推力和馬力之間的 比較
飛行中的發(fā)動機推力
前飛速度的影響
加力燃燒對發(fā)動機 推力的影響
高度的影響
溫度的影響
推進效率
油耗和功率重量的關系
Fuel consumption and power-to-weight relationship 225
INTRODUCTION
1. The performance requirements of an engine are obviously dictated to a large extent by the type of operation for which the engine is designed. The power of the turbo-jet engine is measured in thrust, produced at the propelling nozzle or nozzles, and that of the turbo-propeller engine is measured in shaft horse-power (s.h.p.) produced at the propeller shaft. However, both types are in the main assessed on the amount of thrust or s.h.p. they develop for a given weight, fuel consumption and frontal area.
2.
Since the thrust or s.h.p. developed is dependent on the mass of air entering the engine and the accel-eration imparted to it during the engine cycle, it is obviously influenced, as subsequently described, by such variables as the forward speed of the aircraft, altitude and climatic conditions, These variables influence the efficiency of the air intake, the compressor, the turbine and the jet pipe; conse-quently, the gas energy available for the production of thrust or s.h.p. also varies.
3.
In the interest of fuel economy and aircraft range, the ratio of fuel consumption to thrust or s.h.p. should be as low as possible. This ratio, known as the specific fuel consumption (s.f.c.), is expressed in pounds of fuel per hour per pound of net thrust or
s.h.p. and is determined by the thermal and propulsive efficiency of the engine. In recent years considerable progress has been made in reducing
s.f.c. and weight. These factors are further explained in para. 46.
5.熱效率和推進效率在很大程度上還影響著壓氣機和渦輪的尺寸，因而在輸出給定的情況下，這些效率能決定發(fā)動機的重量和直徑。
4.熱效率常被稱作發(fā)動機的內部效率，而推進效率稱作外部效率。在第37段中描述的這個推進效率解釋了為什么飛機在低速時純噴氣發(fā)動機的效率不如渦輪螺槳發(fā)動機，因而導致渦輪風扇發(fā)動機以及最近的槳扇發(fā)展。
7.為了能對相似的一些發(fā)動機的性能進行比較，有必要以常用的形式使隨高度和天氣條件而發(fā)生的空氣溫度和壓力的變化標準化。在在使用中，對標準大氣有幾種不同的定義，最為通用的是國際標準大氣（I.S.A.).這是根據(jù)溫度的遞減率約為每1000英尺1.98K，導致從海平面的288.15K（15℃）降至36，089英尺（對流層頂）的216.65K（-56.5℃）。在這個高度以上，直至65，617英尺國際標準大氣溫度不變。在海平面國際標準大氣的標準壓力為14.69磅/平方英寸，在對流層頂，它降至3.28磅/平方英寸（參閱圖21-10國際標準大氣表）
6.這些和其他因素在曲線和圖表中做了介紹，它們是從基本的氣體定律（第2章）中計算出來的，并在試車臺和試飛或在高空試驗臺上模擬飛行條件下得到實驗證明。為了進行這些計算，采用專門的符號來表示發(fā)動機各部位的壓力和溫度；例如，利用圖21-1中表示的符號，壓氣機的總增壓比為P3/P1。這些符號因發(fā)動機類型不同而稍有不同；例如，對高涵道比發(fā)動機，還要當采用加力（第16章）時，就使用更多的符號。
4.
Whereas the thermal efficiency is often referred to as the internal efficiency of the engine, the propulsive efficiency is referred to as the external efficiency. This latter efficiency, described in para. 37, explains why the pure jet engine is less efficient than the turbo-propeller engine at lower aircraft speeds leading to development of the by-pass principle and, more recently, the propfan designs.
5.
The thermal and the propulsive efficiency also influence, to a large extent, the size of the compressor and turbine, thus determining the weight and diameter of the engine for a given output.
6.
These and other factors are presented in curves and graphs, calculated from the basic gas laws (Part 2), and are proved in practice by bench and flight testing, or by simulating flight conditions in a high altitude test cell. To make these calculations, specific symbols are used to denote the pressures and tem-peratures at various locations through the engine; for
Performance
instance, using the symbols shown in fig. 21-1 the P3
overall compressor pressure ratio is . These
P1 symbols vary slightly for different types of engine; for instance, with high by-pass ratio engines, and also when afterburning (Part 16) is incorporated, additional symbols are used.
7. To enable the performance of similar engines to be compared, it is necessary to standardize in some conventional form the variations of air temperature and pressure that occur with altitude and climatic conditions. There are in use several different definitions of standard atmospheres, the one in most common use being the International Standard Atmosphere (I.S.A.). This is based on a temperature lapse rate of approximately 1.98 K. degrees per 1,000ft,, resulting in a fall from 288.15 deg.K. (15 deg.C) at sea level to 216.65 deg.K (-56.5 deg.C.) at 36,089 ft. (the tropopause). Above this altitude the
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