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Gas turbine development
The idea of using a gas turbine to produce jet propulsion was developed quite independently by Whittle in England and von Ohain and others in Germany in the 1930s. Neither Whittle nor the other pioneers actually invented the gas turbine; the concept had been around for some time. Their genius lay in realising that such an apparently unpromising and inefficient form of engine would provide the basis for high speed and high altitude flight.
Whittle filed his original jet-propulsion patent in 1930, and his experimental engine first ran in April 1937. Von Ohain’s records were lost during the war, but it is thought that a von Ohain/Heinkel engine actually ran in the previous month. This engine was however a preliminary experimental arrangement running on gaseous hydrogen.
The first jet-engined aircraft was the Heinkel He-178 shown in Fig. 6.17. Using a von Ohain engine, its maiden flight was on 27th August 1939, some 21 months before that of the British Gloster/Whittle E28/39.
Some gas turbines use a centrifugal compressor as shown in Fig. 6.18. This form was used on early British jet engines and is similar to the type used in superchargers. Air enters the rotating disc at the centre and is spun to the outside at increased pressure and a considerable whirl speed. A diffuser downstream, consisting of fixed curved blades or passages, is used to slow the flow down by removing the whirl component. The reduction in speed is accompanied by a further rise in pressure.
Both the Whittle and the von Ohain engines used a centrifugal compressor, but by 1939 rival British and German teams were already working on axial compressors which offer higher efficiency and reduced frontal area.
As shown in Fig. 6.19, an axial compressor consists of a series of multi – bladed fans separated by rows of similar-looking fixed stator blades. The moving blades are used to increase the pressure and density rather than the speed. The stator blades remove the swirl and produce a further pressure rise.
The rise in pressure obtainable through a single row or stage is not as great as for a centrifugal compressor, and many stages are required. Despite a trend to higher overall pressure ratios, modern engines are able to use fewer stages because of improved design.
The earliest successful turbo-jet with an axial compressor was the Junkers Jumo 004 which was developed by a team led by the little-known Anselm Franz. In 1942 this engine was used to power the Messerschmitt Me 262, the
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Centrifugal
compressor
Fig. 6.20 A turbo-prop engine
The design illustrated uses centrifugal compressor stages. For turbo-prop engines, it is still common practice to use at least one centrifugal compressor stage.
The gearbox and accessory drives represent a significant proportion of the total engine weight (Illustration courtesy of Rolls-Royce pic)
world’s first jet-propelled combat aircraft (Fig. 2.18). The Jumo engine, with its axial compressor and annular combustion chamber, was much more like a modern engine than the Whittle or von Ohain engines, and was developed quite independently, with no knowledge of Whittle’s work.
Whittle’s heroic efforts are well documented in his book Jet (1953) and in a later book by Golley (1987). A full account of the early jet engines is given by Glyn Jones in The Jet Pioneers (1989).
Although the axial compressor is always used for large turbo-jet engines, smaller engines and those designed for turbo-prop propulsion often have at least one centrifugal stage (see Fig. 6.20, and Fig. 6.21). The centrifugal compressor is simpler, and considerably cheaper than the axial type, and in applications such as helicopter propulsion, the increased diameter is of little significance.
