WO2011155850A1

WO2011155850A1 - Inflatable wing with inflatable spar spaced from the leading edge

Info

Publication number: WO2011155850A1
Application number: PCT/NZ2011/000091
Authority: WO
Inventors: Richard John Roake
Original assignee: Aquadria Kite Design Limited
Priority date: 2010-06-02
Filing date: 2011-06-02
Publication date: 2011-12-15
Also published as: NZ585881A

Abstract

An inflatable wing for a traction kite, the wing including: a flexible envelope having a first sheet and a second sheet, the first and second sheets joined together at a leading edge, a trailing edge and two spaced apart wing tips; and an inflatable spar located within the envelope and extending substantially spanwise between the two wing tips, characterised in that the inflatable spar is located within the flexible envelope in a spaced apart relationship with the leading edge.

Description

IMPROVEMENTS TO AN INFLATABLE WING

TECHNICAL FIELD

The present invention relates to an inflatable propulsive wing of the type used in traction kites to provide a traction force and/or for lifting or pulling a load via lines attached to the wing. In particular the invention relates to an inflatable wing having an upper and a lower outer skin joined to form a flexible envelope having a leading edge and a trailing edge, and an inflatable rib extending from the leading edge towards the trailing edge of the wing, the wing having a curved profile across the span when inflated. The inflatable wing may be configured as a kite to provide traction to an attached load, and as such may be generally used in a similar manner to other inflatable kites, such as, without limitation, Leading Edge Inflatable (LEI) kites, bow kites and hybrid kites. However, it is envisaged that the inflatable wing may also find applications in devices other than kites and reference to its use as a wing for a kite should not be seen as limiting.

BACKGROUND ART

The use of Inflatable wings is increasingly common in many activities, especially sport and recreational activities involving the use of an inflatable wing configured as a traction kite to provide traction to a person on a device for moving or sliding over water (kitesurfing, kiteboarding etc) or land (kite buggying, kiteskiing, etc). Most traction kites commonly used in these activities use a wing either of the leading edge inflatable (LEI) type or a variant thereof. Sporting activities using LEI kites (and related variants) have become extremely competitive and there is very high demand for a wing/kite that can provide improved performance over existing wings/kites.

The basic concept of an LEI wing was disclosed by D and M Legaignoux in US 4,708,078 ("Legaignaux"). Legaignaux discloses a wing configured like a spherical segment, or arc shaped spanwise. The wing has a light weight skin formed from a flexible material. Typically an LEI kite wing has a single skin which forms the canopy for the kite.

An LEI kite wing includes inflatable tubes preformed to create a leading edge spar and generally one or more ribs extending chordwise between the leading edge and trailing edge of the wing. The spar and ribs of an LEI kite wing are generally formed by enclosing an airtight bladder within a more robust outer part that is configured to provide the desired shape of the spar or rib.

The leading edge spar is attached to, or more generally enveloped by the flexible skin forming the leading edge of the kite, the shape of the inflatable leading edge spar being chosen and preformed to create the desired spanwise shape of the leading edge of the kite. Likewise, the shape of the outer part of a rib may be preformed to provide a desired shape.

A traction kite is formed by attaching one or more control lines to the wing tips of the LEI kite wing, the other end of the lines generally terminating in a control bar. The control bar is typically attached to a harness for holding the person operating the kite.

The LEI kite, while very successful, has a number of disadvantages, including difficulties experienced by users in controlling the kite in flight and in maintaining the desired aerodynamic performance for the wing at all times.

A further disadvantage, common to all LEI kites, is the amount of time and effort required to inflate the leading edge spar and the ribs. The leading edge spar, for example, can have a diameter of 80mm or more in the central, midspan, portion. Even with modern pumps it can take many minutes to inflate the spar to the required pressure, which is not only time consuming but can be generally frustrating and inconvenient to a user.

This problem is exacerbated when ribs are inflated as well. It is common for an LEI kite wing to have 5 -7 ribs spaced across the span of the wing. Inflation of the ribs as well as the leading edge spar can increase the inflation time considerably. Partly because of this the size of the rib bladders is generally a trade off between the desire to provide shape and stiffness to the chordwise profile and the need to keep inflation time at an acceptable level. As a consequence the ribs are generally much smaller than required to hold the chordwise profile of the kite wing in an aerodynamically efficient shape. Further, the shape of the leading edge, without further modification, is generally restricted to a semi circular arc shape. This may not be the best shape for all purposes, and maintaining even that shape in changing conditions, as can be common in flight, can be difficult. Failure to control the shape of the wing correctly can lead to loss of boundary layer stability /lift and/or in extreme cases collapse of the wing, which can lead to injury or fatality, especially if the user was airborne at the time.

Various modifications to the Legaignaux design have been made since it was patented in 1987. For example, US 2002/0185570 A1 (to Winner) discloses the addition of cross bridle lines attached to the leading edge of the wing enable the turning radius of the kite to be altered to suit different wind conditions. Other developments include adding further bridle lines attached to the leading and trailing edges of the wing to improve control and to maintain a desired shape to the wing in varying conditions.

US 2009/0277997 (to Shogren et al.) discloses a leading edge formed from a plurality of spaced segments fabricated from a material having a high elasticity in order to allow greater twisting and bending of the leading edge. The shape of the leading and trailing edges in Shogren is controlled by a plurality of additional bridle lines attached at various points along the edges.

While this construction may improve the flexibility of the leading edge, it comes at the cost of increased complexity of construction and hence cost of the wing/kite to the user.

Moreover the additional control/bridle lines add complexity to the control required of a user, as well as adding weight to the kite and increasing the drag forces experienced by the kite, both of which are undesirable where high performance is a requirement.

US 6,837,463 (to Lynn) discloses a double skin, bladderless wing including a plurality of cells formed between the skins by chordwise-extending ribs. The cells are formed as walls of flexible material extending in a chordwise direction between the skins. The wing includes valved openings to the cells, situated near the leading edge, to allow ram air to inflate the wing. The shape of the wing in chordwise profile is held under tension by the walls of the cells.

One disadvantage of the "ram air" wing is that, while the shape may be held while the pressure difference between the skins holds the cell walls in tension, there is little resistance to compression, which can lead to partial or full collapse of the pressure inside the cell under certain conditions.

Another disadvantage is the large number of lines used to maintain the shape of the ram air wing and to control it during flight. The lines add weight to the kite, can generate considerable additional drag forces and can become tangled, all of which reduce the performance and usability of the ram air wing. A further disadvantage is that it can be difficult for a user to control the shape of the wing section during flight.

Maintaining the spanwise shape of the wing during changing flight conditions is a problem all of the above prior art inflatable wings have in common. The problem arises at least in part from the design of the wings as they typically lack spanwise stiffness across the entire wing. For example LEI kite wings rely on stiffness provided by the leading edge spar only, with the remainder of the wing left largely unsupported across the span. It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.

All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country. Throughout this specification, the word "comprise", or variations thereof such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

DISCLOSURE OF THE INVENTION

According to one aspect of the present invention there is provided an inflatable wing for a traction kite, the wing including:

a flexible envelope having a first sheet and a second sheet, the first and second sheets joined together at a leading edge, a trailing edge and two spaced apart wing tips; and an inflatable spar located within the envelope and extending substantially between the two wing tips ,

characterised in that

the inflatable spar is located within the flexible envelope substantially in a spaced apart relationship with the leading edge.

Reference to an inflatable wing throughout this specification should be understood to mean a wing constructed in such a way that the wing is formed when air (or some other gas) is pumped into a compartment or compartments within the wing. Reference to a first and/or second sheet throughout this specification should be understood to refer to an outer skin of the inflatable wing. With reference to a wing in normal (i.e., not inverted) flight, a first sheet may form the upper skin or surface of the wing, and a second sheet may form the lower skin or surface of the wing (or vice versa). The lower sheet forms the primary tension member of the present invention.

Reference to a sheet includes the situation where a surface of the wing is formed from a plurality of panels of flexible material which are joined together to form the sheet.

Accordingly, reference to the first sheet and second sheet being joined together at a leading edge and a trailing edge should be understood to mean that the sheets forming the upper and lower surfaces of the wing are joined at the leading and trailing edges of the wing. This should be understood to include the situation where the join between the first sheet and the second sheet is formed from a continuous section of flexible material - i.e., it may be the surfaces that join in a continuous manner (along an imaginary line), rather than strictly a join in the material forming the sheets.

The sheets of the present invention are formed from flexible material and joined together as described to form a flexible envelope, i.e. having a substantially enclosed space between the first and second sheets.

In one aspect of the present invention an inflatable spar is located within the substantially enclosed space between the first and second sheets and attached to them, the spar located at a distance from the leading edge, for most of the span at least. An exception may occur when an end of the spar nears a wing tip, when it may come close to the leading edge (or leading edge spar).

In some embodiments the inflatable spar may include an airtight flexible bladder, or a series of such bladders, which, when inflated, substantially fills a volume between the sheets of the envelope. The bladder may be encased in an outer sheath formed from a more durable and stiffer material than that used to form the airtight bladder. The outer sheath may be

constructed to contain the inflated bladder and to provide a desired shape to the spar.

An advantage of this embodiment is that the spar, when inflated, may provide separation between the sheets and latitudinal (spanwise) stiffness to the inflated wing. The shape of the spar may be chosen to provide an aerodynamically efficient shape spanwise (i.e. from across the wing from wingtip to wingtip) to the sheets of the wing attached to the spar, thus improving the flight characteristics of the wing.

However, generally the size of such a spar may be too large to be practical in everyday use, especially if the wing is used sporadically (as in sporting applications for example) and has to be inflated before each use, in which case the amount of time spent inflating the spar may be such that it detracts from the usability of this embodiment.

According to another embodiment of the present invention there is provided an inflatable wing substantially as described above wherein the inflatable spar includes a first inflatable chord and a second inflatable chord spaced apart from the first chord for at least a length of the spar, and one or more inflatable struts extending between the first chord and the second chord, the spar configured such that, when inflated, the first chord, second chord and the strut(s) form a truss.

Reference to a truss should be understood to mean a structural member in the form of a web composed of contiguous triangles. A truss typically has a number of structural members, including longitudinal members, in the form of an upper and a lower chord, and a plurality of straight struts which form vertical of diagonal support members between the upper and lower chords.

An early and relatively simple form of truss, known as the Warren Truss, used only diagonal struts arranged to create a web of contiguous equilateral triangles. However, as those skilled in the art will appreciate, numerous variants of this basic arrangement have been developed and used, and reference to a truss throughout this specification should be understood to refer to any of the numerous variants.

A truss is designed so that the members only act in compression or tension. The region where the straight struts meet with a chord will be referred to as a node. The construction of the truss is such that the forces at a node are balanced (i.e., when all the tensile and compressive forces of each member at the node are added together the result is zero).

A significant feature of all truss structures is that they provide a relatively stiff and stable structure which is able to support considerable loads, while using relatively small amounts of material (in comparison to other load bearing structures).

In preferred embodiments the inflatable wing includes an inflatable leading edge spar as is well known to those skilled in the art, the inflatable leading edge spar configured to provide shape to the leading edge as well as latitudinal (spanwise) stiffness to the leading edge. The leading edge spar may be of the type commonly used in a wing of a Leading Edge Inflatable (LEI) kite, such as a C kite, and in more recent variants of the LEI kite, such as bow kites and hybrid kites. LEI kites typically have only a single sheet to which the leading edge spar and a plurality of ribs are attached. The present invention differs from such single sheet kites by including two sheets joined together at the leading and trailing edges to form a flexible envelope. The inflatable leading edge spar is attached to the interior of the flexible envelope along the leading edge of the envelope. The leading edge spar may be formed as a single armature, as in the early LEI kite designs, or the spar may be formed from a plurality of segments as in more recent designs.

A leading edge spar according to the present invention may be in the form of a conventional leading edge spar as described above, or it may be in the form of an inflatable truss. The inflatable wing of the present invention includes at least one, and generally a plurality of, inflatable rib(s) which extend from the leading edge, or from the inflatable leading edge spar, towards the trailing edge of the wing. A rib may be in the conventional form of an inflatable, flexible bladder contained within an outer sheath.

However, in preferred embodiments the rib is formed as an inflatable truss which will be referred to as a rib truss. A spar according to the present invention will usually span a space between adjacent rib trusses and typically will be connected to each of the adjacent rib trusses to form a relatively stiff internal framework for the wing.

In preferred embodiments of the present invention the inflatable spar forms a truss (to be referred to as a spar truss) when inflated. The spar truss includes a first inflatable chord and a second inflatable chord, which for convenience of visualisation will be referred to as the upper and lower chords respectively, and a plurality of struts, all of which are inflatable.

A key feature of the spar truss is that the dimensions of the members that make up the truss are small in comparison with the separation of the chords, especially in the vicinity of the centre of the truss. In a preferred embodiment the diameter of an inflated chord of the truss is in the range from 8 mm to 15 mm.

In a preferred embodiment the diameter of an inflated strut of the truss is in the range from 8 mm to 15 mm.

Use of a chord or strut having a diameter within the above ranges in a spar truss may provide a significantly stiffer structure than a bladder having a diameter of the same width as the spar truss when inflated to the same pressure.

As a result, the volume of a spar truss (i.e. the inflatable volume of the chords and struts) needed to provide a structurally stiff spar may be significantly lower than the inflatable volume of a comparable spar formed from a bladder. This may reduce the amount of material required to form the spar, thus saving cost and weight, but most importantly may significantly reduce the amount of time required to inflate the spar. The applicants estimate that an inflatable spar truss may be formed with 8%- 30% (for 8mm - 15mm diameter struts and chords respectively) of the volume of a comparable spar formed from a bladder, with commensurate reduction in inflation times.

In a preferred embodiment a plurality of inflatable struts and an inflatable chord are connected together to form a node, the configuration of the struts and chord at the node being arranged such that, when inflated, the forces exerted at the node by the struts and the chord are balanced.

A spar truss may have a plurality of nodes along the upper and lower chords, the specific design depending on the size of the spar and the required stiffness produced by the truss. The amount of stiffness required may vary along the length of the spar and the design of the spar truss may be chosen to achieve the desired stiffness of each section of the spar truss.

In preferred embodiments a spar truss is connected at each end to a rib truss, the connection being made at a node of the rib truss. The nodes forming the connection between the rib and spar trusses is 3D, unlike the remaining nodes of a rib or spar truss which are essentially 2D (i.e. lie substantially in a single plane). In practice a spar is typically oriented substantially orthogonally to a rib (at least in the midspan region of the wing). As a consequence, the structural members of the rib truss lie in a plane substantially orthogonal to the plane of the structural members of the spar truss. Hence the node connecting both sets of structural members extends in all three dimensions.

In a preferred embodiment at least one spar includes a node located at or in close proximity to the design centre of lift of the wing.

In a preferred embodiment a central rib is located along a chord line corresponding to the centre of the span of the wing.

In at least one model for analysing the flight characteristics of a wing the sum of the lift forces are considered to act at a single point on the mid-span chord line, known as the centre of lift. The centre of lift is not a fixed point but moves along the mid-span chord line under different flight conditions as the angle of attack (the angle between the apparent wind direction (or wing velocity vector) and the chord line) varies. However, designers may typically design a wing for optimal performance under a prescribed set of conditions, including a preferred angle of attack for a particular wing section. The centre of lift under the design set of conditions will be referred to as the design centre of lift.

In a preferred embodiment the central rib includes a node located at or in close proximity to the design centre of lift of the wing, the node configured to interconnect the central rib truss with a spar. An advantage of placing a node configured to interconnect a rib and a spar at the design centre of lift (in practice at either or both the upper or/and lower chords above and below the design centre of lift respectively) is that it may provide the required to the wing at the design centre of lift and enable the load at the design centre of lift to be distributed appropriately throughout the rib and spar trusses.

In a preferred embodiment at least one of the first and second inflatable chords of the spar truss is pneumatically interconnected to an inflatable rib truss located within the flexible envelope.

As described above, the spar truss will typically be connected to a rib truss at a node. An advantage of connecting a spar truss, or at least a chord of it, to an inflatable leading edge spar is that the wing may then be inflated from a single air inlet. This may save time as a pump need only be connected (and disconnected) once to inflate the wing. However, in some embodiments several inflation points may be used.

In a preferred embodiment the inflatable strut is pneumatically interconnected to at least one of the first and second inflatable chords.

Once again, the advantage of pneumatically interconnecting each strut to at least one of the chords is to enable the spar truss to be inflated from a single point, since in preferred embodiments at least one chord is pneumatically connected to an inflatable rib truss which in turn is pneumatically connected to the inflatable leading edge spar. With this arrangement the entire infrastructure of the wing, including the leading edge spar, the rib trusses and the spar trusses may all be inflated from a single air inlet.

In a preferred embodiment the first inflatable chord is attached to the first sheet of the flexible envelope.

In a preferred embodiment the second inflatable chord is attached to the second sheet of the flexible envelope.

Attaching the chords to the sheets not only locates the spar truss in the desired location and orientation within the flexible sheet, but also enables the spar truss to be configured to define a desired shape of the wing along the spar.

In a preferred embodiment the first chord is curved when inflated.

In a preferred embodiment the second chord is curved when inflated.

In a preferred embodiment the first chord and second chord form an aerodynamic shape when inflated. Reference to an aerodynamic shape should be understood to mean a shape which provides a high aerodynamic efficiency to the wing.

This ability to design the shape of the chords of the spar truss is a significant advantage in enabling the design of wings (and kites) having high aerodynamic efficiency, something which is difficult to obtain with single skin wings such as used in most traction kites. This is a significant advantage when the spar trusses are interconnected with the rib trusses, each of which may also be shaped to provide an aerodynamically efficient profile to the wing along a chord line between the leading and trailing edges. In this way both the spanwise and chordwise profiles of the wing may be designed to have a desired shape when the ribs and spars are inflated, something which is difficult to achieve in the inflatable wings of the prior art.

In particular the increased stiffness of the wing provided by the spar and rib trusses may enable the use of a leading edge spar having a smaller diameter than is commonly used with LEI kites. This may enable a designer to produce an aerodynamic shape having a relatively small leading edge diameter, rather than the large leading edge diameters common to LEI kite wings (where a relatively large diameter leading edge spar is required to provide the required stiffness to the spar so that the leading edge holds its shape).

The ability to use a smaller diameter leading edge spar, and hence sharper leading edge, may significantly decrease the turbulent flow under and over the wing surface in comparison with wings having larger diameter leading edge spars. A further advantage may be the ability to shape the spar truss, and therefore the spanwise profile of the wing, to further reduce drag forces caused by turbulent flow around the wing. In particular, the use of an inflatable spar truss in conjunction with a rib truss may allow the sheets forming the surface of the wing to have both positive curvature (i.e. convex) and/or negative (i.e. concave) in profile across the span and chordwise. The curvature may be chosen potentially to induce laminar flow over a larger portion of the wing (measured from the leading edge in a chordwise direction), or at least increase the distance from the leading edge over which the air is accelerating (thus increasing lift) before turbulent flow is initiated (which occurs when the air velocity begins to decrease).

In some embodiments the leading edge spar is formed as a truss.

The advantages of the spar truss may be utilised in the leading edge spar by forming it as a truss. In order to keep the leading edge diameter small, it is preferable for the leading edge spar truss to be a horizontal truss; in other words, the plane of the leading edge spar truss members is at right angles to the plane of the members of the rib trusses and other spar trusses (which lie in a vertical plane). The leading edge spar truss may be formed into a single truss configured to the desired shape of the leading edge, or may be in the form of a plurality of interconnected trusses forming segments of the leading edge.

Forming the leading edge spar as a truss may reduce the inflated volume of the leading edge spar from that of the conventional bladder construction, thus reducing the time required to inflate it. Further, the increased stiffness provided by the truss may enable the diameter of the leading edge of the wing to be reduced considerably, thus improving the aerodynamic properties of the wing.

In a preferred embodiment the flexible envelope includes a closable air vent.

A closable air vent may be a valve, or an opening having a closable cap (such as a screw on cap, an opening in a sheet which is closable with a flap, or any other such device.

A closable air vent in the flexible envelope may be used, when open) to allow air into the flexible envelope when the wing is being inflated (i.e., when the spar and rib(s) are being inflated). This may reduce the suction which may otherwise occur within the deflated envelope when pressurised air (or other gas) is pumped into the spar and ribs. The air vent may be closed when the wing is inflated, so that the air inside the envelope is at atmospheric pressure.

In a preferred embodiment the closable air vent is an access panel.

An access panel may be any (closable) opening in the sheet which allows access into the inside of the envelope. An access panel may enable maintenance to be carried out on the rib and spar inside the envelope without the entire envelope being opened. This may reduce the time taken to repair a spar or rib (for example to repair a puncture or replace a damaged part of the spar or rib) by providing direct access to the damaged area, as well as cost savings as the envelope can be restored simply by closing the access panel (as against the cost of resealing the envelope).

In a preferred embodiment one or more lines are attached to the inflatable wing to form a kite. A traction kite typically includes two or more control lines attached to the wing of the kite (usually at the wing tips but in some cases including bridle lines attached to the leading and trailing edges of the wing to assist with maintaining a desired profile to wing). These lines are commonly attached at the load end to a control bar which is manoeuvred by an operator to control the flight of the wing.

The applicants envisage that one application for the inflatable wing of the present invention may be as the wing of a kite of the type commonly used in many recreational and sporting activities. These include the use of a kite to provide traction to a person on a device configured to move over water (kite surfing and kite boarding), snow (kite skiing) and land (land yachts and buggies).

An inflatable wing according to the present invention may provide many advantages over wings of the prior art, including: · reduced time to inflate the rib as the inflatable volume of the spar truss is

significantly less (8% - 30%, depending on the diameter of the chords and struts used in the truss) than the volume of a comparable spar formed from a bladder; increased stiffness of the spar due to the truss structure which may assist the wing to maintain its form spanwise, thus improving the aerodynamic profile of the wing while also reducing the dependence on bridle lines to the leading and trailing edges to maintain the shape, which may result in less weight and drag in comparison with prior art wings; the increased stiffness spanwise may enable designers to design a variety of wing shapes other than the traditional semicircular arc or C shape, , which may provide a user with a wider choice of a wing tailored to a particular performance under various conditions - these shapes may include flattened wing shapes which may hold their shape with less reliance on additional bridle lines across the leading and trailing edges; and providing designers greater freedom in designing the shape of the spar truss, together with any inflatable rib trusses, to produce an aerodynamically efficient aerofoil profile for the wing both spanwise and chordwise, thus increasing performance of the wing during flight.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present invention will become apparent from the ensuing description which is given by way of example only and with reference to the accompanying drawings in which:

Figure 1 shows a schematic view of an inflatable wing according to one embodiment of the present invention; and

Figure 2 shows an inflatable spar truss according to one embodiment of the present

invention; and Figure 3 shows an inflatable spar truss according to another embodiment of the present invention; and

Figure 4 a schematic view of part of a wing according to one embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

An inflatable wing according to one embodiment of the present invention is generally indicated by arrow 1 in Figure 1. The wing 1 is configured as a traction kite having control lines 2, 3 attached to the wing tips 4, 5 of the wing. Further control lines 6, 7, 8 and 9 are attached to the leading edge 10 of the wing.

The wing 1 has a first sheet 11 and a second sheet 12 which define an upper and lower outer surface for the wing, the sheets joined at the leading edge 10 and a trailing edge 13 to form a flexible envelope. The sheets (11 and 12) are formed from a light weight fabric made from any suitable material, such as ripstop nylon, Cuben Fibre™ or other synthetic material as is well known in the art. When the wing is used as a traction kite for use on or over water (as in kitesurfing for example), it is important that the material is waterproof.

The leading edge 10 is formed by an inflatable spar typical of the prior art leading edge inflatable wings. The leading edge spar 15 is enclosed within the envelope formed by the sheets (11 and 12) and therefore is not shown in Figure 1 , but is shown in part in Figure 4. The leading edge spar 15 is typically in the form of an air (or other gas) filled bladder encased in a stronger external casing attached to the leading edge 10 of the wing. The leading edge spar 15 may be formed as a single bladder where the outer casing is configured to provide the spanwise shape of the leading edge, or may be composed of a series of sections joined together to form the spar. The wing 1 includes an inflatable spar in the form of a truss (a spar truss), generally indicated by arrow 16 in Figure 2 for an arc-shaped wing. The spar truss 16 includes a first chord 17 and a second chord 18 and a plurality of struts which form vertical 19 and diagonal 20 members of the truss between the first and second chords. The regions where the vertical and diagonal members join with a chord form the nodes 21 of the truss. The structure of the spar truss 16 is a web composed of contiguous triangular shapes 22, each triangular shape formed by a node on one chord which is linked to two nodes on the other chord by diagonal and/or vertical members. The first and second chords (17 and 18) and the vertical and diagonal members (19 and 20) are formed from thin walled flexible tubes having a diameter in the range from 8 to 15 mm, depending on the design and the size of the spar truss/wing. The tubes have a wall thickness of around 0.08 mm and are formed from extruded polyether TPU. The tubes are joined together at the nodes 21 so that the spar truss 16 can be inflated by pumping air into the spar through a single valve (not shown). To achieve this the spar truss 6 is pneumatically connected at a node 25 to an adjacent rib truss, generally indicated by arrow 26 in Figure 4, which in turn is pneumatically connected to the inflatable leading edge spar 15, so that the leading edge spar and the rib truss(es) and the spar truss(es) can be inflated through a single valve (not shown).

The first chord 17 is attached to the first sheet 11 by loops of material 27 fixed to the first sheet and tied or otherwise attached to the first chord. Similarly, the second chord 18 is attached to the second sheet 12 by loops of material 28 fixed to the second sheet and tied or otherwise attached to the second chord. A spar truss for a wing having a relatively flat spanwise profile is indicated by arrow 29 in Figure 3. Similar features in Figure 3 are labelled with the same numeral as discussed above for the spar truss 16 shown in Figure 2.

Comparison of the spar trusses 16 and 29 provides an indication of how a suitably designed spar truss may be used to dictate the shape of a wing. Furthermore, the spanwise stiffing of the wing provided by the spar truss may reduce the need for the relatively large number of bridle lines used in some prior art wing designs, thus reducing the additional weight and drag forces that these lines create.

The spar trusses 16 and 29 are configured to define aerodynamically efficient shapes and to provide the required stiffness to the spar truss when inflated. This is achieved by configuring each node such that the sections of tube connecting linked nodes, either between adjacent nodes on the same chord or between linked nodes on different chords, are angled to provide the desired shape for the first and second chords (to provide the desired spanwise shape to the attached first and second sheets) and to distribute the loads experienced by the wing in flight along the vertical and diagonal members. In this way a relatively stiff spar truss is formed having the desired curvature for the first and second chords (and hence the spanwise profile of the first and second sheets of the wing respectively).

The spar truss (16, 29) shown in Figure 4 has a node in the vicinity of the design centre of lift, indicated by arrow 34. The node 25 in this position is connected to a vertical member 30 (as well as diagonal members 31 and 32) of the spar truss, as well as to a chord and strut members of the adjacent rib truss 26.

A plurality of spar trusses (16, 29) are attached to the first and second sheets in a spanwise direction spaced apart in a chordwise direction, the number of spar trusses and their spacing being determined by the span of the wing and design criteria. Each of the plurality of spar trusses is pneumatically connected to the inflatable leading edge spar via one or more interconnected rib trusses, so that the entire internal structure of the wing (ie leading edge spar plus the plurality of spar trusses and rib trusses) can be inflated through a single valve (not shown). A plurality of closable air vents, in the form of access panels 33 are located in the first and second sheets towards the trailing edge 13 in the vicinity of a spar truss as illustrated in Figure 1. The access panels are formed as apertures in the sheet and are closable by a flap using Velcro™ strips. An access panel is opened to allow air to enter the interior of the envelope during inflation, thus reducing the suction within the closed envelope that could occur otherwise. The placement of the access panels 33 in proximity to a spar truss 16, 29 or rib truss 26 also enables a person to access the spar and spar members to carry out maintenance (repair a puncture or replace a damaged truss member) as and when required.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof as defined in the appended claims.

Claims

WHAT I CLAIM IS:

1. An inflatable wing for a traction kite, the wing including: a flexible envelope having a first sheet and a second sheet, the first and second sheets joined together at a leading edge, a trailing edge and two spaced apart wing tips; and

an inflatable spar located within the envelope and extending substantially spanwise between the two wing tips , characterised in that

the inflatable spar is located within the flexible envelope in a spaced apart relationship with the leading edge.

2. An inflatable wing for a traction kite as claimed in claim 1 wherein the spar is

attached to the first and second sheets.

3. An inflatable wing for a traction kite as claimed in either one of claims 1 or 2 wherein the inflatable spar includes a first inflatable chord and a second inflatable chord spaced apart from the first chord for at least a length of the spar, and one or more inflatable struts extending between the first chord and the second chord, the spar configured such that, when inflated, the first chord, second chord and the strut(s) form a truss.

4. An inflatable wing as claimed in claim 3 wherein a diameter of an inflated chord of the truss is in the range from 8 mm to 15 mm.

5. An inflatable wing as claimed in either one of claims 3 or 4 wherein a diameter of an inflated strut of the truss is in the range from 8 mm to 15 mm.

6. An inflatable wing as claimed in any one of claims 3 to 5 wherein a plurality of

inflatable struts and an inflatable chord are connected together to form a node, the configuration of the struts and chord at the node being arranged such that, when inflated, the forces exerted at the node by the struts and the chord are balanced.

7. An inflatable wing as claimed in claim 6 wherein at least one spar includes a node located at or in close proximity to the design centre of lift of the wing.

8. An inflatable wing as claimed in any one of claims 3 to 7 wherein at least one of the first and second inflatable chords is pneumatically interconnected to an inflatable rib truss located within the flexible envelope.

9. An inflatable wing as claimed in any one of claims 3 to 8 wherein the inflatable strut is pneumatically interconnected to at least one of the first and second inflatable chords.

10. An inflatable wing as claimed in any one of claims 2 to 9 wherein the first inflatable chord is attached to the first sheet of the flexible envelope.

11. An inflatable wing as claimed in any one of claims 2 to 10 wherein the second

inflatable chord is attached to the second sheet of the flexible envelope.

12. An inflatable wing as claimed in any one of claims 3 to 11 wherein the first chord is curved when inflated.

13. An inflatable wing as claimed in any one of claims 3 to 12 wherein the second chord is curved when inflated.

14. An inflatable wing as claimed in any one of claims 3 to 13 wherein the first chord and second chord form an aerodynamic shape when inflated.

15. An inflatable wing as claimed in any one of claims 1 to 14 wherein the flexible

envelope includes a closable air vent.

16. An inflatable wing as claimed in claim 15 wherein the closable air vent is an access panel in the flexible envelope.

17. An inflatable wing as claimed in any one of claims 1 to 16 including an inflatable leading edge spar.

18. An inflatable wing as claimed in any one of claims 1 to 17 including one or more lines attached to, or in the vicinity of, the wing tips to form a kite.

19. An inflatable wing substantially as described herein and with reference to and as illustrated by the accompanying description and drawings.