[NSRCA-discussion] Rudder pull-pull

[Lance Van Nostrand]

Jim,
I've done this on heavier IMAC style planes but never thought it could be done light enough for pattern.  If you have thoughts on this, I am interested.
Thanks,
--Lance

  ----- Original Message ----- 
  From: J N Hiller 
  To: General pattern discussion 
  Sent: Sunday, May 11, 2008 8:58 PM
  Subject: Re: [NSRCA-discussion] Rudder pull-pull


  Lance I use a bell crank to get equal length arms and control horns. This allows continued use of full servo travel and removes the cable tension from the servo shaft and mounting grommets. The servo is connected with a short link with ball end fittings. It works well especially on elevators.

  Jim Hiller

   

  -----Original Message-----
  From: nsrca-discussion-bounces at lists.nsrca.org [mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of Lance Van Nostrand
  Sent: Sunday, May 11, 2008 5:38 PM
  To: General pattern discussion
  Subject: Re: [NSRCA-discussion] Rudder pull-pull

   

  You all are so fast on this stuff that by the time I read the post the thread is old.

   

  Ackerman exists if the rudder has control horns (as opposed to a wheel, which is very hard to build into a rudder).  The only ideal way to have constant tension is to use a symmetrical servo arm that is offset from its pivot (servo screw) the same amount as the rudder control horn attachment point.  This will work every time and is simple, however with a 1" thick rudder post and two 3/4" control horns you will need a 2.5" servo arm (1.25" each side of center) and the servo throw will only be equal to your rudder throw (maybe about 30 degrees) which gives you poor mechanical advantage and poor use of your servo.  If you use a shorter servo arm and more servo throw the cables will go slack.  The round servo wheel is a compromise by offsetting this slack tendency and letting you get more servo throw.  It's not a perfect match but it can be very close.

   

  Anyone that can do the math on this and present a complete answer can get a recommendation to advance from a shade tree aerodynamicist to an apprentice.

   

  --Lance

   

  ----- Original Message ----- 


  From: John Pavlick 

  To: General pattern discussion 

  Sent: Friday, May 09, 2008 9:02 AM

  Subject: Re: [NSRCA-discussion] Rudder pull-pull

   

  Chris,

   The only benefit that I know of is that you don't have to deal with the Ackerman angle like you do with arms / horns. The cable is always the same distance from the hinge center no matter what the angle is.

   

  John Pavlick
  http://www.idseng.com

  ----- Original Message ----- 


  From: chris moon 

  To: nsrca-discussion at lists.nsrca.org 

  Sent: Friday, May 09, 2008 9:13 AM

  Subject: [NSRCA-discussion] Rudder pull-pull

   

  Can someone explain the benefits (real or otherwise) of the pull-pull 
  wheel type rudder servo arm vs. a regular servo arm? Is there some real 
  benefit, or is it bling?
  Thanks

  Chris




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