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Need help diagnosing elevator stiffness

LarryT

Well Known Member
My RV-6A is currently undergoing the annual condition inspection. therefore among other things I wanted to address the SB about elevator spar cracking.

1. It was difficult to tell if there was a crack in the left outboard elevator hinge doubler, so I made a decision to remove the left elevator for dye penetrant inspection.

2. The good news - no crack. the bad news - the elevator seems stiffer now that things are reassembled. Before, movement was free and the elevator would rest where it was balanced. After, while movement is smooth there is just enough friction to hold the elevator in place wherever I let go of it.

3. I was careful to count the threads on the rod end when I unscrewed it, wrote the number of turns down and tried to replicate. We have disconnected the push/pull tube from the bell crank and determined the stiffness is in the elevator itself. Loosening the center fastener bolts and the L/S outboard hinge bolt has not made any difference.

Am I going to have to remove the VS, take the elevator off and set it up the alignment on a bench? Any other suggestions before it gets that complicated?

Thanks,

Larry Tompkins
N544WB -6A
W52 Battle Ground, WA
 
A good thread on how to correctly install/rig the elevators:

http://www.vansairforce.com/community/showthread.php?t=79222

I will also add that all hinge bolts should be a "slip fit" so try installing the OB and center bolts first, tighten them down and check for freedom of movement, the elevator should move freely. The center bolt should now slip in without having to wrangle it, if you can't get it in adjust the center and OB bearings until everything lines up. Build the correct shims for the center bearing after you have both elevators adjusted. The elevator TT should be disconnected for all of the above.
 
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Walt, I looked at the drawing...

... and the drawing made it look (to me, anyway) like the center bearing had shoulders on it, as opposed to washer/spacers. I will look at the drawing and the builders manual again. I presume I am the guilty party because it was fine before I took it apart. although I did not check for loose fasteners because I was removing them to Spot Check the outboard rod-end nutplate.

Larry
 
reopening this thread to ask Walt A. a question

Because Walt indicated that he finds the elevator center bearing being used as a bushing about 75% of the time, I thought it could benefit many if I ask Walt publicly instead of PM.

I have removed the rudder and VS from the plane so I could look directly down at the center bearing. It does not move when I move the elevators and the elevators get stiffer as I start to torques the center fastener.

Does penetrating oil work to loosen the center bearing?

Or will I probably have to remove the center bearing from the aircraft and exercise it, e.g., clamp the shoulders in a soft-jawed vise, liberally apply penetrating oil and begin working the bracket around the clamped center bearing?

Or should I just replace the center bearing assy?

Anything I overlooked?

Thanks for your help Walt. I am determined to get this fixed properly.

Larry Tompkins
544WB -6A
W52 Battle Ground WA
 
Because Walt indicated that he finds the elevator center bearing being used as a bushing about 75% of the time, I thought it could benefit many if I ask Walt publicly instead of PM.

I have removed the rudder and VS from the plane so I could look directly down at the center bearing. It does not move when I move the elevators and the elevators get stiffer as I start to torques the center fastener.

Does penetrating oil work to loosen the center bearing?

Or will I probably have to remove the center bearing from the aircraft and exercise it, e.g., clamp the shoulders in a soft-jawed vise, liberally apply penetrating oil and begin working the bracket around the clamped center bearing?

Or should I just replace the center bearing assy?

Anything I overlooked?

Thanks for your help Walt. I am determined to get this fixed properly.

Larry Tompkins
544WB -6A
W52 Battle Ground WA

I'm not Walt, but I might still be of help....

Regarding your previous post, the bearing does have shoulders on it, but since every elevator is slightly different in dimension, it is expected that spacers will be needed between the bearing shoulder and the elevator horn.

A proper installation includes spacers (typically washers) between the elevator horns and the center bearing.

If you see a gap, when the nut is loosened on teh bolt, that is at least part of your problem.

The required spacers need to be a precise fit. If you force in a spacer that is just a bit too big, or use one that is just a bit too small, when you tighten the center bolt it will side load the two rod end bearings on the elevator and induce friction.

Your center bearing may be just fine. If you have a very long needle nose pliers you could try and rotate it to see if it is turning freely. If it is, install some closely fitting spacer washers and see if it is still ok after torquing the bolt.
 
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The center bearing is basically just a sealed bearing, the inner race is locked in place to the elevators with the bolt and spacers while the outer race is retained by the elevator support bracket.

51UKZCs7npL._AA220_.jpg


HS-29.jpg


Here's a pick I snagged from google to show the spacers installed:

emp_horizontal_bearing1.jpg


If the spacers are not a perfect fit on both sides of the bearing when you tighten the bolt it will try to pull the elevators together placing a side load on the outer and middle rod end bearings, that is what causes the binding.

When I get the outer and middle rod ends adjusted correctly then I use a feeler gauge set to measure the gap between the elevator horn and the bearing on each side and make spacers to fit. Sometimes you get lucky and a combination of washers will work, other times you have to make a custom spacer from aluminum tubing to get the exact dimension you need.

When you get it all set up correctly all the bolts are tight and the elevator floats like a butterfly :D
 
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That is the explanation I needed.

Thanks guys.

Larry Tompkins
544WB -6A
W52 Battle Ground WA
 
This issue is one that has been kicked around a long time. First off, I try to never argue with Walt. I always lose, and this would be no exception, and I am not arguing.
However, I find a few things about this long known issue interesting;

There isn't anywhere that I am aware of in any of Van's designs that call out for a precision, custom machined spacer. Please correct me.

It has been inferred in this thread that most of the fleet has an issue here, however, there has never been a flight safety issue noted, only that the bearing is not being utilized as a bearing, but becomes a bushing, and the nut may not be torqued to limits.

It is quite common for bushings to be used in this specific application, and indeed bushings are used in this system in RV's, specifically the stick to control weldment. (steel bolt, brass bushing)

The bushings in my Aeronca and Bucker are seeing 50 and 70 years of continuous operation showing no or minimal signs of wear.

While the nut may not be torqued properly, it is highly unlikely it is going to leave the bolt.

What does that leave us with?
If you want to do it "right", you most likely will need a precision machined spacer. If your airplane was not built this way, you will not get smooth "floats like a butterfly" operation unless you don't torque the nut to limits (shear). If you don't torque the nut, you have a bushing, and an under torqued nut.

If your reading this, it probably affects your airplane. Will you ground it? Will you address it at the next CI? Will you simply ignore this and keep it the way it is?

Larry and I will be making some custom spacers for his airplane.
 
I recommend making custom spacers and attaching them with a flush rivet to the elevator horns, this way they never fall out during disassembly for painting or whatever. If memory serves correctly it is a hair less than 3/16 thick spacer. More than several have it done this way using a small teardrop piece of Aluminum.

Larry
 
Jon,
I don't think anyone has said it needs to be a precision bushing. It has been said that if you want to have your elevators operate properly, you need to accurately shim the gap that by design will exist between the elevator horn and the bearing. I am usually able to do this to a high enough level of precision with a combination of thick and thin washers, but it does occasionally require making a custom wash from aluminum to an specific thickness.
This may not have been detailed in the RV-6 plans/manual when you built your airplane, but I am pretty sure it has been in plans for RV's for many years now.

I am not aware of any data to imply that most of the fleet has an issue here.
Yes, a lot of RV's have more friction in the elevator circuit than is desirable for best handling qualities and stability. This bearing connection is not the only sorce of that friction (but it is one of the common ones). They seem to fly ok.... I think it is one of those things where people don't know, what they don't know.

I think choosing to not properly torque a fastener because we are pretty sure that the nut wont come off is a rather slippery slope to travel. Not that I disagree, but the reason we safety anything is for the "just in case" situations. As a standard practice we say that any bolt that could be subject to rotation must have a drilled bolt and cotter pin. This should not be considered any differently. Having said that, I do not think that is really a good choice for resolving this when simply installing the correct spacers will fix it the right way.

Say we decide it is ok. Then we must decide how big of a gap is ok. What if it is 3/16" on each side (not common, but not unheard of either)?

What it really comes down to is that if someone is going to be building and maintaining an airplane, it is smart to do it to the highest level practical. Properly doing an assembly process so that a bolt can be properly torqued falls within that standard.

There isn't anywhere that I am aware of in any of Van's designs that call out for a precision, custom machined spacer. Please correct me.

It has been inferred in this thread that most of the fleet has an issue here, however, there has never been a flight safety issue noted, only that the bearing is not being utilized as a bearing, but becomes a bushing, and the nut may not be torqued to limits.

It is quite common for bushings to be used in this specific application, and indeed bushings are used in this system in RV's, specifically the stick to control weldment. (steel bolt, brass bushing)


While the nut may not be torqued properly, it is highly unlikely it is going to leave the bolt.

What does that leave us with?
If you want to do it "right", you most likely will need a precision machined spacer. If your airplane was not built this way, you will not get smooth "floats like a butterfly" operation unless you don't torque the nut to limits (shear). If you don't torque the nut, you have a bushing, and an under torqued nut.

If your reading this, it probably affects your airplane. Will you ground it? Will you address it at the next CI? Will you simply ignore this and keep it the way it is?

Larry and I will be making some custom spacers for his airplane.
 
Here's a picture I took many years ago of my HS center bearing right after I drilled the center bolt hole. There's certainly nothing "precision" about those spacers, I just worked them down with a hand file until they fit. The assembly probably still needs some more work, though; there's still a tiny bit of friction when the center bolt is tightened.

Dave
 
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Jon,
I don't think anyone has said it needs to be a precision bushing. It has been said that if you want to have your elevators operate properly, you need to accurately shim the gap that by design will exist between the elevator horn and the bearing. I am usually able to do this to a high enough level of precision with a combination of thick and thin washers, but it does occasionally require making a custom wash from aluminum to an specific thickness.
This may not have been detailed in the RV-6 plans/manual when you built your airplane, but I am pretty sure it has been in plans for RV's for many years now.

I am not aware of any data to imply that most of the fleet has an issue here.
Yes, a lot of RV's have more friction in the elevator circuit than is desirable for best handling qualities and stability. This bearing connection is not the only sorce of that friction (but it is one of the common ones). They seem to fly ok.... I think it is one of those things where people don't know, what they don't know.

I think choosing to not properly torque a fastener because we are pretty sure that the nut wont come off is a rather slippery slope to travel. Not that I disagree, but the reason we safety anything is for the "just in case" situations. As a standard practice we say that any bolt that could be subject to rotation must have a drilled bolt and cotter pin. This should not be considered any differently. Having said that, I do not think that is really a good choice for resolving this when simply installing the correct spacers will fix it the right way.

Say we decide it is ok. Then we must decide how big of a gap is ok. What if it is 3/16" on each side (not common, but not unheard of either)?

What it really comes down to is that if someone is going to be building and maintaining an airplane, it is smart to do it to the highest level practical. Properly doing an assembly process so that a bolt can be properly torqued falls within that standard.

Thanks Scott. I was just giving "food for thought" based on what was being said previously. I absolutely agree with you and Walt. I reworked mine some time ago after losing the argument with Walt ;) (and I thought I was winning!, DARN!)
I was able to do it with a combination of washers.
I know I have told you this before, but thanks for being a valued resource on all things RV.
Carry on.
 
That's a picture I took many years ago of my HS center bearing right after I drilled the center bolt hole. There's certainly nothing "precision" about those spacers, I just worked them down with a hand file until they fit. The assembly probably still needs some more work, though; there's still a tiny bit of friction when the center bolt is tightened.

Dave

Well thanks for letting me "borrow" your picture, its the first good one I found doing a google search for images of the assy. Obviously I can't tell if you made the shims the correct size, but the "process" looks good :D
 
It may not be just the center bearing.

Good thread - I would like to add a little information. My right elevator had friction without the center. It floated with zero friction until the HS bolts were tight. It was clear that the steel brackets (or one of them) was causing an axial load on the heim bearings when tightened. So, the bolts were loosened and gap checked (measured) with a feeler gage. In this case, hand bending one bracket .015" reduced the friction by about 80%. Surely with more fiddling it could be minimized.

This is just to add to the possible measurements and adjustments future builders may consider to reduce friction.
 
I recently came across this as well - my plane came out of paint about three weeks ago and on the first long cross-countries I made I noticed the autopilot (Dynon) asking for a LOT of trim changes, first one way then the other. I hand flew for a while and played with the trim and determined I had either a sticky trim tab or a sticky elevator, was difficult to tell which while in flight but either way I chalked it up to paint overspray in a hinge somewhere.

After getting back home I first cleaned the trim tab hinge but it was moving freely already. The elevators were definitely stiff - relatively speaking. It was a small change but noticeable to me, and apparently to the autopilot too. I pulled the elevators and checked everything, and found that the "A&P" hired by my painter to disassemble and reassemble the airplane had removed the elevators and the washer spacers I had on the center bearing, and reinstalled without the spacers on the center bearing. The resulting sideloads and stiffness disappeared as soon as I reassembled it correctly.

I needed to pull the elevators anyway, to rebalance them after paint. Might be a minor detail ignored by many, but I did it anyway.
 
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If someone with a lathe is not ready available to part off a spacer of the correct dimensions (use feeler gauges so your lathe friend only needs to make one set), brass and stainless shim stock is available from MSC or McMaster Carr in assorted packs for little money. They start at .005" and go up to .05 in .01 increments. Inexpensive punch sets are available from the same sources to match the through bold diameter. A simple office paper punch will work with the brass so long as you don't ask it to cut through more then .02. Standard AN washers as available in .031 thickness also.

These will come in handy in the future for lots of other situations. If you don't have a set of feeler gauges the shim packwill act as a rough one.
 
I recently came across this as well - my plane came out of paint about three weeks ago and on the first long cross-countries I made I noticed the autopilot (Dynon) asking for a LOT of trim changes, first one way then the other. I hand flew for a while and played with the trim and determined I had either a sticky trim tab or a sticky elevator, was difficult to tell which while in flight but either way I chalked it up to paint overspray in a hinge somewhere.

After getting back home I first cleaned the trim tab hinge but it was moving freely already. The elevators were definitely stiff - relatively speaking. It was a small change but noticeable to me, and apparently to the autopilot too. I pulled the elevators and checked everything, and found that the "A&P" hired by my painter to disassemble and reassemble the airplane had removed the elevators and the washer spacers I had on the center bearing, and reinstalled without the spacers on the center bearing. The resulting sideloads and stiffness disappeared as soon as I reassembled it correctly.

I needed to pull the elevators anyway, to rebalance them after paint. Might be a minor detail ignored by many, but I did it anyway.

Exact same here. I'm glad i came across this thread. I had stiff elevator after paint and similar auto pilot issues. I loosened this bolt a tad and issue solved. Now I think I will look at this again; install a thin washer and torque to spec and hopefully be back to normal.
 
I just did this for my RV9A. It only took one AN960-416L washer on the right side. This was in addition to the metal washers(don't have the part number, but they are silver in color) the plans call out to install on either side of the bearing.
 
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been fiddling with this for a few days. at the moment it appears my elevator horns are not quite squarely welded to their tubes. so torquing the 1/4 bolt causes an axial load outboard. I think I can resolve this by using slightly oversized spacers to offset the flex.
 
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