高压涡轮出口
低压涡轮出口
排气
推进喷管
大气
进口
涡轮入口
图21-1 一种典型的涡喷发动机的温度和压力符号
Fig. 21-1 Temperature and pressure notation of a typical turbo-jet engine.
10．如前面所述，大气压力和温度的变化对发动机的推力影响相当大。这是因为它们影响空气的密度，因此在一定的发动机转速时影响进入发动机的空气质量。为了能对相似的一些发动机在不同的气候条件或不同高度时的性能进行比较，计算时必须采用修正系数，使观察到的值回到国际标准大气条件下来。例如，对涡轮喷气发动机的推力修正是：推力(磅)(修正值)=推力(磅)(观测值)×30/P0
9．考虑第20章推导出来的用于发动机在“壅塞”喷口条件下工作的公式，……
可以看到，推力还能受到通过发动机的空气的质量流量的变化和喷气速度变化的影响。采用喷水的办法可使质量流量增加(第17章)，采用加力可使喷气速度增加(第16章)。
Performance
轴马力(修正值)=轴马力(观察值)
not in pounds of thrust the factors are different. For example, the correction for s.h.p. is:
S.h.p. (corrected) = 30 273 + 15
s.h.p. (observed) x x
P0
273 + T0 where P0 = atmospheric pressure (in.Hg.) (observed) T0 = atmospheric temperature in deg.C. (observed) 30 = I.S.A. standard sea level pressure (in.Hg.) 273 + 15 = I.S.A. standard sea level temperature in deg.K. 273 + T0 = Atmospheric temperature in
deg.K. In practice there is always a certain amount of jet thrust in the total output of the turbo-propeller engine and this must be added to the s.h.p. The correction for jet thrust is the same as that in para. 10.
12.
To distinguish between these two aspects of the power output, it is usual to refer to them as s.h.p. and thrust horse-power (t.h.p.). The total equivalent horse-power is denoted by t.e.h.p. (sometimes e.h.p.) and is the s.h.p. plus the s.h.p. equivalent to the net jet thrust. For estimation purposes it is taken that, under sea- level static conditions, one s.h.p. is equivalent to approximately 2.6 lb. of jet thrust.
13.
The ratio of jet thrust to shaft power is influenced by many factors. For instance, the higher the aircraft operating speed the larger may be the required proportion of total output in the form of jet thrust. Alternatively, an extra turbine stage may be required if more than a certain proportion of the total power is to be provided at the shaft. In general, turbo-propeller aircraft provide one pound of thrust for every 3.5 h,p. to 5 h.p.
Therefore :
t.e.h.p. = s.h.p. + 2.6 jet thrust lb.
Comparison between thrust and horse-power
14. Because the turbo-jet engine is rated in thrust and the turbo-propeller engine in s.h.p., no direct comparison between the two can be made without a power conversion factor. However, since the turbo-propeller engine receives its thrust mainly from the propeller, a comparison can be made by converting the horse-power developed by the engine to thrust or the thrust developed by the turbo-jet engine to t.h.p.; that is, by converting work to force or force to work. For this purpose, it is necessary to take into account the speed of the aircraft.
temperature is constant up to 65.617ft. The I.S.A. standard pressure at sea level is 14.69 pounds per square inch falling to 3.28 pounds per square inch at the tropopause (refer to I.S.A. table fig. 21-10).
ENGINE THRUST ON THE TEST BENCH
8.
The thrust of the turbo-jet engine on the test bench differs somewhat from that during flight. Modern test facilities are available to simulate atmospheric conditions at high altitudes thus providing a means of assessing some of the performance capability of a turbo-jet engine in flight without the engine ever leaving the ground. This is important as the changes in ambient temperature and pressure encountered at high altitudes consider-ably influence the thrust of the engine.
9.
Considering the formula derived in Part 20 for engines operating under 'choked' nozzle conditions,
WVJThrust = (P . P0) . A + g
it can be seen that the thrust can be further affected by a change in the mass flow rate of air through the engine and by a change in jet velocity. An increase in mass airflow may be obtained by using water
injection (Part 17) and increases in jet velocity by using afterburning (Part 16).
10. As previously mentioned, changes in ambient pressure and temperature considerably influence the thrust of the engine. This is because of the way they affect the air density and hence the mass of air entering the engine for a given engine rotational speed. To enable the performance of similar engines to be compared when operating under different climatic conditions, or at different altitudes, correction factors must be applied to the calculations to return the observed values to those which would be found under I.S.A. conditions. For example, the thrust correction for a turbo-jet engine is: Thrust (lb.) (corrected) =
30thrust (lb.) (observed) x
大气压力，以水银柱的英寸数计(观察值) 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：劳斯莱斯喷气引擎－中英(104)
