Pushrods "falling over"

Paul Eastham · #1 04-22-2007, 12:22 AM

Hi VAF,
At various times during construction and fiddling with my pushrods, I have noticed that they can get in a state where the heim bearings allow the whole pushrod to fall over from one side of the bearing's rotation to the other.

You know how you can twist a pushrod one way and the other, exercising the bearing? That's what's happening, except it happens on it's own as you move the control surface, every time you move it through a certain position. Usually this is accompanied by a small bang and a vibration in the control stick. It could easily be confused for an obstruction or something rubbing on the pushrod.

Currently this is happening on one of my aileron stick-to-bellcrank pushrods, though I have seen it happen on the aft elevator pushrod as well. Between the condition itself and the fact that it might mask a real problem, should I worry and is there anything I can do to resolve?

Paul

AlexPeterson · #2 04-22-2007, 01:06 PM

Paul, can you be a little more descriptive or specific? I am not sure I understand, maybe others do. Control systems are high up on the "important" list...

gvgoff99 · #3 04-22-2007, 01:28 PM

Paul, The movement should be smooth and free flowing. If you are hearing a "bang" something is probably not aligned correctly or is binding somewhere along the push rod. If you are unable to diagnose it and get it to "flow" in movement get one of the CA guru's (Dan Checkoway comes to mind http://www.vansairforce.com/community/member.php?u=1049 ) to come take a look. Push rods do not do well with shearing forces and a "bang" to me implies there is a shearing force somewhere that could cause the push rod to fatigue and fail at the worst possible moment. It would be somewhere between an annoying and a life threatening problem if it broke.

Paul Eastham · #4 04-22-2007, 01:34 PM

I do not believe there is any binding or interference. It's just that as I move the controls, the balance of the pushrod changes at some point, causing it to abruptly rotate from one extreme to the other. When it reaches the other extreme, it makes a noise.

L.Adamson · #5 04-22-2007, 02:04 PM

Check the angle of the bearing, where it connects to the stick. Get it set in a neutral angle, to where the pushrod can roll side to side evenly as it moves the bell crank to both extremes. If it's angled too much to one side, it will pop.

I'm going from memory here, but it seems that I noticed a problem like this, when final adjusting the aileron pushrods.

L.Adamson

rv8ch · #6 04-22-2007, 02:11 PM

If my understanding of what you are talking about is correct, this should not be a problem. I do not recall if mine did this when I had the wings fitted, but I can easily see how it would happen. Once some dust and grime gets into those bearings, it probably won't happen any more!

gvgoff99 · #7 04-22-2007, 02:35 PM

Paul, Without seeing it we are all kind of guessing. The adverb "abruptly" implies to me that there is a bind and when the force overcomes that bind the energy release is attenuated via the "abrupt" movement and "bang" as it pops over the top.

LAdamson's suggestion of alignment of the heads of the push rods sounds like a good starting place for trouble shooting. This is the sort of thing that would always be in the back of my mind if I did not find out the "why." I presume it is the same for you or you would not have posted the question.

I like to not have too many unknowns in the control system.........make that zero unknowns.

Paul Eastham · #8 04-22-2007, 03:22 PM

Quote:

Originally Posted by gvgoff99

The adverb "abruptly" implies to me that there is a bind and when the force overcomes that bind the energy release is attenuated via the "abrupt" movement and "bang" as it pops over the top.

It is abrupt in that it is caused by gravity and stops suddenly when the bearings can rotate no more. You can easily rotate the pushrod back to the position it came from. If you hold the pushrod to prevent it from rotating (in any position), it makes no noise and there is no binding.

gvgoff99 · #9 04-22-2007, 05:38 PM

Paul,, What you are describing is a bit more clear now and assuming that you have no binding at any position there should be no problem. I guess you have checked this by moveing the joy stick while rotating the push rod back and forth by hand through out the entire range of travel. Good job and good flying. L.Adamson's suggestion should "fix" the noise. Hope you are flying soon.

AlexPeterson · #10 04-22-2007, 10:58 PM

I'm still not following the "gravity" part of this - aren't pushrods more or less rotationally balanced? What is motivating it to "go over center" as it were?

Smartest thing is to have another RV builder or two look it over.