Servo Issue on Carbon Cub

[John_K]

I had a strange experience with my Carbon Cub yesterday, when I was setting up to practice taxiing it. When I got to the field and powered the radio and plane, the elevator was in the down position and there was no up elevator. Returning back home and removing the wing, I could see the elevator servo arm was in the full-forward position, rather than centered. I unscrewed the servo arm and repositioned it in the center position. Then the elevator controls worked normally – up and down travel the same. My concern is not knowing what caused it to be full forward when the battery is connected. Have you ever seen something like this? If the servo were to do this while it was flying, it would be disastrous. Thanks for your help!

 

[Gray Harlequin]

I have had similar things happen before. One problem could be stripped splines on either the servo or the servo horn. Check to make sure the splines on either are not stripped. One time I had used a servo horn that was just slightly too large. It seemed to work fine when testing but under load would spin on the servo shaft. A more serious issue would be broken teeth on the servo gears. The other gears may jump the gap of broken teeth and seem to work normally but is very unreliable. Unfortunately in that case the servo is a goner unless you want to replace gears. (Mine was a $2 servo, so it got trashed.)

 

[Merv]

...My concern is not knowing what caused it to be full forward...

Hard to say for sure what caused the problem. My guess, you bumped the elevator during transport which caused the shift. When something like that happens, I always test the servo using the Tx or a servo tester. While the servo is moving back and forth through the full range of motion, put some pressure on it. If there is a weak spline, the pressure will expose it. Be sure to test both directions during the full range of motion.

 

[John_K]

Thanks Merv and Gray. I reached out to my instructor who has been flying RC for 40+ years. He said some of the same things you guys have, but also said if it were his plane he would replace the servo, so I ordered a new servo today which is cheaper than a crash from the failed servo.