uby wrote:
It was helpful, but the calculations are for a very small surface (rudder). The force on the surface can never vary lineraly.
What is not linear?
I had some old figure lying around showing the results of measurements of the hinge moment coefficient for a plain full span hinged flap where the flap chord is 25% of the total chord. On the vertical axis is the hinge moment coefficient, on the horisontal axis the control surface deflection is shown and the different curves shows the results for different wing angle of attacks. If I remeber correctly the flying surface was rectangular with a NACA009 symmetrical airfoil.
As you can see the hinge moment coefficient varies almost linearly with control surface deflection.
The hinge moment can be calculated from the following formula:
Moment = 1/2 * air density * (air speed)^2 * (control surface chord)^2 * (control surface length) * hinge moment coeffiecient.
In order to simplify your calculations you can use the calculator available at [link=http://www.csd.net/~cgadd/eflight/calcs_servo.htm]http://www.csd.net/~cgadd/eflight/calcs_servo.htm[/link]. The results of this calculator are in good agreement with results obtained from the figure below.
One warning: The calculator available at
http://www.csd.net/~cgadd/eflight/calcs_servo.htm seem to underestimate the torque by almost a factor of 2. I haven't checked why.
I agree with Dick Hanson that the error margin for simplified calculations will be quite big.
/Red B.