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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 30 in front of the hold-down screw and to rake the aileron horns back so that the angle of the control horn is 90.

     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 15 while the other one drops 10.

     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 


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