[NSRCA-discussion] Control surface/linkage setup

[Woodward, Jim R (US SSA)]

Instead of setting the arm at 90 degrees then trying to work equal throws from there, instead, set the servo arm position at the center of the control surface travel.  For instance, if you set the arm at 90 degrees, adjust your pushrod to center the aileron control surface, then measure throws (like with a Jerry Budd laser meter), you'll find that the up and down throws probably do not match.  90 degrees on the servo arm is just one variable in the geometry of the setup.

Instead. Set your ATV to 140% (or the max that will not bind the surface), then measure up/down on the ailerons.  Move up/down and take note.  The Up travel might be more than the Down travel (or pull direction has more throw than push direction). This is because one direction is gaining mechanical advantage, and one side is losing it.  If you move the servo horn "ahead" in the direction the gain was occurring and reset the pushrod length to center the surface then measure again.  You will find that the total throws are closer together.  Now that the "result" of the servo motion is equal,

Work this method until the throws are equal up/down and you will not have to use the Left/Right ATV or differential settings to achieve equal throw.
Thanks,
Jim W.


From: nsrca-discussion-bounces at lists.nsrca.org [mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of J N Hiller
Sent: Wednesday, November 10, 2010 8:32 PM
To: General pattern discussion
Subject: Re: [NSRCA-discussion] Control surface/linkage setup

Be careful selecting servo arms. A believe there are 3 different spline drives in use. When selecting aftermarket arms get the ones specifically intended for the servo brand you are using such as 'JR' or 'Futaba'. Someone did a really nice write-up on this some months back but I don't remember where I dead it.
Jim Hiller

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org [mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of Michael Cohen
Sent: Wednesday, November 10, 2010 5:12 PM
To: NSRCA
Subject: [NSRCA-discussion] Control surface/linkage setup

I have a question for the list about control surface/linkage setup.  I remember a K-factor article which mentioned making the servo arm is as close to a perfect 90 degrees as possible at neutral.  This is what the little 1-2-3-4 on the stock plastic servo arms are for.  You simply rotate the arm until 90 degrees is achieved.  However, with aftermarket heavy duty plastic or metal arms, is there any other solution other than sub-trim?  I am concerned about the "stock" arms deflection under load and like to use the aftermarket products, but on some servos on my airplane, this leads to a lot of sub-trim, which is also not preferable.  Thoughts?

Mike Cohen
D4
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