GB2584668A

GB2584668A - An aircraft wing with a moveable wing tip

Info

Publication number: GB2584668A
Application number: GB1908245.2A
Authority: GB
Inventors: Wilson Thomas
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2019-06-10
Filing date: 2019-06-10
Publication date: 2020-12-16
Also published as: GB201908245D0

Abstract

An aircraft wing a fixed wing 200, 300 with a wing tip 210, 310 rotatably mounted at the tip thereof, the wing tip comprising a balance mass 501, 601 for reducing aeroelastic flutter. The wing is operable between a flight configuration in which the wingtip forms an extension of the wing and a load alleviating configuration for load alleviation during flight, wherein the wingtip is allowed to rotate about the hinge under the action of aerodynamic forces acting on the wingtip towards an equilibrium position. The balance mas sis located such that the polar moment of inertia of the wing tip, about the hinge axis, is increased relative to the wingtip without the mass. The balance mass may be located on the upper surface of the wingtip and be suspended on a spacer 502, 602. A method of reducing aeroelastic flutter using such a wingtip arrangent is also provided.

Description

AN AIRCRAFT WING WITH A MOVEABLE WING TIP

BACKGROUND OF THE INVENTION

100011 The present disclosure relates to an aircraft wing comprising a moveable wing tip.

[0002] The present invention also concerns an aircraft comprising a wing with a moveable wing tip, a wing tip for use in such an aircraft wing and a method of reducing aeroelastic flutter.

[0003] Aircraft with moveable wing tips are known. The wing tips may be moved from a flight configuration in which the wing tip contributes to the lift generated by the wing, but also contributes to the forces experienced by the wing inwards of the wing tip, to a load-alleviating configuration in which the wing tip does not contribute to the load experienced by the wing inwards of the wing tip. The wing tip is typically moved to a load-alleviating configuration in response to a high load event caused, for example, by a gust of wind or a particular aircraft manoeuvre. Such wing tips, under certain circumstances, may be subject to flutter, which is undesirable. Flight crew may have to undertake remedial action to stop the flutter, such as changing flight speed to one in which flutter is reduced.

100041 W02017118832 and GB2546246 disclose an arrangement in which a wing tip is moveable between such a flight configuration and a load-alleviating configuration. Flutter is mitigated by providing a biasing member, damper or choosing a suitable hinge orientation. Using these approaches to mitigate flutter has been found to have some drawbacks. For example, springs and dampers may limit the ability for the wing tip to freely rotate, and the hinge orientation is determined in a design phase and is therefore difficult to adjust in due course once it has been set.

[0005] The present invention may address the problem mentioned above.

Alternatively or additionally, the present invention seeks to provide an improved aircraft wing with a moveable wing tip.

SUMMARY OF THE INVENTION

[0006] According to a first aspect of the invention, there is provided an 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 hinge axis, the wing is operable between: (i) a flight configuration for use during flight in which the wing tip forms an extension of the fixed wing; and (ii) a load-alleviating configuration for load alleviation during flight, wherein the wing tip is allowed to rotate about the hinge 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, wherein the wing tip is provided with a balance mass for reducing aeroelastic flutter, thereby providing a weighted wing tip, the balance mass being located so such that the polar moment of inertia of the wing tip, about the hinge axis, is increased relative to the wing tip without the mass.

[0007] Polar moment of inertia is known to those skilled in the art, for example, see orgiveikiTheiar moment of inertia. The contribution of the balance mass to the polar moment of inertia of the weighted wing tip is the mass of the balance mass multiplied by the square of the distance from the hinge axis to the centre of gravity of the balance mass. Furthermore, the provision of a balance mass may lower the wing tip flapping frequency relative to the wing bending frequency.

[0008] In the load-alleviating configuration, no bending moment is transferred from the wing tip to the fixed wing. The wing may, for example, be operable into the load-alleviating configuration when the wing experiences a flight condition, such as a high wind or particular aircraft manoeuvre, that is likely to exert a large lifting force on the wing tip which would lead to a correspondingly large bending moment being transferred to the fixed wing if the wing were not in the load-alleviating configuration.

[0009] The balance mass may be located on the inboard portion of the wing. The balance mass may be located so that the centre of gravity of the weighted wing tip is inboard of the centre of gravity of the wing tip without the balance mass.

[0010] The angle of rotation of the wing tip between the load-alleviating configuration and the ground configuration may optionally be at least 10 degrees, optionally at least 20 degrees, and optionally at least 30 degrees. The angle will depend on the aerodynamic forces exerted on the wing tip during flight.

[0011] 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 and lower surfaces of the wing tip may be continuations of the upper and lower surfaces of the fixed wing. 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.

[0012] In the load-alleviating configuration, the wing tip device may be moved relative to the fixed wing such that at least one of the upper and lower surfaces of the wing tip device is moved away from the respective surface of the fixed wing.

[0013] Optionally, the wing may be provided with a restraining assembly for resisting movement of the wing tip from the flight configuration to the load-alleviating configuration.

[0014] The restraining assembly may be operable between a restraining mode in which the wing tip is held in the flight configuration using a restraining force and a releasing mode in which the restraining force on the wing tip is released, such that the wing tip may adopt the load-alleviating configuration. Such a restraining assembly is described in GB2546246, The restraining assembly may incorporate any of the features of the restraining assembly described in GB2546246, the content of which is incorporated herein.

[0015] Optionally, the wing may comprise a biasing member arranged such that when the wing tip is in the flight configuration, the biasing member exerts a biasing force to urge the wing tip towards the load-alleviating configuration. Optionally, when the restraining assembly is in the restraining mode, the biasing force is overcome by the restraining force, but when the restraining assembly is in the releasing mode, the biasing force urges the wing tip towards the load-alleviating mode. Optionally, the biasing member may be selectively disengaged from exerting the biasing force on the wing tip. A clutch may be provided for selectively disengaging the biasing member. Optionally, the aircraft comprises a control system arranged to control operation of the restraining assembly between the restraining and release modes.

100161 Optionally, the wing may be provided with an actuator for moving the wing tip from the load-alleviating configuration to the flight configuration.

[0017] The wing tip may be operable to a ground configuration in which configuration the wing tip is moved away from the flight configuration such that the span of the aircraft wing is reduced. Optionally, the wing comprises an actuator for moving the wing tip to the ground configuration. This actuator may be used to move the wing tip from the load-alleviating configuration to the flight configuration.

[0018] If the wing tip has a length, x, defined along a midpoint of the wing, it is optional that the centre of gravity of the combination of the mass and wing tip may be no more than a distance of 0.4x outboard of the hinge axis when the wing tip is in the flight configuration, optionally no more than 0.3x outboard of said hinge axis, optionally no more than 0.25x outboard of said hinge axis, optionally no more than 0.2x outboard of said hinge axis, optionally no more than 0.15x outboard of said hinge axis, optionally no more than 0: Ix x outboard of the hinge axis and optionally no more than 0.05x outboard of the hinge axis The centre of gravity of the weighted wing tip may be inboard of said hinge axis.

100191 Said balance mass may be located within the wing tip, for example.

However, it is preferred that the balance mass is located above the upper surface of the wing tip or below the lower surface of the wing tip, optionally above the upper surface of the wing tip.

[0020] A spacer may be provided to hold the balance mass in spaced relationship to the wing tip. The spacer helps increase the polar moment of inertia of the weighted wing tip about the hinge axis, the contribution of the mass to the polar inertia being the mass of the balance mass multiplied by the square of the distance from said hinge axis to the balance mass.

[0021] The distance between the centre of gravity of the balance mass and said hinge axis may optionally be at least 0.1x (x being defined as indicated above), optionally at least 0.15x, optionally at least 0.2x, optionally at least 0.25x and optionally at least 0.3x.

[0022] The length of the spacer may optionally be at least 0.05x (x being defined as indicated above), optionally at least 0. I x, optionally at least 0.12x, optionally at least 0.15x, optionally at least 0.18x and optionally at least 0.20x.

100231 The spacer may be elongate and aerodynamic in cross-section along its length, and may optionally be configured to present an aerodynamic form when the wing tip is in the flight configuration.

[0024] The balance mass is optionally aerodynamic in shape and optionally configured to present an aerodynamic form when the wing tip is in the flight configuration. The balance mass may be teardrop-shaped, for example.

[0025] The balance mass may have a density of at least 1Ogcm-3, optionally at least 11gcm-3, optionally at least 13gcm-3, optionally at least 1510gcm-3, optionally at least 15gcm-3, optionally at least 17gcm-3 and optionally at least 19gcm-3.

[0026] The balance mass may comprise one or more of lead, gold, iridium, tungsten, depleted uranium and osmium.

[0027] The balance mass may optionally be elongate. The longitudinal axis of the balance mass may be substantially parallel to the flight direction of an aircraft to which the wing is fitted, and/or substantially parallel to the longitudinal axis of an aircraft to which the wing is fitted.

100281 The hinge axis is preferably oriented non-parallel to the longitudinal axis of the aircraft. The hinge axis is preferably oriented non-parallel to the line of flight direction. The hinge axis is preferably oriented such that the hinge axis intersects the wing in a trailing edge region inboard of where the hinge axis intersects the wing in a leading edge region. In embodiments, the hinge axis may intersect the trailing edge and/or leading edge of the wing (for example when viewed from above).

[0029] The hinge 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 hinge 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.

[0030] The hinge axis is preferably oriented such that the mean angle of incidence of the wing tip changes when the wing tip is rotated about the hinge axis. More preferably, the hinge axis is oriented such that the mean angle of incidence of the wing tip is reduced as the wing tip rotates away from the flight configuration.

100311 The hinge 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 hinge axis may be chosen such that the direction of airflow over the wing in the region of the wing tip remains nonparallel to the first axis during flight. The orientation of the hinge 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.

[0032] The angle between the hinge axis and the longitudinal axis of the aircraft may be referred to as the "flare angle". The flare and may be between 10 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.

[0033] Rotation of the wing tip about the hinge axis may be referred to as folding of the wing tip. The hinge axis may be a fold axis.

[0034] The length of the wing tip as defined along a midpoint of the wing may optionally be at least 10% of the length of the wing, optionally at least 15%, optionally at -7 -least 20%, optionally at least 25%, optionally at least 30%, optionally at least 35% and optionally at least 40% of the length of the wing.

[0035] The length of the wing tip as defined along a midpoint of the wing may be no more than 60% of the length of the wing, optionally no more than 50%, optionally no more than 40% and optionally no more than 30% of the length of the wing.

[0036] The length of the wing tip as defined along a midpoint of the wing may optionally be at least 2.5m, optionally at least 3.0m, optionally at least 4.0m, optionally at least 5.0m, optionally at least 6.0m and optionally at least 7.0m.

[0037] The length of the wing tip as defined along a midpoint of the wing may be no more than I 5m, optionally no more than I Om, optionally no more than 8.0m, optionally no more than 7.0m, optionally no more than 6.0m and optionally no more than 6.0m.

[0038] The length of the wing from root to tip in the flight configuration (in particular, but not, exclusively for a two-engine aircraft) as measured along the midpoint of the wing may be at least I 5m, optionally at least 20m, 25m, optionally at least 30m, optionally at least 35m, optionally at least 40m and optionally at least 45m. It is envisaged that the aircraft wing of the present invention may facilitate the use of longer and optionally larger area movable wing tips 100391 The aircraft wing may be for use in an aircraft optionally having an operating empty weight of at least 20,000kg, optionally at least 25,000kg, optionally at least 30,000kg and optionally at least 40,000kg.

[0040] As mentioned above, the wing may optionally be operable into and out of (iii) a ground configuration for operation of the aircraft on the ground. In the ground configuration, the wing tip is positioned away from the flight configuration such that the span of the aircraft wing is reduced. For example, if the wing tip is operable into the ground configuration, the angle of rotation about the hinge axis of the wing tip between the flight configuration and the ground configuration may optionally be at least 60 degrees, optionally at least 80 degrees, and optionally at least 90 degreesTn the first 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. -8 -

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 vertical. The wing tip may be moved from the flight configuration or the load-alleviating configuration to the ground configuration by rotating the wing about the hinge axis. The position of the wing tip when the wing is in the ground configuration may be referred to as the ground position.

[0041] . Tn the ground configuration, if the wing tip comprises upper and lower surfaces, then at least one of the upper and lower surfaces of the wing tip are optionally moved further away from the respective surface of the fixed wing than when the wing is in a load-alleviating configuration. In the ground configuration, the effective length of the wing is reduced compared to when the wing is in the flight configuration and when in a load-alleviating configuration.

[0042] 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.

100431 According to a second aspect of the present invention there is provided an aircraft comprising at least one wing, and optionally two wings, in accordance with the first aspect of the present invention.

[0044] According to a third aspect of the present invention, there is provided an aircraft wing tip provided with a balance mass for reducing aeroelastic flutter for use the in the aircraft wing of the first aspect of the present invention and/or for use in the aircraft of the second aspect of the present invention.

[0045] According to a fourth aspect of the present invention, there is provided a method of reducing aeroelastic flutter in a wing tip which is attached for rotational motion at the end of a fixed wing, the method comprising providing a balance mass to the wing tip, thereby providing a weighted wing tip, the polar moment of inertia of the weighted wing tip being greater than the polar moment of inertia of the wing tip without the balance mass.

100461 The centre of gravity of the weighted wing tip may optionally be inboard of the centre of gravity of the wing tip without the balance mass.

[0047] The balance mass may comprise those features described above in relation to the aircraft wing of the first aspect of the present invention.

[0048] The method may comprise those features described above in relation to the aircraft wing of the first aspect of the present invention.

[0049] According to a fifth aspect of the present invention, there is provided an aircraft wing comprising a fixed wing with a wing tip rotatably mounted at the tip thereof, the wing tip comprising a balance mass for reducing aeroelastic flutter. The aircraft of the fifth aspect of the present invention may comprise those features described above in relation to the aircraft wing of the first aspect of the present invention.

[0050] In accordance with a sixth aspect of the present invention, there is provided an aircraft comprising a wing in accordance with the fifth aspect of the present invention.

[0051] It will of, course, be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa.

DESCRIPTION OF THE DRAWINGS

100521 Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 100531 Figure 1 shows a front-on view of an example of an embodiment of an aircraft; 100541 Figure 2 shows a plan view of the aircraft of Fie, 1 when the wing tip is in the flight configuration; [0055] Figure 3 shows a close-up schematic view of the wing tip of the aircraft of Fig. 1; and [0056] Figure 4 shows an example of a method of reducing aeroelastic flutter in a wing tip according to another embodiment of the invention.

-10 -

DETAILED DESCRIPTION

[0057] An example of an aircraft wing and an aircraft in accordance with an embodiment of the present invention will now be described by way of example only with reference to Figs. 1 to 3. The aircraft is denoted generally by reference numeral 100 and comprises two aircraft wings 200, 300. Each wing 200, 300 comprises a fixed wing 201, 301 and a wing tip 202, 302 mounted at an end of the fixed wing 201, 301. Each wing tip 202, 302 is rotatable relative to the respective fixed wing 201, 301 about a hinge axis 205, 305. Referring to Figure 1, each wing 200, 300 is operable between (i) a flight configuration (FC) for use during flight in which the wing tip 202, 302 forms an extension of the fixed wing 201, 301 and (ii) a load-alleviating configuration (LAC) for load alleviation during flight, wherein the wing tip 202, 302 is allowed to rotate about the hinge axis 205, 305, under the action of aerodynamic forces exerted on the wing tip 202, 302 during flight, towards an equilibrium position, such that the load on the wing 200, 300 is reduced, wherein the wing tip 202, 302 is provided with a balance mass 501, 601 for reducing aeroelastic flutter, thereby providing a weighted wing tip, the balance mass being located so such that the polar moment of inertia of the wing tip, about the hinge axis 205, 305, is increased relative to the wing tip 202, 302 without the mass.

[0058] Each balance mass 501, 601 is located above a respective wing tip 202, 302 and is made from tungsten which has a density of 19.3gcm-3. Each balance mass 501, 601 is teardrop shaped and is elongate, as is best seen in Fig. 2. The longitudinal axis of each balance mass 501, 601 is parallel to the longitudinal axis of the aircraft 100. Such an arrangement provides an aerodynamic balance mass 50 I, 601. Each balance mass 501, 601 is attached to the respective wing tip 202, 302 with an elongate spacer 502, 602. Each spacer 502, 602 is teardrop shaped in cross-section, such as shape being relatively aerodynamic. As mentioned above, the balance mass 501, 601 increases the polar moment of inertia of the wing tip 202, 302. The use of a relatively long spacer gives rise to a relatively high polar moment of inertia contribution from the mass balance 501, 602 because the contribution of the balance mass 501, 601 to the polar moment of inertia of the weighted wing tip is the mass of the balance mass 501, 601 multiplied by the square of the distance from the hinge axis 205, 305 to the centre of gravity of the balance mass 501, 601. Increase in the polar moment of inertia is desirable in order to lower the wing tip flapping frequency with respect to the wing bending frequency, thereby reducing aerodynamic flutter, especially when the wing tip is in the load-alleviating condition.

[0059] Those skilled in the art will be familiar with aeroelastic flutter, for example, see littps:/: er ikipedia.omiwiki./Aeroelasticitv4Pluiter.

[0060] Each balance mass 501, 601 is attached to the respective wing tip 502, 602, but is located close to the hinge axis 205, 305. Locating the balance mass 501, 601 close to the hinge axis 205, 305 limits wing bending loads if the wing is in the flight configuration. Locating the balance mass 501, 601 towards the end of the wing tip 501, 601 would give increased wing bending loads.

[0061] As indicated above, each wing 200, 300 is operable between a flight configuration FC and a load-alleviating configuration LAC. In the flight configuration FC, each wing tip 202, 302 is a continuation of the fixed wing 201, 301, and each wing tip 202, 302 is effectively rigidly coupled to the respective fixed wing 201, 301 so that any lift force generated by the wing tip 202, 302 during flight exerts a bending moment on the wing. During certain flight-related events (e.g. gust of wind or when the aircraft undertakes certain manoeuvres) the lift generated by the wing tip 202, 302 and the associated bending moment may become undesirably large, and under such circumstances the wing 200, 300 is operable to the load-alleviating condition (LAC) in which the wing tip is allowed to rotate about the hinge 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. Operation into the load-alleviating configuration LAC is discussed below.

[0062] Each wing 200, 300 is provided with a restraining assembly 350 that is operable to allow the wing tip 201, 301 to rotate about the hinge axis 205, 305. The restraining assembly is described in detail in W02017118832 and GB2546246, but will be described here for convenience.

[0063] Referring to Fig. 3, the aircraft 100 is provided with a restraining assembly 350. The restraining assembly 350 comprises a brake 351, a clutch 352, a rotational -12 -spring 353 and a rotational damper 354, and will now be described in more detail. Note that balance mass 601 has been omitted from Fig. 3.

[0064] The brake 351 comprises two pads configured to selectively clamp against a shaft 342 to restrain its rotation. The restraining assembly 350 is operable between a restraining mode (in which the brake 351 is deployed to brake the rotation of the shaft 342), and a releasing mode (in which the brake 351 is released by pulling the pads away from the shaft 342 to allow its free rotation (and thus the rotation of the wing tip 302)).

[0065] The default (passive) mode of the restraining assembly 350 is the restraining mode in which the shaft 342 is braked. When the wing tip 302 is in the flight configuration, the power to the restraining assembly 350 is switched OFF (i.e. the assembly is passive) and the restraining assembly 350 is left with the shaft 342 braked. Such an arrangement is attractive as it ensures an active command (e.g. an ON signal) is required to move the wing tip device) [0066] The restraining assembly 350, including the brake 351, is controlled by a control module 343 of the Electronic Flight Control System (EFCS). The control module 343 is shown as a box in the schematic of Figure 3.

[0067] The module 343 is configured to receive a measurement of the local angle of attack from an alpha vane (not shown) on the nose of the aircraft 100. During cruise flight, the restraining assembly 350 is OFF and the brake 351 is braked onto the shaft 342. However, when the measurement from the alpha detector indicates an oncoming gust (i.e. a significant change in angle of attack) the control module 343 switches the restraining assembly ON, which releases the brake 351.

[0068] Such an arrangement enables the wing tip 302 to be securely held in the flight configuration during normal cruise flight, but by switching the releasing assembly ON to release the brake 351, the wing tip 302 is movable quickly to the load alleviating configuration. This means the wing can avoid being subjected to high gust loadings. This in turn may enable the wing 300 to have a relatively large span, without necessarily having to incur the associated weight penalty, because it can be designed for a lower magnitude of maximum load.

-13 - 100691 The wing tip 302 may, at least partially, be moveable to the load-alleviating configuration purely under the action of aerodynamic force acting on it during flight, or under gust loads. The restraining assembly 350 comprises a rotational spring 353 and damper 354 arrangement to assist this movement. The rotational spring 353 and damper 354 are located at one end of the hinge 305. The rotational spring 353 is preloaded such that when the wing tip 302 is in the flight configuration FC, it exerts a biasing force that urges the wing tip 302 towards the load-alleviating configuration LAC. That biasing force is unable to overcome the restraining force exerted by the brake 351 when it is deployed. However, when the brake 351 is released, the biasing force acts to rotate the wing tip 302 about the hinge 305. The rotational spring 353 is sized such that it rotates the wing tip 302 by around 30 degrees of rotation, but once the wing tip 302 has rotated that far, the spring 353 is fully unwound and does not urge any further rotation. Providing a pre-loaded spring 353 in this manner has been found to be beneficial as it quickly moves the wing tip device 302 to the load alleviated configuration LAC, as soon as the brake 351 has been released.

100701 The damper 354 is configured to damp movement of the wing tip 302 as it rotates under the action of the spring 353 (and any aerodynamic forces). Such an arrangement has been found to be beneficial, especially when the wing tip 302 is quickly moved to the load alleviating configuration LAC, as it tends to damp down transient, oscillatory, movements.

[0071] The restraining assembly also comprises a clutch 352 located on the hinge 305. The clutch 352 serves to selectively engage/disengage opposing ends of the hinge, such that the spring 353 can be selectively chosen to exert the biasing force on the wing tip 302. Such an arrangement has been found to be beneficial because, in the event of failure of the restraining assembly 350, it may enable unwanted movement of the wing tip 302, by the restraining assembly 350 to be prevented. It may also enable the spring 353 to be selectively disengaged to enable easier maintenance of the wing tip 302.

[0072] The aircraft 100 also comprises a motor 341. When the wing 300 is in the flight configuration FC, the motor 15 is in a passive state such that it does not actively contribute to restraining the wing tip 302 in the flight configuration (except for resistance -14 -as a result of rotational inertia). When the wing tip 302 has been moved to the load alleviating configuration LAC, the motor 341 may, however, be activated such that it rotates the wing tip 302 back to the flight configuration FC and re-compresses the spring 353. Once in that position, the restraining assembly 350 is switched back into restraining mode such that the brake 351 is applied, and the motor 341 is again returned to its passive state. Thus the motor 341 is used to move the wing tip 302 between the flight and load alleviating configuration.

[0073] The wing 300 is also operable to a ground configuration GC in which the wing span of the aircraft 100 is reduced so that the aircraft can comply with airport gate limits. The motor 341 is arranged to rotate the wing tip 302 between the flight configuration FC and the ground configuration (see Figure 2d) by actuation of the motor 341.

[0074] It is worth noting the orientation of the hinge axis 205, 305. Each of said axes 205, 305 is at an angle of about 17 degrees to the longitudinal axis of the aircraft and the flight direction of the aircraft, denoted by "F." in Fig. 2. The flight direction corresponds to the longitudinal axis of the aircraft 100. This so-called "flaring" of the hinge axis away from the flight direction F means that the mean angle of incidence of the wing tip changes when the wing tip device is rotated about the hinge axis. The hinge axis is oriented such that the mean angle of incidence of the wing tip is reduced as the wing tip device rotates away from the flight configuration.

[0075] 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.

[0076] The example of the aircraft described above in relation to Figs. 1 to 3 describes a wing tip that is movable to a load-alleviating configuration and to a ground configuration. Those skilled in the art will realise that the aircraft may have a wing tip that is movable to only the load-alleviating configuration.

-15 - 100771 The example of the aircraft described above shows the balance mass positioned above the upper surface of the wing tip. Those skilled in the art will realise that the balance mass may be positioned below the lower surface of the wing tip.

[0078] 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. Tt 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

-16 -CLAIMSI An 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 hinge axis, the wing is operable between: (i) a flight configuration for use during flight in which the wing tip forms an extension of the fixed wing; and (ii) a load-alleviating configuration for load alleviation during flight, wherein the wing tip is allowed to rotate about the hinge 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, wherein the wing tip is provided with a balance mass for reducing aeroelastic flutter, thereby providing a weighted wing tip, the balance mass being located so such that the polar moment of inertia of the wing tip, about the hinge axis, is increased relative to the wing tip without the mass.
2. An aircraft wing according to claim 1 wherein the balance mass is located on the inboard portion of the wing.
3. An aircraft wing according to claim 1 or claim 2 wherein the balance mass is located so that the centre of gravity of the weighted wing tip is inboard of the centre of gravity of the wing tip without the balance mass.
4. An aircraft wing according to any preceding claim wherein the wing is provided with a restraining assembly for resisting movement of the wing tip from the flight configuration to the load-alleviating configuration.
5. An aircraft wing according to claim 4 wherein the restraining assembly is operable between a restraining mode in which the wing tip is held in the flight configuration using a restraining force and a releasing mode in which the restraining force on the wing tip is released, such that the wing tip may adopt the load-alleviating configuration.
6 An aircraft wing according to any preceding claim wherein the balance mass is located within the wing tip.
7 An aircraft wing according to any of claims I to 5 wherein the balance mass is located above the upper surface of the wing tip or below the lower surface of the wing tip, preferably above the upper surface of the wing tip.
8 An aircraft wing according to any of claims I to 5 and claim 7 comprising a spacer to hold the balance mass in spaced relationship to the wing tip.
9 An aircraft wing according to claim 8 wherein the spacer is elongate and aerodynamic in cross-section along its length.
10. An aircraft wing according to any of claims I to 5 and 7 to 9 wherein the balance mass is aerodynamic in shape.
II. An aircraft wing according to any of claims 1 to 5 and 7 to 10 wherein the balance mass is elongate and the longitudinal axis of the balance mass is substantially parallel to the longitudinal axis of the aircraft.
12. An aircraft wing according to any preceding claim wherein the balance mass has a density of at least llgcm-3.
13. An aircraft wing according to any preceding claim wherein the balance mass comprises one or more of lead, gold, iridium, tungsten, depleted uranium and osmium.
14. An aircraft wing according to any preceding claim wherein the hinge axis is oriented non-parallel to the longitudinal axis of the aircraft.
15. An aircraft wing according to any preceding claim operable into and out of (iii) a ground configuration for operation of the aircraft on the ground, in which ground configuration, the wing tip is positioned away from the flight configuration such that the span of the aircraft wing is reduced.
16. An aircraft comprising at least one wing, and optionally two wings, in accordance with any preceding claim.-18 -
17. An aircraft wing tip provided with a balance mass for reducing aeroelastic flutter for use the in the aircraft wing of any of claims 1 to 15
18. A method of reducing aeroelastic flutter in a wing tip which is attached for rotational motion at the end of a fixed wing, the method comprising providing a balance mass to the wing tip, thereby providing a weighted wing tip, the polar moment of inertia of the weighted wing tip being greater than the polar moment of inertia of the wing tip without the balance mass.
19. An aircraft wing comprising a fixed wing with a wing tip rotatably mounted at the tip thereof, the wing tip comprising a balance mass for reducing aeroelastic flutter.