Servo Load

This above graph is from the Model Aircraft Performance Calculator software.
Ex I/4 scale light plane. Aileron area 25 x 3 , scale speed 40 mph, surface deflection 30 Deg.
      Required torque = 22 Oz-In
     1/4 Scale fighter with the same size control surface and deflection  and a scale speed of 120 mph
     Required torque = 226 Oz in.

     Reduce the Fighter control deflection to 5 deg,
     Required torque = 38 Oz-In

Theory
 

The calculated results compare favorably to the practical test results results published in 1933 by NACA (Report #278),  for a 18"x1.8" control surface at 40 mph.
20 Deg =   6.4 Oz-In
10 Deg =   3.2 Oz-In
5 Deg = 2 Oz in

And to practical test results published May 1998 in Model Airplane News(tm)

If the Ch values derived in NACA technical report 441 are applied the the "hinge moment" equation (Clark Y with sealed aileron) good agreement is also found.

Formulae for the torque load are theoretical,  one that is based on the "hinge moment" equation follows;

 H = 1/2*p*V^2*C^2*L*Ch           (McCormick)

p = Air density
V = airspeed
C = chord
L = length
Ch = Coefficient of the hinge moment

Ch consists of three parts               b1*a1 + b2*a2 + b3*a3

b1*a1 = the angle of attack of the control surface * hinge moment factor b1
b2*a2 = the angle of deflection of the control * hinge moment factor b2
b3*a3 = the angle of deflection of the trim tab * hinge moment factor b3

b2*a2 is the only parameter of interest for this program.

The value for torque;

Force = H/A, where A is approx 1/3 surface chord.

The formula used for this program is of the form conceived Craig Tenney  and has been modified agree with measured results and comparisons to other methods of calculating servo toque;

 Toque = ((Sin(Angle of surface deflection) (Chord-In) ^ 2 * Length-In * (mph) ^ 2)) * 0.0000085