Abstract:
A gas turbine powerplant for a V/STOL aircraft in which a vectorable exhaust nozzle at a forward part (eg. nose) of the aircraft selectably receives a supply of cold fan air through a duct leading from the fan through a passageway ( 28 ) coaxial with the engine axis and located within the hub of the fan ( 12 ), with the aid of controllable flaps and diverters ( 26, 30 ).

Description:
FIELD OF THE INVENTION 
   The present invention relates to gas turbine power plants, and in particular to gas turbine powerplants for use in aircraft of the kind known as vertical or short take-off and landing (V/STOL) aircraft. 
   BACKGROUND OF THE INVENTION 
   V/STOL aircraft are well known and commonly employ at least one gas turbine powerplant with one or more vectorable exhaust nozzle(s) which can be directed aft for forward propulsion in wing-borne flight and downwardly for jet-borne flight (hovering) and also in directions and conditions intermediate those mentioned. 
   One such aircraft has a centrally located powerplant with two fore and two aft vectorable or swivellable lift nozzles mounted directly on the powerplant so as to be symmetrically disposed about the centre of gravity of the aircraft. This has the advantage that the thrusts from the lift nozzles are readily balanced about the centre of gravity and the ducting to the lift nozzles can be made short and therefore efficient. 
   However, the central location of the powerplant in such aircraft is much further forward than in most other aircraft and imposes severe constraints on aircraft configuration, notably the centre fuselage/wing attachment structure and the undercarriage location and supporting structure. In addition, the forward location of the powerplant reduces the room for the air intake, which is compensated by providing sharp bends with consequent loss of efficiency. Likewise, the jet pipe of the powerplant leading to an outlet nozzle for providing thrust for horizontal flight is lengthened, which is a problem especially in an aircraft incorporating an afterburner for enhanced performance. 
   In a development of such aircraft, the gas turbine powerplant is located in the more conventional location towards the rear of the aircraft and one or more lift device(s) or nozzle(s) is or are located separately from the main body of the powerplant for supporting the aircraft during jet-borne flight. The supply to the lift devices is provided, on the one hand, by the main exhaust efflux from the powerplant and, on the other, by bleeding off cold low pressure air from the front of the powerplant and leading it through a pipe to a remote lift device. This arrangement is known as a remote lift system (RLS). 
   The advantage of such an arrangement is the greater freedom in locating the powerplant whilst still permitting thrust balance to be attained for jet-borne flight. However, the designs of RLS which are currently available all involve pipes situated externally of the main body or casing of the gas turbine powerplant for carrying air forward to the remotely situated lift device or devices; such an arrangement has drawbacks in various aspects of the aircraft design in terms of fuselage cross-section, airframe drag, fuel consumption and aircraft mass, at least. 
   It is also known in gas turbine powerplants to bleed off a small proportion of the low pressure compressor air through the compressor hub for the purposes of pressurizing seals and providing cooling for lubricant oil and turbine blades. Such air is utilised for assisting in compressor efficiency but has no utilisation for lift devices or nozzles. 
   It is an aim of the present invention to provide a gas turbine powerplant including a supply for one or more lift devices or nozzles which overcomes the various problems described above and combines efficient and effective lifting with a compact powerplant design. 
   SUMMARY OF THE INVENTION 
   The gas turbine powerplant of the present invention employs a passage leading from the powerplant to the nose of the aircraft, coaxially with the main axis of the powerplant to supply low pressure compressor air forwards to a remote lift device. 
   According to one aspect of the present invention, there is provided a gas turbine powerplant for V/STOL aircraft, wherein means are provided for directing a sufficient proportion of the low pressure compressor air forwardly, preferably via a space within the low pressure by-pass fan hub of the powerplant, substantially along or adjacent to the central axis of the compressor to supply air to a remote lift means of the aircraft effective to support the aircraft in jet-borne flight. 
   The invention has a number of advantages and, in particular, enables a simplified powerplant installation and permits a simpler and more compact aircraft structure around the powerplant by comparison with the known designs featuring large pipes external to the main body of the powerplant. This saves mass and reduces the cross-sectional area of the aircraft thus improving flight performance and fuel consumption and reducing aircraft cost and maintenance. 
   By providing a passage extending along the axis of the main powerplant, a more direct supply to the remote lift device is possible. This reduces the mass and improves the aerodynamic efficiency of the lift device, and avoids the compromise of both intake duct path and aircraft cross-section which normally results from having duct(s) external to the engine. Even if the diameter of the air intake for the powerplant has to be increased, such increase will not be substantial and will not significantly affect the operation of the powerplant. 
   In a preferred embodiment of the invention, the gas turbine powerplant further comprises a by-pass fan and a by-pass duct surrounding the housing for the compressor, and the means for directing air forwardly are selectably operable to direct the by-pass air forwardly instead of rearwardly along into the by-pass duct or high-pressure compressor. 
   The forwardly directed air may be employed during jet-borne flight only for operation of the remote lift device, or alternatively, low pressure gas air may continue to be forwardly directed during wing-borne flight, for example for the supply on an auxiliary powerplant or for operation of a reaction control valve for pitch and/or yaw control of the aircraft at low speed or for supply to any other device requiring flow such as additional nozzles, blown flaps etc. 

   
     DESCRIPTION OF THE DRAWINGS 
     The invention is described further, by way of example only, with reference to the accompanying drawing which is a diagrammatic section through one embodiment of gas turbine powerplant according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to the drawing ( FIG. 1 ), which is an axial section through a gas turbine powerplant featuring the present invention, the lower half of the drawing illustrates the powerplant in a conventional flight mode for horizontal or wing-borne flight whereas the upper half of the drawing illustrates the powerplant in a remote lift mode for hovering or jet-borne flight. The lower half of the drawing will be described first. 
   As illustrated, the gas turbine powerplant comprises an air intake  10  supplying a low pressure or by-pass fan  12  within a fan housing  14 . Downstream of the low pressure fan  12  is a high pressure compressor  16  in a compressor housing  18 . A by-pass duct  20  surrounds the compressor housing  18 . A combustion chamber  22  is situated downstream of the compressor for injecting fuel into the compressed air and igniting it, the hot combustion air being then led to a turbine  24 , and from there to non-illustrated jet nozzles which may be vectorable nozzles. The by-pass air from duct  20  is directed to a non-illustrated flight nozzle which may also, in wing-borne flight mode, receive the exhaust air from the turbine. An afterburner section may be provided upstream of this flight nozzle. In this mode, a selectively operable diverter flap  26  or other suitable flow control device, located in the by-pass duct  20  at the upstream end of the compressor, is open. The powerplant operates conventionally to provide forward propulsion and need not be described further. 
   Turning now to the upper half of the drawing, it will be seen that the diverter  26  has been operated to restrict flow of air along duct  20 . This has the effect of forcing air compressed by the fan  12  to flow into a forwardly directed passage  28  in the hub of the fan  12 , to carry a supply of air forwardly to a remote lift device or vectorable nozzle (not shown). In this mode, the amount of air diverted into the forwardly directed passage  28  may be such a substantial proportion as to enable the remote lift device(s) to operate so as to enable the aircraft carrying the powerplant to sustain vertical or jet-borne flight. 
   A means to prevent air from flowing forwardly during conventional wing-borne flight mode must be provided. By way of example, the lower half of the drawing shows the provision in the forwardly directed passage  28  of a blocking baffle  30  which is arranged to block the passage  28  and prevent air from flowing forwards during the conventional flight mode. The baffle  30  or other blocking device may be situated at any convenient point along the flow path of the forwardly directed air. 
   As described with reference to the drawing, the illustrated gas turbine powerplant features a selectably operable arrangement for diverting low pressure compressor air from flowing through the by-pass duct  20  and into the forwardly directed passage  28 . 
   In an alternative embodiment of the invention, low pressure compressor air may be directed forwardly from part-way along the compressor  16  through the compressor hub. 
   In both instances, the forwardly directed flow takes place along the axis of the gas turbine powerplant, which has significant advantages in terms both of powerplant design and of overall aircraft design. 
   The passage  28  may include radial support structures for the hub itself or for the front bearing housing of the powerplant, and these may, if desired, be designed to promote efficient flow. For example, such radial support structures may be in the form of static vanes to generate a reaction force or in the form of rotating vanes  35  to do work on the flow, for eliminating swirl in the airflow or increasing the total enthalpy of the diverted air to increase the available thrust at the remote lift device. By increasing the proportion of vertical thrust available at the forward lift device an improved balance between power plant cycles may be achieved in wing-borne (forward flight) and jet-borne (vertical lift) flight modes. 
   If the vanes  35  are static vanes, they may be designed to have part or all of their chord variable in incidence, so as to either preserve engine matching over the range of conditions encountered, or to control the relation between thrust available from the diverted air, and thrust available from the air which passes through the downstream elements of the engine. This latter may be of particular value in maintaining effective balance and control of the aircraft throughout the jet-borne flight regime, and at the instant of change between forward (wing-borne) flight and vertical lift (jet-borne flight) modes.