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Aviation History
1948
1948 - 0681.PDF
MAY 13TH, 1948 FLIGHT The Art of the Aviation Engine 531 w -1-: the 24-cylinder, opposed type,liquid-cooled, sleeve-valve engine with two crankshafts. The actual manufac-turing times for each of these three types war of the order of 3,000, 3,300 and6,io° man-hours respectively, at a pro- duction rate per factory of 400 to 500engines per month. There is one very important factorwhich is not often appreciated. This concerns the ability and capacity of afirm to develop engines intensively. In other words, when buying an engine,- itis power and performance which are actually being purchased; and the firmmo't active in the technique of develop- ment will produce better performancefrom an engine of given size and weight d t!:in another firm which may offer only* iii? same performance from an engine of, say, 25 per cent larger capacity. Inten-sive development, however, always pays and finally leads to the production ofengines of lower cost. The Whittle-type jet turbine, with adouble-sided single-staac compressor and In service for many years from 1919, th? 450 h.p. Napier Lion. a single-stage turbine giving a static sea-level thrust of 5,ooolb, requires some- what less manufacturing or man-hoursthan a. 60 deg V-type, 12-cylinder, liquid-cooled engine of about 2,500 h.p.take-off rating. But the jet turbine costs -considerably more ,per pound of struc-ture weight than the piston engine. Man-hours involved in the design and", manufacture of a prototype airscrew tur- .bine with an axial compressor are from10 to- 15 per cent greater than those needed for a large 24-c\'linder, twin-crankshaft, liquid-cooled, sleeve-valve T£ pistcn-engine, assuming the same maxi-mum power for both engines to be of the 3,500 h.p. order. Cost of a prototype engine varies some-what with its size or maximum power, and complexity, but is between /io and£20 per pound of structure weight. As an example, a large liquid-cooled engineof 3,500 h.p. which weighs 3,700 1b, will cost about £15 a pound, or £55,000. Anairscrew turbine of about the same maxi- mum power will weigh much less butwill absorb more design hours and cost more per pound to manufacture inprototype form. Therefore, the total cost will be of the same order as that ofthe equivalent piston engine, assuming a weight of 29,000 lb at £20 per pound, or£58.000. The lecturer said that he had fre-quently been asked what he considered should be the average number of enginehours run per year in developing a proto- type engine and, in this connection,stated that the total running time on fee test bed and in flight before thenrst "Mark" of a prototype engine is ready for type test will be5,000 or 6,080 engine-hours in a period of three to threeand a half years, involving about ten engines. Equipment required forcalibrating and testing the components of a gat? turbineis far more elaborate and ex- pensive than any used forpiston engines. In the past three or four years, equip-ment has been installed at some of the British and American firms which runsinto hundreds of thousands of pounds sterling and tens of millions of dollars. Insome cases, the equipment was supplied at government expense, but one or twofirms provided comprehensive equipment at their own cost. A. Cdre. Banks thenmade the trenchant observation that air, in the quantity and condition requiredfor testing gas turbines, is a very expen- sive commodity. He then went on to say that, in orderto test complete and full-scale jet engines at conditions of altitude and temperaturesimulating 11,000 metres (tropopause) and minus 60 deg C, the cost of the testplant is equivalent to £20,000 for every pound of air per second capacity pro-vided for the engine. For instance, if an engine can consume, say, fifteenpounds of air per second at 11,000 metres, the total cost of the plant willbe in the order of £300,000. Subsidized Research The lecturer gave it as his opinion thatthe best and only practical way to help the aviation engine industry is for agovernment to provide its research estab- lishment with the more expensive andelaborate facilities, such as the high- altitude test tunnel and compressor andturbine test houses with an available power of, say, 40,000 h.p. Aerodynamictest equipment for investigating blade forms, etc., must be provided and, also,comprehensive facilities for combustion research. Some of this equipment alreadyexists in the research establishments and in the firms, both in Great Britain andAmerica. But America has the most complete research and testing facilitiesfor gas turbines in the world. Chief among the points made by thelecturer in his summing-up were that: (a) If successful aviation engines areto be built, the industry must have properly controlled government supportin the form of sound technical require- ments, followed by orders. The industryshould not be nationalized because of its highly competitive nature and the fact type The present-day Bristol Centaurus which produces 2,700 h.p. The Rolls-Royce Merlin which first powered our 8-gun fighters in 1936 and is still in service. The Merlin 130 is shown. that, in times of peace, manufacturinglicences and engines are sold to other countries who rray prefer to deal directwith the firm, or firms. These countries may also have a government-supportedaviation engine industry which would re- sent competition with one that wasgovernment controlled, or nationalized. (b) Engines should always be built tomeet specific aircraft requirements and, therefore, very good liaison with the air-craft manufacturer and his designer is essential. There must be mutual trustand confidence ii each other's products. But it is more important that the enginemanufacturer enjoys the full confidence of the aircraft builder. (c) Very heavy expenditure is involvedwhen providing complete development and testing facilities for gas turbine com-ponents ; much heavier than that re- quired for the equivalent facilities for apiston engine. Between £2,000,000 and £3,000,000 would be necessary to manu-facture and install the equipment. Some firms prefer to be independent of thegovernment in regard to capital expendi- ture and provide their own, less ela,bor-ate. equipment. Others pay for the buildings (test houses) and also own theland upon which these stand, but ask the government to provide the experimentalequipment installed, on the basis that this equipment may be rendered quicklyobsolete. (d) More time is at present required(between 10 and 20 per cent.) to design and manufacture a prototype airscrewturbine of the axial compressor type, ready for test, than that required fora piston engine of equivalent power. Even a small airscrew turbine with atwo-stage centrifugal compressor exceeds in total design hours, the time taken todesign the more conventional piston engine of somewhat greater power. Themost economical engine in terms of design and manufacturing hours, is theclassic Whittle-type jet turbine, which absorbs about one quarter of the designhours of a large airscrew turbine. (e) The life of a piston engine, as aie_, h^cs been about ten years, some- more, before becoming obsolete. ng this time, its performance willve been much improved by continuous development. This is important, sinceit takes five years to create an engine and it would not be very economical todesign and develop an entirely new engine whenever an increase in perform-ance was required. This also applies to the gas turbine but it is unlikelythat there will be the same large per- centage increases in power which have (Concluded on page 534)
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