Propellers
Overview
Propellers tests blade angle, pitch, advance ratio, efficiency and the asymmetric effects that influence multi-engine handling.
Core rules
- A propeller blade is a rotating wing: it produces lift (thrust) and drag (torque).
- Blade angle is measured from the plane of rotation; geometric pitch is the theoretical advance per revolution. Slip is the difference between geometric and effective pitch.
- Advance ratio J = V/(nD), where n is revolutions per second and D is diameter. Efficiency η = (thrust × V)/(shaft power).
- A constant-speed propeller uses a governor to hold RPM by varying blade angle as power or speed changes.
- Feathering aligns the blade with the airflow to reduce drag; reverse pitch produces negative thrust.
- P-factor: at high AoA the descending blade takes a larger bite, shifting thrust to one side.
Worked example
D = 1.8 m, n = 40 rev/s, V = 100 m/s. J = 100/(40 × 1.8) = 1.39. If thrust is 3,000 N and shaft power is 400 kW, η = (3,000 × 100)/400,000 = 0.75.
Traps
- Increasing blade angle at low speed can stall the blades and reduce thrust.
- Manifold pressure alone does not set power; RPM and blade angle matter.
- P-factor depends on AoA, so it is greatest at high AoA and low speed.