凈推力
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
磅小時
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/小時/磅推力
帶有進氣道沖壓的推力
無進氣道沖壓推力
Fig. 21-3 Thrust recovery with aircraft圖21-3 推力隨飛機速度得以上恢復
speed.
The effect of aircraft speed on
thrust and fuel consumption.
圖21-4飛機速度對推力和油耗的影響
海平面國際標準大氣條件
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.前飛速度對典型的渦輪螺槳發(fā)動機的影響在圖21-5的趨勢曲線上表示出來。雖然凈噴氣推力降低，由于“沖壓比”對增加質(zhì)量流量和與之匹配的
燃油流量的影響使軸馬力提高。因為用渦輪螺槳發(fā)動機
的耗油率相對于軸馬力來表示是一種標準做法，由圖可見，耗油率有了改進。不過，這并沒有提供一個與圖21-4顯示的典型渦輪噴氣發(fā)動機的曲線的真正比較，約為軸馬力被螺旋槳吸收并轉(zhuǎn)變成推力，在高亞音速前飛時，不考慮軸馬力的增大，螺旋槳效率以及凈推力衰減。這樣，在與渦輪噴氣發(fā)動機的總體比較中，渦輪螺槳發(fā)動機相對于凈推力的耗油率在低速前飛時有了改進，但在高速時迅速衰減。
加力燃燒對發(fā)動機推力的影響
25.在起飛條件下，氣流通過發(fā)動機的動量阻力可忽略不計，這樣可以認為，總推力等于凈推力。如果選用加力燃燒（第16章），對純噴氣發(fā)動機來說，起飛推力增加30%是可能的，對內(nèi)外涵發(fā)動機來說，增長量要大得多。這種加大基本推力的做法對某些特定的使用來說具有很大的優(yōu)點。
23.在低空告訴前飛時，“沖壓比”效應(yīng)在發(fā)動機上造成非常高的應(yīng)力。為防止應(yīng)力過大，燃油流量自動減少，以限制發(fā)動機轉(zhuǎn)速和空氣流量。燃油控制的方法已在第10章中介紹過。
22.隨前飛速度的增加，由于“沖壓比”的影響，空氣的質(zhì)量流量增加，這必須與燃氣流量（第10章）相匹配。結(jié)果造成油耗增大。因為凈推力隨著前飛速度趨于減少，如圖21-4中典型的渦輪噴氣發(fā)動機的曲線所示，其最終結(jié)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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