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Tore Loodin dropped me an interesting note relating to my Citabria project being covered (slowly!) on work in progress.

"Looking upon your Citabria in progress, I remember the first flight I did some years ago with my quarter scale Piper J3 Cub, which also had ailerons. When trying to do a left aileron turn just after start, it didn't turn at all, instead it side-slipped, rapidly losing height. At the last moment I remembered that I also had a rudder in the left stick and tried it. Now it turned left in a correct way and saved the model from the seemingly unavoidable imminent crash .As your Citabria also has a long wing and a large span/chord ratio, I reckon that the effectiveness of the ailerons is rather low. For that reason you may have to use the rudder as the principle turning surface and only use ailerons to tilt it to "hold the ball in the middle" during the turn. After that I always turn models with blended rudder and ailerons, which makes the turns much more nice and realistic. But with the modern computer transmitters it is easy to blend these functions. But as my tx is a simple one, I have to use the computer in my head. To improve the ailerons try to make the movement of the down-going aileron only half as long as the up-going aileron. The down-going aileron brakes that wing half to a yaw, which counteracts the turn and makes the model slip"

Thinking that the only way I could adjust the relative up/down movement of the ailerons would be through the ATV control on my transmitter, I concluded it would not be possible since the ailerons on the Citabria are controlled from a single servo. I was wrong, of course, and Tore sent me some information he dug up from a very old copy of Model Airplane News showing that it can be done mechanically, not electronically. For anyone else who has never come across this information before, here's the know-how, quoted (mostly) from the magazine itself..

"In many flat-bottom wing designs, it is necessary to use differential aileron movement in order to have the ailerons work properly. In this case, the "down" aileron causes drag which turns the model in the direction opposite to that called for by the ailerons. By limiting the "down" aileron travel, this effect can be eliminated.

"Differential movement can be provided at the control horn, or by using a bellcrank that has the proper neutral angle. The method is the same in either case and is illustrated below. It is important to note that the differential movement is controlled by the angle between the control rod and the line through the hinge and clevis pins at neutral. If this angle is 90º, no differential occurs. More or less will vary the differential between "up" and "down" as shown. The control horn is shaped to provide this angle; otherwise its shape has no significance."

I found that sentence which I've underlined interesting, since I have not come across control horns or bellcranks with any other angle than 90º, although I'm told that some kits actually supply non-standard ones where the design calls for it. However, insofar as control horns are concerned, the same effect can be produced when using a standard right-angled horn by adjusting the location of the horn with respect to the hinge-line, as this further item explains:

" . . . Let's take a look at what happens when the control horn is mounted in different locations with respect to the hinge-line. Assuming the clevis is located in the outer hole of the horn, with an average servo travel one can readily see the effect (diag. B and C below). With the horn located ahead of the hinge-line (B), down control surface is greater than up. With the horn located behind the hinge-line (C), up control surface is greater than down. The further away from the hinge-line  the horn is located, the greater the differential travel. The shaded areas show the added difference beyond equal travel.

"In diag. D, the control horn is shown located 1/16" away from the hinge-line with very little consequence of differential. What did surprise me was the distance the control horn had to be moved in order to gain any appreciable differential."