GB2583499A

GB2583499A - Aircraft wing with a moveable wing tip for load alleviation

Info

Publication number: GB2583499A
Application number: GB1906062.3A
Authority: GB
Inventors: Wilson Thomas
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2019-04-30
Filing date: 2019-04-30
Publication date: 2020-11-04
Also published as: GB201906062D0

Abstract

An aircraft wing 104 comprising a fixed wing 106 for mounting to a fuselage, and a wing tip 108 mounted at an end of the fixed wing. The wing tip is rotatable relative to the fixed wing about a first axis 126 and a second axis 114. The wing is operable between: a flight configuration, wherein the wing tip is restrained to prevent rotation of the wing tip about the first axis; and a load alleviating configuration, wherein the wing tip can rotate about the first axis assisted by aerodynamic forces exerted on the wing tip during flight such that the load on the wing is reduced. Rotation of the wing tip about the second axis results in a change in the angle of incidence of the wing tip; this may be achieved by changing the pitch of the wing tip and causing the wing tip to adopt a different position in the load alleviating configuration during flight. Rotation of the wing tip about the second axis can be used to restore the wing tip to the flight configuration following a high load event or move the wing tip to a ground configuration during landing.

Description

AIRCRAFT WING WITH A MOVEABLE WING TIP FOR LOAD ALLEVIATION

TECHNICAL FIELD

100011 The present invention concerns aircraft wings having a movable wing tip.

More particularly, this invention concerns aircraft wings having a wing tip that is movable in flight so as to reduce the aerodynamic load on the wing. The invention also concerns methods of controlling an aircraft wing.

BACKGROUND OF THE INVENTION

[0002] Aircraft may be equipped with moveable wing tip devices attached to fixed wings. The wing tip devices may be used to allow load-alleviation during flight, for example, in the event of wind gusts. Typically, large loads cause the wing tip device to move from a flight configuration in which the wing tip device is effectively a continuation of the fixed wing portion to a load-alleviating configuration in which the load on the wing is reduced. Once the cause of the requirement to enter the load-alleviating configuration has passed, it is often desirable to move the wing tip device back into the flight configuration. W02017118832, the contents of which is fully incorporated herein by reference, discloses such a wing tip device that is moveably mounted about a hinge.

100031 Further, there is a trend towards increasingly large passenger aircraft, for which it is desirable to have correspondingly large wing spans. However, the maximum aircraft span is effectively limited by airport operating rules which govern various clearances required when manoeuvring around the airport (such as the span and/or ground clearance required for gate entry and safe taxiway usage). Moveable wing tip devices may be used to enable the wing span to be reduced in a ground configuration.

[0004] An actuator, for example an electro-mechanical or hydraulic actuator, may be used to move the wing tip device between different configurations, for example between a flight configuration and a ground configuration when the aircraft is on the ground. However, such an actuator may require a substantial load and/or torque capacity, -2 -particularly where the actuator is to be used during flight. Typically, the larger the load and/or torque capacity required by the actuator, the larger and heavier the actuator. The inclusion of a large, heavy (and potentially complex) actuator system on the aircraft may lessen or erode some or all of the aerodynamic benefits brought about by the moveable wing tip device itself Further, on-board space is an important consideration on aircraft. Some aircraft may not have enough space for a sufficiently powerful actuator capable of withstanding and/or generating torques as desired.

100051 The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved aircraft wing.

SUMMARY OF THE INVENTION

[0006] The present invention provides, according to a first aspect, an aircraft wing comprising a fixed wing, for example for mounting to a fuselage, and a wing tip mounted at an end of the fixed wing. The wing tip is rotatable relative to the fixed wing about a first axis. The wing is operable between: (i) a flight configuration for use during flight, wherein the position of the wing tip is restrained; and (ii) a load alleviating configuration for load alleviation during flight, wherein the wing tip is allowed to rotate about the first axis, under the action of aerodynamic forces exerted on the wing tip during flight, towards an equilibrium position, such that the load on the wing is reduced. When the wing is in the load alleviating configuration, at least part of the wing tip, and preferably all of the wing tip, is rotatable relative to the fixed wing about a second axis. The second axis is orientated relative to the first axis such that rotation about the second axis results in a change to the angle of incidence of that part of the wing tip, thereby urging the wing tip towards a different equilibrium positon in the load alleviating configuration during flight.

100071 In flight, when the wing tip is allowed to rotate about the first axis for the purposes of load alleviation, there will be an equilibrium position, at any given set of conditions, at which the various forces acting on the wing tip would be balanced. In -3 -embodiments of the invention, the equilibrium position of the wing tip can be altered whilst the wing is in the load alleviating configuration, by rotating the wing tip about the second axis so as to change the angle of incidence of the wing tip. It is therefore possible to alter the aerodynamic forces on the wing tip by such a change in angle of incidence to cause the equilibrium position to move and thus cause the wing tip to move towards a new equilibrium position, via a rotation about the first axis. It may thereby be possible to cause the wing tip to adopt a positon which is at, or is close to, the flight position of the wing tip with the assistance of such aerodynamic forces. It may therefore be possible to reduce the size of the actuator required to restore the wing tip to the flight position, and in some embodiments it may be possible to remove the actuator completely.

100081 Changes to the angle of incidence of the wing tip may, in some embodiments, be considered as a change to the pitch angle of the wing tip relative to a local frame of reference of the wing tip/fixed wing. In most flight conditions, the change in angle of incidence / pitch angle of the wing tip may cause a corresponding change in the angle of attack of the wing tip. In the flight configuration, the wing span of the aircraft may be extended by the position of the wing tip. In embodiments of the invention, the wing may have its greatest span when the wing is in its flight configuration, with the wing tip positioned accordingly.

[0009] In the flight configuration, the wing tip may act as an extension of the fixed wing. The flight configuration may be a configuration in which the wing tip is at an aerodynamically efficient position (possibly the most aerodynamically efficient position) during I g flight. It may be that, in the flight configuration, lift loads generated by the wing tip is transferred to the fixed wing. Lift loads generated by the wing tip in the flight configuration thereby contributes to the overall lift generated by the wings of the aircraft.

100101 Preferably, in the flight configuration, the position of the wing tip is restrained so as to restrict rotation of the wing tip about the first axis. It may be that, in the flight configuration, the wing tip is prevented from rotating about the first axis, for example by being locked in position.

100111 The fixed wing may have an upper surface and a lower surface, and the wing tip may have an upper surface and a lower surface. In the flight configuration, the upper -4 -and lower surfaces of the wing tip may be continuations of the upper and lower surfaces of the fixed wing.

[0012] In the flight configuration, the trailing edge of the wing tip is preferably a continuation of the trailing edge of the fixed wing. The leading edge of the wing tip is preferably a continuation of the leading edge of the fixed wing. There is preferably a smooth transition from the fixed wing to the wing tip. It will be appreciated that there may be a smooth transition even when the shape of the wing is such that there are changes in sweep or twist at the junction between the fixed wing and wing tip. However, there are preferably no discontinuities at the junction between the inner wing and wing tip. The position of the wing tip when the wing is in the flight configuration may be referred to as the flight position of the wing tip.

100131 In the flight configuration, the line of 50% chord (i.e. the line which connects the 50% chord positions) of the fixed wing and the line of 50% chord of the wing tip may be substantially aligned.

[0014] When the wing is in the load alleviating configuration, the aircraft incorporating the wing, is still suitable for flight, but the wing tip is preferably moved to a position in which the load on the wing is alleviated. It will be appreciated that the load alleviating configuration may encompass the wing tip being in a range of positions (all of which alleviate load to some degree). The position (for example the magnitude of rotation about the first axis) may be dependent on the magnitude of load that is sought to be alleviated. The wing tip may be repeatably moveable between the load alleviating configuration and the flight configuration. The load on the wing in the load alleviating configuration is preferably reduced as compared to the load on the wing in the flight configuration.

[0015] In the load alleviating configuration, the wing tip may be moved relative to the fixed wing such that at least one of the upper and lower surfaces of the wing tip is moved away from the respective surface of the fixed wing. In the load alleviating configuration, the wing tip may be moved relative to the fixed wing such that at least one of the leading and trailing edges of the wing tip is no longer a continuation of the respective edge of the fixed wing. In the load alleviating configuration, the line of 50% -5 -chord of the fixed wing and the line of 50% chord of the wing tip may move out of alignment [0016] In the load alleviating configuration, the wing tip may, in flight, move towards a position of equilibrium in which the angular positon about the first axis is determined by the lift generated by the wing tip and the weight of the wing tip. In the load alleviating configuration, the wing tip may be free to move about the first axis. In the load alleviating configuration, the positon of the wing tip about the first axis is preferably not set or controlled by an actuator.

[0017] In the load alleviating configuration, preferably substantially no bending moment is transferred across a mounting between the wing tip and the fixed wing; thereby the lift loads generated by the wing tip are not transferred to the fixed wing. Lift loads generated by the wing tip in the load alleviating configuration may thereby provide substantially no contribution to the overall lift generated by the wings of the aircraft.

[0018] In some embodiments, the wing tip device may be entirely free to rotate about the first axis when the wing is in the load alleviating configuration. I.e. In the load alleviating configuration, there may be substantially no resistive forces acting to prevent rotation about the first axis (other than frictional forces in the mounting between the wing tip and the fixed wing). When the wing tip device is free to rotate in this manner, it may be referred to as 'coasting' or being arranged to 'coast'. In such embodiments, the centre of gravity of the wing tip device may be positioned such that no substantive shear loads are passed into the wing tip and to ensure the flutter speed is sufficiently high.

[0019] The wing may be arranged in the load alleviating configuration if a high load event is predicted and/or detected. High load events may include gusts, turbulence, and certain aircraft manoeuvres.

[0020] The first axis is preferably oriented non-parallel to the longitudinal axis of the aircraft. The first axis is preferably oriented non-parallel to the line of flight direction. The first axis is preferably oriented such that the first axis intersects the wing in a trailing edge region inboard of where the first axis intersects the wing in a leading edge region. In embodiments, the first axis may intersect the trailing edge and/or leading edge of the wing (for example when viewed from above). -6 -

[0021] The first axis may be located in a vertical first plane, wherein the first plane intersects the trailing edge of the wing and the leading edge of the wing. Preferably, the first axis is oriented such that the first plane intersects the trailing edge of the wing inboard of where the first plane intersects the leading edge of the wing.

[0022] The first axis is preferably oriented such that, in the load alleviating configuration, the mean angle of incidence of the wing tip changes when the wing tip is rotated about the first axis. More preferably, the first axis is oriented such that the mean angle of incidence of the wing tip is reduced when the wing is in the load alleviating configuration during flight. As the wing tip rotates about the first axis, and the angle of incidence reduces, the lift generated by the wing tip may reduce to a point that the aerodynamic forces are balanced by the weight of the wing tip. There may therefore be an equilibrium position at which there is zero moment about the first axis. In such a position, the moment due to the aerodynamic forces (e.g. lift and drag) acting on the wing tip and the moment due to the weight of the wing tip are balanced. The wing tip may thereby settle to a stable load alleviating position.

[0023] It will be understood that, for example in a gust, where the apparent wind speed over the wing changes, the aerodynamic loads on the wing tip may change. For example, more lift may be generated by the wing tip. This may cause the equilibrium position of the wing tip to change, and the wing tip may thereby be caused to move towards the new equilibrium position by rotating about the first axis. The wing tip may therefore not remain in the same position relative to the wing whilst the wing is in the load alleviating configuration.

[0024] The first axis may be orientated substantially perpendicular to the swept mean chord axis of the wing. The swept mean chord axis may be parallel to the longitudinal direction of the wing box. The orientation of the first axis may be chosen such that the direction of airflow over the wing in the region of the wing tip remains non-parallel to the first axis during flight. The orientation of the first axis may be chosen such that, for a given orientation of the wing tip, the direction of airflow remains incident on the same side of the wing tip despite the yaw of the aircraft. The sign (positive or negative) of the angle of orientation of the first axis relative to the direction of airflow during normal flight conditions, when viewed from above, preferably remains the same. This may reduce the risk of the wing tip abruptly changing orientation during flight due to the airflow suddenly becoming incident on the opposite side of the wing tip, for example in the event of the aircraft experiencing large amounts of movement about the yaw axis, such as during a 'Dutch roll'.

[0025] The angle between the first axis and the longitudinal axis of the aircraft may be referred to as the "flare angle". The flare and may be between I 0 and 20 degrees. An example flare angle is 17 degrees. A flared axis of rotation may refer to an axis of rotation of the wing tip that is non-parallel to the longitudinal axis of the aircraft.

[0026] Rotation of the wing tip about the first axis may be referred to as folding of the wing tip. The first axis may be a fold axis. For example, it may be that rotation of the wing tip about the first axis results in the wing tip moving towards a position in which, if such movement were sufficiently continued, the wing tip would overlay the fixed wing. It will be appreciated that the wing tip may be prevented from moving to a position in which it actually overlays the fixed wing.

[0027] One or more stops may be provided to prevent the wing tip from moving beyond a certain range of angles. There may be a stop to prevent the wing tip from rotating upwardly about the first axis past a substantially vertical orientation (wherein the free end of the wing tip is substantially above the mounted end of the wing tip). There may be a stop to prevent the wing tip from rotating downwardly about the first axis past the position of the wing tip in the flight configuration (e.g. past a substantially horizontal orientation). The stops may include dampers. The dampers may dissipate the kinetic energy and slow the wing tip until stationary at the limits of its range of motion.

[0028] In the flight configuration, the second axis may be oriented in a generally spanwise direction. The second axis may be substantially perpendicular to the first axis. In the flight configuration, the second axis may be orientated substantially parallel to the swept mean chord axis of the wing. In the flight configuration, the second axis may be substantially parallel to the longitudinal axis of the fixed wing and/or the wing tip. The second axis may pass proximate the leading edge of the wing in the region at which the wing tip is mounted to the fixed wing. -8 -

[0029] The wing tip may be rotatable relative to the fixed wing about a second axis so as to change the pitch of the wing tip relative to the fixed wing. In other words, rotation of the wing tip about the second axis may change the pitch of the wing tip in a local frame of reference relative to the fixed wing.

[0030] The flight position may be said to define a position of zero rotation about the first and/or second axis. The wing tip may be rotatable by up to 20 degrees, up to 30 degrees, or up to 40 degrees in total about the second axis. (I.e. the wing tip may have a total range of motion of up to 20, 30 or 40 degrees). It may be that the wing tip can rotate in one direction about the second axis (e.g. in a nose-down direction so as to decrease the angle of incidence) further than the wing tip can rotate in the opposite direction about the second axis.

100311 The wing preferably comprises an actuator arranged to control rotational movement of the wing tip about the second axis. The actuator may set the angular position of the wing tip about the second axis. In some embodiments, in the load alleviating configuration, the rotation of the wing tip about the second axis may only be changed by actuating the actuator. The wing tip may be prevented from moving (i.e. not free to move) about the second axis under the action of aerodynamic forces during flight. The actuator may be a motor, for example an electric motor. The actuator may be a hydraulic actuator. Preferably, the actuator is provided in the fixed wing. This may be beneficial as the weight associated with the actuator is provided closer to the fuselage.

[0032] A brake and/or a lock may be provided to prevent rotation about the second axis unless the actuator is activated so as to drive said rotation. In some embodiments, the actuator may be connected to non-backdriveable gearing having self-locking properties to prevent rotation about the second axis unless the actuator is activated so as to drive said rotation.

100331 The actuator may be arranged to actuate a rack and pinion mechanism. The pinion may be mounted to the fixed wing and the rack may be mounted to the wing tip, or vice versa. Rotation of the pinion gear by the actuator may thereby cause movement of the wing tip relative to the fixed wing about the second axis. The rack gear may have a -9 -curved shape. The rack gear may be at least partially accommodated in a faring extending below the lower aerodynamic surface of the wing.

[0034] The wing tip may be mounted to the fixed wing via a joint comprising one or more pivots. There may be a first pivot which provides for rotation about the first axis and a second pivot which provides for rotation about the second axis. The one or more of the pivots may be provided by a hinge. The hinges may each define a hinge line about which the wing tip rotates. The direction of the hinge line may define the direction of the first axis.

[0035] The joint may comprise a forward part and an aft part. The forward part of the joint may be proximate the leading edge of the wing. The aft part of the joint may be proximate the trailing edge of the wing.

100361 A part of the joint may provide one rotational degree of freedom. For example, the joint may comprise a hinge comprising a pin which rotates within an aperture, to allow pivoting about a single axis of rotation. A part of the joint may provide two rotational degrees of freedom. For example, the joint may comprise a ball joint (e.g. a ball and socket joint) to allow pivoting about more than one axis of rotation (e.g. both the first and second axis).

[0037] The joint is preferably orientated such that the joint at the trailing edge of the wing is further inboard than the joint at the leading edge of the wing.

[0038] The wing may comprise a restraining assembly arranged to restrain the wing tip. The restraining assembly may be operable between a restraining mode and a releasing mode. In the restraining mode, the wing tip may be prevented from rotating about the first axis using a restraining force. In the restraining mode, the wing tip may be held in the flight position. In the releasing mode, the restraining force on the wing tip may be released such that the wing tip is able to adopt a load alleviating position (by rotating about the first axis).

100391 The wing tip may be securely held in the flight position by the restraining assembly during normal cruise flight, but if the aircraft encounters gusts, or other high load events, the restraining force can be released such that the wing is movable quickly to -10 -the load alleviating configuration. This may mean the wing can avoid being subjected to high gust loadings.

[0040] The restraining assembly may comprise a brake arranged to hold the wing tip.

The default (passive) mode of the restraining assembly is preferably the restraining mode. For example, in embodiments incorporating a brake, the default (passive) state of the brake is preferably such that it is acting to restrain the wing tip. The restraining assembly preferably needs activating, for example via an input signal, in order to switch to the releasing mode. Such an arrangement has been found to be beneficial because it ensures an 'active' step will be required to move it to the releasing mode, thereby removing the risk of uncommanded actuation to the releasing mode. For example, in embodiments incorporating a brake, the brake mechanism preferably needs to be switched ON and/or receive power in order to release the brake and release the wing tip.

[0041] The restraining assembly may comprise a lock and/or a latch. For example, the restraining assembly may comprise a latch member, for example a latch pin, carried by the fixed wing that is arranged to engage with the wing tip, or vice versa, to restrain the wing tip. The restraining assembly may comprise one or more stops that are movable into a position so as to prevent movement of the wing tip about the first axis.

[0042] The wing tip may be entirely passively actuated in the load-alleviating configuration once the restraining assembly is in releasing mode. For example, the wing tip may be moved under the action of aerodynamic forces.

[0043] It will be appreciated that when the restraining assembly is in the releasing mode, this does not necessarily preclude there still being resistive forces per se present between the wing and wing tip (for example from a damper).

[0044] The wing may comprise a biasing member arranged to exert a biasing force to urge the wing tip away from its position in the flight configuration.

100451 When the restraining assembly is in the restraining mode, the biasing force is preferably overcome by the restraining force. When the restraining assembly is in the releasing mode, the biasing force is preferably sufficient to assist in moving the wing tip into a position for alleviating load on the wing. Such an arrangement may provide reassurance that the wing tip can move away from its position in the flight configuration to a load alleviating position, if needs be, even in the absence of aerodynamic forces acting on the wing tip.

[0046] The biasing member may be able to be selectively disengaged from exerting the biasing force on the wing tip. For example, the wing may comprise a clutch for selectively disengaging the biasing member from exerting the biasing force on the wing tip. Such an arrangement may enable the biasing member to be selectively disengaged to enable easier maintenance of the wing tip.

100471 The wing may comprise a damping system arranged to damp movement of the wing tip about the first axis. Such an arrangement may be beneficial if the wing tip moves quickly away from its position in the flight configuration, as the damping system tends to damp down transient, oscillatory, movements. A damping system may also mitigate aeroelastic instabilities such as flutter, and/or may limit cycle oscillations.

[0048] In other embodiments, the wing need not necessarily comprise any damper and/or biasing member.

[0049] The wing tip may be a wing tip extension; for example the wing tip may be a planar tip extension. In other embodiments, the wing tip may comprise, or consist of, a non-planar device, such as a winglet. It will be appreciated that in the context of the present invention, the terms 'wing tip' and 'wing tip device' may be used interchangeably.

[0050] The wing is preferably operable to (iii) a ground configuration for use during ground-based operations. In the ground configuration, the wing tip is positioned away from the flight configuration such that the span of the aircraft wing is reduced. In the flight configuration, the span may exceed an airport compatibility gate limit. In the ground configuration the span is reduced such that the span (with the wing tip in the ground configuration) is less than, or substantially equal to, the airport compatibility gate limit. In the ground configuration, the wing tip may be positioned such that the wing has its shortest span. In the ground configuration, the wing tip may be oriented substantially vertically. The wing tip may be moved from the flight configuration to the ground configuration by rotating the wing about the first axis. The position of the wing tip when the wing is in the ground configuration may be referred to as the ground position.

-12 - [0051] Prior art wing arrangements typically use an actuator to move the wing from the flight configuration to the ground configuration. Typically it is necessary to wait for the aircraft to slow down to a certain speed (for example about 40-50kts) before actuating the actuator to move the wing tips. In some cases this may mean that the wings will not be in the ground configuration (e.g. with the wing tips fully raised) by the time the aircraft comes to leave the runway, which could cause a delay to the progress of the aircraft towards its gate. In order to move the wings to a ground configuration at a higher speed, a larger actuator must be used to withstand the aerodynamic loads generated by the wing tips. Use of a larger actuator conies with a weight penalty.

[0052] The present invention may allow the aerodynamic forces on the wing tip to provide assistance with moving the wing to the ground configuration, for example during landing. In some embodiments, it may even be possible to eliminate the actuator completely and move the wing to the ground configuration completely using aerodynamic forces. This may be effected by, whilst the aircraft is moving, rotating the wing tip about the second axis in a direction so as to increase the aerodynamic force acting to move the wing tip towards its position in the ground configuration. For example, if in the ground configuration the wing tip is folded upward, the wing tip may be rotated about the second axis in a direction which acts to increase the lift on the wing tip. For example, the wing tip may be pitched upwards. The wing tip may thereby be urged upwards (so as to rotate about the first axis) towards its position in the ground configuration by the lift being generated.

[0053] Moving the wing to the ground configuration by pitching the wing tip upwards may enable the wing to be moved to the ground configuration earlier on in the landing process and whilst the aircraft is travelling at a higher speed. The upper speed limit at which the wing can be moved to the ground configuration may thereby be dictated by the speed below which a touch and go landing is not possible. In some embodiments, the wing may be moved to the ground configuration whilst the aircraft is travelling at about 100kts 100541 It will be appreciated that in some embodiments the wing may still comprise an actuator arranged to move the wing tip from the flight position to the ground position.

-13 -The actuator may, however, be less powerful (and less heavy) than may be required in prior art wings. The actuator may be useful if, for example, the actuator for rotating the wing tip about the second axis fails and it is not possible to configure the wing tip so as to generate sufficient aerodynamic loads to lift the wing tip using aerodynamic forces alone. The same actuator may also be arranged to move the wing tip from the ground position to the flight position, for example prior to take off In some embodiments, the wing may be arranged such that the wing tip can move from the ground positon to the flight position using gravity alone. The same actuator may also be arranged to assist in moving the wing from the load alleviating configuration to the flight configuration.

[0055] When it is desired to move the wing from the flight configuration to the ground configuration the restraining assembly may be placed into a releasing mode.

100561 When the wing is in the flight configuration, the actuator may be in a passive state such that it does not actively contribute to the restraining force that hold the wing tip in the flight position, the restraining force instead being provided by the restraining assembly. The function of restraining of the wing tip (during flight) may thereby be de-coupled from the function of the actuator (which may be reserved for moving the wing tip between the flight and ground configurations).

[0057] The actuator may be used, either in an active state or a passive state, to damp rotation of the wing tip about the first axis.

[0058] The joint between the fixed wing and wing tip may comprise a ratchet mechanism arranged to prevent retum movement of the wing tip towards the flight position when attempting to move the wings to the ground configuration. For example, the ratchet may prevent downward movement of the wing tip due to a sudden gust of wind when attempting to raise the wing tip towards a ground position. The ratchet may be en gagable for use during landing.

100591 The wing tip may be arranged such that it can move from the ground position to the flight position under its own weight. In the ground configuration, the centre of mass of the wing tip may be such that the wing tip rotates about the first axis from the ground position towards the flight position under its own weight without an impulse from -14 -an actuator. In alternative arrangements, the actuator may initiate movement of the wing tip from the ground position to the flight position.

[0060] The present invention provides, according to a second aspect, an aircraft comprising a wing in accordance with the first aspect of the invention. The aircraft preferably comprises two wings according to the first aspect of the invention, a wing being provided on each of the port and starboard side of the aircraft.

[0061] The aircraft is preferably a passenger aircraft. The passenger aircraft preferably comprises a passenger cabin comprising a plurality of rows and columns of seat units for accommodating a multiplicity of passengers. The aircraft may have a capacity of at least 20, more preferably at least 50 passengers, and more preferably more than 50 passengers. The aircraft is preferably a powered aircraft. The aircraft preferably comprises an engine for propelling the aircraft. The aircraft may comprise wing-mounted, and preferably underwing, engines.

[0062] The span ratio of the fixed wing relative to the wing tip may be such that the fixed wing comprises at least 70%, 80%, 90%, or more, of the overall span of the aircraft wing.

100631 The aircraft may comprise a control system arranged to control operation of the wing between the flight configuration and the load alleviating configuration, and optionally also the ground configuration. The control system may be part of the Electronic Flight Control System (EFCS).

[0064] The control system may be arranged to determine the positon of the wing tip relative to the fixed wing. The wing may comprise sensors for detecting the angular position of the wing tip about the first axis and/or the second axis. The control system may be arranged to instruct the wing tip to rotate about the second axis in order to alter the position of the wing tip. More specifically, the control system may be arranged to instruct an actuator to rotate the wing tip about the second axis, on the basis of the angular position of the wing tip about the first axis, in order to alter the position of the wing tip about said first axis.

100651 The control system may control operation of the restraining assembly between the restraining mode and the releasing mode.

-15 - [0066] The aircraft may comprise a detector for detecting when the wing should be switched from the flight configuration to the load alleviating configuration. The detector may be arranged to detect a force on the aircraft. For example, the detector may be arranged to detect a bending force in the wing. The control system may be configured to switch operation of the wing from the flight configuration to the load alleviating configuration in response to the detected force exceeding a predetermined threshold.

[0067] The aircraft may comprises an alpha detector (also known as an Alpha vane, or angle of attack detector) for detecting the angle of attack of the aircraft relative to the freestream flow. The control system may be configured to switch operation of the wing from the flight configuration to the load alleviating configuration in response to the detected angle of attack exceeding a predetermined threshold. Thus, the wing may be placed in the load alleviating configuration when an event indicative of a high load is detected (for example a gust may be registered using the alpha detector).

[0068] The control system may be arranged to also control other load alleviating elements on the aircraft (for example elevators or ailerons) in response to the alpha detector. Such elements may provide complementary load alleviation to that provided by the moveable wing tip.

[0069] The alpha detector may be located on the aircraft nose such that it is located upstream of the wing tip. Locating the alpha detector upstream of the wing tip enables the event (for example the gust) to be detected prior to that event encountering the wing tip. Thus the wing may be allowed to adopt the load alleviating configuration before that event reaches it.

[0070] The control system may be configured to switch operation of the wing into the load alleviating mode in response to a roll signal. The roll signal may be indicative of the aircraft undergoing a roll manoeuvre (e.g. in response to oppositely actuated ailerons on either wing).

100711 The present invention provides, according to a third aspect, a control system for an aircraft according to the second aspect of the invention.

100721 The present invention provides, according to a fourth aspect, a method of controlling an aircraft wing, for example a wing according to the first aspect of the -16 -invention. In such a method, the wing may be moved from a load alleviating configuration, in which the wing tip is in a load alleviating position and is allowed to rotate about a first axis under the action of aerodynamic forces exerted on the wing tip, to a flight configuration, in which the wing tip is in a flight position and the position of the wing tip is restrained. The method may comprise a step of rotating the wing tip in a first direction about a second axis to change the angle of incidence of the wing tip so as cause the wing tip to rotate about the first axis under the resulting aerodynamic forces on the wing tip that arise from the rotation about the second axis and move closer to flight position. The method may comprise a step of restraining the wing tip so as to prevent rotation of the wing tip about the first axis. The method may comprise a step (preferably subsequent to the preceding step) of rotating the wing tip in a second direction about the second axis so as to adopt the flight position.

[0073] The first direction is preferably a direction that pitches the wing tip downwards of its initial position. The first direction is preferably a direction that decreases the angle of incidence of the wing tip. The second direction may be opposite to the first direction.

100741 The method may comprise a step of configuring the restraining assembly into the restraining mode in which the wing tip is prevented from rotating about the first axis using a restraining force.

[0075] The present invention provides, according to a fifth aspect, a method of controlling an aircraft wing, for example a wing according to the first aspect of the invention. In such a method, the wing is moved from a flight configuration wherein the wing tip is in a flight position and the position of the wing tip is restrained, to a ground configuration, in which the wing tip is in a ground position. The method may comprise a step of allowing the wing tip to rotate about a first axis under the action of aerodynamic forces exerted on the wing tip. The method may comprise a step of rotating the wing tip about a second axis to change the angle of incidence of the wing tip in a direction so as to increase the aerodynamic force acting to move the wing tip towards the ground position (preferably via a rotation about the first axis).

-17 - [0076] The wing tip is preferably rotated about the second axis in a direction that pitches the wing tip upwards of its initial position so as to increase the lift generated by the wing tip. The wing tip is preferably rotated about the second axis in a direction that increases the angle of incidence of the wing tip [0077] The method may comprise a step of configuring the restraining assembly into a releasing mode. The method may further comprise a step of restraining the wing tip in the ground configuration. For example, a lock may be activated to lock the wing tip in the ground configuration.

[0078] The present invention provides, according to a sixth aspect, a method of landing an aircraft, for example an aircraft according to the second aspect of the invention. The method may comprise a step of determining whether the speed of the aircraft is below a threshold speed for initiating movement of the wing tip to the ground configuration. The method may comprise a step of, when the speed has been determined to be below the threshold speed, allowing the wing tip to rotate about the first axis under the action of aerodynamic forces exerted on the wing tip. The method may comprise a step of rotating the wing tip about a second axis to change the angle of incidence of the wing tip in a direction so as to increase the aerodynamic force acting to move the wing tip towards the ground position. The method may comprise a step of holding the wing in the ground configuration.

[0079] The threshold speed may be the speed below which a touch and go landing is not possible. The method may comprise a step of determining whether the aircraft is on the ground and allowing the wing tip to rotate about the first axis if the aircraft is determined to be on the ground.

[0080] The method may comprise a step of configuring the restraining assembly into a releasing mode so as to allow the wing tip to rotate about the first axis. The method may comprise a step of locking the wing tip in position in the ground configuration. The method may comprise a step of using an actuator to assist in moving the wing tip into the ground configuration (for example to provide a force to urge the wing tip in a direction in which it moves about the first axis towards its position in the ground configuration).

-18 - [0081] The wing tip may be rotated about the second axis before the wing tip is allowed to rotate about the first axis (i.e. before the restraining assembly is put in the releasing mode). The wing tip may be rotated about the second axis whilst the speed of the aircraft is above the threshold speed and/or before the aircraft has touched down on the ground.

[0082] The rotation of the wing tip about the second axis and the releasing of the restraining assembly may be performed automatically, for example on the basis of aircraft speed. The holding of the wing tip when it reaches its ground position may also be automatic. Confirmation that wing tips on both wings of the aircraft are securely held may be made available to the pilot via an indicator in the cockpit.

100831 The releasing of the wing tips prior to take off, such that they move to the flight position, may be manually instigated to the pilot or automatically controlled. The wing tips may be moved from the ground configuration to the flight configuration when the aircraft is stationary. The restraining assembly may be configured to the restraining mode automatically prior to take off Confirmation that wing tips on both wings of the aircraft are restrained may be made available to the pilot via an indicator in the cockpit.

100841 The present invention provides, according to a further aspect, an aircraft wing, the wing comprising: a fixed wing for mounting to a fuselage, a wing tip mounted at an end of the fixed wing; wherein wing tip is foldable relative to the fixed wing about a first axis of rotation so as to change the wing span of the aircraft; wherein the first axis is located in a first plane, wherein the first plane passes through the trailing edge inboard of where the first plane passes through the leading edge; and wherein the wing tip is rotatable relative to the fixed wing about a second axis so as to change the pitch of the wing tip, the second axis being non-parallel to the first axis.

[0085] It will be appreciated that any features described with reference to one aspect of the invention are equally applicable to any other aspect of the invention, and vice versa. For example, features described with reference to the wing of the first aspect may also be applicable to the aircraft, control system and/or method of the other aspects of the invention.

-19 - [0086] The term 'or' shall be interpreted as and/of unless the context requires otherwise.

DESCRIPTION OF THE DRAWINGS

[0087] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: [0088] Figure 1a shows a plan view of an aircraft comprising a wing according to a first embodiment of the invention; [0089] Figures I b and lc show a plan view of the wing according to the first embodiment of the invention; [0090] Figures 2a to 2c show a front view of the wing according to the first embodiment of the invention, a wing tip being rotated about a first axis; [0091] Figures 3a to 3c show a front view of the wing according to the first embodiment of the invention, the wing tip being rotated about a second axis; 100921 Figure 4 shows a plan view of the wing according to the first embodiment of the invention showing a joint between a fixed wing and the wing tip; 100931 Figure 5 shows an exploded perspective view of the wing according to the first embodiment of the invention showing the joint between a fixed wing and the wing tip; 100941 Figures 6a to 6d show a sequential front view of the wing according to the first embodiment of the invention, the wing being moved from a load alleviating configuration to a flight configuration; and [0095] Figures 7a to 7d show a sequential front view of the wing according to the first embodiment of the invention, the wing being moved from the flight configuration to a ground configuration.

DETAILED DESCRIPTION

[0096] Figure I a shows a plan view of an aircraft 100 comprising a fuselage 102 and two wings 104 according to a first embodiment of the invention. The wings 104 are swept -20 -backwards with respect to the fuselage 102. Each wing 104 comprises a fixed wing 106 and a wing tip 108. The fixed wing 106 is mounted to the fuselage 102 at the wing root 110. The wing tip 108 is mounted to the outboard end of the fixed wing 106 via a joint 112.

[0097] Figure I b and lc show the end of the wing 104 in greater detail. The joint 112 defines a first axis 114 and a second axis 124 about which the wing tip 108 can rotate relative to the fixed wing 106.

100981 The first axis 114 is oriented perpendicular to the swept mean chord axis 116 of the wing 102. The first axis 114 is therefore non-parallel to the longitudinal axis of the aircraft 100 and will also be non-parallel to the direction of airflow 118 over the wing 102 during flight. The angle 120 between the first axis 114 and the direction of airflow 118 is termed the "flare angle" 120. In this example, the flare angle is approximately 23 degrees. As can also be seen in Figures I b and l c, the first axis 114 intersects the training edge of the wing 104 inboard of where the first axis 114 intersects the leading edge of the wing 104. When the wing tip 108 rotates about the first axis 114 the wing tip 108 folds relative to the fixed wing 106 and the wing span of the aircraft 100 is reduced.

100991 The second axis 124 is oriented perpendicular to the first axis 114. When the wing tip 108 is rotated about the second axis 124, the angle of incidence of the wing tip change (which may be considered as a change in pitch of the wing tip 108 in a local reference frame). When the wing tip 108 rotates about the second axis 124 the wing tip 108 does not perform any folding motion, instead, the wing tip 108 performs a twisting motion relative to the fixed wing 106.

[00100] The second axis 124 is proximate the leading edge of the wing 104 at the interface between the fixed wing 106 and the wing tip 108. The trailing edge of the wing tip 108 therefore moves away from the trailing edge of the fixed wing 106 when the wing tip 108 rotates about the second axis 124; however, the leading edge of the wing tip 108 becomes only slightly misaligned with the leading edge of the fixed wing 106.

[00101] Figures 2a to 2c and 3a to 3c are a head-on views of the end of the wing 104.

Figures 2a to 2c and show the wing tip 108 rotating about the first axis 114. Figures 3a to 3b show the wing tip rotating about the second axis 124.

-21 - [00102] Figures 4 and 5 shows the joint 112 in more detail. The joint comprises a forward part comprising a pivot in the form of a ball joint 126. The ball joint 126 is provided proximate the leading edge of the wing 104. A ball part of the ball joint 126 is mounted to the fixed wing 106 and a socket part of the ball joint 126 is mounted to the wing tip 108. Both the first axis 114 and the second axis 124 pass through the ball joint 126.

[00103] The joint 112 also comprises an aft part comprising a pivot in the form of a pin hinge 128. The pin hinge 130 comprises a first part rotatably mounted to a second part about a central pin. The first axis 114 also passes through the central pin of the pin hinge 128.

1001041 The first part of the pin hinge 128 is mounted to the wing tip 108. The second part of the pin hinge 128 is mounted to a curved rack gear 130. The rack gear 130 is generally downwardly extending (see figure 5) and is slidably mounted to the fixed wing 106. The rack gear 130 is engaged with a pinion gear 132 which is driven by an actuator in the form of an electric motor 134. By actuating the electric motor 134, the rack gear 130 moves relative to the fixed wing 106, and therefore the pin hinge 128 moves relative to the fixed wing 106. The wing tip 108 is thereby caused to rotate about the ball joint 126 and second axis 124. In the orientation shown in Figure 4, the rack gear 130 moves in and out of the page.

[00105] The rack gear 130 extends below the lower aerodynamic surface of the wing by a distance of approximately 25% of the local wing chord in order to provide the wing tip 108 with the desired range of rotational movement about the second axis 124. The rack gear 130 is accommodated within a faring to protect the rack from the elements and improve aerodynamic efficiency. The wing tip 106 has a range of motion comprising approximately 20 degrees of rotation about the second axis. By way of example, in embodiments, the wing tip 108 may be pitched in a nose-down direction by approximately 4 degrees during normal cruise conditions when restoring the wing 104 to the flight configuration. In embodiments, and at slower speeds, the wing tip 108 may need to be pitched in a nose-down direction by up to approximately 15 degrees when restoring the wing 104 to the flight configuration.

-22 - [00106] The wing 104 also comprises a restraining assembly shown schematically as a box 136. The restraining assembly is operable between a restraining mode and a releasing mode. When the restraining assembly is in the restraining mode, the wing tip 108 is prevented from rotating about the first axis 114 using a restraining force.

[00107] In embodiments, the restraining assembly 136 may be a brake which acts to hold a part of the wing tip 108 in place. For example, the brake may be similar to that disclosed in International patent application number W02017118832. In other embodiment, the restraining assembly 136 may comprise a latch and/or lock, for example using a latch pin which engages with both the fixed wing 106 and wing tip 108 to prevent rotation of the wing tip 108 about the first axis 114.

1001081 The wing 104 is operable between (i) a flight configuration, (ii) a load alleviating configuration, and (iii) a ground configuration.

[00109] In the flight configuration, the wing tip 108 is in a flight positon. In the flight position, the wing tip 108 is an extension of the fixed wing 106. The upper and lower surfaces of the fixed wing 106 are continuous with the upper and lower surfaces of the wing tip 108. Similarly, the leading and trailing edges of the fixed wing 106 are continuous with the leading and trailing edges of the wing tip 108. The wing span of the aircraft is maximised. At the interface between the fixed wing 106 and the wing tip 108, the mean chord line of the fixed wing 106 is aligned with the mean chord line of the wing tip 108. Figures la to lc, 2a, and 3a show the wing tip 108 in the flight positon.

[00110] In the flight configuration, the wing tip 108 is restrained by the retraining assembly (the restraining assembly is in the restraining mode) so as to prevent rotation of the wing tip 108 about the first axis 114.

[00111] In the load alleviating configuration, the wing tip 108 is allowed to rotate relative to the fixed wing 106 about the first axis 114. The restraining assembly 136 is placed in the releasing mode and the restraining force on the wing tip 108 is released. During flight, the wing tip 108 can rotate about the first axis 106 under the action of aerodynamic forces exerted on the wing tip 108. Since the wing tip 108 can rotate, the lift forces generated by the wing tip 108, and transferred to the fixed wing 106, will be significantly reduced or eliminated. The arrangement is such that no bending moment is -23 -transferred across the joint 112. Therefore the wing tips 106 will contribute very little, or not at all, to the load on the wing 104.

[00112] For a given velocity of air flow over the wing 104, the wing tip 108 has a certain position of equilibrium in which the forces on the wing tip 108 are balanced. The wing tip 108 will tend to move towards this equilibrium position by rotation about the first axis 114. When released by the restraining assembly 136 during flight, the wing tip 108 is initially likely to move upward relative to the fixed wing 106 due to the lift the wing tip 108 is generating. Since the first axis 114 is at an angle to the direction of airflow 118, the angle of incidence of the wing tip 108 will reduce as the wing tip 108 rotates upwardly. The lift generated by the wing tip 108 will therefore tend to reduce as the wing tip 108 rotates further upward until equilibrium is achieved. It will be appreciated that, in practice, the airflow over the wing 104 may not have a constant velocity, and thus the wing tip 108 may not necessarily settle to any particular position when the wing is in the load alleviating configuration.

[00113] In the load alleviating configuration, the wing tip 108 can adopt one of a range of load alleviating positions. In embodiments, the wing tip 108 may adopt an angle between approximately 10 and 70 degrees from the horizontal, depending on the speed and angle of attack of the aircraft. In said load alleviating positions, the upper and lower surfaces of the fixed wing 106 and wing tip 108 are no longer continuous with each other. Similarly, the leading and trailing edges of the fixed wing 106 and wing tip 108 are no longer continuous with each other. The wing span of the aircraft is slightly reduced as compared to the flight configuration. At the interface between the fixed wing 106 and the wing tip 108, the mean chord line of the fixed wing 106 is no longer in alignment with the mean chord line of the wing tip 108. Figure 2b shows the wing tip 108 in an example load alleviating position.

1001141 During flight, when the wing 104 is in the load alleviating configuration, rotation of the wing tip 108 about the second axis 124 changes the angle of incidence of the wing tip 108 (i.e. the pitch angle of the wing tip in a local frame of reference is changed). The aerodynamic forces on the wing tip 108, such as the lift the wing tip 108 generates, will therefore change. The equilibrium position of the wing tip 108 will -24 -therefore shift and, in an attempt to move towards that equilibrium position, the wing tip 108 rotates about the first axis 114.

[00115] The range of movement of the wing tip 108 about the first axis 114 is limited by stops (not shown). In embodiments, a damper may be provided to damp the rotational movement of the wing tip 108 about the first axis 114.

[00116] In the ground configuration, the wing tip 108 is in a ground position in which the wing tip 108 is oriented vertically upward so as to minimise the wing span of the aircraft 100. When the wing 104 is in the ground configuration, the wing span of the aircraft 100 is sufficient to meet airport compatibility rules and gate limits. Figure 3b shows the wing 104 in the ground configuration.

1001171 The aircraft further comprises a control system 138 arranged to control operation of the wing 104 between the flight configuration, the load alleviating configuration and the ground configuration. The control system 138 is arranged to operate the restraining assembly 136 and configure the restraining assembly 136 between the restraining mode and the releasing mode. The control system 138 is arranged to control actuation of the actuator 134 so as to control the angular position of the wing tip 108 about the second axis 124. Sensors are provided to determine the position of the wing tip 108 about the first axis 114 and the second axis 124. The sensors are coupled to the control unit 136. An angle of attack sensor 140 provided on the nose of the aircraft 100 is arranged to determine the angle of attack and relay the angle of attack to the control system 138.

[00118] The present invention is of particular use when configuring the wing 104 from the load alleviating configuration to the flight configuration when it is necessary to recover the wing tip 108 to the flight position, and when configuring the wing 104 from the flight configuration to the ground configuration when it is necessary to move the wing to the ground position.

1001191 A method of controlling the aircraft wing 104 will now be described, with reference to Figures 6a to 6d, according to a second embodiment of the invention; the method being a method of moving the wing 104 from the load alleviating configuration to the flight configuration.

-25 - [00120] The wing 104 is initially provided in a load alleviating position with the restraining assembly in the releasing mode; the wing 104 having previously been configured into the load alleviating configuration due to a high load event being detected, the high load event having since finished. In this example, the wing tips 108 are folded slightly upwardly in said load alleviating position. The sensors detect the angular position of the wing tip 108 about the first axis 114 and feed the detected positon back to the control unit 138.

1001211 The control unit 138 instructs the actuator 134 to rotate about the second axis 124 so as to pitch the wing tip 108 in a downwards direction and decrease the angle of incidence of the wing tip 108 (Figure 6a). Pitching the wing tip 108 downwards reduces the lift on the wing tip 108 and thereby causes it to rotate about the first axis 114 and move towards the flight position (Figure 6b).

[00122] When the wing tip 108 is detected to have reached a point such that the angular positon of the wing tip 108 about the first axis is the same as the angular position of the wing tip 108 about the first axis 114 in the flight position, the restraining assembly is moved to the restraining mode so as to prevent further rotation about the first axis 114.

1001231 The control system then instructs the actuator 124 to rotate the wing tip 108 about the second axis so as to pitch the wing tip 108 in an upwards direction and increase the angle of incidence of the wing tip 108 (Figure 6c) so as to adopt the flight position and thus place the wing 104 in the flight configuration (Figure 6d).

[00124] A method of landing an aircraft will now be described, with reference to Figures 7a to 7d, according to a third embodiment of the invention; the method comprises a method of controlling the aircraft wing 104 comprising moving the wing 104 from the flight configuration to the ground configuration.

[00125] The wing 104 is initially provided in flight position with the restraining assembly in the restraining mode. The speed of the aircraft 100, which is determined by one or more sensors, is fed to the control unit 138 and the control unit determines whether the speed is below a threshold speed. In embodiments, the threshold speed is the speed at which a touch and go landing is no longer possible.

-26 - [00126] When the speed of the aircraft 100 is below the threshold speed, the restraining assembly 136 is switched to the releasing mode and the wing tip 108 is allowed to rotate about the first axis 114. At the same time, or shortly before or after, the control unit 138 instructs the actuator to rotate the wing tip 108 about the second axis 124 so as to pitch the wing tip 108 in an upward direction and increase the angle of incidence (Figure 7a). Pitching the wing tip 108 upwards increases the lift generated by the wing tip 108 and thereby causes it to rotate about the first axis 114 and move upwards towards the ground position (Figure 7b).

[00127] When the wing tip 108 is detected to have adopted an angular position about the second axis 124 equal to that of the wing tip 108 in the ground position, the wing tip 108 is held in position. In embodiments, the restraining assembly may be put in the restraining mode to hold the wing tip 108 in the ground position.

[00128] The control system then instructs the actuator 124 to rotate the wing tip 108 in the opposite direction about the second axis so as to reduce the aerodynamic loads on the wing tip 108 (Figure 7c) and move the wing tip 108 to the ground position (Figure 7d).

1001291 Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

[00130] In alternative embodiment, the joint between the wing tip and the fixed wing does not comprise a ball joint. Instead, the forward part of the hinge mechanism comprises two parts which each provide one rotational degree of freedom. For example, in embodiments, the forward part of the first hinge also comprises a pin hinge, a first part of the pin hinge being directly mounted to the wing tip, the second part of the pin hinge being pivotally mounted to the fixed wing and thereby defining the second axis.

1001311 In embodiments, the actuator for rotation about the second axis is mounted within the wing tip.

1001321 In embodiments, the wing comprises an actuator arranged to rotate the wing tip about the first axis. Said actuator could have a variety of uses. For example, the -27 -actuator could be a back-up way to move the wing tip from the flight configuration to the load alleviating configuration. The actuator could be used to initiate lowering of the wing tips from the ground configuration to the flight configuration (the primary lowering force being provided by gravity). The actuator could be used to assist in restraining movement of the wing tip about the first axis in the load alleviating configuration; in particular, when the wing tip is at or near the flight position, yet before the restraining assembly is fully moved to the restraining mode. The actuator may be useful in ensuring the wing tip is in the correct angular positon for restraining via the restraining assembly, particularly if the restraining assembly comprises a lock which requires alignment of two parts for engagement of the lock to occur.

1001331 In embodiments, rotation of the wing tip about the second axis can additionally or alternatively be used as a supplementary means for load alleviation, as a means for wake vortex interruption, and as a means for managing the spanwise load distribution to minimise drag.

1001341 Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

-28 -CLAIMSAn aircraft wing, the wing comprising: a fixed wing, and a wing tip mounted at an end of the fixed wing; wherein the wing tip is rotatable relative to the fixed wing about a first axis, the wing is operable between: (i) a flight configuration for use during flight, wherein the position of the wing tip is restrained; and (ii) a load alleviating configuration for load alleviation during flight, wherein the wing tip is allowed to rotate about the first axis, under the action of aerodynamic forces exerted on the wing tip during flight, towards an equilibrium position, such that the load on the wing is reduced; and wherein when the wing is in the load alleviating configuration, at least part of the wing tip is rotatable relative to the fixed wing about a second axis, the second axis being orientated relative to the first axis such that rotation about the second axis results in a change to the angle of incidence of that part of the wing tip thereby urging the wing tip towards a different equilibrium positon in the load alleviating configuration during flight.
2. An aircraft wing according to claim I, wherein the first axis is oriented nonparallel to the longitudinal axis of the aircraft.
3. An aircraft wing according to claim 2, wherein the first axis intersects the wing in a trailing edge region inboard of where the first axis intersects a leading edge region of the wing.
4. An aircraft wing according to any preceding claim, wherein the second axis is oriented perpendicular to the first axis.
-29 - 5. An aircraft wing according to any preceding claim, comprising a restraining assembly operable between a restraining mode in which the wing tip is prevented from rotating about the first axis by a restraining force, and a releasing mode in which the restraining force on the wing tip is released such that the wing is able to adopt the load alleviating configuration.
6. An aircraft wing according to any preceding claim, comprising an actuator arranged control the rotation of the wing tip about the second axis.
7. An aircraft wing according to any preceding claim, wherein in the load alleviating configuration the wing tip is freely rotatable about the first axis.
8. An aircraft wing according to any preceding claim, wherein the wing tip is mounted to the fixed wing via a joint, wherein a forward part of the joint allows rotation of the wing tip about both the first axis and the second axis.
9 An aircraft wing according to claim 8, wherein the joint comprises a ball joint.
10. An aircraft wing according to claim 9, wherein the ball joint is provided proximate the leading edge of the wing.
1 1. An aircraft wing according to any preceding claim, further comprising an actuator arranged to rotate the wing tip about the first axis.
12. An aircraft wing according to any preceding claim, wherein the wing tip is prevented from rotating about the first axis past a vertical orientation and/or past a horizontal orientation.
13. An aircraft wing according to any preceding claim, wherein the wing tip is also operable to: -30 - (iii) a ground configuration for use during ground-based operations, wherein the wing tip is moved away from the flight configuration such that the span of the aircraft wing is reduced.
14. An aircraft comprising an aircraft wing according to any preceding claim.
An aircraft according to claim 14, comprising a control system, the control system being arranged to determine the angular position of the wing tip relative to the fixed wing about the first axis and the second axis, the control system being arranged to instruct the wing tip to rotate relative to the fixed wing about the second axis so as to cause the wing to rotate about the first axis under the resulting aerodynamic forces on the wing tip that arise from the rotation about the second axis.
16. An aircraft according to claim 15, the aircraft being arranged to detect or predict an event which may load the wings above a threshold level, the control system being arranged to configure the wing into the load alleviating configuration upon detection or prediction of the event, and being arranged to configure the wing into the flight configuration, following the event, by rotating the wing tip about the second axis.
17. A control system for an aircraft, the control system being arranged to: determine the angular position of a wing tip relative to a fixed wing about the first axis; instruct the wing tip to rotate relative to the fixed wing about a second axis so as to cause the wing tip to rotate about the first axis under the resulting aerodynamic forces on the wing tip that arise from the rotation about the second axis; and operate a restraining assembly between a restraining mode in which the wing tip is prevented from rotating about the first axis using a restraining force, and a releasing mode in which the restraining force on the wing tip is released.
-31 - 18. A method of controlling an aircraft wing, wherein the wing comprises a wing tip movably mounted at an end of a fixed wing, whereby the wing is moved from: (i) a load alleviating configuration wherein the wing tip is in a load alleviating position and is allowed to rotate about a first axis under the action of aerodynamic forces exerted on the wing tip, to (ii) a flight configuration wherein the wing tip is in a flight position and the position of the wing tip is restrained; the method comprising the steps of: rotating the wing tip in a first direction about a second axis to change the angle of incidence of the wing tip so as cause the wing tip to rotate about the first axis under the resulting aerodynamic forces on the wing tip that arise from the rotation about the second axis and move closer to flight position; restraining the wing tip so as to prevent rotation of the wing tip about the first axis; and rotating the wing tip in a second direction about the second axis so as to adopt the flight position.
19. A method according to claim 18, wherein the first direction is a direction that decreases the angle of incidence of the wing tip.
20. A method of controlling an aircraft wing, wherein the wing comprises a wing tip movably mounted at an end of a fixed wing, whereby the wing is moved from: (i) a flight configuration wherein the wing tip is in a flight position and the position of the wing tip is restrained, to (ii) a ground configuration wherein the wing tip is in a ground position; the method comprising the steps of: allowing the wing tip to rotate about a first axis under the action of aerodynamic forces exerted on the wing tip; and -32 -rotating the wing tip about a second axis to change the angle of incidence of the wing tip in a direction so as to increase the aerodynamic force acting to move the wing tip towards the ground position.
21. A method according to claim 21, wherein the wing tip is rotated about the second axis in a direction that pitches the wing tip upwards of its initial position so as to increase the lift generated by the wing tip.
22. A method of landing an aircraft, the aircraft having a wing comprising a wing tip movably mounted at an end of a fixed wing, whereby the wing is movable from: (i) a flight configuration wherein the wing tip is in a flight position and the position of the wing tip is restrained, to (ii) a ground configuration wherein the wing tip is in a ground position in which the span of the aircraft is reduced; the method comprising the steps of: determining whether the speed of the aircraft is below a threshold speed for initiating movement of the wing to the ground configuration; when the speed has been determined to be below the threshold speed, allowing the wing tip to rotate about the first axis under the action of aerodynamic forces exerted on the wing tip; rotating the wing tip about a second axis to change the pitch angle of the wing tip in a direction so as to increase the aerodynamic force acting to move the wing tip towards the ground position; and holding the wing in the ground configuration.
23. A method of operating a folding wing tip on a wing of an aircraft, the folding wing tip being configured (a) to provide load alleviation during flight by allowing the wing tip to rotate, from a lower position, about a flared hinge axis, thus enabling the loads that would otherwise be sustained by the aircraft wing to be reduced, and (b) to be locked in the lower position in which the wing tip acts as an extension to the wing, -33 -the method comprising recovering the folding wing tip during flight to the lower position, at least in part by twisting the wing tip in the nose-down direction about an axis, different from the flared hinge axis, so that aerodynamic loads on the wing cause the wing tip to fold downwards towards the lower position.
24. A method of operating a folding wing tip on a wing of an aircraft, the folding wing tip being configured (a) to provide load alleviation during flight by allowing the wing tip to rotate, from a lower position, about a flared hinge axis, thus enabling the loads that would otherwise be sustained by the aircraft wing to be reduced, and (b) to be locked in an upper position to reduce the span of the wing as compared to a lower position, the method comprising recovering the folding wing tip to the upper position, at least in part by twisting the wing tip in the nose-up direction about an axis, different from the flared hinge axis, so that aerodynamic loads on the wing cause the wing tip to fold upwards towards the upper position.