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.
軸馬力 功率
Performance
圖21-5 飛機速度對軸馬力和油耗的影響
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，在飛行條件下，這種優點甚至更大，約為不管有沒有加力燃燒，動量阻力都是一樣的。由于沖壓效應，通過發動機的每一磅氣流都更好地得到利用。下面的例子，利用第16章給出的靜態值，說明了為什么加力推力在飛行條件下有了改進。
220
27．假定飛機的速度為600英里／小時(880英尺／秒)，那么動量阻力為：
880/32.2=27.5(近似值)
這就是說，每秒鐘流經發動機的每磅空氣并加速到飛機的速度時約產生27.5磅的阻力。
28．假定流過發動機的每磅空氣給出的總推力為77.5磅。那么對于每秒每磅空氣來說，發動機給出的凈推力為77.5-27.5=50磅。
30．這樣更大地增加推力對于獲得更高的速度和高度性能來說是非常寶貴的。總油耗和耗油率是高了。但對性能這樣大的提高是合算的。
Performance
29．當選用加力時，假定根據第25段給出的靜推力增長為30％，總推力將是1.3×77.5=100.75磅。那么，在600英里／小時的飛行條件下，每秒每磅空氣的凈推力將是100.75-27.5=73.25磅。因此，由于加力，凈推力的比率是73.25/50=1.465。換句話說，在靜態條件下推力增長30％，在速度600英里／小時時推力就增長46.5％。
31．對可獲得的推力的限制取決于加力燃燒溫度和排氣流中剩余的可用氧氣量。因為在內外涵發動機的外涵道中，在此以前沒有發生過燃燒加熱，對于這些還有大量氧氣剩余的發動機來說，特別適合采用加力燃燒，靜推力增長可達70％。在高速前飛時，可達此數字的幾倍之多。
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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