Trim tab tension

[cmsusllc]

Is there any way to adjust the trim tab system so that it will stay trimmed for anything over 130 MPH? I often want to trim for 140 when I drop down out of the mountains but it will roll itself back to around 130. Cable tension is correct and I don't see anything else to do shy of less lubrication. ( It was fine a few years ago when I brought her home. With lots of TLC and lubrication the system works very smooth, maybe too smooth.)
Scott.....53B

[n2582d]

Scott,
I' not sure if you're saying it will initially trim to 140 mph but then re-trims (without pilot input) to 130 mph or are you saying you can no longer get it to trim at 140 mph in a descent--i.e. the fastest it will trim to is 130 mph.

If it is re-trimming in flight I would inspect the actuator. As you probably know the trim wheel actuates a jackscrew. A jackscrew by design should not allow the tab to move even if the trim cable was completely disconnected. I don't think you can over-lubricate the actuator. Looking at the picture below one should not be able to make the sprocket (7) turn by pushing the rod (13) in or out.

The rod can only be extended or retracted by turning the sprocket. There are only two things I can think of that would be cause your trim to change without turning the trim wheel. One would be if the threads in the jackscrew were so worn they were no longer engaging between the male and female threads. I've never heard of this happening before. But if you can force the actuator rod in our out by hand you need to ground your plane until the actuator is rebuilt or replaced. A second, and equally remote possibility, is that your actuator is not clamped in position tightly. Open up the inspection panel on the bottom of the right horizontal stab and make sure the actuator cannot move fore or aft. The circlips on the actuator should prevent fore and aft movement even if the bracket (26) was loose. Cessna Service Letter SE75-23 tells how to overhaul the actuator. A copy of it is found here in the MX Library.

Trim Actuator Mount.jpg (38.74 KiB) Viewed 3505 times

If you can't trim it for anything faster than 130 mph in a descent anymore check the trim tab travel. It should be 10 degrees up and 27 degrees down. If that is correct slide your seat all the way forward when descending or get rid of all that junk in the baggage compartment!

[cmsusllc]

I can trim for any speed I want, It just won't stay trimmed for anything over 130. Trimmed for 140 and lightly hold the wheel and it slowly rolls back. The actuator is tightly mounted and properly adjusted but shows signs of wear. It looks like a rebuild is in order. Lubrication was mentioned "tounge in cheek".
Thanks, Scott.....53B

[n2582d]

... Trimmed for 140 and lightly hold the wheel and it slowly rolls back. The actuator is tightly mounted and properly adjusted but shows signs of wear. It looks like a rebuild is in order.

Wow, that's interesting. Contrary to what I wrote above, it sounds like the rod is driving the sprocket. How much play do you have at the end of the trim tab?

[cmsusllc]

Trim tab freeplay with the elevator in the mid position is minimal, well within tolerences. Torington bearing and bolt for rod attachment to the tab have very recently been replaced. Freeplay increases with the elevator at either stop due to sideways freeplay in the end bearing of the actuator where the rod passes thru. I suspect that is where I need to start.
Scott.....53B

[GAHorn]

I can trim for any speed I want, It just won't stay trimmed for anything over 130. Trimmed for 140 and lightly hold the wheel and it slowly rolls back. The actuator is tightly mounted and properly adjusted but shows signs of wear. It looks like a rebuild is in order. Lubrication was mentioned "tounge in cheek".
Thanks, Scott.....53B

I have to ask what is likely an obvious question..... Do you have an autopilot installed?

[cmsusllc]

Autopilot, whats that?
Scott.....53B

[wingnut]

I don't know why I woke up at 2am thinking about this. But would like to throw a couple thoughts out there to you. First, a jackscrew can drive a sprocket, although very unlikely unless it's 1) worn out, 2) has limited "thread" engagement at it's extreme travel limit.
Any vibration would magnify either above problem. I would look really close at how much engagement the male and female threads have, at the extreme limit where your having the problem. If I remember right, if it's not rigged correctly, it's possible for the chain/sprocket to actually drive the screw right out the end. Your problem may be that you only have a half turn before that jackscrew falls out (limited engagement=less friction to inhibit the jackscrew "creeping")

Vernier type engine controls are a type jackscrew, and they creep when worn.
A Beech landing gear actuator is a motor driving a worm gear, that drives a sector gear. The inertia of the sector will continue to drive the worm if it were not for a dynamic braking relay, which momentarily turns the electric motor into a generator.