, as it takes off vertically,
flies forward at reduced speeds, and slowly
floats down when the gas runs out. You can
fly it in a limited space with less danger of
cracking it up. And it will teach you about
rotary-wing aircraft and their problems.
With all the advantages of helicopter flyingwhy haven't we seen more of them? Controlling
a model helicopter involves some complex dynamic problems. Flight stability has been a big
stumbling block, as early models either crashed
within seconds after take-off or rose drunkenly
into the air before tipping over to one side and
plummeting downward. Helicopters gained the
reputation of being tricky, hard-to-control, and
requiring an expert's patience to adjust all the
factors that would keep it in the air for a few
minutes.
Actually the trouble was not with the helicopter
idea, but with the approach to the problem. A
rotating wing observes not only aerodynamic
laws, but also the laws which govern gyroscopics. Rotors behave in much the same fashion as a toy gyro top.
Applying a force at one
point on the rim tilts the
rotor, not at that point, but
90 degrees from that point.
This is a basic law of
gyroscopic action. A rotor
that is rigid and stiff, as
most of the early rotors
were, would react at 90° to
any deflection in flight.
If a wind struck the front
of
the
rotor,
for
example, it
Counter-rotating
props
solve torque problem as small
blade attaches to engine
output shaft while main
rotating wings are fastened to
engine frame.
TOYS AND GAMES
tilted on its side. Then, if the
same wind struck the side (due to
the slip induced by the first
displacement), the rotor would
pitch fore and aft. The net result
was a wild series of dips and
rolls of increasing amplitude
ending in a crash. These
reactions happen so rapidly that
it appeared the model simply
'went crazy' and crashed. Solving
this stability problem calls for
freeing the rotor blades so they
can tilt, increasing or decreasing
their pitch angle according to the
aerodynamic load upon them. To
obtain this motion
Helicopter Adjustments Made Easy
FIRST, remember that the spinning rotor is essentially a
gyroscope. Since the large rotor has the most mass, it
rules the system in this type of model. A gyro reacts
to an adjustment or disturbance at 90 degrees from
where the disturbance takes place. Thus it might be
said that we adjust model helicopters "around the
corner." For example, if the center of gravity is in the
correct position and the model tends to nose up, we
correct this by bending the rotor mast so that the rotor
mechanism tilts slightly to the left. If the model tends
to dive, then we tilt the mast somewhat to the right. By
varying the sidewise tilt of the rotor mechanism we can
make the model fly forward to right or left or rise
vertically.
On the other hand, suppose we load the model tail
heavy. This puts a side load on the rotor, which is
processed (moved 90 degrees) by the stabilizing tip
weights and changes the pitch (cycles the blades) and
produces FORWARD flight. If we load the model nose
heavy it will tend to back up, but due to the keel
surface it will swing around quickly at high speed
and may dive because of sudden change in altitude.
165
automatically, small flyweights
are attached to the blade tips to
integrate the gyroscopic
forces with the aerodynamic
forces.
When
the
rotor
encounters a tilting force it
reacts at 90°, causing the tip
weights to bob up or down.
These weights change the angle
of attack of that particular
blade. Since this reaction
occurs at 90° from the
original displacement, the control reaction, which also moves
90°, travels back to the original
point of displacement to cancel
out of the disturbance.
The flyweight solution also
provides
automatic
autorotation when the motor stops.
Otherwise the rotor would
slow down and stop, then
start to spin backwards, which
nine times out of
166
No.
1
1
1
2
3
4
1
5
20
2
5
1
1
1
TOYS AND GAMES
MATERIALS LIST—HELICOPTER
All Dimensions in Inches
Size and Material
1/8 x 3 x 18 balsa
1/16x 3 x 36 balsa (spare blade allowance)
1/4 sheet balsa, approximately 1-1/2 x 3 (mast fairing)
10 x 12 stiff paper for fuselage covering
small jar covers (press-on type)
paper clips
3/32 hole washers
length 1/16 piano wire, 36 long
5-in lengths soft iron wire, 1/16 diameter
sq in tin can stock
1-diameter wheels
2/56 x % nuts and bolts
1/8 leather washer or equivalent
Wasp. .049 or Atwood .049 model engine
2 or 3 pitch prop 5 to 7 diameter for engine
Misc. colored fuel proof dope, cement, solder, etc.
ten would tumble the model into a crash, to say
nothing of presenting a very sloppy performance
having little in common with full-scale machines.
While our helicopter appears to be, and actually
is, an extremely simple design, the design
considerations behind it are based on a working
knowledge of some rather complex factors, so don't
alter the plans if you want the best performance.
While everybody likes to make minor changes here
and there, you'll find it to your advantage to build this
model exactly as shown. Later on, with more
experience and the understanding of gyro forces
learned from this model, plus actual flying
experience and observation, you can design an
original. Understanding of what goes on is the
keynote to success with helicopters.
Our helicopter uses a 'torque reaction drive',
which bypasses all problems of clutches and gearing.
Two rotors supply lift—a small one attached to the
engine's output shaft and a large one, which spins in
the opposite direction. Along
with
this
system's
mechanical simplicity is its
more dynamic complexity.
The small rotor does the
major lifting task while
the large rotor takes care
of the stability and autorotation.
The fuselage is a good
place to begin construction.
Bulkheads slip on a simple
keel and the resulting frame
is covered with stiff
construction paper. The
rotor mast and the
landing gear legs must be
ruggedly attached. Cabin
windows are painted on with
hot fuel-proof dope in a
contrasting color. The tail
surfaces are fixed, as it is
not necessary to adjust
them (we'll explain this
later).
The rotor unit, the
heart of the helicopter,
begins with two small can covers—we used caps
from baby food jars. These should be about 1-1/4
in. diameter and no larger than 2 in. diameter
Clean out the seal or cardboard and accurately
center a hole to fit the rotor mast in each. The
blade arms are 1/16 in. diameter music wire.
Bend to shape (Fig. 4) and solder in place inside
one of the caps. Good balance and accurate layout
are important here.
Next drill holes for the engine mount, taking
care to center the engine exactly. Solder the
mounting nuts in place over the holes.
The blade clips are bent from tin can stock.
Brass or copper tubing forms the pivots on two of
them (Fig. 4) while the third is soldered directly to
the blade arm at a slight positive angle. A #3-48 or
#2-56 nut and bolt holds the blades in place. With
the rotor stems in place, take the other jar cap,
butt it up against the first (which has the hardware
in it) and run a bead of solder around the seam.
This makes a very light and friction-free rotor
bearing. Hold it to the fuselage mast with a
washer soldered to the mast. Make up the rotor
blades now. The woodwork is identical for all
three; simply sand them to a 'glider wing' section
and dope. Two of the blades are fitted with tip
weights (Fig. 4). The third blade is assembled
without weights and goes in the fixed holder.
The only weight we put on this is whatever is
required to balance the engine cylinder.
Assemble the whole works and check for balance. Then make up the pivot prop and put it
on the engine (Fig. 3). Use a small wood block
to provide take-up for the shaft nut.
You're now ready to test fly it. Check the
balance of the fuselage. The center of gravity
should lie about 1/8 in. behind the vertical axis
of the rotor. This is why: In order for the
TOYS AND GAMES
helicopter to fly forward while it is rising we must
have some force that makes it do so. The
design of the fuselage causes it to present
more resistance to the high-speed downwash of
the small rotor in the direction in which we
wish it to travel. This means the fuselage
will tend to tilt down unless we move the CG
aft to compensate for the downwash effect. In
flight the fuselage will tilt forward only slightly,
because the reaction to this tilting force is
precessed gyroscopically to the blades, which
shift position slightly and propel the model
forward.
Start the engine and run it up to top speed
with the mixture set a bit on the rich side to
compensate for the centrifugal force on the
tank. Allow the main rotor to come up to its
maximum speed, and then allow the model to
rise under its own power from your hand.
Be sure it is level and never throw it. The
machine should rise up steadily and when
about 10 feet up it should start to move
forward, flying in a large circle to the left, and
gaining altitude until the engine stops, when it
will drop vertically on its spinning rotor.
Bending rotor shaft to left, forward or to right
controls direction of flight.
This model has one fixed blade on the rotor,
which is used to control forward speed and
eliminate diving tendencies. The fixed pitch
angle should be about the same as the
normal angle of the feathering blades,
otherwise the model may 'walk' a bit as it
flies. If your model refuses to fly forward,
decrease the pitch of the fixed blade, and at
the same time bend the flyweights up on the
other two blades so they rotate at the same
average pitch.
If, on the other hand, your model tends to
dive, increase the pitch of this fixed blade by
167
bending the wire slightly and bend the
flyweights on the other two blades down a bit
further so that when they ride up under
centrifugal force the pitch on these blades will
'seek' about the same angle as the fixed
blade. Since the feathering (or pivoted)
blades automatically tip up when the engine
stops, the fixed pitch blade does not interfere
with auto-rotation.
The function of the special stabilizer
arrangement, which tends to twist the
fuselage to the right if the forward speed
becomes high, is to lift the nose and slow the
model down. This feature makes the model
easier to handle, acting as a sort of built-in
'governor.' For this reason it may be flown at
considerably higher forward speed than the
usual run of model helicopters. Do not reduce
the size of this set of surfaces, and in
stubborn cases (if, for example, your model is
very heavy) it may be necessary to increase the
size slightly. The recommended engines for
this model are the Wasp .049 or the Atwood
.049. Use either STA dopes or Aerogloss
finishes throughout.
• Craft Print No. 202, in enlarged size for building
the Powered Helicopter is available at $1. SPECIAL
QUANTITY DISCOUNT! If you order two or more
craft prints (this or any other print), you may
deduct 25 ⊄ from the regular price of each print.
Hence, for two prints, deduct 50⊄
⊄ ; three prints,
deduct 75⊄
⊄ , etc. Order by print number. To avoid
possible loss of coin or currency in the mails, we
suggest you remit by check or money order (no
C.O.D.'s or stamps) to Craft Print Dept. 5561,
Science and Mechanics, 450 East Ohio Street,
Chicago 11, Illinois. See coupon on page 192. Now
available, our new illustrated catalog of "186 Do It
Yourself Plans," 10⊄
⊄ . Please allow three to four
weeks for delivery
Scanned in from:
Toys and Games You Can Make
Science and Mechanics Handbook Annual No. 6, 1958—No. 556
Autobiography of ROY L. CLOUGH, Jr.
Modeler, Author, Novelist, Magazine Contributor,
Designer, AMA Hall of Fame Life Member
Modeler since 1931
Birth Date: November 18, 1921 AMA: 3254
Written & Submitted by RLC; Updated 7/02
Transcribed by NR (5/97)
Edited by SS (2002)
Career:
● Designed the Berkeley “Cloud Copters”
● Designed the first ground effect vehicle in late 1930s
● Built successful rubber powered helicopters and gas powered versions in the 1950's
● Built ducted fan delta in late 1940s
● Was an early builder of reso-jets
● Designed and built glow and diesel engines
● Built a steam-powered control liner
● Designed the original free flight version of the Martian Spaceship
● Have built and flown Flettner-type rotor planes as early as the late 1950's
● Originated the slotted flying saucer design made popular by Fran McElwee
● Built and flown several types of autogyro
● In 1948, built a liquid fuel rocket with compressed air oxidizer
● 22 years as Chief Eng. of high tech firm
● Four years as an Ind. Consultant
● Worked as reporter/writer/editor New Haven Journal-Courier, Eastern Pilot
● At present a freelance writer/designer, author of four books, including 1930s period
novel: A Brief History of the Ashmont Town Team etc.
● Long time C&W musician, pedal steel, Dobro. Designer and builder of many
instruments
Honors:
● 1999 – AMA Hall of Fame
I was about 10-years-old when I built my first flying model out of pine sticks, gray building
paper and inner-tube rubber. It flew about 25 feet and I was hooked.
My first kit was a Guillow 10-cent Spad, given to me by a little girl who drew my name in a
school Christmas party. Spad wound up as a midwing that would not fly. Early inspiration to
design my own sparked by Gordon Light's Wakefield winner. It proved a good pattern.
I have built models with all sorts of power including rubber, compressed air, steam, rocket, park
ignition, glow engines and electric motors. One early model, best forgotten, even had a clock
spring motor. .
I developed an early interest in unusual, unorthodox, new types etc., and in early 1940's built
successful rubber powered helicopters and gas powered versions in the 1950's. Both types were
widely copied.
I got into magazine projects by accident when Popular Science's Workshop Editor, the late Harry
Clough
Page 2 of 3
Walton, was told by his secretary, Ria Nichol, that during a visit to my wife and I in Now Haven
she had seen me repeatedly fly a model helicopter from one table top to another. When I
discovered I could get paid for this stuff I published a lot of it in MAN, FM, Pop Science, Air
Trails, Science & Mechanics, Popular Mechanics and Mechanix Illustrated. I also designed the
Berkely “Cloud Copters.” My rotor system was widely copied, including by Cox Manufacturing
who never acknowledged or paid for it. Considering the job they did on Jim Walker I figured I
lacked the resources to buck them. Later involvement with Big John Elliot and Larry Renger
was pleasant and productive, but they fared no better at the hands of Cox management.
I had a lot of firsts including perhaps the first ground effect vehicle. It was a small gas engine
powered model about three feet long. It worked great, but all the dirt, grass and twigs it threw up
convinced me the idea was practically useless. Talk about blowing it!! I could have had a basic
patent. Later on I built a .049 powered model for Popular Mechanics, which they reprinted in
one of their workshop books.
I built several early ducted fans, maybe the first to achieve worthwhile thrust. I was flying Free
Flight versions when Bill Effinger and Don McGovern could not get their Control Liners off the
ground, but what they told me was helpful to my later designs. Also I built two or three pressure
Jets. Only one of these was published but I might get back to the idea. Jets published in Air
Trails & Popular Science. I was an early builder of reso-jets; the smallest had 7/16 diameter and
a tailpipe made from telescoping tubing sections from a golf club. I slid sections back and forth
like a trombone to locate the proper resonant frequency. I designed and built glow and diesel
engines. Little Dragon, published in Model Airplane News is the best known. I converted an
OK C02 motor to run very poorly as a diesel. I also built a small steam boat and a steam race car
for Popular Science. I built a steam-powered Control Liner that blew up on take-off.
I designed a gas helicopter with 48" self-powered rotor that flew well on Cox .02. Pop Science.
Originated the slotted flying saucer design, (Air Trails,) but it was made popular by Fran
McElwee who carried the design forward. Much later I published a couple of versions in Model
Builder.
While I designed the original Free Flight version of the Martian Spaceship, (Air Trails,) Skip
Ruff should get the lion's share of the credit for the bigger radio control versions that have made
such a hit on the West Coast.
I built and flew Flettner-type rotor planes as early as the late 1950's. Model Aviation News,
Popular Science, latest “Rotorplane!” in MAN. (*)See bibliography
Built and flown several types of autogiro. MAN. Autogyro kites for PS and S~&M. Built early
but not greatly successful electric in the 50's.
In 1948 I built a liquid fuel rocket with compressed air oxidizer that blew up with a very
satisfactory bang.
Clough
Page 3 of 3
Despite early AMA membership have, until recently usually been a loner. Now, in the
springtime of my senility have joined up with the Winnipesaukee Radio Controllers, and
acquired a great bunch of flying buddies.
I'm likely to try any construct that gets my interest. As a result of this I know more things that
won't work than anybody.
Background includes:
22 years as Chief Eng. of high tech firm,
Four years as an Ind. Consult.
Worked as reporter/writer/editor New Haven.Journal-Courier & Eastern Pilot.
Co-authored a McGraw-Hill text on Industrial Psych.
At present a freelance writer/designer, novelist and semi-pro C&W musician: pedal steel/ Dobro/
keyboards. Have worked with Paul Main, Hank Thompson, Dick Curless, and ran my own
“Stateliners” for a few years, but enforcement of tough driving and drinking laws has wreaked
havoc on the cabaret scene.
Although I believe I am widely known as a designer of “weirdos,” I wonder what these same
readers would think if they had seen some of the stuff I tried but did not publish. Reason? It has
always been my policy to publish only those things that were easy to build, were not dangerous
and would work on the first try for anybody. This ruled out my regen engine that developed so
much ungovernable power it kept blowing up; my fuelless flying machine that would have to
dump energy continuously to avoid melt-down, and a new system of propulsion that pilot and
passengers probably could not live with. Fun stuff, but practically just curiosities.
Currently I have a couple of dozen hot model projects in the works, which, now being retired, I'll
finish doing whenever so will not interfere with goofing-off.
Bibilography:
MAN= Model Airplane News
AM= American Modeler
PM= Popular Mechanics
AST= Astounding Science Fiction
SM= Science & Mechanics
MODAV= AMA'S Model Aviation Magazine
CL= Car Life
MI= Mechanix Illustrated
PMA= PM Shop Man
Date
June 1945
Sept. 1945
Oct. 1945
Magazine
Hunt. Fish
Bas. Des.
AP
AT= Air Trails
FM=Flying Model
PS= Popular Science
AZ= Amazing Stories
BL= Boy's Life
AP= Airports Magazine
MN=Merchandising News
RTV= S&M's RAD.TV.EXP.
Title, Subject or Content
Several Hum. Verses anent H&F
Problems of Model Helis
Dear Mr. Small Operator
Clough
Page 4 of 4
Oct. 1945
Oct. 1945
Jan. 1946
Jan. 1946
March 1946
April 1946
Summer 1946 ?
Sept. 1946
Sept. 1946
Oct. 1946
March 1947
May 1947
May 1947
Sky Raiders
PS
PS
MAN
MAN
PS
PS
PS
MAN
AT
PS
MAN
PS
Aug. 1947
Sept. 1947
Feb. 1948
Jan. 1948
March 1948
April 1948
June 1948
Aug. 1948
Nov. 1948
May 1949
Aug. & Sept.
1949
April 1950
Mo.? 1950
Mo.? 1950
Sept. 1950
Oct. 1950
Nov. 1950
Mo.? 1950
April 1951
June 1951
Aug. 1951
Aug. 1951
Mo.? 1951
July 1952
Aug. 1952
Sept. & Oct.
1952
Dec. 1952
Feb. 1953
AT
MAN
MAN
MAN
MAN
PS
MAN
MAN
AT
MAN
MAN
Is German.Sec. Weapon a Rocket Plane?
Skyhook Coax Helicopter
Air Freighter of the Future?
SOS to Model Manufacturer
Unorthodox Design
“Naclio” Steamboat Model
Roadable Plane Model
Triad (Radial Wing FF Model)
Speed Made Easy
Cage Drive Rubber Co-Ax Helicopter
Steam Powered Model Race Car
More About Model Helicopters
Stressed Paper U-Control (Atom .05) Car and Plane in
Home Workshop Annual for 1950
Evolution of the Model Plane
Hoverbug Rubber Powered Helicopter
Improving CO2 Performance – Wat. Jack.
Autogiro Theory
Autogiro Free Flight Rubber Job
WHIZZER CO2 Racing Boat Model
Experiments with Expansion Engines
English Gyro to Clough Specs
Beginners’ Goat (Still Flying, March 1990)
Theory of Rotor Planes
Reworking Old Engines
MAN
AZ
AZ
MAN
MAN
MAN
AZ
PS
AST
FM
MAN
MAN
MN
AT
AT
WhirliCO2pter
Social Obligation – STF
How the Saucers Fly
Flying Saucer Design
Little Dragon Glow Engine
Part 2
Micrometer in Your Brain
Why Shacks Come Apart (Foundations)
Bait (Picked up for Anth. Space Police)
Try a Helicopter
Why Models Land in Trees (Prof. Tate)
Flying Barrel – Description and pictures only
Small Dealers on the Carpet
Build Flying Saucer (Famous slot job)
What’s the Score on Helicopters?
PS
FM
Fly a Plane in the Living Room
Sikorsky S51 TR Helicopter
Clough
July 1953
Aug. 1953
Sept/ 1953
Nov. 1953
Aug. 1954
June 1954
Mo. ? 1954
Not. Pub.
Pub. 54-55
April 1954
July 1954
Nov. 1954
Aug. 1954
Annual 1955
Mo. ? 1954
Mo.? 1954
July 1954
July 1954
Mo.? 1954
Sept. 1954
Oct. 1954
JORA 1954 S
Dec. 1954
Dec. 1954
Jan. 1955
Mo.? 1954-55
Mo.? 1954-55
Feb. 1955
March 1955
Aug. 1955
Aug. 1955
Sept.1955
Oct. 1955
Oct. 1955
Dec. 1955
Dec. 1955
Mo.? 1955
Dec. 1955
March 1956
June 1956
June 1956
Pub. 1956
March 1957
Dec. 1957
Page 5 of 5
AT
AT
PS
AT
PS
SM
?
PS to BERK
AT
AT
AT
AST
FM
AT
PS
PS
PS
AT
AT
AT
AT
BL
PS
PS
PS
PS
PS
PS
AT
AT
PM
AT
AT
AT
PS
AT (YM)
CL
SM
AT
MAN
AT (YM)
AT Ann
AM
AM
Tubine (Duct. Fan.) Jets for Models
Below Bug Pressure Jet Model
Jetex Driven Turbie – FF Model
Two-Part Helicopter Design Dope
Hydrojet Powers Tiny Boat
TR Helicopter .049
Reprinted Model Craft V3, 1954
Oil Can Reso-Jet
Doggerel Verse Fillers
Martian Spaceship
Typhoon Expansion Engine
It Didn’t Come From Mars
Stunt Goat CL Model
Sikorsky R-6 Helicopter TR
Co-axial Autogiro Kite
More Fillers per Request
Fourth of July Noisemakers
Channel Wing Control Line Plane
Tri-Yi Rubber Model
January Alouette’ .049 Duct Fan – Ukie
Wind Wagon Air Drive Race Car
Mille Diesel Endurance – Ukie
Ceiling Repair Fillers
Uses for umbrella ribs
Electric Shop Heater from Oil Can
Things to do with Coffee Cans
Turbine Jet Race Car Model
Model Submarine Bautilus
Venusian Scout
Saturanian Space Skimmer
Slat Wing Control Line Trainer
TanGiro Twin – CL Autogiro
Teenie Genie
Tri-Yil
Aerial Tramway
TEE JAY’Super Delta Ducted Fan
Phantom of the Turnpike
Rebec Medieval Three String Violin
Sheet Metal Susie
Lil’ Dragon Used as School Project
Cement Drives Crazy Water Gadgets
Special Case of TR Helicopters
Tumblewing CL Flettner Rotor Plane
Famous Firsts (Yet to be Made)
Clough
July 1960
Sept. 1960
Feb. 1961
June 1961
June 1961
Sept. 1961
Nov. 1961
Jan. 1962
Annual 1963
Sept. 1962
Sept. 1962
Mo.? 1961-62
Feb. 1962
March 1962
May 1962
July 1962
Aug. 1962
Sold Aug. 1962
Nov. 1962
Nov. 1962
1962
Sold Dec. 1962
Dec. 1962
Dec. 1962
Mo.? 1962
Mo.? 1962
Pub. 1962-63
Jan. – Feb. 1963
March 1963
March 1963
March 1963
April 1963
June 1963
June 1963
Aug. 1963
Fall 1963
Fall 1963
Nov. 1963
Nov. 1963
Mo.? 1963
Mo.? 1963
Page 6 of 6
PS
AM
SM
SM
PS
PS
SM
PM
PMSHOP
PS
PM
MAN
A.E. Esty Machine
and Tool
Employees Manual
MAN
PS
PM
AM
PM
SM
AM
Crestwood
PM (Pub?)
MI
AM
SM
PM
Sam Bierman
AM
MI
PM
PS
PM
AM
PM
PM
SMRTVEXP
SMRTVEXP
SM
PM
PM
PM
Rebuild of Cecil Peoli Twin Pusher
Typhoon’s Expansion Engine
Two String Splinter Bass
Triple Neck Steel Guitar
Tethered Ducted Fan Jet Plane
Boat that Flips to Go (Porp. Tail Dr)
Reader Report on Guitar Projects
Ground Effect Vehicle Air Car
Reprint GEV
Platter Plane-Non-Slot Saucer
Old Mill Waterwheel Table Centerpiece
Model Helicopter Dynamics
Nervous Nellie .05 Stunt Job
Self-Powered Rotor Helicopter
Remember the Spinning Wing Autogiro?
Spinning Disk Model Ven. Scout
Control Line Kite
Whirlybird Co-Axial Autogiro Kite
Snapper .49 FF Speed Job
Please Shake Carefully (Humor)
Tetra Four Radial Wings Ukie (X-Wing)
String Phones Deluxe
Peter O’Dactyl
Homemade Vernier Dials
Vapor Pressure Drivers Model Boat
How to Open Anything (Humor)
Yankee Flea Tandem
Darkroom Light Box
Mystery Moving Box and Kite, p. 164
Styrofoam Model Oos on Cox .010
Hoopskirt Annular Wing FF
Turkey Buzzard Flying Plank Type
Water Skating Model Boat
Wind Rotor Drives Boat Propeller
Speaker Box Does Everything
Electric Amp for Banjo
Moon Scout
Electric Centrifugal Cannon
Harlequin Stressed Paper Glider
Control Line Stunt Kite
Clough
Page 7 of 7
As a result of being bugged by Skip Ruff and Bill Northrop in 1990. First published was in
MAN, July 1992 a review of “Modelcad.” Then “Multiwiz” in August 1992 in MAN. More
articles are due in MAN, FM and MB.
Date
July 1992
Aug. 1992
June 1993
July 1993
Oct. 1993
Feb. 1994
May 1994
May 1994
July 1994
July 1994
Aug. 1994
Nov. 1994
May 1995
June 1995
July 1995
Aug. 1995
Oct. 1995
Nov. 1995
Dec. 1995
Dec. 1996
Dec. 1996
Dec. 1996
Dec. 1996
Magazine
MAN
MAN
MB
MAN
MB
MB
FM
MB
MB
FM
MB
MB
FM
MAN
MAN
MAN
MAN
MB
MAN
MAN
FM
MODAV
FM
Title, Subject or Content
CAD for Your Model Designs
Multiwiz Model 2-Channel From 1 Servo
Pushcart .049 RC Pusher
Rotorplane! Flettner Wing-Rotor
Saucer Mania Two Slot Saucers
Ment. RLC Award Winner
Morles 1915 .049 FMBD RC
Pix Reader Built Zoomslot
Ment. Adv. For Martian SS
Ment. FM Helic. Of August 1951
Mention
Traysvite
Ment. Page 7 and Class, Gas and Jazz
Ment. Page 28 and 57 Martian Spaceship
Ment. Hal DeBolt’s Col. On Early RC
Stringer Wing-Warp Control 02 RC
Designed 2D 6.0 Critique
R.K. Hicks FF Electric
Air Age RC Book Chapter “Mindset”
Ringer Annular Wing .049 RC
Faux Fighter FMBD ‘Pilchard’
Aileron Trainer
Materia Aeronautica Modela
The following list of articles are missing from my files and have incomplete information.
Date
Early 1960s
Early 1960s
Sold April 1967
Early 1960s
Magazine
AT or MAN
PS
PM
Lost by AT
Title, Subject or Content
Flying Barrel
Gyro Glider Kite
Foilplane, Flying Body Aircraft
Maple Seed Jetex Powered One-Bladed Helicopter
Other Projects
● Constructed the steam-powered race car model made for Minimax importer
● Cloud Copters Designed for Berkley Models
● Hummingbird Helicopter Design for Hamp. Research Foundation]
● Helicopter engine design with K & B
● Helicopter blade system for Sikorsky Aircraft Patent Inter.
Clough
Page 8 of 8
● Co-authored text on Ind. Psych. With Dr. Brian Kay, McGraw-Hill
● Wrote novel: “A Brief History of the Ashmont Town Team vs Equestrian Statue.”
(Amazon.com Barnesandnoble.com)
(signed) Roy Clough, Jr.
Updated July 15, 2002
- End -
BASIC DESIGN
PROBLEMS OF MODEL
HELICOPTERS
by R. L. CLOUGH JR.
A timely, interesting Article on a subject that has
been stumping the experts for years
EXPERIMENTERS in the model helicopter field soon
discover they are working with a tricky breed in which
instability is inherent and where such terms as
"balance," "keel surface" and "power transmission" take
on a deeper and at times maddening significance.
If one approaches the holy trio—dihedral, down thrust
and balance—with the proper degree of reverence in
designing a fixed-wing model, it is a fairly simple task to
plan a ship that will fly and fly well However, this is not
so with helicopters. In the model helicopter, and we are
speaking of those intended to represent the real thing
and not the familiar whirligig of the "freak" contest, one
soon discovers dihedral, or coning angle of the blades,
does not assure stability; that balance may be a
variable due to gyroscopic action when "stiff" rotor
blades are used; and that "down-thrust" has no true
counterpart.
Unlike most scale models, the fact that a full size
machine has flown does not necessarily mean a model
helicopter built to the same pattern will be successful.
This discrepancy is due in part to what is commonly
called "scale effect," and because in a larger machine
there is a pilot along to constantly correct flight
aberrations as they appear. In power transmission,
friction losses in a model run far higher in direct
proportion than in full-scale machines. Torque effect is
also more pronounced because more power is needed
proportionately to fly a model.
Therefore, in order to secure successful flights, the
modeler must design his little ship in such a manner as
to insure inherent stability—something which makers of
full-scale machines have not been too successful in to
date.
The greatest single problem in helicopter design is:
What to do with torque? Shall we concentrate on using
it, nullifying it, or plot such a design wherein it may be
successfully ignored?
Since this article is dealing primarily with rubber
powered helicopter models we shall concentrate on the
first two; either to nullify torque or use it. The latter
method of plotting a design wherein torque may be
ignored involves self-motivated rotors, propelled by jets
at their tips, and confronts the modeler with many
mechanical difficulties.
Perhaps we speak rather loosely of "using torque."
What is meant is that torque is being "used" in a model
when the method of eliminating it contributes to the
overall lift. When this equalization does not contribute to
the lift it is considered nullification,
Gyroscopic action is another bugbear. It will always be
present to a certain extent, but evidence at hand
indicates it is not an insurmountable problem.
14
MODEL AIRPLANE NEWS
• September, 1945
Flexible blades and articulated rotor hubs
do much to alleviate this effect Proof of the
power of this effect was made quite apparent to
the writer in an early model design. This model
was of the single-rotor and torque prop design
and featured a heavy, non-flexible main rotor. It
was discovered that when the model was hand
launched it would maintain the position in
which it took the air until the motor had wound
down enough to permit the weight of the
machine to overcome the gyro action and
return to an even keel This effect was so
pronounced that the model would fly on edge
for several seconds before leveling off when
launched in that position Subsequent
experiments with a flexible main rotor definitely
laid the blame for this condition at demon
gyro's door
When choosing the type of design to work
with, the experimenter should make UP his mind
to stick to that type until he is thoroughly
familiar with its intricacies
There are five basic types of helicopters and
many modifications of each. There are
certain disadvantages to each type and all
lack the simplicity of rigid wing aircraft
Probably the most familiar is the Sikorsky
type which corrects for torque by means of a
smaller rotor at right angles to the larger in
such ratio as to amply compensate for torque
at all speeds
Second is the contra rotating, in which two
main rotors revolve in opposite directions
around a common center The De Bothezat,
Hiller-copter and Bleriot machines are good
representative types
Third is the twin-rotored helicopter utilizing
two main rotors of opposite rotation extended
on booms from the side of the aircraft A
variation of this principle is to put the rotors at
opposite ends of the fuselage, thus doing away
with the booms. The Landgraf, Platt-LePage
and German Foeke-Achgelis are examples of
this trend of thought. This type is probably the
oldest.
Fourth is a fairly recent innovation control could be effected through the
and the writer has been unable to large single rotor which would
secure information as to .whether the undoubtedly
throw
dangerous
machine has actually been built or stresses onto the smaller rotors. This
was merely proposed Three rotors type is definitely not recommended
are employed, a large main rotor in for model experimentation.
the middle and two smaller ones on
The fifth type is the newest and has
booms, rotating in opposition from the received quite a bit of attention. In
main rotor to counterbalance its this type there is but one main rotor,
torque. From casual inspection it which is activated by jets located in
would seem the gearing necessary to the tips. Thus thrust is contained
accomplish this would result in within the rotor and, acting directly
something of a plumber's nightmare, upon it, automatically eliminates
with more power being absorbed by torque effect.
gear boxes than by the rotors.
It is interesting to note in relation to
Control, too, would offer quite a full sized ships that this idea appears
problem. Either all three rotors must basically sound for two very good
be controllable entailing a great deal of reasons.
Tip
speeds
approach
weight and machinery, or possibly velocities at which jet propulsion
works best and with the power
being applied to the tip of the rotor
instead of the hub the mechanical
advantage is much greater, permitting
concentration of thrust where drag is
heaviest and allowing rotor structures
to be lighter
As far as model helicopters are
concerned, however, this method
offers many difficulties The writer has
succeeded in making a compressed
air jet powered rotor lift its own weight
(and no more) under 90 Ibs pressure,
and a steam jet rotor lift its own
boiler—but not its heat source
Powder rockets will supply enough
thrust and are not overly heavy, but
their extremely short duration is
discouraging
Therefore the remainder of this
article will deal with the first three
types Power utilized for these
experiments is rubber, chiefly because
of the simplicity of hookup and the high
power-to-weight ratio Helicopter gas
models will probably be built, but for
the present it is undoubtedly best to
leave out the added complexities of
internal combustion until familiarity
with the stability problem is gained.
A gas engine would produce a fine
steady source of power, but would
also mean slip-clutches and gearing.
Unless one has access to a machine
shop these items are rather difficult to
produce
The design of the model helicopter
poses the question- "Straight up, or
straight ahead 9" If the model is,
designed to fly vertically and attain
the greatest possible elevation it
seldom can be adjusted for "cross
country" flights of any great duration
One exception to this rule is the
contra rotating type with a freewheeler which may be adjusted to
move forward as it climbs by adding
weight to the nose It will continue to
move forward during its free-wheeling
descent but "glide" ratio will be small
Since model helicopters are designed
primarily
to
fly
vertically,
the
experimenter would do well to
concentrate upon arriving at a design
capable of a steady climb and slow
descent with good stability throughout
the flight Then, and only then,
should he attempt "cross country"
flights.
The
following
sketches
are
presented primarily to stimulate the
imagination of model experimenters;
however, if the general proportions
are followed throughout these models
will fly well, though in no case is any
sketch intended to represent a
completely "perfect" solution.
Fig. 1 is perhaps the simplest
possible form. It is a direct takeoff on
the familiar contest whirligig and is the
easiest to build and fly. Directional
stability is only fair, but "glide" is quite
good if a free-wheeler is employed.
The long nose-wheel strut protects
the lower prop and brings the center of
gravity forward. Rubber hook-up is
simple
and
contra-rotation
is
automatic.
In this, as in all types of model
helicopters, too much emphasis
cannot be laid upon the importance of
making the rotor blades flexible. For
every foot of radius the blade should
have a "spring" of at least 1-1/2". This
enables them to bounce and helps to
destroy the aforementioned gyro
effect.
This model is described first to
point out the effect of keel surface
upon the flying qualities of the
helicopter. Note the very narrow rear
section and pointed nose. This is
because
when
displaced
air
produced by the upper rotor strikes
the fuselage it has a marked tendency
to rotate it in the direction of the rotors
movement. Therefore, the larger the
keel-surface, the greater the turning
moment. A friction brake on the lower
prop is, in theory, the best way to
counteract this effect. In practice,
however, it proves tricky to adjust. A
felt washer on the lower prop will
often turn the trick, but a fin hinged
on a fore and aft axis works better.
This same effect is why pylon gas
jobs turn to the right under full power,
when one might be inclined to think
they should swing to the left because
of the torque.
One tricky phase of keel-surface, or
lateral area, should be mentioned
before going any further. In fixed-wing
models one usually attempts to get
the center of lateral area as low as
possible. The reverse of this
ordinarily good rule is true in model
helicopters for this reason:
At the top of the flight the rotors
come to a stop, then reverse for a
freewheeling descent. The rudder
effect of the fuselage side area is very
pronounced at this moment, and if the
area below the center of gravity
exceeds that above it the model will flip
over on its back and descend inverted.
Therefore, in designing a model
helicopter one must work out a good
compromise with sufficient area above
the center of gravity to permit a rightside-up descent and sufficient area
below the c. g. to permit a stable
climb. About 60% above and 40%
below is about right, although on some
types it is advisable to have as much
as 75% of the lateral area above the c.
g.
Fig. 2 is the contra rotating type. Two
rotors of equal diameter revolve in
opposed directions. Hook-up is simple
but care must be exercised in building
the "cage," and all bearings must be
true. Balance of all moving parts is the
keynote to success with this model.
Directional stability is very good and
this sketch points up another phase of
helicopter design. In this model,
forward flight may be secured by
adding weight to the nose, and the
mass of lateral area must be well back
of the rotor axis to keep it headed right.
However, this brings in another factor:
top-of-fuselage area. Since with this
type there is more area on top of the
fuselage exposed to the downwash of
the rotors, behind the rotor axis, if the
model balances directly on the rotor
axis the down wash will force the tail
down and give the model all the
symptoms of tail-heaviness. This is
best counteracted by balancing the
model slightly ahead of the center of
lift.
The upper rotor should be
equipped with a freewheeling device
for easy descents, and it has been
found that best results are obtained if
the lower rotor has a slightly greater
pitch—about 2°. The climbing ability of
this type apparently exceeds that of all
others. This is probably due to the
direct utilization of available thrust
where it will do the most good with a
minimum of fuselage or deflection
interference.
Fig. 3 illustrates the dual rotor
helicopter. Principal problem here is
to equalize the thrust of the two
rotors. The simplest and most positive
way to accomplish this is by an
equalizer beam. Hook-up should be
clear from the sketch. In this model it is
highly important to keep the center of
lateral area as high as possible and
to make the two rotors as nearly
identical as possible. This helicopter is
the simplest to adjust for forward
flight as it climbs. Simply add a bit
of weight to the nose.
Fig. 4 of the Platt-LePage pattern is
basically the same idea as Fig. 3.
Power transmission of some sort is
needed for this type; therefore it
requires a lot of work in building and
excellent balance for good results.
Pulley and belt, of the kind described
in Fig. 5, seems to work better than the
connecting-rod type of transmission.
Bevel gearing might be the ideal
solution if a set of the same, light
enough for practicality, could be
obtained. A horizontal stabilizer
seems to be necessary on this model
and a rudder often helps. The best
way of winding is by a small crank in
the nose section as shown on
sketch.
Fig. 5 is based on the Sikorsky
design. This is the model shown in the
accompanying photograph. Power
transmission to the rear prop was a
great problem in designing the original.
After numerous experiments the
pulley and belt system was adopted
as the most simple and efficient.
Ordinarily one might think such an
arrangement would result in slippages
so great as to obviate the possibility of
any constant ratio between the main
rotor and torque propeller. This
problem, however, was solved very
nicely by facing the pulleys with a fine
grade of sandpaper. The belt is
common twine, tied snugly in place
and shrunk with water.
A four bladed rotor is used to
absorb as much thrust as possible
within a small area to keep the antitorque rotor boom as short as
possible. The main disadvantage of
this type is the short rubber length,
but due to the proportionately slow
revolutions of the main rotor, longer
flights than one might be inclined to
think possible may be had.
This model works best under power
dropping quite rapidly after achieving
maximum altitude. It is presented here
chiefly as an experiment in power
transmission. The model will fly well
only if weight is kept down. This
method of nullifying torque rather than
"using" it does not seem to be very
efficient, more rubber being required
proportionately to fly this type of
model than one featuring dual, or
contra rotating props.
One interesting fact about the antitorque propeller was discovered: It
does not need to produce a thrust
anywhere near equal to the torque
reaction produced by the main rotor
in order to hold the ship steady. This
is probably due to a keel-surface
effect produced by the spilling of air
from the tips of the main rotor
against the apparent disk of the antitorque propeller, which would of
course tend to push the boom in the
direction of the main rotor.
Adjusting this model so that torque
is evenly balanced is quite simple.
With a pulley ratio of 3-1 start with the
blade area of the little prop equal to
% the area of the main rotor. This
will cause a slight over-correction
and cause the boom to swing
around in the direction of the big
prop. Then trim the small rotor, a
little at a time until it balances. This is
considered the best way, even if it
amounts to cut and try, because it has
been found that a difference in
bearings and pulley alignment is
peculiar to each builder, with a natural
slight difference in results. If the
blades are over-trimmed, add a small
fin to the boom in the slipstream of
the larger prop and trim it to fit.
Once adjusted this type will stay
adjusted. Varying power used will
not upset the ratio between the two
rotors.
It is a good idea for the serious
experimenter to keep a record of his
experiments for future reference.
Patience is the keynote to success.
Do not give up any design type until
you are certain you have tried
everything that can be done with it.
Often a very simple "bug" will prevent
a model helicopter from performing
well. Once this is located the model
will often turn in a surprising
performance. Remember that in these
little jobs a somewhat different set of
conditions holds sway from those of
conventional models. Respect those
conditions and success will be yours.
VICTORY