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air intake is provided (Part 23); an efficient air intake
is necessary to obtain maximum benefit from the ram
ratio effect.
21. As aircraft speeds increase into the supersonic
region, the ram air temperature rises rapidly
consistent with the basic gas laws (Part 2). This
Performance
219
Fig. 21-3 Thrust recovery with aircraft
speed.
Fig. 21-4 The effect of aircraft speed on
thrust and fuel consumption.
g
(P P )A W(v V) J
0
− + −
temperature rise affects the compressor delivery air
temperature proportionately and, in consequence, to
maintain the required thrust, the engine must be
subjected to higher turbine entry temperatures. Since
the maximum permissible turbine entry temperature
is determined by the temperature limitations of the
turbine assembly, the choice of turbine materials and
the design of blades and stators to permit cooling are
very important.
22. With an increase in forward speed, the
increased mass airflow due to the ’ram ratio’ effect
must be matched by the fuel flow (Part 10) and the
result is an increase in fuel consumption. Because
the net thrust tends to decrease with forward speed
the end result is an increase in specific fuel
consumption (s.f.c.), as shown by the curves for a
typical turbo-jet engine in fig, 21-4.
23. At high forward speeds at low altitudes the ’ram
ratio’ effect causes very high stresses on the engine
and, to prevent overstressing, the fuel flow is automatically
reduced to limit the engine speed and
airflow. The method of fuel control is described in
Part 10.
24. The effect of forward speed on a typical turbopropeller
engine is shown by the trend curves in fig.
21 -5. Although net jet thrust decreases, s.h.p.
increases due to the ’ram ratio1 effect of increased
mass flow and matching fuel flow. Because it is
standard practice to express the s.f.c. of a turbopropeller
engine relative to s.h.p., an improved s.f.c.
is exhibited. However, this does not provide a true
comparison with the curves shown in fig. 21-4, for a
typical turbo-jet engine, as s.h.p, is absorbed by the
propeller and converted into thrust and, irrespective
of an increase in s.h.p., propeller efficiency and
therefore net thrust deteriorates at high subsonic
forward speeds. In consequence, the turbo-propeller
engine s.f.c, relative to net thrust would, in general
comparison with the turbo-jet engine, show an
improvement at low forward speeds but a rapid deterioration
at high speeds.
Effect of afterburning on engine thrust
25. At take-off conditions, the momentum drag of
the airflow through the engine is negligible, so that
the gross thrust can be considered to be equal to the
net thrust. If afterburning (Part 16) is selected, an
increase in take-off thrust in the order of 30 per cent
is possible with the pure jet engine and considerably
more with the by-pass engine. This augmentation of
basic thrust is of greater advantage for certain
specific operating requirements.
26. Under flight conditions, however, this advantage
is even greater, since the momentum drag is the
same with or without afterburning and, due to the
ram effect, better utilization is made of every pound
Performance
220
Fig. 21-5 The effect of aircraft speed on
s.h.p. and fuel consumption.
of air flowing through the engine. The following
example, using the static values given in Part 16,
illustrates why afterburning thrust improves under
flight conditions.
27. Assuming an aircraft speed of 600 m.p.h. (880ft.
per sec.), then Momentum drag is:
This means that every pound of air per second
flowing through the engine and accelerated up to the
speed of the aircraft causes a drag of about 27.5 lb.
28. Suppose each pound of air passed through the
engine gives a gross thrust of 77.5 lb. Then the net
thrust given by the engine per lb. of air per second is
77.5 - 27.5 = 50 lb.
29. When afterburning is selected, assuming the 30
per cent increase in static thrust given in para. 25,
the gross thrust will be 1.3 x 77.5 - 100.75 lb. Thus,
under flight condition of 600 m.p.h., the net thrust per
pound of air per second will be 100.75 - 27.5 = 73.25
lb. Therefore, the ratio of net thrust due to
afterburning is = 1.465. In other words, a 30
per cent increase in thrust under static conditions
becomes a 46.5 per cent increase in thrust at 600
m.p.h.
30. This larger increase in thrust is invaluable for
obtaining higher speeds and higher altitude performances.
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