Propeller
Core concepts
Propeller fundamentals: blade element theory, blade angle, geometric pitch, effective pitch and slip; the relationship between pitch, rpm and forward speed; thrust, torque and the blade forces - centrifugal, thrust bending, torque bending and aerodynamic twisting. Fixed-pitch, two-pitch and constant-speed propellers; the governor, its flyweights and speeder spring, and how it controls blade angle to hold rpm. Feathering, reverse pitch, and the safeguards against inadvertent operation. Propeller vibration and balance, tracking, blade damage limits, lightning-strike damage, ice protection, and the inspection requirements including NDT and the inspection after a ground strike. Also synchronising and synchrophasing of multi-engine installations, and the maintenance records and overhaul intervals.
Traps in questions
- Blade angle is measured between the blade chord and the plane of rotation, not the axis of rotation.
- Increasing blade angle (coarser pitch) reduces rpm at constant power; the governor's flyweights move outward as rpm rises.
- Centrifugal force acts along the blade and is the dominant root stress; it is independent of thrust.
- Feathering is not the same as reverse - one stops rotation, the other produces negative thrust.
- Slip is not aerodynamic inefficiency to be minimised at any cost; it always exists.
Worked example
A propeller has a geometric pitch of 60 in and advances 54 in per revolution. Slip = 60 − 54 = 6 in, i.e. 6/60 = 10 %. If the engine turns at 2400 rpm, the theoretical speed is 60 in × 2400 = 144 000 in/min ≈ 219 km/h, and the actual speed is 90 % of that, about 197 km/h.
Rule of thumb
Slip % = (geometric pitch − effective pitch) ÷ geometric pitch × 100.