Propeller Aerodynamics

Simulation

Blade element at 75% radius

Propeller blade-element velocity and force diagram A spinner, propeller blade cross-section and velocity triangle show RPM, TAS, relative airflow, helix angle, blade angle and blade angle of attack. A force triangle shows thrust, torque and total reaction. PROPELLER BLADE · AIRFLOW AND FORCES RPM TAS RELATIVE AIRFLOW THRUST + TORQUE + TOTAL REACTION Helix φ 20.8° Blade β 30.0° AOA α 9.2°
Velocity and force arrows use one scale within their respective diagrams, so their relative changes remain proportional.
Helix angle20.8°
Blade angle30.0°
Blade AOA9.2°Positive AOA · thrust + / torque +

Method

How the velocity triangle is built

01

Velocity triangle

RPM creates the rotational component. TAS creates the forward component. Together they define the relative-airflow direction.

02

Helix angle

The helix angle is formed between the plane of rotation and the relative airflow.

φ = 20.8°
03

Blade angle of attack

Blade angle is measured from the plane of rotation. A variable-pitch blade rotates around its pitch-change axis as this angle changes.

α = blade angle − helix angle 30.0° − 20.8° = 9.2°

Relationship

How the aerodynamic force is estimated

Relative speedRotational airflow and TAS combine: W² = U² + V².
Lift and dragL and D use dynamic pressure q = ½ρW². Twice the relative speed produces four times the aerodynamic force.
ComponentsThrust = L cos φ − D sin φ. The tangential component, L sin φ + D cos φ, produces torque.

Simplified blade-element model at one reference section. Lift is perpendicular to relative airflow and drag is parallel to it; thrust and torque are their resolved components. Induced airflow, slipstream, spanwise blade twist and compressibility are omitted.

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