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Old 06-30-2004, 10:25 PM
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bojangle
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Default RE: Differential Aileron

Just means the amount of up deflection and down deflection are "different". You need more up than down. say 1/2" up, 1/4" down. A down deflected aileron creates drag, tends to make the plane yaw in the direction of the down aileron. This is a momentary condition, as the increased lift will soon overcome the drag, it just makes turns sluggish. If you were seated in the plane, you would feel the slip in the seat of your pants.

The easiest way to visualize this is to look at the plane from the front, at an angle slightly below the wing (because the plane flies at a positive angle of attack). If you give full right aileron, you can barely see the up right aileron, but you can see the down left aileron completely, and this is what the "relative wind" also sees. It's like a huge flap hanging down.
Perhaps a better way of describing the action is to say the wing with the down aileron slows down, at a time when it needs to speed up in the turn. As the turn progresses the slow wing tries to play catch up and rushes forward, sometimes resulting in over banking. It just ain't smooth.

Proper turns also require coordinated rudder. The rudder does not create the turn, it merely helps to counteract the adverse yaw in the wrong direction. Piper Cubs in particular are said to be hard to turn without differential and rudder. With today's modern computer radios, you can mix in the rudder for turns, but this is not a proper way to learn correct flying techniques. Years ago, all trainers were 3 channel, and the use of the rudder to "turn" created the wrong concept for the pilot. Even then, the rudder didn't make the turn, it simply created yaw, making the outboard wing move faster, and the relative wind was then able to act on the dihedral, and the turn resulted. Etch in your mind...Rudder does not turn the plane.

It is best to set up the differential at the servo by having the linkage point forward of the servo center. (On a high wing) Then when the servo pulls the rod for down aileron, the path of the servo arm connection point is an arc instead of a "straight" line. Setting the servo arm on the spline about 30 degrees forward of the center line is usually sufficient.

Bob