Propellers

The function of a propeller is to convert the power developed by the engine into thrust, which propels the aircraft through the air.

Propeller blades are shaped as aerofoils and produce an aerodynamic force as they rotate through the air.

Fixed-Pitch Propeller

The blade angle of a fixed-pitch propeller is set during manufacture and cannot be changed in flight.

A fixed-pitch propeller is designed to give its best efficiency at a particular combination of:

• engine RPM
• airspeed
• phase of flight

Because the pitch cannot be changed, a fixed-pitch propeller is a compromise.

It may be optimised more towards:

• take-off and climb, or
• cruise performance

but cannot provide maximum efficiency under all flight conditions.

It can be compared with a car having only one gear.

Key point

A fixed-pitch propeller is most efficient at the RPM and airspeed for which it was designed.

Fine Pitch

A fine-pitch propeller has a relatively small blade angle.

It takes a smaller "bite" of air with each revolution and places less load on the engine.

This allows the engine to develop higher RPM at relatively low airspeed.

Fine pitch is suited to: