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

[jonlowe at aol.com]

Jim,
This only manifests itself at neutral, and both cables are tight.  
Nothing is slipping, and I don't use ferrules.  In this particular 
case, I don't see how ackerman would be involved.

Jon

-----Original Message-----
From: jivey61 at bellsouth.net
To: nsrca-discussion at lists.nsrca.org
Sent: Thu, 24 Aug 2006 7:21 AM
Subject: Re: [NSRCA-discussion] DS 8411sa Servo Oscillation on Rudder

Jon
Read this about pull-pull.
This will also happen if your cable ends slip. If you put the cable 
through the ferrule just one time it may be slipping. Bad geometry will 
cause this also.
 
Jim Ivey
 
 
 
http://members.cox.net/bdfelice/Ackerman/ackerman.htm
----- 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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