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Aviation History
1959
1959 - 0784.PDF
20 March 1959 383 is driven by a mechanically independent low-pressure turbine(there are reasons why a single-shaft engine with front fan is undesirable). G.E. have produced an aft-fan engine by adding,behind the original turbine, a complete wheel with an inner ring of turbine blades and an outer ring of fan blades, the latter beingcarried on the tip-shroud platforms of the former, This raises severe structural and aerodynamic design problems, but has theadvantage of requiring very little modification to the basic gas- producer (and thus is attractive as a retroactive modification). Even the front-fan configuration can be applied in more thanone way. Pratt & Whitney, below, have chosen to discharge the fan air overboard through propulsive nozzles on either side of theengine immediately behind the fan. Rolls-Royce have always carried it round to the rear in a by-pass duct, so that the hot andcold streams can be mixed in a single final nozzle. This can result in a theoretical improvement in specific consumption, and it alsopermits a thrust-reyerser to be fitted. The American arrangement reduces engine weight, although this advantage is probably con-fined to the uninstalled engine. This leads to an underlining of the fact that—as in the case withall engines—the only performance that matters is installed per- formance. Brochuremanship can be excellently served by quotingthe figures obtained with a bare engine on the bed, but the aircraft- builder, or operator, needs to know what the engine will really dowhen it is hung in the airframe, coupled up to accessories taking 200 s.h.p., bled of air to de-ice the wings and intakes and todrive cabin turbo-compressors, and—perhaps the most important aspect of all—creating its own installed drag. In the latter con-nection, momentum drag is particularly worthy of careful study, theoretical gain in specific fuel consumption of up to four percent. This, of course, does not apply to engines in which such mixing does not take place. Of these factors, that of turbine-inlet temperature is the mostintractable. The higher the by-pass ratio, the hotter should the engine be, and both the established turboprops—which have avery high theoretical by-pass ratio—and the fan units are un- remitting in their demands upon the technology of blade materials.The problem is felt most keenly in the cruising regime, and cruise power has to be strictly limited to that at which turbine tempera-ture, and hence rate of creep, are appropriate to the achievement of a comfortable blade life. Much is being learned by the opera-tion of air-cooled turbine blades, and in this field Rolls-Royce appear to have a commanding lead. In fact, from the meagreevidence available—precise details are classified on military grounds—it may well be that such blading is not at presentamenable to mass-production in the American manner, and that the high skilled-labour content could make it a very expensivebusiness at over $2.50 per man-hour. The obvious alternative is to employ a better material, but the law of diminishing returns isputting a brake on progress in this direction. The latest creep- resistant alloys in use are already causing furrowed brows in theblade-fabricating shops; where only 10 deg C separate the "fluid" from the "unforgeable" states, there is barely time for one quicksmack with a press before the piece has to be returned to the furnace. In consequence, a very great deal will have to be done beforeair-cooled blades capable of making fan engines really worth while can be manufactured on a large scale and at an economic A manufacturer's drawing of the Pratt & Whitney JT3D-1 turbofan. A direct development of the JT3/J57 two-spool turbojet, the JT3D incor- porates two of the compressor stages from the big J91 nuclear turbojet (now defunct) which replace the first three stages of the existing low- pressure compressor. To supply the increased shaft-power required, the low-pressure shaft has been strengthened and modifications made to the low-pressure turbine, the first stage being enlarged and an additional second stage being added since a large-diameter fan engine with a bench-rating of, say,15,000 lb, may actually impart a net propulsive force on the aircraft at 100 kt during take-off of barely 13,000 lb.Regarding the internal design of the engine itself, certain con- clusions may be drawn merely by studying the fundamentalperformance curves common to all gas turbines. Unlike the simple turbojet, or the low ratio by-pass engine—but like the turboprop—the optimum transport unit achieves its best propulsive efficiency with an exceedingly high flame temperature. This renders theachievement of hot parts with acceptable commercial life (say 5,000 hr minimum) no easy task, and implies the adoption of air-cooled high-pressure turbine rotor blades in an advanced creep- resistant material. By the same token, it is more than ever neces-sary to obtain high-intensity combustion with minimum pressure drop. Another fundamental factor is the choice of by-pass ratio, andthe choice of low-pressure and high-pressure ratios (a ratio of ratios), which combine to determine the manner in which theaggregate propulsive thrust is shared between the cool fan air and the hot central core. Where there is wide divergence between themean energies of the two flows it is impossible to mix them without incurring appreciable losses, but approximate parityenables mixing to be efficient; moreover, the additional heat trans- fer from the hot jet to the cold surrounding flow can produce a price. It can certainly be suggested that an optimum engine cannotbe produced merely by sticking a fan on the front of an existing turbojet, and still less by adding a fan at the back. This is notintended to be a partisan attack on Pratt and Whitney and G.E.; everybody wants to add a fan today, and the facts of life hit bothsides of the ocean with equal severity. Direct Lift Already responsible for more curiously contrived heavier-than-air vehicles than anything else in history, the need for the genera- tion of direct lift by engine thrust is now recognised by a hostof military and commercial operators. It is almost an impossible task to assess the relative merits of the widely varying systemsalready suggested, unless a common requirement is postulated. Such a requirement was investigated in recent years by Canadair,two of the engineers involved subsequently outlining the pros and cons in a C.A.I, paper (reprinted in full in the Canadian Aero-nautical Journal for November last). The work was done under a study contract from the Defence Research Board, the missioninvolving the "carrying of an intermediate cargo load in a pre- scribed envelope, or an equivalent passenger load, over a pre-scribed short range, returning light without refuelling." The profile specified was 80 per cent at optimum altitude and sealevel for the remainder (assumed to be in the target area). Canadair
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