📖 The lesson
Types of propeller
- Fixed-pitch — one-piece wood or metal, with the blade angle fixed at manufacture. It is efficient at only one airspeed and rpm. A climb propeller (low pitch) gives best take-off and climb; a cruise propeller (high pitch) gives best cruise; a standard propeller is a compromise.
- Test club — a short, wide, flat-bladed propeller used on engines in test cells. It loads the engine correctly and provides cooling airflow; it is not a flight propeller.
- Ground-adjustable — the blade angle can be changed on the ground with the engine stopped, by loosening the hub clamps and turning the blades, but not in flight.
- Controllable-pitch — the pilot can change blade angle in flight. A two-position propeller has only fine (take-off) and coarse (cruise) settings. Other types allow any angle between the stops.
- Constant-speed — a governor automatically changes blade angle to hold the rpm the pilot selects, regardless of airspeed or power. It gives near-best efficiency over a wide range. The pilot sets rpm with the propeller lever and power (manifold pressure or torque) with the throttle.
- Feathering — a constant-speed propeller that can turn its blades to about 90° when an engine fails, stopping windmilling and minimising drag. Used on multi-engine aircraft.
- Reverse-pitch — the blades can turn to a negative angle so the propeller produces rearward thrust for braking and ground manoeuvring. Common on turboprops.
Propeller location
- Tractor — the propeller is ahead of the engine and pulls the aircraft. It works in undisturbed air, and the structure behind it is in compression. Most aircraft are tractors.
- Pusher — the propeller is behind the engine/wing and pushes. It sits in disturbed air and is more prone to damage from stones and debris thrown up by the wheels, as well as being harder to cool the engine.
- Contra-rotating — two propellers on concentric shafts from one engine (or gearbox), turning in opposite directions. Torque reaction and slipstream swirl cancel, and more power can be absorbed in a smaller diameter.
- Counter-rotating — on a twin, the propellers on the two engines turn in opposite directions (e.g. one left-hand engine), which removes the critical engine problem caused by asymmetric thrust.
Propeller removal and installation
- Flanged shafts — most horizontally opposed engines. The propeller hub bolts to a flange on the crankshaft (or the flange carries studs). Dowel pins or an index mark position the propeller correctly relative to the crankshaft (for balance and for magneto timing on some engines). A wooden propeller uses a steel face plate so the bolts do not crush the wood.
- Tapered shafts — older engines. The hub has a matching taper and a key. A retaining nut pulls it on; a snap ring acting as a puller removes it.
- Splined shafts — larger and older radial engines. A front cone and rear cone centre the propeller on the shaft. The rear cone is a one-piece bronze cone; the front cone is a two-piece (split) steel cone. Check for front-cone bottoming with marking compound; fit a spacer if required. Hub contact with the rear cone should be at least about 70 %.
- Safety first: before working on a propeller, make sure the magnetos are off (P-leads grounded), mixture is at idle cut-off and fuel is off. Treat every propeller as if the engine could fire.
- Removal: remove the spinner, release safety wire or cotter pins, loosen the bolts or retaining nut, and support the propeller (large propellers need a hoist and sling). Drain oil from constant-speed hubs as required.
- Installation: clean mating surfaces; check for damage. Fit new O-rings where the hub seals against the crankshaft flange (for oil to the pitch-change mechanism). Torque the bolts or retaining nut in the specified sequence to the specified value, then safety them. Check blade track after installation.
- Spinner — streamlines the hub and directs cooling air. The spinner bulkhead (rear plate) is fitted with the propeller; the dome screws to it, aligned with the index marks. Check for cracks around screw holes and the bulkhead, and for loose screws. A cracked spinner can cause vibration and may separate in flight.
- After installation of a constant-speed propeller: run the engine to check operation, rpm range, governor response and (where applicable) feathering, and check for oil leaks.