Propellers

A propeller blade is a rotating wing. Its blade angle is between the blade's chord and the plane it turns in. Its angle of attack is between the chord and the air it meets, which depends on blade angle, rpm and airspeed. The tip travels much faster than the root, so blades are twisted, with less angle toward the tip, to keep the angle of attack about the same along the blade. Positions along a blade are blade stations, measured in inches from the center of the hub.

Kinds of propeller

• Fixed pitch: one blade angle. A climb propeller (low angle) or a cruise propeller (high angle).
• Ground adjustable: the angle is set on the ground.
• Constant speed: a governor changes the blade angle in flight to hold the rpm the pilot selects.
• Feathering: on multi-engine aircraft, blades can turn edge-on to the airflow (about 90°) so a failed engine's propeller stops windmilling and makes little drag.
• Reversing: turboprops and some others can go past flat to a negative angle for braking on the ground.

How a governor holds rpm

Flyweights spun by the engine push against a speeder spring that the pilot's control sets. If the engine overspeeds, the flyweights fly outward and move a pilot valve that changes oil flow to increase blade angle; the bigger bite slows the engine. If it underspeeds, they fall inward and the blade angle decreases. When the two balance, it's on speed.

Care

Inspect blades for nicks, cracks and erosion. Small nicks in aluminum blades are dressed with files to smooth, shallow curves within the manufacturer's limits; afterwards the blade is checked for track (each tip in the same plane) and the propeller for balance. A mechanic can do minor propeller repairs, but major propeller repairs and alterations go to a propeller repair station. After a propeller strike, follow the engine manufacturer's instructions, which usually mean a teardown inspection of the crankshaft and gears.