Electric Trim Binding

[kiwibob]

We've just re-assembled a new FX-3 in New Zealand and the only serious problem is that the electric trim appears to be binding on each revolution with a the motor making a characteristic whir whir noise. The circuit breaker popped after takeoff on the first flight but it operated fine after that.
The jack screw is observed to be running true and the motor runs fine when detached from the yoke. Manually turning the jack screw it seems stiff in one particular segment of rotation. If the pressure is taken off the elevator however the screw turns much more freely.
Disassembly of the jack screw involves cutting a hole in the fabric so I'm less than enthusiastic about that.
Any suggestions or experiences welcome.

[Pete D]

Make sure the tail wires are rigged properly. How-to should be in the AMM. But if the tail brace wires are pulling asymmetrically it can cause that.

The key points to not miss are leveling the fuselage and making sure both horizontals are level when the tail brace wires are tightened.


Likely the tail was disassembled for shipping without loosening the ends of the tail wires but possible left and right wires got mixed up.

A quick check to see if this would help would be to disconnect one or two of the tail brace wires and run the trim, see if it is acceptable, if so then proceed with fine tuning the rigging.

[David H]

Bob,

Hopefully Pete is correct on your issue.

if not:
See Daves excellent post on this issue here:

post #49
http://forum.cubcrafters.com/showthr...mp-Hints/page5

I had the same issue on my EX-2. I suspect the vertical tube deforms slightly during the welding process and requires an operation similar to what Dave describes above. (or the Jack screw shaft needs to be turned down in diameter)

If your not careful and thorough with this process the jack screw will bind on a portion of its revolution when the trim motor is running, over current, and the circuit breaker will pop.

The fix, once covering and paint is done is well... not a fun prospect. There is no way to get the Jack screw out without major surgery.

David

[kiwibob]

Hopefully Pete is correct on your issue.

Thank you David and Pete, much appreciated.

The motor was surging even with the stabiliser disconnected but I'll try running the jack screw up and down a 'few' times to see if it frees up. I'd hate to have to do surgery on the fabric.

I'll let you know how I get on.

[Daveembry]

Hopefully it’s not what I talk about in post #49 here when discussing installing the jack screw.

http://forum.cubcrafters.com/showthr...p-Hints/page10

On all 3 I’ve built, I’ve had to sand open the tube where the jack screw goes down. They alway bind in one little spot and don’t turn freely.

Also, I had one once that seemed to sporadically work good and then just drag or slow down/surge, whatever. It was a bad trim motor. Worked fine when testing it when building the fuse and just running it up and down by itself to adjust the limit screws but when time to go flying, it worked sporadic. Checked everything else and last thing was to replace it and that was it.

Thank you David and Pete, much appreciated.

The motor was surging even with the stabiliser disconnected but I'll try running the jack screw up and down a 'few' times to see if it frees up. I'd hate to have to do surgery on the fabric.

I'll let you know how I get on.

[kiwibob]

Thanks for all the input.

Video of the trim operation is here https://youtu.be/jZdWcCSCa2A

All reassembled and tail wires correctly rigged. The jackscrew is free to turn and could be turned manually when not attached to the motor and stabiliser.

It’s certainly better than it was but you can hear the surging and there is a little ‘clonk’ apparent on some revolutions which appears to be the sleeve moving, it’s not a particularly tight fit.

I don’t see how it could not have been like this before it was disassembled so perhaps it's acceptable?

[Daveembry]

Bob, Maybe Pete or others could comment also, but other than the “click sounds” at the first of the video, it sounds normal. Those motors do make that sound where it sounds like they are surging but I think it’s just the. Way those low rpm, high torque electric motors sound.

Good to know it isn’t binding in the sleeve.

Thanks for all the input.

Video of the trim operation is here https://youtu.be/jZdWcCSCa2A

All reassembled and tail wires correctly rigged. The jackscrew is free to turn and could be turned manually when not attached to the motor and stabiliser.

It’s certainly better than it was but you can hear the surging and there is a little ‘clonk’ apparent on some revolutions which appears to be the sleeve moving, it’s not a particularly tight fit.

I don’t see how it could not have been like this before it was disassembled so perhaps it's acceptable?

[Pete D]

I agree, other than the clicking that sounds normal. I'm not sure what the clicking is, maybe something in the connection at the yoke where the nuts are between the machined aluminum piece and the welded steel piece with the ears? Does it click when the nuts are on those bolts and everything is together?

The trim will be slightly faster than that when the engine is running and you get the extra couple of volts with the alternator running.

Looks like the upper roll pin was removed and reinstalled-the safety wire is missing?

[kiwibob]

I agree, other than the clicking that sounds normal. I'm not sure what the clicking is, maybe something in the connection at the yoke where the nuts are between the machined aluminum piece and the welded steel piece with the ears? Does it click when the nuts are on those bolts and everything is together?

The trim will be slightly faster than that when the engine is running and you get the extra couple of volts with the alternator running.

Looks like the upper roll pin was removed and reinstalled-the safety wire is missing?

I'll re-examine the security of everything tomorrow and re-test. The upper roll pin was not fully reinstalled as we were running out of time and wanted to get the video. We'll finish the job and retest.

Thanks again for everyones assistance.