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  #1  
Old 04-21-2018, 12:22 PM
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Default AP servo mechanically locked

Preflighting today found my ailerons locked in place. All power to the A/C was off. Troubleshooting led to finding the AP roll servo mechanically locked. Disconnecting the servo arm from the aileron belcrank resumed normal operation of the ailerons. Servo itself remained locked until power was turned on. At that point the servo seemed to act normally, no error codes. I removed the servo. Since removing the servo, I canít replicate the mechanical locking. Itís going back to Dynon.

Has anyone else experienced mechanical locking of a servo without power? I should note that the AP was used on the previous flight to this one, and functioned normally.
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  #2  
Old 04-21-2018, 01:21 PM
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Question

Could the linkage have moved (bumped) into an over-center condition?
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  #3  
Old 04-21-2018, 03:11 PM
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Default Travel limit bracket was in place

Gil, the travel limiting bracket was in place. The servo locked in about the neutral position of both ailerons and servos. Part of me wonders if the limit bracket screws may be path of the question, but I used the Dynon provided screws and spaced with a washer. Iím sending it as installed to Dynon.

Iím worried they wonít find root cause.
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RV-9A tip-up, O320H2, Ellison, Dynon D180
First Flight 2017apr23 - Phase 1 is dragging....
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  #4  
Old 04-22-2018, 12:44 AM
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Default

I will bet, a small chocky frog, that your servo has been corroded internally from condensation build up, and has seized.

It has happened before, and on ROLL.

Make sure you alert this to Dynon's AP engineer Lawrence Doan.

I am not sure how you stop it. These days they have a metallic sticker covering the hole in the end shield of the motor to stop drips getting in, but condensation is hard to stop.

All that you can do is put a drain hole in to let moisture out.

Having done Beta Testing for Dynon in the past, this was not part of the test plan, but I did participate
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  #5  
Old 04-22-2018, 04:24 AM
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Default Corrosion

David, corrosion is possible. My hanger isn’t the best when it comes to rain intrusion, and while my airplane doesn’t get wet, it’s humid, especially with the temp swings we’re having day to night. The servo was manufactured in 2010 if I’m reading the motor date code correctly, and it has no sticker over the motor shaft hole. I will contact Dynon as you suggest.

Update: servo Locked up overnight sitting in my kitchen in a box. I didn’t imagine the whole thing. Last evening, it moved freely, this morning locked tight. There is the smallest amount of gear lash in the arm, suggesting the output shaft is free, but beyond that.....

On the other hand it was kinda nice hand flying it this smorning, not having the temptation to use the AP.
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RV-9A tip-up, O320H2, Ellison, Dynon D180
First Flight 2017apr23 - Phase 1 is dragging....
2011-2018 VAF contributor
Dog is my copilot, rest in peace 2015APR28. I miss that dog.

Last edited by rjcthree : 04-22-2018 at 10:53 AM.
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  #6  
Old 04-22-2018, 10:26 AM
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Default

Quote:
Originally Posted by rjcthree View Post
David, corrosion is possible. My hanger isnít the best when it comes to rain intrusion, and while my airplane doesnít get wet, itís humid, especially with the temp swings weíre having day to night. The servo was manufactured in 2010 if Iím reading the motor date code correctly, and it has no sticker over the motor shaft hole. I will contact Dynon as you suggest.

Update: servo Locked up overnight sitting in my kitchen in a box. I didnít imagine the whole thing. Last evening, it moved freely, this morning locked tight. The is the smallest amount of gear lash in the arm, suggesting the output shaft is free, but beyond that.....

On the other hand it was kinda nice hand flying it this smorning, not having the temptation to use the AP.

Just a suggestion - when you pack it up, put labels and signs and notes that are imp0ossible for the ďunpackerĒ at Dynon to ignore that say ďdonít move the output arm until on a test benchĒ. Since youíve managed to recreate the problem, wouldnít it be nice for the technician to see it that way, and not have someone in the mail room grab it and ďunbind itĒ without thinking......

Curious about the root cause for this!

Paul
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  #7  
Old 04-22-2018, 11:04 AM
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Default Thanks and ?

Paul, good suggestion. I see cardboard surgery in my future! It beats cleaning the garage.

David, what in the great googly-moogly is a chocky frog? Iím leaping to a Harry Potter reference?
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Rick 90432
RV-9A tip-up, O320H2, Ellison, Dynon D180
First Flight 2017apr23 - Phase 1 is dragging....
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Dog is my copilot, rest in peace 2015APR28. I miss that dog.
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  #8  
Old 04-22-2018, 01:33 PM
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THis type of situation is exactly why these servos have sheer screws. I havenít seen this before on a Dynon servo, but I have with another brand, although not a tight lock up, just significant drag. Whoever at Dynon gives you the RMA, make sure hey know the symptoms and make sure they note in the RMA notes that it sometimes does and sometimes doesnít lock up.
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  #9  
Old 04-22-2018, 03:43 PM
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Default Lock torque

I can’t tell you exactly what the breakaway torque was, but while it was still installed I opted to remove the push rod from the servo arm with a torque wrench. While not perfect in technicqe, as it’s a click type, it’s calibrated and the removal torque was greater than the 25 in-lb setting, and the servo did not move. The other end connecting to the bell crank was already free. I admit I didn’t lean on it too hard while it was still connected. I wasn’t going to fly it that way even if it broke loose.

It’s boxed, pushrod protected from bumping.
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Rick 90432
RV-9A tip-up, O320H2, Ellison, Dynon D180
First Flight 2017apr23 - Phase 1 is dragging....
2011-2018 VAF contributor
Dog is my copilot, rest in peace 2015APR28. I miss that dog.
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  #10  
Old 04-22-2018, 09:44 PM
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https://www.cadbury.com.au/products/...ry/freddo.aspx
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