the used lubricating oil is ejected overboard, can be used.
15. Lift engines can be designed to operate in the vertical or horizontal position and a thrust deflecting nozzle fitted to provide some of the advantages of thrust vectoring. Alternatively, the engine may be mounted so that it can swivel through a large angle to provide thrust vectoring. The lift-jet engine will have an extremely hot, high velocity jet exhaust and to reduce ground erosion by the jet the normal
exhaust nozzle may be replaced by a multi-lobe nozzle to increase the rate of mixing with the surrounding air.
16. The lift-fan engine is designed to reduce the jet exhaust velocity, to reduce ground erosion and allow operation from unprepared ground surfaces. It also reduces the jet noise significantly. A range of design options have been considered for this type of engine and some are shown on fig. 18-11.
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Fig. 18-9 Vectored thrust engine. Fig. 18-8 Switch-In deflector system.
Remote lift systems
17. Direct lift remote systems duct the by-pass air or engine exhaust air to downward facing lift nozzles remote from the engine. These nozzles may be in the front fuselage of the aircraft or in the wings. The engine duct is blocked by means of a diverter similar to that described in para. 10.
18. The remote lift-fan (fig. 18-12) is mounted in the aircraft wing or fuselage, and is driven mechanically or by air or gas ducted into a tip turbine, The drive system is provided by the main propulsion power plant or by a separate engine.
19. The advantage of the remote lift system is that it gives some freedom to the aircraft to position the
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Fig. 18-11 Lift-fan engine configurations. Fig. 18-10 A lift-jet engine.
propulsion system to the best advantage whilst still maintaining the resultant thrust near the aircraft centre of gravity in the jet lift mode. This freedom is achieved at a cost of increased volume, particularly with the gas driven systems, due to the size of the ducts to feed the gas to the remote lift system. Although the mechanically driven remote lift-fan eliminates the need for these large gas ducts, it is done at the expense of long shafts and high power gearboxes and clutch systems.
Swivelling engines
20. This method consists of having propulsion engines which can be mechanically swiveled closed
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 the control system more complex.
Bleed air for STOL
22. Fig. 18-14 shows one method how STOL can be achieved with a form of 'flap blowing'. The turbo-fan engine has a geared variable pitch turbo-fan and an oversized low pressure (L. P.) compressor from the exit of which air is bled and ducted to the flap system in the wing trailing edge. The variable pitch fan enables high L.P. compressor speed and thus high bleed pressure to be maintained over a wide range of thrusts. This gives excellent control at greatly different aircraft flight conditions.
LIFT THRUST AUGMENTATION
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 lift rating.
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Fig. 18-12 Remote lift fan.
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.
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.
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.
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Fig. 18-14 Flap blowing engine.
Fig. 18-15 Thrust increases with short lift ratings.
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Fig. 18-16 Plenum chamber burning.
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 con-figuration. A rapid response rate is essential to enable the engines to be used for aircraft stability and control. It is usually necessary to combine differ-ential throttling with differdiffer-ential 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.
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