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

[Grow Pattern]

It is actually a 1" o-ring. 

If you have a slight "pinch" in the pull wires it has the same effect. If your rudder is very free and the cable-exits offer no friction at all, it can oscillate. 

I also moved the clevises in on the servo arm and it had the effect of taking away the death-dance, as I call it. (I still think that is what has been killing Impacts in the air!!!!).

An 8311 does better on the rudder and an 8611 is a no-brainer as Ed said.


Eric.
  ----- Original Message ----- 
  From: Scott Smith 
  To: NSRCA Mailing List 
  Sent: Thursday, August 24, 2006 7:57 AM
  Subject: Re: [NSRCA-discussion] DS 8411sa Servo Oscillation on Rudder


  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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