If It Flies - 2008/08

Symmetry is a life pursuit; does it matter with an airplane?
August 2008 91
If It Flies ... Dean Pappas | [email protected]
IF IT FLIES, it probably has a propeller in
front of it. Propellers make horsepower into
thrust, and they also just stir things up. It’s
the price we pay ...
I’m going to write about symmetry. Forget
airplanes for a second; let’s discuss it in the
general sense. We humans are bilaterally
symmetric; that is, barring industrial
accidents, we look the same on the left as on
the right. This generally works out well,
because if one leg were shorter than the
other, we would run around in circles.
Still, you can be sure that the wildebeest
in the herd with shorter legs on the left side
will be the first one the lions eat. On top of
that, I don’t know of any birds that have
unequal-length wings, so symmetry must be
good for flight.
Yet, our airplanes are far from
symmetric, even if everything lines up when
you fold the building plans down the middle
and hold them up to the light. For one thing,
the propeller spins one way, and that creates
a spiral-airflow tornado along the entire
length of the airplane. Even a fairly simple
analysis of the interaction of that horizontal
tornado with the rest of the airframe gets
involved quickly. (That won’t stop us!)
As important as that airflow is, there is a
more obvious asymmetry on almost all
propeller-driven models: they have engines
and exhaust systems that hang mostly on one
Also included in this column:
• What engine placement
does to your airplane
• That troublesome horizontal
tornado
• A revolutionary power option
for CL electric fliers
A practical, relatively hassle-free all-in-aline
exhaust system results when a rearexhaust
system is tucked up against the
underbelly. There was only a small
interference issue between the landing
gear and header pipe.
The Tiger 60 ARF, which was reviewed in
MA’s Sport Aviator, balanced perfectly
from side to side because the left wing was
almost an ounce heavier out of the box!
side. Depending on the setup, a fair amount
of weight and drag hangs on one side of the
airplane—typically the right.
This simple, normally unquestioned fact
caused my new model-airplane pen pal Mert
Thayer to write an honest to goodness paperand-
envelope letter asking (I want to get this
right, so let me get the letter), “Aside from
the obvious streamlining and symmetry, are
there any advantages to a rear exhaust/
muffler engine?”
I had a physics professor in college who
preached a set of rules for problem solving
that serve me well to this day. One is that
nothing is limited to the obvious. Besides,
what qualifies as obvious? I guess that
depends on how closely you look. So I sat
down to compose an answer for Mert.
Almost all rear-exhaust engines are
intended for tuned-pipe use, so this could be
a question about engine types and the
relative merits of tuned vs. untuned exhaust
systems. That is a good subject, and we’ll
discuss it some in the next column. We are
certainly going to run out of space this
month. Yet another day, we’ll have to have a
long discussion about tuned-pipe setup.
It turns out that the real question was
about the benefits of getting the weight and
drag of the cylinder and muffler onto the
airplane centerline rather than hanging off to
one side. We will definitely cover this.
The third question rides Oedipus-like in
the Trojan Horse belly of the second: Can
we make the airplane completely
symmetric? Does it really matter, and why?
This is an interesting question, so even
though that’s not what Mert meant, we’ll
go over it too! This part of the discussion
will spill over into the next installment too.
Asymmetric Muffler-and-Cylinder Mass:
The typical 60-size sport airplane with an
upright engine has a muffler that weighs
maybe 6 ounces, and it hangs
approximately 3 inches from the model’s
centerline. If the engine is side-mounted,
the muffler moves in to maybe 2 inches
from the centerline, and you can add
another 3 or 4 ounces of aluminum
cylinder head, brass, or steel cylinder liner
and aluminum piston, all centered maybe 3
inches from the airplane centerline.
All told, that could add up to 18-24
ounce-inch of torque trying to roll the
airplane off to the right while in level
flight. An 8-ounce weight at a distance of
3 inches makes 24 ounce-inch of torque.
If the airplane is flown and trimmed
with this imbalance, left-aileron trim will
be necessary for straight and level flight.
The aileron trim will cause three bad
things to happen.
First, the model will wander off to the
right anytime it is flown more slowly than
the cruise speed where the ailerons were
set. A prime example is during landing.
That’s because the aileron effectiveness is
reduced by slower airspeed while landing,
while the weight is the same, no matter
how fast or slow the aircraft flies.
The second thing that happens is that the
airplane will not stay wings-level during
inverted flight, even if you correctly set the
aileron trim for level flight, because the
heavy side of the model is on the same side
as the direction of the aileron trim you put
in.
Think about it for a second. You don’t
like it either, do you? It turns out that a small
lump of weight will mostly fix the problem.
These two unwanted effects bother
everybody—especially beginning pilots.
After all, who needs an airplane that veers
off to one side just when you slow it for
landing?
The almost-perfect cure is easy to carry
out. All you need to do is balance the
airplane from side to side. This will fix the
problem for the typical sport and/or training
mission: one in which the model is generally
flown through a wide range of airspeeds but
without being maneuvered into many
unusual attitudes. We balance the airplane
from side to side by putting a small amount
of weight on the opposite wingtip.
Balance the airplane by hanging it from a
pair of strong threads or thin wires. Hang the
front of the model from a string or wire
looped under the crankshaft. This will
probably require removing the spinner and
propeller. Support the back of the airplane
from a point that is approximately halfway
up the rudder. If there is an imbalance, the
airplane will hang with one wing low. Sticky-backed lead weights or finishing
nails are often used to add weight to the light
wingtip. This is terribly difficult to do with
any wind, so do it in the workshop before
your next flying session.
Now that you have balanced the model,
the aileron trim should be able to return to
center. This simple balancing act works well
but is less than perfect.
The third undesirable effect is caused by
the remaining side-to-side imbalance in
rotational inertia. Anytime you change the
“G” loading, such as at the beginning of a
loop, the wingtip with the small weight will
drop slightly. That’s because the small
weight at a large distance has more rotational
inertia than the 8 ounces of engine and
muffler, 3 inches away from the centerline.
The loop forces the airplane to rotate
around the center of the loop, and the side
with more rotational inertia will hang out of
the loop. Once the higher G loading during
the loop has been established, the balance
will return. But a small roll will result every
time the G loading changes. This inertial
imbalance will annoy anybody who is trying
to fly loops precisely but is not a big deal for
most of us.
If you really want to gather a better
understanding of this third effect, look up the
terms “moment of inertia” and “center of
percussion” in Wikipedia. You’ll also learn
about what gives a baseball bat its sweet
spot.
Spiral Airflow—That Troublesome
Horizontal Tornado: The spiral airflow that
swirls along the model’s length destroys any
hope of having a truly symmetric airplane.
The jet pilots (pure jet and ducted fan) know
this, and they enjoy the benefits of straight,
symmetric airflow.
As soon as it starts turning, the spiral
airflow, of a normal “right-hand” propeller,
strikes the right side of whatever “chin” the
airplane has and the left side of the fin and
rudder. The center of the fin and rudder area
is normally centered above the aircraft’s
thrustline, so the net action is that the tail is
pushed to the right and the nose is pushed to
the left. That’s why your airplane yaws left
on takeoff and requires right thrust to
compensate.
There are other effects, such as pure
torque reaction, the rolling effect of spiral
airflow on the wings and stabilizer, and the
tough-to-explain but unimportant P-Factor.
The preceding explanation is the reason
why the model needs right thrust. I will
attempt to justify that statement in the next
installment.
News Flash: When I was a kid, the
aeromodeling press actually disseminated
news. In this age of the Internet forum and
irrefutable anonymous authority, a magazine
such as MA rarely gets to the “scoop.” The
following might be an exception.
The Bob Hunt/Dean Pappas skunkworks
has been busy since following Mike Palko’s
lead and switching to electric propulsion for
our CL Aerobatics (Stunt) efforts. After
refining the power plant enough to convince
ourselves that electric power was already at
parity with wet power, we turned some of
our attention to airframe design.
What design changes would suit electric
power best? Torque and all the unwanted
trim effects because of propeller rotation
have been a pain for Stunt fliers around the
world.
The first idea was to adopt a torquesharing
contrarotating gearbox, as is sold in
Europe for electric-powered RC Aerobatics
by my pen pal Michael Rammel of
Germany. The gearbox sounded like it might
be too heavy, so we came up with the idea of
a counter-rotating twin.
94 MODEL AVIATION
Bob has the airframe roughly half done,
and the drawings look snazzy. Both of these
schemes would allow a Stunt aircraft to trim
out better.
Then Kaz Minato sent word, through
mutual friend Frank McMillan, to Bob about
an interesting experiment he had just tried.
Kaz is the many-time Japanese National CL
Stunt champion, as well as a member of the
Japanese national team and an all-around
nice guy.
Remember what I wrote at the beginning
of this column about how what we call
obvious depends on how carefully you look?
Kaz looked carefully, and he did the
obvious, and smart, thing. Reversing the
running direction is easy with brushless
motors, so he reversed his motor rotation,
put a pusher propeller of the same size and
pitch on the airplane, and flew it. The result
was astonishing.
Remember that with a normal right-hand
propeller, the spiral airflow yaws an airplane
to the left? The spiral airflow and yaw
tendency both get worse when the airspeed
drops. (I’ll cover that in detail in the next
installment.) In CL, the speed drops anytime
you fly toward the top of the hemisphere.
Using a left-hand propeller now causes
the airplane to subtly yaw to the right as it
slows, and this reduces the loss of line
tension when flying in the upper half of the
hemisphere. It’s obvious. There’s that word
again!
We duplicated Kaz’s tests, swapped the
APC 12 x 6E propeller for a 12 x 6EP
(pusher), and started test-flying. Sure
enough, line tension improved everywhere,
but even more at the top of the circle. Bob
has even started flying with slightly slower
lap times, achieving a more pleasantly paced
presentation. Poor line tension usually forces
us to keep up the speed to maintain good
control at the top of the circle.
Flying in calm conditions has improved
too. Now when flying through his or her own
turbulence in nearly calm conditions, the
airplane rolls out and away from the pilot
instead of into the circle, as usually happens.
Remember: you saw it here first!
That’s it for now. I’m going flying, how
about you? Have fun, and do take care. MA