Page 235 - The Jet Engine

Performance

reference to fig. 21-9 it can be seen that for aircraft pass ratios in the order of 15:1, and reduced 'drag'
designed to operate at sea level speeds below results due to the engine core being 'washed' by the
approximately 400 m.p.h. it is more effective to low velocity aircraft slipstream and not the relatively
absorb the power developed in the jet engine by high velocity fan efflux.
gearing it to a propeller instead of using it directly in
the form of a pure jet stream. The disadvantage of 45. The improved propulsive efficiency of the
the propeller at the higher aircraft speeds is its rapid bypass system bridges the efficiency gap between
fall off in efficiency, due to shock waves created the turbo-propeller engine and the pure turbo-jet
around the propeller as the blade tip speed engine. A graph illustrating the various propulsive
approaches Mach 1.0. Advanced propeller efficiencies with aircraft speed is shown in fig. 21-9.
technology, however, has produced a multi-bladed,
swept back design capable of turning with tip speeds FUEL CONSUMPTION AND POWER-TO-WEIGHT
in excess of Mach 1.0 without loss of propeller RELATIONSHIP
efficiency. By using this design of propeller in a 46. Primary engine design considerations, particu-
contra-rotating configuration, thereby reducing swirl larly for commercial transport duty, are those of low
losses, a 'prop-fan' engine, with very good propulsive specific fuel consumption and weight. Considerable
efficiency capable of operating efficiently at aircraft improvement has been achieved by use of the by-
speeds in excess of 500 m.p.h. at sea level, can be pass principle, and by advanced mechanical and
produced.
aerodynamic features, and the use of improved
43. To obtain good propulsive efficiencies without materials. With the trend towards higher by-pass
the use of a complex propeller system, the by-pass ratios, in the range of 15:1, the triple-spool and
principle (Part 2) is used in various forms. With this contra-rotating rear fan engines allow the pressure
principle, some part of the total output is provided by and by-pass ratios to be achieved with short rotors,
a jet stream other than that which passes through the using fewer compressor stages, resulting in a lighter
engine cycle and this is energized by a fan or a and more compact engine.
varying number of LP. compressor stages. This 47. S.f.c. is directly related to the thermal and
bypass air is used to lower the mean jet temperature
and velocity either by exhausting through a separate propulsive efficiencies; that is, the overall efficiency
propelling nozzle, or by mixing with the turbine of the engine. Theoretically, high thermal efficiency
stream to exhaust through a common nozzle. requires high pressures which in practice also means
high turbine entry temperatures. In a pure turbo-jet
44. The propulsive efficiency equation for a high by- engine this high temperature would result in a high
pass ratio engine exhausting through separate jet velocity and consequently lower the propulsive
nozzles is given below, where W and V J1 relate to efficiency (para. 40). However, by using the by-pass
1
the by-pass function and W and v J2 to the engine principle, high thermal and propulsive efficiencies
2
main function. can be effectively combined by bypassing a
proportion of the L.P. compressor or fan delivery air
Propulsive efficiency = to lower the mean jet temperature and velocity as
W V v ( − V )+ W V v ( − V ) referred to in para. 43. With advanced technology
1 J 1 2 J 2 engines of high by-pass and overall pressure ratios,
2
W 1 V v ( J 1 − V )+ W 2 V v ( J 2 − V )+ 1 2 W 1 V v ( J 1 − V ) + 1 2 W 2 V v ( J 2 − V ) 2 a further pronounced improvement in s.f.c. is
By calculation, substituting the following values, obtained.
which will be typical of a high by-pass ratio engine of
triple-spool configuration, it will be observed that a 48. The turbines of pure jet engines are heavy
propulsive efficiency of approximately 85 per cent because they deal with the total airflow, whereas the
results. turbines of by-pass engines deal only with part of the
V = 583 rn.p.h. flow; thus the H.P. compressor, combustion
W = 492 lb. per sec. chambers and turbines, can be scaled down. The
1
W = 100 lb. per sec. increased power per lb. of air at the turbines, to take
2
V J1 = 781 m.p.h. advantage of their full capacity, is obtained by the
V J2 = 812 m.p.h. increase in pressure ratio and turbine entry
temperature. It is clear that the by-pass engine is
Propulsive efficiency can be further improved by lighter, because not only has the diameter of the high
using the rear mounted contra-rotating fan configura- pressure rotating assemblies been reduced but the
tion of the by-pass principle. This gives very high by- engine is shorter for a given power output. With a low

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