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Aviation History
1977
1977 - 0126.PDF
116 J* TOMORROW'S POWER tained approximately constant. A similar trend is evident for simpler engines with about half this number of stages. Progress along such lines is by no means at an end: we can certainly look forward to reaching pressure ratios considerably higher than 30 in a 20-stage engine with current efficiency levels. Alternative courses are the achievement of current levels of cycle pressure ratio with fewer stages, and the retention of current pressure ratios and numbers of stages while component efficiency improves. High maximum cycle temperature offers gas-generator thermal efficiency and specific power benefits. Moreover, of commercial and military significance is the fact that an increase of turbine-entry temperature frequently forms the basis for developing an existing engine to give greater power. The development of high-temperature materials and cooling techniques has permitted a steady rise in the tem peratures attained in engines. Figure 4 shows the trends up to the present on military and civil engines. Also shown is a projection to higher temperatures on the basis of cur rently foreseen developments of nickel alloys and the further exploitation of convection and film cooling. Refinement will largely take the form of greater com pactness, fewer parts, and better reliability and life. Technical developments will allow some increase in cycle temperatures, though maximum power ratings will be chosen on the basis of more detailed studies of the balance between performance and life. There will be more investigation of intake/engine compatibility, with the object of reducing the risk of expensive troubles at an advanced stage of project development or in service. We can also expect a strong emphasis on the design-to- cost approach, whereby very strict discipline is exercised right from the start of design to ensure that a produc tion cost target is met. This target is itself the outcome of careful assessment during the project-definition phase, and will usually involve "backing off the ultimate" in the performance specification. Although in most areas of gas-turbine development there remains plenty of scope for future technical advance, we are in fact approaching a limit in the design of military engines. Intensive development of jetpipe reheat has brought us to the point at which modern high- boost systems use most of the oxygen available in the jetpipe. For applications requiring the maximum of specific thrust, some biasing of engine design towards high jetpipe pressure is now indicated, implying a low or zero bypass ratio and high turbine-entry temperature. It is of course possible! to exceed the current limit by adding oxidant, but the associated complication and extra tankage requirements would make this unattractive except for very special applications. Civil engines First we shall see extensive exploitation of the current turbofan, with bypass ratios in the 4-6 bracket and pressure ratios of 25 to 33. This process is already well in train. All three current large engines are being stretched to give substantially higher thrust. Refinements are being incorporated where possible to alleviate service problems and pick up gains in perform ance. For example, Eltis and Wilde report an improve ment of between 3*2 and 5J2 per cent in specific air range of the L.1011 as a result of modifications to the RB.211 core engine afterbody. The problem is what the next major step in design concept shall be. The basic aim will be a further significant improvement in operating costs. In view of the current importance of fuel cost, this must mean lower fuel consumption. At the same time, the operational virtues of reliability and good component life will be important, as will environmental characteristics. In seek ing further reductions in fuel consumption, we can con sider changes in specific thrust, installation design, the Figure 5: three future subsonic-engine possibilities, scaled to the same installed cruise thrust FLIGHT International, 15 January 1977 thermodynamic cycle, and required component efficiency. Which of these, or what combination, should we go for? Component inefficiencies and other losses have a signi ficant effect. For example, improvements equivalent to a 2 per cent increase over all the turbo-machinery efficiencies would reduce s.f.c. by about 5 or 6 per cent. Such improvements should be feasible but would not come easily: more stages and some alteration of com ponent layout might be needed. Mixing of the exhaust flows, as in engines of lower bypass ratio, offers a poten tial gain of perhaps 4 per cent, but the net benefit would probably be half this value because of the penalties in drag and weight associated with the longer nacelle required. Detailed improvements centred on current cycles, though of great value for derivatives of existing engines, seem unlikely to provide enough of a fuel- consumption advantage to justify a new engine genera tion. Figure 5 shows three distinctly different possibilities, all scaled to a common level of installed thrust at Mach 0-8 and altitude cruise. The upper illustration is a direct extension of the current turbofan design concept, with no major new features. Pressure ratio would be in the region 40-45, cruise turbine-entry temperature 1,450- 1,500K, and bypass ratio 8 to 10. Specific thrust would be about 100-120N/kg/sec (10-121b/lb/sec), so that for a given thrust the engine would be about 10 to 20 per cent bigger in diameter than current engines. Using a fan tip^speed of about 500m/sec (a little higher than current values), direct drive of the fan with four low-pressure turbine stages should be feasible. Fan noise from the nacelle would be reduced, as in current installations, by acoustic liners in the intake and bypass ducts. The installed specific fuel consumption of such an engine would be 10-15 per cent better than current turbofans, depending on the balance of fuel economy versus weight and the success achieved in further improving the efficiency of highly loaded turbo-machinery. The middle illustration shows a change of concept. By going to a gear drive for the fan, we could use a much lower fan tip-speed, leading to reduced fan noise. This *- >- page 134
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