Page 204 - The Jet Engine

Vertical/short take-off and landing

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 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
Fig. 18-12 Remote lift fan. the control system more complex.
propulsion system to the best advantage whilst still Bleed air for STOL
maintaining the resultant thrust near the aircraft 22. Fig. 18-14 shows one method how STOL can
centre of gravity in the jet lift mode. This freedom is be achieved with a form of 'flap blowing'. The turbo-
achieved at a cost of increased volume, particularly fan engine has a geared variable pitch fan and an
with the gas driven systems, due to the size of the oversized low pressure (L. P.) compressor from the
ducts to feed the gas to the remote lift system. exit of which air is bled and ducted to the flap system
Although the mechanically driven remote lift-fan in the wing trailing edge. The variable pitch fan
eliminates the need for these large gas ducts, it is enables high L.P. compressor speed and thus high
done at the expense of long shafts and high power bleed pressure to be maintained over a wide range of
gearboxes and clutch systems. thrusts. This gives excellent control at greatly

Swivelling engines different aircraft flight conditions.
20. This method consists of having propulsion LIFT THRUST AUGMENTATION
engines which can be mechanically swiveled closed
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
Fig. 18-13 Jet lift with swivelling nozzles. lift rating.

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