Originally posted by Steve Campbell
Keep in mind that it is EXTREMELY important, when doing a closed-loop (push-pull, pull-pull, whatever) system, that the distance between attachment points at the servo is exactly the same as between the two rudder horn attachment points. IOW, the linkage should describe a parallelogram. Failure to do this will result in assymetrical response, cable slack, the common cold, dandruff, etc.
Steve, actually, as long as the line drawn between the two attachment points intersect the hinge point exactly on both servo end and control surface end, it's okay if the driven end moment arm does not equal the driving end. The geometry will be such that the servo arm will push on one side of the control horn exactly the same distance as it will pull on the other side. The caveat is that the servo must be sufficient distance away from the control horns so that the nonlinear effect of the two different arc radii is minimized.
Sorry for being geeky about this, I've done this with both solid pushrods and cables and haven't caught a cold because of it yet.
BTW, there was an article on the geometry of pull-pull linkages recently in one of the RC mags. Forgot which one it was.