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Aviation History
1961
1961 - 0065.PDF
FLIGHT, 13 January 1961 ..i. T64 In the design of the 764 great care has been taken to make the engine singularly easy to maintain. All casings are split into halves, and fuel manifolds and nozzles are all externally mounted. Rotor blading is of the balanced-moment type, so that individual blades can be replaced without any machining or rebalancing of the complete assembly Design Philosophy Behind GE's New Shaft Turbine FOR some years the gas-turbine design policy of the AmericanGeneral Electric Co has been to achieve high pressure-ratioon a single shaft, at the same time obtaining satisfactory handling by the use of variable stators. The first engine to incor-porate these principles was the big J79 turbojet, but the company's Small Aircraft Engine Department at Lynn, Mass, is now com-mitted to two families of small turboshaft engines of similar configuration. The first of these is the T58, which has been widelyadopted by airframe designers and is being licence-produced by de Havilland Engines as the Gnome. In 1953 the accepted formula for a small engine presupposedindifferent component efficiencies, low pressure-ratio and modest top temperature. Consequently sueh engines were poor per-formers, and compared unfavourably with their piston-engine livals. It was at this time that the designers at Lynn formed theopinion that something could be done to break a vicious circle: small-engine development was starved of funds owing to lowmilitary priority, which in turn stemmed largely from the unattrac- tiveness of the engines themselves. "Surely," they thought, "asmall gas turbine can be built with big-engine quality." Moreover—and this is die crux of the argument—the company'sstudies showed that such engines, although relatively costly, made sound economic sense. Only a limited market could be envisagedso long as the established piston engines continued to perform their task in a satisfactory manner. Arithmetic showed that a turbineengine of advanced design, fitted in either a helicopter or aero- plane, would enable substantially improved payload/range curvesto be plotted on short ranges; but, inevitably, the piston engine came back into the picture above a given endurance; the T58'spressure ratio of 8.3:1 was insufficiently high. During 1954 work began on a small turbojet, now known asthe J85. Emphasis was laid on the achievement of minimum specific weight. It is well known that, if specific weight for a rangeof theoretical engines widi similar parameters is plotted against engine-size, the result is a loop with minimum values in theneighbourhood of 2,0OO-3,0001b. In the final reckoning installed weight must be considered, and this tends to favour the largeengine. On the other hand, GE were able to demonstrate that a small engine can be produced with overall quality comparable tothat of a large engine. For example, while the T58 was on the drawing board many engineers doubted that the compressorefficiency would exceed 78 per cent; but the production compressor surpasses 85 per cent. Bearing in mind that the blades in thefinal stages are smaller than the average thumbnail, this is a remarkable achievement. The centre bearing holds clearancesadequately, blade chord is a maximum (and costs nothing) and advanced rolling and coining methods have been evolved for rapidand relatively cheap production. By 1956 the T58 was well into its bench tests, and several helicopters were being prepared to use it.In each case the airframe designer had concurred with GE's con- tention that the engine should be assessed as part of an integratedtransport system. In the ultimate equation regarding the number of vehicles required to fulfil the operator's task, the employment ofa lighter and more efficient engine became of supreme importance: particularly in the helicopter world, every ounce of weight savedwas found to have a measurable effect on overall economics. Such was the climate surrounding the basic design of the T64:i the winter of 1953-54. Teams from Lynn were touring the American airframe industry, and their studies showed that, whilethe T58 turboshaft engine and J85 turbojet were both correctly sized and timed, there was an increasing need for a shaft machinerated at some 2,500 h.p. widi the highest possible thermodynamic efficiency. A competition for such an engine was held by the USNavy, and after an early closure GE were announced a winner and went ahead on preliminary design during 1954.Perhaps the most basic parameter in the design of any gas turbine is compressor pressure-ratio. As this is raised above12:1 the losses must increase disproportionately; and at the same time additional turbine stages have to be brought in, causinga substantial increase in powerplant weight. For an appreciable period GE pursued a 2,500 h.p. study with five turbine stages;but, while this was regarded as a promising "growth" engine for future development, it was decided that, haying due regardfor the law of diminishing returns, the correct choice for the initial product would be a lighter engine with a total of four turbinestages (two compressor and two power). Time of flight, always an important factor in engine design, was clearly going to exceedby a wide margin that of the many aircraft using the T58. None of the applications envisaged could approach sonic speed, so thata high inbuilt pressure-ratio could not prove an embarrassment. Moreover, the fact that most of the applications involved expen-sive aircraft made engine cost relatively less important. Curves for specific fuel consumption were plotted against aggregateengine-plus-fuel weight for a range of missions. It was clear that the weight of the propeller, gearbox and turbine would be muchgreater than that of the compressor, and that in any case the weight increase consequent upon raising the pressure-ratio (nearthe point at which an additional turbine stage became necessary) would be relatively insignificant. Altogether the calculationsshowed that the weight-penalty of a high pressure-ratio was well worth paying. Accordingly it was decided to choose a pressure ratio of 12.5 : 1,the highest yet adopted in any single-spool turboshaft engine. In order to achieve the desired level of 2,500 h.p. the mass-flow wasfixed at 251b/sec. Immediately these figures were established, component evaluation was put in hand. Power is a function ofairflow, temperature and component efficiency (in that order), with a high pressure-ratio giving a small bonus. During 1954and 1955 GE took the project "down the road" without waiting for any further competition. Early in 1956 the company made anunsolicited proposal to the Navy, pointing out that the engine would suit complete families of aircraft, including many typesalready in production with piston engines. It was emphasized that its adoption would actually return money to the taxpayer,owing to the much-increased capability of helicopters and fixed- wing aircraft and the consequent reduction in the number ofvehicles required. "For the first time in history," said GE, "it would become an economic proposition to start junking new pistonengines." Independendy the Navy had been making their own investiga-tions, and concurred with GE's claims. A competition for the T64 was thereupon initiated, and after almost a year GE weredeclared the winners, and an instruction to proceed was received early in 1957. The contract was of the cost-plus-fixed-fee type;valued at $58m, it covered the development up to 150hr qualifica- tion of turboprop, turboshaft and direct-drive engines, in both
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