GB2587429A

GB2587429A - Wingtip device for an aircraft

Info

Publication number: GB2587429A
Application number: GB1914098.7A
Authority: GB
Inventors: cosentino Enzo; Llewellyn-Jones Catherine
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2019-09-30
Filing date: 2019-09-30
Publication date: 2021-03-31
Also published as: GB201914098D0

Abstract

An aircraft wing tip device 101 comprising a main aerofoil 110 having a first portion 111 with local dihedral angle increasing monotonically from less than +20 degrees at inboard location to more than +50 degrees in outboard direction. A second portion 113 outboard of the first potion has a local dihedral angle decreasing monotonically from more than +50 degrees at inboard location to less +20 degrees in outboard direction. The device may have a planar transition intermediate portion 112 and substantially horizontal tip. The curved wing tip device may be an S-shaped swanlet, resembling skewed or stretched apart letter S or sigmoidal shape. A strut or supporting structure 120 (e.g. secondary aerofoil structure) may brace the wingtip under or beneath the main aerofoil portion, forming a closed loop wingtip defining an airfow channel 140. The brace may comprise an actuator (e.g. cable systems) for twisting, elastically deforming, altering, flexing or morphing the wingtip main aerofoil shape between first and second geometric configurations. The first wingtip configuration may have higher fuel efficiency during take-off. The second wingtip configuration may be optimised for cruise fuel efficiency.

Description

WINGTIP DEVICE FOR AN AIRCRAFT BACKGROUND OF THE INVENTION

[0001] The present disclosure relates to a wing tip device for an aircraft. The invention also concerns an aircraft wing comprising a wing tip device, and a method of operating an aircraft including a wing tip device.

[0002] The efficiency of a fixed wing aircraft can be improved by the addition of a wingtip device. Such devices typically improve aerodynamic performance by means of reducing induced drag that would otherwise be caused by vortices downstream of the wingtip of the aircraft wing. One such wingtip device is used on the Airbus A320 Neo, the device commonly being referred to as a "sharklet". The extra mass that is added by such a sharklet can exceed 100 kg, yet the addition of a sharklet can still provide significant fuel efficiency advantages despite such extra mass. Much of the mass of a sharklet is at the outboard end of the wing tip device, due to the vertical planar region at the outboard end. Much of the total mass of the sharklet is dictated by the need for sufficient structural strength over the whole device.

[0003] It has previously been proposed that a spiroid shaped wingtip device might provide improved performance. An early proposal concerning a spiroid tipped wing" was the subject of US 5,102,068.

[0004] U52017/0029094 discusses an aircraft wing having a strut-braced wing tip device. The wing tip device is rotatable about a pivot, by the stmt moving laterally on the wing tip surface.

[0005] US2017/0073062 purports to disclose a variable geometry wingtip having an upper aerofoil surface and a lower aerofoil surface which, according to the disclosure, change shape according to the flight conditions of the aircraft. For example it is suggested that a ram air effect between the two aerofoil surfaces acts to inflate the structure, leading to increased stability. There are no moving parts in the upper aerofoil structure. The inventors of the present application have doubts over the feasibility of constructing a wingtip device, in accordance with US '062, for use on a commercial passenger aircraft [0006] W02008155566A1 discusses a winglet having a long planar portion immediately outboard of the inboard end of the wing tip device. Outboard of the long -2 -planar portion is a curved portion which curves through a 90 degree curve and which terminates in a vertical planar portion. At the tip of the planar portion is a further curved section which curves back through at least 90 degrees to a position perpendicular or at a decline to the vertical planar portion.

[0007] It is generally desired to improve the fuel efficiency of an aircraft. This may typically be achieved by a combination of improving aerodynamic efficiency and/or reducing mass. Thus, any devices added to the aircraft in order to improve aerodynamic efficiency should ideally do so in a way that either does not increase mass or provides an overall benefit despite an overall increase in mass. It may be desirable to obtain a preferential distribution of mass in the wing tip device, and to bear in mind the effect of extra outboard mass on the loads sustained at the root of the wing. Consideration may need to be given to distributing mass of the wingtip so that its centre of gravity is located as far inboard as is feasible, bearing in mind other factors, such as aerodynamic benefits. One added complication is that optimising fuel efficiency for take-off conditions of an aircraft might result in a solution that is not optimised for the aircraft when flying at cruising altitude, and vice versa.

[0008] 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 wingtip device for an aircraft. Alternatively or additionally, the present invention seeks to provide a way in which the fuel efficiency of an aircraft might be improved.

SUMMARY OF THE INVENTION

[0009] The present invention provides, according to a first aspect, an aircraft wing comprising: a main body having an outboard end, a wing tip device extending from the outboard end of the main body of wing, the wing tip device comprising a main aerofoil.

The main aerofoil has a first portion extending across a significant portion of the total length of the wing tip device (for example at least 25% of the total length of the wing tip device and optionally 30% or more). In the context of the present invention, the length of the main aerofoil is measured in the outboard direction along the uppermost surface at the median chord line of the main aerofoil (i.e. such that the length of the main aerofoil will be greater than the straight line separation of the root from the tip). -3 -

The first portion is shaped such that the angle of the local dihedral varies, with increasing distance in the outboard direction, from a value of less than +40 degrees, optionally less than +30 degrees (and possibly less than +20 degrees) at an inboard location to a value that is at least 25 degrees greater, possibly at least 30 degrees greater, at an outboard location. For example, the first portion may be shaped such that the angle of the local dihedral reaches a value of greater than +45 degrees, possibly greater than +50 degrees at an outboard location. Preferably, the local dihedral varies gradually and/or monotonically for substantially the entire extent of the first portion. The main aerofoil also has a second portion outboard of the first portion. The second portion also extends across a significant portion of the total length of the wing tip device (for example extending across at least 25% of the total length of the wing tip device, and optionally 30% or more). The second portion is shaped such that the angle of the local dihedral varies, with increasing distance in the outboard direction, from a value at an inboard location that is the same or higher than the maximum dihedral of the first b portion (e.g. an angle of greater than +45 degrees, possibly greater than +50 degrees) to a value that is lower by at least 25 degrees, possibly to a value that is at least 30 degrees lower. For example, the second portion may be so shaped that the angle of the local dihedral drops to a value of less than +40 degrees, optionally less than +30 degrees (and possibly less than +20 degrees) at an outboard location. Preferably, the local dihedral varies gradually and/or monotonically for substantially the entire extent of the second portion. The main aerofoil may have a maximum angle of the local dihedral of less than 90 degrees, possibly less than 85 degrees, for example 75 degrees or less. It will be understood that the dihedral angle is the angle as measured when the aircraft is on the ground at rest with the wing tip device adopting the first geometrical configuration. If the angle of inclination of the wing surface varies according to the chordwise position at which it is measured and/or on which of the upper and lower surfaces of the wing tip device is used to define the angle, then the dihedral angle may be taken as the angle when measured with reference to the surface midway between the upper and lower surfaces of the body concerned and as averaged across the chordwise direction.

[0010] The main aerofoil may follow a sigmoidal profile when viewed from the front and/or rear (i.e, when viewed in a direction which is parallel to the line of flight). The main aerofoil following a sigmoidal profile may have advantages over prior art wing -4 -tip devices through induced drag reduction, weight reduction and/or preferential mass distribution. A wing tip device with a main aerofoil having a sigmoidal profile may advantageously have a greater proportion of the mass of the winglet inboard in comparison to a "sharklet" wing tip device. A wing tip device with a main aerofoil having a sigmoidal profile may have advantages structurally over a traditional sharklet.

It may be that a wing tip having a sigmoidal profile may be mounted to a wing rib inboard of the wing rib to which a sharklet would be mounted. A wing tip device with a main aerofoil having a sigmoidal profile may be easier to manufacture.

[0011] The term "sigmoid-, or "sigmoidal" is used throughout this description to refer to the overall shape of the wing tip device. It will be appreciated that the shape essentially follows a characteristic S"-shape. It will thus be understood that the shape need not follow exactly a shape that would be deemed as being mathematically identical to a curve defined by a sigmoid function. A sigmoid has two curved sections separated by a transition section. The gradient of the sigmoid line may always be non-negative (i.e. zero or positive). The wing tip device on the port side of the aircraft may have a sigmoidal profile when viewed from the front. The wing tip device on the starboard side of the aircraft may have a sigmoidal profile when viewed from the rear. The wing tip device may be referred to as a "swanl et" owing to its "S"-shaped profile.

[0012] The wing tip device may have a shape when viewed in a line-of-flight direction that resembles the letter "S" with a horizontal skew which increases the distance between the start and end of the letter (e.g. a shape that looks like the letter "S" having been stretched apart by its free ends). The wing tip device is preferably sufficiently structurally rigid that it retains the same overall shape (i.e. having an S-shaped profile) in all flight conditions, yet sufficiently flexible to morph and/or flex when subjected to typically operational loads / or displacements.

[0013] The wing tip device may have a root portion, a first curved portion, an intermediate portion (comprising a transition portion), a second curved portion and a tip. The root portion may be inboard of (and preferably directly adjacent to) the first curved portion, which in turn may be inboard of (and preferably directly adjacent to) the transition portion, which in turn may be inboard of (and preferably directly adjacent to) the second curved portion, which in turn may be inboard of (and preferably directly adjacent to) the tip. These portions (and the tip) may be portions of the main aerofoil, preferably making up substantially the entire main aerofoil. The first curved portion -5 -curves in the opposite direction to the curve of the second curved portion. In comparison to a traditional "sharklet", in an embodiment of the present invention, the lowest radius of curvature of the first curved portion of the wing tip device is significantly greater than the lowest radius of curvature of the curved section in the sharklet. The lowest radius of curvature of the first curved portion may be greater than 0.75m, optionally greater than 1m, and possibly greater than 1.5m.

[0014] It may be the case that at all points on the first curved portion of the wing tip device (of embodiments of the present invention), the angle of the local dihedral is less than +75 degrees. A sharklet (of the prior art) has a first planar portion, a curved portion and a second planar portion. The first curved section of the wing tip device of the present invention may start at the end of the root portion of the wing tip device. The root portion may be less than 10% of the length of the wing tip device. The root portion may be less than 5% of the length of the wing tip device. The first curved portion may be defined as the portion of the main aerofoil between the location at which where the dihedral increases from the dihedral of the outboard end of the fixed wing and the location at which the dihedral stops increasing in the outboard direction. The transition portion may begin where the first curved portion reaches the maximum angle of local dihedral of the main aerofoil. The second curved portion may curve in an opposite way to the first curved portion. The maximum angle of the local dihedral in the second portion may be less than +75 degrees (the maximum being at an inboard location). The minimum angle of the local dihedral in the second portion may be more than 0 degrees (the minimum angle being at an outboard location). The second curved portion may terminate at a dihedral of less than +10 degrees, or less than +5 degrees. The second curved portion may end at an angle relative to the horizontal axis of about 0 degrees, for example more than -5 degrees, optionally more than +1 degrees.

[0015] It may be that the first portion and the second portion of the aircraft wing are directly adjacent to each other, the] unction between them including a point of inflection for example. Alternatively, it may be that the first portion and the second portion of the aircraft wing are joined by an intermediate portion extending across at least 10% of the total length of the main aerofoil. It may be that the intermediate portion includes an essentially planar portion (i.e. having a constant dihedral within +/-1 degrees) extending across at least 10% of the total length of the main aerofoil. It may be that the intermediate portion includes both an inboard region in which the local dihedral -6 -increases in the outboard direction and an outboard region in which the local dihedral decreases in the outboard direction. It may be that the essentially planar portion extends across no more than 20% of the total length of the main aerofoil. The intermediate portion may have a length between 10% and 30% of the total length of the main aerofoil.

It may be that the first portion, second portion and intermediate portion together extend along at least 90%, possibly at least 95%, of the length of the main aerofoil and optionally along substantially the entire length of the main aerofoil. The intermediate portion may have features of the above described transition portion and/or vice versa. [0016] It may be that the main aerofoil terminates at a substantially horizontal tip. It may be that the outboard end of the main aerofoil is a substantially horizontal tip.

[0017] Preferably, the local dihedral varies gradually for substantially the entire extent of the main aerofoil of the wing tip device. For example, it may be that the magnitude of the rate of change of the dihedral angle with increasing distance in the outboard direction is such that the maximum variation in angle over any portion of the main aerofoil extending along 10% of the length of the main aerofoil (as measured in the outboard direction along the uppermost surface of the main aerofoil) is less than 40 degrees, optionally less than 30 degrees, and possibly less than 25 degrees. It may be that there is no portion of the main aerofoil for which the local dihedral angle changes by more than 15 degrees over a distance of 5% of the total length of the main aerofoil.

It may be that there is no portion of the main aerofoil for which the local dihedral angle changes by more than 60 degrees (possibly more than 50 degrees) over a distance of 20% of the total length of the main aerofoil.

[0018] It may be that the wing comprises a blended region blending between the first portion of the wing tip device and the outboard end of the main body of wing. It may be that the wing comprises a blended region blending between the first portion of the main aerofoil and the outboard end of the main body of wing. The chordwise cross-section of the main aerofoil at an inboard end of the wing tip device may be substantially identical to the chordwise cross-section of the outboard end of the fixed wing. The chordwise cross-section of the wing tip device at an inboard end of the wing tip device may be substantially identical to the chordwise cross-section of the outboard end of the fixed wing.

[0019[ The main aerofoil of the device may have a shape that includes a twist (for example so that there is a change in angle between two spaced apart chord lines). -7 -

[0020] The height (vertical dimension) of the wing tip device may 2.5 meters or less. The height of the wing tip device may be greater than 0.75 meters. The height of the wing tip device may be between 0.75 meters and 2 meters. The length of the wing tip device may be up to 4 meters. The straight-line distance between the root portion and the tip may be between 2.5 meters and 4 meters.

[0021] It may be that the height of the main aerofoil never decreases in the outboard direction. Advantageously, this may reduce or prevent detachment of airflow from the aerofoil surface and have consequential benefits in preventing stalling and/or reducing induced drag.

[0022] The inboard face of the wing tip device may connect to an outboard face of a fixed wing of the aircraft. The chordwise cross-section of the main aerofoil at an inboard end of the wing tip device may be substantially identical to the chordwise cross-section of the outboard end of the fixed wing.

[0023] The main body of the wing may be understood to be the "fixed wing".

[0024] The wing tip device may comprise a main aerofoil and a supporting structure, for example the main aerofoil being separately discernible from the supporting structure. The wing tip device may be braced by the supporting structure. The supporting structure may extend from the first portion or a position inboard of the first portion to the second portion or a position outboard of the second portion. The supporting structure may extend below the wing tip device. The supporting structure may extend below the main aerofoil. The supporting structure may be so shaped that its chordwise dimension at any position along at least 90% of its length is less than the chordwise dimension of the main aerofoil at the same position in the spanwi se direction. The supporting structure may be so shaped that for at least 90% of its length the angle of the local dihedral varies monotonically from a value of greater less than +20 degrees at an inboard location to a value of greater than +70 degrees at an outboard location. The supporting structure may attach at a point which is in the same plane as the wing. The supporting structure may extend in the outboard direction from a region on the underside of the main aerofoil which is substantially coplanar with the main body of the wing. The supporting structure may assist in strengthening the main aerofoil in a way that enables the main aerofoil to be lighter. The supporting structure may also assist in reducing flexure of the main aerofoil during use, in a way that reduces -8 -delamination or other damage of the wing tip device in the region of the junction with the main wing for example.

[0025] It may be that the supporting structure is so shaped that with increasing distance in the outboard direction the maximum change in angle over any portion of the supporting structure extending across 10% of its length is less than 30 degrees, possibly less than 25 degrees.

[0026] The wing tip device may further comprise a structural support. The supporting structure may comprise the structural support. The structural support may comprise a strut, for example a bracing member.

[0027] It may be that less than 25% of the length of the trailing edge of the structural support extends rearwards of the trailing edge of the main aerofoil. It may be that less than 10% of the length of the trailing edge of the structural support extends rearwards of the trailing edge of the main aerofoil. It may be that there are no spanwise positions, at which the trailing edge of the structural support extends rearwards of the trailing edge of the main aerofoil.

[0028] The main aerofoil may entirely overlay the supporting structure when viewed along the centreline of the wing. The supporting structure may occupy a volume of space which is less than half the volume of space occupied by the main aerofoil.

[0029] The supporting structure may be aerodynamically shaped. The supporting structure may be a secondary aerofoil structure. The main aerofoil and the supporting structure may together form a closed surface wing tip device (e.g. a closed loop wing tip device).

[0030] The structural support may extend between an inboard end and an outboard end of the main aerofoil on the underside of the wing tip device. In the case where the main aerofoil has a sigmoidal profile, the structural support may extend from the base of the sigmoid to the top of the sigmoid. The structural support may attach to the main aerofoil at a point which is in the same plane as the wing. The structural support may attach at the tip of the outboard end of the main aerofoil. The structural support may attach at a point inboard of the tip of main aerofoil. At the outboard end of the wing tip device the structural support may be attached normal (i.e. at or close to perpendicular) to the main aerofoil. At the inboard end of the wing tip device the structural support may be attached at an acute angle to the main aerofoil. The structural support may be attached at its inboard end to the underside of the first curved portion of main aerofoil. -9 -

The structural support may be attached at its outboard end to the underside of the tip of the main aerofoil. The structural support may have a single curved portion which curves in one direction only (i.e. monotonically varying gradient) and which accounts for at least 90% of the length of the structural support. The curved portion of the structural support may be shaped such that the separation from the main aerofoil increases to a maximum and then decreases. The structural support may follow a substantially constant curve. The dihedral of the structural support may increase in the outboard direction. The structural support and the main aerofoil may define an airflow hole and/or airflow channel. The curvature of the structural support may provide for a relatively large airflow hole / channel without extending outboard of the main aerofoil.

The structural support may extending outboard of the main aerofoil, of at all, by less than 10% of the length of the wing tip device. The curvature of the structural support may be shaped so as to assist with the support given to the main aerofoil by the structural support.

[0031] There may be a junction between the main aerofoil and the supporting structure. The main aerofoil and the supporting structure may together, possibly with other parts, form a closed loop, for example corresponding to the closed surface of a closed surface wing tip device. In some embodiments, the main aerofoil and the supporting structure may be joined by a blended region, there being no easily discernible single junction between the main aerofoil and the supporting structure. The main aerofoil and the supporting stnicture may be connected at two separate spaced apart regions, such that together they form the closed surface. It may be that a joint is formed between the main aerofoil and the supporting structure of the wing tip device. Such a joint may be a fixed joint. Such a joint may be a pivoting joint. The wing tip device may include an overhang portion, for example at the tip of the wing tip device.

Thus, in some embodiments, one or both of the main aerofoil and the supporting structure may form a part of the wing tip device, which is not also a part of a closed surface. The wing tip device may be rigidly attached to the main wing, for example via one or more bolts or other fixings, and thus -for example -not being pivotally mounted to the main wing.

[0032] The main aerofoil of the device may have a maximum dimension in the chordwise direction that is between 120% and 500% of the maximum dimension in the chordwise direction of the supporting structure. The supporting structure may be -1 0 -significantly less massive than the main aerofoil. For example, the supporting structure may have a mass that is less than half of the mass of the main aerofoil, optionally less than less than one third of the mass of the main aerofoil, possibly less than 20% of the mass of the main aerofoil. It may be that the mass is greater than 10?,'O of the mass of the main aerofoil.

[0033] The structural support and the main aerofoil have leading and trailing edges. It may be that the foremost point of the leading edge of the structural support does not extend forwards of the foremost point of the leading edge of the main aerofoil. It may be that the aftmost point of the trailing edge of the structural support does not extend rearwards of the aftmost point of the trailing edge of the main aerofoil.

[0034] The structural support may be a lower structure and the main aerofoil may be an upper structure. It may be that the foremost point of the leading edge of the lower structure does not extend forwards of the foremost point of the upper structure of the wing tip device. It may be that the aftmost point of the trailing edge of the lower structure does not extend rearwards of the aftmost point of the upper structure of the wing tip device.

[0035] It may be that the main aerofoil substantially overlays the structural support when viewed from above. It may be that the main aerofoil entirely overlays the structural support when viewed in the outboard direction along the centreline of the 20 wing.

[0036] The structural support may be a secondary aerofoil structure. It may be that the structural support is aerodynamically shaped to reduce / minimise drag. It may be that the structural support is aerodynamically shaped to provide lift.

[0037] The wing tip device may include an element within the wing tip device which is arranged to deform the shape of the wing tip device, for example when the aircraft is stationary and on the ground. There may be more than one such element within the wing tip device. The one or more elements may be configured to deform the shape from a first geometrical configuration to a second geometrical configuration, for example such that the second geometrical configuration has different aerodynamic properties from the first geometrical configuration.

[0038] The supporting structure (or the structural support) may comprise an actuator arranged to alter the shape of the main aerofoil.

-H -

[0039] Embodiments of the present invention, for example an aircraft featuring such wing tip devices, may enable the aerodynamic performance of the aircraft to be adjusted to suit a typical set of flight conditions. On a given occasion, the wing tip device may for example be set to optimise fuel efficiency for a short haul flight where fuel consumption during take-off has a greater effect on overall fuel efficiency than fuel consumption during steady state cruise mode. On a different occasion, the wing tip device may for example be set to optimise fuel efficiency for a longer haul flight where fuel consumption during steady state cruise mode has a greater impact.

[0040] It may be that at least one point on the wing tip device main aerofoil is displaced by at least lOmm, preferably by at least 50mm, and possibly by more than 100mm, as between the first and second geometrical configurations. There may be many different geometrical configurations of the wing tip device.

[0041] The one or more elements may be arranged to be able to deform actively the overall shape of the wing tip device while the aircraft is in flight. It may be that the wing tip device is arranged such that the one or more elements deform the shape of the wing tip device while the aircraft is stationary and on the ground. It may for example be the case that a particular configuration of the wing tip device is selected before the aircraft takes off for a particular destination.

[0042] The shape of the wing tip device may be arranged to be deformed with two degrees of freedom. For such a feature, it may be preferred to have more than one independently movable element for deforming the shape of the wing tip device.

[0043] The one or more elements may be arranged to deform the overall shape of the wing tip device by means of an elastic deformation of the wingtip device. The one or more elements may be arranged to deform the overall shape of the wing tip device such that substantially the majority of the outer surfaces, by surface area, are displaced from one configuration to another. The wing-tip device may for example undergo a global, or macroscopic, deformation of substantially its entire shape. The wing-tip device may comprise a morphing surface. Although the shape of the wing tip device may change due to the force applied by the element within the supporting structure, the main aerofoil is preferably configured to maintain a sigmoidal profile. It may be that the sigmoidal profile of the wing tip device is stretched or contracted in the horizontal direction and/or the vertical direction.

-12 - [0044] The element may be arranged to exert a range of forces wherein the difference between the maximum force and the minimum force is greater than 100N. The element may be under tension when exerting the force. The element may for example comprise one or more cables. The element may for example comprise one or more links, chains, hinged portions, or the like. The element may be under compression when exerting the force. The element may for example comprise one or more actuators. The element may for example comprise one or more pistons, rods, or the like. The element may directly exert a force at various positions along the length of the wing tip device. The element may be substantially enclosed by the outer skin of the wingtip device, for example such that no part of the element is directly exposed to airflow when the device is in use on an aircraft in flight.

[0045] Certain embodiments of the present invention may have application where the main aerofoil is provided primarily to perform the function of a winglet, whereas the supporting structure is provided primarily to perform the function of bracing the winglet and/or actively manipulating the aerodynamic shape of the winglet. The supporting structure may be actively moveable (by integrated actuators for example) whereas the main aerofoil may be a passive structure having a global shape that is adaptable by external forces. This may simplify design and manufacture of a new aircraft wing. [0046] In certain embodiments of the invention, the supporting structure of the wing tip device is designed primarily having in mind one or more of (i) a desire to reduce mass of the main aerofoil, by providing structural strength by using the supporting structure as a support strut and (ii) a desire to provide a means of controllably deforming the shape of the lower section by means of one or more elements contained in the supporting structure. In certain embodiments, the one or more elements may be contained only within the supporting structure. This arrangement may simplify the aircraft wing design process. The supporting structure may have a mass greater than 10Kg.

[0047] The supporting structure of the wing tip device may be a morphing structure, the shape to which the supporting structure is morphed being controllable by the one or more elements contained within the supporting structure. The shape adopted by the main aerofoil of the wing tip device may depend on and be controlled by the shape to which the supporting structure is morphed The main aerofoil of the wing tip device may be a morphing structure, the shape to which the main aerofoil is morphed being I-3 -controllable by the one or more elements contained within the supporting structure. There may be elements for deforming the shape of the wing tip device in both the supporting structure and the main aerofoil. The supporting structure of the wing tip device may have a length that can be controllably changed (increased or decreased) by the one or more elements contained within the supporting structure.

[0048] The present invention provides, according to a second aspect, a wing tip device of or for a wing of an aircraft. The wing tip device may be a wing tip device for use in the first aspect. The wing tip device may comprise a main aerofoil structure, which follows a sigmoidal profile when viewed in a line-of-flight direction. The wing tip device may be configured so as to be suitable for, or configured to form at least part of the wing tip device of the aircraft wing of the first aspect of the invention.

[0049] According to a third aspect of the invention there is provided an aircraft incorporating or otherwise comprising a wing according to the first aspect or a wing tip device according to the second aspect of the invention. The aircraft may be a single aisle aircraft. The aircraft may be a passenger aircraft, for example an aircraft suitable for transporting at least 50, for example at least 100, for example at least 200 passengers. For the purposes of the present specification the term commercial passenger aircraft also covers aircraft of an equivalent size configured for cargo and/or used on a non-commercial basis. The aircraft may have a maximum take-off weight (MTOW) of at least 20 tonnes, optionally at least 40 tonnes, and possibly 50 tonnes or more. The aircraft may have an operating empty weight of at least 20 tonnes, optionally at least 30 tonnes, and possibly about 40 tonnes or more.

[0050] According to a fourth aspect of the invention there is provided a method of operating, for example flying, an aircraft, the aircraft for example being in accordance with the third aspect mentioned above. The aircraft comprises wings which each include a wing tip device, for example according to any aspect of the invention as described and claimed herein. The wing tip device may have a sigmoid-shaped upper aerofoil extending from a root to a tip. The root of the wing tip device may be located at, for example blending in with, the outboard end of a main wing structure. The method includes using the upper aerofoil to reduce the drag that would otherwise be induced by wingtip vortices, and providing support to the upper aerofoil with a separate support member. The support member is preferably aerodynamically shaped. The support member extends beneath the upper aerofoil, for example from an inboard location at or -14 -proximate to the root of the wing tip device to an outboard location at or proximate to the tip of the wing tip device.

[0051] The method may include deforming the shape of each wing tip device from a first geometrical configuration in which the wing tip devices are set up in a manner to suit a first flight profile, for example in accordance with a given planned destination and route, to a second geometrical configuration in which the wing tip devices are set up in a manner to suit a second flight profile, different from the first flight profile (for example having a different route, planned flight time, distance or the like). In certain embodiments, it may be possible for the fuel efficiency of the aircraft if set up in the second configuration to be worse for the first flight than when set up in the first configuration, and for the fuel efficiency of the aircraft if set up in the first configuration to be worse for the second flight than when set up in the second configuration. For example, the first configuration may suit short-haul flights over particularly short distances, whereas the second configuration may suit longer distance flights [0052] The present invention also provides, according to a further aspect, a method of manufacturing an aircraft having a wing tip device according to any other aspect of the invention. The wing tip device may be retro-fitted to the aircraft. The method may comprise a step of attaching an aircraft wing according to any other aspect of the invention to an aircraft fuselage.

[0053] 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

Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure la shows a top view of a prior art aircraft; Figure lb shows a frontal view of a wing tip device according to a first embodiment of the invention; -15 -Figure lc shows a frontal view of the wing tip device according to a first embodiment of the invention and a "sharklet" wing tip device; Figure Id shows a frontal view of the wing tip device according to a first embodiment of the invention and a "sharklet" wing tip device; Figure 2 shows a frontal view of a wing tip device according to a second embodiment of the invention; Figure 3 shows a perspective view of the wing tip device from the outboard side according to the second embodiment of the invention, Figure 4 shows a perspective view of the wing tip device from the inboard side according to the second embodiment of the invention; Figure 5 shows a perspective view of the wing tip device from below according to the second embodiment of the invention; Figure 6 shows a perspective view looking down and aftwards of the wing tip device according to the second embodiment of the invention; Figure 7 shows a perspective view of the wing tip device from the outboard side looking down the centreline of the wing according to the second embodiment of the invention; Figure 8 shows a further perspective view of the wing tip device according to the second embodiment of the invention; Figure 9 shows a yet further perspective view of the wing tip device according to the second embodiment of the invention; and Figure 10 shows a flowchart of the steps of a method according to a third embodiment of the invention

DETAILED DESCRIPTION

[0054] Fig. la shows the location of a wing tip device 1 on a wing 2 of an aircraft 3, namely at the outboard end of its wing. The wing is shown with a centreline 4 (median chordline), which is used in the context of this description to define the "sweep angle' of the wing, which in this case is about 25 degrees.

[0055] According to an embodiment of the invention shown in Fig. lb, a wing tip device 1 has a main aerofoil 10 including a tip 30 and a root portion 50. The tip 30 is at -16 -an outboard end 35 of the wing tip device 1 and the root portion 50 is at an inboard end 55 of the wing tip device 1.

[0056] When viewed from the front (the view shown in Fig. lb -i.e. in the line of flight direction) or rear, the main aerofoil 10 follows a sigmoidal curve. The height of the main aerofoil 10 does not decrease in the outboard direction. The height of the main aerofoil increases monotonically in the outboard direction. The main aerofoil is self-support and has a mass of the order of about 110 kg.

[0057] The main aerofoil 10 extends from the root portion 50 to the wing tip 30. The main aerofoil 10 has a first curved portion 11 extending from the root of the wing tip device, and a second curved portion 13, curving in the opposite direction and extending to the tip of the wing tip device. The first curved portion 11 and the second curved portion 13 are separated by a generally planar (within +/-2 degrees) transition portion 12. Fig. lb shows four horizontal lines A, B, C, and D, which notionally define the regions of the different portions of the wing tip device. The height of the tip of the main aerofoil above the horizontal position of the root of the main aerofoil is at least 2,000 mm (the height being about the same as the distances between lines A and D). A maximum gradient line G is also shown. Along the length of the upper surface of the aerofoil 10, the first curved portion 11 (between lines C and D) represents about 40% of the length of the aerofoil. The generally planar transition portion 12 (between lines B and C) represents about 20% of the length of the aerofoil 10. The second curved portion 13 (between lines A and B) represents about 40% of the length of the aerofoil. In the first curved portion 11, the gradient of the main aerofoil 10 increases in the outboard direction relative to the horizontal (lateral) axis. It will be seen that the first portion is shaped such that the angle of the local dihedral varies monotonically from a value of about 0 degrees at the root (intersection with line D) to a value of about +70 degrees at a location further outboard (intersection with line C). The generally planar transition portion 12 (between lines B and C) has a local dihedral that is substantially constant and about +70 degrees. It will be seen that the second curved portion 13 is shaped such that the angle of the local dihedral varies monotonically from a value of about +70 degrees at its most inboard part (intersection with line B) to a value of about +2 degrees at the tip (intersection with line A). The gradient of the main aerofoil 10 at the tip 30 is thus substantially the same as the gradient of the main aerofoil at the root portion 50.

-17 - [0058] The maximum gradient (line G) is the maximum gradient of the transition portion 12 (between lines B and C) -the local dihedral at the maximum gradient is labelled as 0 (theta) in the Figures. At -70 degrees, 0 is less than the local dihedral at the maximum gradient portion of a sharklet style winglet which may have a local dihedral of 75 degrees or higher (i.e. even closer to the vertical). A lower 0 (e.g. less than 75 degrees) may nevertheless provide a benefit, that compares well to that provided by a sharklet, in managing the airflow over the upper surface of the main aerofoil and thus reducing induced drag that might others be caused by vortices of the tip of the wing.

[0059] The angle of the local dihedral defined by the sigmoid-like shape of the aerofoil 10 varies gradually, thus having no sharp edges that might otherwise cause problems aerodynamically and / or structurally, The dihedral angle varies by 70 degrees over about 40% of the length of the main aerofoil (i.e. an average of almost 20 degrees for every 10% of length), but sufficiently gently that, for any section being 10% of the length of the main aerofoil, the change in the dihedral angle is less than 30 degrees.

[0060] It will be appreciated that there may be a smooth transition, even where there are changes in sweep or twist at the junction between the fixed wing 2 (shown in part only in dashed line in Fig lb) and the wing tip device I. In this embodiment, there are no discontinuities at the junction between the fixed wing and wing tip device. The upper and the lower surfaces of the wing tip device are continuations of the upper and lower surfaces of the fixed wing. The span ratio of the fixed wing relative to the wing tip device may be such that the fixed wing comprises 70%, 80%, 90%, or more, of the overall span of the aircraft wing.

[0061] The first curved portion 11 of the wing tip device 1 is immediately outboard of the root portion 50. There is no significant planar portion between the root portion and the first curved portion 11. This allows for a large proportion of the mass of the wing tip device 1 to be located close to the root portion 50 of the wing tip device 1. The second curved portion 13 of the wing tip device 1 is immediately inboard of the tip 30. [0062] Figures 1 c and Id show the wing tip device 1 of the embodiment alongside a "sharklet" wing tip device 5 for the same aircraft wing (and being shown at the same scale). The wing tip device 1 has substantially the same height as a sharklet wing tip device 5. The aerodynamic benefits of the wing tip device 1 resulting from the height difference between the end of the wing tip and the wing is therefore similar in comparison to a sharklet 5. It can be seen from Fig. lc that the wing tip device 1 is longer in the outboard direction in comparison to the sharklet 5, and therefore mounts onto a rib inboard of the rib to which a sharkl et would typically be mounted. The shape of the wing tip device 1 is such that its centre of mass is further inboard in comparison to a sharklet.

[0063] Fig. Id shows more clearly that the wing tip device 1 has a maximum dihedral less than that of the sharklet 5.

[0064] A second embodiment of the invention is shown in Fig. 2. A wing tip device 101 has a main aerofoil 110, a support structure in the form of a support brace / member 120, a join 160, a tip 130 and a root portion 150. The main aerofoil HO has the same general sigmoidal shape as the main aerofoil 10 of Fig. lb, so will only be described briefly here. (Similar reference numbers are used for similar parts, the reference numbers for the second embodiment being in the form "'MN", where NN is the reference number of the corresponding integer of the first embodiment). The main aerofoil 110 extends from the root portion 150 to the wing tip 130 and has a first curved portion 111 (between lines C and D), a transition portion 112 (between lines B and C) and a second curved portion 113 (between lines A and B).

[0065] The support structure 120 and the main aerofoil 110 form a closed loop and define an airflow channel 140. The wing tip device 100 is thus in the form of a closed loop wing tip device. The tip 130 is at an outboard end 135 of the wing tip device 100 and the root portion 150 is at an inboard end 155 of the wing tip device 100.

[0066] When viewed from the front (the view shown in Fig. 2) or rear, the main aerofoil 110 follows a sigmoidal curve, and has a shape and size similar to that of the first embodiment (albeit with slightly less structure -afforded by the strucutural support provided by the support member 120). The height of the main aerofoil 110 continuously increases in the outboard direction.

[0067] The supporting structure 120 is joined to the underside of the main aerofoil 110. The supporting structure 120 extends from the inboard end 155 ofthe main aerofoil 110 to the outboard end 135 of the main aerofoil 110. Although the term "joined" has been used to describe the attachment of the support structure to the main aerofoil, it may be that the supporting structure 110 is integrally formed with the main aerofoil 120. The supporting structure 120 is joined at the outboard end to the underside of the main aerofoil 110 substantially normal to the main aerofoil 110, thus providing good -19 -support from underneath, with a join that need not be long in the spanwise direction. Reducing the proportion of the weight of the wing tip which is outboard advantageously reduces the overall bending moment of the wing. Reducing the bending moment of the wing tip has a consequential benefit in reducing the strength needed for attaching the wing tip device to the fixed wing.

[0068] The supporting structure 120 essentially consists of a single curved portion curving in one direction only when viewed front-on (as in Figure 2). Thus the structure 120 has at its root (join 160) a local dihedral of about 0 degrees, which gradually increases to about +90 degrees at its tip (an average change of almost 10 degrees for every 10% step along the length of the supporting structure 1 20. The change in dihedral is however gentle (with no sharp comers) such that the maximum change in angle over any 10% of the length of the supporting structure 120 is less than 25 degrees.

[0069] The relative changes in curvature of the supporting structure 120 as compared to the second curved portion 113 of the main aerofoil 110 creates a large airflow channel 140 between the main aerofoil 110 and the supporting structure 120. The supporting structure 120 having a single curved portion has a benefit in how it performs as a structural support, in comparison to a supporting structure having more than one curved portion. The geometry of the main aerofoil 110 and the supporting structure 120 thus defining a large airflow channel 140 may have multiple benefits. The supporting structure 120 can support an increased amount of the mass of the main aerofoil 110, by following a structurally strong curve, which terminates at a vertically extending end which meets the main aerofoil 110 at an angle which is 90 degrees or close thereto (i.e. substantially perpendicular). The supporting structure supporting a greater amount of the weight of the main aerofoil 110, enables the main aerofoil 110 and the wing tip device 1 to be lighter. By having a large airflow channel 140, drag is decreased in comparison to a smaller airflow channel.

[0070] It will be noted that the wing tip device 100 according to the embodiment shown in Fig. 2 has the main aerofoil 110 as the upper section, and the supporting structure 120 as the lower section. The supporting structure 120 according to this embodiment is joined to the main aerofoil 110 at the join 160 located on the bottom surface of the main aerofoil 110. The inventors have found that in aircraft wing tips there is supersonic airflow on the top surface of the main aerofoil and subsonic airflow on the bottom surface of the main aerofoil. Having a junction, such as a join, in the -20 -supersonic airflow on the top surface of the main aerofoil has been found to induce more drag than having a junction on the bottom surface of the main aerofoil. Providing a "clean" top surface of the main aerofoil and having the join 160 on the bottom surface of the main aerofoil has been found to reduce drag in comparison to arrangements where the main aerofoil forms a lower section supported by supporting structure as the upper section (imagine a braced sharklet structure for example) [0071] The inboard end 155 of the wing tip device 100 attaches to an outboard end of a fixed wing of an aircraft (not shown). The root portion 150 of the wing tip device 100 is a continuation of the outboard end of the fixed wing of the aircraft. The root portion 150 of the wing tip device has the same sweep, cant angle and angle of attack as the outboard end of the fixed wing of the aircraft. The leading edge of the root portion 150 of the wing tip device is a continuation of the leading edge of the outboard end of the fixed wing of the aircraft. The trailing edge of the root portion 150 of the wing tip device is a continuation of the trailing edge ofthe outboard end of the fixed wing of the aircraft.

[0072] The supporting structure 120 attaches to the root of the main aerofoil at a point which is in the same plane as the wing. Each supporting structure 120 has a mass of the order of at least 30 kg (optionally, in the range of 25 kg to 40 kg). By comparison, the main aerofoil may have a mass of the order of at least 80 kg.

[0073] Figures 3-9 show perspective views of the wing tip device 101 of a starboard aircraft wing with the direction of flight being indicated by the arrow 180. It will be seen from Figures 5 and 7 that the tailing edge of the supporting structure 120 does not extend rearwards of the trailing edge of the main aerofoil 110. Also, as shown in Figure 7, the main aerofoil 110 entirely overlays the supporting structure 120 when viewed along the centreline of the wing.

[0074] By way of a brief summary, it will be seen that embodiments of the invention are able to provide an aircraft wing tip device with a sigmoid shaped (e.g. S-shaped) aerofoil structure blending in with a main wing of the aircraft. The aerofoil structure may be braced with a structural support such as a curved aerodynamically shaped structural support or strut, that is located beneath. The main aerofoil and structural support together form a closed loop wing tip device. The upper aerofoil reduces the drag that would otherwise be induced by wingtip vortices. The curved strut may include one or more actuators for changing the shape of the wing tip device so as to morph from a first geometrical configuration which suits a flight plan to a second geometrical -21 -configuration in which the wing tip devices are set up to suit a different flight plan. A further example will now be described.

[0075] According to an embodiment of the present invention (not illustrated separately), the structural support may comprise an actuator. The shape of the main aerofoil 110 can be adapted. The structural support 120 may comprise an actuator, wholly contained within its shape, as shown in Figure 2 to 9. For example, the lower structural support may comprise two integrally formed cable systems which are arranged within the structure of the structural support in such a way as to enable its shape to be deformed elastically under the control of the cable systems, and thus cause a change in geometry of the upper main aerofoil. The integrated cable systems may be designed to be adjusted by ground crew when attending to the aircraft when on the ground and stationary. Once the geometry of the wingtip device has been set up by ground crew by means of making adjustments to the integrated cable systems and the aircraft is moving, the setup of the wingtip device, insofar as its global geometry is concerned, is fixed and cannot be altered or controlled by the pilot during flight. A first configuration of the wingtip device might require the tension in each cable system to be approximately equal, at about 250 N. [0076] Further, different configurations, may require the cable systems to be under increased tension for example each exerting a tension of about 500 N, resulting in the tip of the support structure to be drawn more inboard with a consequent movement of the tip of the upper main aerofoil inboard also. There may be a case where one of the cable systems has a tension (for example 100 N) significantly lower than the tension in the other of the cable systems (for example 700 N), causing a twist. There may be portions on the wingtip device which are moved by more than 50 mm as between such different configurations. The movement from one configuration to another may be such that the wingtip device is deformed elastically and is thus able to return to its previous shape, as and when the internal stresses are returned to their previous values. The extent of the cable systems in the (lower) support member may follow a path that curves. The curvature of that path can change as between the various configurations of the wingtip device. Such a curvature, and changes in curvature, can be accommodated by means of the cable passing via channels, pulleys, wheels or the like.

[0077] Figure 10 illustrates a flowchart of 500 illustrating a method according to a fourth embodiment of the invention. The aircraft may be one as shown in Figure 1 a -22 -incorporating wingtip devices of an embodiment of the invention. As a first step 501 the wingtip devices are set up so as to be suitable for a short haul flight, in which the time the aircraft is expected to fly at cruising altitude is around 30 minutes. The aircraft undertakes the flight (step 502). When back on the ground, the wingtip devices are adjusted by means of changing the geometry of the wingtip devices (as in step 503) so as to be more suited to a longer flight. This step may include imparting a twisting force in the wingtip device to provide a different aerodynamic profile at the wingtip. The aircraft then undertakes a further flight (step 504), in which the aircraft flies at cruising aptitude for three hours or longer. The fuel efficiency of the aircraft when set up in the first configuration is better for the first flight and it would be for the second flight if retained in that first for configuration. Likewise, the fuel efficiency of the aircraft when set up in the second configuration is better for the second flight than it would be for the first flight if retained in that second configuration.

[0078] 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.

[0079] It will be appreciated that the wingtip device shown in the Figures might include fairings and/or additional fairing services in order to smooth out any sharp changes in curvature. The wingtip device according to the second embodiment may be a closed surface wingtip device without any other parts protruding from the smoothly faired structure that provides the closed surface.

[0080] The term 'or' shall be interpreted as 'and/or' unless the context requires otherwise.

[0081] 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 -23 -in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments

Claims

-24 -CLAIMSI. An aircraft wing comprising: a main body having an outboard end, a wing tip device extending from the outboard end of the main body of wing, the wing tip device comprising a main aerofoil, the main aerofoil having a first portion extending across at least 25% of the total length of the wing tip device, and a second portion outboard of the first portion, and extending across at least 25% of the total length of the wing tip device, the first portion being shaped such that the angle of the local dihedral varies monotonically from a value of less than +20 degrees at an inboard location to a value of greater than +50 degrees in the outboard direction, the second portion being shaped such that the angle of the local dihedral varies monotonically from a value of greater than +50 degrees at an inboard location to a value of less than +20 degrees in the outboard direction.
2. An aircraft wing according to claim 1, wherein the magnitude of the rate of change of the dihedral angle with increasing distance in the outboard direction is such that the maximum variation in angle over any portion of the main aerofoil extending along 10% of the length of the main aerofoil is less than 30 degrees.
3. An aircraft wing according to claim 1 or claim 2, wherein the first portion and the second portion are joined by an intermediate portion extending across at least 10% of the total length of the main aerofoil.
4. An aircraft wing according to claim 3, wherein the intermediate portion includes an essentially planar portion extending across at least 10% of the total length of the main aerofoil.
5. An aircraft wing according to claim 3 or claim 4, wherein the intermediate portion includes both an inboard region in which the local dihedral increases in the outboard direction and an outboard region in which the local dihedral decreases in the outboard direction.
-25 - 6. An aircraft wing according to any of claims 3 to 5, wherein the essentially planar portion extends across no more than 30% of the total length of the main aerofoil.
7. An aircraft wing according to any preceding claim, wherein the main aerofoil terminates at a substantially horizontal tip.
8. An aircraft wing according to any preceding claim, wherein the wing comprises a blended region blending between the first portion of the wing tip device and the outboard end of the main body of wing.
9. An aircraft wing according to any preceding claim, wherein the angle of the greatest local dihedral of the main aerofoil of the wing tip device is less than 75 15 degrees.
10. An aircraft wing according to any preceding claim, wherein the main aerofoil of the wing tip device is braced by means of a supporting structure extending from the first portion or a position inboard of the first portion to the second portion or a position outboard of the second portion.
11. An aircraft wing according to claim 10, wherein the supporting structure extends below the main aerofoil
12. An aircraft wing according to claim 10 or claim 11, wherein the supporting structure is so shaped that its chordwise dimension at any position along at least 90% of its length is less than the chordwise dimension of the main aerofoil at the same position in the spanwise direction.
13. An aircraft wing according to any of claims 10 to 12, wherein the supporting structure is so shaped that for at least 90% of its length the angle of the local dihedral varies monotonically from a value of greater less than +20 degrees at an inboard location to a value of greater than +70 degrees at an outboard location.-26 - 14.
An aircraft wing according to any of claims 10 to 13, wherein the magnitude of the rate of change of the dihedral angle of the supporting structure with increasing distance in the outboard direction is such that the maximum variation in angle over any portion of the supporting structure extending over 10% of its total length is less than 30 degrees.
An aircraft wing according to any of claims 10 to 14, wherein the supporting structure extends in the outboard direction from a region on the underside of the main aerofoil which is substantially coplanar with the main body of the wing.
16. An aircraft wing according to any of claims 10 to 15, wherein the trailing edge of the supporting structure does not extend rearwards of the trailing edge of the main aerofoil.
17. An aircraft wing according to any of claims 10 to 16, wherein the main aerofoil entirely overlays the supporting structure when viewed along the centreline of the wing.
18. An aircraft wing according to any of claims 10 to 17, wherein the supporting structure is a secondary aerofoil structure.
19 An aircraft wing according to any of claims 10 to 18, wherein the supporting structure comprises an actuator arranged to alter the shape of the main aerofoil
20. An aircraft wing according to any preceding claim, wherein the chordw se cross-section of the main aerofoil at an inboard end of the wing tip device is substantially identical to the chordwise cross-section of the outboard end of the fixed wing.
21. An aircraft wing according to any preceding claim, wherein the main aerofoil of the wing tip device has a shape when viewed in a line-of-flight direction that -27 -resembles the letter" S " with a horizontal skew which increases the distance between the start and end of the letter.
22. An aircraft wing according to any preceding claim, wherein the main aerofoil of the wing tip device has a substantially sigmoidal shape when viewed in a line-of-flight direction.
23. A wing tip device of the aircraft wing according to any preceding claim.
24. A wing tip device of or for a wing of an aircraft, wherein the wing tip device comprises a main aerofoil structure, which follows a sigmoidal profile when viewed in a line-of-flight direction.
25. The wing tip device according to claim 24, wherein the height of the main aerofoil structure continuously increases in the outboard direction.
26. The wing tip device according to claim 24 or claim 25, wherein the angle of the local dihedral of the main aerofoil structure does not exceed +75 degrees.
27 The wing tip device according to any of claims 24 to 26, wherein the wing tip device comprises a structural support, which extends between an inboard end on the underside of the wing tip device and an outboard end of the main aerofoil on the underside of the wing tip device
28. The wing tip device according to claim 27, wherein the structural support occupies a volume of space which is less than half the volume of space occupied by the main aerofoil.
29. An aircraft comprising the aircraft wing of any of claims 1 to 22 or the wing tip device of any of claims 23 to 28.
30. A method of operating an aircraft, the aircraft comprising wings which each include a wing tip device having an sigmoid shaped upper aerofoil extending from a -28 -root to a tip, the root of the wing tip device being located at, and blending in with, the outboard end of a main wing structure, the method including using the upper aerofoil to reduce the drag that would otherwise be induced by wingtip vortices, and providing support to the upper aerofoil with an aerodynamically shaped support member which extends, beneath the upper aerofoil, from an inboard location at or proximate to the root of the wing tip device to an outboard location at or proximate to the tip of the wing tip device.
31. A method according to claim 30, including using the support member to change the shape of the upper aerofoil.
32. A method according to claim 31, wherein the step of using the support member to change the shape of the upper aerofoil includes deforming the shape of each wing tip device from a first geometrical configuration in which the wing tip devices are set up in a manner to suit a first flight to a second geometrical configuration in which the wing tip devices are set up in a mariner to suit a second flight, different from the first flight, such that the fuel efficiency of the aircraft if set up in the second configuration would be worse for the first flight than when set up in the first configuration, and the fuel efficiency of the aircraft if set up in the first configuration would be worse for the second flight than when set up in the second configuration.
33. A method according to claim 31 or claim 32, wherein the step of using the support member to change the shape of the upper aerofoil includes changing the shape or length of the support member with an actuator housed at least partly within the support member.
34. A method according to any of claims 30 to 33, wherein during flight the support member and the upper aerofoil together form a closed surface wing tip device.
35. A method according to any of claims 30 to 34, using an aircraft according to claim 29.