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.
轴马力 功率
Performance
图21-5 飞机速度对轴马力和油耗的影响
s.h.p. and fuel consumption.
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
26.然而，在飞行条件下，这种优点甚至更大，约为不管有没有加力燃烧，动量阻力都是一样的。由于冲压效应，通过发动机的每一磅气流都更好地得到利用。下面的例子，利用第16章给出的静态值，说明了为什么加力推力在飞行条件下有了改进。
220
27．假定飞机的速度为600英里／小时(880英尺／秒)，那么动量阻力为：
880/32.2=27.5(近似值)
这就是说，每秒钟流经发动机的每磅空气并加速到飞机的速度时约产生27.5磅的阻力。
28．假定流过发动机的每磅空气给出的总推力为77.5磅。那么对于每秒每磅空气来说，发动机给出的净推力为77.5-27.5=50磅。
30．这样更大地增加推力对于获得更高的速度和高度性能来说是非常宝贵的。总油耗和耗油率是高了。但对性能这样大的提高是合算的。
Performance
29．当选用加力时，假定根据第25段给出的静推力增长为30％，总推力将是1.3×77.5=100.75磅。那么，在600英里／小时的飞行条件下，每秒每磅空气的净推力将是100.75-27.5=73.25磅。因此，由于加力，净推力的比率是73.25/50=1.465。换句话说，在静态条件下推力增长30％，在速度600英里／小时时推力就增长46.5％。
31．对可获得的推力的限制取决于加力燃烧温度和排气流中剩余的可用氧气量。因为在内外涵发动机的外涵道中，在此以前没有发生过燃烧加热，对于这些还有大量氧气剩余的发动机来说，特别适合采用加力燃烧，静推力增长可达70％。在高速前飞时，可达此数字的几倍之多。
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: 880
= 27.5 (approximately)
32 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
73.25
afterburning is = 1.465. In other words, a 30
50 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 perform-ances. The total and specific fuel consumptions are high, but not unduly so for such an increase in performance.
31.
The limit to the obtainable thrust is determined by the afterburning temperature and the remaining usable oxygen in the exhaust gas stream. Because no previous combustion heating takes place in the duct of a by-pass engine, these engines with their large residual oxygen surplus are particularly suited to afterburning and static thrust increases of up to 70 per cent are obtainable. At high forward speeds several times this amount is achieved.
Effect of altitude
32. With increasing altitude the ambient air pressure and temperature are reduced. This affects the engine in two interrelated ways:
The fall of pressure reduces the air density and hence the mass airflow into the engine for a given engine speed. This causes the thrust or
s.h.p. to fall. The fuel control system, as described in Part 10, adjusts the fuel pump output to match the reduced mass airflow, so maintaining a constant engine speed.
The fall in air temperature increases the density of the air, so that the mass of air entering the compressor for a given engine speed is greater. This causes the mass airflow to reduce at a lower rate and so compensates to some extent for the loss of thrust due to the fall in atmospheric pressure. At altitudes above 36,089 feet and up to 65,617 feet, however, the temperature remains constant, and the thrust or s.h.p. is affected by pressure only.
Graphs showing the typical effect of altitude on thrust, s.h.p, and fuel consumption are illustrated in fig. 21-6 and fig. 21-7.
Effect of temperature
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