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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
Vertical/short take-off and landing
191
Fig. 18-7 Nozzle duct configuration.
the used lubricating oil is ejected overboard, can be
used.
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 design
options have been considered for this type of engine
and some are shown on fig. 18-11.
Vertical/short take-off and landing
192
Fig. 18-9 Vectored thrust engine.
Fig. 18-8 Switch-In deflector system.
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
Vertical/short take-off and landing
193
Fig. 18-11 Lift-fan engine configurations.
Fig. 18-10 A lift-jet engine.
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
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.
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 considerations
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 turbofan
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-
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