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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 takeoff
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
lift rating.
Vertical/short take-off and landing
194
Fig. 18-12 Remote lift fan.
Fig. 18-13 Jet lift with swivelling nozzles.
25. At high ambient temperatures, the engine may
run into a turbine temperature limit before reaching
its maximum r.p.m. and suffer a thrust loss as a
result. Restoration of the thrust can be achieved by
means of water injection into the combustion
chamber (Part 17) which allows operation at a higher
turbine gas temperature for a given turbine blade
temperature. If desired, water injection can also be
used to increase the thrust at low ambient temperatures.
Lift burning systems
26. The thrust of the four nozzle lift/propulsion
engine may be boosted by burning fuel in the bypass
flow in the duct or plenum chamber supplying the
front nozzles. This is called plenum chamber burning
(P.C.B.) (fig. 18-16) and thrust of the by-pass air may
be doubled by this process. This thrust capability is
available for normal flight as well as take-off and
landing and so can be used to increase manoeuvrability
and give supersonic flight.
27. The thrust of a remote lift jet can also be
augmented by burning fuel in a combustion chamber
just upstream of the lift nozzle (fig. 18-17). This
system is commonly known as a remote augmented
lift system (R.A.L.3.). The thrust boost available from
the burner reduces the amount of airflow to be
supplied to it and therefore reduces the size of the
ducting needed to direct the air from the engine to
the remote lift nozzle.
Ejectors
28. The principle of the ejector is that a small, high
energy jet entrains large quantities of ambient air by
viscous mixing and an increase in thrust over that of
the high energy jet results. A number of projected
V/STOL aircraft have incorporated this concept using
either all the engine exhaust air or just the bypass
flow.
Vertical/short take-off and landing
195
Fig. 18-14 Flap blowing engine.
Fig. 18-15 Thrust increases with short lift
ratings.
Vertical/short take-off and landing
196
Fig. 18-16 Plenum chamber burning.
Fig. 18-17 Remote augmented lift system.
AIRCRAFT CONTROL
29. The low forward speeds of V/STOL aircraft
during take-off and transition do not permit the
generation of adequate aerodynamic forces from the
normal flight control surfaces, it is therefore
necessary to provide one or more of the following
additonal methods of controlling pitch, roll and yaw.
Reaction controls
30. This system bleeds air from the engine and
ducts it through nozzles at the four extremities of the
aircraft (fig. 18-18), The air supply to the nozzles is
automatically cut off when the main engine swivelling
propulsion nozzles are turned for normal flight or
when the lift engines are shut down. The thrust of the
control nozzles is varied by changing their area
which varies the amount of airflow passed.
Differential engine throttling
31. This method of control is used on multi-engined
aircraft with the engines positioned in a suitable configuration.
A rapid response rate is essential to
enable the engines to be used for aircraft stability
and control. It is usually necessary to combine differential
throttling with differential thrust vectoring to
give aircraft control in all areas.
Automatic control systems
32. Although it is possible for the pilot to control a
V/STOL aircraft manually, some form of automation
can be of benefit and in particular will reduce the pilot
workload. The pilot’s control column is electronically
connected to a computer or stabilizer that receives
signals from the control column, compares them with
signals from the sensors that measure the attitude of
the aircraft, and automatically adjusts the reaction
controls, differential throttling or thrust vectoring
controls to maintain stability.
Vertical/short take-off and landing
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