Patent Publication Number: US-11643183-B2

Title: Spar arrangement in a wing tip device

Description:
RELATED APPLICATION 
     This application claims priority to United Kingdom Patent Application (GB) 1709523.3 filed Jun. 15, 2017, the entirety of which is incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an aircraft having a wing and a wing tip device. The present invention also relates to a wing assembly and to a wing tip device. The present invention also relates to a method of manufacture of an aircraft, wing assembly and wing tip device. 
     Wing tip devices, for example winglets, are used to improve the efficiency of fixed wing aircraft, predominantly by reducing lift induced drag. Although this may increase the profile drag, the overall lift to drag ratio is increased. This increases fuel efficiency, which is particularly important with passenger aircraft. 
     Conventional winglets comprise front and rear spars that are attached to each other by ribs distributed along the span-wise direction of the winglet. 
     However, as the size and sweep of wing tip devices increase, the loading on the wing tip devices increases. With a conventional wing tip device design, such as the above conventional winglet design, this can present a number of design obstacles. 
     The present invention seeks to address or mitigate at least some of the above mentioned problems. Alternatively, or additionally, the present invention seeks to provide an improved aircraft having a wing tip device. Alternatively, or additionally, the present invention seeks to provide an improved wing assembly comprising a wing and a wing tip device. Alternatively, or additionally, the present invention seeks to provide an improved wing tip device for an aircraft. Alternatively, or additionally, the present invention seeks to provide an improved method of manufacture of a wing tip device for an aircraft. Alternatively, or additionally, the present invention seeks to provide an improved method of manufacture of a wing assembly comprising a wing and a wing tip device. Alternatively, or additionally, the present invention seeks to provide an improved method of manufacture of an aircraft having a wing tip device. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided an aircraft comprising a wing and a wing tip device at the tip of the wing, the wing tip device having front and rear spars, wherein the wing tip device comprises a cross-brace spar that links the front and rear spars. In embodiments of the invention the cross-brace spar is oriented such that it is oblique to the front and rear spars. 
     This is advantageous in that the cross-brace spar may transmit aerodynamic loads between the front and rear spars, thereby reducing the load on at least one of the spars. Furthermore, the cross-brace spar may help to provide a more balanced load path into the wing (i.e. into a wing box of the wing). The cross-brace spar may allow the wing tip device to more effectively resist the external aerodynamic loads, for example during gust and manoeuvre. 
     The cross-brace spar may allow for a reduction in the number of ribs that would otherwise be required in order to resist the required aerodynamic loads. In this respect, the cross-brace spar may advantageously react the brazier load that would usually be carried by ribs. Similarly, the cross-brace may allow for the reduction in the number of other components that may otherwise be required in order to resist the aerodynamic loads, for example stringers and rib posts. The cross-brace spar may allow for a reduction in the thickness of the wing assembly, and of the number of bolts required, to transfer load at the connection of the wing tip device to the wing. 
     In view of the above, the cross-brace spar may facilitate a reduction in weight of the wing tip device and therefore of the aircraft, thereby providing performance advantages. Due to the reduction in parts required, the cross-brace spar may provide a reduction in manufacturing time and cost. 
     In embodiments of the invention the cross-brace spar links a first location on the rear spar to a second location on the front spar. 
     In embodiments of the invention the cross-brace spar links the first location on the rear spar to the second location on the front spar such that it transmits aerodynamic loads between the front and rear spars. In embodiments of the invention the cross-brace spar is oriented at said oblique angle such that it transmits aerodynamic loads between the front and rear spars. It will be appreciated that references to the transmittal of aerodynamic loads is in relation to when the aircraft is in use, i.e. in flight. 
     In embodiments of the invention the cross-brace spar is configured to provide a primary load path between the front and rear spars. The aerodynamic loads may be external forces and moments on the wing tip device, namely bending moment, shear force and torque. 
     In embodiments of the invention the first and second locations are spaced in the span wise direction. In embodiments of the invention the cross-brace spar is directly attached to the rear spar at the first location and is directly attached to the front spar at the second location. This is advantageous in that it may provide a relatively continuous load path. 
     It will be appreciated that the term ‘oblique’ refers to any inclined orientation that is not perpendicular (or parallel). 
     In embodiments of the invention the front and rear spars are configured to transmit aerodynamic load into the wing. Optionally the front and rear spars are attached to a root rib of the wing tip device. Optionally the front and rear spars are attached to front and rear spars of the wing (for example via a root rib of the wing tip device). Optionally the front and rear spars of the wing tip device are attached to a wing box of the wing. 
     In embodiments of the invention the cross-brace spar is oriented such that: 
     a plane that passes through the first and second locations and is parallel to the thickness direction of the wing tip device at the first location is oriented at an oblique angle to a plane that passes through the first location and a location on the rear spar that is at a corresponding span wise position to the second location and is parallel to the thickness direction of the wing tip device at the first location; and 
     a plane that passes through the first and second locations and is parallel to the thickness direction of the wing tip device at the second location is oriented at an oblique angle to a plane that passes through the second location and a location on the front spar that is at a corresponding span-wise position to the first location and is parallel to the thickness direction of the wing tip device at the second location. 
     In embodiments of the invention the cross-brace spar is oriented such that: 
     a straight line that passes through the first and second locations is oriented at an oblique angle to a straight line that passes through the first location and a location on the rear spar that is at a corresponding span wise position to the second location, when the wing tip device is viewed looking along an axis that is parallel to the thickness direction of the wing tip device at the first location; and 
     a straight line that passes through the first and second locations is oriented at an oblique angle to a straight line that passes through the second location and a location on the front spar that is at a corresponding span-wise position to the first location, when the wing tip device is viewed looking along an axis that is parallel to the thickness direction of the wing tip device at the second location. 
     In embodiments of the invention the cross-brace spar extends along a longitudinal direction from a rear end, attached to the rear spar at the first location, to a front end attached to the front spar at the second location. Optionally the rear spar extends along a longitudinal direction from the first location to a location on the rear spar that is at a corresponding span wise position to the second location and the front spar extends along a longitudinal direction from the second location to a location on the front spar that is at a corresponding span wise position to the first location; 
     wherein at the first location the local longitudinal direction of the cross-brace spar is oblique to the local longitudinal direction of the rear spar when the wing tip device is viewed looking along an axis that is parallel to the thickness direction of the wing tip device at the first location; and 
     at the second location the local longitudinal direction of the cross-brace spar is oblique to the local longitudinal direction of the front spar when the wing tip device is viewed looking along an axis that is parallel to the thickness direction of the wing tip device at the second location. 
     It will be appreciated that reference to a ‘local’ longitudinal direction refer to the direction of a tangent to the longitudinal direction at that location. 
     In embodiments of the invention the oblique angles referred to above are the respective internal angles defined by the respective notional planes/lines, i.e. between adjacent and opposing faces of the respective planes. In embodiments of the invention these internal angles are acute angles, i.e. less than 90°. 
     Optionally the first location is an inboard location and the second location is an outboard location. In this respect, optionally the cross-brace spar links an inboard location on the rear spar to an outboard location on the front spar. Optionally the cross-brace spar is directly attached to the rear spar at the inboard location and is directly attached to the front spar at the outboard location. 
     In embodiments of the invention the longitudinal direction, of the cross-brace spar, extends outboard in the span wise direction as it extends forwards in the chord wise direction. 
     Optionally the cross-brace spar is curved. In this regard, the cross-brace spar may curve as it extends from its rear end to its front end. Accordingly the longitudinal direction of the cross-brace spar may be curved. 
     Optionally the wing tip device comprises a rib that links the front and rear spars and the cross-brace spar is linked to the rib. 
     In embodiments of the invention the cross-brace spar is linked to the rib such that it transmits aerodynamic loads between the front and rear spars and the rib. 
     The cross-brace spar may be directly attached to the rib. In this respect, the cross-brace spar may be directly attached to the rib part way along the length of the rib or to an end of the rib. 
     Alternatively, or additionally, the cross-brace spar may not be directly attached to the rib, but may be linked to the rib by its attachment to the front and/or rear spars. 
     Optionally the cross-brace spar is oriented such that it is oblique to the rib. 
     Optionally the wing tip device comprises an inboard rib and an outboard rib that each link the front and rear spars and wherein the cross-brace spar is linked to the inboard and outboard ribs. 
     In embodiments of the invention the cross-brace spar is linked to the inboard and outboard ribs such that the cross-brace spar transmits aerodynamic loads between the inboard and outboard ribs and the front and rear spars. 
     Preferably a rear end of the inboard rib is directly attached to the rear spar at the first location and a front end of the outboard rib is directly attached to the front spar at the second location. 
     Preferably the cross-brace spar is directly attached to the inboard rib and/or outboard rib. Preferably the cross-brace spar is directly attached to a rear end of the inboard rib at the inboard location and to a front end of the outboard rib at the outboard location. 
     Alternatively, or additionally, in respect of either or both of the inboard and outboard ribs, the cross-brace spar may not be directly attached to the rib but may be linked to the rib by its attachment to the front and/or rear spars. 
     Optionally the inboard and outboard ribs and the front and rear spars together form a structural box and the cross-brace spar extends diagonally across the structural box. 
     Preferably the cross-brace spar extends diagonally across the box from a corner of the box to a diagonally opposite corner of the box. Alternatively, the cross-brace spar may extend diagonally across the box from and/or to a non-corner position of the box. 
     Optionally the cross-brace spar extends from a location at which the inboard rib is attached to the rear spar to a location at which the outboard rib is attached to the front spar. 
     The cross-brace spar may be directly attached to the inboard rib, for example at the inboard location, and the cross-brace spar may be directly attached to the outboard rib, for example at the outboard location. 
     Optionally the wing tip device comprises an intermediate rib, positioned between the inboard and outboard ribs, and wherein the cross-brace spar is linked to the intermediate rib. In embodiments of the invention the cross-brace spar is linked to the intermediate rib such that it transmits aerodynamic loads between the intermediate rib, the inboard and outboard ribs and the front and rear spars. 
     Preferably the cross-brace spar is directly attached to the intermediate rib. 
     Alternatively, the cross-brace spar may not be directly attached to the intermediate rib but may be linked to the intermediate rib by its direct attachment to one or more of the front spar, rear spar, inboard rib and outboard rib. 
     Optionally the cross-brace spar extends through the intermediate rib. Optionally the cross-brace spar is directly attached to the intermediate rib at a position intermediate the front and rear ends of the cross-brace spar. 
     Optionally the cross-brace spar is oriented such that it is oblique to the, or each, rib. 
     Optionally the, or each, rib is substantially perpendicular to at least one of the front and rear spars. Optionally the, or each, rib is substantially perpendicular to one of the front and rear spars and is oriented at an oblique angle to the other of the front and rear spars. 
     Preferably the, or each, rib is substantially perpendicular to the rear spar and is oriented at an oblique angle to the forward spar. Alternatively, optionally the, or each rib is substantially perpendicular to the front spar and is oriented at an oblique angle to the rear spar. 
     Optionally the cross-brace spar is a single continuous spar. 
     Optionally the cross-brace spar is substantially continuous from its rear end to its front end. 
     Optionally the wing tip device is swept back. 
     Optionally the wing tip device has a high sweep angle. Optionally the wing tip device has a sweep angle in the range 30° to 60°. 
     The cross-brace spar is particularly advantageous when the wing tip device is highly swept back because this produces a high external yaw component of loading during high manoeuvre and gust conditions. This high degree of loading would otherwise be taken by the rear spar. Therefore the cross-brace spar decreases the loading on the rear spar. 
     Optionally the wing tip device extends upwardly as it extends outboard in the span wise direction. 
     Optionally the wing tip device is curved along its length in the span wise direction. 
     Optionally the wing tip device is twisted about a span wise axis of the wing tip device. 
     Optionally the wing tip device twists anti-clockwise when looking from the root to the tip of the wing tip device. In this regard, optionally the winglet twist is washed out. 
     In embodiments of the invention the wing tip device is configured to reduce induced drag on the wing. 
     In embodiments of the invention the wing tip device is fixed relative to the wing. 
     Optionally the wing tip device comprises a winglet. Alternatively, or additionally, the wing tip device may comprise a wing tip fence, a raked wing tip or a wing tip extension, for example. 
     Optionally the wing and the wing tip device form a wing assembly and the wing tip device is located within the outboard most 40% of the span wise extent of the wing assembly, preferably within the outboard most 30% of the span wise extent of the wing assembly, more preferably within the outboard most 20% of the span wise extent of the wing assembly, even more preferably within the outboard most 10% of the span wise extent of the wing assembly. 
     Optionally the wing is swept back. Optionally the wing has a high sweep angle. 
     In embodiments of the invention the aircraft comprises a pair of said wings and wing tip devices. The aircraft may be any air vehicle such as a manned aircraft or a UAV. More preferably the aircraft is a passenger aircraft. The passenger aircraft preferably comprises a passenger cabin comprising a plurality of rows and columns of seat units for accommodating a multiplicity of passengers. The aircraft may have a capacity of at least 20, more preferably at least 50 passengers, and more preferably more than 50 passengers. The aircraft is preferably a powered aircraft. The aircraft preferably comprises an engine for propelling the aircraft. The aircraft may comprise wing-mounted, and preferably underwing, engines. 
     According to a second aspect of the invention there is provided a wing assembly comprising a wing and a wing tip device at the tip of the wing, the wing tip device having front and rear spars, wherein the wing tip device comprises a cross-brace spar that links the front and rear spars and is oriented such that it is oblique to the front and rear spars. 
     Optionally the cross-brace spar links an inboard location on the rear spar to an outboard location on the front spar. 
     In embodiments of the invention the wing tip device of the wing assembly of the second aspect of the invention is for use as the wing tip device in the first aspect of the invention. The wing tip device of the wing assembly of the second aspect of the invention may have any, or all, of the features of the wing tip device in the first aspect of the invention. 
     According to a third aspect of the invention there is provided a wing tip device for use with a wing of an aircraft, the wing tip device having front and rear spars, wherein the wing tip device comprises a cross-brace spar that links the front and rear spars and is oriented such that it is oblique to the front and rear spars. 
     Optionally the cross-brace spar links an inboard location on the rear spar to an outboard location on the front spar. 
     In embodiments of the invention the wing tip device of the third aspect of the invention is for use as the wing tip device in the first aspect of the invention. The wing tip device of the third aspect of the invention may have any, or all, of the features of the wing tip device in the first aspect of the invention. 
     According to a fourth aspect of the invention there is provided method of manufacture of a wing tip device for use with a wing of an aircraft, the method comprising linking a front and rear spar of the wing tip device with a cross-brace spar that is oblique to the front and rear spars. 
     Optionally the cross-brace spar links an inboard location on the rear spar to an outboard location on the front spar. 
     According to a fifth aspect of the invention there is provided a method of manufacture of a wing assembly comprising a wing and a wing tip device at the tip of the wing, wherein the method comprises manufacturing a wing tip device according to the fourth aspect of the invention and attaching the wing tip device to a tip of a wing for an aircraft. 
     The front and rear spars of the wing tip device may be linked by the cross-brace spar before or after the wing tip device is attached to the tip of the wing. However, preferably the front and rear spars of the wing tip device are linked by the cross-brace spar before the wing tip device is attached to the tip of the wing. 
     According to a sixth aspect of the invention there is provided a method of manufacture of an aircraft comprising a wing and a wing tip device at the tip of the wing, wherein the method comprises manufacturing a wing assembly according to the fifth aspect of the invention and attaching the wing assembly to a fuselage of an aircraft. 
     The front and rear spars of the wing tip device may be linked by the cross-brace spar before or after the wing tip device is attached to the tip of the wing and before or after the wing is attached to the fuselage of the aircraft. However, preferably the front and rear spars of the wing tip device are linked by the cross-brace spar before the wing tip device is attached to the tip of the wing and before the wing is attached to the fuselage of the aircraft. 
     In embodiments of the invention the method comprises attaching the cross-brace spar to the inboard location on the rear spar and to the outboard location on the front spar. 
     According to a seventh aspect of the invention there is provided an aircraft comprising a wing and a wing tip device at the tip of the wing, the wing tip device having front and rear spars, and an inboard rib and an outboard rib that each link the front and rear spars, the inboard and outboard ribs and the front and rear spars together forming a structural box, wherein the wing tip device further comprises a structural brace support that links an inboard location on the rear spar to an outboard location on the front spar and extends diagonally across the structural box. 
     According to an eighth aspect of the invention there is provided a wing assembly comprising a wing and a wing tip device at the tip of the wing, the wing tip device having front and rear spars, and an inboard rib and an outboard rib that each link the front and rear spars, the inboard and outboard ribs and the front and rear spars together forming a structural box, wherein the wing tip device further comprises a structural brace support that links an inboard location on the rear spar to an outboard location on the front spar and extends diagonally across the structural box. 
     According to a ninth aspect of the invention there is provided a wing tip device for use with a wing of an aircraft, the wing tip device having front and rear spars, and an inboard rib and an outboard rib that each link the front and rear spars, the inboard and outboard ribs and the front and rear spars together forming a structural box, wherein the wing tip device further comprises a structural brace support that links an inboard location on the rear spar to an outboard location on the front spar and extends diagonally across the structural box. 
     According to a tenth aspect of the invention there is provided a method of manufacture of a wing tip device for use with a wing of an aircraft, the method comprising linking front and rear spars of the wing tip device with inboard and outboard ribs to form a structural box, wherein the method further comprises linking an inboard location on the rear spar to an outboard location on the front spar with a structural brace support that extends diagonally across the structural box. 
     According to an eleventh aspect of the invention there is provided a method of manufacture of a wing assembly comprising a wing and a wing tip device at the tip of the wing, wherein the method comprises manufacturing a wing tip device according to the tenth aspect of the invention and attaching the wing tip device to a tip of a wing for an aircraft. 
     According to a twelfth aspect of the invention there is provided a method of manufacture of an aircraft comprising a wing and a wing tip device at the tip of the wing, wherein the method comprises manufacturing a wing assembly according to the eleventh aspect of the invention and attaching the wing assembly to a fuselage of an aircraft. 
     The structural brace support may be a spar, preferably a cross-brace spar, for example. 
     In embodiments of the invention the structural brace support links the inboard location on the rear spar to the outboard location on the front spar such that it transmits aerodynamic loads between the front and rear spars and the inboard and outboard ribs. 
     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 any aspect of the invention may incorporate any of the features described with reference to the apparatus of any aspect of the invention and vice versa. 
     Other preferred and advantageous features of the invention will be apparent from the following description. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings of which: 
         FIG.  1    is a front view of an aircraft according to an embodiment of the invention; 
         FIG.  2    is a schematic plan view of a port wing  3  of the aircraft of  FIG.  1    (where the wing tip device is omitted for illustrative purposes); 
         FIG.  3    is a front perspective view of a wing tip device of the port wing assembly of the aircraft shown in  FIG.  1   ; 
         FIG.  4    is a side perspective view (looking inboard and rearward) of the wing tip device shown in  FIG.  3   ; 
         FIG.  5    is a perspective view (from above and looking inboard) of the wing tip device shown in  FIGS.  3  and  4   ; 
         FIG.  6    is schematic perspective view (from above and looking outboard) of the wing tip device shown in  FIGS.  3  to  5   , where the upper skin of the wing tip device has been omitted for illustrative purposes and showing a pair of notional geometric planes (P 1 , P 2 ) defined by a cross-brace spar and a rear spar of the wing tip device; 
         FIG.  7    is a view corresponding to that of  FIG.  6   , but showing a pair of notional geometric planes (P 1 ′, P 3 ) defined by the cross-brace spar and a front spar of the wing tip device; 
         FIG.  8   a    is a view of the notional geometric planes (P 1 , P 2 ) in  FIG.  6    when viewed looking along the thickness direction (t) of the winglet  4  (from above) at an inboard location L 1 ; 
         FIG.  8   b    is a view of the notional geometric planes (P 1 ′, P 3 ) in  FIG.  7    when viewed looking along the thickness direction (t) of the winglet  4  (from above) at the outboard location L 2 , and 
         FIG.  9    is a flow-chart showing the steps of a method of manufacture of an aircraft, according to a further embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    illustrates an aircraft  1  according to an embodiment of the invention. The aircraft  1  is a commercial jet aircraft that comprises a pair of wing assemblies  2  attached on opposite sides of a fuselage  50 . The aircraft  1  is a passenger aircraft comprising a passenger cabin comprising a plurality of rows and columns of seat units for accommodating a multiplicity of passengers, in this case more than 50 passengers. The aircraft  1  is a powered aircraft and comprises a pair of engines, mounted under the wing assemblies  2 , for propelling the aircraft  1 . 
     Each wing assembly  2  comprises a wing  3  and a wing tip device in the form of a winglet  4  attached to the outboard tip of the wing  3 . The winglet  4  is fixed relative to the wing  3 . 
     Referring to  FIG.  2   , the wing  3  extends outboard in a span wise direction, from an inboard end  61 , attached to the fuselage  50 , to a tip  62 . The wing  3  also extends in a chord wise direction from a trailing edge  64  to a leading edge  63 . The wing  3  is swept back. It will be appreciated that  FIG.  2    is a schematic view and is not to scale. 
     Referring to  FIGS.  3  to  7   , the winglet  4  extends outboard in a span wise direction (S), from an inboard end  7 , attached to the outboard tip  62  of the wing  3 , to a tip  8 . The winglet  4  also extends in a chord wise direction (C) from a trailing edge  10  to a leading edge  9 . The winglet  4  comprises upper and lower skins  11 ,  12  (see  FIG.  5   ). The upper and lower skins  11 ,  12  provide the aerodynamic surfaces that are exposed to the air flow during flight. The winglet  4  extends in a thickness direction (t) (see  FIG.  3   ) between the upper and lower wing skins  11 ,  12 . It will be appreciated that references to the thickness direction (t) refer to the local thickness direction. 
     The winglet  4  is slightly cambered. 
     The winglet  4  curves upwardly as it extends outboard in the span wise direction S. The curvature of local dihedral increases from a low angle, or an angle of approximately zero, at or near the inboard end  7  of the winglet  4  and increases in the outboard direction. The outboard end of the winglet  4  is almost vertical but is inclined at a small angle with respect to the vertical plane. 
     The winglet  4  is swept back. In the currently described embodiment the wing tip device is swept back by a sweep angle (X) of 35° (see  FIG.  6   ). 
     The winglet  4  is twisted about a span wise axis of the winglet  4 . The winglet twist is washed out. In this regard, the winglet twists anti-clockwise when looking from the inboard end  7  to the outboard tip  8  of the wing tip device  4 . 
     The leading and trailing edges  9 ,  10  of the winglet  4  are continuations of the leading and trailing edges of wing  3 . Furthermore, the upper and lower surfaces of the winglet  4  are continuations of the upper and lower surfaces of the wing  3 . Thus there is a smooth transition, from the wing  3  to the winglet  4 . 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 wing  3  and winglet  4 . However, there are preferably no discontinuities at the junction between the wing  3  and winglet  4 . 
     The winglet  4  is configured to reduce the induced drag on the wing assembly  2  (and on the aircraft  1  as a whole), leading to improve fuel efficiency and reduced carbon emissions. 
     The winglet  4  comprises rear and front spars  13 ,  14  (see  FIGS.  6  and  7   ) that are spaced in the chord wise direction C of the winglet  4 . It will be appreciated that in relation to the rear and front spars  13 ,  14 , the terms ‘rear’ and ‘front’ are relative to each other, i.e. the front spar is forward of the rear spar, in the chord wise direction of the winglet  4  (and vice versa). In the described embodiment, the rear and front spars  13 ,  14  are the rear-most and front-most spars of the winglet  4  respectively. Alternatively, there may be additional spars in front of the front spar  14  and behind the rear spar  13 . However, preferably the rear and front spars  13 ,  14  are the rear-most and front-most spars of the winglet  4  respectively. 
     The rear and front spars  13 ,  14  are located proximal the trailing and leading edges  10 ,  9  of the winglet  4  respectively. Each spar  13 ,  14  extend outboard, in the span wise direction, from the inboard end  7  of the winglet  4 . Each of the rear and front spars  13 ,  14  has a generally C-shaped cross-sectional shape, where the C-shape faces inwardly (i.e. towards the centre-line of the winglet  4 ). However it will be appreciated that the rear and front spars  13 ,  14  may have other cross-sectional shapes, including I-shaped, H-shaped, square or rectangular cross-sectional shapes, for example. 
     The rear and front spars  13 ,  14  are located internally of the upper and lower skins  11 ,  12  and are attached (e.g. by bonding or fasteners) to the upper and lower wing skins  11 ,  12  to increase the rigidity of the wing skins  11 ,  12 . 
     Each of the rear and front spars  13 ,  14  extends from an inboard end  13 A,  14 A, to an outboard end  13 B,  14 B along a longitudinal direction  13 C,  14 C. Each of the rear and front spars  13 ,  14  twist anticlockwise slightly (when looking from their inboard end to the outboard end) and curve upwardly, as they extend outboard, in order to match the shape of twist and upward curvature of the winglet  4 . Since the wing tip device  4  is swept back, each of the rear and front spars  13 ,  14  is swept back accordingly. The front spar  14  has a slightly higher sweep angle than the rear spar  13 . 
     The inboard ends  13 A,  14 A of the rear and front spars  13 ,  14  are respectively attached to a winglet root rib  51  (shown schematically in  FIGS.  6  and  7   , for illustrative purposes), which is attached to outboard ends of rear and front spars  40 ,  41  of the wing  3  (shown schematically in  FIGS.  6  and  7   ), which form part of the wing box of the wing  3 . Accordingly the rear and front spars  13 ,  14  of the winglet  4  are attached to the rear and front spars  40 ,  41  of the wing  3 , via the winglet root rib  51 . In this respect, the rear and front spars  13 ,  14  of the winglet  4  are attached to the wing box of the wing  3 . The rear and front spars  13 ,  14  are configured to transmit aerodynamic loads into the wing  3 , specifically into the wing box of the wing  3 . 
     The winglet  4  also comprises an inboard rib  15 , an outboard rib  17  and an intermediate rib  16  positioned between the inboard rib  15  and the outboard rib  17 , in the span wise direction. It will be appreciated that the terms ‘inboard’ and ‘outboard’ are relative to each other, i.e. the inboard rib  15  is inboard, in the span wise direction (S), of the outboard rib  17  (and vice versa). There may be additional ribs inboard of the inboard rib  15  and outboard of the outboard rib  17 , for example. In this respect, in the currently described embodiment there are a plurality of ribs (not shown) that are outboard of the outboard rib  17  and the winglet root rib  51  is inboard of the inboard rib  15 . The inboard rib  15  is located adjacent to the winglet root rib  51 . 
     The ribs  15 ,  16 ,  17  are located internally of the upper and lower wing skins  11 ,  12  and are attached (e.g. by bonding or fasteners) to the upper and lower wing skins  11 ,  12  to increase the rigidity of the wing skins  11 ,  12 . 
     Each rib  15 ,  16 ,  17  is substantially perpendicular to the rear spar  13  (at the location of attachment of the rib to the rear spar  13 ) and is oriented at an oblique angle to the front spar  14 . 
     Each rib  15 ,  16 ,  17  structurally links the rear and front spars  13 ,  14  together. In this regard, for each rib  15 ,  16 ,  17 , rear and front ends of the rib are directly attached to the rear and front spars  13 ,  14  respectively. 
     In this respect, a rear end of the inboard rib  15  is directly attached to the rear spar  13  at the inboard location L 1  and a front end of the outboard rib  17  is directly attached to the front spar  14  at the outboard location L 2 . 
     In the currently described embodiment this attachment is by bolts that pass through suitable brackets (not shown), with the bolts being fastened by suitable nuts. However it will be appreciated that any suitable form of attachment may be used. 
     Each rib  15 ,  16 ,  17  is provided with a plurality of circular cut-outs  33  along its length, to reduce its weight. 
     The ribs  15 ,  16 ,  17  are configured to transmit aerodynamic loads between the rear and front spars  13 ,  14 . Each rib  15 ,  16 ,  17  is arranged to increase the stiffness of the winglet  4  in the direction of the depth of the winglet  4  and to increase the stiffness of the winglet  4  in the chord wise direction (i.e. to prevent fore-aft deformation). Each rib  15 ,  16 ,  17  is also arranged to increase the stiffness of the winglet  4  in the direction of twisting of the winglet  4  about the span wise direction in which the winglet  4  extends. In this regard, each rib  15 ,  16 ,  17  is arranged to maintain the camber, i.e. the profile shape, of the winglet  4 . 
     Referring to  FIGS.  6  and  7   , the winglet  4  further comprises a cross-brace spar  18 . The cross-brace spar  18  is oriented such that it links an inboard location L 1  on the rear spar  13  to an outboard location L 2  on the front spar  14 . The outboard location L 2  is spaced forward of the inboard location L 1  in the chord wise direction C of the winglet  4 . 
     It will be appreciated that in relation to the inboard and outboard locations L 1 , L 2 , the terms ‘inboard’ and ‘outboard’ are relative to each other, i.e. the inboard location is inboard, in the span wise direction (S) of the winglet  4 , of the outboard location (and vice versa). 
     It will be appreciated that a ‘cross-brace’ spar refers to any type of spar that extends in both the chord wise and span wise directions of the wing tip device  4  and links the rear and front spars  13 ,  14  such that it transmits aerodynamic loads (e.g. force and/or torque) between the rear and front spars  13 ,  14 . The cross brace spar  18  is a structural brace support. 
     The cross-brace spar  18  is elongate and extends from a rear end  30 , directly attached to the rear spar  13  at the inboard location L 1 , to a front end  31 , directly attached to the front spar  14  at the outboard location L 2 . 
     In this respect, at each of the inboard and outboard locations L 1 , L 2 , the respective end of the cross-brace spar  18  is directly attached to the respective spar  13 ,  14  by bolts that pass through suitable brackets (not shown) on the cross-brace spar  18  and into the spar  13 ,  14 , with the bolts being fastened by suitable nuts. However, it will be appreciated that any suitable type of attachment, may be used. 
     The cross-brace spar  18  extends from its rear end  30  to its front end  31  along a longitudinal direction  18 C. The cross-brace spar  18  curves as it extends from its rear end  30  to its front end  31 . In this respect, the cross-brace spar  18  curves when looking at the winglet  4  in plan form. Accordingly, the longitudinal direction  18 C of the cross-brace spar  18  is curved. The cross-brace spar  18  also extends upwardly, as it extends from its rear end  30  to its front end  31  (i.e. its longitudinal direction  18 C extends upwardly). In the currently described embodiment the cross-brace spar  18  is not twisted (about its longitudinal direction  18 C). 
     The cross-brace spar  18  has a substantially rectangular cross-sectional shape (about the longitudinal direction  18 C). The cross-brace spar  18  is substantially solid. However, it will be appreciated that the cross-brace spar may have any suitable cross-sectional shape (including an I-shaped, H-shaped or C-shaped cross-sectional shape, for example). 
     The cross-brace spar  18  is made of CFRP. However, it will be appreciated that any suitable material may be used, including a composite or metallic material, for example. 
     The cross-brace spar  18  is located internally of the upper and lower wing skins  11 ,  12  of the winglet  4 . The cross-brace spar  18  is attached (e.g. by bonding or fasteners) to the upper and lower wing skins  11 ,  12  to increase the rigidity of the wing skins  11 ,  12 . 
     The cross-brace spar  18  is oriented such that as it extends forwards in the chord wise direction (C), it extends outboard in the span wise direction (S). In this regard, as its longitudinal direction  18 C extends from the rear end  30  to the front end  31  of the cross-brace spar  18 , the longitudinal direction  18 C extends forwards in the chord wise direction (C) and outboard in the span wise direction (S). 
     It will be appreciated that the ‘forward’ chord wise direction refers to the chord wise direction (C) in the direction from the rear spar  13  to the front spar  14 . 
     The cross-brace spar  18  is configured to transmit aerodynamic loads between the rear and front spars  13 ,  14 . The cross-brace spar  18  is configured to provide a primary load path between the rear and front spars  13 ,  14 . The aerodynamic loads may be force and/or torque. 
     The cross-brace spar  18  is oriented such that it is oblique to the front and rear spars  14 ,  13 . It will be appreciated that the term ‘oblique’ refers to any inclined orientation that is not perpendicular (and not parallel). 
     In this regard, the cross-brace spar  18  is oblique to the rear spar  13  in that that a plane (P 1 —see  FIG.  6   ) that passes through the inboard and outboard locations L 1 , L 2  and is parallel to the thickness direction (t) of the winglet  4  at the inboard location L 1  is oriented at an oblique angle (Ø) to a plane (P 2 —see  FIG.  6   ) that passes through the inboard location L 1  and a location (A 1 ) on the rear spar  13  that is at a corresponding span wise position to the outboard location L 2  (on the front spar  14 ) and is parallel to the thickness direction (t) of the winglet  4  at the inboard location L 1 .  FIG.  8   a    shows a view of these planes P 1 , P 2  when viewed looking along the thickness direction (t) of the winglet  4  (from above) at the inboard location L 1 . 
     In the currently described embodiment the oblique angle (Ø) is approximately 40°. However, it will be appreciated that any oblique angle may be used. 
     The cross-brace spar  18  is oblique to the front spar  14  in that a plane (P 1 ′—see  FIG.  7   ) that passes through the inboard and outboard locations L 1 , L 2  and is parallel to the thickness direction (t) of the winglet  4  at the outboard location L 2  is oriented at an oblique angle (W) to a plane (P 3 —see  FIG.  7   ) that passes through the outboard location L 2  and a location (A 2 ) on the front spar  14  that is at a corresponding span-wise position to the inboard location L 1  (on the rear spar  13 ) and is parallel to the thickness direction (t) of the winglet  4  at the outboard location L 2 .  FIG.  8   b    shows a view of these planes P 1 ′, P 3  when viewed looking along the thickness direction (t) of the winglet  4  (from above) at the outboard location L 2 . 
     In the currently described embodiment the oblique angle (W) is approximately 40°. However, it will be appreciated that any oblique angle may be used. 
     In this regard, it will be appreciated that a straight line that passes through the inboard and outboard locations L 1 , L 2  is oriented at an oblique angle (Ø) to a straight line that passes through the inboard location L 1  and the location A 1  on the rear spar  13  that is at a corresponding span wise position to the outboard location L 2 , when the winglet  4  is viewed looking along an axis that is parallel to the thickness direction (t) of the winglet  4  at the inboard location L 1 . 
     Similarly, a straight line that passes through the inboard and outboard locations L 1 , L 2  is oriented at an oblique angle (W) to a straight line that passes through the outboard location L 2  and a location A 2  on the front spar  14  that is at a corresponding span-wise position to the inboard location L 1 , when the winglet  4  is viewed looking along an axis that is parallel to the thickness direction (t) of the winglet  4  at the outboard location L 2 . 
     At the inboard location L 1  the local longitudinal direction  18 C of the cross-brace spar  18  is oblique to the local longitudinal direction  13 C of the rear spar  13  when the winglet  4  is viewed looking along an axis that is parallel to the thickness direction (t) of the winglet  4  at the inboard location L 1 . 
     Also, at the outboard location L 2  the local longitudinal direction  18 C of the cross-brace spar  18  is oblique to the local longitudinal direction  14 C of the front spar  14  when the winglet  4  is viewed looking along an axis that is parallel to the thickness direction (t) of the winglet  4  at the outboard location L 2 . 
     It will be appreciated that a reference to a ‘local’ longitudinal direction refers to the direction of a tangent to the longitudinal direction at that location. 
     The oblique angles ( 0  and W) referred to above are the respective internal angles defined by the respective notional planes/lines, i.e. between adjacent and opposing faces (adjacent and opposing in the chord wise direction) of the respective planes (P 1 , P 2 ) (P 1 ′, P 3 ). These internal angles are acute angles, i.e. less than 90°. 
     The rear end of the inboard rib  15  is also attached (in addition to its attachment to the rear spar  13 ) to the rear end  30  of the cross-brace spar  18 , at the inboard location L 1 . This attachment is by a corner bracket and bolts (not shown). However, it will be appreciated that any suitable type of fastener, or any suitable type of attachment, may be used. 
     Similarly, the front end of the outboard rib  17  is also attached (in addition to its attachment to the front spar  14 ) to the front end  31  of the cross-brace spar  18 , at the outboard location L 2 . This attachment is by a corner bracket and bolts (not shown). However, it will be appreciated that any suitable type of fastener, or any suitable type of attachment, may be used. 
     The cross-brace spar  18  passes through the intermediate rib  16  and is attached to the intermediate rib  16  part way along the length of the intermediate rib  16  (i.e. between the ends of the intermediate rib  16 ) This attachment is by bolts and a suitable bracket (not shown). However, it will be appreciated that any suitable type of attachment may be used. 
     The cross-brace spar  18  is a single continuous spar. In this respect, it is not formed of discrete sections that are connected together (e.g. discrete sections separated by the intermediate rib  16 ). In this respect, the cross-brace spar  18  passes uninterrupted through a slot in the intermediate rib  16 . 
     The cross-brace spar  18  is oriented at an oblique angle to each rib  15 ,  16 ,  17 . 
     The inboard rib  15 , the outboard rib  17  and the portions of the rear and front spars  13 ,  14  that extend between the inboard and outboard ribs  15 ,  17  together form a winglet box  42  (i.e. a structural box) and the cross-brace spar  18  extends diagonally across the winglet box  42 , from one corner of the winglet box  42 , at the inboard location L 1 , to a diagonally opposite corner of the winglet box  42 , at the outboard location L 2 . 
     The cross-brace spar  18  structurally links each of the ribs  15 ,  16 ,  17  and the rear and front spars  13 ,  14  together. The cross-brace spar  18  is configured to transmit aerodynamic loads between the rear and front spars  13 ,  14  and the ribs  15 ,  16 ,  17 . 
     The cross-brace spar  18  is advantageous in that it may transmit aerodynamic loads between the rear and front spars  13 ,  14 . This may, in particular, reduce the load on the rear spar  13 . Furthermore, the cross-brace  18  spar may help to provide a more balanced load path into the wing  3  (i.e. into a wing box of the wing  3 ). The cross-brace spar  18  may allow the winglet  4  to more effectively resist external aerodynamic loads, for example during gust or manoeuvre. 
     The cross-brace spar  18  may allow for a reduction in the number of ribs that would otherwise be required in order to resist aerodynamic loads. In this respect, the cross-brace spar  18  may advantageously react the brazier load that would usually be carried by the ribs. Similarly, the cross-brace spar  18  may allow for the reduction in the number of other components that may otherwise be required in order to resist the aerodynamic loads, for example stringers and rib posts. The cross-brace spar  18  may allow for a reduction in the required thickness of the wing assembly, as well as a reduction in the number of bolts required to transfer load, at the connection of the winglet  4  to the wing  3 . 
     The cross-brace spar  18  is particularly advantageous when the winglet  4  is highly swept because a highly swept winglet  4  produces a high external yaw component of loading during high manoeuvre and gust conditions. This high degree of loading would otherwise be taken by the rear spar  13 . Therefore the cross-brace spar  18  decreases the loading on the rear spar  13 . 
     In view of the above, the cross-brace spar  18  may facilitate a reduction in weight of the winglet  4  and therefore of the aircraft  1 , thereby providing performance advantages. Due to the reduction in parts required, the cross-brace spar  18  may also provide a reduction in manufacturing time and cost. 
     Referring to  FIG.  9   , there is shown a flowchart illustrating a method  1000  of manufacture of an aircraft  1  (i.e. the aircraft  1  of the above described embodiment), comprising a wing  3  and a winglet  4  at the tip of the wing  3 , according to a further embodiment of the invention. 
     The method comprises the step ( 1001 ) of manufacturing a winglet  4 , which comprises linking an inboard location L 1  on a rear spar  13  of the winglet  4  to an outboard location L 2  on a front spar  14  of the winglet  4  with a cross-brace spar  18  that is oblique to the front and rear spars  14 ,  13 . 
     The winglet  4  is then attached to a tip of an aircraft wing  3 , to form a wing assembly  2  (step  1002 ). The wing assembly  2  is then attached to an aircraft fuselage  50  to form an aircraft  1  (step  1003 ) (it will be appreciated that a pair of wing assemblies  2  are attached to the fuselage to form a complete aircraft  1 ). 
     The rear and front spars  13 ,  14  of the winglet  4  may be linked by the cross-brace spar  18  before or after the winglet  4  is attached to the tip of the wing  3 . However, preferably the rear and front spars  13 ,  14  of the winglet  4  are linked by the cross-brace spar  18  before the winglet  4  is attached to the tip of the wing  3 . 
     The rear and front spars  13 ,  14  of the winglet  4  may be linked by the cross-brace spar  18  before or after the wing  3 , or the wing assembly  2 , is attached to the fuselage  50  of the aircraft. However, preferably the rear and front spars  13 ,  14  of the winglet  4  are linked by the cross-brace spar  18  before the wing  3 , or the wing assembly  2 , is attached to the fuselage of the aircraft  1 . 
     The method ( 1000 ) may comprise linking the front and rear spars  14 ,  13  with the inboard and outboard ribs  15 ,  17  such that the inboard and outboard ribs  15 ,  17  and the front and rear spars  14 ,  13  together form the structural box  42 . The cross-brace spar  18  may form a structural brace support that extends diagonally across the structural box  42 . 
     Unless otherwise stated, references to the wing  3  and winglet  4  are in relation to their in-flight shape. 
     Unless otherwise stated, where ‘or’ has been referred to this means ‘and/or’. 
     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. 
     For example, in a reverse orientation, the cross-brace spar  18  may be oriented such that it links an outboard location on the rear spar  13  to an inboard location on the front spar  14  (with the method of manufacture altered accordingly, i.e. to link an outboard location on the rear spar  13  to an inboard location on the front spar  14 ). However, it is preferred that the cross-brace spar  18  is oriented such that is links an inboard location L 1  on the rear spar  13  to an outboard location L 2  on the front spar  14  (as in the described embodiment) as this reduces the load on the rear spar  13 . 
     The cross-brace spar  18  may not be directly attached to the front and rear spars  13 ,  14 , for example it may instead be directly attached to the ribs  15 ,  16 ,  17  such that it links the front and rear spars  13 ,  14 . However, it is preferred that the cross-brace spar  18  is directly attached to the front and rear spars  13 ,  14 , as this provides a more continuous load path. 
     In the described embodiment, the inboard and outboard ribs  15 ,  17  are directly attached to the cross-brace spar  18  (at said ends of the ribs  15 ,  17 ). Alternatively, the inboard and outboard ribs  15 ,  17  may not be directly attached to the cross-brace spar  18 , but may be linked to the cross-brace spar  18  by being directly attached to the front and rear spars  13 ,  14  (at different positions along the spars  13 ,  14  to the cross-brace spar  18 ). 
     The shape of the cross-brace spar  18  may vary. In the currently described embodiment the cross-brace spar  18  curves as it extends from its rear end  30  to its front end  31 . Accordingly, its longitudinal direction  18 C is curved. Alternatively, the cross-brace spar may be substantially straight. In this regard, the cross-brace spar may have a longitudinal axis that is substantially straight. In the currently described embodiment the cross-brace spar  18  extends upwardly, as it extends from its rear end  30  to its front end  31 . Alternatively it may be substantially horizontal, or it may curve upwardly as it extends in the outboard direction, for example. The cross-brace spar may twist (about its longitudinal direction  18 C) or it may not twist. 
     In the described embodiment the cross-brace spar  18  is continuous. Alternatively, it may comprise a plurality of sections, for example a plurality of length wise sections. In this respect, where the wing tip device comprises a plurality of ribs, the cross-brace spar may be divided into a plurality of length wise sections by the ribs. 
     The winglet  4  may comprise a different number and/or arrangement of ribs. In this respect, the winglet  4  may comprise no ribs (with the cross-brace spar providing all of the necessary structural link between the front and rear spars  14 ,  13 ). 
     In the currently described embodiment the wing tip device is a winglet  4 . Alternatively, or additionally, the wing tip device may comprise a wing tip fence, a raked wing tip or a wing tip extension, for example. 
     The angle of sweep of the winglet  4  may be varied. In this respect, the winglet  4  preferably has a sweep angle (X) in the range 30° to 60°. As stated above, the cross-brace spar  18  is particularly advantageous when the winglet  4  is highly swept back. Alternatively, the winglet  4  may not be swept. 
     The twist of the winglet  4  may be varied. For example, the winglet twist may be washed in. 
     The aircraft  1  may be any air vehicle such as a manned aircraft or a UAV, for example. 
     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. 
     While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.