VTOL aircraft with variable wing sweep

Abstract: An aircraft (10) capable of vertical take off with the fuselage generally vertical and normal cruise flight with the fuselage generally horizontal has a fuselage (12) and a pair of wings (13,14), the wings (13,14) being movable relative to the fuselage (12) from a rearwardly swept position to which the wings (13,14) are moved for vertical take off, to a spread position to which the wings (13,14) are moved for normal cruise flight.

Description: This invention relates to an aircraft and more particularly to an aircraft which is capable of vertical take off and landing (VTOL). Such aircraft are known which are of configurations which have come to be known as "tailsitters", an example being disclosed in US-A-5289994. In this previous specification, a VTOL aircraft is described which has a pair of fixed wings which provide lift in normal horizontal or cruise flight, and helicopter-like rotors or propellers to provide lift during vertical take off and landing, and thrust in cruise flight. On the ground, the aircraft is adapted to sit on a tail structure thereof with a fuselage extending generally vertically. Other examples of similar aircraft are disclosed in US-A-2622826, US-A-2859003, and US-A-5516060. In the latter two prior proposals, jet thrust instead of rotor/propeller thrust is utilised. In each of these examples, the wings which provide lift during normal horizontal flight are fixed relative to a fuselage thereof, although the wings may be provided with ailerons and the like for cruise flight control. Such fixed wings having a relatively high aspect ratio are capable of achieving good cruise efficiency especially in the generally low speed horizontal flight regime of such aircraft but the fixed spread position impairs hover and low speed stability and control in the vertical take off and landing mode. According to one aspect of the invention we provide an aircraft having a fuselage and a pair of wings and being capable of vertical take off and landing with the fuselage generally vertical and normal cruise flight with the fuselage generally horizontal, the wings being movable relative to the fuselage from a rearwardly swept position to which the wings are moved for vertical take off, to a spread position having a higher aspect ratio to which the wings are moved for normal cruise flight. By virtue of the wings being so movable, improved low speed performance and control, particularly during and immediately after vertical take-off can be achieved with the wings in the rearwardly swept position. It is well known in conventional aircraft which take off and land generally horizontally to provide so called swing wings, by means of which a wing spread position can be provided during take-off and landing, and a rearwardly swept positon can be provided during high speed cruise flight. In an aircraft in accordance with the invention which is capable of at least vertical take off, an opposite regime is adopted. By moving the wings during vertical take off, to a rearwardly swept position, the distance of wing control surfaces, such as ailerons/flaps or the like from a centre of gravity of the aircraft, is increased, to provide a lengthened moment arm to increase the control forces which can be achieved by moving the control surfaces. Furthermore, with the wings in the rearwardly swept position, the space required for storage of the aircraft is reduced. In general, VTOL aircraft of the kind with which the invention is primarily concerned are not required to achieve very high speed forward cruise flight, but may achieve cruise flight speeds in the range 150 to 200 knots. At such relatively low speeds the higher aspect ratio of the wings in the spread position provides an improved lift/drag ratio to optimise the efficiency of the aircraft during normal cruise flight. If the invention was applied to an aircraft which was capable of higher speed forward flight, if desired in such high speed mode of operation, the wings may be swept rearwardly to an interim or fully swept position to reduce the aspect ratio and to improve efficiency at such higher forward speeds. The aircraft of the invention may have an e.g. cruciform configuration tail structure including tail wings and, prior to take off, the aircraft may sit on the tail structure. In a preferred arrangement, the aircraft is capable of both vertical take-off and landing. Thus preferably the wings are moveable to the rearwardly swept position during landing as well as take-off. Thrust at least for take-off, and landing where the aircraft is capable of vertical landing, may be provided by a rotor/propeller system, which may include a pair of contra-rotating rotors which mutually counteract torque reactions. Such rotor system may be provided at or adjacent a nose structure of the aircraft. Alternatively or additionally, thrust at least for take-off, and landing where the aircraft is capable of vertical landing, may be provided by a jet thrust preferably provided in the fuselage e.g. at a tail end of the aircraft. If desired, a rotor system may be used to achieve thrust during at least take-off, and landing where the aircraft is capable of vertical landing, and thrust may be provided by a jet thrust during normal cruise flight. A rotor system is preferred for at least take-off, and landing where the aircraft is capable of vertical landing, for finer stability and control, for manoeuvring and hovering. The aircraft may be used as an un-manned aerial vehicle. According to a second aspect of the invention we provide a method of operating an aircraft having a fuselage and a pair of wings, and being capable of vertical take-off and landing with the fuselage generally vertical and normal cruise flight with the fuselage generally horizontal, the method including moving the wings relative to the fuselage to a rearwardly swept position for vertical take-off, operating a thrust means to achieve vertical take-off, manoeuvring the aircraft to the normal cruise flight condition, and moving the wings relative to the fuselage to a spread position for normal cruise flight. Preferably the wings are moved relative to the fuselage from the swept position to the spread position during the transition from vertical flight to normal cruise flight. The method of the invention may include moving the wings from the spread position to the rearwardly swept position during transition from normal cruise flight to vertical flight prior to vertical landing. The aircraft to which the method of the second aspect of the invention is applied, may have any of the features of the aircraft of the first aspect of the invention. The invention will now be described with reference to the accompanying drawings in which:- FIGURE 1 is an illustrative side view of an aircraft in accordance with the invention in a vertical position prior to vertical take-off; FIGURE 2 is an illustrative plan view of the aircraft of figure 1 in normal cruise flight. Referring to the drawings, an aircraft 10 of the kind which is capable of vertical take-off and landing, includes a fuselage 12, and a pair of wings 13, 14. At a tail end 15 of the fuselage there is a cruciform tail structure 18 which includes four tail wings 19 each having an aerodynamic control surface 20. The aircraft 10 further includes a nose structure 22 which in this example mounts a rotor/propeller system 25 which has a first multi-bladed rotor 28 which in use rotates in a first direction e.g. clockwise about a longitudinal axis A of the aircraft and is located axially outermost, and a second multi-bladed rotor 29 which in use rotates in a second opposite direction about axis A to the first rotor 28, the second rotor 29 being located axially innermost. Thus the rotors 28, 29 are generally parallel and rotate in opposite directions, thus mutually counteracting torque reactions produced by the other of the rotors 28, 29. Referring now to figure 1, the aircraft 10 is shown with the fuselage 12 in a generally vertical position prior to take-off. The aircraft 10 sits on the cruciform tail structure 18 which is adapted to support the weight of the aircraft 10 on the ground. The wings 13, 14 are in a rearwardly swept position in which the wings are moved close to the fuselage 12 and a wing quarter chord line L is swept rearwardly at an angle  from a line perpendicular to a fuselage (longitudinal) centre line 21. By virtue of the rearwardly swept wing position, during vertical flight, better control and stability of the aircraft 10 is achievable, than for an aircraft having wings in a spread position in which the wing quarter chord line L is perpendicular or approximately perpendicular to the fuselage centre line 21. To achieve take-off, the rotors 28, 29 are rotated about axis A at a sufficient speed to achieve the necessary thrust for take-off. Thus there may be provided an engine within the fuselage 12, connected to the rotors 28, 29 by a transmission which may be of a similar construction to that used for a helicopter main sustaining rotor system. By virtue of the rotor system 25, the aircraft 10 may be made to hover in vertical flight, in the same manner as a helicopter. Once the aircraft 10 has taken-off, the aircraft 10 may be manoeuvred for normal cruise flight as seen in figure 2, in which the fuselage 12 is generally horizontal. Such manoeuvring may be achieved by adjusting the rotors' blades, and/or operating control surfaces such as ailerons/flaps 40, 20 provided on the wings 13, 14 and the tail structure 15. In normal cruise flight, two of the tail wings 19 may or may not be generally vertical or horizontal as desired. During transition to normal cruise flight, the wings 13, 14 are moved relatively to the fuselage 12 outwardly to a spread position seen in figure 2 to reduce the angle  from that shown in Figure 1, to an angle in which the wing quarter chord line L extends perpendicularly or more nearly perpendicularly to the aircraft fuselage (longitudinal) centre line 21, than when the wings 13, 14 are in the rearwardly swept position. In this position, the wings 13, 14 present a higher aspect ratio than when wings 13, 14 are in the rearwardly swept position. At least in normal cruise, relatively slow forward flight, the higher aspect ratio of the wing improves the control of the aircraft 10, by providing an improved lift/drag ratio to optimise efficiency of the aircraft 10. During normal cruise flight, the rotors 28, 29 operate as a propeller system to provide thrust for forward flight. For landing the aircraft, the fuselage 12 is manoeuvred into vertical flight and the wings 13, 14 are again moved to the rearwardly swept position shown in figure 1. Thus the aircraft 10 is landed vertically. Although in the example described, thrust for vertical and normal cruise flight is achieved by a rotor/propeller system 25, in another example thrust at least for normal cruise flight, but if desired for vertical flight too, may be achieved by a jet thrust means such as a jet engine provided in the fuselage 12 e.g. as indicated at J in figure 2, at or adjacent the tail end 15 thereof. Thus the aircraft may have both a rotor system 25 and a jet thrust means J to develop thrust, or a jet thrust means J or rotor/propeller system 25 only. Mechanisms for achieving wing movement outwardly and inwardly relative to the fuselage are known for use on aircraft which take-off and land conventionally i.e. in a generally horizontal condition. It is envisaged that a similar mechanism may be used for moving the wings 13, 14 in an aircraft in accordance with the invention.

Claims: An aircraft (10) having a fuselage (12) and a pair of wings (13, 14), and being capable of vertical take off and landing with the fuselage (12) generally vertical and normal cruise flight with the fuselage (12) generally horizontal, the wings (13, 14) being movable relative to the fuselage (12) from a rearwardly swept position to which the wings (13, 14) are moved for vertical take off, to a spread position having a higher aspect ratio to which the wings (13, 14) are moved for normal cruise flight. An aircraft according to claim 1 characterised in that the aircraft )10) has a tail structure (18) including tail wings (19) and, prior to take off, the aircraft (10) sits on the tail structure. An aircraft according to claim 2 characterised in that the tail structure (18) is of cruciform configuration. An aircraft according to claim 1 or claim 2 or claim 3 characterised in that the aircraft (10) is capable of both vertical take-off and landing and the wings (13, 14) are moveable to the rearwardly spread position during landing as well as take-off. An aircraft according to any one of the preceding claims characterised in that thrust for at least take-off, and landing where the aircraft is capable of vertical landing, is provided by a rotor system (25). An aircraft according to claim 5 characterised in that the rotor system (25) includes a pair of contra-rotating rotors (28, 29) which mutually counteract torque reactions. An aircraft according to claim 5 or claim 6 characterised in that the rotor system (25) is provided at or adjacent a nose structure (22) of the aircraft (10). An aircraft (10) according to any one of the preceding claims characterised in that thrust for at least take-off, and landing where the aircraft is capable of vertical landing, is provided by a jet thrust means (J). An aircraft according to claim 8 characterised in that the jet thrust means (J) is provided in the (12) at a tail end (15) of the aircraft (10). An aircraft according to claim 9 characterised in that a rotor system (25) is used to achieve thrust during at least take-off, and landing where the aircraft (10) is capable of vertical landing, and thrust means are provided by a jet thrust (J) during normal cruise flight. An aircraft according to any one of the preceding claims characterised in that the (10) is an un-manned aerial vehicle. A method of operating an aircraft (10) having a fuselage (12) and a pair of wings (13, 14) and being capable of vertical take off and landing with the fuselage (12) generally vertical and normal cruise flight with the fuselage (12) generally horizontal, the method including moving the wings (13, 14) relative to the fuselage (12) to a rearwardly swept position for vertical take off, operating a thrust means (25, 33) to achieve vertical take-off, manoeuvring the aircraft (10) to the normal cruise flight condition, and moving the wings (13, 14) relative to the fuselage (12) to a spread position for normal cruise flight. A method according to claim 12 characterised in that the wings (13, 14) are moved relative to the fuselage (12) from the swept position to the spread position during the transition from vertical flight to normal cruise flight. A method according to claim 12 or claim 13 characterised in that the wings (13, 14) are moved from the spread position to the rearwardly swept position during transition from normal cruise flight to vertical flight prior to vertical landing. A method according to claim 12 or claim 13 or claim 14 characterised in that the aircraft (10) has any of the features of the aircraft of any one of claims 1 to 11.

Inventors: INGRAM DAVID BARRY, INGRAM, DAVID BARRY
Applicants: WESTLAND HELICOPTERS, WESTLAND HELICOPTERS LIMITED
Application year: 2000
Main classification: B64C29
IPC codes: B64C29, B64C5, B64C3