Differential ailerons - radio controlled aircraft

    Radio controlled aircraft with ailerons can benefit from differential throws of ailerons. Aileron differential means that the aileron that moves upward needs to be deflected by a larger angle, and the one that moves downward is being deflected by a smaller angle. This results in a more coordinated roll of the aircraft because if the aileron throws were the same, the equal angle of downward deflection causes more drag than lift for the upward moving wing, slowing the wing down and causing the aircraft to yaw in the direction opposite to the desired direction of turn (if any).
    While desirable, differential aileron throws is not really a critical flight function and is not implemented in many lower-cost computer radios. And, of course, it is not at all available in simple, non-computer radios. And if you sensibly choose to drive both ailerons with one servo then transmitter programming is of no use at all.
    There is a simple way of accomplishing differential aileron throws mechanically, by positioning control horns away from the hinge of the control surface. On this page we provide simple charts that allow one to measure and position aileron control horns to get precise maximum throw angles while achieving the desired differential.

To use the charts, first make a note of the deflection angle you need and of what percentage of the upward deflection angle you want the downward deflection angle to be (the differential). Next critical parameter is servo arm travel. This parameter is what you can change by placing the aileron linkage in the different holes in the servo arm.
Next, follow the links in the table below to find the servo arm travel that will enable you to select the horn offset and clevis height, while giving the desired deflection angle and percentage of differential. Make sure that you use charts for the same servo arm travel.

1. The horn is attached to the underside of the aileron some distance from the hinge, or
2. The horn is positioned on the top side of the wing but is made so that the linkage attachment point is forward of the aileron hinge line.

For example, we want maximum upward deflection angle of 50º and downward deflection angle that is 60% of the upward deflection. Checking the charts for 5 mm linkage travel (see examples of angle and differential charts marked accordingly), we find a suitable combination of clevis height (10 mm) and control horn offset (5 mm). Other options would be 12 mm and 8 or 9 mm (or one can try 12 mm and 6 mm with the 7 mm linkage travel, or 7 mm and 4 mm with the 4 mm linkage travel). From these options, higher clevis would be better because that setup is less sensitive to imprecise installation.

The calculations presented on the plots are based on the kinematics similar to these:

If something is not quite clear, or you have questions or need a chart for some parameters not listed above, feel free to drop us a message.
Differential aileron throws are calculated using a small IDL program involving simple kinematics calculations with IDL, the Interactive Data Language by RSInc, the Kodak Company.