44．下面给出的是通过分别喷口排气的高涵道比发动机的推进效率方程。这里W1 和 VJ1。与外涵道功能有关，W2 和 vJ2与发动机主功能有关。
推进效率=……
代入下列数值，进行计算。这些数值对三转子布局的高涵道比发动机是典型值。将会看到，结果是推进效率约为85％。
……
采用后置式内外涵道原理的对转风扇布局可进一步提高推进效率。这得出很高的涵道比，达15：1。由于发动机核心机被低速飞机的滑流而不是速度相当高的风扇气流“冲刷”，结果是阻力减小。
45．内外涵道系统推进效率的提高填补了涡轮螺桨发动机和纯涡轮喷气发动机之间的效率差距。图21-9表示了推进效率随飞机速度变化的图表。
reference to fig. 21-9 it can be seen that for aircraft pass ratios in the order of 15:1, and reduced 'drag'
油耗和功率重量的关系
designed to operate at sea level speeds below approximately 400 m.p.h. it is more effective to absorb the power developed in the jet engine by gearing it to a propeller instead of using it directly in the form of a pure jet stream. The disadvantage of the propeller at the higher aircraft speeds is its rapid fall off in efficiency, due to shock waves created around the propeller as the blade tip speed approaches Mach 1.0. Advanced propeller technology, however, has produced a multi-bladed, swept back design capable of turning with tip speeds in excess of Mach 1.0 without loss of propeller efficiency. By using this design of propeller in a contra-rotating configuration, thereby reducing swirl losses, a 'prop-fan' engine, with very good propulsive efficiency capable of operating efficiently at aircraft speeds in excess of 500 m.p.h. at sea level, can be produced.
43.
To obtain good propulsive efficiencies without the use of a complex propeller system, the by-pass principle (Part 2) is used in various forms. With this principle, some part of the total output is provided by a jet stream other than that which passes through the engine cycle and this is energized by a fan or a varying number of LP. compressor stages. This bypass air is used to lower the mean jet temperature and velocity either by exhausting through a separate propelling nozzle, or by mixing with the turbine stream to exhaust through a common nozzle.
44.
The propulsive efficiency equation for a high by-pass ratio engine exhausting through separate nozzles is given below, where W1 and VJ1 relate to the by-pass function and W2 and vJ2 to the engine main function.
Propulsive efficiency =
W1V(v 1 .V)+W2V(vJ .V)
J2
W1V(vJ .V)+W2V(vJ .V)+
12W1V(vJ .V)2 +
12W2V(vJ .V)2
12 1 2
By calculation, substituting the following values, which will be typical of a high by-pass ratio engine of triple-spool configuration, it will be observed that a propulsive efficiency of approximately 85 per cent results.
V = 583 rn.p.h.
W1 = 492 lb. per sec.
W2 = 100 lb. per sec.
VJ1 = 781 m.p.h.
VJ2 = 812 m.p.h.
Propulsive efficiency can be further improved by using the rear mounted contra-rotating fan configura-tion of the by-pass principle. This gives very high by-results due to the engine core being 'washed' by the low velocity aircraft slipstream and not the relatively high velocity fan efflux.
45. The improved propulsive efficiency of the bypass system bridges the efficiency gap between the turbo-propeller engine and the pure turbo-jet engine. A graph illustrating the various propulsive efficiencies with aircraft speed is shown in fig. 21-9.
FUEL CONSUMPTION AND POWER-TO-WEIGHT RELATIONSHIP
46.
Primary engine design considerations, particu-larly for commercial transport duty, are those of low specific fuel consumption and weight. Considerable improvement has been achieved by use of the by-pass principle, and by advanced mechanical and aerodynamic features, and the use of improved materials. With the trend towards higher by-pass ratios, in the range of 15:1, the triple-spool and contra-rotating rear fan engines allow the pressure and by-pass ratios to be achieved with short rotors, using fewer compressor stages, resulting in a lighter and more compact engine.
47.
S.f.c. is directly related to the thermal and propulsive efficiencies; that is, the overall efficiency of the engine. Theoretically, high thermal efficiency requires high pressures which in practice also means high turbine entry temperatures. In a pure turbo-jet engine this high temperature would result in a high jet velocity and consequently lower the propulsive efficiency (para. 40). However, by using the by-pass principle, high thermal and propulsive efficiencies can be effectively combined by bypassing a proportion of the L.P. compressor or fan delivery air to lower the mean jet temperature and velocity as referred to in para. 43. With advanced technology engines of high by-pass and overall pressure ratios, a further pronounced improvement in s.f.c. is obtained.
48.
The turbines of pure jet engines are heavy because they deal with the total airflow, whereas the turbines of by-pass engines deal only with part of the flow; thus the H.P. compressor, combustion chambers and turbines, can be scaled down. The increased power per lb. of air at the turbines, to take advantage of their full capacity, is obtained by the increase in pressure ratio and turbine entry temperature. It is clear that the by-pass engine is lighter, because not only has the diameter of the high pressure rotating assemblies been reduced but the engine is shorter for a given power output. With a low
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