Can't get auto trim to work ...

[colinrasmussen]

I have a Bonanza V35B. Installed the trim amp, confirmed the electric trim switch rolls the trim correctly (although had to flip the wires to get that to work correctly). Also interesting to note that the clutch on the trim servo (Bendix 272) gets balky at less than 13 volts - motor runs fine. Programmed the software to agree with the direction the trim tab was moving. But, keep getting the AP Trim Malfunction warning. Electric trim still operates but when the autopilot is engaged keep getting the warning and the autopilot won't trim - have to do it manually.

Anyone seen a similar behavior?

 

[PYoder]

Also using a Bendix trim servo. The malfunction warning appears whenever the servo does not achieve the desired positioning within 3 seconds. It will still work (keep working) until it achieves the proper position. There is one adjustment in the setup software that ‘maximizes’ the time actuating-time before warning - but it is not enough for large trim-change-demands. (I will have to go to the airport to detail that setting for you)
You can either (a) fly such that only small adjustments are required, (b) ‘help’ it manually when large trim adjustments are required, (c) put up with the warnings and cancel them, or (d) turn off the warnings in the setup menu.
I choose to use (c) because it assures me that the servo is working and that I should make it work less ‘strenuously’ by flying more smoothly!

 

[colinrasmussen]

Thanks for the info. So if I understand you correctly, the auto trim continues to work, it just throws out the warning because it thinks things aren't going quickly enough for its liking?

 

[PYoder]

Essentially. Yes. Watch your trim wheel to verify.

 

[colinrasmussen]

Thanks. Going to go out for a spin today and I will check that out.

Essentially. Yes. Watch your trim wheel to verify.

Didn't work that way. Warning goes off, autopilot won't turn the wheel - manually or with the electric trim switch works. Weird.

 

[PYoder]

Thanks. Going to go out for a spin today and I will check that out.

Didn't work that way. Warning goes off, autopilot won't turn the wheel - manually or with the electric trim switch works. Weird.

I’ll check mine again ….

 

[colinrasmussen]

Figured out one part of the problem - servo clutch seems to need 14V to operate properly.

 

[PYoder]

Figured out one part of the problem - servo clutch seems to need 14V to operate properly.

Do you have a wiring diagram?

 

[airguy]

Figured out one part of the problem - servo clutch seems to need 14V to operate properly.

Sorry, no. The clutch is magnetic and requires no voltage to operate. The trim amp, on the other hand, very well may - which would lead to an electrical installation problem.

 

[Rhino]

Check to make sure that both pins 7 and 8 on the autopilot are connected to pins 4 and 3 on the trimamp, and that pins 11 and 12 on the trimamp are both connected to the servo. I'm not saying it's what happened here, but I've seen some people assume the 'low' side can be simply connected to ground, and that isn't true. If the autopilot does not get the feedback needed through both those wires, it will trigger an AP Trim Malfunction warning. The only other thing I can think of is power. The servo requires 13-24v at 5 amps max, so the trimamp is only working at the bottom end of that range, and that's only when conditions are perfect. Actual maximum motor current out of the trimamp is 3 amps.

 

[Rhino]

Dynon says you don't need the clutch, but the trimamp has pins for it.

 

[colinrasmussen]

Sorry, no. The clutch is magnetic and requires no voltage to operate. The trim amp, on the other hand, very well may - which would lead to an electrical installation problem.

The clutch is run by a solenoid - which is an electromagnet.

 

[colinrasmussen]

Dynon says you don't need the clutch, but the trimamp has pins for it.

Well you do need the clutch. If the clutch doesn't engage the motor doesn't drive the cable.

 

[airguy]

The clutch is run by a solenoid - which is an electromagnet.

Whose clutch and servo? Not Dynon's....

 

[colinrasmussen]

Look at my first post - Bendix servo.

 

[airguy]

Look at my first post - Bendix servo.

Sorry, missed that part. Sitting down and shutting up now.

 

[colinrasmussen]

Sorry, missed that part. Sitting down and shutting up now.

No worries I'm getting the sense that the servo might be getting a little long in the tooth and not responding as quickly as the Trim Amp expects. Changed the malfunction time setting from 16 to 32 (1/16 sec increments), and that seemed to solved most of the problem. Doesn't spit out a warning when using electric trim on its own, and it was doing a lot better when the autopilot was engaged.

 

[Rhino]

Well you do need the clutch. If the clutch doesn't engage the motor doesn't drive the cable.

Yeah, I don't know why Dynon says you don't.

 

[colinrasmussen]

Yeah, I don't know why Dynon says you don't.

Could be their servos don't have a clutch. In this case the trim servo I have is the pre-existing Bendix 272 which does have a clutch.

 

[colinrasmussen]

So it turns out Dynon has created a wiring scheme for Bendix 272 servos that is different than what's in the installation manual. Got in touch with them and they sent me the diagram. My installer said would have been nice if they would tell dealers about these things. Going to get my avionics shop to make the changes and see if it solves the issues.