圖18-10 一種升力噴氣發動機
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
排氣裝置
Fig. 18-11 Lift-fan engine configurations.
21.偏轉式發動機系統只能與兩臺或多臺發動機一起使用。但這在一臺發動機發生故障時會帶來安全方面的問題。所以，盡管重量的代價雖小且也不增加燃油消耗，但從安全方面考慮，與某些其它動力升力系統相比這些優勢就體現不出來了。提供低速下的飛機控制的正常方法是差動節流和發動機轉向，它簡化了基本發動機的設計，但使控制系統更為復雜。
偏轉幾臺發動機
20.這一方法包括具有若干臺推進發動機，它們至少可做90度的機械偏轉以使推力轉向(圖18-13)。除這些推進發動機以外，還可以安裝一臺或多臺升力發動機，以便在起飛和降落時提供輔助升力。
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 圖18-13 偏轉幾臺發動機產生噴氣升力
垂直起飛
空氣進氣門
輔助進氣口
打開
向前飛行
關閉
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.
圖18-12 遠距升力風扇
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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