Propeller theory (basics)

Propeller Theory — Foundations

A propeller is essentially a rotating wing that produces thrust instead of lift. The aerodynamic principles are identical — only the geometry of application differs.

Operating principle

Each propeller blade is a small airfoil. In rotation:

• The blade accelerates air rearward (Newton III).
• → Thrust forward (reaction).
• Simultaneously the blade produces torque resistance that the engine must overcome.

Terms

Angle of attack at the propeller

The relative wind at the propeller blade is the vector sum of:

1. Rotational velocity (tangential).
2. Aircraft forward speed (axial).

Consequence:

• At low forward speed (take-off): relative wind mainly tangential → small blade α → little thrust per revolution.
• At high forward speed (cruise): more axial component → larger α → more effective energy transfer.

Tip speed

Tip speed of the propeller:

V_tip = π · D · n

with D = diameter, n = rotation (Hz).

Example C172 (D = 1.9 m, n = 2400 RPM = 40 Hz):

• V_tip = π × 1.9 × 40 = 239 m/s ≈ 0.7 Mach at MSL.

Above Mach 0.8 wave-drag effects arise → efficiency drops drastically. Therefore propeller RPM is limited.

Propeller types

1. Fixed-pitch propeller

• One fixed geometry — no adjustable blade angle.
• Efficient only at one speed: typically optimised for cruise or take-off, never both.
• Cruise-optimised: good at 110 KTAS, worse in climb.
• Climb-optimised: good at take-off, worse in cruise.
• Example: Cessna 152, older PA-28.

2. Constant-speed propeller

• Blade angle automatically adjustable via governor.
• RPM stays constant across the forward-speed range.
• Efficiency high over a wide speed range.
• Pilot operates via prop lever (separate from throttle).
• Example: Cirrus SR22, DA-40, Bonanza, propeller airliners.

3. Feathering propeller

• In emergency (e.g. engine failure in a twin): blades parallel to wind → minimal drag.
• Important in multi-engine for asymmetric thrust effects.

Propeller effects (in flight)

These four propeller effects influence flight behaviour:

1. Torque effect

• Engine rotates propeller clockwise (viewed from pilot).
• Newton III: aircraft tries to rotate counter-clockwise (roll left).
• Result: slight roll tendency to left at high power.
• Compensation: right rudder + aileron.

2. Spiral slipstream

• Propeller throws air spirally rearward.
• This spiral flow hits the vertical fin at an angle → produces left yaw.
• Result: yaw tendency to left, especially at low speed (take-off).
• Compensation: right rudder.

3. P-factor (asymmetric blade effect)

• At high α (climb): rising blade side (right) has larger effective α than descending (left).
• → more thrust right, less left → left yaw.
• Stronger at low speed + high α.
• Compensation: right rudder.

4. Gyroscopic precession

• Rotating propeller acts like a gyro.
• Pitch-up (yoke back): produces right yaw (90° ahead).
• Pitch-down: produces left yaw.
• Stronger on tail-draggers (take-off from three-point to two-point).

Propeller efficiency

η = T · V / P

with T = thrust, V = TAS, P = shaft power.

• Max η at design speed: typ. 0.85-0.90.
• At low speed: η drops (slip large).
• At high speed: η drops (tip-Mach effects, AoA too small).
• Constant-speed prop: η constantly high over wide range.

Propeller limits

• Vne due to propeller: at very high speed propeller spins free → thrust becomes negative (windmill).
• RPM limit: over-RPM → structural stress on prop hub, noise.
• Manifold pressure: with constant-speed prop → MAP not over limit (POH).

Cross-reference

• P-factor and spiral slipstream also affect directional stability (see lesson "Directional Stability").
• Propeller icing and carburettor icing: Subject 050.