Servo Torque Calculator

Courtesy of Chuck Gadd

Over the years we’ve provided access to “Chuck Gadd’s” easy to use Servo Torque Calculator.  Although the pivot table was based on solid aircraft engineering principles and formula, the equation was simplified for use in model airplanes.

Pattern, Scale and Acrobatic R/C Aircraft Flyers loved it for quick bench marks on the various projects they worked on. The formula has proven itself over and over for the past 15 years to work very well for R/C Aircraft Modelers.

The self-calculating pivot table was removed based on potential miss application … it may not be accurate for full-sized aircraft!  If you’re in need of this type of data for a full-size aircraft application, we suggest that you contact a professional aeronautical engineer.

Servo Torque Calculator - Formula Used;

The maximum torque requirement does not always occur at full deflection. This calculator determines the torque at every control position, from 1 degree to the maximum deflection specified.

The result is the max torque found and the position of the control surface when the max torque was reached.

The formula used to calculate the torque is as follows:

Torque (oz.-in) = 8.5E-6 * (C² V² L sin(S1) tan(S1) / tan(S2))

                        Where:

§  C = Control surface chord in cm

§  L = Control surface length in cm

§  V = Speed in MPH

§  S1 = Max control surface deflection in degrees

§  S2 = Max servo deflection in degrees

Reducing the servo deflection from the default 60 degrees is similar to using ATV / Dual Rates to reduce the control throws. If you vary the servo deflection from the normal 60 degrees, you will see that using "Dual rates / ATV" to set the proper control surface deflection greatly increases the load on the servo.

Note the following assumptions:

·   The angle of incidence of the wing, stab, or fuse is zero (relative to the airflow).

·   Angular velocity and acceleration of the aircraft is zero.

·   Air flow may be modeled using Bernoulli's equation for dynamic pressure.

·   Conditions are: sea level, zero humidity, moderate (~55 F) temperature.

·   Control linkages have zero offset at hinge line and are perpendicular to horns at neutral.

·   Control mechanisms are frictionless and surfaces are mass-balanced.

·   The wing, stab, fuse, and control surfaces are basic scale shape.

·   No aerodynamic counterbalances are used. (Account for these manually, if desired.)

·   The pushrods are longer than the servo and control horns.

Please note:

o    The calculations are completely theoretical. No empirical "tweaking" has been done.

o    The assumptions (except #6) should generally yield conservative (high) predicted torques.

o    Extreme control throws are probably not practical at high speeds.

o    This model is best used for comparisons. No guarantees are made of its validity.

o    Maximum required servo torque may occur at LESS than maximum throw.