How much throw should you use on each control surface of your new model airplane?
You are getting ready to do a maiden flight on a new model airplane. Where should you put the center of gravity? How much throw do you need on each control surface?
If the manufacturer has recommendations for these, follow them. They will be in the building manual or written right on the plans. They have flown the specific model before. They know about any quirks that it might have.
But what if you do not have any guidance for your specific model? What if the model is your own design? That is where these recommendations come into play. They are good and reasonably safe values to use in most model airplanes. The idea is to get into and out of the air safely. After that you will want to adjust the values up or down as needed.
I have talked about this one before, so I will be brief here. The center of gravity (CG) determines the pitch or longitudinal stability of the airplane. You want to be about 25% of the way back from the leading edge of the wing.
You need to take into account wing taper and sweep. Polyhedral wings also complicate the formula.
Rudder Control Throw
Rudders are normally designed for 45 degrees of throw in either direction. That is usually how much they can deflect without hitting the elevator.
Use 30 degrees for the elevator. You will also need a small amount of up elevator when neutral, say 5 degrees or so, to account for the nose down pitching force or moment.
Ailerons are similar to elevators. Use 30 degrees. If you have aileron differential, then try 30 degrees up and 15 degrees down.
When you are taking off, you want to increase the amount of lift without increasing the amount of drag very much. A reasonably safe amount of flaps to use is 15 degrees. If you want to be conservative about this, I would use a little less, like 10 degrees.
The right amount of flaps depends also on how much thrust you have available from the motor. If you have lots of power, then you can get away with using a little more flaps during take-off.
The main goal during a landing approach is to lose energy quickly and controllably. Use flaps during a landing to increased drag.
There is one big caveat: you need to be able to apply full power and climb out with landing flaps. You see, in an emergency aborted landing situation, forgetting to raise the flaps is very common.
As a starting point, use 45 degrees deflection for landing. After you know the airplane better, you might be able to increase the flaps up to 90 degrees. With the typical overpowered model airplane, I’ll bet you will end up with lots of flaps for landing.
Measuring the Angles
I picked the angles I recommended above partly because they are relatively easy to measure. You can buy plastic drafting triangles with 30, 45, 60, and 90 degree angles. To measure 15 degrees, just eyeball half of a 30 degree angle.