推力和馬力之間的 比較
飛行中的發(fā)動(dòng)機(jī)推力
前飛速度的影響
加力燃燒對(duì)發(fā)動(dòng)機(jī) 推力的影響
高度的影響
溫度的影響
推進(jìn)效率
油耗和功率重量的關(guān)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(jìn)效率在很大程度上還影響著壓氣機(jī)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(dòng)機(jī)的重量和直徑。
4.熱效率常被稱作發(fā)動(dòng)機(jī)的內(nèi)部效率，而推進(jìn)效率稱作外部效率。在第37段中描述的這個(gè)推進(jìn)效率解釋了為什么飛機(jī)在低速時(shí)純噴氣發(fā)動(dòng)機(jī)的效率不如渦輪螺槳發(fā)動(dòng)機(jī)，因而導(dǎo)致渦輪風(fēng)扇發(fā)動(dòng)機(jī)以及最近的槳扇發(fā)展。
7.為了能對(duì)相似的一些發(fā)動(dòng)機(jī)的性能進(jìn)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(biāo)準(zhǔn)化。在在使用中，對(duì)標(biāo)準(zhǔn)大氣有幾種不同的定義，最為通用的是國(guó)際標(biāo)準(zhǔn)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(dǎo)致從海平面的288.15K（15℃）降至36，089英尺（對(duì)流層頂）的216.65K（-56.5℃）。在這個(gè)高度以上，直至65，617英尺國(guó)際標(biāo)準(zhǔn)大氣溫度不變。在海平面國(guó)際標(biāo)準(zhǔn)大氣的標(biāo)準(zhǔn)壓力為14.69磅/平方英寸，在對(duì)流層頂，它降至3.28磅/平方英寸（參閱圖21-10國(guó)際標(biāo)準(zhǔn)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(jì)算出來(lái)的，并在試車臺(tái)和試飛或在高空試驗(yàn)臺(tái)上模擬飛行條件下得到實(shí)驗(yàn)證明。為了進(jìn)行這些計(jì)算，采用專門的符號(hào)來(lái)表示發(fā)動(dòng)機(jī)各部位的壓力和溫度；例如，利用圖21-1中表示的符號(hào)，壓氣機(jī)的總增壓比為P3/P1。這些符號(hào)因發(fā)動(dòng)機(jī)類型不同而稍有不同；例如，對(duì)高涵道比發(fā)動(dòng)機(jī)，還要當(dāng)采用加力（第16章）時(shí)，就使用更多的符號(hào)。
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
中國(guó)航空網(wǎng) m.k6050.com
航空翻譯 www.aviation.cn
本文鏈接地址：勞斯萊斯噴氣引擎－中英(103)
