GB2320477A

GB2320477A - Tail rotorless helicopters

Info

Publication number: GB2320477A
Application number: GB9213471A
Authority: GB
Inventors: Darrell Lee Mann; Martin Berry; Richard Cyril Adkins
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1991-06-29
Filing date: 1992-06-25
Publication date: 1998-06-24
Anticipated expiration: 2012-06-25
Also published as: GB9213471D0; GB9114085D0; GB2320477B

Abstract

The helicopter has a main body portion, a main rotor, a gas turbine engine 14, including a variable area exhaust nozzle 16, for driving the main rotor, and a hollow tail boom 20 extending from the main body portion. Exhaust efflux from the engine (Fig 3) is directed into the boom and the boom is provided at an end remote from the main body portion with a means (Figs 4-8) for directing exhaust efflux from within the boom into the atmosphere at an angle that may be controlled with respect to the axis of the boom. The means is arranged in a first mode of operation to direct the efflux sideways from the boom so as to provide anti-torque, and in a second mode of operation is arranged to direct the efflux rearwards in the direction of the axis of the boom so as to provide forward thrust. The tail boom may have an aerofoil cross section (Fig 9) which interacts with the main rotor downwash to supplement the side thrust.

Description

IMPROVEMENTS IN OR RELATING TO HELICOPTERS This invention concerns improvements in or relating to helicopters, and in particular to a means for countering torque produced by the helicopter's main rotor system.

Conventionally, a helicopter is provided with a tail rotor to counter torque produced by the main rotor system. The tail rotor adds significant complexity and weight to the overall helicopter because it is necessary to provide gearing and shafting to drive the rotor from the engine. In order to adequately counter the torque produced by the main rotor system it is necessary to mount the tail rotor as far as possible from the helicopter's centre of gravity. This requirement, in conjunction with the weight of the tail rotor drive mechanism, further adds to the weight of the overall airframe in that the tail boom is a structural member of the helicopter.

Further, the complexity of the conventional tail rotor system can give rise to problems which are factors in a significant number of helicopter failures. For this reason, the weight factor and the fact that the tail rotor contributes significantly to aircraft noise, it would appear to be desirable to be able to eliminate the tail rotor system altogether.

At high forward speeds of the helicopter, although the anti-torque required from the tail rotor is reduced, the system remains as a contributor to the drag of the vehicle and hence only acts to inhibit increasing forward speed. It is desirable to eliminate this source of drag, especially for compound type helicopters where it is hoped to introduce greater possible maximum speeds.

A further problem of helicopters, not necessarily concerned with the tail rotor system of the aircraft although the rotor gearbox is a potentially large heat source, is the infra-red signature. In the case of military aircraft, much effort has been expended in reducing infra-red signature levels, which result mainly from the efflux of the powerplant system. The usual means of overcoming this problem is by one or more of the following: (i) shielding hot engine parts; (ii) diluting the engine exhaust plume; and (iii) keeping the diluted plume away from other parts of the airframe.

Item (iii) usually means that the plume has to-be pointed away from the airframe. In doing this a further drag penalty is introduced to the helicopter, but using an ejector system to dilute the plume in the first instance will introduce a drag penalty which will become progressively worse as forward speed is increased due to the increasing amount of air which is ducted into the ejector system by ram action. Clearly, this drag conflicts with a requirement for a very high speed helicopter.

Regarding aircraft controllability, a tail rotor system provides only relatively unsophisticated directional control. The tail rotor provides a thrust which is at or nearly perpendicular to the direction of travel, primarily to counteract main rotor torque; thus yaw control may be provided by varying this thrust. However, pitch control has to effected by the main rotor, thus adding further to the overall complexity of the aircraft.

Helicopters with tail rotors are also susceptible to main rotor tip vortex ingestion, a phenomenon which may be of particular concern when attempting to yaw whilst in the hover mode. Swallowing of the rotor tip vortex causes fluctuations in tail rotor thrust which require pilot input to correct. Thus, for example, turning about a point may require skilful manipulation of the controls by the pilot.

A recently proposed solution to the above problems involves replacing the conventional tail rotor by essentially the same rotor arrangement mounted within the aircraft tail boom. The fan within the boom serves to feed low pressure ratio air to two mechanisms which provide the required aircraft anti-torque.

In the first of these two mechanisms pressurised air generated by the fan within the boom is forced round a cascade bend and through a nozzle system to be ejected at right angles to the tail boom.

In the second mechanism two slots aligned along the tail boom eject air supplied by the fan within the boom over the skin of the boom. This air, blown over the skin of the airframe, serves to entrain air being forced down from the main rotor in a manner which generates a net sideways force, thus providing anti-torque.

As the forward speed of the aircraft is increased, the anti-torque load is progressively taken by the tail fin, to a point where at cruise forward flight, 90% of yaw control is provided by the fin. In this way the prior proposal has as its objective reduction of the fan power requirement.

However, the prior proposal fails in several respects to adequately address the problems discussed above, for the following reasons: 1) The tail rotor has not in fact been removed, but has been relocated within the tail boom. This means that system complexity has not been significantly reduced, in that the necessary gearing, shafting and fan pitch controls are still present.

2) The anti-torque system so described is inherently unable to provide forward thrust.

3) The controllability of the aircraft has not been enhanced significantly and there is no means provided to control pitch from the tail.

4) In the second of the two mechanisms described above the anti-torque system suffers from the need to maintain a constant pressure air feed to the slots.

It is accordingly an object of the present invention to provide a helicopter without a tail rotor system but nevertheless with provision for anti-torque.

In its most general aspect the invention comprises a gas turbine-driven tail-rotorless helicopter wherein exhaust efflux from the gas turbine is used instead of a tail rotor to provide anti-torque.

According to the present invention there is provided a helicopter having a main body portion, a main rotor, a gas turbine engine, including a variable area exhaust nozzle, for driving the main rotor, and a hollow tail boom extending from the main body portion, wherein means is provided to direct exhaust efflux from the engine into the boom and the boom is provided at an end remote from the main body portion with a first means for directing exhaust efflux from within the boom into the atmosphere at an angle that may be controlled with respect to the axis of the boom.

Preferably, said first means is arranged in a first mode of operation to direct the efflux at right angles to the axis of the boom and at right angles to the axis of the rotor so as to provide anti-torque, and in a second mode of operation is arranged to direct the efflux rearwards in the direction of the axis of the boom so as to provide forward thrust.

An air inlet may be provided upstream of and surrounding the variable area engine exhaust nozzle, and arranged to entrain ambient air into the boom, thereby to cool the engine exhaust and augment available thrust power. The air inlet may be configured so as to eliminate line-of-sight viewing of the engine nozzle.

The tail boom is preferably of double skin construction and provision may be made to pass cooling air between the skins. The outer skin of the boom may be constructed from a light composite material.

Preferably, the cooling air is provided by said air inlet.

An ejector system may be provided to eject the cooling air from between the skins into the means for directing the exhaust efflux from the boom into the atmosphere.

The air inlet to the boom may be provided with a power driven fan and variable inlet guide vanes upstream of the fan, to pressurise ambient air entrained into the boom. The fan may be driven by the helicopter engine, and may be directly attached to the engine main shaft.

A second means may be provided within the boom for directing exhaust efflux into the atmosphere in a direction opposed to that produced by the first means when in the first mode of operation, so as to provide improved yaw control.

The first exhaust efflux ejector means may be provided by a swan neck duct having a fixed first section substantially at right angles to the boom axis and an Lshaped second section rotatably mounted on the first section for rotation about the axis of the first section whereby exhaust may be effluxed at right angles to the boom axis in the first mode of operation to provide anti-torque and rearwards parallel to the boom axis in the second mode of operation to provide forward thrust.

In a first alternative the first exhaust efflux ejector means may be provided by a first nozzle directed sideways from the boom and a second nozzle directed rearwards from the boom, together with valve means to effect split flow of the exhaust gas stream between the two nozzles.

The valve means may be provided by a gate valve, or, in a further embodiment, by a variable pitch cascade of vanes.

In a second alternative the first exhaust efflux ejector may be provided by a plug member located on the axis of the boom at the remote end of the boom and surrounded by an annular section of the boom, an aperture being provided in the wall of the boom upstream of the plug and annular section, and a hollow frusto-conical member within the boom and translatable along the axis of the boom, the frusto-conical member having its larger end towards the remote end of the boom, whereby in a first position when the frusto-conical member abuts the plug member the remote end of the frusto-conical member is blocked by the plug member and the exhaust gases are constrained to exit sideways from the boom via said aperture, and in a second position when the frustoconical member is spaced upstream from the plug member said aperture is blocked by the frusto-conical member and the exhaust gases are constrained to pass axially through the frusto-conical member and to exit between the plug member and said annular section of the boom in a direction generally parallel to the axis of the boom.

The external cross section of the tail boom may be an airfoil aligned so as to employ main rotor downwash to provide a force which provides a measure of anti-torque.

Preferably, the interior of the boom is generally convergent downstream of the engine exhaust nozzle.

Preferably, the external surface of the boom is provided with one or more fins to extend the area available for cooling.

Preferably, the tail boom is provided internally with one or more vanes or baffles to limit line-of-sight viewing of hot engine exhaust gases.

The invention will now be described by way of example only with reference to the accompanying non-scale purely diagrammatic drawings, in which, Figure 1 is a general representation of a tail-rotorless helicopter according to the invention; Figure 2 is a part sectional plan view of a tail boom in the helicopter of Figure 1; Figure 3 is a general arrangement showing a configuration of an engine with an air inlet to a tail boom according to the invention; Figures 4 - 6 and 8 show longitudinal sections through alternative tail boom exhaust configurations; Figure 7 is a cross section through the configuration of Figure 6 taken at line VII-VII; Figure 9 is a cross-section profile of a further embodiment of a tail boom according to the invention; Figure 10 is a perspective view of a variable area 2-D nozzle for use in the invention; and Figure 11 is a cross-section of a tail boom containing two gas turbine engines.

Referring to the drawings, there is shown in Figure 1 a helicopter 10 having a main body section or fuselage 12, a gas turbine engine 14 as a power source for the helicopter, an exhaust nozzle 16 mounted on the engine, a main rotor 18, and a rotorless tail boom 20 extending from the fuselage. The tail boom 20 is provided with control surfaces 102, 104 to provide a pitch and yaw capability at high speed. The exhaust nozzle 16 is of variable area construction, as will be discussed below with reference to Figures 2 and 3.

As shown in Figure 2, the tail boom 20 is hollow and receives engine exhaust gases from the engine exhaust nozzle 16. In general, by passing this engine exhaust longitudinally through the tail boom 20 and discharging it at right angles to the axis of the boom and at right angles to the axis of the main rotor 18, a proportion of the required main rotor anti-torque will be provided, as shown by arrow 22. Provision is also made, as will hereinafter be described with reference to further drawings, to direct exhaust gas rearwards (arrow 24) so as to provide direct forward thrust in forward flight as anti-torque is progressively provided by a tail fin, or sideways (arrow 26) in a direction opposite to that of arrow 22 to provide yaw trim, as required.

In order both to cool the engine exhaust passing through the boom 20 and to augment the available thrust power, an air inlet 28 is provided upstream of the boom and surrounding the engine final nozzle 16. This creates an ejector system and ambient air will be entrained into the boom 20 through inlet 28. It will be necessary to entrain, typically, 1.5 to 2 times engine exhaust flow in order to cool the plume temperature to about 200"C above ambient temperature.

As shown in Figure 3, the ambient air entering the inlet 28 to the boom 20 may be pressurised by means of a fan 11 connected to the power turbine of the engine 14 and a set of variable inlet guide vanes 13 located in the air inlet 28.

The boom 20 is convergent downstream of the engine final nozzle 16 not only to improve thrust augmentation but to reduce the size and weight of the system. The air inlet 28 is configured in such a manner as to prevent a direct external view of the engine nozzle 16 and thereby to eliminate a possible source of infrared radiation.

However, in alternative embodiments (not illustrated) the boom 20 need not be convergent.

The externally visible surfaces of the tail boom 20 need to be close to ambient temperature not only to minimise their infra-red signature but to provide a surface temperature that is within safety limits for handling by maintenance personnel and others when the helicopter is on the ground. This objective is achieved by making the tail boom 20 of double skin construction 30. Cooling air enters between the skins at the air inlet 28 and leaves by an ejector system 32 near the end of the boom remote from the engine final nozzle 16. This cooling makes it possible to construct the outer skin of the boom 20 of a light composite material. Further cooling of the boom may be achieved by increasing the surface area (eg by finning) and utilising the main rotor downwash air.

The engine nozzle 16 is a variable area device, so as to enable easy transition of shaft power to thrust power.

The variable nozzle is preferably of the known circular petalled arrangement exemplified in, for instance, the Rolls-Royce RB199 engine. This petalled arrangement is illustrated in "The Jet Engine", 4th Edition, 1986, ISBN 0 902121 04 9, page 171, published by Rolls-Royce plc.

Possible configurations of the variable area nozzle 16 which may be adopted by the nozzle are illustrated in Figures 2 and 3 at 106 and 108, and in Figure 10. In configuration 106 the nozzle has a restricted cross-section area, whereas in configuration 108 the cross-section area is increased. This variability allows for better control of the air entrained into the tail boom, for instance by controlling the mass-flow of gas passing through the boom, and this will assist in control of the aircraft in both the hover mode and in high speed flight. Scheduling arrangements may be made to provide coordination of the tail boom ejector system with the variable area nozzle.

An alternative way of varying the area of the nozzle 16 may be by means of the arrangement known as "2-D", which will be described below with reference to Figure 10.

Figure 4 shows a simple nozzle system for controlling the direction of exhaust gases leaving the remote end of the boom 20. In this system, air passing along the tail boom 20 is directed firstly downwards and then rearwards through a swan neck shaped duct 34. The duct 34 is in two parts: the first part, 36, is an L-shaped downwardly facing extension of the boom 20; the second part, a final nozzle 38, is a further L-shaped extension rotatably mounted on the end of the first part so that the exhaust angle may be varied from sideways to rearwards to provide anti-torque and thrust respectively, as indicated by arrow 40.

A potential problem arising in some circumstances from this arrangement is that an unwanted forward thrust may result if yaw is demanded when flying in the hover mode and the nozzle 38 is then rotated.

The potential problem mentioned above is overcome in an alternative embodiment shown in Figure 5 wherein a gate valve 42 effects a flow split between nozzles 44, 46 pointing sideways and rearwards respectively from the boom 20. A second sideways nozzle 48, provided with a gate valve 50, is located opposite the main sideways nozzle 44 and serves to improve yaw control, thus solving the potential unwanted forward thrust problem mentioned above with reference to Figure 4.

Figures 6 and 7 show a further embodiment based on the concept of Figure 5. In this embodiment a variable pitch cascade of vanes 52 is provided to effect a flow split between nozzles 44 and 46. Likewise, as in the embodiment of Figure 4, yaw control is provided by a further cascade of variable pitch vanes 54 in association with the second sideways nozzle 48. Control of pitching may be provided by supplementary nozzles 56 and 58 shown in Figure 7 arranged at right angles to nozzles 44 and 48. Nozzles 56 and 58 will be controlled by their own respective sets of variable pitch vanes or gate valves and allow a pitching moment which improves the controllability and manoeuvrability of the helicopter.

The embodiment of Figure 8 shows the invention in two configurations: the first, above the centre line is in the thrust condition; the second, below the centre line, is in the yaw condition. There is shown provided a plug 60 located on the axis of the boom 20 at the remote end of the boom and surrounded by an annular section 62 of the boom.

A single aperture 64 or diametrically opposed apertures 64, 66 are provided in the wall of the boom 20 upstream of the annular section 62. A hollow frusto-conical member 68 is provided within the boom 20, has its larger end towards the remote end of the boom, and is arranged to translate along the axis of the boom.

In the first configuration, when the frusto-conical member 68 abuts the plug 60 the end of the frustoconical member remote from the engine is blocked by the plug and the exhaust gases are constrained to exit sideways from the boom 20 via one or both of said apertures 64, 66.

In the second configuration, when the frusto-conical member 68 is spaced upstream from the plug 60, the aperture 64 or apertures 64, 66 are blocked by the frusto-conical member and the exhaust gases are constrained to pass axially through the frusto-conical member and to exit between the plug and said annular section 62 of the boom 20 in a direction generally parallel to the axis of the boom.

The embodiment of Figure 8 has the inherent advantage of preventing, by means of the plug 60, any direct view into the tail boom of the aircraft and hence any direct view of the engine exhaust. A similar result may be achieved in the other embodiments by including, for example, swirl-deswirl vanes or baffles within the tail boom 20.

Further cooling of the vanes, plugs and other exhaust control surfaces of the embodiments discussed above, if required for military purposes, may be effected by the provision of cooling techniques known in the art of cooling hot turbine engine components such as blades and vanes. Such cooling techniques include film cooling whereby cool ambient air may be blown into the interior of the component and forced out through minute cooling holes in the surface of the component to form a film of cool air over the surface of the component. This is illustrated schematically in Figure 8 by film-cooling holes 124 in the surface of plug 60. Alternatively, the plug 60 (for instance) may be provided with a double skin through which cool ambient air is passed to cool the surface. Cooling of the downstream surface of the plug 60 is important in order to reduce detectable infra-red emissions.

The engine exhaust plus entrained air provides a proportion of the total required main rotor anti-torque.

All of the required anti-torque could in principle be provided by the power output alone of the turbine plant, but this is inefficient in that it requires of the order of 25% of engine power, as against 11% engine power required for the conventional tail rotor system in the worst case condition (hovering flight).

A solution to this problem is shown in Figure 9, wherein main rotor downwash is used in conjunction with a tail boom 20 having an external cross section profile 70 in the shape of an airfoil aligned so as to provide a net side force resulting from the main rotor downwash opposing in part the torque of the main rotor. - This provides a purely passive means of inducing an antitorque force which supplements that available from the engine exhaust gas elf fluxing for example through nozzle 44. The interior of the airfoil is provided with a hollow core 71 through which exhaust gases pass to efflux through the nozzles described above.

The trailing edge of the airfoil 70 is provided with a flap 72 whereby trim on the tail boom may be improved.

Figure 10 shows a "2-D" variant 110 of the variable area nozzle 16 of Figure 2. The 2-D nozzle 110 provides a circular-to-rectangular engine exhaust pipe and comprises a pair of opposed movable generally rectangular flaps 112, 114 rotatable in the direction of arrows 116 about a pair of spaced parallel axes that lie at right angles to the longitudinal axis of the exhaust nozzle. Fixed opposed side-walls 118, 120 located in planes at right angles to the parallel axes of rotation of the flaps 112, 114 extend downstream from a circular cross-section portion 122 of the nozzle 110 to provide sealing members at the sides of the flaps. Hence, the engine exhaust gases are constrained to enter the tail boom 20 via a rectangular aperture. This can be an advantage if the tail boom has a cross-section that departs markedly from the circular, as for instance in the tail boom shown in Figure 9.

The 2-D nozzle 110 will of course be provided with operating mechanisms well known in the art and accordingly not illustrated here. In some embodiments the 2-D nozzle may be preferred to the petalled arrangement referred to above in that the 2-D nozzle is mechanically simpler and lighter, despite having recognised disadvantages in that under some conditions flow vortices can occur in the corners of the rectangular opening.

In terms of controllability and manoeuvrability of the helicopter the present invention provides potentially great benefits, including reduced complexity - variable pitch rotors being replaced by simple valve movements, and reduced overall weight. The weight of the tail boom itself can be reduced significantly because it no longer has to support the weight of control mechanisms associated with a tail rotor.

Elimination of a tail rotor automatically reduces drag, and by employing engine exhaust in an augmenting ejector system in the manner described which allows an axially rearward plume discharge, it becomes possible to produce useful thrust. As mentioned above, incorporation of a variable area engine final nozzle accentuates this effect for compound helicopter application.

The invention enables the helicopter to have an inherently low infra-red signature. All views of the engine nozzle system are hidden by the ambient air intake, entrainment of ambient air and double skinning of the tail boom keeps it cool, and the final plume discharge occurring at the extreme rear of the overall airframe means that airframe heating is insignificant.

Supplementary cooling of externally visible components which are at plume temperature may be achieved by either active or passive means.

In a further embodiment of the invention, illustrated in Figure 11 there may be provided two, or more, parallel engines 126, 128 ejecting exhaust gases into the tail boom 20, one engine preferably being positioned above the other, and at least one engine, preferably both, having a variable area exhaust 16 as discussed with reference to Figures 2 and 3.

Claims

1A helicopter having a main body portion, a main rotor, a gas turbine engine, including a variable area exhaust nozzle, for driving the main rotor, and a hollow tail boom extending from the main body portion, wherein means is provided to direct exhaust efflux from the engine into the boom and the boom is provided at an end remote from the main body portion with a first means for directing exhaust efflux from within the boom into the atmosphere at an angle that may be controlled with respect to the axis of the boom.

2 A helicopter as claimed in claim 1 wherein said first means is arranged in a first mode of operation to direct the efflux at right angles to the axis of the boom and at right angles to the axis of the rotor so as to provide anti-torque, and in a second mode of operation is arranged. to direct the efflux rearwards in the direction of the axis of the boom so as to provide forward thrust.

3 A helicopter as claimed in claim 1 or 2 wherein an air inlet is provided upstream of and surrounding the variable area engine exhaust nozzle, and arranged to entrain ambient air into the boom, thereby to cool the engine exhaust and augment available thrust power.

4 A helicopter as claimed in any preceding claim wherein the tail boom is of double skin construction and provision is made to pass cooling air between the skins.

5 A helicopter as claimed in claim 4 wherein the cooling air is provided by said air inlet.

6 A helicopter as claimed in claim 4 or 5 wherein an ejector system is provided to eject the cooling air from between the skins into the means for directing the exhaust efflux from the boom into the atmosphere.

7 A helicopter as claimed in claim 3 wherein the air inlet to the boom is provided with a power driven fan and variable inlet guide vanes upstream of the fan, to pressurise ambient air entrained into the boom.

8 A helicopter as claimed in claim 7 wherein the fan is driven by the helicopter engine.

9 A helicopter as claimed in claim 8 wherein the fan is directly attached to the engine main shaft.

10 A helicopter as claimed in claim 2 or in any one of claims 3 to 9 as dependent on claim 2 wherein a second means is provided within the boom for directing exhaust efflux into the atmosphere in a direction opposed to that produced by the first means when in the first mode of operation, so as to provide improved yaw control.

11 A helicopter as claimed in claim 10 wherein the first exhaust efflux ejector means is provided by a swan neck duct having a fixed first section substantially at right angles to the boom axis and an L-shaped second section rotatably mounted on the first section for rotation about the axis of the first section whereby exhaust may be effluxed at right angles to the boom axis in the first mode of operation to provide anti-torque and rearwards parallel to the boom axis in the second mode of operation to provide forward thrust.

12 A helicopter as claimed in claim 10 wherein the first exhaust efflux ejector means is provided by a first nozzle directed sideways from the boom and a second nozzle directed rearwards from the boom and there is provided valve means to effect split flow of the exhaust gas stream between the two nozzles.

13 A helicopter as claimed in claim 12 wherein the valve means is provided by a gate valve.

14 A helicopter as claimed in claim 12 wherein the valve means is provided by a variable pitch cascade of vanes.

15 A helicopter as claimed in claim 10 wherein the first exhaust efflux ejector is provided by a plug member located on the axis of the boom at the remote end of the boom and surrounded by an annular section of the boom, an aperture being provided in the wall of the boom upstream of the plug and annular section, and a hollow frusto-conical member within the boom and translatable along the axis of the boom, the frusto-conical member having its larger end towards the remote end of the boom, whereby in a first position when the frusto conical member abuts the plug member the remote end of the frusto-conical member is blocked by the plug member and the exhaust gases are constrained to exit sideways from the boom via said aperture, and in a second position when the frusto-conical member is spaced upstream from the plug member said aperture is blocked by the frusto-conical member and the exhaust gases are constrained to pass axially through the frusto-conical member and to exit between the plug member and said annular section of the boom in a direction generally parallel to the axis of the boom.

16 A helicopter as claimed in claim 15 wherein there is provided means to cool the surface of the plug.

17 A helicopter as claimed in claim 16 wherein the cooling means is provided by film cooling holes in the surface of the plug and a source of ambient air to provide cooling air to enter the plug and exit via the cooling holes.

18 A helicopter as claimed in claim 16 wherein the cooling means is provided by a double skin surface of the plug through which cooling ambient air is passed.

19 A helicopter as claimed in any preceding claim wherein the external cross section of the tail boom is an airfoil aligned so as to employ main rotor downwash to provide a force which provides a degree of anti- torque.

20 A helicopter as claimed in claim 3 wherein the air inlet is configured so as to eliminate line-of-sight viewing of the engine nozzle.

21 A helicopter as claimed in claim 4 wherein the outer skin of the boom is constructed from a light composite material.

22 A helicopter as claimed in any preceding claim wherein the interior of the boom is generally convergent downstream of the engine exhaust nozzle.

23 A helicopter as claimed in any preceding claim wherein the external surface of the boom is provided with one or more fins to extend the area available for cooling.

24 A helicopter as claimed in any preceding claim wherein the tail boom is provided internally with one or more vanes or baffles to limit line-of-sight viewing of hot engine exhaust gases.

25 A helicopter as claimed in any preceding claim wherein the variable area engine nozzle is of circular petalled construction.

26 A helicopter as claimed in any one of claims 1 to

24 wherein the variable area engine nozzle is a 2-D nozzle.

27 A helicopter as claimed in any preceding claim wherein the tail boom is provided with control surfaces to provide a pitch and yaw capability at high speed.

28 A helicopter as claimed in any preceding claim wherein there are provided two parallel engines ejecting exhaust gases into the tail boom.

29 A helicopter substantially as hereinbefore described with reference to any one of the accompanying drawings.

Amendments to the claims have been filed as follows 1A helicopter comprising a main body portion, a main rotor, a gas turbine engine, including a variable area exhaust nozzle, for driving the main rotor, and a hollow tail boom extending from the main body portion, the nozzle being arranged to direct engine exhaust efflux into the boom, and the boom being provided with a first means, at an end remote from the main portion, for directing the exhaust efflux into the atmosphere at an angle that may be controlled with respect to the axis of the boom.

2 A helicopter as claimed in claim 1 wherein said first means is arranged in a first mode of operation to direct the efflux at right angles to the axis of the boom and at right angles to the axis of the rotor so as to provide anti-torque, and in a second mode of operation is arranged to direct the efflux rearwards in the direction of the axis of the boom so as to provide forward thrust.

5 A helicopter as claimed in claim 4 as dependent on claim 3 wherein the cooling air is provided by said air inlet.

6 A helicopter as claimed in claim 4 or 5 wherein an ejector system is provided to eject the cooling air from between the skins into the means for directing the exahsut efflux from the boom into the atmosphere.

7 A helicopter as claimed in claim 3 wherien the air inlet to the boom is provided with a power driven fan and varaible inlet guide vanes upstream of the fan, to pressuirse ambient air entrained into the boom.

10 A helicopter as claimed in claim 2 or any one of claims 3 to 9 as dependent on claim 2 wherein a second means is provided within the boom for directing exhaust efflux into the atmosphere in a direction opposed to that produced by the first means when in the first mode of operation, so as to provide improved yaw control.

11 A helicopter as claimed in claim 10 wherein the first exhaust efflux ejector means is provided by a swan neck duct having a fixed first section substantially at right angles to the boom axis and an L-shaped second section rotatably mounted on the first section for rotation about the axis of the first section whereby exhaust may be effluxed at right angles to the boom axis in the first mode of operation to provide anti-torque and rearwards parallel to the boom axis in the second mode of operation to provide fnrward thrust.

25 A helicopter as claimed in any preceding claim wherein the variable area engine nozzle is of a variable area circular petalled construction.

26 A helicopter as claimed in any one of claims 1 to

24 wherein the engine nozzle is a variable 2-D nozzle.