trim noses up during bumpy flight

[CGinSD]

Hello all, I've noticed that during turbulence I'm continually correcting for increased nose-up trim in my 1962 M20C. The more turbulence the more often I have to roll the wheel forward to correct. 

Love to hear anyone's ideas, similar experiences, cautionary tales or estimated costs of repair?

[Marauder]

Now that is a new one. Only thing I can think of is that something is not snug on the jackscrew assembly. How much resistance is there on the ground when you rotate the trim wheel?

[Andy95W]

Definitely weird.

It would be nice if you could compare your trim movement to someone else's. There are some adjustments to the trim assembly which can tighten the chain and add some tension/friction to the system if yours is considerably looser than it should be.

[CGinSD]

Thanks Marauder and 95W for confirming the weirdness. First time I squawked this our mechanic tested the tensions and reported all normal but there has to be something in that whole trim contraption that is loose or slippy. Just hoping someone point me to a likely culprit... its gotten old dealing with the problem and I really don't want to take out an 'exploratory maintenance' mortgage

[Hank]

Definitely sounds wrong. I rarely adjust my trim during cruise unless the winds aloft change direction or speed. The worst turbulence I've ever flown in [maybe moderate?] was in western SD/eastern WY, and I had both hands on the yoke then [it ended around the Philipi VOR] as a sub-200-hour VFR pilot. "Normal" turb/chop does not require a trim adjustment.

 

Good luck in your investigation!

[Danb]

Main reason to turn off altitude hold in turbulence

[47U]

Once the slop is taken out by running the trim wheel forward, it shouldn't come back. Worm gears don't work that way. Are you absolutely sure you don't have a u-joint where the roll pins have sheared? Try running the trim all the way to the stop each direction and see if it slips once it hits the stop.

I'm curious what your friction is set at, too... I agree with Andy... definitely wierd.

[N201MKTurbo]

My plane does it too. It is a very subtle effect. In continuous moderate I notice that I trim nose down more then I trim nose up. I figure that after a while I'm going to unscrew the jack screw.

 

Here is my theory:

 

In normal flight there is a constant downward pressure on the horizontal stabilizer. This puts a compression force on the jack screw and linkage which takes all the slop out of the trim system. In turbulence the force is constantly changing which has the same effect as wiggling a stuck bolt to make it turn easier. The constant aerodynamic force along with loading and unloading caused by the bumps, back drives the jack screw a little bit.

[N601RX]

I was also be sure that the hing shim AD from a couple of years ago had been completed. Might even be a good idea it to look at it again.

[Oscar Avalle]

Yes, it does not sound right to me...

[Hank]

Main reason to turn off altitude hold in turbulence

If only our C's had that available! Manual trim, don't you know.

[CGinSD]

Thanks for all the thoughts: I guess there are enough people who don't have to do this tells me it can be fixed... which is nice.

[CGinSD]

Once the slop is taken out by running the trim wheel forward, it shouldn't come back. Worm gears don't work that way. Are you absolutely sure you don't have a u-joint where the roll pins have sheared? Try running the trim all the way to the stop each direction and see if it slips once it hits the stop.

I'm curious what your friction is set at, too... I agree with Andy... definitely wierd.

I'll ask our mechanic to look again at the assembly and check the joints. The problem with testing the trim limits is that in smooth air or on the ground the trim stays where it is, but I find the idea of flying around in turbulence with the trim set to the stops a bit hair-raising

 

Is there a quantitative way to set the friction? I'm not a mechanic in any sense, but this sounds like a concrete test of what could be wrong if there is.

[CGinSD]

I was also be sure that the hing shim AD from a couple of years ago had been completed. Might even be a good idea it to look at it again.

Yep - will check, thanks

[CGinSD]

My plane does it too. It is a very subtle effect. In continuous moderate I notice that I trim nose down more then I trim nose up. I figure that after a while I'm going to unscrew the jack screw.

 

Here is my theory:

 

In normal flight there is a constant downward pressure on the horizontal stabilizer. This puts a compression force on the jack screw and linkage which takes all the slop out of the trim system. In turbulence the force is constantly changing which has the same effect as wiggling a stuck bolt to make it turn easier. The constant aerodynamic force along with loading and unloading caused by the bumps, back drives the jack screw a little bit.

OK, so its not just me! Now my trim slipping is by no means subtle, yesterday I was having to work it down every 30 second or so for over an hour of flight but I was coming to the same theory that the tail 'waggling' was just unwinding the screw. Thanks

[Marauder]

If only our C's had that available! Manual trim, don't you know.

 

Oh you can have electric trim. This here button cost me $11,000 (in 1998 dollars):

 

[N201MKTurbo]

Once the slop is taken out by running the trim wheel forward, it shouldn't come back. Worm gears don't work that way.

 

Actually in this case they do. You can easily rotate the trim torque tube by hand which back drives the trim wheel. The electric trim motor drives the torque tube which back drives the trim wheel.

[47U]

Actually in this case they do. You can easily rotate the trim torque tube by hand which back drives the trim wheel. The electric trim motor drives the torque tube which back drives the trim wheel.

Of course you are correct. Rotating the torque tube by hand is quite easy. I think your theory is correct. The turbulence wiggles horizontal enough that the jack screw turns slightly and the slop in the linkage prevents the trim wheel friction from preventing the trim change.

I meant to check the trim stops to the limits on the ground, not airborne. Sorry I did not clarify.

[CGinSD]

Of course you are correct. Rotating the torque tube by hand is quite easy. I think your theory is correct. The turbulence wiggles horizontal enough that the jack screw turns slightly and the slop in the linkage prevents the trim wheel friction from preventing the trim change.

I meant to check the trim stops to the limits on the ground, not airborne. Sorry I did not clarify.

 

Ah. Did that and it seems normal.

 

If I understand correctly though, the diagnosis for me is a bit different than this as the trim wheel itself turns as part of the nosing up so it is not just the slack in the linkage. So if I extrapolate, you would say then that the friction on the trim wheel is just to light? Would be the kind of Doh! solution that wouldn't surprise me but I was not aware there was a specific way of tuning it.

[N201MKTurbo]

There is no friction mechanism in the trim system. I think the effect described does not indicate a broken trim system it indicates a very well maintained trim system.

 

You could fix it by lubricating the jack screw with roofing tar.

[CGinSD]

There is no friction mechanism in the trim system. I think the effect described does not indicate a broken trim system it indicates a very well maintained trim system.

 

You could fix it by lubricating the jack screw with roofing tar.

Tempting, or kicking a door wedge under the trim wheel

 

Seriously though it needs something doing: as you can imagine being forced to fly one hand off the yolk in turbulence is sub-optimal 

[N201MKTurbo]

Are you saying that yours moves that much? Mine moves about 100 feet of trim in five minuets.

[47U]

Tempting, or kicking a door wedge under the trim wheel

 

Seriously though it needs something doing: as you can imagine being forced to fly one hand off the yolk in turbulence is sub-optimal 

Yeah, flying with one hand on the yoke and one hand on the trim wheel just doesn't sound right. 

 

I think there is an adjustable friction on the trim wheel, though.  I had the cover off last annual to clean under there and I had felt pad that rides on the left (inside) surface of the trim wheel.  It was choked with carpet fuzz.  My IPC doesn't show it, but I have a later version from the mid-70s that I think is similar.  It goes back as far as the '65C.  Take a look.  Under index 14 is listed a "BRAKE ... NI" (NI - not illustrated).  I haven't found any adjustment procedures, though. 

Your the '62B had a little different trim wheel installation.  But, since you've got a 'C' I am guessing it's similar to the attachment.

 

Trim Wheel.pdf

[CGinSD]

Are you saying that yours moves that much? Mine moves about 100 feet of trim in five minuets.

Yes. Yesterday was particularly choppy but for an hour it was trying to give me over 500'pm of climb in under 2 minutes of reseting it at 8,500'... tiring

[N201MKTurbo]

Wow, you learn something every day. I guess we need to tighten our brake screws a little.