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M20C elevator trim rolling forward in flight


dk580

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I'm curious -- which models have the trim brake? It's not on my J and the trim stay's put. I'm somewhat at a loss to see how a force on the tail would cause a rotation since it's a jackscrew mechanism. Does anyone have a drawing or description from a manual of this brake?

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From the ‘65C POH... ‘62, ‘63, ‘64 data included.

There was an earlier thread about this, I remember because I stuck my foot in my mouth.

My royalite trim wheel tower cover has a hole on the left side to access the friction lock screw.

image.thumb.png.b5c8a9ac42b58c609b8016dfab0fc7b1.png

Edited by 47U
Clarification...
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The down force on the tail does back drive the jackscrew. Turbulence helps by jostling it around. A well lubricated jackscrew and gearbox makes it worse. If your plane doesn't do it, you should probably lubricate your system. Mine did it pretty bad. When I looked at the trim gearbox, there was no brake. The hole and the screw were there. I owner produced a brake by taking a 1/2 inch Delrin rod and drilling a hole in the back of it slightly larger than the screw and putting a spring behind the rod around the screw. I got all the parts from the parts bins at work, so it cost nothing. It took a few hours to R & R the gearbox. Problem solved, no more phantom down trim. I can't even tell there is any friction on the trim wheel when manually trimming.

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It’s interesting that the service letter came out in 1958, but apparently it wasn’t incorporated by the factory until 1963. Also, the friction lock isn’t on later models, so whatever caused the rotation was apparently solved by some other means. I also notice that the design has changed: In the service letter drawing, there is no bicycle chain.

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I think the generalization of this solution on all models un 1965 (C, D, E & F).

11 hours ago, carusoam said:

Great pic JR!

Thanks for sharing it.

Any idea what year Mooney that came from? (Your M20F?)

Best regards,

-a-

 

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Yeah, that's a later model year fix.  I have had the difficulties with my trim moving on my '62 as well.  We even paid a mooney service center to "tighten up" the trim, which did nothing...not even certain what they did.

One thing we looked at was retrofitting the later friction device.   We decided that a newer trim wheel would be required (as the original was not designed for the added wear).  A hangar elf put some friction material between the trim wheel and the riser and I haven't had nearly as much of a problem since.

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1 hour ago, PT20J said:

I'm still curious when Mooney quit using this and how it otherwise solved the problem. My J doesn't have it. How about Es and Fs?

I was always under the impression that with the electric trim, it was no longer needed because that added sufficient friction to the system.

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39 minutes ago, Andy95W said:

I was always under the impression that with the electric trim, it was no longer needed because that added sufficient friction to the system.

That's an interesting thought. Clearly it was an issue. It's a little hard to understand the root cause. A jackscrew under constant tension cannot transmit motion from the nut to the screw. But, under vibration it could. In cruise, there should be a few hundred pounds downforce on the tail, so it's hard to see how it would vibrate enough to cause this effect. There was period of time when my trim electric was inop and and I never had a problem with the manual trim moving on my M20J.

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