图18-10 一种升力喷气发动机
Fig. 18-10 A lift-jet engine.
Remote lift systems
17. Direct lift remote systems duct the by-pass air or engine exhaust air to downward facing lift nozzles remote from the engine. These nozzles may be in the front fuselage of the aircraft or in the wings. The engine duct is blocked by means of a diverter similar to that described in para. 10.
18.
The remote lift-fan (fig. 18-12) is mounted in the aircraft wing or fuselage, and is driven mechanically or by air or gas ducted into a tip turbine, The drive system is provided by the main propulsion power plant or by a separate engine.
19.
The advantage of the remote lift system is that it gives some freedom to the aircraft to position the
排气装置
Fig. 18-11 Lift-fan engine configurations.
21.偏转式发动机系统只能与两台或多台发动机一起使用。但这在一台发动机发生故障时会带来安全方面的问题。所以，尽管重量的代价虽小且也不增加燃油消耗，但从安全方面考虑，与某些其它动力升力系统相比这些优势就体现不出来了。提供低速下的飞机控制的正常方法是差动节流和发动机转向，它简化了基本发动机的设计，但使控制系统更为复杂。
偏转几台发动机
20.这一方法包括具有若干台推进发动机，它们至少可做90度的机械偏转以使推力转向(图18-13)。除这些推进发动机以外，还可以安装一台或多台升力发动机，以便在起飞和降落时提供辅助升力。
Vertical/short take-off and landing
propulsion system to the best advantage whilst still maintaining the resultant thrust near the aircraft centre of gravity in the jet lift mode. This freedom is achieved at a cost of increased volume, particularly with the gas driven systems, due to the size of the ducts to feed the gas to the remote lift system. Although the mechanically driven remote lift-fan eliminates the need for these large gas ducts, it is done at the expense of long shafts and high power gearboxes and clutch systems.
Swivelling engines
20. This method consists of having propulsion engines which can be mechanically swiveled closed 图18-13 偏转几台发动机产生喷气升力
垂直起飞
空气进气门
辅助进气口
打开
向前飞行
关闭
through at least 90 degrees to provide thrust vectoring (fig. 18-13). In addition to these propulsion engines, one or more lift engines may be installed to provide supplementary lift during the take-off and landing phase of flight.
图18-12 远距升力风扇
21. The swivelling engine system can only be used with two or more engines. This then introduces the problem of safety in the event of an engine failure. So, although there is only a small weight penalty and no increase in fuel consumption, safety considera-tions tend to offset these advantages compared to some of the other powered lift systems. The normal method of providing aircraft control at low speeds is by differential throttling and vectoring of the engines which simplifies the basic engine design but makes the control system more complex.
Bleed air for STOL
22. Fig. 18-14 shows one method how STOL can be achieved with a form of 'flap blowing'. The turbo-fan engine has a geared variable pitch fan and an oversized low pressure (L. P.) compressor from the exit of which air is bled and ducted to the flap system in the wing trailing edge. The variable pitch fan enables high L.P. compressor speed and thus high bleed pressure to be maintained over a wide range of thrusts. This gives excellent control at greatly different aircraft flight conditions.
LIFT THRUST AUGMENTATION
23. In many cases on V/STOL aircraft augmentation of the lift thrust is necessary to avoid an engine which is oversized for normal flight with the consequent effects of higher engine weight and fuel consumption than would be the case for a conventional aircraft-This lift thrust augmentation can be achieved in a number of different ways:
(1)
Using special engine ratings.
(2)
Burning in the lift nozzle gas flow.
(3)
By means of an ejector system.
Special engine ratings
24. Experience has shown that an engine rating structure can be devised which provides high thrust levels for short periods of time without reducing engine life. Operation in ground effect and the take-off and landing manoeuvres require maximum thrust for less than 15 seconds so that use of a short lift rating for that time is feasible. Fig. 18-15 shows an example of thrust permissible with a 15 second short lift rating compared to that with a 2.5 minute normal lift rating.
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