100-inCHES Of R/C SCaLE fLyinG BOat

SPRUCE GOOSE
build
HUGHES H-4 HERCULES
BY: Mike MacFarland
SPRUCE
GOOSE
100-inches of R/C Scale Flying Boat
BY: Mike MacFarland
As the safety shunt, the scale-sized
Hughes pilot sits within the cockpit
mated to the female port. Flight only
takes place with him installed, true to
the original.
On November 2, 1947, Howard
Hughes proved his design genius
by piloting his Hercules H-4 — the
world’s largest flying boat — into the
air from the waters off Long Beach,
CA on its maiden and only flight.
With its stadium-sized wingspan,
curvaceous
lines,
and
eight
enormous propellers, the triumph of
the Iconic Hughes Hercules H-4 has
fascinated me since I began building
model airplanes in the 1980s. I
remember poring over a remotecontrol airplane model magazine
article at the time that featured a
glow-engine-powered version of
Howard Hughes’ Spruce Goose (the
media-given nickname of the H-4)
and wishing I had one of my own.
At the time, unfortunately, neither
my paper-route income nor my
beginning building skills could turn
the wish into reality. It would require
25 years from my first fascination
and the impetus of an online contest
to perform the miracle: Design
and create a flying scale model of
Hughes’ Spruce Goose.
Fast forward to 2007, when I
entertained the idea of competing
in a contest proposed in the
Scale Electric Airplane forum of
RCGroups.com. It was the second
of such contests called Build-Off
II, where adventurous modelers
committed to creating an original
scale design and then building and
flying it within a specified six-month
period. Those models which were
successfully built and flown were
entered into the contest voting. The
contest stipulated that the models
must be electric-powered scale
models of a multi-engine aircraft
with a maximum wingspan of 100
in. The Spruce Goose qualified, and
since I wanted a large-scale flying
boat for our club’s annual float fly,
the concept of a 100-in. wingspan
Hercules H-4 was settled.
Design Phase
Then it was time to take inventory
of the skills and materials needed
for the project. My model airplane
design experience had been limited
to a couple of single-engine sport
models, both much smaller and less
complex than the Spruce Goose.
Also, this model would be my first
attempt
at
Three-Dimensional
Computer Aided Design (3D
CAD). I had many years of 2D CAD
experience, but virtually none using
3D. If the model was constructed
completely within the computer, in
SPRUCE GOOSE
build
The model was completely
three dimensions by first
parts, then giving each part
and locating the part where
the 3D computer workspace.
designed in
creating 2D
its thickness
it belongs in
all three dimensions, prior to the
building portion of the contest, it
could be built and flown within the
exiguous contest building period.
The 3D CAD process seemed simple
enough: utilizing software created for
this purpose, the designer creates a
flat, 2D (X and Y axis) of each part
of the aircraft. Using the “Extrude”
command, the designer gives the
part its thickness on the Z axis (1/8
in. for a wing rib, for example), then
locates the part where it belongs in
3D workspace. Though it was simple
in concept, it became clear with
the first lines drawn that this design
adventure would require a great deal
of patience, education, and time.
I began with an online search for
the Hercules and found some original
engineering and and three-view
drawings. With help from friends at
RCGroupswhosuppliedphotographs
and documentation of the original
Hercules at its resting place in
McMinnville, Oregon, a very trueto-scale rendition of this American
icon soon took shape. The model
uses the same wing and empennage
The fuselage construction utilizes plywoodlaminated, balsa-core stringers at 90-degree
intervals to lock each former into place. A
plywood fixture “jig” supports the assembly at
pre-determined intervals at a precise distance
above the building plane.
airfoils as its full-scale counterpart,
with a thickness modification to the
wing airfoil for better performance
at lower Reynolds numbers. The
slotted flaps are built up, shaped,
and sheeted like the original with
five prominent built-up flap hinges
beneath the wing. All hinge lines
are scale in design and capture the
scale pivot points. In addition, all
control actuations are concealed
within the structures, so as to not
reveal the remote-controlled nature
of the model.
In retrospect, the 3D CAD pathway
proved to be the best method for
this unique design. After starting the
process with a popular, industrystandard program, it became obvious
that the software was limited in its
ability to create complex 3D curving
surfaces. Since the Spruce Goose is
constructed extensively from these
surfaces, the switch was made to a
wonderful design program, Rhino
3D. Many hours were invested in trial
and error commands and in reading
a companion book to understand the
program’s capabilities. Though the
process is simple enough, it takes
longer to draw in 3D than in 2D;
however, the 3D model is much more
versatile. With time and experience,
I discovered how to utilize the 3D
virtual model to create unique and
accurate design assemblies and
building fixtures, which greatly
enhanced the form and function of
the finished model.
The Build
The construction of the model
involved extensive use of laser-cut
parts located in tab and slot fashion.
A unique facet of this build was that
the model was not built over plans
in the traditional sense. Although
an alignment sheet was used for
portions of the construction, for the
most part, laser-cut fixtures were
used to locate parts accurately in
3D space above the building board.
The fuselage, for example, used
plywood-laminated,
balsa-core
stringers at 90-degree intervals
to lock each former into place.
A plywood fixture jig supported
the assembly at pre-determined
The rudder servo directly connects to
the rudder through a plywood “key” that
engages into the bottom of the rudder and
permits easy servicing through the use of a
removable music wire hinge pin.
intervals at a precise distance above
the building plane. Another use of a
large fixture was the wing support
jig, which supported every wing rib
and wing spar in place. This scaffoldlike jig allowed the wing to be built
while attached to the fuselage with
the carbon fiber plug-in wing tubes
in place, permitting very accurate
alignment. Since the wing is far
from being a flat-bottom airfoil, and
has a geometric twist built into it,
attempting to build it accurately
using traditional methods would
have been problematic. The fixtures
proved to be a huge assistance
in maximizing the accuracy of all
parts fit and alignment. In addition,
I credit a good part of the model’s
well-behaved flying success to the
accuracy of the fixture assembly
SPRUCE GOOSE
build
The center fuselage hatch allows for connecting
a two-piece, plug-in-style wing and all of the
electrical connections for each wing’s motors,
lights and servos, in addition to access to the tail
cone securing “cam” system.
method.
Howard Hughes was handed a
challenge when his government
contract stipulated construction
of the largest aircraft ever built
without allowing use of traditional
aircraft aluminum metals. Hughes
pioneered the use of extensive
lamination of woods in the structure
to create his behemoth from mostly
Birch plywood laminations. It felt like
a small-scale tribute to the original to
construct this model completely out
of wood and to utilize laminations of
woods like in the original. As such,
the model is constructed mainly
from balsa and Finnish plywood
laminations with a little spruce and
some light plywood. The airframe
is fully balsa sheeted except for the
ailerons, which are open framework
covered in Silkspan and nitrate
dope. The wings were covered in
0.5 oz/sq yd fiberglass cloth, while
the fuselage was a combination of 2
oz/sq yd on the hull and lighter cloth
above the waterline. The empennage
sheeting was covered with Silkspan
and nitrate dope. All cloth was
adhered with coats of nitrate dope,
then the cloth weave was filled with
polyester-based primer and finishpainted with Rustoleum® aluminum
paint from a home improvement
store.
One of the questions early in the
design was how important it would
be for modelers to be able to break
down a large model for transport
and storage. Those experienced in
large-aircraft ownership felt that a
two-piece wing was important, as
was the ability to separate portions,
or all, of the empennage from the
fuselage for transportation. This led
to the use of a two-piece, plug-instyle wing with a center fuselage
hatch for connecting the wing and all
of the electrical connections for each
wing’s motors, lights, and servos.
In addition, the full-scale version
had been made to break down, as
was seen when it traveled through
town in sections on huge transport
vehicles. Hughes had incorporated
an angled parting line for the tail
section to separate cleanly at the
rear of the fuselage directly beneath
The slotted flaps are built up, shaped,
and sheeted like the original with five
prominent built-up flap hinges beneath
the wing.
The wing support jig supported every wing rib and wing spar, allowing the wing to be
built while attached to the fuselage. The 5/8-in. I.D. carbon fiber plug-in wing tubes permit
very accurate alignment. Since the wing is far from being a flat-bottomed airfoil and has
a geometric twist built into it, attempting to build it accurately using traditional methods
would have been problematic.
the leading edge of the stabilizer. A
unique part of the model design is
the use of this same, angled parting
line, with an actuating carbon-fiber
shaft and interlocking plywood cam
system that secures the empennage
in place, all from the forward fuselage
hatch area. This allows the tail section
to be removed and replaced without
the use of tools and, since the surface
controlling servos are located within
the tail cone, only standard electrical
connections need to be made each
time. This complex joint has proven
to be watertight, stable, and strong
in flight and water operations and
also permits the aircraft to break
down for transport inside smaller
vehicles.
Another self-imposed challenge
undertaken in an effort to remain
old-school in design and cost was
to power this large aircraft with a
simple, inexpensive, and lightweight
power system. Speed 400 “long
can” brushed electric motors
were selected for the job, rather
than more modern and powerful
brushless motors. Due to the quantity
of motors on the airplane, having to
purchase eight of anything would be
expensive, so it was figured that the
cheaper the motors used, the more
attractive the design would be to
prospective builders. In addition,
using brushed motors and just
two Jeti 80-amp speed controllers
connected together in series-parallel
wiring from one common 5S, a 5000mAh Lithium Polymer (LiPo) battery
pack simplified the total component
count and further reduced the cost
from its theoretical brushless-motor
counterpart. A computer mixing of
the two speed controls slaved to the
rudder servo enabled “turning on a
dime” water-taxi operations, even in
strong winds.
In selecting the motors, it was
decided to remain true to the scalepropeller size and number of blades.
The full-scale version used 4-blade,
17-ft. x 2-in. diameter propellers,
The parts count increases quickly with
eight motor nacelles under construction.
The frameworks are built, covered with
rolled balsa and 1/64-in. plywood, then
fiber-glassed. The motor attaches to the
front framework, then the assembly slides
into the hollow tube.
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The flap hinges lines are scale in design and capture the scale pivot points. The JR
DS3421 digital mini servo provides nice flap control actuation and are fully concealed
within the structure so as to not reveal the radio-controlled nature of the model.
The ailerons are scale hinged and
controlled by a direct linkage connection
to the JR DS3421 digital mini servos.
The brass tube soldered to the modified
EZ connector enables not only surface
rotation but easy removal and installation
of the aileron from the exterior by simply
sliding it into place.
which would closely scale to 5.5in. diameter props in this 1:38 scale
model. The small diameter also meant
a gearbox would not be needed,
and a direct-drive motor system was
settled on. The long-can, 400-speed
motors selected were also known as
“480” motors and were marketed
for use in direct-drive applications
with a two-blade, 6x4-in. propeller.
The final propeller selection was a
ground-adjustable pitch, 5.6- x 3.5in., 4-blade propeller system from
Varioprop. I purchased 6.2-in. scale
blades and trimmed them to 5.6 in.
(as determined through MotoCalc™
analysis to be the most efficient use
of this setup). Static testing proved
the analysis correct with the eight
motors providing approximately
7 lb of thrust while consuming
approximately 1100 watts of power.
With the finished weight of the model
at 15 lb, this power has proven to fit
the Spruce Goose very well and fly it
with authority.
When the model is on the water,
steering is accomplished with
the use of differential thrust to the
motors controlled by the computer
transmitter mixing. The three
outboard motors and one opposite
side inboard motor are wired
together in series-parallel wiring into
each of the two speed controllers. In
the unlikely event of one of the speed
controllers failing in flight, one wing
would have three remaining motors
running, with just one remaining on
the opposite side. It seemed like the
best idea to split the wiring this way
to give the best steering authority
and a small margin of safety. On the
water, as the throttle and rudder
stick is applied, fully proportional
steering is accomplished by a 0 to
100% reduction of power to the side
of the wing in the direction of the
turn, as indicated by the rudder stick
deflection. This mix is controlled by
a transmitter switch and is only used
for water taxiing, never in flight.
There is approximately 15% mix
applied this way in the flight mode,
however, and it helps to correct any
changes in the takeoff run.
Wing tip floats connect to the wing with a spring retainer and two 1/8-in. carbon fiber
tubes. These tubes are used in the event of excessive side force, when the tubes will
break and permit easy servicing without causing more extensive wing damage.
The wing tip floats are constructed from laser-cut, lightweight 1/16-in. balsa formers
and planking. Each completed float, as shown, weighs less than 1/2 ounce.
SPRUCE GOOSE
build
The entire tail assembly is of fully sheeted
1/16-in. balsa construction with 1/64-in. plywood
edges creating the sharp transitions at the
mating lines of the control surfaces. In this photo
the elevators are shown fiber-glassed and primed
after sheeting.
A unique part of the model design is the angled parting line, with an actuating
carbon fiber shaft and interlocking plywood cam system that secures the
empennage in place, all from the forward fuselage hatch area. This allows the
tail section to be removed and replaced without the use of tools and, since
the surface controlling servos are located within the tail cone, only standard
electrical connections need to be made each time.
The front doors on either side of the fuselage
were made and hinged to open and close. A
plastic domed lens creates the porthole glass in
this little side door.
The motor wires exit the wing and are plugged into the motor “sub-panel” on
each fuselage side, permitting wing removability. The sub panel connects all of
them together in series-parallel wiring into a Jeti 80-amp speed controller on
each side into one common 5S, 5000-mAh Lithium polymer battery pack.
A good exercise in boat building, with many planks of 3/16-in. balsa making
up the hull of this curvaceous fuselage.
The final propeller selection was a
ground-adjustable pitch, 5.6- x 3.5in., 4-bladed propeller system from
Varioprop. Shown are the 6.2-in. scale
blades before they were trimmed
down to size.
A Deans Ultra Male connector
shunt circuit was surgically installed
within the scale figure of Howard
Hughes, who, fittingly, controls the
power to the motors in the safety
arming circuit.
SPRUCE GOOSE
build
Spruce Goose in Flight
Flying this airplane has been
surreal and surpassed my wildest
expectations. With a smooth
application of throttle and nose into
the wind, the model determinedly
obtains step with only an occasional
bit of spray passing through the
inboard propellers at the start of the
run. Once on step, the model builds
speed smoothly. The rhythmic
sound of the eight roaring propellers
changes to a symphonic drone as the
distance increases from the operator.
Then, in an amazing metamorphosis,
the weight of the speeding boat shifts
from the hull to the wings of a beautiful
flying ship in an effortlessly smooth
exit from the water. One comment
I’ve heard more than any other from
those who have observed its
flight is “she loves to fly.” The
A computer mixing of the two speed
controls slaved to the rudder servo
enables “turning on a dime” water taxi
operations, even in strong winds.
Spruce Goose is a graceful, gentleflying lady with wonderful curves
in all different flight attitudes and
almost glider-like behavior. She
seems most fond of being flown and
performing takeoff and landings with
15 degrees of flap deflection. Once
the airplane is airborne, most of the
flight is performed at an average
of half-throttle, and very scale-like
lumbering flight is the norm. An
onboard camera in the passenger
seat of the cockpit has produced
some amazing videos from the slowflying, steady platform of this large
airplane.
Even with the Spruce Goose’s finished weight of 15 lb, the direct drive brushed motor
power fits the airplane very well and flies it with authority. Flying this airplane has been
a surreal experience, surpassing the builder’s wildest expectations.
The Spruce Goose is most fond of
performing takeoffs and landings with 15
degrees of flap deflection. Once the plane
is airborne, most of the flight is performed
at an average of 1/2 throttle, and very
scale-like lumbering flight is the norm.
I’ve found that, after a typical six to eight
minutes of flying, a low and flat approach
is best, since the throttle is slowly reduced
and transferred proportionally to the back
pressure on the elevator stick. Once the
Goose is skimming only inches above the
water in full ground effect at minimum
flight speed, the power is reduced to just
above idle and full flare will settle her
in for a smooth transition to the water
surface.
With 30 oz/sq ft wing loading, the
Spruce Goose handles moderate
winds with authority and seems to
prefer a slight chop to the water at
takeoff. On one demonstration flight,
some light winds helped her leap into
the air so quickly that I had to remind
myself that the Goose was flying!
When bringing her in after a typical
six to eight minutes of flying, a low,
flat approach is best as the throttle
is slowly reduced and transferred
proportionally to the back pressure
on the elevator stick. Once she is
skimming inches above the water in
full ground effect at minimum flight
speed, the power is reduced to just
above idle and full flare will settle her
in for a smooth transition to the water
surface. After a few flights, it became
apparent that any skipping upon
touchdown simply means that more
speed needs to be bled off in “water
effect” prior to the landing flare. Be
careful to resist the urge to chop the
power fully at low airspeeds, since
the drag from the rotating propellers
could cause a large loss of flying
speed.
Finally, the design, construction,
and
flights
of
this
model
representation of an icon of American
ingenuity has been an amazing
journey and experience. By fall
2008, the production run of the lasercut components for the 100-in.-span
Spruce Goose will be available. If
you have a great deal of building
experience, time, and patience for a
long-term project, the Spruce Goose
may help you relive childhood
dreams and become your favorite
flying boat, as it has for me.
SPRUCE GOOSE
build
SPRUCE GOOSE SPECIFICATIONS
Product type
REFERENCES
Laser-Cut Advanced Builders Kit
TFC Aeroplanes
Aircraft type
Scale Multi-Engine Flying Boat
Builder Skill
Advanced
Pilot Skill
Intermediate
9461 Deschutes Road Suite 10
Palo Cedro, CA 96073
Phone: 503-547-1703
Web site: tfcaeroplanes.com
Wing Span
100 in.
Length
68 in.
Wing Area
1120 sq in.
Airfoil Root
NACA 63-412 (Modified)
Airfoil Tip
NACA 65-415 (Modified)
Airfoil Horizontal tail
NACA 0012-64
Airfoil Vertical tail
NACA 0012-64
Weight
14.9 lb (ready to fly)
Wing loading
30.6 oz / sq. ft.
Controls
Aileron, elevator, rudder, throttle, flaps. Optional:
landing lights, navigation lights, onboard video
Construction
Built-up balsa and thin plywood laminations,
balsa-sheeted/fiberglass-covered airframe,
fabric-covered control surfaces, plastic engine
scoops and cowlings (parkflyerplastics.com).
Transmitter
Futaba 14 MZ 2.4-GHz FASST
Receiver
R6014
Receiver battery
Duralite #7212, 7.4-V 2150-mAh Li.-ion
Radio Channels
6 required, 12 used
Servos (6)
(5) DS3421 JR Digital Mini MG Servo, (1) Spektrum DS-821
Motors (8)
Brushed Long Can 400 (Hobby Lobby FK5202)
wired series/parallel (4/4)
Propellers (8)
VP-06A, 4 blade Varioprop with 6.2-in. scale
blades (trimmed to 5.6 in.)
Speed Controller (2)
Jeti 80A (35A minimum needed)
Motor Battery (1)
Thunder Power 5S 5000 mAh
RPM
12,375
Static Thrust
110 oz
Thrust/Weight
0.46:1
Flight times
6 to 8 minutes
Flight speeds
22 mph stall, 31 mph level flight (flaps up)
Manufacturer
TFCaeroplanes.com (cottage industry)
Website
www.TFCaeroplanes.com
Availibility
Fall 2008
Price
To be announced
Contact
[email protected]