PC roll trim

[Groundhog]

I have a new to me 1970 M20F, and the roll trim on the PC autopilot turn coordinator is all the way to the left. Most of the time it’ll hold level unless the fuel is imbalanced. I read a lot of old threads about the PC but was wondering if anyone had personal experience with this issue that could save me some time. I looked at the servos on the ailerons, and the right one looks older than the left, but I didn’t see any noticeable holes, cracks or tears. I’m trying to find a mechanic that knows something about these systems, but no luck so far (Chicago area). I thought maybe if someone had the same experience with running out of roll trim they could point me in the right direction and maybe I could guide a local A&P on what I’d like to try to troubleshoot. Thanks in advance!

[N201MKTurbo]

There are two tubes that run along the left side wall. They come up behind the instrument panel on the left side. Take the hose apart somewhere like where they connect to the turn coordinator. Suck on them one at a time as hard as you can then stick your tongue on the hose end. If it holds vacuum, the servos are good. If they blead off quickly, you have a leaky boot. If sucking on the hoses grosses you out, get one of those hand vacuum pumps from the auto parts store. 

[David Lloyd]

There are some manuals in the download section that you should read through.  There are many threads concerning PC misbehavior that you may search. I remember none that really have the answer you are looking for.  First take care of the things in the trouble shooting section, one that may pertain is verifying the airplane flies true with the PC disengaged.  The 1965s had a remote gyro for the PC. At the back of the gyro where the tubing attaches is a lever which might be called a bias adjustment. A bump during maintenance might cause the problem you describe.  Haven't seen this mentioned anywhere.  Later models used the panel mounted turn co-ordinator and had the bias adjustment on the back.  Easy to see that being moved during any work behind the panel.  Not sure if the 1970 models had the same adjustment.

Please keep in mind, I just know enough to be dangerous.  About a lot of stuff.

[211º]

I compiled a lot of information over here. I hope that it is helpful.

https://www.windfield.farm/positive-control

Dave

[Groundhog]

Thanks for the helpful info! I’ll try to get a few things checked out and see if we can find the problem.

[0TreeLemur]

Tears in Servo membranes can be extremely difficult to see.  Here's one I found on the step retract boot last year.  I couldn't see it until I spread it with my finger.   Fixed with a bicycle inner tube patch.

BTW Section 6, Table 2 of the M20 S&MM provides important troubleshooting info.

One last thing, whether or not and how much air a slit like this leaks can depend on whether or not the slit is flexed around a bend in the membrane.  Taking the servos out to work on them is a major pain in da arse.  You can diagnose them one at a time with a vacuum pump, accessing the line to each individually.  Good luck.

[Mooneymite]

When my turn knob finally ended up in the fully clockwise position to keep the wings level, it was a small slit near the rim of the boot on the left servo (red line).

[Groundhog]

That makes sense. I was thinking the right side is suspect on mine since I have the trim all the way left. Thanks for sharing that.

[Joshua Blackh4t]

I found that I can reach the red bias lever on my 65 E and adjust it in flight to work. It works (my gyro isn't working so I just get trim) so I havent looked further. It will get upgraded someday.

Start with making sure it flies balanced without it though. It makes life so much easier, i can use the roll trim to correct left and right of track errors.

[N201MKTurbo]

You can’t go to the esoteric problems until you know if your boots leak. That is it 99% of the time.

[carusoam]

Boot failure…

Hose failure…

New hoses are easy to find…

-a-

[Bob R]

On 3/27/2022 at 9:36 PM, carusoam said:

Boot failure…

Hose failure…

New hoses are easy to find…

-a-

Agreed. Fixed mine which had several issues. I have the original Parker part numbers, supplied by Brittian, for the poly flex tubing and compression nuts. May have uploaded them already but reach out if you want them. Hoses and Fittings, as well as others carry them. Just don’t tell them it’s for an airplane.

[0TreeLemur]

On 5/9/2022 at 5:55 PM, Bob R said:

 Just don’t tell them it’s for an airplane.

Truer words never spoken...