Page 201 - The Jet Engine

Vertical/short take-off and landing

pipe, the joint faces of which are so angled that, 10. The switch-in deflector consists of one or a pair
when the sections are counter-rotated, the nozzle of heavily reinforced doors which form part of the jet
moves from the horizontal to the vertical position (fig. pipe wall when the engine is operating in the forward
18-5). To avoid either a side component o! thrust or a thrust condition. To select lift thrust, the doors are
thrust line offset from the engine axis during the moved to blank off the conventional propelling nozzle
movement of the nozzle it is necessary that the first and direct the exhaust flow into a lift nozzle (fig. 18-
joint face is perpendicular to the axis of the jet pipe. 8). The lift nozzles may be designed so that they can
be mechanically rotated to vary the angle of the
If it is desired that the nozzle does not rotate, as may thrust and permit intermediate lift/thrust positions to
be the case if it is a variable area nozzle, a third joint be selected.
face which is perpendicular to the axis of the nozzle
is required. 11. A second type of switch-in deflector system is
used on the tandem fan or hybrid fan vectored thrust
9. The two and four nozzle deflector systems use engine (fig. 18-9). In this case the deflector system is
side mounted nozzles (fig. 18-6) which can rotate on situated between the stages of the fan of a mixed
simple bearings through an angle of well over 90 flow turbo-fan engine. In normal flight the valve is
degrees so that reverse thrust can be provided if positioned so that the engine operates in the same
required. A simple drive system, for example, a manner as a mixed flow turbo-fan and for lift thrust
sprocket and chain, can be used and by mechanical the valve is switched so that the exhaust flow from
connections all the nozzles can be made to deflect the front part of the fan exhausts through downward
facing lift nozzles and a secondary inlet is opened to
simultaneously. For forward flight, to avoid a high provide the required airflow to the rear part of the fan
performance loss and consequent increase in fuel and the main engine. On a purely subsonic V/STOL
consumption, careful design of the exhaust unit and aircraft where fuel consumption is important the
nozzle aerodynamic passages are essential to valve may be dispensed with and the engine
minimize the pressure losses due to turning the operated permanently in the latter high by-pass
exhaust flow through two close coupled bends (fig. mode described above.
18-7).
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
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
Fig. 18-7 Nozzle duct configuration. simplified and a total loss oil system (Part 8), in which

191