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M20C elevator trim rolling forward in flight


dk580

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1 hour ago, PT20J said:

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.

If the stab downforce is (relatively) evenly distributed forward and aft of the stabilizer pivot point, then would not light turbulence (or, even engine/propellor harmonics?) maybe be enough to jostle (technical term) the worm gears just enough that it could change the trim setting?  I don’t think you’d want a lot of downforce transferred directly to the trim mechanism as it would induce friction making it more difficult to move (without mechanical advantage), and potentially cause more wear in every trim system component from front to back. 

Obviously, the drag from the vertical stab will amplify the applied downforce, another (speed dependent) variable?

 

image.thumb.png.6006c4a33bf578392d9b4e394c667995.png

 

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5 minutes ago, 47U said:

If the stab downforce is (relatively) evenly distributed forward and aft of the stabilizer pivot point, then would not light turbulence (or, even engine/propellor harmonics?) maybe be enough to jostle (technical term) the worm gears just enough that it could change the trim setting?  I don’t think you’d want a lot of downforce transferred directly to the trim mechanism as it would induce friction making it more difficult to move (without mechanical advantage), and potentially cause more wear in every trim system component from front to back. 

Obviously, the drag from the vertical stab will amplify the applied downforce, another (speed dependent) variable?

 

image.thumb.png.6006c4a33bf578392d9b4e394c667995.png

 

Excellent point -- I hadn't really thought that through. The hinge point is at or very near the quarter chord point and it's a symmetrical airfoil (no pitching moment) so the force on the trim system due to aerodynamic forces should be minimal.

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Actually, you do want some force on the trim system. It will take up the backlash, otherwise the tail would be constantly banging back and forth on the backlash of the jack screw. I’ve flown Mooneys where you could move the tail up and down on the ground as much as 1/4 inch,  in flight it was perfectly stable.

Besides, if you couldn’t back drive the ring and pinion in the trim gearbox, how does an electric trim motor do it?

Edited by N201MKTurbo
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Just now, N201MKTurbo said:

Actually, you do want some force on the trim system. It will take up the backlash, otherwise the tail would be constantly banging back and forth on the backlash of the jack screw. I’ve flown Mooneys where you could move the tail up and down on the ground as much as 1/4 inch, hey in flight it was perfectly stable.

Besides, if you couldn’t back drive the ring and pinion in the trim gearbox, how does an electric trim motor do it?

The motion of the forward gearbox (worm and worm wheel) is reversible. The jackscrew in the tail should not be. That's why jackscrews are used for this type of application. I didn't mean to say that there is no force on the trim system in flight -- just not hundreds of pounds as I first implied.

 

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On 3/2/2021 at 10:39 PM, N201MKTurbo said:

I’ve flown Mooneys where you could move the tail up and down on the ground as much as 1/4 inch,  in flight it was perfectly stable.

 

I seem to recall a story told to me by the late, may he rest in peace, Bill Wheat during a Mooney fly-in about 15 years ago.  He talked about a time when they loosened up the tail on an A model, I believe, to the point where it was just basically flopping around.  When he flew it, he said it flew just as stable as it did when the tail was tight with no mobility.  He was the most interesting Mooney guy to talk to.  

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I think Bill described the experiment as using a bolt one size smaller in diameter to simulate wear...

That much wear wouldn’t happen in years...  :)

It was probably a story written in a MAPA article... where I got that from...

Best regards,

-a-

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