Dan Tandberg, MD

Most of us mechanically adjust the throws of our control surfaces by trial and error. We pick a hole on the servo horn and on the control surface horn and try it out on the model. If it isn’t right, we move the linkage to a different hole and then try that. This works well for simple things like rudder and elevator, but what about something more complicated like flap and aileron control horns that your are making yourself. These surfaces are typically hinged on the top of the wing (ailerons) or the bottom (flaps) and the question of where to best put the control horn holes becomes more complicated. You can calculate the actual resulting change in angle with trigonometry, but this is enough trouble that few actually do it. Here is a practical, low-cost mechanical solution that I thought I’d share.

First, tape a piece of graph paper to your building board and draw a horizontal centerline. Turn on your transmitter, plug the actual servo you are going to use into the receiver, and measure the servo throw angle away from zero with a protractor. You can then mark this angled line on the graph paper as shown.

Next, drill about a dozen holes precisely 1/8" apart in two Popsicle sticks as shown. (You can tape them together during drilling to insure that they’re identical.) Make the holes the same size as the wire you are going to use for your trial linkage (I used 0.046" wire here.) You’ll also need a couple of T-pins and some music wire that both snugly fit your holes, so get these together before actually drilling. Cut the wire about 1/8" longer than the linkage you are trying to model. Bend a 90-degree angle at each end of the wire and cut the bends to 1/16" (the thickness of the Popsicle sticks.

Next, pin one of the sticks to the left edge of the graph paper to represent the servo horn.

Place the bent tips of the linkage wire into the holes you want to try (here 8/8^ths and 6/8^ths are shown) and then T-pin the stick representing the control surface horn to the board.

Now can move the servo horn stick and see how the control horn stick angle will change. Experimenting with different combinations of holes will give you the desired control surface throw, and you can then use the ratio of the distances on the sticks exactly reproduce this on your model. You can now pick the best combination of holes on stock horns. You can also drill new control horn holes at the exact place that gives the desired result before mounting them on your model.

You can easily use a protractor and ruler to measure the resulting angle and displacement of the control surface horn that results from a given angle of servo movement. When you are done, put the paper, sticks, and pins into a folder or envelope to use next time.

For added precision, you can make several photocopies of your protractor and use two of these for your layout. Just cut off the sticks as shown and mark their centers.