Wednesday, March 21, 2012

Vertical/short take-off and landing - Lift engines

Lift engines
Fig. 18-10 A lift-jet engine.
13. The lift engine is designed to produce vertical thrust during the take-off and landing phases of V/STOL aircraft. Because the engine is not used in normal flight it must be light and have a small volume to avoid causing a large penalty on the aircraft. The lift engine may be a turbo-jet which for a given thrust gives the lowest weight and volume. Should a low jet velocity be necessary a lift fan may be employed.
14. Pure lift-jet engines have been developed with thrust/weight ratios of about 20:1 and still higher values are projected for the future. Weight is reduced by keeping the engine design simple and also by extensive use of composite materials (fig. 18-10). Because the engine is operated for only limited periods during specific flight conditions i.e. during take-off  and  landing,  the  fuel  system  can  be simplified and a total loss oil system (Part 8), in which the used lubricating oil is ejected overboard, can be used.

Fig. 18-11 Lift-fan engine configurations.

15. Lift engines can be designed to operate in the vertical or horizontal position and a thrust deflecting nozzle fitted to provide some of the advantages of thrust vectoring.  Alternatively, the engine may be mounted so that it can swivel through a large angle to provide thrust vectoring.  The lift-jet engine will have an extremely hot, high velocity jet exhaust and to reduce ground erosion by the jet the normal exhaust nozzle may be replaced by a multi-lobe nozzle  to  increase  the  rate  of  mixing  with  the surrounding air.
16. The lift-fan engine is designed to reduce the jet exhaust velocity, to reduce ground erosion and allow operation from unprepared ground surfaces. It also reduces the jet noise significantly. A range of designoptions have been considered for this type of engine and some are shown on fig. 18-11.

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