How much torque do I need for ailerons?

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.

Why does airspeed matter so much?

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.

How does the servo horn length affect the torque needed?

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.

What is the difference between analogue and digital servos?

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.

ModelCalcs

Servo Torque Calculator — Control Surface Sizing

Size the servo for your control surface from its dimensions, airspeed and linkage.

Surface Length (mm)

Span of the control surface along its hinge line

Surface Chord (mm)

Width of the surface, hinge to trailing edge

Airspeed (km/h)

Max/cruise speed — torque scales with its square

Max Deflection (degrees)

Full surface throw angle each way

ApplicationSets the linkage friction allowance

Speed RequirementTarget servo transit speed (informational)

Supply VoltageServo power supply voltage (affects rating to buy)

Servo TypeDigital holds position with more authority

How We Calculate This

This calculator estimates the aerodynamic hinge moment on a control surface, then converts it to the torque your servo must supply through its linkage. It is an engineering estimate, not a guarantee — friction, hinge gaps and surface mass-balance all vary in practice.

Method

Hinge moment (oz-in) = Length × Chord² × Speed² × Deflection ÷ 430,000 (length and chord in inches, speed in mph) — the validated empirical RC formula, in which load rises with the square of both chord and airspeed.
Servo torque = Hinge moment × (Servo-horn arm ÷ Control-horn arm) — the linkage ratio.
× Application allowance for linkage friction and binding (steering and flaps run higher).
Recommended = Required × Safety margin (default 1.3).
Result shown in kg-cm and oz-in (1 kg-cm = 13.887 oz-in).

Sources: empirical hinge-moment formula (RCUniverse / Radio Control Info); hinge-moment theory H = ½·ρ·V²·S·c·Ch; kg-cm↔oz-in = 13.887.

Frequently Asked Questions

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Last updated: June 2026

All calculations are estimates. Always verify quantities before purchasing materials.