Thrust and propulsion
It is tempting to try to divide the conventional aircraft propulsion systems into two neat categories; propeller and jet. Real propulsion devices, however, do not always fall into such simple compartments. In particular, gas-turbine propulsion covers a wide range from turbo-props to turbo-jets. To simplify matters, we shall look first at the two ends of this spectrum; by considering propeller propulsion at one end, and simple turbo-jet propulsion at the other. Later on, we shall look at the intermediate types such as turbo-fans and prop – fans, and also some unconventional systems.
At one time, it looked as though the propeller was in danger of becoming obsolete. Since the early 1960s, however, the trend has been reversed, and nowadays nearly all subsonic aircraft use either a propeller or a ducted fan. Even the fan has lost some ground to advanced propellers, and we shall therefore pay more attention to propeller design than might have seemed appropriate a few years ago. It is worth noting, that in 1986, half a century after the first successful running of a jet engine, 70 per cent of the aircraft types on display at the Farnborough Air Display were propeller driven.
The blades of a propeller like those of the helicopter rotor can be thought of as being rotating wings. Since the axis of rotation of the propeller is horizontal, the aerodynamic force produced is directed forwards to provide thrust rather than upwards to generate lift. The thrust force is therefore related to the differences in pressure between the forward – and the rearward-facing surfaces of the blades.
Fig. 6.1 The flow past a propeller in flight
In the process of producing this pressure difference, the propeller creates a slipstream of faster-moving air. In Fig. 6.1, the dashed lines represent the streamlines that pass through the tips of the propeller. In three dimensions we have to imagine a stream-tube that encloses or surrounds the propeller disc. Downstream of the propeller, this surrounding stream-tube roughly defines the boundary of the slipstream. The rate of change of momentum of the air within this stream-tube gives a good indication of the overall thrust.