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

[Scott Smith]

I believe this may be an issue for you after 75 flights due to the rudder
hinges freeing up.  All you need is a little drag put back into the system.
I did this with my last airplane by putting a small rubber band around both
rudder cables about 6" aft of the servo (per Eric H.) and the problem
completely went away.

Try it!

Scott
  -----Original Message-----
  From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of
JonLowe at aol.com
  Sent: Thursday, August 24, 2006 7:38 AM
  To: nsrca-discussion at lists.nsrca.org
  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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