Servo Torque Calculator — Control Surface Sizing

It depends mostly on airspeed and chord, because hinge moment rises with the square of both. A slow sport plane (≈80 km/h) with shallow 50mm-chord ailerons may only need under 1 kg-cm at the servo, while a fast 150 km/h model with deep ailerons can need several kg-cm. Enter your actual length, chord, speed and deflection — and always keep the 30% safety margin above the calculated minimum.

Aerodynamic hinge moment scales with the SQUARE of airspeed (H = ½·ρ·V²·S·c·Ch). Doubling speed quadruples the load on the surface, so a fast model needs roughly four times the servo torque of a slow one for the same surface. This is the single biggest driver of servo sizing, which is why it is a required input here, not an afterthought.

What matters is the linkage ratio — servo-horn arm divided by control-horn arm — not the servo horn on its own. A longer servo horn (or shorter control horn) gives more throw but means the servo works at a mechanical disadvantage against the hinge moment, so it needs MORE torque. A shorter servo horn needs less torque but gives less throw. Set both arms equal (ratio 1.0) for a neutral baseline.

Digital servos update their position far more often (≈300Hz vs ≈50Hz), giving better holding power, faster response and less deadband, at the cost of higher idle current draw. For primary control surfaces on anything beyond a basic trainer, digital servos are the sensible choice. Both are rated the same way (kg-cm at a stated voltage), so this calculator’s torque figure applies to either.