Does the servo have to be centred between the horns or can it be placed to the side of the servo tray?

regards
Tim

Doesn't have to be, but it helps to simplify the geometry involved in getting your throws to match both ways.

Let me assume you are talking about a pull-pull system on a rudder.

I would suggest that the servo be mounted on the centerline of the fuselage if at all possible. There will be no question this will work without concerns. However if you must mount the servo 'off center' within the fuselage then hook the pull-pull up with something like dental floss and make a little measuring tool from cardboard that clamps to the fin and is positioned behind the rudder such that you can see the amount of throw of the rudder from neutral to right and from neutral to left (that is make a rotation guage). Move the rudder to extreme left and mark the rudder position. Now move the rudder to the extreme right and mark the rudder position. Move the rudder to neutral and mark the neutral position. Now see how much difference there is between the right and left throw.

Sometimes you can adjust 'throw control' or 'differential' in the transmitter to balance the amount of throw from right to left to compensate for the servo being off center. Make the transmitter adjustments until you obtain equal movement in both directions.

To visualize what is actually happening if the servo is mounted off of the center line of the fuselage draw two circles one above the other separated by a few inches. Draw a line horizontally through the center of each circle. The lines represent the servo and pull-pull arms. The circles represent the possible positions of the ends of the arms. Now draw lines that represent the cables. When the servo is centered within the fuselage the servo will rotate the pull-pull equally in both directions.

Now draw another two circles but this time place the bottom one further off to the left or to the right. Draw the horizontal lines through each circle and then draw the cables attaching to those horizontal lines. You will see that the movement of the pull-pull arm will not be the same when being rotated clockwise then counterclockwise (both from neutral). If this 'natural' imbalance in movement can be compensated for with the transmitter then this may work fine. If the servo is too far from the center of the fuselage the system may prove too difficult to use.

Since the servo arm and pull-pull connections rotate on center points (they scribe circles) it causes problems the further off center the servo is placed. In your drawings rotate the control arms within the circles and see what I mean. At some point moving the servo off center too much will not provide adequate control.

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Probably not. Depends on the use of the airplane more than anything else. How often will you need symmetrical rudder response?
It's more important that the pull wires leave the servo wheel at the same angle , and arrive at the surface horn at the same angle.
IOW, the neutral position at the servo should oompensate for any position offset of the servo in the fuselage.
Dead on centerline, a 90 degree angle between the cables and the servo neutral. Offset one side, the servo neutral should be adjusted.

As seen above, centering the servo is a far easier solution than playing with angles and neutral points.

Are you not able to place the servo in the center of the fuse for some reason?

What plane are you setting up?

Theoretically it's simple. In practice, it's a little different. Servos are pretty stupid and don't know which way the airplane is flying, so they don't care.

If the servo is offset at angle "alpha", then offset the servo arms and control horns by angle alpha, too.

The trick is, making yourself drill crooked holes for the control horn. This is actually pretty easy to make if you use a 4-40 rod threaded through a hardwood insert in the rudder.

Thanks for the input - I was thinking about this because it is for a narrow fuz and I cant get the elevator pushrod past the rudder servo arms but I think I will use a tiller bar which I can drop below the level of the servos , driven by a single arm off the rudder servo.

Isn't Ackerman great !!

Tim

Or install both servos inline and raise the rudder servo. Dead simple to install and maintain.

http://www.geocities.com/roger_forgues/PullPull.html
http://webpages.charter.net/bobad/pullpull.htm

check out these links
Cheers
Hisham