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Control geometry?

rjcthree

Well Known Member
We noted something unusual(?) on my RV-9A during the airworthiness inspection today (passed ;)). Center the elevator and ailerons. Move the elevator trailing edge up(nose up elevator) and both aileron trailing edges rise a smidge, maybe 1/8". Go trailing edge down on the elevator, the aileron trailing edges drop about 1/8". Return the neutral, everything is neutral. Repeatable, no binding, no slop. You have to go stop to stop on the elevator to see it. If you hold one aileron at neutral, the effect at the other tip is magnified.

So I have to ask, anybody who is at their hanger, can you try this, please? Center controls and move the elevator at the elevator from stop to stop. See if the ailerons move.

Looking at the centers, I would expect a little of this....but would like confirmation from the brain trust.

Thanks in advance. Rick.
 
Normal. It makes sense if you think about it. By moving the stick forward or aft you are effectively lengthening the distance between the pushtube attach point below the stick and the aileron bellcrank because the pushtube is now at an angle. The ailerons have to move to account for that increasing distance.
 
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Steve has described it perfectly. If you imagine that the push rod attached to the stick were very short, the effect would be easy to see. Because of this effect, it is important that when adjusting the ailerons, the elevator be clamped at neutral.
 
Having done a handful of control system design, I can say that a tiny amount of interaction between the stick axes is almost unavoidable unless you go to heroics to separate them. In practice, the effects are almost completely unnoticeable.

On my HP-24 racing sailplane kit, where you see this most strongly is when the stick is held at the right or left stop and cycled through the elevator travel. The full-span flaperons change position ever so slightly. Of course, if you're in a flight regime where you have the stick at the roll stop and you're using substantial pitch inputs, a tiny change in the roll deflection is the least of your worries... :)

Congrats on the successful inspection!

Thanks, Bob K.
 
Interesting. That interaction works *against* the control inputs as well, so if you want to pull up you need to pull more than you would if the ailerons didn't move as well. Ditto for pushing the nose down.

Of course, maybe this is a feature, as it works to make the -9 less twitchy? Those designers are a clever lot... :)
 
Interesting, indeed.

Rob, that was exactly what I was thinking.....essentially you know there will be some interactions as a resul of the out of plane motion in the non moving axis.....maybe it was a choice. A totally trivial one, the range of aileron motion with full up to down elevator travel is less than 1/4".

I didn't remember to go the other way, I got too excited to reassemble.
 
Because of this effect, it is important that when adjusting the ailerons, the elevator be clamped at neutral.

Learn something new everyday. 3.5 years of building and I never ran across this tip anywhere. Next time I'm at the hangar I'll have to see where my ailerons are when elevators are neutral. Maybe correcting any error could reduce the slight down-elevator I have in cruise, and add a few knots.

Chris
 
Interesting observation! But a concern that a DAR inspecting airplanes didn't know this (ie aileron movement wrt elevator)..... :(
 
Closing this out...

Slight aileron motion with full travel of elevator is consistent on an RV-7, and RV-6 and and RV-9A. It's looking completely normal. Yeah geometry!
 
Yea I moved my elevator up and down yesterday with ailerons at neutral - if there was any movement it was extremely small.

Chris
 
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