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

[jivey61 at bellsouth.net]

Jon
The prob i had with this servo on rudder was the top bearing would wear because of cable tension. I solved this with a JR top bearing brace. I don,t think they make it anymore. Search the disc archives for Ackerman and all the post will show up for pull-pull.
Jim Ivey
  ----- Original Message ----- 
  From: JonLowe at aol.com 
  To: nsrca-discussion at lists.nsrca.org 
  Sent: Thursday, August 24, 2006 7:38 AM
  Subject: [NSRCA-discussion] DS 8411sa Servo Oscillation on Rudder


  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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