[NSRCA-discussion] Rudder pull-pull

[J N Hiller]

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 <mailto:jpavlick at idseng.com>
To: General pattern discussion <mailto:nsrca-discussion at lists.nsrca.org>
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 <mailto:cjm767driver at hotmail.com>
To: nsrca-discussion at lists.nsrca.org
<mailto: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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