GB2238995A

GB2238995A - Compound helicopters

Info

Publication number: GB2238995A
Application number: GB9026433A
Authority: GB
Inventors: David Ernest Hall Balmford; David Vincent Humpherson
Original assignee: Westland Helicopters Ltd
Current assignee: AgustaWestland Ltd
Priority date: 1989-12-08
Filing date: 1990-12-05
Publication date: 1991-06-19
Anticipated expiration: 2010-12-05
Also published as: GB2238995B; FR2655611B1; FR2655611A1; GB8927785D0; IT9048546A1; IT1242176B; DE4039027A1; GB9026433D0; IT9048546A0

Abstract

A compound helicopter having a lift rotor assembly (13) for providing lift in vertical take-off/landing, hover and slow speed flight and a fixed wing (15) for providing lift in high speed forward flight, has its operating efficiency improved by a combination of features. Air is blown through slots (24, 25) in a tail boom structure to control circulation of lift rotor downwash air whereby a lateral force is obtained on the tail boom (12) for counteracting the effect of lift rotor torque. Air is selectively vented from air outlet ports (27) provided at either side of the tail boom (12) to provide lateral forces for yaw control. In high speed flight yaw control is obtained from a fin (31) and rudder (32) mounted on the tail boom. An engine driven fan (20), which may be an engine by-pass fan, provides low pressure ratio air to the interior of the tail boom structure during vertical take-off/landing, hover and slow speed flight, and provides higher pressure ratio air for propulsion in high speed forward flight. The higher pressure ratio air may be vented from the aft end (29) of the tail boom structure and may be mixed with engine exhaust gases. in an embodiment of the invention (Figures 5 and 6) gas turbine engines (51) having engine by-pass fans (53) are mounted on the fixed wing (15) and in high speed forward flight higher pressure ratio air is delivered rearwardly of the engines for propulsion, <IMAGE>

Description

v - 1 1

Description of Invention

Title: Compound Helicopters This invention relates to compound helicopters.

A compound helicopter may be broadly defined as a helicopter having some means for producing lift and/or propulsion in addition to a lift rotor.

A particular compound helicopter is one in which during forward flight some component of lift is produced by a fixed wing and at least a component of propulsive force is produced by means other than the lift rotor.

In operation of such a compound helicopter the lift rotor is used to produce forces for vertical take-off/landing and hover modes of operation and is progressively off-loaded as forward speed builds up and the lift component produced by the wing increases. With the lift rotor off-loaded and propulsive forces produced by other means a compound helicopter is capable of achieving higher forward speeds than a conventional helicopter but depending upon a number of factors this may be at the expense of efficiency and a number of penalties may be incurred. For example, if the lift rotor is driven by an engine through a gearbox there continues to be a requirement for anti-torque means to counteract the effect of lift rotor torque in vertical take-off/landing, hover and slow speed flight modes and for some means of providing yaw control. If a conventional tail rotor is used for these purposes penalties are incurred because in forward flight the tail rotor absorbs power which could otherwise be used for propulsion and, also, gives rise to drag loads.

A compound helicopter has been proposed, see for example GB-A-1032771, in which propulsive forces are produced by a ducted propeller mounted at the end of a rearwardly extending tail boom. Vertical vanes or rudders mounted on the outlet side of the duct deflect propulsive air for yaw control and, when required, to produce a lateral force to counteract the effect of lift rotor torque. whilst a -W 2 propeller is a reasonably efficient means of producing propulsive thrust it requires a drive shaft extending along the length of the tail boom and places weight at a position where it is most detrimental to.a favourable location of the centre of gravity.

To achieve forward flight speeds in excess of the maximum forward speed of some present day conventional helicopters, e.g. in excess of 370km/h (200 knots), it is necessary that the propeller of such acow helicopter be capable of absorbing the maximum amount of available engine power when the lift rotor is off-loaded. In meting. this requirement the size of the propeller and its associated duct may, be extremely large and the drive shaft extremely heavy thereby adding to the aforementioned weight problem.

It is known in conventional helicopters to control circulation of air over a tail boom surface by blowing air from the tail boom so asto produce a lateral force of magnitude and direction appropriate to counteracting the effect of lift rotor torque. Two such disclosures. are to be found in GB-A-959075 and GB-A-2012223. GB-A-959075 further discloses a fin and rudder assembly for producing lateral forces for yaw control during forward flight. These being disclosures of. conventional helicopters, propulsive forces for forward flight-are derived entirely from the lift rotor.

The aft end of the tail boom of a helicopter disclosed in US-A-3,807,662 is of boat tail configuration and incorporates articulated vanes for controlling the direction of air expelled from the tail boom. The air is supplied to the interior of the tail boom by an axial-flow variable pitch fan and the vanes may be set such that air is expelled in sideways directions to provide lateral thrusts for counteracting the effect of lift rotor torque and for yaw control, or they may be set such that air is expelled rearwardly to provide propulsive thrust. There is further disclosure in this reference of the provision of slots in the tail boom through which air is blown to control circulation of air around the tail boom in obtainment of additional sideways force for counteracting the effect of lift rotor torque. However, the most efficient use of fan air for providing propulsive forces for flight at higher speeds is not obtained by the disclosure of this reference because air is used to provide lateral forces for yaw control in forward flight so reducing the propulsive

3 component and creating additional drag. This drag arises from a momentum change of the deflected fan air in the direction of flight. This effect is most apparent when the air is vented at 90 degrees to the direction of flight to effect a rapid turn at high forward speed.

in the disclosure of US-A-3,957,226 a helicopter having a pair of small wings is provided with three nozzles at the aft end of a tail boom structure. Each nozzle incorporates a butterfly valve for controlling venting of air supplied to the interior of the tail boom by a variable pitch fan. A first one of the nozzles mounted on one side of the tail boom vents air to generate a lateral thrust which counteracts the effect of lift rotor torque and by adjustment of its butterfly valve this thrust is varied to control yaw during hover and slow speed flight. A second one of the nozzles mounted on the opposite side of the tail boom is used in combination with the first nozzle to provide yaw control only during autorotation. The third nozzle is located in the end of the tail boom so as to be rearward facing and is used to vent air to generate thrust for propulsion in high speed flight. This nozzle arrangement does not make the most efficient use of fan air in hover and low speed flight because a flaw of air of higher energy than is required for some alternative solutions, must be supplied to the first nozzle to generate the lateral force necessary to counteract the effect of lift rotor torque. This requirement for higher energy air can only be met by increased mass flow at the same jet velocity which demands a larger diameter fan or increased jet velocity at the same mass flow. Both these solutions require increased power to the fan and, in addition, a large fan is difficult to install and will give rise to a weight penalty.

It is an object of the present invention to improve the efficiency of operation of a compound helicopter.

It is another object of the present invention to provide compound helicopter in which lateral forces for counteracting the effect of main rotor torque and for yaw control are produced by means other than a conventional tail rotor, and propulsive forces for high speed forward flight are produced by means other than a ducted propeller, so that the weight and drag of a tail boom structure forming part of the compound helicopter are minimised and substantially all of the engine power is available for propulsion in high speed forward flight.

4 Accordingly, in its broadest aspect the present invention provides. a compound helicopter comprising a fuselage structure including a tail boom structure extending rearwardly therefrom, a lift rotor assembly including a plurality of rotor blades mounted above the fuselage structure for providing a major portion of lift in vertical take-off/ landing, hover and slow speed flight, a fixed wing extending laterally from either side of the fuselage structure for providing a major portion of lift in high speed forward flight, gas turbine engine means mounted m the fuselage structure, transmission means including a gearbox connected between the engine means and the lift rotor whereby the lift rotor may be driven to produce lift, characterised by slot mans provided in the tail boom structure through which air may be blown to control circulation over an external surface of the tail boom whereby a lateral force is produced on the tail boom of magnitude and direction appropriate for counteracting the effect on the fuselage structure of lift rotor torque in vertical take-off/landing, hover and slow speed flight; air outlet port mans including pivotal vane means provided at either side of the tail boom structure near to the aft end thereof through which air may be vented to provide lateral forces for yaw control in vertical take-off/landing, hover and slow speed flight.; engine driven fan means adapted to deliver a flow of appropriate low pressure ratio air for supply to the slot means and the air outlet port means in vertical take-off/landing, hover and slow speed flight and: further adapted to deliver a flow of appropriate higher pressure ratio air for propulsion in high speed flight; and fin and rudder means mounted on the tail boom structure, the rudder means being operable to provide lateral forces for yaw control in high speed forward flight.

I n one embodiment of the invention higher pressure ratio air delivered by the engine driven fan means for propulsion is ducted through the tail bom structure and vented from the aft end thereof through a rearward facing nozzle.

In another embodiment of the invention gas turbine engine weans including engine by-pass fan means are mounted on each of the fixed wings and higher pressure ratio air is delivered rearwardly of the engine means for propulsion in high speed flight.

A compound helicopter in accordance with the present invention provides an extremely clean tail boom structure which produces minimm p k drag in forward flight. The requirement for fan air to generate lateral thrust forces for counteracting the effect of lift rotor torque and for yaw control in vertical take-off/landing, hover and slow speed flight is reduced by blowing air to control the circulation of lift rotor downwash air around the tail boom structure thereby taking advantage of the energy available in the downwash air to obtain the lateral force required for counteracting the effect of lift rotor torque. At the saw time, the fan can be sized to absorb the maximum amount of engine power which becomes available when the rotor is off-loaded in order to meet the requirement for higher pressure ratio air for propulsion, with yaw control forces in high speed flight being provided by the fin and rudder.

In another aspect the present invention provides a compound helicopter comprising a fuselage structure including a tail boom structure extending rearwardly therefrom, a lift rotor assembly including a plurality of rotor blades mounted above the fuselage structure for providing a major portion of lift in vertical take-off/ landing, hover and slow speed flight, a fixed wing extending laterally from either side of the fuselage structure for providing a major portion of lift in high speed forward flight, gas turbine engine means mounted on the fuselage structure, transmission mans including a gearbox connected between the engine means and the lift rotor whereby the lift rotor may be driven by the engine means to produce lift, variable pitch axial fan means located in the fuselage structure near to a forward end of the tail boom structure, transmission means connected between the gearbox and the fan means whereby the fan means may be driven by the engine means through the gearbox, air inlet means in the fuselage structure for allowing ambient air to be drawn in by the fan means, duct means located in the fuselage structure for receiving a flow of air at increased pressure ratio from the fan means and delivering said air to the tail boom structure, slot means in the tail boom structure for blowing low pressure ratio fan air over an external surface of the tail boom structure so as to control circulation of air thereover whereby a lateral force is produced m the tail boom structure of magnitude and direction appropriate for counteracting the effect on the fuselage structure of lift rotor torque, air outlet port means located at either side of the tail boom 6 structure near to an end thereof which is remote from the fan means, the air outlet port means including pivotal vane means operable to open. the air outlet port means to allow low pressure ratio fan air to vent therefrom whereby lateral forces are produced on the tail boom structure for yaw control during vertical take-off/landing, hover and slow speed flight; fin and rudder means mounted on the tail boom structure near the remote end thereof,, the rudder means being operable to provide lateral forces for yaw control during high speed forward flight; and rearward facing air exhaust nozzle means provided at the remote end of the tail boom structure, including door closure means operable between a position in which the nozzle means is closed and a position in which the nozzle means is open to allow higher pressure ratio fan air to exhaust therefrom to produce propulsive forces for high speed forward flight.

In a further aspect the present invention provides a compound helicopter comprising a fuselage structure having a tail boom structure extending rearwardly therefrom, a lift rotor assembly including a plurality of rotor blades mounted above the fuselage structure for providing a major portion of lift in vertical take-off/landing, hover and slow speed flight, a fixed wing extending laterally from either side of the fuselage structure for providing a major portion of lift in high speed forward flight, gas turbine engine means including engine by-pass fan means mounted on the fuselage structure, transmission means including a gearbox connected between the engine means and the lift rotor whereby the lift rotor may be driven by the engine means to produce lift, duct mans for ducting air from the engine by-pass fan means over the engine to encompass hot gas exhausting from power turbine means of the engine means and to mix therewith downstream of the engine means and produce a by-pass fan air-exhaust gas mixture which is ducted by said duct means to the tail boom structure, slot mans in the tail boom structure for blowing said mixture at low pressure ratio over an external surface of the tail boom structure so as to control circulation of air thereover whereby a lateral force is produced m the tail boom structure of magnitude and direction appropriate for counteracting the effect on the fuselage structure of lift rotor torque, outlet port means located at either side of the tail boom structure near to an end thereof which is remote from the engine c 7 k means, the outlet port means including pivotal vane means operable to open the outlet port means to allow said low pressure ratio mixture to vent therefrom whereby lateral forces are produced on the tail boom structure for yaw control during vertical take-off/landing, hover and slow speed flight; fin and rudder means mounted on the tail boom structure near the remote end thereof, the rudder means being operable to provide lateral forces for yaw control durAng high speed forward flight; and rearward facing air exhaust nozzle means provided at the remote end of the tail boom structure, including door closure means movable between a position inwhich the nozzle means is closed and a position in which the nozzle means is open to allow higher pressure ratio by-pass fan air- exhaust gas mixture to exhaust therefrom to produce propulsive forces for high speed forward flight.

In yet another aspect the present invention provides a compound helicopter comprising a fuselage structure having a tail boom structure extending rearwardly therefrom, a lift rotor assembly including a plurality of rotor blades mounted above the fuselage structure for providing a major portion of lift in vertical take-off/landing, hover and slow speed flight, a fixed wing extending laterally from either side of the fuselage structure for providing a major portion of lift in high speed forward flight, gas turbine engine means including engine by-pass fan means mounted on each fixed wing for providing propulsive thrust for high speed forward flight, transmission means including a gearbox connected between the engine mans and the lift rotor whereby the lift rotor way be driven by the engine means to produce lift, duct means connecting with the engine by-pass fan means for delivering a bleed of low pressure ratio air from the engine by-pass fan mans to the interior of the tail boom structure, slot means in the tail boom structure for blowing said low pressure ratio air over an external surface of the tail boom structure so as to control circulation of air thereover whereby a lateral force is produced on the tail boom structure of magnitude and direction appropriate for counteracting the effect m the fuselage structure of lift rotor torque, air outlet port means located at either side of the tail boom structure near to a remote end thereof, the outlet port mans including pivotal vane mans operable to open the outlet port means to allow low pressure ratio air to vent therefrom whereby lateral forces are produced on the tail boom k a structure for yaw control during vertical take-off/landing, hover, and. slow speed flight; valve means associated with the duct means for.. closure thereof whereby bleed of low pressure ratio by-pass fan air to the tail bow structure is terminated and by-pass fan air at higher pressure ratio is directed rearwardly of the engine means to provide propulsive thrust; and fin and rudder means mounted on the tail boom structure near the remote end thereof, the rudder means being operable to provide lateral forces for yaw control during high speed forward flight.

The invention will now be further described by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a schematic side view of a compound helicopter in accordance with one embodiment of the invention; Figure 2 is a section on line II-II in Figure 1; Figure 3 is a section on line III-III in Figure 1; Figure 4 is a schematic side view of a compound helicopter in accordance with another embodiment of the invention; Figure 5 is a schematic side view of a compound helicopter in accordance with a further embodiment of the invention; and Figure 6 is a schematic plan of the compound helicopter shown in Figure 5 when viewed from above.

A cow helicopter 10 in accordance with one embodiment of the invention, reference Figure 1, comprises a fuselage structure 11 including a rearwardly extending tail boom structure 12. A lift rotor assembly 13 including a plurality of rotor blades 14 is mounted for rotation above the fuselage and a fixed wing 15 extends laterally from either side of the fuselage. ngine means which in this embodiment comprise me or more gas turbine engines 16 are mounted on the fuselage and each engine is connected for driving the lift rotor assembly 13 through transmission means comprising an engine drive shaft 17, a gearbox 18, and a main rotor drive shaft 19.

Pan means comprising a variable pitch axial fan 20 is located in the fuselage near to a forward end of the tail boom structure 12. Transmission mans comprising a fan drive shaft 21 connects the fan 20 with the gearbox 18 whereby the fan my be driven. Air inlet means comprising one or more inlet openings 22 are provided in the fuselage for allowing the fan 20 to draw in ambient air. The fan 20 delivers 9 air at increased pressure to a duct 23 through which the air flows to the interior of the tail boom structure 12.

Longitudinally extending slots 24 and 25 are provided on one side of the tail boom structure near the top and bottom thereof to permit fan air at an appropriate pressure ratio to be vented from the tail boom structure. As is shown in Figure 2 the slots are arranged so that the air is vented in a downward direction over the starboard side surface of the tail boom structure. Venting of air from the slots 24 and 25 is controlled by valve means which in this embodiment comprises a pivotal flap valve 26 (reference Figure 2),which may be actuated by a rotary actuator or other suitable mans (not shown).

Air outlet ports 27 are provided at either side of the tail boom structure 12 near that end which is remote from the fan 20, the outlet ports 27 being closable by pivotal vanes 28 (reference Figure 3).

That end of the tail boom structure 12 which is remote from the fan 20 is formed as an exhaust nozzle 29 and is provided with pivotal doors 30 for closing the exhaust nozzle.

A fin 31 and rudder 32 are mounted on the tail boom structure 12 near the remote end thereof.

In operation of the compound helicopter 10 shown in Figure 1, for takeoff, landing, hover and slow speed flight modes the gas turbine engine(s) transmit power to the gearbox 18 to drive the lift rotor assembly 13 and the fan 20, a major portion of the available engine power being used to power the lift rotor and a minor portion being used to power the fan. The collective pitch of the rotor blades 14 is set to produce appropriate lift forces and cyclic pitch is applied to the rotor blades to produce lateral and/or fore and aft forces for trim and man oeuvre purposes.

In these modes of operation the pitch of the blades of the fan 20 is set such that ambient air is drawn in through the inlet openings 22 and air of appropriate pressure ratio, say 1.05, is delivered to the duct 23 and hence to the interior of the tail boom structure 12 which acts as an overpressure plenum. The flap valves 26 associated with the slots 24 and 25 are moved to an open position allowing air to vent from the interior of the tail boom structure through the slots. with the main rotor assembly 13 rotating in a clockwise direction, as viewed from above, the fuselage will tend to rotate in the same direction in reaction to the torque applied by the main rotor assembly. At the same time there will be a dowrrwash of air from the rotor blades over the - tail bow structure. The air venting from the slots m the starboard;, side of the tail bom will be moving faster than the downwash air so that less pressure will be exerted m the starboard side surface of the tail boom structure than is exerted on the port side surface. Thus a lateral force will be exerted m the tail boom structure from port to starboard which will counteract the tendency of the fuselage to rotate in reaction to main rotor torque.

In these modes of operation lateral forces for yaw control are produced by selective opening of the pivotal vanes 28 associated with the outlet ports 27 to allow air to vent from one or the other side of the tail boom structure thereby producing a lateral force which supplements or detracts from the lateral force reacting lift rotor torque. If desired the pivotal vanes associated with the outlet port on the port side of the tail boom structure may be opened to vent air to produce a lateral force which is additive to the lateral force produced on the tail boom structure by venting air from the slots whereby the sum of these forces is that required for counteracting main rotor torque.

Forward flight is initiated by applying forward cyclic pitch to the rotor blades. As forward speed builds up the wings 15 provide increasing lift allowing the collective pitch of the rotor blades to be progressively decreased thereby off-loading the rotor. As the rotor is off-loaded the torque reaction of the fuselage decreases resulting in a reducing requirement for venting air from the slots to produce a lateral force counteracting the effect of lift rotor torque. Forward speed is built up by increasing the pitch of the fan blades so that the fan delivers higher pressure ratio air, say 1.5, to the interior of the tail bom structure. The doors 30 of the exhaust nozzle 29 at the remote end of the tail bom structure are opened so that the higher pressure ratio air can vent from the tail boom to produce propulsive forces for high speed forward flight. At the same time the cyclic pitch of the rotor blades is adjusted to maintain a substantially horizontal rotor disc. As forward speed builds up the velocity of the air flowing over the fin 31 and rudder 32 increases so that lateral forces for yaw control can be obtained by operation of the rudder.

11 The compound helicopter 10 in accordance with this embodiment of the invention has the advantage of providing an extremely clean tail boom structure which produces minimum drag in forward flight. At the same time, in meeting the requirement for higher pressure ratio air demanded for propulsion, the fan 20 can be sized to absorb the maxim= unt of engine power which becomes available when the lift rotor is off-loaded.

An added benefit of this embodiment is the use of blowing air over the tail boom to control circulation so as to take advantage of energy available in lift rotor downwash in obtainment of a lateral force for counteracting lift rotor torque. This provides the opportunity to generate the required anti-lift rotor torque forces with the expenditure of less power than is the case if the required lateral force is obtained by expelling air through a jet nozzle.

A compound helicopter 40 in accordance with another embodiment of the invention illustrated in Figure 4, has a by-pass fan gas turbine engine 41 connected by an engine drive shaft 42 projecting from the front of the engine to a gearbox 18 which is connected by a lift rotor drive shaft 19 to a main rotor system 13. Features of the compound helicopter 40 illustrated in Figure 4 which are common with the features of the compound helicopter 10 illustrated in Figure 1 are given like reference numerals and are not described in detail.

The engine 41 has a by-pass fan 43 having variable pitch fan blades not shown. Air delivered by the by-pass fan 43 is ducted over the engine through a duct 44 which joins with a duct 45 connected at its downstream end with the interior of the tail boom structure 12. An advantage of this arrangement is that the by-pass fan air encompasses hot gases exhausting from the engine power turbine stage (not shown) and then mixes with these exhaust gases downstream of the engine to provide an air/exhaust gas mixture which is vented from the slots 24/25, the outlet ports 27, or the exhaust nozzle 29, as may be required.

In operation of the helicopter 40, during vertical take-off/ landing, hover and slow speed flight when the rotor system is being driven to generate lift forces, the fan blades of the by-pass fan 43 are set at a minimm pitch angle so that the fan produces a flow of low pressure ratio air suitable for venting from the slots 24/25 and the 12 k outlet ports 27. For high speed forward flight the pitch of the fan, blades is increased to a maxinum pitch angle so that the fan produces%a flow of higher pressure ratio air suitable for exhausting from the exhaust nozzle 29 at the remote end of the tail boom structure to produce propulsive forces.

A cce apound helicopter 50 in accordance with a further embodiment- of the invention illustrated in Figures 5 and.6, has a by-pass fan gas turbine engine 51 built into each of its laterally extending fixed wings 15. Features of the cund helicopter 50 which are coomon with features of the com pound helicopter 10 illustrated in Figure 1, are given like reference numerals and are not described in detail.

Each engine 51 is connected by transmission means including an engine drive shaft 52 for driving the lift rotor system 13 through a gearbox 18. The wings 15 are mounted high up on the fuselage 11 so that the drive shafts 52 are substantially horizontal in extending from the engines through the wings and fuselage to connection with the gearbox.

A by-pass fan 53 at a forward facing end of each engine has variable pitch fan blades (not shown). Provision is made for a bleed of low pressure ratio air delivered by each by-pass fan to be supplied to ducts 54 which extend through the wings and fuselage to the interior of the tail boom structure 12.

In operation of the compound helicopter 50, when the engines 51 are driving the rotor system 13 to produce lift forces the fan blades (not shown) of each engine by-pass fan 53 are set to minimum pitch angles so that the fans deliver low-pressure ratio air to the interior of the tail boom structure 12 by way of the ducts 54. This air is vented from the slots 24/25 to control circulation of air over the tail boom structure whereby a lateral force of magnitude and direction for counteracting the effect of lift rotor torque is produced on the tail boom structure. The low pressure ratio air is also selectively vented from the outlet ports 27 at either side of the tail boom structure to produce lateral forces foryaw control. In high speed flight with the lift rotor off-loaded, the fan blades are set to maximum pitch angles so that the fans 53 deliver higher pressure ratio air. Valve mans (not shown) associated with each duct 54 are closed to prevent higher pressure ratio air bleeding to the ducts because with the rotor i 13 off-loaded there is no requirement for air to be delivered to the interior of the tail boom structure for venting for the slots 24/25. Also, in high speed flight yaw control forces are obtained by operation of the rudder 32 so that air is not required for venting from the outlet ports 27. Thus substantially all of the higher pressure ratio air produced by the by-pass fans is exhausted from the rear of the engines to produce propulsive forces.

The compound helicopter 50 in accordance with this embodiment of the invention has the aforementioned advantages and, also, by locating the engines on the fixed wings a considerable reduction in cabin noise is obtained which makes a compound helicopter configured in accordance with this embodiment suited for use as a passenger transportation aircraft.

14

Claims

1. A cam helicopter comprising a fuselage structure including a tail boom structure extending rearwardly therefroa, a lift rotor assembly including a plurality of rotor blades mounted above the fuselage structure for providing a major portion of lift in vertical take-off/landing, hover and slow speed flight, a fixed wing extending laterally from either side of the fuselage structure for providing a major portion of lift in high speed forward flight, gas turbine engine means mounted on the fuselage structure, transmission means including a gearbox connected between the engine means and the lift rotor whereby the lift rotor may be driven to produce lift, characterised by slot means provided in the tail boom structure through which air may be blown to control circulation over an external surface of the tail boom whereby a lateral force is produced on the tail boom of magnitude and direction appropriate for counteracting the effect on the fuselage structure of lift rotor torque in vertical take-off/landing, hover and slow speed flight; air outlet port means including pivotal vane weans provided at either side of the tail boom structure near to the aft end thereof through which air may be vented to provide lateral forces for yaw control in vertical take-off/landing, haver and slow speed flight; engine driven fan means adapted to deliver a flow of appropriate low pressure ratio air for supply to the slot means and the air outlet port mans in vertical take-off/landing, haver and slow speed flight and further adapted to deliver a flow of appropriate higher pressure ratio air for propulsion in high speed flight; and fin and rudder mans mounted on the tail boom structure, the rudder means being operable to provide lateral forces for yaw control in high speed forward flight.

2. A coo helicopter as claimed in Claim 1, characterised in that the engine driven fan means comprise a variable pitch axial fan mounted in the fuselage and connected for being driven by the engine means through the gearbox.

3. A compound helicopter as claimed in Claira 1, further characterised in that the engine driven fan means comprise gas turbine engine by-pass A is fan means.

4. A compound helicopter as claimed in any one of the preceding claims, further characterised in that higher pressure ratio air delivered by the engine driven fan means is ducted to the tail boom structure and vented from the aft end thereof to produce propulsive thrust.

5. A compound helicopter as claimed in Claim 3, further characteri sed in that gas turbine engine means including engine by-pass fan means are mounted on each of the fixed wings and higher pressure ratio air is delivered rearwardly of the engine means for propulsion in high speed flight.

6. A compound helicopter comprising a fuselage structure including a tail boom structure extending rearwardly therefrom, a lift rotor assembly including a plurality of rotor blades mounted above the fuselage structure for providing a major portion of lift in vertical take- off/landing, hover and slow speed flight, a fixed wing extending laterally from either side of the fuselage structure for providing a major portion of lift in high speed forward flight, gas turbine engine means mounted on the fuselage structure, transmission means including a gearbox connected between the engine means and the lift rotor whereby the lift rotor may be driven by the engine means to produce lift, variable pitch axial fan means located in the fuselage structure near to a forward end of the tail boomstructure, transmission means connected between the gearbox and the fan means whereby the fan means may be driven by the engine mans through the gearbox, air inlet means in the fuselage structure for allowing ambient air to be drawn in by the fan means, duct means located in the fuselage structure for receiving a flow of air at increased pressure from the fan mans and delivering said air to the tail boom structure, slot means in the tail boom structure for blowing low pressure ratio fan air over an external surface of the tail boom structure so as to control circulation of air thereover whereby a lateral force is produced on the tail boom structure of magnitude and direction appropriate for counteracting the effect m the fuselage structure of lift rotor torque, air outlet port 16 means located at either side of the tail boom structure near to an-end thereof which is remote from the fan means, the air outlet port means.. including pivotal vane means operable to open the air outlet port means to allow low pressure ratio fan air to vent therefrom whereby lateral forces are produced m the tail boom structure for yaw control during vertical take-off/landing, hover and slow speed flight; fin and rudder means mounted on the tail bom structure near.the remote end thereof, the rudder means being operable to provide lateral forces for yaw control during high speed forward flight; and rearwardly facing air exhaust nozzle means provided at the remote end of the tail boom structure including door closure means operable between a position in which the nozzle means is closed and a position in which the nozzle means is open to allow higher pressure ratio fan air to exhaust therefrom to produce propulsive forces for high speed forward flight.

7. A comd helicopter comprising a fuselage structure having a tail boom structure extending rearwardly therefrom, a lift rotor assembly including a plurality of rotor blades mounted above the fuselage structure for providing a major portion of lift in vertical take-off/landing, hover and slow speed flight, a fixed wing extending laterally from either side of the fuselage structure for providing a major portion of lift in high speed forward flight, gas turbine engine mans including engine by-pass fan means mounted on the fuselage structure, transmission means including a gearbox connected between the engine mans and the lift rotor whereby the lift rotor may be driven by the engine means to produce lift, duct mans for ducting air from the engine by-pass fan means over the engine to encompass hot gas exhausting from power turbine means of the engine mans and to mix therewith downstream of the engine mans and produce a by-pass fan air-exhaust gas mixture which is ducted by said duct means to the tail boom structure, slot mans in the tail boom structure for blowing said mixture at low pressure ratio over an external surface of the tail boom structure so as to control circulation of air thereover whereby a lateral force is produced m the tail boora structure of magnitude and direction appropriate for counteracting the effect on the fuselage structure of lift rotor torque, outlet port means located at either side of the tail boom structure near to an end thereof which is remote -R 1 17 from the engine means,the outlet port means including pivotal vane means operable to open the outlet port means to allow said low pressure ratio mixture to vent therefrom whereby lateral forces are produced on the tail boom structure for yaw control during vertical take-off/landing, hover and slow speed flight; fin and rudder means mounted on the tail boom structure near the remote end thereof, the rudder means being operable to provide lateral forces for yaw control during high speed forward flight; and rearward facing air exhaust nozzle means provided at the remote end of the tail boom structure including door closure means movable between a position in which the nozzle means is closed and a position in which the nozzle means is open to allow higher pressure ratio by-pass fan air- exhaust gas mixture to exhaust therefrom. to produce propulsive forces for high speed forward flight.

8. A compound helicopter comprising a fuselage structure having a tail boom structure extending rearwardly therefrom, a lift rotor assembly including a plurality of rotor blades mounted above the fuselage structure for providing a major portion of lift in vertical take- off/landing, haver and slow speed flight, a fixed wing extending laterally from either side of the fuselage structure for providing a major portion of lift in high speed forward flight, gas turbine engine means including engine by-pass fan means mounted m each fixed wing for providing propulsive thrust for high speed forward flight, transmission mans including a gearbox connected between the engine means and the lift rotor whereby the lift rotor may be driven by the engine means to produce lift, duct means connecting with the engine by-pass fan means for delivering a bleed of low pressure ratio air from the engine by-pass fan mans to the interior of the tail boom structure, slot means in the tail boom structure for blowing said low pressure ratio air over an external surface of the tail boom structure so as to control circulation of air thereover whereby a lateral force is produced on the tail boom structure of magnitude and direction appropriate for counteracting the effect on the fuselage structure of lift rotor torque, air outlet port means located at either side of the tail boom structure near to a remote end thereof, the outlet port means including pivotal vane mans operable to open the outlet port means to A 1 18 allow low pressure ratio air to vent therefrom whereby lateral forices are produced m the tail boom structure for yaw control during vertical take-off/landing, hover, and slow speed flight; valve means associated. with the duct means for closure thereof whereby bleed of low pressure ratio by-pass fan air to the tail boom structure is terminated and bypass fan air at higher pressure ratio is directed rearwardly of engine means to provide propulsive thrust; and fin and rudder means mounted on the tail bo= structure near the remote end thereof, the rudder means being operable to provide lateral forces for yaw control. during high speed forward flight.

9. A com helicopter substantially as hereinbefore described with reference to and as shown in Figures 1, 2 and 3 of the accompanying drawings.

10. A coo helicopter substantially as hereinbefore described with reference to and as shown in Figure 4 of the accompanying drawings.

11. A cow helicopter substantially as hereinbefore described with reference to and as shown in Figures 5 and 6 of the accompanying drawings.

12. Any new or improved features, combinations and arrangements described, shown and mentioned or any of them together or separately.

Published 1991 atIte Patent Office. State House. 66171 High Holborn, London WCAR4TP. Further copier. may be obtained frorn Sales Branch. Unit 6. Nine Mile Point, Cwmfc!linfach. Croas Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray. Kent.