GB2238996A

GB2238996A - Compound helicopters

Info

Publication number: GB2238996A
Application number: GB9026434A
Authority: GB
Inventors: David Vincent Humpherson; David Ernest Hall Balmford
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: IT9048545A1; FR2655612A1; FR2655612B1; IT9048545A0; IT1242175B; GB2238996B; DE4039028A1; GB9026434D0; GB8927784D0

Abstract

A lift rotor (13) providing a major portion of lift for vertical take-off/landing, hover and slow speed flight of a compound helicopter is driven through a gearbox (18) by a gas turbine engine (16) having a variable area final exhaust nozzle (33). The engine also provides power for driving a fan (21) to produce a flow of air internally of a tail boom structure (12). Air is vented through slots (23) in the tail boom to control circulation of lift rotor downwash air over the tail boom thereby producing a lateral force for counteracting the effect of lift rotor torque. Air is also vented from the tail boom through air outlet ports (24) to produce lateral forces for yaw control. In high speed flight the major portion of lift is provided by a fixed wing (15). The exhaust nozzle is set such that substantially all of the engine power is used for propulsion and the lift rotor and fan are off-loaded. For high speed flight yaw control is provided by a fin (27) and rudder (28) mounted on the tail boom. This maximises use of engine power for propulsion and provides a low drag tail boom allowing higher forward speeds to be obtained. <IMAGE>

Description

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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 being 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 ail 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 1 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 a compound helicopter be capable of absorbing the maximum amount of available engine power when the lift rotor is off-loaded. In meeting 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 as to 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.

other disclosures of helicopters having an engine driven fan providing a flow of air internally of and rearwardly along a tail boom structure, the air being vented from the aft end thereof for torque reaction and propulsive purposes, are to be found in US-A-2,486,272; US-A-3,807,662 and US-A-3,957,226.

The aft end of the tail boom of the 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

3 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 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 alr 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 flow 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.

GB-A-2130984 discloses a compound helicopter including a variable cycle gas turbine engine having a variable area exhaust nozzle, the engine driving a lift rotor through a gear box. Engine shaft power to the rotor may be reduced in forward flight to off-load the rotor and thus reduce the lift and propulsive components provided by the rotor.

4 The variable area nozzle is adjusted to produce propulsive thrust offsetting the reduction in the propulsive force component provided by the rotor. This reference does not disclose the provision of means for counteracting the effect of lift rotor torque or means for producing lateral forces for yaw control.

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 a 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 mans 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.

In its broadest aspect the present invention provides a co mpo und helicopter comprising a fuselage structure including a rearwardly extending tail boom structure, a lift rotor assembly for providing a major portion of lift in vertical take-off/landing, hover and slow speed flight, a fixed wing for providing a major portion of lift during high speed forward flight, engine means connected through the gearbox means for driving the lift rotor assembly, variable area final exhaust nozzle means connected with said engine means for producing propulsive forces for high speed forward flight, means for producing a flow of air internally of and rearwardly along said tail boom structure, means for venting said air from said tail boom structure to produce lateral forces appropriate for counteracting the effect of lift rotor torque on said fuselage structure and for yaw control at least during vertical take-off/ landing, hover and slow speed flight, and fin and rudder means mounted on said tail boom structure for producing yaw control forces during high speed forward flight.

The means for venting air to produce lateral forces for counteracting the effect of lift rotor torque and for yaw control may be combined and may comprise one or more slots extending longitudinally of a rearwardly extending tail boom through which air is blown to control circulation of lift rotor downwash air over the tail boom F whereby suitable lateral forces acting on the tail boom are produced.

In such a combined arrangement means are provided for controlling the venting of air so that the lateral force can be varied to produce an out of balance with lift rotor torque for yaw control.

Alternatively, the means for venting air from the tail boom may comprise slots extending along at least a part of the length of the tail boom through which air is blown to control circulation of lift rotor downwash air over the tail bow whereby a lateral force of magnitude and direction suitable for counteracting the effect of lift rotor torque is produced, said air venting means further comprising air outlet ports including pivotal closure vanes located at each side of the tail boom whereby air may be selectively vented to produce lateral forces for yaw control.

The means for producing a flow of air within the tail boom structure may comprise a fan driven by transmission means connected with the engine and gearbox means, or a fan driven by an auxiliary power unit.

The fan may be an axial fan having Variable pitch blades.

The engine means may comprise at least one gas turbine engine having a variable area final exhaust nozzle provided as a part of the engine or, alternatively, the variable area final exhaust nozzle may be provided as part of another device connected to or co-operating with the engine.

The or each engine may be a variable cycle engine and means may be provided for controlling the engine shaft power delivered for driving the lift rotor whereby power to the lift rotor is substantially reduced as forward speed increases.

A particular variable cycle gas turbine engine suited for use in a compound helicopter in accordance with the present invention comprises a low pressure compressor, a core gas generator which drives the compressor, a power turbine driven by the exhaust of the core gas generator and connected to drive a power output shaft, and a variable area final exhaust nozzle downstream of the power turbine, the nozzle receiving the exhaust from the power turbine and being operable to vary the power absorbed by the power turbine and simultaneously vary the propulsive thrust produced by the nozzle.

In another aspect the invention provides a compound helicopter 6 comprising a fuselage structure including a rearwardly extending tail bow structure, a lift rotor assembly including a plurality of rotor blades 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 for providing a major portion of lift during high speed forward flight, gas turbine engine means connected through gearbox means for driving the lift rotor assembly, variable area final exhaust nozzle means co-operating with said engine means for producing propulsive forces for high speed forward flight and whereby engine power supplied for driving said lift rotor assembly may be controlled, means for producing a flow of air internally of and rearwardly along said tail boom structure, means on said tail boom structure for venting said air to control circulation of main rotor downwash air over an external surface of said tail boom structure so as to produce a lateral force on said tail boom structure of magnitude and direction appropriate to counteracting the effect on said fuselage structure of lift rotor torque when said lift rotor is being driven to provide the major portion of lift, air outlet ports including pivotal closure mans for selectively venting said air from said tail boom structure to produce lateral forces for yaw controlwhen said lift rotor is being driven to provide the major portion of lift, and fin and rudder means mounted on said tail boom structures said rudder means being operable to produce yaw control forces during high speed forward flight when said fixed wing is providing the major portion of lift.

A compound helicopter in accordance with the present invention may include an onboard computer which may receive signals from sensors sensing engine and flight operating parameters together with control demands input by aircrew and output signals for optimisation of vehicle operation.

The invention will now be further described by way of example only and with reference to the accying drawings in which:- Figure 1 is a schematic side view of a cor apound helicopter in accordance with one embodiment of the invention; Figure 2 is a section on line I-I in Figure 1.

Referring first to Figure 1, a compound helicopter 10 comprises a fuselage structure 11 having a rearwardly extending tail boom structure 12. A lift rotor assembly 13 including a plurality of rotor blades 14 ?9 7 1 is located above the fuselage 11 and a fixed wing 15 extends laterally from either side of the fuselage 11. Gas turbine engine means comprise one or more gas turbine engines 16 mounted on the fuselage 11 and connected for driving the lift rotor assembly 13 through an engine drive shaft 17, a gearbox 18, and a main rotor drive shaft 19. Each engine 16 is also connected through the gearbox 18 and a fan drive shaft 20 for driving a fan 21 which is suitably mounted in the tail boom structure 12. Fan air inlet openings (not shown in Figure 1) are provided in the structure at either side of the compound helicopter and are located forward of the fan 21. A duct 22 is provided within the tail boom 12 for carrying air drawn in by the fan 21 along the tail boom.

means for venting air to control air circulation over the tail boom comprise in this embodiment longitudinally extending slots 23 provided in one side surface of the tail boom 12 near the top and bottom thereof. Whilst in this embodiment each slot 23 is continuous in length the vent means could be provided by a number of slots arranged in line. Valve means (not shown) are provided for communicating the slots 23 with the duct 22 so as to control venting of fan air through the slots 23. In operation, when the lift rotor assembly is being driven to provide lift there is a downward flow of air through the rotor over each external side surface of the tail boom. By venting air through the slots in a downward direction the dowrward velocity of air flow over that side of the tail boom is increased and a lower pressure obtains compared with the pressure on the opposite side of the tail boom. This produces a net lateral force on the tail boom of magnitude and direction suitable for counteracting the tendency of the fuselage to rotate in reaction to the lift rotor torque. If the lift rotor assembly 13 rotates in an anti-clockwise direction as viewed from above then the fuselage tends to rotate in a clockwise direction and the slots 23 are located on the starboard side of the tail boom so that the resultant lateral force on the tail boom is from port to starboard.

In vertical take-off/landing, hover and slow speed flight, lateral forces for yaw control about the datum of the balance between lift rotor torque and the reacting lateral force on the tail boom are produced by venting air from air outlet ports 24 provided at either 8 side of the tail boom near that end 26 remote from the fan 21, the ports 24 being closable by pivotal vanes 25 (reference Figure 2).

A tail fin assembly 27 including a rudder 28 is provided at the remote end 26 of the tail boom 12, the rudder 28 being operable to provide yaw cohtrol during high speed forward flight.

In this embodiment each engine 16 comprises a variable cycle gas turbine engine having a low pressure compressor 30, a core gas generator 31 which drives the low pressure compressor 30, a power turbine 32 which is driven from the exhaust of the core gas engine and a variable area final exhaust nozzle 33. The power turbine 32 is connected to the drive shaft 17.

In operation of the compound helicopter 10, for take-off, landing, hover and slow speed flight modes each engine is run at or near maximum power with the variable area nozzle 33 in its maximum area position. High engine shaft power is input to the gearbox by drive shaft 17 and used to power the lift rotor assembly 13 which has the collective pitch of the rotor blades 14 set to produce appropriate lift forces. At the same time cyclic pitch may be applied to the rotor blades to produce lateral and/or fore and aft forces for trim and manoeuvre purposes. Power is also transmitted through the gearbox and drive shaft 20 to the fan 21 which is preferably of axial type having variable pitch fan blades. The fan draws in air through the fan air inlets (not shown) and forces this air at increased pressure along the duct 22 from which it is vented through the slots 23 downwardly over the external side surface of the tail boom so as to increase the velocity of air flowing over that surface and thereby to produce a net sideways force on the tail boom of magnitude and direction appropriate to counteracting the reaction of the fuselage to lift rotor torque. In assisting directional control the pivotal vanes 25 on one or other side of the tail boom are opened to vent air from the associated air outlet port 24, as required, to produce a lateral force for yawing the fuselage.

A compound helicopter in accordance with the present invention obviates the requirement for a conventional tail rotor by the use of blowing air over the tail boom 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 A 9 power than is the case if the required lateral force is obtained by expelling air through a jet nozzle.

Forward flight is initiated by applying forward cyclic pitch to the rotor blades. As forward speed builds up and the wings 15 provide increasing lift, collective pitch is progressively decreased thereby off-loading the rotor which results in a reduced requirement for circulation control of the tail boom to produce a lateral force to counteract the effect of lift rotor torque. At the same time cyclic pitch is adjusted to maintain a substantially horizontal rotor disc. Forward speed is built up by reducing the area of the final exhaust nozzle 33 to produce increasing propulsive thrust from the exhaust nozzle. As forward speed builds up the velocity of airflow over the tail fin assembly 27 increases so that lateral forces for yaw control may be obtained by operation of the rudder 28. With the lift rotor off-loaded there is no requirement to counteract lift rotor torque and with the rudder available for yaw control, the fan 21 may be shut down thereby making additional engine power available for propulsion.

Since a compound helicopter in accordance with the present invention does not have an air propeller mounted at the end of the tail boom to produce propulsive forces for high speed forward flight, as has been proposed in some other compound helicopter configurations, the requirement for a drive shaft extending along the tail boom is avoided thereby saving weight and assisting in maintaining the centre of gravity in line with the centre of lift.

An added advantage is that drag loads produced by venting air from the air exhaust ports for yaw control are not incurred in high speed forward flight of a compound helicopter in accordance with the present invention.

A further advantage of a compound helicopter in accordance with the present invention is that it is able to maintain a substantially horizontal attitude in forward flight and during acceleration/ deceleration.

Claims

1. A compound helicopter comprising a fuselage structure including a rearwardly extending tail boom structure, a lift rotor assembly for providing a major portion of lift in vertical take-off/landing, hover and slow speed flight, a fixed wing for providing a major portion of lift during high speed forward flight, engine means connected through gearbox means for driving the lift rotor assembly, variable area final exhaust nozzle means connected with said engine means for producing propulsive forces for high speed forward flight, means for producing.a flow of air internally of and rearwardly along said tail boom structure, mans for venting said air from said tail boom structure to produce lateral forces appropriate for counteracting the effect of lift rotor torque on said fuselage structure and for yaw control at least during vertical takeoff/landing, hover and slow speed flight, and fin and rudder means mounted m said tail boom structure for producing yaw control forces during high speed forward flight.

2. A compound helicopter as claimed in Claim 1, wherein the means for venting air from said tail boom structure comprise slots extending along at least a part of the length of said tail boom structure through which said air is blown to control circulation of lift rotor downwash air over the tail bow structurewhereby a lateral force of magnitude and direction suitable for counteracting the effect of lift rotor torque is produced, said air venting means further comprising air outlet ports including pivotal closure vanes located at each side of the said tail boom, structure whereby said air may be selectively vented to produce lateral forces for yaw control.

3. A compound helicopter as claimed in Claim 1 or Claim 2, wherein the means for producing a flow of air internally of said tail boom structure comprise a fan driven by transmission means connected with said engine and gearbox means.

4. A compound helicopter as claimed in Claim 3, wherein said fan comprises an axial fan having variable pitch blades.

1 P, A 11

5. A compound helicopter as claimed in any preceding claim, wherein the engine means comprise at least one gas turbine engine having a variable area final exhaust nozzle provided as a part of the engine.

6. A compound helicopter as claimed in Claim 5, wherein the or each engine is a variable cycle engine and means are prcided for controlling engine shaft power delivered for driving said lift rotor assembly whereby power to said lift rotor assembly is substantially reduced as forward speed of said compound helicopter increases.

7. A compound helicopter as claimed in Claim 6, wherein the or each variable cycle engine comprises a low pressure compressor. a core gas generator for driving the compressor, a power turbine driven by the exhaust of the core gas generator and connected for driving a power output shaft, said variable area final exhaust nozzle being connected for receiving the exhaust from the power turbine and being operable to vary the power absorbed by the power turbine and to simultaneously vary the propulsive thrust produced by the nozzle.

8. A compound helicopter comprising a fuselage structure including a rearwardly extending tail boom structure, a lift rotor assembly including a plurality of rotor blades 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 said fuselage structure for providing a major portion of lift during high speed forward flight. gas turbine engine means connected through gearbox mans for driving the lift rotor assembly, variable area final exhaust nozzle means co-operating with said engine means for producing propulsive forces for high speed forward flight and whereby engine power supplied for driving said lift rotor assembly may be controlled, means for producing a flow of air internally of and rearwardly along said tail boom structure, means on said tail boom structure for venting said air to control circulation of lift rotor downwash air over an external surface of said tail boom structure so as to produce a lateral force on said tail boom structure of magnitude and direction appropriate to counteracting the effect on said fuselage structure of lift rotor torque when said lift rotor is being driven to provide the major portion of lift, air outlet 12 ports including pivotal closure means for selectively venting said air from said tail bom structure to produce lateral forces for yaw control when said lift rotor is being driven to provide the major portion of lift, and fin and rudder means mounted on said boom structure, said rudder means being operable to produce yaw control forces during high speed forward flightwhen said fixed wing is providing the major portion of lift.

1

9. A coo pound helicopter substantially as hereinbefore described with reference to and as shown in the accaq:)anying drawings.

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

Published 1991 at Ihe Patent Office, State House. 66171 High Holbom. London WC] R 41P. Further copies rnay be obtatnedfrorn Sales Branch. Unit 6. Nine Mile Point Cwmfelinrach. Cross Keys, Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray, Kent.