净推力
磅
19. Since reference will be made to 'ram ratio' and Mach number, these terms are defined as follows:
Ram ratio is the ratio of the total air pressure at the engine compressor entry to the static air pressure at the air intake entry.
Mach number is an additional means of measuring speed and is defined as the ratio of the speed of a body to the local speed of sound. Mach 1.0 therefore represents a speed equal to 油耗
磅小时
the local speed of sound.
20. From the thrust equation in para. 18, it is apparent that if the jet velocity remains constant, independent of aircraft speed, then as the aircraft speed increases the thrust would decrease in direct proportion. However, due to the 'ram ratio' effect from the aircraft forward speed, extra air is taken into the engine so that the mass airflow and also the jet velocity increase with aircraft speed. The effect of this tends to offset the extra intake momentum drag 耗油率 磅/小时/磅推力
带有进气道冲压的推力
无进气道冲压推力
Fig. 21-3 Thrust recovery with aircraft图21-3 推力随飞机速度得以上恢复
speed.
The effect of aircraft speed on
thrust and fuel consumption.
图21-4飞机速度对推力和油耗的影响
海平面国际标准大气条件
Fig. 21-4
due to the forward speed so that the resultant loss of net thrust is partially recovered as the aircraft speed increases. A typical curve illustrating this point is shown in fig. 21-3. Obviously, the 'ram ratio' effect, or the return obtained in terms of pressure rise at entry to the compressor in exchange for the unavoidable intake drag, is of considerable importance to the turbo-jet engine, especially at high speeds. Above speeds of Mach 1.0, as a result of the formation of shock waves at the air intake, this rate of pressure rise will rapidly decrease unless a suitably designed 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 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.
24.前飞速度对典型的涡轮螺桨发动机的影响在图21-5的趋势曲线上表示出来。虽然净喷气推力降低，由于“冲压比”对增加质量流量和与之匹配的
燃油流量的影响使轴马力提高。因为用涡轮螺桨发动机
的耗油率相对于轴马力来表示是一种标准做法，由图可见，耗油率有了改进。不过，这并没有提供一个与图21-4显示的典型涡轮喷气发动机的曲线的真正比较，约为轴马力被螺旋桨吸收并转变成推力，在高亚音速前飞时，不考虑轴马力的增大，螺旋桨效率以及净推力衰减。这样，在与涡轮喷气发动机的总体比较中，涡轮螺桨发动机相对于净推力的耗油率在低速前飞时有了改进，但在高速时迅速衰减。
加力燃烧对发动机推力的影响
25.在起飞条件下，气流通过发动机的动量阻力可忽略不计，这样可以认为，总推力等于净推力。如果选用加力燃烧（第16章），对纯喷气发动机来说，起飞推力增加30%是可能的，对内外涵发动机来说，增长量要大得多。这种加大基本推力的做法对某些特定的使用来说具有很大的优点。
23.在低空告诉前飞时，“冲压比”效应在发动机上造成非常高的应力。为防止应力过大，燃油流量自动减少，以限制发动机转速和空气流量。燃油控制的方法已在第10章中介绍过。
22.随前飞速度的增加，由于“冲压比”的影响，空气的质量流量增加，这必须与燃气流量（第10章）相匹配。结果造成油耗增大。因为净推力随着前飞速度趋于减少，如图21-4中典型的涡轮喷气发动机的曲线所示，其最终结果是耗油率（s.f.c.）增大。
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 auto-matically 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 turbo-propeller 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 turbo-propeller 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 dete-rioration at high speeds.
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