16th May 09:46
There is a servo size calculator on Multiplex website
The poor man rule is 1 ounce of torque per square inch of control
surface. If an aileron is 1" wide and 20" long than 20 ounces of
torque should be enough.
However, the faster you go the more torque you need to move the
With all the infinite variables involved the only true way to know if
a servo is big enough is to fly the plane and see if it gets mushy or
erratic at maximum speed, if so you need a bigger servo.
The next question is how well the servo holds its position. Holding
torque and moving torque is very different. You need an servo that can
both move and hold position.
This is were digital servo are becoming increasingly popular, they
have excellent holding power but at a sacrifice of battery life.
So in the end you need a servo big enough to move and maintain a
deflected position of the control surface when the plane is a maximum
Unless the plane is going to fly faster than 60, 70 mph then 1 ounce
of torque per square inch of control surface is probably big enough.
20th May 13:55
Used the 1" x 20" aileron example at 70 mph in the calculator the
result was 16.3 ounces. My guess factor was 20 ounces
Also tried 1.25" x 24" at 70 mph the calculator result was 31.3, my
guess factor was a 30 ounces.
So 1 ounce of torque per square inch of control surface is in the ball
park for any plane not exceeding 70 mph.
I think one part of the calculator has a bad value. The default servo
deflection from center is set at 45 degrees. When I plug in 1.25" x
24" at 70 mph. 45 degree control surface deflection and 45 degree
servo deflection the result is 18.09 ounces.
But unless I am mistaken a JR servo only travels 60 degrees stop to
stop. So thats 30 degrees deflection from center. Change the servo
deflection to 30 degrees and the torque required jumps to 31.3 ounces.