M20C elevator trim rolling forward in flight

[dk580]

I have a 1962 M20C. In flight my elevator trim slowly rolls nose down requiring constant adjustment. On the ground the trim moves easy nose down but not the same force to nose up. Anyone ever run into this issue? 

[carusoam]

Are you familiar with the friction adjustment?

Iirc.... in my 65C... There is some leather like material under the floor board to be adjusted...

Really old memory, could be a figment of imagination...

Best regards,

-a-

[N201MKTurbo]

It just proves that your trim system is in great shape.

there is supposed to be a trim brake on your trim wheel. Most are missing. It puts spring pressure on a plastic rod that keeps the wheel from doing that.

There are a few threads on here about it.

[Andy95W]

The trim brake didn't get added until '63 or '64.

You could increase the tension slightly on the chain that drives the system.

 

 

[PT20J]

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?

Skip

[Prior owner]

Wow. I find it so improbable that the tail could turn the screw and turn the trim wheel gearbox.... but it must be happening.

I’m Following this thread with great interest.

[47U]

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.

Edited February 28, 2021 by 47U
Clarification...

[47U]

This is the old cover, Plane Plastics sold me a new one which is much better quality.

[PT20J]

It's kind of a mystery. The 1966 Service Manual page posted by @Andy95W doesn't show it. I have a M20C Owners Manual for 1962-1964 models that includes the same text as posted by @47U and includes the note: "The friction adjustment for the trim control wheel does not apply to 1962 models."

Skip

 

[47U]

Found the thread from long ago...

Link to SB to add the friction lock (from the last post). 

https://www.mooney.com/wp-content/uploads/2020/12/SBM20-33.pdf

[N201MKTurbo]

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.

[N201MKTurbo]

Wow, I just read the SB from 1958!

I wouldn't use the spring plunger, it will eventually wear through your trim wheel. I love how they say to just put it on a lathe and take some material off till it fits. I don't think they would get that through the process today.

[PT20J]

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.

Skip

[Raymond J1]

small screw under the sprocket to rotate to increase friction on the wheel.

[carusoam]

Great pic JR!

Thanks for sharing it.

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

Best regards,

-a-

[lamont337]

Just now, Raymond J said:

small screw under the sprocket to rotate to increase friction on the wheel.

 

Are you referring to the set screw that is holding the upper sprocket in place or something else?

[N201MKTurbo]

Just now, lamont337 said:

Are you referring to the set screw that is holding the upper sprocket in place or something else?

No, the screw in the center of the u shaped bracket. It presses a rod on the back of the trim wheel.

[carusoam]

Just now, lamont337 said:

Are you referring to the set screw that is holding the upper sprocket in place or something else?

the circled screw seems to be a friction device... see the light marks caused on the trim wheel in the corresponding area...

 

[Raymond J1]

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-

 

[cctsurf]

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.

[carusoam]

See if @Alan Fox might have one... (trim wheel with friction lock adjustment bolt)

In case somebody wants to go through the effort to modernize...

Best regards,

-a-

[dk580]

Thanks for all the input guys. I am hopefully getting into the plane later today to see if any of these suggestions are the culprit. 

[PT20J]

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?

[Andy95W]

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.

[PT20J]

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.