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Trim control friction

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Apr 4, 2022
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I had a great time at the Madison, Indiana flying today. There was a Stearman, Waco, tiger moth, and my Starduster, amongst some other neat airplanes. At one point a brief shower came through and soaked all of us without a hangar to park in. I didn’t think much of it, and took off shortly later before some bigger storms arrived.

After taking off and getting a few miles from the airport, I buried my head in the cockpit to play with the GPS and my phone and my water bottle, etc. all of a sudden the plane pitched suddenly and got my head out of my butt.

I settled the plane down, set the trim and went back to scratching the back of my knees. Half a minute later, it did the same thing. This time I paid more attention and saw the nose steadily drop, the air speed pick up, and not come back to my trim position.

It didn’t take too much fooling around to figure out that the trim control was slipping, going to fall nose down in about 20 seconds. Once I understood what was going on it was merely annoying, not difficult or dangerous, to fly the hour back home. (In fact, I took several detours to see some interesting things along the way)

While the additional cockpit load was manageable, if anything else had been demanding my attention on the flight, this would’ve quickly escalated to a significant risk increase.

When I got back to my hanger, I took the trim control apart, and found a brass sleeve that rotates on a fixed metal tube. I assume water got in there and reduced the friction. Even during the flight I was trying to adjust the friction by tightening the knob, but it just doesn’t work that way.

I think the root cause is either a flaw in the design, or more likely a missing washer behind the brass bushing. You can see in the pictures below the bolt clamps only on the fixed steel tube, which stands slightly proud of the bushing. The plate under the bolt head contacts the inner, fixed tube, and does not actually contact the brass fitting.

I intend to put a washer behind the handle forcing it to stand proud of the fixed tube. This will allow the bolt clamp down on the moving fitting, instead of the inner tube.

Initially I considered machining some of these parts on the lathe to achieve the same result, but in the end I suspect assembly is simply missing a washer.

Does that sound reasonable? Am I missing something?

Beyond that, and simply drying the bushing out, is there something else I should be doing here? I drop off oil? Dry lube? Or just clean it and leave it dry?


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