Vertical/short take-off and landing - AIRCRAFT CONTROL

AIRCRAFT CONTROL

Fig. 18-18 Reaction control system.

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

Vertical/short take-off and landing - Lift burning systems

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 manoeuvra- bility and give supersonic flight.

Fig. 18-16 Plenum chamber burning.

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.

Vertical/short take-off and landing - Special engine ratings

Special engine ratings

Fig. 18-15 Thrust increases with short lift

ratings

24. Experience has shown that an engine rating  structure can be devised which provides high thrustlevels 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 lift rating.

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 tempera- tures.