The gas turbine

The gas turbine was originally developed primarily as a practical device for providing jet propulsion, since it was realised that this would overcome the speed limitations imposed by propeller propulsion. The other factor that prompted its original development, was the realisation that it would operate satisfactorily at high altitudes. As with the high speed ducted fan described earlier, the air slows down as it enters a gas turbine in high speed flight, which means that the air pressure and density increase at inlet. This increase can com­pensate for the low atmospheric air density at high altitude. Both high speed and high altitude flight have obvious advantages for military aircraft.

A major feature of the gas turbine is the considerable amount of power that it can produce at high forward speeds. The effective power produced is the product of the thrust and the forward speed. For example, a large turbo-jet engine giving 250 kN (approx. 50,000 lb) of thrust would be producing around 60 megawatts (approx 80,000 bhp) at 240 m/s (approx 500 mph). The most powerful piston engines produced no more than about 2.5 megawatts (approx 3400 bhp). On the first experimental turbo-jet flight by the Heinkel He-178 (Fig. 6.17) in 1939, the engine was producing about as much equiva­lent power at maximum speed as the most powerful production piston engines of that time.

Other advantages of the gas-turbine engine compared to reciprocating engines are the high power-to-weight ratio, the virtual absence of reciprocating parts, and simpler less frequent maintenance.