Low-performance aircraft have ‘fixed props’ that are milled from one piece of metal or wood and are simply bolted to the engine’s crankshaft.
Almost all higher-performance aircraft have ‘constant speed’ props, sometimes misnomered ‘variable pitch’, these days.
Constant speed props have a complex mechanism in the hub that lets the pilot set the desired RPM for the prop and the mechanism varies the pitch of the propeller’s blades to maintain that RPM.
The old ‘variable pitch’ props had a much simpler mechanism, often electric, for the pilot to set the angle of pitch of the propeller blades, not the RPM of the engine. Props are set at fine pitch for take off, then the throttle is pushed forward to takeoff RPMs and nudged forward to maintain it as the aircraft rolls, then pulled back to coarse pitch for cruise and the engine RPM would be set to cruise with the throttle.
It’s not inexpensive for constant speed props. A constant speed prop can cost more than half what the engine costs. Constant speed props require overhaul at intervals like 1500 hours, and they’re constantly checked on preflight walkaround for signs of wear like oil leaking out of the hub, or blades that wiggle a tiny bit when you tug on their tips.
This lets a pilot push prop levers forward to takeoff and the engine turns full rpm with the prop at flat pitch, and the constant speed mechanism gradually increases the pitch to maintain rpm as the throttles are pushed forward and the plane accelerates into the wind. With a constant speed prop the engine produces RPM for full takeoff power at the beginning of the roll and maintains it through lift off and into the climb. A fixed pitch prop, the engine doesn’t produce full takeoff power until late in the takeoff roll.
When cruise altitude is reached, the pilot pulls the throttles back to cruise power and the prop levers back to cruise RPM, and the props move to a coarser pitch for better cruise performance at lower RPMs. On final approach for landing, after the throttles have been pulled back, the prop levers are pushed forward so full RPM so full power will be available if a ‘go around’ is needed.
So, this is a benefit of ‘feathering’, or changing the pitch on the blades for routine flying.
Some, probably most, constant speed props can vary the pitch way past ‘coarse’ to ‘feathered’ if an engine is shut down or otherwise fails, so that the prop is streamlined with the wind and makes less drag. This is so the remaining engine can be more effective for climb out or to maintain altitude.
Pilots of multi-engine aircraft are taught to accelerate to Vmc, minimum controllable airspeed with an engine out, before they rotate and take the weight off the gear. They learn phrases like ‘dead foot = dead engine’, since they’ll almost instinctively push the rudder on the side of the remaining engine to keep the plane from yawing hard into the dead engine. They’ve learned to pull the throttle back on the dead engine first, to verify that they got the dead foot thing correct, and then to pull the prop lever back past the detent to feather it.
Feathering the prop on the dead engine keeps it from ‘windmilling’ and adding the drag from both the prop and the engine compression. A windmilling prop vs feathered can make the difference between a shallow climb and a slow descent with the remaining engine at full power.
Turbines don’t have much ‘compression’ at slow speeds like pistons do, so they often feather their props as part of the shutdown procedure so that they won’t windmill violently in the breeze or from prop or jet blast from nearby aircraft.
Aircraft that are used for multi-engine training usually have ‘unfeathering accumulators’ that will move the prop out of feather, let it start ‘windmilling’, and the engine will restart itself smoothly when the ignition is switched on and the mixture and throttles are pushed forward.
Without the accumulators, restarting an engine with a feathered prop by cranking the engine is kinda scary and puts a lot of stress on the engine mounts as the prop slowly comes out of feather and paddles the air. Aircraft that will seldom be used for single engine practice typically don’t have the defeathering accumulators and wouldn’t restart a dead engine with a feathered prop in flight.
Airplanes with fixed pitch props ‘love the ground’ on takeoff and then suck up more fuel at cruise because the props’ pitch is a compromise between takeoff and cruise performance.
A variable pitch or constant speed prop can be adjusted to give the best takeoff power _and_ economical cruise.
Author – G Saunders
