Propellers

081 Principles of FlightChapter 4

Propeller fundamentals

Key fact

Thrust = component of aerodynamic force parallel to rotational axis. Propeller torque opposes engine torque (acts in plane of rotation). Reference section at 75% of blade radius; most effective thrust area 60–90% of tip radius (max at ~70%).

Key fact

Blade twist (root to tip): maintains constant AoA along blade length. Helix angle = blade pitch angle − blade AoA (angle of advance). Geometric pitch — theoretical forward distance per revolution at 0° AoA. Effective pitch — actual distance advanced. Slip = geometric − effective pitch.

Formula — Propeller efficiency

η = (Thrust × TAS) / (Torque × RPM)
η = Thrust HP / Brake HP

Most efficient blade AoA ≈ 3–4°.

Fine vs coarse pitch

Key fact

Fine pitch — small blade angle, significant twist; more drag, lower L/D, higher ROD. Coarse pitch — large blade angle; better cruise efficiency. Fixed pitch (FPP): optimal AoA at one flight phase only — coarse setting often used to prevent windmilling in cruise.

Key fact

RPM lever forward (take-off): finer pitch, max power, more drag. Constant-speed prop (CSP): governor maintains optimal AoA — RPM increase → coarser pitch; RPM decrease → finer pitch.

Windmilling & feathering

Key fact

Windmilling: engine torque less than prop torque; drag highest condition — greater than feathered or non-rotating prop. AoA can become negative (high-speed idle descent). Feathered (~90° blade angle): minimum drag; improves OEI handling. Prop runaway: first action — close throttle.

Forces on propeller blades

Key fact

Five forces: (1) Centrifugal turning moment — toward fine pitch; (2) Aerodynamic turning moment — toward coarse pitch (CP ahead of torsion axis); (3) Centrifugal — blades pulled outward; (4) Thrust bending — tips bend forward; (5) Torque bending — opposite to rotation.

Propeller effects (all props in flight)

Effect	Description
Torque reaction	Roll opposite to prop rotation; highest at low speed + max power
P-factor (asymmetric blade)	Downgoing blade more thrust → left yaw (clockwise prop); worst at high AoA/RPM
Slipstream	Spiralling airflow hits fin → yaw; fin placed as far aft as possible
Gyroscopic precession	Pitch/yaw inputs on spinning prop → reaction 90° ahead; noticeable tailwheel take-off

Key fact

Counter-rotating (two engines): cancels torque and gyro effects. Contra-rotating (one engine, two rotors): same. Left engine = critical engine for clockwise props (greater yaw arm). Tailwheel: all four effects on take-off; tricycle: slipstream + torque only.

Exam trap

Icing reduces prop efficiency ~20%; occurs at low RPM, root/thick sections first. Wing icing is more critical than prop icing. More blades → power absorption ↑, efficiency ↓, noise ↓, solidity ↑.

Design limits

Key fact

Solidity = total frontal area of blades ÷ disc area. Tip speed to supersonic → noise ↑, efficiency ↓. Blade following preceding blade too closely → efficiency loss per blade.