The propeller or airscrew
Of the various systems of propulsion, the propeller has been most used in the past, and for many types of aircraft it is likely to be a long time in dying. More and more gas turbines, rather than reciprocating engines, are being used for driving propellers but that does not in any way affect the aerodynamic problems involved. It is right, therefore, that we should give brief consideration to those problems. Some of them also are common to those of the helicopter, some too to the blades of compressors and fans and turbines, and these are further reasons why we should consider them.
The object of the propeller is to convert the torque, or turning effect, given by the power of the engine, into a straightforward pull, or push, called thrust.
If an airscrew is in front of the engine it will cause tension in the shaft and so will pull the aeroplane – such an airscrew is called a tractor. If, on the other hand, it is behind the engine, it will push the aeroplane forward, and it is called a pusher (Figs 4F and 4G, later). In Fig. 4H (later) there is an unusual combination of both pusher and tractor propellers.