Propeller-Electric Brushless Motor Sizing Tool

Welcome, guys!

This tool designs a propeller for your flight conditions and estimates the parameters of a matching brushless motor — stator dimensions and KV rating. Use it to explore how thrust, RPM, diameter and material affect the final design.

Use the baseline values below as a starting point, or enter your own.

Inputs

Please use realistic values

The design algorithm assumes physically achievable conditions. Extremely high thrust from a small diameter (e.g. 20 N from a 5-inch propeller at 3 000 RPM) will produce unrealistic geometry and misleading numbers. A good starting point:

  • Thrust: 0.5–5 N for 5–10″ props; up to 15 N for 14″+
  • RPM: 3 000–8 000 (large slow props) or 15 000–25 000 (small fast props)
  • Diameter: 5–16 inches for typical UAVs
  • Speed: 10–30 m/s at cruise

If the result looks physically impossible (chord larger than the propeller radius, efficiency above 90 %), check your inputs.

How this tool works

Blade element momentum (BEM) analysis

The propeller is divided into small radial elements. At each element, the local flow angle is found by iterating the axial and tangential induction factors until momentum and blade forces balance. Prandtl tip and root loss factors correct for the finite number of blades.

Minimum-induced-loss design (Larrabee / Betz)

Chord and twist are chosen so the slipstream velocity is constant along the blade — the classic Betz condition for minimum induced power. This is the method used by Larrabee (1979) for hand-built racing propellers.

NeuralFoil 2D polars

Lift and drag coefficients at each blade section are predicted by NeuralFoil — a trained neural-network surrogate for XFOIL. It is accurate across the Reynolds-number range typical of small UAV propellers (5×10⁴ – 5×10⁵).

Snel 3D stall correction

In the rotating frame, stall is delayed by centrifugal and Coriolis effects. The Snel correction blends the 2D post-stall lift back toward its inviscid value using a factor f(c/r) that depends on local chord-to-radius ratio.

Propeller mass estimate

Blade mass is estimated by integrating the local airfoil cross-sectional area along the span and multiplying by the effective density of the chosen material. A 2.5× structural factor accounts for root reinforcement, hub hardware, and folding mechanism — real propellers weigh more than the aerodynamic minimum.

Motor sizing — Essen's rule (D²L)

The torque capability of an electric motor scales with rotor volume D²L. Rearranging the D²L equation:

D²·L = Pout / (Co · nrps)

where Co is the output coefficient [kW·s/m³]. For small BLDC outrunners with natural convection cooling, we use Co = 300, validated against real racing motors (a 500 W / 20 000 RPM case yields a 22 × 7 mm stator — matching a T-Motor F80 Pro).

Motor KV estimate

KV = RPM / (Vpack × 0.80). The 0.80 factor accounts for voltage sag under load and mechanical RPM drop.

Accuracy and limitations

Good accuracy (5–15 % vs. wind-tunnel and flight data):

  • Propeller efficiency at the design point
  • Pitch angle and pitch distance
  • Chord and twist distributions

Moderate accuracy (10–25 %):

  • Absolute thrust and power at off-design conditions
  • Propeller mass (2.5× structural factor applied)
  • Motor stator dimensions (empirical D²L rule)

Limitations:

  • Uses 2D airfoil polars with 3D stall corrections — a common engineering approximation, not full CFD
  • Does not model blade-blade interference, tip vortices beyond Prandtl's correction, or compressibility above M = 0.7
  • Does not include vibration, flutter, or structural analysis
  • Mass and motor sizing are estimates for preliminary design — verify with real components before building

This tool is intended for teaching and preliminary sizing, not for final hardware specification.