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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 bypass
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 =
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 configuration
of the by-pass principle. This gives very high bypass
ratios in the order of 15:1, and reduced ’drag’
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, particularly
for commercial transport duty, are those of low
specific fuel consumption and weight. Considerable
improvement has been achieved by use of the bypass
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
Performance
225
2
2 2 J
2 1
2 1 J
1
1 J 2 J
1 J 2 J
W V(v V) W V(v V) W V(v V) W V(v V)
W V(v V) W V(v V)
1 2 1 2
1 2
− + − + − + −
− + −
Performance
226
Fig. 21-10 International Standard Atmosphere.
by-pass ratio engine, the weight reduction compared
with a pure jet engine is in the order of 20 per cent
for the same air mass flow.
49. With a high by-pass ratio engine of the triplespool
configuration, a further significant improvement
in specific weight is obtained- This is derived mainly
from advanced mechanical and aerodynamic design,
　
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