Roll servo- dead or something else?

HFMan

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Joined
Aug 28, 2019
Messages
80
The last couple weeks, I've been getting a constant "Roll Slip" message at the top of my screen(s). This was at ALL times when the AP was engaged, even in very smooth conditions where very little input was required on the roll. Yet the AP was behaving normally on the roll, as if there was nothing wrong- the AP was managing the roll axis per usual, even in very turbulent conditions. I checked the info screen, and the numbers for roll force (I believe that was the readout, not entirely sure) did seem very sensitive and high. But I can tell you there is NO drag on my roll controls, it is all very smooth and no different than the day I installed the AP.

Yesterday, while AP was engaged on the way home, suddenly just lost roll authority from the AP- with no error. It simply stopped working on the roll channel. After landing, I ran the AP servo test, and could definitely hear the roll servo engaged, but of course no movement on the roll axis. I then went to the servo status screen, and could move the aileron stick axis back and forth and see the roll servo position value update correctly.

Anybody have an idea what is going on there?
 

Marc_J._Zeitlin

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Sep 24, 2007
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284
Location
Tehachapi, CA 93561
Sounds like you broke the shear screw in the servo, due to high roll forces. I'd check your roll trim to see if you were way out of trim, causing high servo forces and eventual breakage of the shear screw. While the servo (with a broken screw) wasn't strong enough to move the control surfaces, moving the controls might be able to move the control arm on the servo and hence the shaft just due to friction. Maybe.

Did you see any particularly high roll forces in either direction when the A/P stopped working in roll?
 

HFMan

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Joined
Aug 28, 2019
Messages
80
Sounds like you broke the shear screw in the servo, due to high roll forces. I'd check your roll trim to see if you were way out of trim, causing high servo forces and eventual breakage of the shear screw. While the servo (with a broken screw) wasn't strong enough to move the control surfaces, moving the controls might be able to move the control arm on the servo and hence the shaft just due to friction. Maybe.

Did you see any particularly high roll forces in either direction when the A/P stopped working in roll?
This aircraft doesn't have any roll trim (it's an LSA, so very light forces). Again, I don't think high roll forces have ever been a factor in this plane. After the autopilot roll servo quit working, I didn't feel any unusual roll forces. In fact, the airplane just did a very subtle drift off course- I didn't really notice the problem for probably 30 seconds.
 

HFMan

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Aug 28, 2019
Messages
80
Removed the roll servo this weekend... turns out something is stripped out in the gear train. Shear screw is fine- so this one is a puzzler. Hard to imagine how a 1320 lb airplane could cause enough torque to blow out a gear train, yet not break the shear screw.
 

RandomSquawk

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Joined
Jul 17, 2020
Messages
34
Removed the roll servo this weekend... turns out something is stripped out in the gear train. Shear screw is fine- so this one is a puzzler. Hard to imagine how a 1320 lb airplane could cause enough torque to blow out a gear train, yet not break the shear screw.
The shear screw is supposed to be the weakest link in the system. That's it only job. Aircraft mass is irrelevant. Roll input force is what matters, and still that shear pin should give way first. How many hours in service? Did Dynon have any insight to add?
 

HFMan

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Aug 28, 2019
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The servos have been in the aircraft 16 months, with about 240 hours on them in that time. I got the servo back from Dynon yesterday, with no further explanation of the fix to the servo- just a tick next to a box with the repair type description of "Electrical/Mechanical component failure not caused by installation."

In any event- BIG KUDOS to Dynon for repairing this swiftly and zero cost to me. I'm a big fan!
 

RandomSquawk

Member
Joined
Jul 17, 2020
Messages
34
The servos have been in the aircraft 16 months, with about 240 hours on them in that time. I got the servo back from Dynon yesterday, with no further explanation of the fix to the servo- just a tick next to a box with the repair type description of "Electrical/Mechanical component failure not caused by installation."

In any event- BIG KUDOS to Dynon for repairing this swiftly and zero cost to me. I'm a big fan!
Nice!
 
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