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Three Ways to Skin This Cat—Piloting Technique: There are three things
we can do, one of which is to do as the full-scale pilots do: use rudder
with aileron all the time. It's called coordinated aileron and rudder,
and it's a basic flying skill.
In a Piper Cub the pilot needs to apply the rudder just a little bit
before the ailerons are moved. With a long-winged sailplane, the
rudder-before-aileron lead may be substantial. That's how powerful the
adverse yaw can be on an airplane with a short tail and long wings.
That's one of the reasons why aerobatic airplanes these days have long
tails and fuselages that are as long as the wing.
Since those airplanes are required to roll cleanly over a wide range of
airspeeds, the best way to keep the aircraft from yawing is to give the
fin and rudder a long moment arm to help keep things straight. And if
the wings are approximately the same length as the fuselage, the
ailerons can't apply as much yawing torque as if the wings were very
long.
Most RC pilots would do well to develop the skill of flying coordinated
aileron and rudder, but we need to help ourselves right now. This would
clearly be asking too much of the student RC pilot.
The second thing we can do is couple the ailerons into the rudder. When
you apply right aileron, right rudder is also applied. This can be done
mechanically or with a programmable transmitter. Your radio may or may
not have this feature, although many medium-priced radios with six
channels and more will do.
If you are a Scale fan, you will probably want to make sure your next
purchase has this feature. If it is not an option, aftermarket control
mixers are available for a moderate price.
Typically, full aileron throw only requires roughly one-quarter rudder
or less. "Roughly" is not a good enough figure; we need a method to test
the amount of coupling. Give me a few moments to describe the next plan
of attack, and I will describe the Dutch roll method.
The third and preferred method is aileron differential. This is what
most of us will use. Some coordinated rudder may still be necessary
during the steepest climbs, but a differential setting that is good for
the entire flight profile can usually be struck.
Aileron differential is easy to describe but requires a little effort to
set up. In simple terms, when you move the aileron stick, the aileron
that goes up must travel farther, in degrees, than the one that goes
down. This is true both left and right. The trick is to do it by
offsetting the linkages in clever ways.
Modern radios also allow for this to be done with
programming, provided you use an independent servo for each aileron. I
will cover how to adjust aileron differential later, but for now let's
go flying to see if and how much adverse yaw we have. The preferred test
method for airplanes that spend most of their flight time upright is the
...
Click on photo to view large image with caption
Dutch Roll Aileron Differential Test (Also For Coupled Aileron Into
Rudder): Let's look at the Dutch roll method. This test is also a bit of
a flying exercise (such as a musician playing scales).
Fly a straight line away from yourself at a safe but low altitude.
Smoothly but quickly rock the aileron stick back and forth so the
airplane banks 45° one way and then the other way.
You want to use as much aileron throw as you can while comfortably
keeping up with the airplane. Ideally the rhythm will be approximately a
half second in one direction and the same back in the other direction.
One of three things will happen. (Everything comes in threes!)
1) Axial Rolling. The airplane will roll back and forth, and the tail
will point straight at you and not wiggle at all. The airplane will
appear to roll on a fixed axis, as if it were riding on a wire. That
means the differential is perfect for level flight.
2) Adverse Yaw. This is typical: the model "duck walks." By that silly
phrase I mean that as the airplane rolls right, the tail wiggles right.
Then as it rolls left, the tail wiggles left. That would mean the nose
is going in the direction opposite the roll—and that's the wrong way!
This means you need more differential or more aileron-into-rudder
coupling.
3) Proverse Yaw. The nose wiggles the same way as the bank. You don't
see it often! You'll see the tail swing out of the Dutch roll in what
looks like the beginning of a sudden turn.
This is not great if you are interested in aerobatics, but it is
perfectly acceptable for training. It adds controllability during all
positive-"G" flight (upright). A moderate amount of proverse yaw
(opposite of adverse) actually helps initiate the turn. If you decide to
fix it, do so by reducing the differential or reducing the
aileron-into-rudder coupling.
Let's Retest in a Climb: As I mentioned, adverse yaw is worst at low
airspeeds, such as in a climb. You'll want to repeat the Dutch roll
test, in a climb, pointed directly away from you. You should use the
steepest climb angle you normally expect to use.
The trick to this test is being able to sight down the tail of the
airplane. The corrective actions are the same as the level-flight Dutch
roll test.
Although this is useful for the student flier, those of you who fly
heavy, slow, or short-tailed Scale airplanes will benefit tremendously
from optimizing their differential for the takeoff climbout. That's the
situation in which so many beautiful airplanes are lost.
The climbing differential test will often uncover an adverse-yaw problem
that requires a lot of differential. It may be too much to practically
put into your control linkages. If so, consider one of several
approaches.
You could learn to move the rudder stick in unison with the ailerons.
You could use coupled aileron into rudder (CAR) or you could install two
separate aileron servos to get more differential adjustment.
This works nicely, but only if your radio is programmable and has an
aileron differential menu. Don't be surprised if some airplanes need
twice as much throw on the rising aileron as on the dropping one.
Feeling Cranky—How to Mechanically Adjust Aileron Differential:
The
differential crank is an ancient mechanical device; that means it is
deceptively simple and sophisticated at the same time. The methods
described work with one servo driving both ailerons or with a separate
servo for each aileron. If you have a radio that allows you to
electronically adjust the differential and used separate aileron servos
in each wing, you might skip the next couple paragraphs.
If your airplane has the servo(s) and control horns on the bottom of the
wing, the proper differential happens if the aileron horns are behind
the hinge line and/or the connections to the servo wheel are in front of
the center of the wheel. This is typically the situation on a high-wing
airplane or a two-servo low-winger. (See the High-Wing Differential
drawing.)
On the other hand, if your airplane has the servo(s) and control horns
on top of the wing, the aileron horns need to be angled forward and/or
the connections to the servo wheel need to be behind the center of the
wheel. This is usually the situation on a single-servo low-winger. It's
that simple. A careful look at the drawings should help untangle the
whole mess. (See the Low-Wing Differential drawing.)
That's how you put in differential. Since it requires a bit of shop
time, we want to leave the workshop with the differential set to a good
guess for starters. Your typical low-wing sport model is usually happy
when the rising aileron goes up approximately 20% more than the other
goes down. All these amounts are for throw angles, in degrees.
A high-wing trainer would like approximately two-to-one, but the
mechanical method shown in the diagram will only get you close. My
recommendation for trainers, especially the ones with flat-bottom
airfoils, is to connect to the servo wheel roughly 30û in front of the
hold-down screw and to rake the aileron horns back so that the angle of
the control horn is 90û.
Let me define the control-horn angle clearly. If you draw a line from
the middle of the hinge line through the little hole that the clevis pin
goes through, it makes an angle with the clevis pin at the vertex with
the pushrod. (See the Horn Angle Measured Through Clevis Hole diagram.)
If you are using a bent-wire strip aileron horn, this is easier when you
use a fitting that does not move the clevis pin forward of the heavy
wire horn. The plastic part that is often included in the kit moves the
clevis pin more than 1/4 inch forward of the bent-wire horn.
Instead Nelson Hobby/Rocket City and Sonic-Tronics make an ideal piece
of hardware. These products place the clevis pin directly in the middle
of the music-wire aileron horn.
The recommendation for how much differential to put into a trainer may
seem to be a lot, but a full-scale Cessna 150 has one-and-a-half-to-one
differential; the up-moving aileron moves 15û while the other one drops
10û.
Even so, in cruise flight the aileron response still requires
coordinated rudder to make the airplane respond properly. On takeoff and
in landing trim it definitely needs aileron-rudder coordination. You
wouldn't expect a high-wing model to be much different from a Cessna
150, now would you?
Remember that if a stable trainer-type model has inadequate
differential, the aileron response will have an initial lag, after which
the control effectiveness will still be sluggish. Control lags lead to
overcontrol and stick thrashing—good for churning butter, but not for
flying.
Tidying Up: This concludes my collection of trim techniques for training
and Sunday flying. I hope I have given you not just a cookbook method
for trimming, but a good start in understanding the whys and hows of
trimming an airplane.
As it turns out, there is a whole body of advanced trimming techniques
for sport, Aerobatics, and 3-D flight regimes. We have a reason to get
back together. MA
—Dean Pappas
deanf3af2b@pappasfamily.net
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