RC Servo Torque Calculator
Calculate required servo torque for RC vehicles and aircraft based on control surface size, speed, and hinge moment.
Choosing the right servo torque for an RC model ensures reliable control surface movement under aerodynamic load. An undersized servo results in flutter, sluggish response, or complete control surface blowback at speed.
Hinge Moment Formula: The aerodynamic force on a control surface creates a hinge moment that the servo must overcome.
Hinge Moment (oz-in) = 0.5 × ρ × V² × S × C_h × Chord_fraction × Arm_length
For practical RC calculations, a simplified approach is used:
Simplified Torque Formula: Required Torque (oz-in) = Surface Area (in²) × Speed Factor × Deflection Factor × Safety Margin
Speed Factors:
| Aircraft Speed | Factor | Typical Models |
|---|---|---|
| Slow (under 30 mph) | 0.15 | Trainers, parkflyers |
| Moderate (30–60 mph) | 0.30 | Sport planes, scale |
| Fast (60–100 mph) | 0.50 | Pattern, EDF jets |
| Very fast (100+ mph) | 0.75 | Turbine jets, pylon racers |
Deflection Factor:
- 20° max deflection: ×0.8
- 30° max deflection: ×1.0
- 45° max deflection: ×1.3
Safety Margin: Always use 1.5× to 2.0× the calculated torque. Servos operating near their maximum torque stall, overheat, and strip gears.
Worked Example — Aileron on a sport plane:
- Control surface: 12" span × 2.5" chord = 30 in²
- Speed: moderate (factor 0.30)
- Deflection: 30° (factor 1.0)
- Raw torque: 30 × 0.30 × 1.0 = 9.0 oz-in
- With 1.5× safety: 9.0 × 1.5 = 13.5 oz-in minimum
- Recommended servo: 20+ oz-in digital servo
Common Servo Sizes:
| Category | Torque Range | Weight | Use |
|---|---|---|---|
| Micro | 10–20 oz-in | 5–10 g | Parkflyers, micro quads |
| Mini | 20–50 oz-in | 10–20 g | Sport planes, small helis |
| Standard | 50–150 oz-in | 40–60 g | Scale, 3D, pattern planes |
| Large | 150–500 oz-in | 60–80 g | Large scale, turbine jets |
| Giant | 500+ oz-in | 80–150 g | Giant scale, 1/4 scale |
Digital vs. Analog: Digital servos provide faster response and higher holding torque at the cost of slightly more current draw. For any speed above “slow,” digital servos are strongly recommended.