WINGTIP DEVICE FOR AN AIRCRAFT

Abstract

An aircraft wing is disclosed including a closed surface wing tip device which includes an element or actuator within the wing tip for deforming/morphing the shape of the wing tip between geometrical configurations having different aerodynamic properties, for example including one with better overall fuel efficiency for a shorter journey and one with overall fuel efficiency better suited for a longer journey. The device includes a lower winglet with an essentially planar portion spaced apart from the main body of the wing by a blended transition region which is shaped such that the curvature of the local dihedral increases in the outboard direction. The device includes an upper aerofoil structure which with the winglet essentially forms the closed surface. There is also disclosed an aircraft wing tip device having a sigmoid shaped (e.g. S-shaped) aerofoil structure blending in with a main wing of the aircraft.

Claims

1. A closed surface wing tip device for an aircraft, wherein the wing tip device includes an element within the wing tip device which is arranged to deform the shape of the wing tip device from a first geometrical configuration to a second geometrical configuration with different aerodynamic properties from the first geometrical configuration.

2. A device according to claim 1, wherein the element is arranged to deform the overall shape of the wing tip device by means of an elastic deformation.

3. A device according to claim 1, wherein the element is arranged to exert a range of forces wherein the difference between the maximum force and the minimum force is greater than 100 N.

4. A device according to claim 1, wherein the closed surface of the wing tip device comprises a lower section being in the form of a winglet having a first portion having a first dihedral angle and a second portion, outbound of the first portion, which has a second dihedral angle which is at least 30 degrees more than the first positive dihedral angle, and an upper section which extends from a region in the first portion of the lower section to a region in the second portion of the lower section, and which forms with the lower section the closed surface.

5. A device according to claim 4, wherein the first portion of the lower section has a dihedral angle of less than +20 degrees and the second portion has a dihedral angle of greater than +60 degrees over a spanwise length of more than 500 mm.

6. A device according to claim 4, wherein the winglet forming the lower section of the closed surface of the wing tip device has a height of at least 500 mm.

7. A device according to claim 4, wherein the upper section includes a portion having a dihedral angle of greater than +45 degrees over a spanwise length of more than 500 mm.

8. A device according to claim 4, wherein there is a point on the lower surface of the upper section for which the shortest distance to the nearest point on the lower section is greater than 500 mm away.

9. A device according to claim 4, wherein when the wing tip device is installed on an aircraft and viewed in the direction of the longitudinal axis of the aircraft, there is first notional straight line joining said region in the first portion of the lower section and said region in the second portion of the lower section, between which regions the upper section extends, and a second notional line which defines the average extent and overall shape of the lower section, the first notional line forming at one end a first included angle with the second notional line and at the other end a second included angle with the second notional line, each of the first and second included angles being greater than 20 degrees.

10. A device according to claim 4, wherein the upper section has a mass of greater than 10 Kg.

11. A device according to claim 1, wherein the element comprises an actuator.

12. A device according to claim 1, wherein the shape of the wing tip device is arranged to be deformed with two degrees of freedom.

13. (canceled)

14. (canceled)

15. An aircraft wing comprising a main body, a winglet at an outboard end of the wing, the winglet having an essentially planar portion spaced apart from the main body by a blended transition region which is shaped such that the curvature of the local dihedral increases in the outboard direction and an upper aerofoil structure extending between a position outboard of a location between the essentially planar portion and the blended transition region to a position inboard of the location between the essentially planar portion and the blended transition region, and a closed loop at the wing tip formed at least in part by at least part of the winglet and at least part of the upper aerofoil structure, the upper aerofoil structure having an actively controllable shape and/or length, which acts in use to manipulate the shape of the winglet by loading the winglet to cause elastic deformation of the winglet from a first aerodynamic configuration suitable for a short-haul flight over a first distance to a second different aerodynamic configuration better suited for a flight over a second distance different from the first difference.

16. An aircraft wing incorporating or otherwise comprising a wing tip device according to claim 1.

17. An aircraft wing comprising: a main body having an outboard end, and 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, and 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.

18. An aircraft wing according to claim 17, 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, the first portion and the second portion are joined by an intermediate, substantially planar, portion extending across between 10% and 30% of the total length of the main aerofoil, the main aerofoil terminates at a substantially horizontal tip, and the angle of the greatest local dihedral of the main aerofoil of the wing tip device is less than 75 degrees.

19. An aircraft wing according to claim 17, wherein the main aerofoil of the wing tip device has a substantially sigmoidal shape when viewed in a line-of-flight direction.

20. An aircraft wing according to claim 17, wherein the main aerofoil of the wing tip device is braced by means of a supporting structure below the main aerofoil 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.

21. An aircraft comprising a wing according to claim 15.

22. (canceled)

23. (canceled)

24. (canceled)

Description

DESCRIPTION OF THE DRAWINGS

[0082] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

[0083] FIG. 1 shows an aircraft of a type suitable for use with a wing tip device according to a first embodiment of the invention;

[0084] FIG. 2 shows a side view of a wing tip according to the first embodiment;

[0085] FIG. 3 shows a perspective view of a wing tip device similar to that shown in FIG. 2, but in accordance with a second embodiment;

[0086] FIG. 4 shows a side view of a wing tip according to a third embodiment;

[0087] FIG. 5 shows a perspective view of the wing tip device shown in FIG. 4 set up in a first configuration;

[0088] FIG. 6 shows a perspective view of the wing tip device shown in FIG. 4 set up in a second configuration;

[0089] FIG. 7 shows a perspective view of the wing tip device shown in FIG. 4 set up in a third configuration;

[0090] FIG. 8 shows a frontal view of a wing tip device according to a fourth embodiment of the invention;

[0091] FIG. 9 shows a frontal view of the wing tip device according to a fourth embodiment of the invention and a “sharklet” wing tip device;

[0092] FIG. 10 shows a frontal view of the wing tip device according to a fourth embodiment of the invention and a “sharklet” wing tip device;

[0093] FIG. 11 shows a frontal view of a wing tip device according to a fifth embodiment of the invention;

[0094] FIG. 12 shows a perspective view of the wing tip device from the outboard side according to the fifth embodiment of the invention;

[0095] FIG. 13 shows a perspective view of the wing tip device from the inboard side according to the fifth embodiment of the invention;

[0096] FIG. 14 shows a perspective view of the wing tip device from below according to the fifth embodiment of the invention;

[0097] FIG. 15 shows a perspective view looking down and aftwards of the wing tip device according to the fifth embodiment of the invention;

[0098] FIG. 16 shows a perspective view of the wing tip device from the outboard side looking down the centreline of the wing according to the fifth embodiment of the invention;

[0099] FIG. 17 shows a further perspective view of the wing tip device according to the fifth embodiment of the invention;

[0100] FIG. 18 shows a yet further perspective view of the wing tip device according to the fifth embodiment of the invention; and

[0101] FIG. 19 shows a flowchart of the steps of a method according to a sixth embodiment of the invention.

DETAILED DESCRIPTION

[0102] FIG. 1 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 aircraft has two such wings 2. Each wing has 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. The wing tip device may be in accordance with any of the embodiments of invention descried below, or variations thereof.

[0103] FIG. 2 shows a wing tip device 200, of a first embodiment, viewed in the longitudinal axis of the aircraft. The device 200 has a root 202 at its inboard end and a tip 204 at its outboard end. The device 200 comprises a lower section 206 which has the general profile of a blended winglet. The lower section 206 in this case has a shape corresponding generally to that of a “sharklet” for the Airbus A320 Neo. Thus the section 206 comprises a generally horizontal inbound portion 208 that is joined to a canted outbound portion 210, via a blended region 212. The dihedral angle of the canted outdone portion is about 70° for at least 1,000 mm of its extent. The height, h, of the tip 204 above the horizontal position of the inbound portion 208 is at least 2,000 mm. Each lower section 206 has a mass of the order of at least 80 kg.

[0104] The device 200 comprises an upper section 214 which acts as a strut extending from a location at or directly adjacent to the root 202 of the device to a location at or directly adjacent to the tip 204 of the device 200. The upper section 214 and the lower section 206 thus define, at least in part, a closed surface. In this embodiment, the upper section 214 is generally planar and is shown in FIG. 2 as having a dihedral angle of about 45° for substantially the entirety of its extent in the spanwise direction. Each upper section 214 has a mass of the order of 25 kg (optionally, in the range of 20 kg to 30 kg).

[0105] It will be noted that the upper section 214 extends from an outboard position set slightly inward from the extreme end of the tip 204 of the device 200 to an inboard position set slightly outward from the root 202 of the device 200. There is thus a slight overhang at the tip 204 of the device 200 which does not form a part of the closed surface.

[0106] The device shown in FIG. 2 thus comprises a blended winglet device 206 (in the form of a sharklet) which is strengthened by means of an upper section 214, which also presents an aerofoil surface. It is believed that substantially the same, or better, aerodynamic benefits may be provided by the lower section 206 as a conventional sharklet structure but as a result of the strengthening provided by the upper section the lower section can be constructed so as to have a lower mass than the conventional sharklet structure, by possibly greater than 25 kg of mass saving per wing tip device. The wing tip device of the first embodiment may therefore be substantially mass neutral as compared to the same aircraft when fitted with a sharklet wingtip device. The angle formed between the lines of the upper section 214 and the lower section 206 in the region of the root 202 is about 50°. The angle formed between the lines of the upper section 214 and the lower section 206 in the region of the tip 204 is about 25°. The region at which the upper section 214 joins the lower section 206 in the region of the root 202 coincides with a rib (not shown separately in FIG. 2). There may be at least one further rib outboard of the rib to which the upper portion 214 so attaches.

[0107] FIG. 3 shows (with a highly schematic diagram) a perspective view of a device 300 according to a second embodiment, which has substantially the same shape and configuration as the device of FIG. 2. The reference numerals as used in FIG. 3 will refer to the same parts as the reference numerals used in FIG. 2, but will start with a “3” instead of starting with a “2”. The upper section 314 of the device 300 includes two elements 330, which are arranged to exert varying forces within the structure of the upper section 314 in a direction substantially parallel with the spanwise direction D.sub.s of the wing, when viewed in plan. (It will be understood that in the present context the spanwise direction at a given location is taken as the mean direction of the leading and trailing edges of the wing when viewed in plan, and not necessarily perpendicular to the longitudinal direction of the aircraft). In this embodiment, the elements 330 may be in the form of hydraulic actuators. By means of varying the force exerted by each element 330 in the upper section 314, it is possible to change the global shape of the upper section 314 and consequently also change the global shape of the lower, winglet, section 306. In use, it is then possible to morph the aerodynamic shape of the lower, winglet, section 306 to suit a particular flight plan. For example for a ½ hour flight, the optimum shape of the winglet part of the wing may be different from the optimum shape of the winglet part for a three hour flight. The elements 330 are independently operable and therefore are able to manipulate the shape of the upper and lower sections 306, 314 with two degrees of freedom. If both elements 330 are contracted, the tip 304 of the device is moved inboard. If one element 330 is extended and the other element 330 is contracted then a twisting stress is induced in the upper and lower sections 306, 314. The hydraulic elements 330 can be controlled from within the aircraft, for example by the pilot of the aircraft, during flight, but it is envisaged that the hydraulic elements 330 would typically be set up in advance of take-off and not adjusted again until landing. However, in more sophisticated embodiments of the present invention, the hydraulic elements 330 could be controlled during flight so as to improve the aerodynamic performance of the wing as between, for example, take-off and cruise conditions.

[0108] FIG. 4 shows a wing tip device 400, of a third embodiment, viewed in the longitudinal axis of the aircraft. The reference numerals as used in FIG. 4 will refer to the same parts as the reference numerals used in FIG. 2, but will start with a “4” instead of starting with a “2”. The device 400 has a root 402 at its inboard end and a tip 404 at its outboard end. The device 400 comprises a lower section 406 which has the general profile of a blended winglet. The lower section 406 has a shape which is substantially the same as the lower section 206 of the first embodiment of the invention. Thus the section 406 comprises a generally horizontal inbound portion 408 that is joined to a canted, essentially planar, outbound portion 410, via a blended transition region.

[0109] The device 400 comprises an upper section 414 extending from a location at or directly adjacent to the root 402 of the device to a location in the region of the tip 404 of the device 400. It will be noted that the upper section 414 extends from an outboard position set slightly inward from the extreme end of the tip 404 of the device 400. In this embodiment, the tip of the upper section 414 extends to an outboard position beyond the tip of the lower section 410.

[0110] In this embodiment, the upper section 414 is generally curved, such that its upper surface 428 is substantially convex. The upper section 414 thus has a dihedral angle that varies along the spun way is extent of the upper section from a value of about 70° to a value of about 30°. It will be seen that the gap between the upper and lower sections of the wingtip can be several orders of magnitude bigger than the thickness of either the upper or lower section. For example, the length represented by the arrow labelled l (such a length representing the shortest distance between the lower surface of the upper section 414 to the upper surface of the lower section 406 at the location of the arrow l) is greater than 500 mm.

[0111] FIG. 5 shows the wingtip device 400 of the third embodiment in a highly schematic manner. The upper section 414 comprises two integrally formed cable systems 430 which are arranged within the structure of the upper section 414 in such a way as to enable the shape of the upper section 414 to be deformed elastically under the control of the cable systems 430. In this embodiment, the integrated cable systems 430 are designed to be adjusted by ground crew when attending to the aircraft when on the ground and a stationary. Once the geometry of the wingtip device 400 has been set up by ground crew by means of making adjustments to the integrated cable systems 430 and the aircraft is moving, the setup of the wingtip device, in so far as its global geometry is concerned, is a fixed and cannot be altered or controlled by the pilot during flight. FIG. 5 shows the wingtip device 400 in a first configuration, in which the tension in each cable system is approximately equal, at about 250 N.

[0112] FIGS. 6 and 7 show the wingtip device 400 in a second configuration and a third configuration, respectively. In both FIGS. 6 and 7, the position of the upper section 414 when in its first configuration is shown in broken line (labelled as 414′) the position of the lower section 406 when in its first configuration is shown in dotted line (labelled as 406′). FIG. 6 shows the cable systems both under increased tension for example each exerting a tension of about 500 N, resulting in the tip of the upper section 414 being drawn more inboard with a consequent movement of the tip of the lower section 406 inboard also.

[0113] FIG. 7 illustrates a case where one of the cable systems 430 has a tension (for example 100 N) significantly lower than the tension in the other of the cable systems 430 (for example 700 N). In this case the upper section 414 and the lower section 406 are twisted from the position of the first configuration. There are portions on the wingtip device which moved by more than 50 mm as between the first configuration and the second configuration. Similarly, there are portions on the wingtip device which move by more than 50 mm as between the first configuration and the third configuration. The movement from the first to the second configurations (and the movement from the first to the third configurations), is 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. It will be seen that the extent of the cable systems 430 in the upper section 414 follows a path that curves, and that 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.

[0114] The embodiments described with reference to FIGS. 2 to 7 may be considered as having a primary aerofoil surface (for example in the form of a wing tip device, winglet or sharklet) and a secondary surface (for example a strut, or bracing structure) that is arranged to deform the shape of the wing tip device and/or which braces/supports the primary aerofoil surface. The primary surface and the secondary surface may cooperate to form a closed loop at the wing tip. In FIGS. 2 to 7 the secondary surface is located above the primary aerofoil surface. In other embodiments, as illustrated in some of the other Figures for example, the secondary surface may be located at least partially below the primary aerofoil surface.

[0115] According to a fourth embodiment of the invention shown in FIG. 8, the wing tip device 1 has a main aerofoil 10 including a tip 30 and a root portion 50. The tip 30 is at 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.

[0116] When viewed from the front (the view shown in FIG. 8—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.

[0117] 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. 8 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.

[0118] 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 θ (theta) in the Figures. At −70 degrees, θ 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 θ (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.

[0119] 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.

[0120] 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. 8) and the wing tip device 1. 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.

[0121] 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 50 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.

[0122] FIGS. 9 and 10 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. 9 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 sharklet 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.

[0123] FIG. 10 shows more clearly that the wing tip device 1 has a maximum dihedral less than that of the sharklet 5.

[0124] A fifth embodiment of the invention is shown in FIG. 11. 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 110 has the same general sigmoidal shape as the main aerofoil 10 of FIG. 8, so will only be described briefly here. (Similar reference numbers are used for similar parts, the reference numbers for the fifth embodiment being in the form “1NN”, where NN is the reference number of the corresponding integer of the fourth 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).

[0125] 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.

[0126] When viewed from the front (the view shown in FIG. 11) or rear, the main aerofoil 110 follows a sigmoidal curve, and has a shape and size similar to that of the fourth embodiment (albeit with slightly less structure—afforded by the structural support provided by the support member 120). The height of the main aerofoil 110 continuously increases in the outboard direction.

[0127] The supporting structure 120 is joined to the underside of the main aerofoil 110. The supporting structure 120 extends from the inboard end 155 of the 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 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.

[0128] The supporting structure 120 essentially consists of a single curved portion curving in one direction only when viewed front-on (as in FIG. 11). 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 120. The change in dihedral is however gentle (with no sharp corners) such that the maximum change in angle over any 10% of the length of the supporting structure 120 is less than 25 degrees.

[0129] 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.

[0130] It will be noted that the wing tip device 100 according to the embodiment shown in FIG. 11 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 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)

[0131] 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 of the outboard end of the fixed wing of the aircraft.

[0132] 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.

[0133] FIGS. 12 to 18 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 FIGS. 14 and 16 that the trailing edge of the supporting structure 120 does not extend rearwards of the trailing edge of the main aerofoil 110. Also, as shown in FIG. 16, the main aerofoil 110 entirely overlays the supporting structure 120 when viewed along the centreline of the wing.

[0134] 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 configuration in which the wing tip devices are set up to suit a different flight plan. A further example will now be described.

[0135] 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 FIG. 11 to 18. 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.

[0136] 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.

[0137] FIG. 10 illustrates a flowchart of 500 illustrating a method according to a sixth embodiment of the invention. The aircraft may be one as shown in FIG. 1 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.

[0138] 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.

[0139] 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 (or fifth?) embodiment may be a closed surface wingtip device without any other parts protruding from the smoothly faired structure that provides the closed surface.

[0140] There may be features of the above described embodiments that have benefit independently of a wing tip device with an upper aerofoil surface forming a closed loop blended wing tip device (in which the upper and lower aerofoil surfaces are similarly dimensioned). For example, there may be embodiments of the invention where a (for example, sharklet-type) winglet has a shape which can be morphed using a moveable bracing structure (with associated integrated actuators for example) that extends from an inboard portion of the winglet to an outboard portion of the winglet. Such a bracing structure may be so shaped that it has a maximum chordwise dimension at a location immediately above a portion of the winglet structure which has a chordwise dimension at least twice the size. Additionally or alternatively, such a bracing structure may be so shaped that its chordwise dimension at the midpoint along its spanwise length, is less than half of the chordwise dimension of the winglet structure at the location immediately below. Additionally or alternatively, such a bracing structure may have an average chordwise dimension (along its spanwise length) which is less than half the average chordwise dimension (along its spanwise length) of the winglet structure.

[0141] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments. The term ‘or’ shall be interpreted as ‘and/or’ unless the context requires otherwise.