Copyright © 2026 Merge Solutions, LLC. All rights reserved except for the Government Purpose Rights granted under the identified contract: FA70002520008. v0.1.49 · 2026-09-01
Aircraft design teaching tool

Prop Picker: Blade Element Momentum Theory (BEMT)

Estimate propeller thrust, torque, shaft power, and efficiency from geometry and an approximate airfoil polar. Explore how RPM and forward velocity determine advance ratio.

Propeller definition

Helical distance advanced in one revolution.

Electrical system

Estimated performance at selected RPM

Blade-element momentum estimate across forward velocity.

Thrust (lbf)Shaft power (hp)Efficiency (%)
Advance ratio, J—
Thrust—
Shaft power— hp— ft·lbf/s— W
Current—
Propulsive efficiency—
Geometric pitch speed—

Propulsive efficiency map

Thrust map

Power required map

Current required map

The solver uses 28 radial elements for the operating point and curve, 10-element previews for the heat maps, approximate embedded airfoil polars, annular momentum theory, and Prandtl hub/tip loss.

Motor and battery selection

Brushless motor

LiPo battery

Changing cells or cell voltage updates Bus voltage. Generic motor-size limits are editable teaching estimates; use manufacturer limits for final hardware selection.

Pack voltage-
Estimated loaded RPM-
KV needed at selected RPM-
Required current-
Available current-
Estimated endurance-
Motor and battery assessment will update with the selected operating point.

Equations used by the solver

SI units are used internally. RPM is converted to revolutions per second, and KEAS is converted to true airspeed before the blade-element calculation. Equations are rendered from LaTeX as embedded SVG.

Rotation rate and angular velocity.
Equivalent-to-true airspeed conversion.
Advance ratio.
Local blade angle from pitch in distance per revolution.
Linear taper normalized at 75% radius.
Local relative airspeed.
Local inflow angle.
Section angle of attack.
Approximate lift polar with a smooth stall cap.
Approximate drag polar.
Lift and drag on one blade element.
Element thrust from all blades.
Element torque from all blades.
Annular momentum balance used to iterate induced velocity.
Prandtl tip- and hub-loss correction.
Integrated thrust, torque, and shaft power.
Electrical input power required by the motor.
Battery current required; infinite when motor efficiency is zero and shaft power is positive.
Propulsive efficiency.
Nondimensional thrust and power coefficients.
Standard-atmosphere density.
Geometric pitch speed without slip.

Preliminary estimate: Embedded analytic airfoil polars are not measured Reynolds/Mach tables. Installation effects, detailed three-dimensional viscous effects, structural limits, and high-Mach corrections are omitted. Validate final designs with measured polars or higher-fidelity analysis.