[NSRCA-discussion] DS 8411sa Servo Oscillation on Rudder

[JonLowe at aol.com]

I'm running a JR 8411sa on rudder on my Aggressor, pull-pull, using the setup 
that Troy Newman recommends (ss control line cable, NMP links at the servo 
end, and MK links at the rudder, etc.). For the first 75 or so flights, I had no 
issues.  Then, the servo started self oscillating around neutral, even with 
the engine not running, on the ground if the rudder stick was bumped.  The 
surface buzzes back and forth about 1/4", until damped out by grabbing the 
surface.  Thinking it was a servo pot going bad, I changed out the servo.  After two 
or three flights, same problem appeared.  Then thinking it must be a natural 
frequency issue in the linkage exciting the feedback loop in the servo, I 
tightened up on the pull-pull somewhat.  It wasn't loose before (these cables, as 
Troy says, don't stretch at all), but I increased tension somewhat.  That 
helped for about 3 flights, then the problem reappeared.  These servos are new 
8411's, self converted to SA with a new plastic gear package.  I've gone thru the 
linkage completely, and find no evidence of anything amiss. I've changed 
nothing in the setup from the first flight. 

I believe I read somewhere in the past, about some 8411's doing this because 
the feedback loop in the servo was too tight, and could be corrected by 
sending them back to Horizon.  Can anyone shed some light on this?  I'm about to 
install an 8611 on rudder, but would like to track down this issue with the 8411s.

Thanks.

Jon Lowe
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