– Propeller Pitch

The Purpose of the Propeller

The purpose of the propeller is to convert the engine torque into axial thrust, or propwash. To provide the necessary force to propel the aircraft forwards, the prop displaces a large volume of air rearwards. Newton’s third law is obeyed by the equal and opposite reaction force of prop thrust acting in a rearward direction. How well the propeller achieves this is measured by the prop’s efficiency. This is determined by a number of factors, which either improve or reduce the propeller’s efficiency. These factors include the pitch, blade angle, diameter, solidity, number of blades, tip speed, drag and the location of the prop in relation to the engine’s nacelle or the fuselage, chord variation along the blade, the shape of each blade element and the prop tips. Also included are the lift and drag coefficients, which are a function of the angle of attack of the propeller blades.

All these factors affect the propeller’s absorption of engine power and its ability to convert the propwash (working fluid) into thrust and will be considered in turn throughout this book, followed by the forces acting on the propeller.