servo question

[JOddino]

Hi Troy,
The reason I asked this question is that I heard a guy claiming his neutrals
were more positive if the servo horn was perpendicular to the pushrod at
neutral.  I see you recognize the system becomes stiffer the closer you get
to the servo arm being parallel to the pushrod.  That is why we used 180
degree servos on landing gear.  So the servo arm perpendicular to the
pushrod doesn't improve holding power at neutral.

In the old days before computer radios we used to offset the servo neutrals
on purpose to get more up than down ailerons for example.  Now we can set it
anywhere we want so the angle isn't critical from that point.

As far as expo (non-linearities) caused by linkages, it is insignificant
(with reasonable linkages) compared to the expo we add.  Forget about linear
distances, look at angles.  The change in lift is a function of the angle of
attack, an angle.  The angle of attack is a function of the surface
deflection, an angle.  The surface deflection is a function of the servo
rotation, an angle.  Therefore the airplane responds to the servo angle not
the linear change of pushrod distance.  It takes a special linkage to get a
linear relationship between servo angle and control surface angle but even a
setup that looks bad really isn't too bad after we adjust end points and add
expo.  It seems to me it is more important to make sure the angles and push
rod lengths are the same (speaking of two elevator servos or two servos
coupled to one surface)  rather than worrying about a non-linearity that is
difficult to measure.  It seems to me it is much easier to set the servos
arms  90 degrees to the case than to try to figure out the angle that will
make them 90 degrees to the pushrod.  In fact I'm not sure how you'd do it
other than by cutting and trying.  I normally strive for both, perpendicular
to the case and the pushrod. However, I'm building a new plane with the
servos on their sides in the wing and stab where I can't have both so I'll
have to look at this a little closer.  I'm still betting it is
insignificant.
Jim
----- Original Message ----- 
From: "Troy Newman" <troy_newman at msn.com>
To: <discussion at nsrca.org>
Sent: Friday, August 01, 2003 5:02 PM
Subject: Re: servo question


> Oh come on Jim....
>
> For the rest of the guys...The servo moves in a  rotational motion...So
the
> linear distance traveled by a pushrod attached to the arm or wheel will be
> greatest right off of center and will reduce the amount of linear travel
of
> the rod as it rotates around. A linkage not setup this will have a funny
> expo like knee in it...But it will be as the arm passes through the square
> position so it would be like adding expo on only one side of the servos
> travel...and reverse expo on the other side of travel.......
>
> Also it could be way up high on the travel of in a funny middle spot...and
> you will get more throw one side than the other....
>
> This sq to the pushrod also gives the servo best mechanical advantage
> through the range...as the servo gets to the endpoints say full travel up
> elevator..the highest force is being applied the surface...and the servo
arm
> is rotated in such a way to reduce the moment arm on the servo by the
> pushrod pushing back.....The arm travels in an arc and the arm gets closer
> to the center of the servo as it rotates off of "center"....This moment
arm
> is leverage for the pushrod to pushback on the servo.....So if you reduce
> this distance when the forces are higher then it will make the servo have
a
> mechanically better shot and holding its position.
>
> did I get it right Jim?
> anything to add Jim?...I'm by far not the expert on this stuff...
>
> Ok Still TMI....
>
> TN
>
>
>
>

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