Elevator trim rod bushings binding

[Pbruce]

I’m a bit flummoxed: I don’t know how this happened or for that matter, how to fix it. Prior to covering, the tail was bolted on and the controls rigged. As supplied, the bushings in the stabilizer ribs holding the elevator trim crank rod were not quite perfectly aligned. This must have been due to minor warping of the steel ribs during welding. Small tweaks and twists of the ribs had the rods turning smoothly and easily. I lubricated the bushings well with white lithium grease (pictured), attached the control horns to the rods and proceeded to cover.

After the stabs were covered I notice considerable tightness in the bushings. It’s very difficult to rotate the rods, whereas it was easy before. There isn’t any particular point where there is acute binding, just generally high friction. I am unable to remove the rods easily because Mark G suggested that I both bolt and JB weld the control horns onto the end of the rods in order to prevent slop. I can move them laterally the width of the control horn slot (about an inch) which I have done to re-lubricate, without success. There is no corrosion evident. No glue entered the bushings. I also tried 5-20 synthetic motor oil, without any noticeable improvement over the white lithium grease. Temperature in my shop is between freezing and 55 F. Since covering it has been bell below freezing for several weeks.

Has anyone else experienced this issue, and if so, what did you do about it? I’m supposing that maybe the shrinking of the covering might have displaced the ribs ever so slightly, which in turn misaligned the bushings. It’s weird though, because the rods were in position in the bushings throughout the covering process. Another concern is if I tear the covering off and do it again, I assume the same issue will present again. Appreciating in advance any comments. IMG_0099.jpg IMG_0100.jpg

[Bcone1381]

We might want a discussion on elevator trim system friction. If I recall correctly, there has been issues with some installed elevator trim systems not having enough friction thus the trim tab could move on its own in flight.

[Pbruce]

I recall that too. I don’t think I’m being anal about insuring the rods move easily in this case though. I’ll except some “snugness”, but this is a bit beyond. I’m going to peel the fabric off them and do it again, taking great care. Hopefully I get them tight enough for longevity but loose enough to prevent warping of the ribs. It looks like I misinterpreted all the instructions I read about insuring the fabric was heated up to 350 in stages. I guess next time I’ll go just a bit past taught. Apparently there are other techniques to guard against over shrinking around thin steel ribs. I’m trying to find them.

[Bcone1381]

This is a link to a thread Colin Campbell started on over shrinking fabric on the elevator. That is different, but make sure the gap for the trim surface has clearance. I had that issue. Keep us updated on what you discover.

https://bearhawkforums.com/forum/model-specific-qb-kit-built/bearhawk-five-quickbuild/69160-over-shrinking-fabric-on-elevators

[Pbruce]

IMG_2777.jpg IMG_2780.jpg I put a straight edge on the root rib and also the next rib (the two with the bushings for trim actuator tube). Both ribs on both stabs are distorted inward toward each other about 1/4” from the tension of the fabric. I’m going to tear it off and re-cover, taking extreme care not to repeat the mistake. Any tips are highly appreciated.

[Bcone1381]

I like your wisdom. You found a data point! I sense more data to come...when the H-stab fabric relaxes will the problem be solved? How much heat will cover #2 accept before distortion begins?

When I covered my elevator I took measurements of trailind edge opening for the trim tab and duplicated Colin's issue. As I shrunk the gap closed. I could equate a shrinking gap with an iron temperature. I got the fabric shrunk as much as it would allow. I wanted it as tight as the physics would allow it to be and could accept some shrinkage. So keep that straight edge in one hand and see how much heat the surface can take before things start to change.

[noema]

Some friction may be preferable and the friction translates to the control inputs less than you may think. Maybe give it a try on the fully rigged system.

I have significant friction too, resulting from a slight misalignment of the bushing that is welded into the stabilizer rib. It was even a bit difficult to get the torque tubes in.

It turned out on the fully rigged system it does not seem to add any unwanted resistance. In fact you *want* to set the trim and have it *stay* there.

[Pbruce]

Yes, I’ve certainly considered that. I discussed this issue with Mark G and he I tried to quantify the friction. Basically, how much is too much? He seemed to think I was in “too much friction” territory. I think what I’ll do is start with the stab that is really stiff. It’s actually hard to move the control horn with my thumb without pain in the pad of my thumb. I’ll strip and recover that stab first, with precautions (more slack, less heat). I’ll gauge the results at that point before recovering the other stab, which is the one you see pictured. Thanks all.

[arborite]

Friction in the movement of the tab has a different outcome than friction in the control lever up front in the LSA. I assume all are similar. you want friction in the lever/wheel to hold the cables in position. the tab in back then can move as an anti servo as the elevator moves. if your linkage to the tab is binding, that implies it can’t move relative to the elevator and the trim lever/wheel is moving as you move the elevator or the elevator is hard to move. that’s not good.

can you clarify exactly what happens when you move the stick from full up to full down? does the trim tab move in the opposite direction as it should or is it binding and causing the cables to move?

[Pbruce]

I cannot say what happens when the stick is moved as I took the control surfaces off to cover, and I have yet to put them back on. I was about to apply Stewart’s ecofill and I happened to notice the tightness in the bushings. I consulted all my resources and each indicated that I needed to recover with less tension. I stripped the covering , stitching, and glue from the first stab this morning. Sure enough, it now moves normally.

Interestingly, I found that the root rib of the stab is actually built with that 1/4” arc in it to better match the aft end of the fuselage. Doesn’t matter much since there’s a fairing there, but in any case, it never was straight. The second rib had a similar arc after covering which disappeared when I stripped the fabric off. Funny thing is, the fabric was not adhering to that rib due to the stitch-reinforcing tape over the fabric, under the pinked tape. Somehow, the whole rig was creating enough tension to bend that rib and thus provide huge friction in the bushings.

I’m presently in the processs of building U-channel braces from .025 2024 to position between the two ribs adjacent the trim actuating tube. This will give me a measure of protection against the stab ribs being squashed toward each other, causing misalignment of the bushings.
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