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engine, either from the engine or by a
separate power unit.
(4) Swivelling the engines.
(5) For STOL aircraft, using bleed air from the
engines to increase circulation around the
wing and hence increase lift.
In several of the projected V/STOL aircraft a
combination of two or more of these methods has
been used.
Lift/Propulsion engines
7. The lift/propulsion engine is capable of providing
thrust for both normal wing borne flight and for lift.
This is achieved by changing the direction of the
thrust either by a deflector system consisting of one,
two or four swivelling nozzles or by a device known
as a switch-in deflector which redirects the exhaust
gases from a rearward facing propulsion nozzle to
one or two downward facing lift nozzles (fig, 18-4).
8. Thrust deflection on a single nozzle is accomplished
by connecting together sections of the jet
Vertical/short take-off and landing
189
Fig. 18-4 Thrust deflector systems.
Fig. 18-3 V/STOL fighter aircraft.
Vertical/short take-off and landing
190
Fig. 18-5 Deflector nozzle.
Fig. 18-6 Side mounted swivelling nozzle.
pipe, the joint faces of which are so angled that,
when the sections are counter-rotated, the nozzle
moves from the horizontal to the vertical position (fig.
18-5). To avoid either a side component o! thrust or a
thrust line offset from the engine axis during the
movement of the nozzle it is necessary that the first
joint face is perpendicular to the axis of the jet pipe.
If it is desired that the nozzle does not rotate, as may
be the case if it is a variable area nozzle, a third joint
face which is perpendicular to the axis of the nozzle
is required.
9. The two and four nozzle deflector systems use
side mounted nozzles (fig. 18-6) which can rotate on
simple bearings through an angle of well over 90
degrees so that reverse thrust can be provided if
required. A simple drive system, for example, a
sprocket and chain, can be used and by mechanical
connections all the nozzles can be made to deflect
simultaneously. For forward flight, to avoid a high
performance loss and consequent increase in fuel
consumption, careful design of the exhaust unit and
nozzle aerodynamic passages are essential to
minimize the pressure losses due to turning the
exhaust flow through two close coupled bends (fig.
18-7).
10. The switch-in deflector consists of one or a pair
of heavily reinforced doors which form part of the jet
pipe wall when the engine is operating in the forward
thrust condition. To select lift thrust, the doors are
moved to blank off the conventional propelling nozzle
and direct the exhaust flow into a lift nozzle (fig. 18-
8). The lift nozzles may be designed so that they can
be mechanically rotated to vary the angle of the
thrust and permit intermediate lift/thrust positions to
be selected.
11. A second type of switch-in deflector system is
used on the tandem fan or hybrid fan vectored thrust
engine (fig. 18-9). In this case the deflector system is
situated between the stages of the fan of a mixed
flow turbo-fan engine. In normal flight the valve is
positioned so that the engine operates in the same
manner as a mixed flow turbo-fan and for lift thrust
the valve is switched so that the exhaust flow from
the front part of the fan exhausts through downward
facing lift nozzles and a secondary inlet is opened to
provide the required airflow to the rear part of the fan
and the main engine. On a purely subsonic V/STOL
aircraft where fuel consumption is important the
valve may be dispensed with and the engine
operated permanently in the latter high by-pass
mode described above.
12. Thrust deflecting nozzles will create an
upstream pressure distortion which may excite
vibration of the fan or low pressure turbine blades if
the nozzle system is close to these components.
Snubbers (Part 3) may be used on the fan blades to
resist vibration. On the low pressure turbine, shrouds
at the blade tips (Part 5) or wire lacing may be used
to achieve the same result.
Lift engines
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
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