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

[Jay Marshall]

Without going into a lot of control systems theory, the response of the
servo is going to depend upon the mass it is trying to move, the "stiffness"
(hinges, push rods, etc.), and the power of the motor (battery voltage). If
changing the servo doesn't fix the problem then I would check the friction
items in the path. The rubber bands sound like a good possible cure as they
will provide more damping. 
 
-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of
DaveL322 at comcast.net
Sent: Thursday, August 24, 2006 10:01 AM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] DS 8411sa Servo Oscillation on Rudder
 
Jon,
 
It is basically a "deadband" issue, which is always a compromise - tighter
deadband yields more accurate centering but increases the probability of
oscillation (especially with heavy surfaces as has been noted).
 
Since introduced, there have been some minor changes to 8411, but Horizon
should be able to increase the deadband for you.
 
Regards,

Dave Lockhart, Team JR
DaveL322 at comcast.net
 
 
-------------- Original message -------------- 
From: 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 co nverted 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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