Patent Publication Number: US-9889920-B2

Title: Actuation assembly for moving a wing tip device on an aircraft wing

Description:
RELATED APPLICATIONS 
     The present application claims priority from Great Britain Application Numbers GB 1502661.0, filed Feb. 17, 2015, GB 1502663.6, filed Feb. 17, 2015, GB 1503980.3, filed Mar. 9, 2015 and GB 1520288.0, filed Nov. 18, 2015, the disclosures of which are hereby incorporated by reference herein in their entireties. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to aircraft wings and wing tip devices, and more specifically, but not exclusively, to aircraft wings with moveable wing tip devices, actuation assemblies for use in such wings, aircraft incorporating such wings, and to methods of moving a wing tip device. 
     There is a trend towards increasingly large passenger aircraft, for which it is desirable to have correspondingly large wing spans. However, the maximum aircraft span is effectively limited by airport operating rules which govern various clearances required when manoeuvring around the airport (such as the span and/or ground clearance required for gate entry and safe taxiway usage). 
     Moveable wing tip devices have been suggested for use on passenger aircraft, where a wing tip device is movable between a flight configuration for use during flight, and a ground configuration for use during ground-based operations. In the ground configuration, the wing tip device is moved away from the flight configuration such that the span of the aircraft wing is reduced, thereby allowing use of existing gates and safe taxiway usage. By way of example, US 2013/0292508 discloses an arrangement in which the wing tip device is rotatable about a hinge located on the fixed (inner) wing. Other arrangements, such as that disclosed in WO2011/051699, enable a more complex movement of the wing tip device. 
     Aircraft having moveable wing tip devices, must be suitable for flight when the wing tip device is in the flight configuration yet must also be able to move the wing tip device, during ground-based operations. This can pose some difficulties: 
     Firstly, in the flight configuration it tends to be desirable to have a seal between the wing tip device and the fixed wing to ensure smooth airflow in this region and to minimise drag losses. However, if seals are used, they tend to be susceptible to significant wear during movement between the flight and the ground configurations. 
     Secondly, it is necessary for flight loads on the wing tip device to be adequately transferred into the main wing. Providing an arrangement in which the flight loads can be adequately transferred, whilst still enabling the wing tip device to be moved from that flight configuration to the ground configuration when required, can pose significant design difficulties. 
     There are also other technical challenges in providing a practical arrangement for moving a wing tip device between the flight and ground configurations. Amongst the issues to be addressed are: the problem of providing a safe and reliable arrangement to enable such movement without impacting unduly on the design of the wing; and the problem of providing a compact and lightweight drive to effect the movement of the wing tip device. 
     The present invention seeks to mitigate at least some of the above-mentioned problems. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided an aircraft wing comprising a fixed wing and a wing tip device at the tip thereof, wherein the wing tip device is configurable between: (i) a flight configuration for use during flight and (ii) a ground configuration for use during ground-based operations, in which ground configuration the wing tip device is moved away from the flight configuration such that the span of the aircraft wing is reduced. The wing comprises an actuation assembly for moving the wing tip device between the flight and the ground configuration, and wherein the actuation assembly is arranged to move the wing tip device from the flight configuration to the ground configuration in a two-stage movement, the two-stage movement comprising a first stage in which the wing tip device is translated away from the flight configuration in a linear movement only, and a second, subsequent stage, in which the wing tip device is rotated to the ground configuration. 
     A two stage movement, particularly a two stage movement in which the first stage is a linear movement only, has been found to be particularly beneficial. For example, having a first stage that is only a linear movement, tends to reduce the wear on any sealing arrangement between the fixed wing and the wing tip device, because it tends to avoid relatively rotation occurring between sealing surfaces. Alternatively or additionally, having a first stage that is only a linear movement, may facilitate engagement between the fixed wing and the wing tip device to ensure an effective load-transfer arrangement. 
     In principle, the second stage of movement may comprise some translational movement in combination with the rotational movement. In preferred embodiments of the invention however, the wing tip device, in the second stage of movement, undergoes substantially no translational movement. In that second stage of movement, the wing tip device is preferably rotated in a rotational movement only. Such an arrangement has been found to be beneficial because it tends to reduce the complexity of the actuation assembly. Furthermore, such an arrangement tends to maximise the span reduction that can occur in the ground configuration (more translational movement my unnecessarily increase span (or at least may offset some of the reduction in span caused by the rotation)). 
     In the flight configuration the trailing edge of the wing tip device is preferably a continuation of the trailing edge of the fixed wing. The leading edge of the wing tip device is preferably a continuation of the leading edge of the fixed wing. There is preferably a smooth transition from the fixed wing to the wing tip device. 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 inner wing and wing tip device. However, there are preferably no discontinuities at the junction between the fixed wing and wing tip device. The upper and the lower surfaces of the wing tip device may be continuations of the upper and lower surfaces of the fixed wing. 
     The first stage of movement is preferably arranged to separate the wing tip device from the fixed wing. When separated, the upper and lower surfaces of the wing tip device are preferably no longer continuations of the respective upper and lower surfaces of the fixed wing. The respective upper and lower surfaces are preferably separated such that they no longer abut, or otherwise contact, each other. The respective upper and lower surfaces are preferably separated such that they are spaced apart in the direction of movement of the first stage. 
     In the flight configuration the fixed wing and the wing tip device may abut along a sealed interface. Such an arrangement tends to be aerodynamically desirable. The first stage of movement is preferably arranged to separate the wing tip device from the fixed wing to break the seal of the sealed interface. The present invention recognises that by breaking the seal using a translational (only) movement, excessive wear on the seal tends to be avoided. 
     One of the fixed wing and the wing tip device may comprise a plurality of male members, such as spigots. The other of the fixed wing and the wing tip device may comprise a plurality of corresponding female members, such as bushes. The female members may be arranged to receive the male members when the wing tip device is in the flight configuration, such that flight loads may be transferred, via the male members, from the wing tip device into the fixed wing. Such an arrangement tends to be beneficial because it enables flight loads to be adequately managed and reacted into the fixed wing. 
     The male and female members may be arranged to transfer bending moments into the fixed wing. The male and female members may be arranged to transfer vertical and/or forward/aft shear loadings into the fixed wing. The male and female members may be arranged such that they are substantially unable to transfer inboard/outboard loads (along the span of the wing) into the fixed wing. For example, the male and female members may be arranged such that they are substantially unable to transfer loads in a direction along the mean chord line, into the fixed wing. 
     The longitudinal axes of the male members may extend in a first direction. The actuation assembly may be arranged such that the first stage of movement is a translation in the first direction only. Such an arrangement has been found to be beneficial because it minimises the wear and/or out-of-plane forces, acting on the male member during movement from the flight to the ground configuration. 
     The actuation assembly may be arranged such that at the end of the first stage of movement, the male members have been removed from the corresponding female members. Such an arrangement may be beneficial because it ensures any rotational movement (in the second stage) only occurs once the male members are clear of the female members. It will be appreciated that the male members may be removed from the female members by moving the male members relative to fixed female members, moving the female members relative to fixed male members, or moving both the male and female members relative to one another. 
     The first stage of movement is preferably a translation in an outboard direction along the length of the wing. The outboard direction is preferably substantially parallel to a main spar of the fixed wing. The outboard direction may be substantially perpendicular to the ribs of the fixed wing. 
     The tip of the fixed wing may comprise an end-surface. The end-surface may, for example, be a notional cut between the fixed wing and the wing tip device. The wing tip device may comprise a corresponding end-surface. The end-surface of the wing tip device is preferably at the inner end of the wing tip device. The end-surface of the wing tip device preferably abuts the end surface of fixed wing. The first stage of movement may be a translation in a direction perpendicular to the end surfaces of the fixed wing and/or wing tip device. 
     The movement between the flight and the ground configurations is a two-stage movement. It may, in some circumstances, be appropriate to consider the wing tip device to have been moved to an intermediate configuration after the first stage of movement has been completed (but before the second stage of movement has commenced). In the intermediate configuration the wing tip device is preferably displaced, in a linear movement only, relative to the flight configuration. 
     The actuation assembly may comprise a hinge about which the wing tip device is rotatable during the second stage of movement. In some embodiments of the invention, the wing tip device may be prevented from rotating during the first stage of movement, but be free to rotate during the second stage of movement. For example, the wing, and more preferably the actuation assembly, may comprise a rotational stop feature arranged to prevent rotation of the wing tip device, about the hinge, during the first stage of movement, whilst allowing rotation of the wing tip device about the hinge during the second stage of movement. The rotational stop feature may comprise a protrusion associated with the wing tip device (and more preferably associated with a part of the wing tip device in the hinge) and an abutment member associated with the fixed wing. The abutment member and protrusion are preferably arranged such that during the first stage of movement, relative rotation between the protrusion and the abutment member is prevented. 
     The actuation assembly may comprise a sliding chassis, slideably moveable relative to the fixed wing. The sliding chassis may be coupled to the wing tip device. The sliding chassis may comprise, or otherwise be associated with, the hinge. The hinge may be fixed, relative to the sliding chassis. The actuation assembly may be arranged such that, during the first stage of movement, the sliding chassis is arranged to translate, thereby creating a linear movement of the wing tip device (for example via the linear movement of the hinge). 
     The actuation assembly may comprise a translational stop feature, arranged to limit the extent of the translational movement, away from the flight configuration. The translational stop feature may be arranged to limit the extent of the translational movement of the sliding chassis. 
     In embodiments of the invention comprising both a rotational stop feature and a translational stop feature, the actuation assembly may be arranged such that, during movement from the flight configuration to the ground configuration, the translational stop feature limits the extent of translational movement of the sliding chassis, at substantially the same time as the rotational stop feature ceases to prevent rotation of the wing tip device about the hinge and instead allows rotation of the wing tip device about the hinge. Such an arrangement may ensure a clear distinction between the two stages of movement. 
     The sliding chassis may contain an articulation mechanism. The articulation mechanism may be arranged to transfer an actuation force acting in a first direction, and acting at a first location, into an actuation force acting in a second direction, and acting at a second location. The actuation force is preferably provided by an actuator (discussed in more detail below). The second direction and second location are preferably such that the wing tip device is urged to rotate. In embodiments comprising a rotational stop feature, the rotational stop feature may prevent the wing tip device actually rotating, unless or until the first stage of movement is complete—thus the articulation mechanism may act as a rigid link whilst the rotational stop feature is preventing rotation of the wing tip device. 
     The articulation mechanism may be arranged to transfer the actuation force into the wing tip device at a location remote from the hinge, thereby creating a moment arm to rotate the wing tip device. For example, the second location may be remote from the hinge and the second direction may be such that the force is offset from the hinge. 
     In some embodiments of the invention, the articulation mechanism comprises a master bell crank and a slave link. The master bell crank may be connected to the actuator. The slave link may connect the master bell crank with the wing tip device, preferably such that actuation of the master bell crank results in a tensile or compressive force along the slave link (that force being arranged to act on the wing tip device). The tensile or compressive force is preferably in the second direction. The slave link may be pivoted, at one end, to the master bell crank about a first pivot. The slave link may be pivoted, at the other end, to the wing tip device about a second pivot. The master bell crank may be pivotably mounted, at its base, about a third pivot. Said third pivot may be slideably moveable relative to the sliding chassis. The first location (on which the actuation force acts) may be located between the third pivot and the first pivot at the other end of the bell crank (at which the link is attached). The locus of the first location, during the second stage of movement, may be curved. Such an articulation mechanism has been found to be particularly beneficial in efficiently converting the actuation force into a rotation of the wing tip device. 
     In principle, the sliding chassis may be mounted on the fixed wing in any manner that enables sliding movement. More preferably however, the actuation assembly comprises a fixed chassis onto which the sliding chassis is slideably mounted. The fixed chassis may be fixedly attached to the fixed wing. Providing a fixed chassis on which the sliding chassis is mounted has been found to be especially beneficial because it enables the actuation assembly to be pre-assembled as a unit, and for example tested, at a location remote from the aircraft wing. Such an articulation assembly may then be able to be installed in the wing in an efficient manner. 
     The wing may include an actuator arranged to act upon the actuation assembly to effect movement of the wing tip device from the flight configuration to the ground configuration. The actuation assembly is preferably arranged such that the same actuator is arranged to effect both the first and the second stages of movement. Such an arrangement is beneficial from both a weight and an airworthiness perspective as it minimises the number of critical components being used. The actuator is preferably a linear actuator. The actuator may be coupled to the fixed wing at one end, and coupled to the actuation assembly (for example the articulation mechanism) at the other end. 
     In the flight configuration, the span of the aircraft may exceed an airport compatibility gate limit. In the ground configuration the span of the aircraft is preferably reduced such that the span (with the wing tip device in the ground configuration) is less than, or substantially equal to, the airport compatibility gate limit. 
     When the wing tip device is in the ground configuration, the aircraft incorporating the wing, may be unsuitable for flight. For example, the wing tip device may be aerodynamically and/or structurally unsuitable for flight in the ground configuration. The aircraft is preferably configured such that, during flight, the wing tip device is not moveable to the ground configuration. The aircraft may comprise a sensor for sensing when the aircraft is in flight. When the sensor senses that the aircraft is in flight, a control system is preferably arranged to disable the possibility of moving the wing tip device to the ground configuration. 
     The wing tip device may be a wing tip extension; for example the wing tip device may be a planar tip extension. In other embodiments, the wing tip device may comprise, or consist of, a non-planar device, such as a winglet. 
     The two-stage movement described herein tends to be described with reference to movement of the wing tip device from the flight configuration to the ground configuration. The actuation assembly is preferably arranged to also move the wing tip device from the ground configuration to the flight configuration in a two-stage movement. The two stage movement is preferably the reverse of the movement from the flight to the ground configuration. In other words, the wing tip device is rotated from the ground configuration, and then, subsequently, translated towards the flight configuration in a linear movement only. For the sake of clarity, each and every feature is not described herein with reference to both directions of movement. Instead, it will be appreciated that any features described with reference to one direction of movement, may be equally applicable, in reverse, to the reverse direction of movement. 
     According to another aspect of the invention, there is provided an actuation assembly as described herein. The actuation assembly may comprise: a fixed chassis for installation into a wing and a sliding chassis, slideably mounted in the fixed chassis. The sliding chassis may comprise a hinge, fixed relative thereto, about which a wing tip device may be mounted for rotational movement. The sliding chassis may carry an articulation mechanism for transferring an actuation force acting at a first location, and in a first direction, into an actuation force acting in a second location and in a second direction, such that a force may be applied to rotate the wing tip device about the hinge. 
     According to another aspect of the invention there is provided a wing tip device coupled to the actuation assembly described herein. 
     According to another aspect of the invention there is provided an aircraft comprising the aircraft wing described herein. The aircraft is preferably a passenger aircraft. The passenger aircraft preferably comprises a passenger cabin comprising a plurality of rows and columns of seat units for accommodating a multiplicity of passengers. The aircraft may have a capacity of at least 20, more preferably at least 50 passengers, and more preferably more than 50 passengers. The aircraft is preferably a powered aircraft. The aircraft preferably comprises an engine for propelling the aircraft. The aircraft may comprise wing-mounted, and preferably underwing, engines. 
     According to yet another aspect of the invention, there is provided a method of moving a wing tip device on an aircraft wing from (i) a flight configuration for use during flight to (ii) a ground configuration for use during ground-based operations, in which ground configuration the wing tip device is moved away from the flight configuration such that the span of the aircraft wing is reduced, wherein the method comprises the steps of moving the wing tip device in a two-stage movement, the two-stage movement comprising a first stage in which the wing tip device is translated away from the flight configuration in a linear movement only, and a second, subsequent stage, in which the wing tip device is rotated to the ground configuration. 
     It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. 
     This application claims priority from UK patent application GB1503980.3 filed on 9 Mar. 2015, and UK patent applications GB 1502661.0 and GB 1502663.6 filed on 17 Feb. 2015. The subject-matter of those applications is incorporated herein by reference. Embodiments of the present invention recognise that a two-stage movement is beneficial. Some embodiments of the present invention may also recognise that a two-stage movement, not necessarily of the exact type disclosed in the embodiments of these earlier applications, is also desirable. Thus, some subject-matter in the earlier applications may be disclaimed from the claims of the present invention. 
     Accordingly, the wing and the aircraft according to aspects of the present invention may, optionally, not comprise: an aircraft wing comprising a fixed wing and a wing tip device at the tip thereof, the wing tip being moveable relative to the fixed wing between: a flight configuration for use during flight; an intermediate configuration; and a ground configuration for use during ground-based operations, wherein in the flight configuration the fixed wing and the wing tip device are locked together via a multiplicity of connectors, the connectors being arranged to transfer loads from the wing tip device to the fixed wing; in the intermediate configuration the wing tip device is displaced in a first direction, relative to the fixed wing, the displacement being such that the connection by the multiplicity of connectors is disengaged and being such that the wing tip device is unlocked from the fixed wing, and in the intermediate configuration the wing tip device is also connected to the fixed wing via a hinge about which the wing tip device may rotate, and in the ground configuration, the wing tip device is rotated about the hinge, such that the span of the wing is reduced. The method according to aspects of the present invention may, optionally, not comprise: the steps of displacing a wing tip device in a first direction, relative to a fixed wing, such that a connection by a multiplicity of connectors is disengaged thereby unlocking the wing tip device from the fixed wing; and subsequently rotating the wing tip device about a hinge such that the span of the aerodynamic structure is reduced. 
     The wing and the aircraft according to aspects of the present invention may, optionally, not comprise: an actuator, the actuator being arranged to effect movement of the wing tip device between: (i) a flight configuration for use during flight and (ii) a ground configuration for use during ground-based operations, in which ground configuration the wing tip device is moved away from the flight configuration such that the span of the aircraft is reduced, characterised in that the aircraft comprises: a carriage guide, fixed relative to the wing, and a carriage arranged to move along the carriage guide as the wing tip device moves between the flight and ground configurations, wherein the carriage carries the wing tip device on a pivot, such that the wing tip device is rotatable relative to the carriage, about the pivot, as the carriage moves along the carriage guide the movement of the wing tip device, between the flight and the ground configurations, thereby comprising both a rotational component of movement of the wing tip device about the pivot, and a translational component of movement, of the pivot, along the carriage guide. The method according to aspects of the present invention may, optionally, not comprise: a method of moving a wing tip device between a flight configuration and a ground configuration, the wing tip device being pivotable on a carriage that is movable along a carriage guide, such that the wing tip device is rotatable relative to the carriage, wherein the method comprises the step of: moving the carriage along the carriage guide, such that the movement of the wing tip device, between the flight and the ground configurations, comprises both a rotational component of movement of the wing tip device about the pivot, and a translational component of movement, of the pivot, along the carriage guide. 
     The wing and the aircraft according to aspects of the present invention may, optionally, not comprise: an actuator, the actuator being arranged to effect movement of the wing tip device between: (i) a flight configuration for use during flight and (ii) a ground configuration for use during ground-based operations, in which ground configuration the wing tip device is moved away from the flight configuration such that the span of the aircraft is reduced, characterised in that the aircraft comprises: a carriage guide, fixed relative to the wing, a carriage arranged to move along the carriage guide as the wing tip device moves between the flight and ground configurations, wherein the carriage carries the wing tip device and the wing tip device is fixed relative to the carriage, such that the path of the wing tip device, during movement of the wing tip device, between the flight and the ground configurations, is defined by the shape of the carriage guide. The method according to aspects of the present invention may, optionally, not comprise: the step of moving a carriage along a carriage guide, the wing tip device being fixed relative to the carriage such that the path of the wing tip device, during movement of the wing tip device between the flight and the ground configurations, is defined by the shape of the carriage guide. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
         FIG. 1  shows a front view of an aircraft having aircraft wings according to a first embodiment of the invention; 
         FIG. 2 a    shows one of the wings of the aircraft of  FIG. 1 , with the wing tip device in the flight configuration; 
         FIG. 2 b    shows the wing of  FIG. 2 a    but with the wing tip device in the ground configuration; 
         FIG. 3 a    shows a frontal view of the wing of  FIG. 2 a    with the wing tip device in the flight configuration; 
         FIG. 3 b    shows a partial cut-away view of the underside of the wing of  FIG. 2   a;    
         FIG. 4 a    is the frontal view of  FIG. 3 a    but with the wing tip device having undergone a first stage of movement, from the flight configuration towards the ground configuration; 
         FIG. 4 b    is a plan view of the wing in  FIG. 4   a;    
         FIG. 5 a    is a frontal view of the wing of  FIG. 4 a    but with the wing tip device having partly undergone a second stage of movement, towards the ground configuration; 
         FIG. 5 b    is a frontal view of the wing of  FIG. 5 a    but with the wing tip device having completed movement to the ground configuration; 
         FIG. 6  is an exploded perspective view of the articulation assembly in the wing of the first embodiment; 
         FIG. 7 a    is a cut-away perspective view of the wing in the first embodiment of the invention, showing the actuation. 
         FIG. 7 b    is a cut-away front view of the wing in  FIG. 7   a;    
         FIG. 7 c    is a cut-away front view of the wing in  FIGS. 7 a  and 7 b   , showing the rotational stop feature; 
         FIGS. 8 a  and 8 b    are cut-away perspective views of the wing in the first embodiment of the invention, showing the actuation assembly when the wing tip device has undergone the first stage of movement; 
         FIG. 8 c    is a cut-away front view of the wing in  FIGS. 8 a    and  8   b;    
         FIG. 8 d    is a cut-away front view of the wing in  FIGS. 8 a -8 c   , showing the rotational stop feature; 
         FIG. 9 a    is cut-away perspective views of the wing in the first embodiment of the invention, showing the actuation assembly when the wing tip device is moving during the second stage of movement, towards the ground configuration; 
         FIG. 9 b    is a cut-away front view of the wing in  FIG. 9   a;    
         FIGS. 9 c  and 9 d    are cut-away front views of the wing in  FIGS. 9 a -9 b    at different moments during the movement towards the ground configuration; 
         FIG. 10 a    is a cut-away perspective view of the wing in the first embodiment of the invention, showing the actuation assembly when the wing tip device has moved to the ground configuration; 
         FIG. 10 b    is a cut-away front view of the wing in  FIG. 10   a;    
         FIG. 10 c    is a cut-away front view of the wing in  FIGS. 10 a  and 10 b   , showing the rotational stop feature; 
         FIG. 11  is a perspective view of the actuation assembly in the wing of the first embodiment of the invention; 
         FIG. 12  is a perspective view of the wing box, having a fixed chassis ready to receive the actuation assembly of  FIG. 11 ; 
         FIGS. 13 a  to 13 c    are perspective views showing the actuation assembly of  FIG. 11  being mounted into the fixed chassis of  FIG. 12 ; 
         FIG. 14  is a perspective view showing part of the wing tip device being attached to the actuation assembly of  FIGS. 13 a -13 c   ; and 
         FIGS. 15 a  and 15 b    are sectional views along a spar-wise cut in the wing, showing the actuation assembly  31 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic drawing showing an aircraft  1  having aircraft wings  3  according to a first embodiment of the invention. The end of one of the wings  3  on the aircraft  1  is shown in more detail in  FIGS. 2 a  and 2 b   , to which reference is now made: 
     The wing  3  comprises a fixed wing  5  extending from the wing root at the aircraft fuselage, to a tip. At the tip of the fixed wing  5  there is a wing tip device  7 . The wing tip device  7  is moveable between a flight configuration (shown in  FIG. 2 a   ) and a ground configuration (shown in  FIG. 2 b   ). 
     In the flight configuration the wing tip device  7  is effectively an extension of the fixed wing  5 , such that the leading and trailing edges  9 ′,  11 ′ of the wing tip device are continuations of the leading and trailing edges  9 ,  11  of the fixed wing  5 , and the upper and lower surfaces of the wing tip device  7  are continuations of the upper and lower surfaces of the fixed wing  5 . The fixed wing and the wing tip device together form a main wing  3  on the aircraft  1 . 
     The wing tip device  7  is moveable from the flight configuration (shown in  FIG. 2 a   ) to a ground configuration (shown in  FIG. 2 b   ). In the ground configuration, the wing tip device  7  is moved such that the span of the aircraft  1  is reduced (relative to the flight configuration). This enables the aircraft  1  to have a relatively large span during flight (which flight span exceeds airport gate limits), whilst still complying with airport gate limits, safe taxiway usage etc., when on the ground. 
     Having a moveable wing tip device per se, to achieve this span reduction on the ground, is known. However, the first embodiment of the invention provides an improved way of moving the wing tip device between the two configurations, as will now be explained with reference to the other Figures. 
     Many of the Figures have been produced from Computer Aided Design (CAD) packages. Thus, it will be appreciated that some of the Figures include constructional lines, and/or some lines showing hidden, or internal, features of the embodiment. 
       FIGS. 3 a    to  6  show the position of the wing tip device  7  at different times (and from different viewpoints) as the wing tip device  7  moves from the flight configuration to the ground configuration. 
       FIGS. 3 a  and 3 b    are both views of the end of the wing  3  with the wing tip device  7  in the flight configuration. As best illustrated in  FIG. 3 a    (which is a frontal view), the upper and lower surfaces of the wing are substantially continuous across the junction between the fixed wing  5  and the wing tip device  7 . The interfacing edges  13 ,  15  of the fixed wing  5  and the wing tip device  7  comprise resiliently deformable “P” seals (not visible in the Figures), which are compressed in the flight configuration to seal the junction and prevent aerodynamic leakage flow across it. 
     The flight configuration is for use during flight, so it is important that the wing tip loads (arising from aerodynamic forces and/or inertial loads) are transferred into the fixed wing  5 . In this respect, the fixed wing  5  of the first embodiment comprises three pairs of fixed spigots  17   a ,  17   b ,  17   c . Two pairs  17   a ,  17   b  protrude from the outboard ends of the main spar  19  and front spar  21 , and one pair  17   c  protrude from an inboard structure  23  of the spars, such that some of the loads can be reacted inboard on the fixed wing  5 . A support frame  25  of the wing tip device  7  comprises corresponding holes  27   a - c , lined with bushes, arranged to receive the spigots  17   a ,  17   b ,  17   c  when the wing tip device  7  is in the flight configuration. The engaging spigots/bushes  17   a - c / 27   a - c  enable loads in the wing tip device  7  to be reacted into the spars  19 ,  21  of the fixed wing  5 . 
     The longitudinal axes of the spigots (and the bushes) extend in an outboard direction, substantially aligned with the spars  19 ,  21 , and substantially in the plane of the fixed wing  5 . Thus, the engaging spigots/bushes  17   a - c / 27   a - c  are particularly effective in transferring vertical and forward/aft loads (which are the predominant loads experienced by the wing tip device  5  during flight). 
     The spigots and bushes  17   a - c / 27   a - c  are best illustrated in  FIGS. 3 b , and 13 a - c   , in which the wing skin has been removed for clarity.  FIG. 3 a    also shows close-up view of one of the pairs of spigots  17   a , in phantom, extending through the bushes  27   a  in the root  25 ′ of the support frame  25 . 
       FIGS. 4 a  and 4 b    show the wing after a first stage of movement of the wing tip device  7  towards the ground configuration. The wing tip device  7  has undergone a translational movement in an outboard direction along the wing (shown by the large arrow in  FIG. 4 b   ). It is important to note that this movement is only a translation and it does not comprise any rotational component of movement. Having this type of initial movement away from the flight configuration, has been found to give rise to two advantages. Firstly, the movement is parallel to the axes of the pairs of spigots  17   a - c . Thus it enables the wing tip device  7  to readily disengage from the spigots  17   a - c  (i.e. the spigots  17   a - c  are moved clear of the bushes  27   a - c ) without requiring secondary mechanisms to retract, or otherwise move, the spigots. Secondly, this translational movement enables the “P” seals to be broken (i.e. separated) without a relative rotational component of movement. Having a rotational component of movement when breaking a seal has been found to increase wear of the seal, so ensuring a linear separation tends to minimise any wear and may enable an improved sealing arrangement. 
     In the first embodiment of the invention, the movement to the ground configuration is a two-stage movement. The first stage is the translational movement described above and with reference to  FIGS. 4 a  and 4 b    (i.e. to separate the seals and move the spigots clear of the bushes). The second stage is a rotational movement described below with reference to  FIGS. 5 a    and  5   b.    
     After the first stage of movement, the wing tip device  7  is arranged to rotate (and only rotate) to the ground configuration. As shown in  FIGS. 5 a  and 5 b   , the rotation is about a hinge  29  located near the upper surface of the wing. The wing tip device  7  is rotated to bring it into a slightly over-vertical position ( FIG. 5 b   ). This position is geometrically stable and also maximises the span reduction that can be achieved. The wing tip device  7  is held in position by a lock (described in more detail below with reference to  FIG. 10 b   ). 
     Having a second stage of movement that is a substantially pure rotation has been found to be beneficial because it avoids any increase in span that might otherwise occur with a translational movement outboard. 
     The wing tip device  7  is also moveable in the reverse of the above-described movement when moving from the ground configuration to the flight configuration. In other words, when moving into the flight configuration (for example is preparation for take-off) the wing tip device  7  is first rotated downwardly about the hinge  29  until it is substantially in-plane with the fixed wing  5 . The wing tip device  7  is then translated onto the spigots  17   a - c  such that they engage with the bushes  27   a - c  and such that the wing tip device  7  abuts the tip of the fixed wing  5 , thereby compressing the seal at the interface. 
     The description above, with reference to  FIGS. 3 a  to 5 b    of the first embodiment of the invention, illustrates the nature of the two-stage movement of the wing tip device  7 . That movement is enabled by an actuation assembly  31  located in the fixed wing. The details of the actuation assembly  31  are shown in  FIG. 6  onwards, and will now be described: 
       FIG. 6  is an exploded view of the actuation assembly. The actuation assembly  31  comprises a sliding chassis  33 , contained in a fixed chassis  35 . The sliding chassis  33  is mounted on two pairs of spring-loaded bogies  37  such that the sliding chassis  33  is slideably moveable along two respective tracks  39  within the interior side faces of the fixed chassis  35 . The sliding chassis  33  comprises a lip  41  that protrudes through an open channel  43  in the top of the fixed chassis  35  thereby forming a translational stop feature (discussed in more detail below). 
     The fixed chassis  35  is itself fixedly attached to a bath-tub fitting  45  located between the front and rear spars of the fixed wing  5  (see  FIG. 7 a   ). 
     The sliding chassis  33  contains an articulation mechanism  47  comprising a master bell crank  49  and a slave link  51 . The slave link  51  is pivoted, at one end, to the master bell crank  49  about a first pivot  52 . The slave link  51  is pivoted, at the other end, to the wing tip device  7  about a second pivot  54 , The master bell crank  49  is pivotably mounted at one end about a third pivot  56  that is mounted on a slider  53 , arranged to move along a central rail  55  in the sliding chassis  33 . Between the ends of the bell crank  49  the centre of the bell crank is connected, at a rotational connection  58 , to a linear actuator assembly  57  comprising two linear actuators  57 . This rotational connection  58  is constrained to move along a drooping groove  61  defined in the sliding chassis structure. 
     The sliding chassis  33  is also connected to the wing tip device  7  via the hinge  29  located at the distal end of the sliding chassis  31 . The outer ends of the hinge include a kidney-shaped end-cap  63 . The end-caps  63  are arranged to abut (in the flight configuration) an abutment surface  65  on the fixed chassis  35  thereby forming a rotational stop feature (discussed in more detail below). 
       FIGS. 7 a  to 7 c    show the actuation assembly  31  when the wing tip device  7  is in the flight configuration. At this time, the actuators  57  are fully retracted and the sliding chassis  33  is at its rearmost (i.e. furthest inboard) position such that it is flush with the end of the fixed chassis  35 . The lip  41  on the sliding chassis  33  that extends out of the open channel  43  in the fixed chassis  35  is a linear distance X from the end of the channel. The articulation mechanism  47  is pulled back as far as possible by the actuators  57  such that the link  51  is almost horizontal. As shown in  FIG. 7 c   , the kidney-shaped end-stop  63  on the hinge  29  is received in a tight fit against the abutment surface  65  of the fixed chassis  35 . 
     To begin the movement to the ground configuration, and more specifically to effect the first stage of that movement, the actuators  57  are extended. The kidney-shaped end-stop  63  is prevented from rotating by the abutment surface  65  on the fixed chassis  35 , and the wing tip device  7  is thus unable to rotate relative to the actuation assembly  31 . Instead, the extension of the actuators  57  pushes the sliding chassis  33  (via the articulation mechanism  47  which is forced to act as a rigid link due to the presence of the rotational stop  63 ,  65 ). This causes a pure translational movement in the actuation assembly  31 , relative to the fixed chassis  35 . Such movement is parallel to the axes of the spigots  17   a - c . Since the wing tip device  7  is coupled to the sliding chassis  33 , along the hinge  29 , this movement acts to push the wing tip device  7  along the length of the spigots  17   a - c  until they are clear of the bushes  27   a - c . The moment at which the spigots  17   a - c  have just cleared the bushes  27   a - c  is shown in  FIGS. 8 a    to  8   d.    
     As most clearly shown in  FIGS. 8 a  and 8 b   , the lip  41  of the sliding chassis  33 , has at this point reached the end of the channel  43  in the fixed chassis  35  (i.e. the distance X in  FIG. 7 a    is only fractionally longer than the length of the spigots  17   a - c  that had been received in the bushes  27   a - c ). The lip  41  abuts the end of the channel  43  and prevents further linear movement of the sliding chassis  33  beyond this point. However, the rotational stop feature  63 ,  65  is designed such that, at the same time the lip  41  abuts the end of the channel  43 , the kidney-shaped end caps  63  simultaneously reach the end of the abutment surface  65  (see  FIG. 8 d   ) such that the wing tip device  7  is freed to rotate about the hinge  29 . 
     Continued extension of the actuators  57  thus ceases to effect a translational movement, and instead effects a rotation of the wing tip device  7  about the hinge  29  (i.e. the second stage of movement). As best illustrated in  FIGS. 9 a  and 9 b   , the sliding chassis  33  now remains stationary but the articulation mechanism  47  moves along the central rail  55 . The locus of the rotational connection  58  on the bell crank  49  (at which the actuator is attached) follows the drooping groove  61  (as best illustrated in  FIG. 10 b   ). The groove  61  is shaped, along its first half, to maintain the link  51  in an orientation that is approximately inline with the rotational connection  58  and the first and second pivots  52 ,  54  and to transfer the actuation force onto the wing tip device  7  at a location remote from (i.e. offset from) the hinge  29 . This creates a moment arm, about the hinge  29 , which acts to rotate the wing tip device  7  upwardly. 
       FIGS. 9 c  and 9 d    show how the end-cap of the hinge no longer prevents rotation, as it is clear of the abutment surface. 
     Continued extension of the actuators  57  moves the wing tip device  7  into the ground configuration (shown in  FIGS. 10 a  and 10 b   ) in which the wing tip device  7  is upright, above the hinge  29 . The end of the groove  61  is drooped so that the primary component of force along the bell-crank  49  continues to act to pull the slider  53  along the rail  55 , rather than merely generating large vertical reactionary forces against the rail  55 . In the ground configuration, the bell-crank  49  and slave link  51  form an over-centre lock (discussed in more detail below). 
       FIG. 10 c    shows how the end-cap  63  of the hinge  29  continues to allow rotation, as it is clear of the abutment surface  65 . 
     The actuation assembly  31  in the first embodiment of the invention has been designed to be quickly and easily installed on the aircraft fixed wing  5 . In particular, the actuation assembly  31  comprises a fixed chassis  35  and a sliding chassis  33  (the latter containing the articulation mechanism  47 ). Since all components in the actuation assembly  31  move relative to the fixed chassis  35 , the fixed chassis  35  can simply be held whilst the assembly  31  is bench-tested prior to installation on the spars  19 ,  21  of the wing. There is no need for all testing to take place during, or after, installation on the fixed wing  5 . This may enable the actuation assembly  31  to be a ‘line replaceable unit’ (LRU). 
     As shown in  FIGS. 11 and 12 , the fixed chassis  35  comprises laterally extending flanges  67  for attachment to a bath tub fitting  45  ( FIG. 12 ) on the fixed wing spars  19 ,  21 . 
     The installation of the actuation assembly  31  takes place in two steps. Referring to  FIGS. 13 a  and 13 b   , the actuation assembly  31  is first secured relative to the fixed wing  5  by inserting the assembly from underneath the wing, and fastening the fixed chassis flanges  67  to the bathtub fitting  45 . Referring now to  FIG. 13 c   , the actuators  57  are then attached at one end to a bar  69  extending between the spars  19 , 21 , and at their other end to the rotational connection  58  on the bell crank  49 . 
     To install the wing tip device  7 , the actuation assembly  31  is set to its configuration in the ground configuration (i.e. the actuators  57  are fully extended). Referring to  FIG. 14 , the wing tip device frame structure  25  is then lowered into the hinge  29  and the slave link  51  is attached to the frame  25  at the offset location from the hinge  29 . 
     Referring back to  FIG. 10 b   , the articulation mechanism is arranged such that when the wing tip device  7  in the ground configuration, the master bell crank  49  and the slave link  51  are in an over-centre position. More specifically, the first pivot  52  (between the bell crank  49  and the slave link  51 ) is out of line with the second pivot  54  (between the link  51  and the wing tip device  7 ) and the third pivot  56  (at which the bell crank  49  is pivoted on the slider  53 ). This creates an over centre lock to lock the wing tip device in the ground configuration. That lock can only be unlocked by retraction of the actuator  57 . It will be appreciated that the over-centre lock is also created by that same actuator  57  as it extends to move the wing tip device into the ground configuration. Thus, the same actuator is arranged to move the wing tip device and make/break the over centre lock that holds the wing tip device in the ground configuration. This removes the need for a separate actuator to lock/unlock the wing tip device in the ground configuration. 
       FIGS. 15 a  and 15 b    are sectional views along a spar-wise cut in the wing, showing the actuation assembly  31 , with particular focus on a track and follower arrangement between the fixed chassis  35  and the sliding chassis  33 , as will now be described: 
     The sliding chassis  33  is received inside the fixed chassis  35  on a follower in the form of two pairs of spring loaded bogies  71   a ,  71   b  (only one of each pair  71   a ,  71   b  is visible in the views of  FIGS. 15 a  and 15 b   ). The bogies  71   a ,  71   b  are received in a C′ track  73  shaped into the inner surface of the fixed chassis  35 , and are moveable along the track  73  to enable a sliding movement of the sliding chassis  33  relative to the fixed chassis  35  (it will be appreciated that a sliding movement need not necessarily be restricted to being via only a sliding contact; this term also encompasses a rolling contact as per this first embodiment of the invention). 
     The track  73  comprises first portions  73   a  which are relatively wide of width Wa (the vertical direction in  FIGS. 15 a  and 15 b   ), and second portions  73   b , located outboard of the respective first portions  73   a , that are relatively narrow of width Wb. 
     When the wing tip device  7  is in the flight configuration (see  FIG. 15 a   ), the spring loaded bogies  71   a ,  71   b  are located along the first portions of the track. They are spring-biased into an expanded orientation in which each bogie is at an angle such that it the wheels of the bogie  71   a ,  71   b  are in contact with both sides of the track  73  (for example the bogie is inclined to the longitudinal axis of the track such that one wheel is in contact with one side, and another wheel is in contact with the other side). The spring biasing force is relatively weak however. Thus, when the wing tip device is subjected to loads, such as flight loads, the spring loaded bogies  71   a ,  71   b  cannot provide any load transfer path, to transfer these forces into the fixed wing  5  (the bogies  71   a ,  71   b  would just change orientation under the action of a force, rather than transfer that force into the fixed wing  5 ). Instead, the loads are only transferred via the three pairs of spigots and bushes ( 17   a - c / 27   a - c ) previously discussed. 
     Such an arrangement ensures that the sliding chassis  33 , and other parts of the actuation assembly  31 , are effectively isolated from the flight loads on the wing tip device  7 , when the device is in the flight configuration. The actuation assembly  31  does not, therefore, need to be sized to cope with the flight loads, enabling the actuation assembly to be relatively small and/or lightweight. 
     As described above, the track  73  comprises both relatively wide portions  73   a  and relatively narrow portions  73   b . The track  73  is shaped such that when the wing tip device has undergone the first stage of translational movement, the sliding chassis moves along the track  73  such that the bogies  71   a ,  71   b  have moved from the wide portions  73   a  of the track to the narrow portions  73   b . The narrow portions of the track are substantially the same width as the diameter of the wheels on the bogies  71   a ,  71   h  such that the bogies  71   a ,  71   b  are urged into alignment with the longitudinal axis of the track  73  and are held in a tight fit. 
     After this translational movement, the spigots  17   a - c  are clear of the bushes  27   a - c  (see description above with reference to  FIGS. 3 a  to 4 b   ). These spigots cannot, therefore act to transfer any loads. However, in the first embodiment of the invention, the actuation mechanism is at this stage no longer isolated from the wing tip loads because the bogies are received in a tight fit in the narrow portion  73   b  of the track  73 ; the bogies cannot move within the width of the track and therefore facilitate load transfer into the fixed wing  5 . 
     Embodiments of the present invention recognise that the loads from the wing tip device are typically lower once it is no longer in the flight configuration, because at that stage there tend to be no flight-induced loads (the loads typically only being the weight of the wing tip device and/or gust loading on the wing tip device when it is folded upwardly). Thus, the actuation assembly can be relatively lightweight yet still be arranged to transfer these loads. 
     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. 
     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.