Ridding our aircraft..

Ridding Our
Aircraft of
P r o t e c t y o u r a i r p l a n e ’s p o w e r
TO D D S. PA RK ER
any sea creatures have their sleek shapes disrupted by the
presence of other creatures seeking free rides. Barnacles and remoras are among these freeloaders.
While not true parasites, biologically speaking, barnacles and remoras suck energy from the host
vessel. How many barnacles or remoras do you have stuck to your vessel—your aircraft?
Drag is the non-useful energy expended while we fly. It
can be thought of literally as the amount of energy we expend dragging air along with us on our journey. You have
probably stood next to a busy road and felt the disturbed
air following behind cars and semitrailers as they passed.
What you felt was the air that had picked up momentum
as the cars and trucks passed through it. You can sense
how much drag those vehicles have by how forcefully the
air blows when they pass. Engineers discovered early on
that they could measure drag from wings and other bod46
FEBRUARY 2008
ies by placing pressure probes behind the object in a wind
tunnel or in flight and measuring how much momentum
was lost compared to the air with no obstructions. This
method is called a wake survey and is an accurate means
of determining the drag of any object.
There are two types of drag affecting the flight characteristics of an airplane: induced drag and parasitic drag.
Induced drag is a byproduct of lift, and it’s at maximum
at the point the aircraft stalls. As speed increases, induced
drag decreases with the inverse square of velocity. In con-
EAA Sport Aviation
47
induced drag. It will likely add cooling drag as well.
Engine cooling is often the largest
single drag component. Reducing this
component of drag was addressed in
the March 2006 issue of EAA Sport
Aviation (“Control the Flow”) and
will not be addressed here. The purpose of this article is to help identify
those little parasites sucking power,
and with it speed, from our aircraft.
It may surprise you how much power
is being sapped away by these seemingly small parasites.
THE RIVET
A metal aircraft is covered with thousands of tiny parasites. The rivet is
probably the most common element
of all aircraft. Individually they are
insignificant, but collectively they
can represent huge power losses. It
should come as no surprise to anyone that a flush-type rivet has the
lowest drag, but it can have a range
of drag coefficients depending on
how it is installed. If the rivet is filled
and matched to the surface, it will offer no more resistance than the skin
would without the rivet. This is the
best condition.
The Germans found that a domehead rivet created 20 times as much
drag as a flush rivet in laminar regions of flow. Recessing the flush
TOP: Wheel pants may cut drag in half, even though they are larger.
rivet about 0.001 to 0.002 inches reBOTTOM: One of the subtle parasites on your aircraft include antennas
BOT
duced the drag by half. If you have
protruding into the smooth airflow across your aircraft. A 1/4-inch diameter,
pr
a laminar airfoil, a non-flush rivet
2-foot long antenna will dissipate approximately 1 hp at 125 knots.
2
is not for you. Once the boundary
layer is about 30 to 40 times thicker
than
the
height
of
the rivet, the differences in drag are
trast, parasitic drag is directly proportional to the square
tra
minimal,
but
not
zero.
of
the
velocity,
increasing
dramatically
with
speed.
Paro
asitic drag is caused by many factors: form (pressure),
cooling, intersection, prop wash, excrescence, and wet- JOINTS, BUGS, AND THINGS THAT STICK OUT
ted area. The lowest drag condition occurs where the in- Another subtle parasite is the type of skin joints used
duced and parasitic drag are equal. This point is usually and the quality with which they are constructed. A lap
much closer to stall speed than cruise speed but moves joint with the top surface pointing into the flow will
farther from stall when the parasitic drag is reduced.
have twice the drag of the lap facing the other direcThe more drag produced, the more power must be tion and 40 times as much drag as a flush- or butt-type
expended to counter it. The power needed for this ef- joint. On a forward lap joint, just rounding the protrudfort directly subtracts from power available for useful ing edge can reduce the drag by a factor of 10. A slightly
flight. (For sailplane folks this means a higher descent lifted forward facing thin edge can have 60 times greater
rate.) Therefore, we either fly slower or add more power drag than the flush joint.
to counter the effects. Adding power will offset the efThe condition of the leading edge of an airfoil can
fects, but it’s not the most efficient means because it make a huge difference in the drag and performance of
will usually require a larger engine and more fuel for the it. When airfoils are rough due to bugs, ice, paint chips,
same range. Both of these add weight and increase the dents, or any other disruption of the leading edge, the
48
FEBRUARY 2008
minimum drag coefficient of the airfoil is typically doubled. On some
highly laminar airfoils, the lift may
also be severely reduced, perhaps as
much as half, because of the rough
edge forcing turbulent transition.
Your propeller is also an airfoil, and
all of those little chips and dents are
robbing you of power right at the
source.
Wire, tubing, and pipes are convenient shapes for many things, but
aerodynamics is not one of them.
A cylindrical shape, compared to a
streamlined shape, creates anywhere
from four to eight times as much drag.
Cylindrical shapes should be avoided
or faired whenever possible. Exhaust
stacks are especially important to fair
or turn into the flow. Aerodynamically speaking, exhaust exiting from
the pipes looks like long poles sticking out into the flow. NACA researchers showed they could place pipes to
represent flows from exhausts when
doing wind tunnel studies. Turning
those poles as close to parallel to the
flow as possible sounds like a good
idea, and it is.
Wire antennas protruding into the
flow also create huge drag considering their small shape. A 1/4-inch diameter, 2-foot long antenna protruding into the flow will dissipate approximately 1 hp at 125 knots. Blade
type or streamlined antennas can
The rivet is one of the most common elements on an aircraft and can collectively
vely
significantly reduce the drag, or betcreate a large amount of drag. The flush-type rivet has the lowest drag if installed
led
ter yet, bury the antenna in the strucproperly, while the dome-head rivet can create 20 times as much drag.
ture. On composite/fiberglass aircraft,
embedding the antennas is common.
Carbon composite and metal aircraft must have non-con- the retract mechanism is heavy (more induced drag).
Control cables, pushrods, torque tubes, control horns,
ducting covers or fairings to achieve the same gains.
Fixed landing gear can be another major source of drag. and bell cranks often look like barnacles stuck to the sides
Frequently, landing gear have round struts. As discussed of our aircraft with the same drag-producing effects. When
earlier, these should be faired whenever possible. More im- possible these items should be contained within airframe
portantly, wheel fairings should be added. Unfaired wheels structures, but the mechanical gearing and control tolerare messy aerodynamically, with tubes and sharp corners, ances may not permit this luxury. These protruding items
small air passages, and interferences everywhere. Unfaired should be faired with gaps kept to a minimum. Some of
wheels are similar to an engine without a cowling, a lot the new Reno racers like the AR-6 or the new Air Force
of messy, useless turbulence. Wheel fairings typically cut fighters, F-22 and JSF, exhibit superb examples of what can
the drag from landing gear in half even though their di- be done to control horn fairings. Many modern aircraft
mensions are larger. Retractable gear may have more net have slotted or Fowler flaps with the axis of rotation bedrag than fixed gear unless the holes the gear fits in are low the wing. The mechanism typically involves a series
closed. Some aircraft whose gear doors are attached only of hinges. Careful fairing of these can pay large benefits in
to the struts do not cover the wheels completely. Such ar- drag reduction. Commercial jets are also good examples of
rangements may not reduce the drag beyond what can be how to do this.
For non-pressurized aircraft, inlets for cabin air should
achieved with a good fairing on a fixed gear, especially if
EAA Sport Aviation
49
Control surface gaps contribute
Cont
to excrescence drag (yes, that’s
explained in the article) and should
ex
be kept as small as possible. Gap
seals—as simple as strips of tape—help
se
tto reduce the effect.
be NACA flush scoops whenever possible. The drag of
these scoops is very low regardless of whether the vent is
open or closed.
WHAT IS EXCRESCENCE DRAG?
This brings us to one of the least obvious parasites of all. It
is broadly known as “excrescence drag,” or drag from air
leaking into or from places you don’t have a need for it to
go. Engine compartments are usually the biggest source
of this type of drag. A loose fitting cowling, or one without a seal strip, can leak copious amounts of relatively
high-pressure air, disrupting the flow around the cowling
50
FEBRUARY 2008
and reducing the air intended for engine
cooling. Sometimes a cowling is not stiff
enough, deforming to create leaks when
in flight. Loose fitting or leaky baffles
inside the engine compartment can rob
you of efficiency in the same way. Seal it
up as tightly as you can. Only let air in and out of places
you want air to flow.
Cabins can also contribute to excrescence drag. Improperly located or designed vents may bring air into or
let air out of places where little benefit is obtained. I remember hot days leaning over in the little Cessna 150
trying to catch the breeze coming from the little pullout
vents bleeding air from the leading edge of the wings.
Ahhh…. Non-pressurized cabins can either be pressurized close to pitot pressure or they can be less than static
pressure, it all depends on where the inlet and outlet air
comes from. This is the reason it is usually not a good
idea to have your static pressure source taken from inside
the cabin.
Again, drawing from experiences with cars, I am sure
most of us have felt the pressure change inside a car when
a window is opened. It may either increase or decrease
the pressure in the car depending upon which window
is opened and where the air source and exits are located.
Sealing the cabin windows, hatches, and doors so that all
of the entrances and exits are controlled is important in
reducing this source of drag. In non-pressurized aircraft,
this problem is made more difficult by the fact that the
controls and other systems are sharing the same volume
with the cabin. This means that anywhere a wire, cable,
pushrod, or other system exits or enters the airframe, there
is the potential for excrescence drag to occur. Where seals
are not practical or desirable, a close fit is encouraged.
Another source of excrescence drag is control surface
gaps. This was a major research area in the ’30s and ’40s,
with many simple and complex solutions developed. This
type of drag is simply the air that leaks from the highpressure side of the wing to the low-pressure side of the
wing. The effects are to increase drag, reduce lift, and reduce control effectiveness. There is nothing good about
control gap leakage. Gaps should be kept as small as possible. Some choose to close the gaps with simple strips of
plastic taped to the wings or fins with vinyl tape. Appar-
ently, the drag from the gaps is larger than the drag from
the tape and plastic. This system is simple and effective.
Some car and airplane designers make the top and side
surfaces with nice sleek lines and flowing streamlined
shapes, and then stick all the messy drag-producing stuff
underneath where it may go unnoticed by the consumer.
Often an assortment of struts, wires, hinges, drains, vents,
antennas, and other protrusions can be found here. Air
is not impressed by aesthetics and will find all of those
drag-producing add-ons no matter where you stash them.
Depending upon how they are arranged, collecting all of
these items into one location or line may either increase
or decrease the net drag effect. A thick, turbulent boundary layer will likely be found downstream from these
“porcupine farms.” Streamlining and reducing the individual effects as much as possible is the best chance of
sorting this mess out.
Next time you walk up to your airplane, think about
the parasites stuck to its surface looking for a free ride.
How many are necessary and how many are just stealing
power from you? I hope that some of the identifiable parasites can be removed or at least changed from barnacles
to remoras.
Todd S. Parker is president of EAA Chapter 23 in Ogden,
Utah, and works as an engineer for the U.S. Air Force.
EAA Sport Aviation
51