Propellers: Thrust and Efficiency, Slip, Blade Angle, Constant-Speed Operation and Asymmetric Blade Effect
This domain tests the propeller as a thrust-producing device: blade angle, pitch, slip, advance ratio, the constant-speed unit, and the asymmetric effects that produce yaw. Items ask for the direction of the effects and the reason for them.
Thrust and efficiency
- Thrust equals mass flow times the change in velocity. A large mass of air accelerated slightly is more efficient than a small mass accelerated a lot.
- Efficiency η = (thrust × TAS)/shaft power. It is zero at zero TAS and falls again at very high TAS.
- Blade element theory: each section produces lift and drag, and the resultant has thrust and torque components.
Pitch, slip and advance ratio
- Blade angle is measured from the plane of rotation. Geometric pitch is the forward distance in one revolution if the blade screwed through solid air.
- Effective pitch is the actual advance per revolution; slip = geometric pitch − effective pitch.
- Advance ratio J = V/(nD); the constant-speed unit works to a blade-angle schedule against J.
Constant-speed operation
- The governor senses RPM and moves the blades: coarse (high) pitch at high power and high speed, fine (low) pitch for take-off and low speed.
- Feathering stops rotation and minimises drag after an engine failure; the beta and reverse ranges are used on the ground.
Asymmetric blade effect
- At high alpha the down-going blade, on the right for a clockwise propeller, meets a greater angle of attack and produces more thrust on that side, giving yaw to the left; right rudder is required.
- Torque reaction, slipstream rotation and gyroscopic precession add their own yaw and pitch effects.
Worked example
Geometric pitch 90 in, 2400 rpm, TAS 160 kt. Advance per minute = 160 × 101.3 = 16,208 ft; per revolution = 16,208/2400 = 6.75 ft = 81 in. Slip = 90 − 81 = 9 in, about 10% of the geometric pitch. If the aircraft slowed with RPM unchanged, the advance per revolution would fall and slip would increase.
Traps
- Slip is not slipstream; the two are unrelated.
- A constant-speed propeller holds RPM, not thrust; thrust still varies with speed and blade angle.
- Fine pitch gives high RPM and good acceleration at low speed but limits high-speed performance.
- P-factor reverses if the propeller rotates the other way.
- Efficiency is zero at zero TAS because no useful work is done.