How much play in the control system is too much?

[RVFan671]

I've got an RV-4 that I'm flying while building my -10. There has generally been s small amount of play in the stick particularly with the elevator control. I went out to pre flight after a few weeks off and it just felt like a slight bit more than before so I tore in to see if there is anything of concern.

What I read in some other threads is the bushing at the stick could be a culprit and I've ruled that out. What it seems is potentially one of maybe 2 of the rod ends with a small amount of play in the joint itself. I took a video and happy to email it since I don't think the site supports more than pictures. Is it common for the rod ends to wear out and need replacement? Is there some leeway for the amount of play?

I'd say at top of stick it moves maybe 1/16th to 1/8th inch. Just enough for a little click fore and aft. The plane has 460 hours on it and was finished in 1991.

 

[AlpineYoda]

I've been amazed since day 1 how little "play" there has been in my controls. Compared to the Cessnas I've been used to flying, the controls seem much tighter in my RV-10. Just touch the stick and a surface moves, while the Cessna always felt like there was a dead zone in the center around the first couple of degrees of yoke movement.

If you can feel that much stick movement, as well as the "click" you described where the pushrod comes into contact and starts to move, that would really concern me. Something has definitely worked loose or worn a round hole oval.

I hate to suggest that you tear everything apart, but I think you need to track down where that is coming from. I suggest that you either clamp the control surfaces in place or have someone hold them still, then move the stick through that dead zone to where it makes contact. If you can remove access covers and see everything, it shouldn't be too hard to track down which of the various bellcranks and other connections are tight and move precisely with the stick, and which allows some travel before movement.

 

[D-Dubya]

I agree with Yoda. There really shouldn't be any discernible play in the controls. Rod ends typically don't wear out (as long as they're properly lubricated), but it's possible. And yes, a bushing could wear out as well, but it would seem to be more likely that something has worked loose.

Depending on who has done the work on the plane, it's also possible that the wrong size bolt was dropped through a rod end and allowing for some wiggle room, so I'd look at that if nothing else is obvious. Just start at one end and work your way through every component until you find something.

The following isn't a horror story, but it could have been. A tech counselor I worked with told me about an inspection he did on an RV. When he got to the elevator pushrod, he pulled on the rod end where it threads into a tapered bushing that is inserted and is ultimately riveted onto the pushrod. While the rivets were there, NONE of them actually penetrated into the tapered bushing, which meant the connection into the pushrod was only a friction fit. My point is that sometimes things can look just fine, but you may have to dig a little deeper to make sure that they are fine.

 

[Ralph Inkster]

......

I hate to suggest that you tear everything apart, but I think you need to track down where that is coming from. I suggest that you either clamp the control surfaces in place or have someone hold them still, then move the stick through that dead zone to where it makes contact. If you can remove access covers and see everything, it shouldn't be too hard to track down which of the various bellcranks and other connections are tight and move precisely with the stick, and which allows some travel before movement.

Yes - do this
Better assured safe rather than a forever nagging feeling.

 

[Roadjunkie1]

Elevator control play

I've got an RV-4 that I'm flying while building my -10. There has generally been s small amount of play in the stick particularly with the elevator control. I'd say at top of stick it moves maybe 1/16th to 1/8th inch. Just enough for a little click fore and aft. The plane has 460 hours on it and was finished in 1991.

I'll agree that there is less play in an RV control system than some "factory built" aircraft. But there are multiple moving joints between the stick and the elevator. Sounds like you have checked those out.

Check this next: where the elevator horn attaches to the horizontal surface fitting (the part numbers are rolled up in my plans tube.....) above the attachment for the torque tube, there is a bearing in the fitting from the horizontal surface that both sides of the horn of the elevator need to be tight against. If it isn't the elevator horn holes will start to wallow out and give the stick some unwanted play. (Take the horizontal/fuselage fairing off to visualize this.) Remove the cotter key from the castellated nut and remove the nut and washers, if present (all of which will fall into the tail of the fuselage.....; have a beach towel or something stuffed down there to catch them) Take the bolt, washers out and look at the elevator horn. If it is wallowed out from round, it has been moving. It is not the end of the world unless it is really out-of-round. After 400+ hours, it shouldn't be. Just put everything back together and tighten it to proper torque and reinsert the cotter key. That should help. The "clunk" might be the elevator horn moving instead of being tight against the bearing. I can look up part numbers if you need them....

Clamp the elevator to the horizontal and move the stick. There is likely still a small amount of play. If you want to isolate it further, take the baggage floor out and see where the play is. Likely you will find the fittings from the stick to the bellcrank "tight" and the laxity at the horn.

It would be odd for it to worsen in a few hours; it might be that you are now just more aware of it.

After 1200+ hours, SuzieQ has a little play as well. During my current "running condition" inspection (also installing new parts!) I did find the bolt not tightened against the bearing with slight wallowing of the horn. I took everything apart and tightened it up. Still a minor bit of play. 1/16" to 1/8" might be "normal"....

 

[DanH]

What it seems is potentially one of maybe 2 of the rod ends with a small amount of play in the joint itself. I took a video and happy to email it since I don't think the site supports more than pictures. Is it common for the rod ends to wear out and need replacement? Is there some leeway for the amount of play?

Is the ball firmly clamped and immobile? Put another way, is the freeplay where the bolt goes through the ball, or between the OD of the ball and the rod end body?

 

[AlexPeterson]

SNIP
What I read in some other threads is the bushing at the stick could be a culprit and I've ruled that out. SNIP

How did you rule this out? This is where I've had the elevator "click" described here since day one. 1800 hours later it is the same. Top of the stick probably moves about as you describe. Since yours is a tandem, perhaps you could push the aft stick all the way forward or aft until lock, then see if there is any play in the forward stick. Might take two people, dunno.

Obviously the integrity of the entire system needs to be carefully inspected also.

 

[JonJay]

I would be curious what you find from Dan’s question.( and Alex as we where typing at the same time,)

All flight control systems have play. They have to or they would bind. I think the question originally asked is “how much play is acceptable?”.
That answer has to come from knowing where the play is and making a determination if it is within tolerance. Since Vans doesn’t publish anything in this regard, that I am aware of, it’s left up to you.

If no “discernible” play was allowed, you would have a bunch of grounded airplanes. As Scott noted, “fixing”, or “ clamping” control surfaces solidly so they can not move is necessary to “discern” where any play may be.

Many, if not most, legacy RV’s, certainly the 6, have some play on the stick. The control weldment is not a precision piece, neither is the brass bushing or the AN bolt. While things can be done to minimize the play, there will still be some, even if you don’t feel it.
To me acceptable play in this example is - you can’t feel it in any phase of flight, there is no safety of flight issue ( the joint is sound) , and there is or should not be, any abnormal wear leading to more play or joint failure. My 6 meets all of these. It has had that play since new, even after trying new bushings and trying end spacers. It is what it is and hasn’t changed since new.

You must know where the play is to make a solid determination on what is acceptable.

 

[rvbuilder2002]

Play in the control stick bronze bushing on an RV-4 or 8 doesn't show itself as slop to nearly the degree that it will on the side by side airplanes.

It is likely caused by something else.

There are 6 rod ends on 3 push-pull tubes between the fwd. stick and the elevators.
If even just a couple of them have wear (very possible if it is an older RV-4), it can be cumulative to equal quite a bit of slop in the system.

I agree that it should be investigated before any more flying.

 

[RVFan671]

Is the ball firmly clamped and immobile? Put another way, is the freeplay where the bolt goes through the ball, or between the OD of the ball and the rod end body?

Between OD of the ball and the rod end body. Happy to share a video I took so you can assess my finding. Will PM.

 

[Webb]

Hold the rear stick stationary and try to move the front stick. If there is slop, you’ll know it quickly. Don’t forget to lift the sticks to check the control column rod end bearings nuts to be sure they are tight. You can do this without even turning a screwdriver.

If no slop up front, move aft.

From your description of stick play, my money is on a loose bolt that holds the rod end bearing on the bottom of the pilot’s stick and the bolt is worn and the holes are wallowed out.

 

[riobison]

I had a 4 with a similar issue. What I found is that there was some play where the stick fit in the tube. I was able to insert some shim stock between the stick and the tube and it completely eliminated 100% of the play I had.

I now have a 9A with something similar but the cause is different and more difficult to rectify.

Tim

 

[bjdecker]

From your description of stick play, my money is on a loose bolt that holds the rod end bearing on the bottom of the pilot’s stick and the bolt is worn and the holes are wallowed out.

Good bet -- which then begs the question "Why did the bolt wear" and/or "why did the rod end hole wear?"

It will be interesting to see the OP's findings...

 

[D-Dubya]

RVFan, I think you just found the smoking gun. If you have play between the OD of the ball and the rod end housing, then I would replace the rod end itself. That ball should be free to spin any which way, but it shouldn't be so loose that it will move laterally.

Dan knows way more about this stuff than I do, so maybe you two have already resolved the issue in your PM.

 

[Webb]

Good bet -- which then begs the question "Why did the bolt wear" and/or "why did the rod end hole wear?"

It will be interesting to see the OP's findings...

If the rod end bearing ball is not bolted tightly in place, then the bolt can also move in the mounting holes and cause wear. As time goes on, the wear progresses faster because there is more movement due to enlarged holes and worn bolts.

 

[Pat Falley]

Similar happened to my RV-4 at about 1200 hrs. here is something to check before tearing things apart.

There is a fore/aft control weldment (I'm not sure what it's officially called, but it mounts the control sticks and the aileron pushrods attach to the side of it), it is suspended by two rod end bearings. Check that the nuts holding the rod ends to the fuselage are tight. The lower nuts are hard to see and probably missed on most condition inspections. Any vertical play of the front or rear stick means one of these nuts is probably loose.

 

[wcalvert]

Is $200 to replace all the bolts and rod ends on the entire assembly too much $ to eliminate a potential failure in a 33 year old airframe?

These kinds of questions like the OP states seem to point to a lurking sense by the owner that there is something wrong. Too many times I've ignored or delayed addressing that nagging feeling and got lucky each time. I wouldn't be able to sleep at night much less fly the craft.

Use all the troubleshooting advice you want, but new parts fix a lot of aging issues!

Cheers

 

[RVFan671]

Post moved and lost track of it. I sent the video to vans tech and they agree that at least the rod end I sent the video of (the one bolting to elevator horns) should be replaced. Bought that one and the other one I thought might show similar signs but was tough to see so didn't record. I'll replace them in a week or so and give a report update if I find any other causes while I'm into it deeper. Thanks for the guidance. I didn't think these rod ends wore out but I was wrong on that.

 

[Ed_Wischmeyer]

When I was in grad school way back when, studying control theory, I learned that just a tiny little bit of slop in the controls makes the plane easier to control. I never got an explanation I understood, and still don't understand it, but a tiny bit of deadband (as it's called) is very desirable. The complete absence of deadband is undesirable, resulting in poorer handling qualities.

 

[Webb]

When I was in grad school way back when, studying control theory, I learned that just a tiny little bit of slop in the controls makes the plane easier to control. I never got an explanation I understood, and still don't understand it, but a tiny bit of deadband (as it's called) is very desirable. The complete absence of deadband is undesirable, resulting in poorer handling qualities.

Imagine balancing a toothpick on a knife’s edge. Then turn the knife over and balance the toothpick on the flat spine of the knife. The edge is harder to balance on.

The tiny bit of deadband is your flat spot like the knife’s spine making control easier by not making the slightest twitch immediately left or right.