How it works

Each part of a propeller blade has a cross-section similar to that of an aero­foil; in fact, in some cases exactly the same shape of section has been used for both purposes. The thrust of the propeller is obtained because the chord at each part of the blade is inclined at a small angle (similar to the angle of attack of an aerofoil) to its direction of motion. Since, however, the propeller is both rotating and going forward, the direction of the airflow against the blade will be at some such angle as is shown in Figs 4.6 (overleaf) and 4.7 (later). This will result in lift and drag on the blade section, just as it does on an aerofoil. Actually in a propeller we are not so much concerned with the forces perpen­dicular and parallel to the airflow, i. e. lift and drag, as the force acting along the axis of the aeroplane (the thrust force) and at right angles to the rotation (the resistance force). So the total force on the blade must be resolved into thrust and resistance forces, as in Fig. 4.7. The difference between these and lift and drag is clearly seen by comparing Figs 4.6 and 4.7.

The total torque force on the propeller blades will cause a turning moment or torque which opposes the engine torque, and also tends to rotate the com­plete aeroplane in the opposite direction to that in which the propeller is revolving. When the propeller is revolving at a steady number of revolutions per minute, then the propeller torque and the engine torque will be exactly equal and opposite.