My Wooden Airplane
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9/8/2010

New Aileron Control Horn

Almost as soon as I posted my last update, a couple of visitors pointed out some potential issues with my aileron control horn. (Thanks, Matt and Roger!) It turns out that the upper aileron cable was rubbing against the top of the rear spar at full up and down deflection.
In order to solve this, I increased the height of the upper aileron control arm 1/2 inch.
This seems to have done the trick.

I should point out that the new hole is 1/2 inch higher than the original hole, but it is in the exact same location fore-and-aft. (I didn't simply extend the upper arm.) This is to keep the angle between the upper aileron cable and the hinge at around 90 degrees.

There was also some concern about lengthening the bottom arm the same amount as the top arm. The argument was that the bottom aileron cable would become slack at the full-up and full-down aileron deflections because the bottom horn wasn't travelling the same linear distance as the top horn. However, I don't think this is going to be an issue, for 3 reasons:

1) The top aileron cable doesn't get connected to the control stick. It get connected directly to the other aileron. As long as both aileron horns are identical, they should move the same distance. If both cables were attached to the control stick, then I would have to either lengthen the bottom arms the same amount as the top (which would then alter the "gear ratio" between the stick and the ailerons), or I could change the attach point of the upper aileron cable at the control stick, which would preserve the "gear ratio". But since the upper aileron cable does not get connected to the stick, I shouldn't have to worry about that.

2) As specified in the plans, the top and bottom arms are already different lengths. If they were originally the same length, I might be more concerned with keeping them equal, but on the plans, the upper horn is 4-1/2 inches (hole to hinge), while the bottom horn is 3 inches.

3) I performed an experiment with string and popsicle sticks. I connected the tops and bottoms of the popsicle sticks with string, then pivoted them around a point exactly in the middle of the two holes, simulating control horns that were exactly the same length. Then I moved the pivot point down towards the bottom of the sticks, simulating the lengthening of the upper control horn. When I moved the sticks, the upper string certainly moved much more then the bottom string, but since the ratios were the same, i.e. the arms were the same length, there was no slack in either of the two strings.

While this certainly doesn't prove anything, it makes me confident that I won't have an issue. However, I will obviously keep an eye on it when I start rigging the ailerons, and if there is a problem, it won't take much effort to make a new set of control horns.

Finally, it occurred to me that if I had to adjust both the intermediate pulley and the control horn in order to avoid rubbing somewhere, perhaps I had installed the main pulley incorrectly. But I double checked the plans, and there is no information about where, specifically, to place the main pulley bracket on the front spar. Mine is centered (top to bottom), but perhaps if it was raised to its upper limit (without drilling a hole in the spar caps), I wouldn't have had any issues with the intermediate pulley or control horn.

Total Time: 1.0 hrs.