Balancing servo throws

from: Setting up your Airplane

 Preferred servo orientation to maximize stiffness:

Linkage setup:

• dimensions "A" and "B" are determined by what the plans call for with respect to the total movement of the control surface (up/down, left/right, etc.) and how much travel your servo has.
  
  set radio for normal, high-rate throws.
• at control surface, mount the control horn so that distance "A" is as large as possible -- about 1" or so for large aircraft, 3/4" or so for smaller airplanes; commensurately larger for giant scale or smaller for park flyers.
• deflect the control surface to the maximum the plans call for, and measure the (horizontal) distance that the hole in the control horn moves. Call this dimension "C".
• Using radio, center servo arm.  run the servo to its extremes and measure the distance some particular hole in the servo arm moves. Choose the hole that is closest to dimension "C" that you measured at the control surface. The distance from the middle of the servo attaching screw to that hole is dimension "B".
• fix control surface in neutral position, and measure the distance between the hole on the servo arm and the hole in the control horn with the servo in its neutral position.
• manufacture a straight rod and attach it to those points.
• mount the servo, making sure that the servo arm is perpendicular to the control rod, as shown, and that the control horn is also perpendicular to the rod.
• turn on your radio again and run that channel to its extremes and measure the throws at the edge of the control surface. Now reference the plans for your model. Are the throws too much, too little, or just right? If too much, increase dimension "A" (or decrease "B").  If too little, reduce "A".
• the throws in each direction should be identical. If not, check that the setup is as shown above, with 90 degree angles at both ends. You may have to adjust the neutral on your servo, or bend the arm at the control horn, but try hard to equalize those throws!
• What you’re doing here is maximizing the utilization of your servo’s power. By using the entire rotation of your servo arm to cause the control surface to move its maximum distance, you’ve used the mechanical advantage of the servo to its fullest extent - all the power your servo has is being used.
• If you merely install your servo and rod without regard to the total desired control throws, and the distance "B" at the servo is longer than at the control arm, you will have to reduce the maximum servo arm rotation allowed in order to keep the control surface movement at the correct distance. Result: poor utilization of your servo's power—you’re only using part of it, and resolution suffers as well.
• If distance "B" at the servo is shorter than at the control arm, the opposite ensues, and you’ll most likely overdrive the servo, perhaps causing binding or other equally devastating problems, or you won’t get the full throws desired at the control surface.