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Aviation History
1950
1950 - 1932.PDF
FLIGHT, 16 November 1950 TWO-EDGED BLADES 7/ie Axial v. Centrifugal Gas-turbine Controversy in the Light of a Formidable Potential Production Problem AT the recent S.B.A.C. Show at Farnborough it was /^L reassuring to find ample evidence that Britain has not only established a lead in turbine-powered air- craft of all types but, technically speaking, is still "on her toes" and determined to maintain that lead. Technical superiority is of great importance—possibly the greatest— but quantity is also a vital factor. In view of the troubled political situation and the vastly extended rearmament pro- gramme, possible requirements must be subjected to the closest study and production must be planned with ex- ceptional foresight. The wider tactical and strategic considerations, however, are beyond the scope and purpose of this article. If it be accepted—and it can scarcely be questioned—that the majority of Service aircraft will henceforward be powered by gas-turbine units, consideration must be given to the production problem entailed. Are production techniques sufficiently well developed? Can supplies of the necessary metals and materials be reasonably ensured? Do adequate production facilities and tools for the purpose exist or could present resources be rapidly expanded? Are the necessary trained personnel available or could suitably experienced workers be rapidly recruited? The answer to these and similar questions can only be known at relatively high levels. To narrow the question it would seem advisable at this stage to devote some consideration to the production problem as it applies respectively to axial and centrifugal types of turbine units. Operationally there would appear to be a valid case for each type for different duties. Any attempt to evaluate the virtues and vices of the types must be rather inconclusive in view of the large number of factors to be taken into consideration. At the present level of development, how- ever, it may fairly be conceded that the axial type possesses a relative advantage as regards compression ratio, thermal jfficiency, specific fuel consumption and overall diameter. For the centrifugal type, it is generally accepted that it is relatively less weighty, of shorter length, more robust, less susceptible to icing, requires fewer man hours to produce and is less costly. In the early days the centrifugal compressor was rapidly advanced by drawing upon the accumulated research and experiment which had been carried out on superchargers for piston engines. Now, however, the advocates of the centrifugal compressor maintain, with some justification, that a disproportionate amount of money and research is being devoted to the axial compressor, to the neglect of further development of the centrifugal type. There wpuld seem to be no inherent reason why centrifugal compressors should not be capable of further advancement and, indeed, no less an authority than Prof. J. Ackeret, of the Zurich Polytechnique, has stated that an efficiency of 90 per cent is quite feasible with a relatively high-pressure unit. Probably, as in other spheres, the two schools will fight their technical battle, with ultimate improvements in both types as the outcome. From the point of view of the Service planners the type that, at any period, offers the highest performance for the AT the present stage of gas-turbine develop- r\ ment the axial type offers certain adiojiages; but what of its production under war conditions, when the building of this class of unit might well require the staggering figure of nearly 5% million precision- made blades a month ? The question now arises as to whether the supply departments are giving this vital aspect the attention it so urgently merits. This photograph suggests that Mr. Strauss, the Minister of Supply, must be well aware of the technical implications of the problem. He is seen examining a Metrovick axial-flow compressor at the School of Gas-Turbine Technology. aircraft in which they are interested will command their support. If it be a high-speed fighter or intercepter they will press for the axial on the grounds of its lower frontal area as a means of reducing drag. Should it be a bomber, the lower specific consumption of the axial, again, will be regarded as essen- tial in order to obtain maximum range. But the supply departments have to reconcile these requirements with pro- duction facilities, material supplies and available man-power. Such a task is hard enough in attempting to meet known conditions; when a decision has to be made to cover the requirements of a hypothetical war situation it be- comes still more hazardous. A major false step may cost untold wealth, occasion a crippling delay in the prosecution of a war or even lose a campaign. In the last analysis—that is, under the stress of actual war conditions—the factor of over- riding importance is producibility. No matter how cleverly conceived or how technically advanced a weapon may be, it is of no military value unless it can be produced in quantities commensurate with the scale of modern operations. It is not unreasonable to enquire if this question of pro- ducibility, as it applies to aircraft turbines, has received sufficient consideration. In an emergency what would be the numerical requirements? Presume, as in 1944, a pro- duction of aircraft of all types of 26,000 per annum or 2,000 per month. At this monthly rate it could be assumed that 500 would be powered by piston engines and 1,500 by turbines. Even if, for the purpose of this estimate, the fact that large bombers and transports will require four or six power units is ignored and it is assumed that 50 per cent are powered by a single unit and 50 per cent by two units, the monthly production would need to be 2,250 units. A relatively modest axial-flow turbojet will have a 12-stage compressor and a two-stage turbine. Each row, rotor and stator, of the compressor could have 80 or more blades and each row of the turbine 100 or more—say a total of 2,400 blades per unit. Thus, if all the turbines had axial-flow compressors a monthly requirement for 5,400,000 blades must be envisaged, Production would necessarily have to
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