EP3589542A1

EP3589542A1 - Kite systems

Info

Publication number: EP3589542A1
Application number: EP18714344.1A
Authority: EP
Inventors: Henry REBBECK; Tim BRODRICK; William Hampton
Original assignee: Kite Power Systems Ltd
Current assignee: Kitemill AS
Priority date: 2017-03-01
Filing date: 2018-03-01
Publication date: 2020-01-08
Also published as: GB2561697A; GB201703322D0; WO2018158576A1; GB201803335D0

Abstract

One type of kite system for the extraction of energy from the wind includes a kite connected to a base unit using a tether. The kite system comprises: an anhedral wing having wing tips and a central part located vertically higher than the wing tips; a frame coupled to the wing by plural bridle lines;a pitch actuator connected to the frame; and a tether attachment mechanism for attaching the kite system to a tether at a tether attachment position. The pitch actuator is configured to move the tether attachment mechanism, and thus the tether attachment position, in the fore and aft direction with respect to a forward flying direction of the kite system.

Description

Kite Systems

Field

This specification related to kite systems and various aspects thereof.

Background

One type of kite system for the extraction of energy from the wind includes a kite connected to base unit using a tether. The tether is wound on a drum. The rotation of the drum as the tether is pulled off is used to generate electricity, and at the end of the power cycle the drum is reversed to wind in the tether.

To generate power efficiently using a kite it is desired to have the kite flying through the air. A static kite on the end of a tether can only produce lift relative to the actual wind speed; however, when the kite is allowed to move lift is increased due to the apparent wind that is created by the motion of the kite relative to the true wind.

A tethered kite has a region of airspace in which it can generate a high tension in the tether. The centre of this region lies directly downwind of the base unit and at an angle of elevation from the base unit dependent on, for example, the design of the kite, the limitations of the power generation equipment and the wind speed. The region in which the appropriate high tension can be generated will hereinafter generally be referred to as 'the centre of the wind'. If the kite moves away from the centre of the wind, in either azimuth or elevation, the amount of tension it can generate in the tether may be less than optimum from the system for extracting energy from the wind. It is therefore desirable to keep the kite near the centre of the wind and also to control the kite to move at a high speed in the wind. It has been suggested that a suitable flight pattern is obtained when the kite is controlled to fly a figure of eight or continuous loop pattern. If the power is to be generated at ground level and not on the wing then during the power generating phase, when the kite is flying in a loop or figure of eight pattern, line is being continuously pulled off of the drum spinning the electrical generator or pump making power.

When the power stroke is finished, the line needs to be retracted. During the retract stage it is necessary to the force generated by the kite, to reduce the energy requirement and increase overall efficiency. This is generally achieved by flying the kite away from the centre of the wind to the edge of the power generating area, particularly towards the zenith or edge of the window. The closer the position is to 90⁰ off of the wind direction, the lower the resistance to the kite being retracted. Some systems have been proposed that modify the kite in some way to reduce tension during the retraction

WO2016/075461 discloses a kite system which includes an actuator mechanism mounted further on a spar arrangement and which can influence the kite in pitch or in pitch and roll combined. Providing kite systems in large scale presents numerous engineering challenges, not least because the forces involved can be very high. The inventors are working towards a system including a wing with 450m² area and capable in a dual kite configuration of generating a power of 3MW. Summary of Various Embodiments

The scope of protection is defined by the appended claims.

Brief Description of the Drawings

Embodiments will now be described with reference to the accompanying drawings, which are by way of example only, and in which:

Figure 1 is an isometric view of a first embodiment of a kite system according to, and incorporating components according to aspects of, the present specification;

Figure 2 is an isometric view of a second embodiment of a kite system;

Figure 3 is an isometric view of a third embodiment of a kite system according to, and incorporating components according to aspects of, the present specification;

Figure 4 is an isometric view of a fourth embodiment of a kite system according to, and incorporating components according to aspects of, the present specification;

Figure 5 is an isometric view of a fifth embodiment of a kite system according to, and incorporating components according to aspects of, the present specification;

Figure 6 is a side view of the Figure 1 kite system;

Figure 7 is an isometric view of a wing of the kite system of Figure 2;

Figure 8 is an isometric view showing details of some components of the Figures 1 to 4 and 5 kite systems, according to, and incorporating components according to aspects of, the present specification; Figure 9 is an isometric view showing some components of the Figures 1, 2, 3, 4 and 5 kite systems, according to, and incorporating components according to aspects of, the present specification;

Figure 10 is an isometric view of a sixth embodiment of a kite system according to, and incorporating components according to aspects of, the present specification;

Figure 11 is an isometric view of a seventh embodiment of the kite according to, and incorporating components according to aspects of, the present specification;

Figure 12 is an isometric view of an eighth embodiment of the kite according to, and incorporating components according to aspects of, the present specification;

Figure 13 is an isometric view of a ninth embodiment of the kite according to, and incorporating components according to aspects of, the present specification;

Figure 14 is a schematic drawing showing a kite system and it's roll and pitch neutral positions; and

Figure 15 is an isometric view of a tenth embodiment of the kite according to, and incorporating components according to aspects of, the present specification.

Detailed Description of Embodiments

In the drawings, like reference numerals denote like elements throughout. In brief, a kite system includes a frame attached to a wing and including roll and pitch actuation mechanisms. The pitch actuator, its attachment to the frame and the attachment of the frame to the wing allows the pitch of the wing to be adjusted, and thus allows the power generated by the kite system to be adjusted (allowing depowering for retracting the kite and repowering for electricity generation). The roll actuator, its attachment to the frame and the attachment of the frame to the wing allows the wing to be rolled, and thus allows the kite to be steered and thus allows control over the trajectory of the kite system. Moreover, through appropriate location and orientation of the roll actuator, steering of the kite system can be achieved through relatively small forces. Additionally, through appropriate location and orientation of the pitch actuator, powering and depowering of the kite system can be achieved through relatively small forces. By allowing pitch and roll control to be achieved using relatively low forces, the use of large (and heavy) actuators and power supplies can be avoided. This is a significant advantage for kite systems where increased kite mass results in an increased wind speed requirement for take-off and increases the forces required for manoeuvring the kite. The pitch and roll actuator mechanisms are quite different to those disclosed in WO2016/075461. Moreover, by locating a tether attachment mechanism approximately coterminous with a roll neutral axis of the kite system, relatively little energy is needed to effect roll control. Improved kite control can be experienced by having tether attachment mechanism located vertically above the roll neutral axis of the kite system,

Advantageously the centre of gravity of the kite system is substantially coterminous with the roll neutral axis of the kite system. Advantageously the centre of gravity of the kite system is below the roll neutral axis of the kite system, since this can provide good stability for the kite system l in low tether tension flight.

Additionally, by providing the pitch actuator with the ability to move the tether attachment position in the fore and aft direction with respect to the forward flying direction of the kite system and placing the pitch actuator at a position below the pitch neutral point of the kite system, good pitch control can be experienced whilst requiring relatively low energy to effect the pitch control.

Referring firstly to Figure l, a kite system l is shown. The kite system l includes a number of components and subsystems, as follows.

Firstly, the kite system l includes a wing loo. The wing loo may be a rigid wing, a flexible (e.g., parafoil) wing or a hybrid wing, for instance. The wing loo includes a trailing edge 102 and a leading edge 101. As such, the forwards direction for flight of the wing 100 is up out of the page and to the left, for the arrangement as illustrated in Figure 1.

The wing 100 has wing tips 103. The wing 100 has a curved profile, with the wing tips being downward of the centre part of the wing 100. The wing 100 thus is anhedral. Because the wing is anhedral, it is highly manoeuvrable and is not prone to excessive side slip when turning corners. However, its anhedral shape means that it does not have natural wing stability. Greater stability is provided with greater tether lengths, but the stability of an anhedral wing is less than that of a straight wing or a dihedral wing. The kite system 1 also includes a frame 300. The frame 300 is attached to the wing 100 by an attachment mechanism 200. The frame 300 is triangular in shape. A substantially horizontal spar 307 is located at the lowermost part of the frame.

Opposite ends of the horizontal spar 307 are connected to lowermost ends of left and rights spars 311, 312 respectively. Uppermost ends of the left and right spars 311, 312 are connected at a top, or uppermost, part of the frame 300. The horizontal spar 307 may also be called a roll slider bar, since a roll slider is mounted thereupon, as is described below.

The spars 307, 311, 312 of the frame 300 can be constructed from aluminium, glass fibre, carbon fibre or other material that is lightweight and good in compression. The spars 307, 311, 312 of the frame 300 can be thinner at the bottom of the frame 300 because of the lower forces than exist there. The spars 307, 311, 312 of the frame 300 can be aerodynamically shaped to minimise drag.

The spars 307, 311, 312 of the frame 300 can be joined together in any suitable way. The frame 300 is generally triangular in shape but need not be exactly triangular. The spars 307, 311, 312 of the frame 300 may be hollow. Hollow spars can house electronics components, to protect them from weather etc.

The kite system 1 furthermore includes a roll actuator 500.

A pitch actuator 501 also is provided as part of the kite system 1.

The kite system 1 is shown connected to a tether 402. The tether 402 is attached to the frame 300 by an attachment mechanism 400. The tether 402 may be considered to be external to the kite system 1. The tether 402 is configured to withstand high tensile forces and may take any suitable form. The tether attachment mechanism 400 may take any suitable form. It provides the function of attaching the uppermost end of the tether 402 to the frame, in particular via a pitch actuator 501. As is described below, the tether attachment mechanism 400 can be moved forwards and aft relative to the frame 300.

The uppermost part of the frame 300 is connected to the wing 100 by the wing and frame attachment mechanism 200. The wing 100 also is connected to the frame 300 by a bridle line arrangement. In the arrangement of the kite system 1, bridle lines 201 are connected between the uppermost part of the frame 300 and the wing 100 at a relatively central portion of the wing 100. Also, further bridle lines 201 are connected from the frame 300 to locations on the wing 100 that are relatively near to the ends of the wing. In the kite system 1 of Figure 1, bridle lines 201 are connected from the lowermost part of the frame 300 to the ends of the wing 100. Furthermore, additional bridle lines 201 are connected to the wing 100 at locations between the bridle line attachment points that are at the ends of the wing and relatively central to the wing. In particular, in the kite system 1, bridle lines 201 are connected from such intermediate positions on the wing 100 to intermediate positions on the left and right spars 311, 312. The bridle lines 201 maybe rigid or flexible. Rigid bridle lines 201 have an advantage that they can be made in an aerofoil shape and thereby reduce drag. Rigid bridle lines 201 also prevent the wing 100 from folding so easily.

Most of the aerodynamic forces on the wing 100 are near the centre of the wing. These forces are dominated by the lift force. Therefore, having the bridles 201 attached near to the top of the triangle of the frame 300 means that the area of high loads can be reduced and therefore weight/mass of the frame 300 can be minimised. With a flexible (soft or non-rigid) wing 100, a hoop tension is required to keep it in the correct shape and therefore the bridles 201 attached to the wingtips 103 need to apply this force by pulling partially tangential to the curvature of the wing 100. As such, the size of the triangle of the frame 300 is based in part on the desire of having the tip bridle lines 201 supported relatively low down and spread out from bridle lines 201 connecting to near the centre of the wing 100. However, load from the tip bridle lines 201 is less than with other bridle lines 201, and so there is a lesser structural requirement on the bottom tips of the triangle of the frame 300. For a rigid or semi-rigid wing 100, there are expected to be at least 2 or 3 bridle attachment points on each side of the triangle of the frame 300 to obtain the optimal balance between having the bridle lines 201 at the optimal angles, reduced structural load on the frame 300, and simplicity of design. There are bridle line attachments to the wing 100 at different positions in forward and aft direction for the wing 100. In particular, in the arrangement of the kite system 1 of Figure 1, there are attachment points at three different locations in the fore and aft direction, one being relatively close to the leading edge 101, one being relatively close to the training edge 102 and one being relatively central. This applies to the bridle connection points at each of the positions in the span-wise direction of the wing 100. During flight, lift provided by the wing as it moves forward in the air places tension on the bridle lines 201, which communicate the tension to the frame 300. Through these forces, the left and right spars 311, 312 are in compression. In particular, both an upper section 301 between the top of the frame and the part of the spars 311, 312 at which the intermediate bridle lines 201 are connected is in compression. Also, a lower section 302 of the left and right spars 311, 312, which is located between the connection point of the intermediate bridle lines 201 and the lowermost bridle lines 201, also is in compression. However, the horizontal spar 307 (and if present, also other horizontal spars) is in tension. It can be imagined that the tips of the wing 100 try to pull the frame apart at the lowermost end. By understanding these forces, the frame 300 can be designed so as to include the needed strength at the relevant locations, both in terms of the structural components and the joints between the structural components.

Although not shown in Figure 1, brace members may be provided as part of the frame 300, to strengthen the frame. For instance, the brace member 304 of Figure 5 may be applied also to other embodiments. Additional brace members may also be applied, in particular to resist bending of the side spars 311, 312 and separation of the side spars 311, 312 from the horizontal spar 307. Because these brace members typically need to carry only tension forces, they may be provided by wire, cord or such like.

As is best seen from Figure 9, the tether 402 is connected at its uppermost end by the tether attachment mechanism 400 to the pitch actuator mechanism 501, which is located at the uppermost part of the frame 300. In essence, the pitch actuator 501 constitutes a mechanism by which the position of connection of the uppermost end of the tether 402 to the frame can be adjusted in the forward and aft direction relative to the wing 100. Various ways of providing the pitch actuator mechanism 501 will be apparent to the skilled person, and one suitable mechanism will now be described.

The pitch actuator 501 comprises an elongate component, for instance a lead screw or belt or linear actuator, pulley, or rack and pinion, which extends from an end that is attached to the uppermost part of the frame 300 in a generally forwards direction relative to the wing 100. The direction is generally forwards in that it may deviate slightly (by up to a few degrees) from the exactly forwards direction. At the forwards end, the pitch actuator 501 is connected a brace 406, which is also connected to the frame 300 at a position that is upwards from the position at which the pitch actuator 501 is connected to the frame 300. Because the tether 402 is in tension when the kite system 1 is in flight, the arrangement of the pitch actuator 501 causes the brace 406 to be in tension. All of the force that is communicated through the tether is

communicated to the frame 300 via the pitch actuator 501, which therefore is constructed with a suitable strength, resistance to bending etc.

As is explained below, the pitch actuator 501 allows the location of the connection of the tether 402 in the forward and aft direction to be adjusted, that is it allows the connection point of the tether 402 to the frame 300 to be adjusted in the forwards and backwards directions. The connection point of the tether 402 is provided by the tether attachment mechanism 400. As is explained below, this allows control of the pitch of the kite system. As can best be seen from Figure 8, the roll actuator 500 includes a number of components. In essence, the roll actuator 500 includes a tether constraining component 403 for at least partially constraining the tether 402 in the horizontal plane. In this example partial constraint is provided by a tether slot 403 that is connected to a secondary pitch actuator 502 (although as explained below the secondary pitch actuator 502 can be omitted), and a mechanism for moving the tether constraining component 403 in a span-wise direction relative to the wing 100. A number of suitable mechanisms will be apparent to the skilled person, and one suitable mechanism will now be described. In the arrangement shown in Figure 8, the roll actuator 500 includes a mechanism that engages with the horizontal spar 307 that allows the roll actuator 500 to move along the horizontal spar 307 in the direction of extension of the spar 307, carrying with it the tether constraining component 403. Put another way, the roll actuator 500 includes a roll slider mechanism that engages with the roll slider spar 307 to move the tether constraining component 403 along the roll slider spar 307 and thus in the horizontal direction (although it may deviate from horizontal by up to a few degrees). Various mechanisms can control this roll slider or roll actuator 500, such as a ball screw, linear actuator, rack and pinion, pulley, lead screw, or belt. The actuator 500 and the spar 307 (or a component that is attached to or integral with the spar) may be considered to cooperate to provide the function of moving the tether constraining component 403 along the horizontal spar 307, and may be considered to be a single mechanism. As can further be seen from Figure 8, the tether constraining component 403 is connected to the secondary pitch actuator 502, and these together provide further control of the pitch of the kite system 1. In particular, the secondary pitch actuator 502 includes a mechanism that allows the location of the tether constraining component 403 to be moved in the forwards and aft direction. Doing so changes the distance from the horizontal spar 307 at which the tether 402 is located. This can be used to control the pitch of the kite system 1. The secondary pitch actuator 502, and may for instance be the same or similar to the pitch actuator 501 as described above and below.

If two pitch actuators 501, 502 are provided, the uppermost pitch actuator is the primary one and the lowermost pitch actuator has the primary function of following the tether 402 and stopping it bouncing back and forth. The lowermost pitch actuator in ideal conditions does not touch or bend the tether 402.

Figure 6 shows the kite system 1 in side cross section. Figure 6 is particularly useful in showing the incidence angle 205 between the wing 100 and the frame 300. It also shows the wing-frame attachment position 206. The wing-frame attachment position 206 can be specified as a percentage of the chord distance between the leading edge 101 and the trailing edge 102.

This Figure also allows the projected tether angle to be visualised. As the pitch actuator 501 actuates and moves the tether attachment mechanism 400 back towards the frame 300, then it will be seen that the wing 100 rotates such that the leading edge 101 moves upwards, increasing the angle of attack, for a given angle of the tether 402.

Figure 6 is useful in showing also that the left and right spars 311, 312 may be tapered such as to have a greater thickness in the forward and back directions at the uppermost end of the spars than at the lowermost end of the spars 311, 312. This allows them to have good strength in the forward direction whilst having an aerodynamic profile.

Referring back to Figure 1, the kite system 1 further includes landing skids 306. These are located at the lowermost part of the kite system 1. They are located at the opposite ends of the horizontal bar 307. The landing skids 306 allow the kite system 1 to be supported in an upright position on a surface, for instance the ground or a landing pad. In the Figure 1 kite system 1, the attachment 200 between the wing 100 and the frame 300 comprises an interface 204. It also comprises an elongate member 200 such as a bar or rod, which extends upwards from the frame 300. The bar or rod 200 is formed rigidly with the frame 300, and is not hinged or otherwise flexibly connected. At the interface 204, there may be a sleeve formed in the wing 100, such as to allow movement in the vertical direction of the rod 200 relative to the wing 100. The sleeve may be almost the same size as the rod or bar, or it may be substantially larger so as to allow some freedom of movement other than in the vertical direction. In any case, the effect of the attachment 200 between the wing and the frame 300 is to result in substantially no vertical forces being communicated between the frame 300 and the wing 100 by the attachment mechanism 200. However, there are side forces, i.e. forces in the forward and aft or span-wise direction relative to the wing 100, communicated between the frame 300 and the wing 100 by virtue of the attachment mechanism 200. Instead of a sleeve, vertical movement between the wing 100 and the frame 300 may be provided by virtue of a ball joint mechanism forming part of the attachment

mechanism 200. For a wing 100 that is rigid or has a rigid centre section, the interface 240 may be a hard (without significant flexibility) mechanical connection. A key function of the attachment 200 is to communicate pitching forces from the frame 300 to the wing 100. However, it can be omitted.

The spar arrangement 2 can be used for the fitting of lighting. Lights can also be fitted so that optical sensing can be used for the final docking manoeuvres. Cavities within the spars can be used for the mounting of avionics and communication equipment, for example sensing platforms incorporating GPS, inertial measurement equipment, radar, radio triangulation transponders, communication equipment, batteries, super capacitors and all other ancillary equipment.

The spar arrangement 2 may store energy storage devices, avionics, sensors, communications equipment, and/or antennas (not shown).

Roll of the kite system 1 is achieved by controlling the roll actuator 500 to move the tether constraining component 403 along the roll slider bar 307, so in a sideways direction relative to the flying direction. This results in force being applied laterally to the tether 402, which may deform by a very small amount. However, the force applied laterally to the tether 402 causes the frame 300, and thus the rest of the kite system 1, to pivot around the tether attachment 400 at the TAL. In particular, the whole kite system 1 rotates about the tether attachment 400 relative to the tether 402. This constitutes roll of the kite system 1. This is achieved by applying force tangential to the tension force in the tether 402, so does not require overcoming any of the tether forces. Instead, it requires overcoming only forces applied by the wing 100 that resist the rolling motion. These forces vary in flight for various reasons, including the gravity effect of the movement of the mass of the kite system 1

This rolling of the kite system 1 is achieved with minimal bending of the tether 402. Avoidance or minimising of bending of the tether is highly desirable considering the high forces that are communicated through the tether.

Pitch of the kite system 1 relies on a moment balance about the location of the tether attachment 400, where the tether 402 attaches to the kite system 1 (in pitch). To control the pitch of the wing 100, the location of the tether attachment 400 is moved fore and aft by the pitch actuator 501. Changing the location of the tether attachment 400 in the fore and aft direction changes the location of the effective (virtual) attachment between the tether 402 and the wing 100, and results in the wing 100 pitching forwards or backwards according to the sign of the change. A similar effect would be experienced by changing the lengths of bridle lines at the front of a kite relative to bridle lines at the rear of a kite, which also changes the virtual tether attachment point and thus the pitch angle of the kite.

Alignment between the frame 300 and the wing 100 is provided by the lift force of the wing 100 keeping the bridle lines 201 in tension, although there is some influence also from the wing-frame attachment mechanism 200 and the interface 204. The pitch actuator 501 does not need to move the tether 402 to change the pitch of the wing 100. The pitch actuator 501 does however hold the position of the tether attachment 400 at the correct location and resist or prevent external forces from moving it further. This can prevent oscillation that might otherwise result from the change in angle of attack of the wing 100. Because the movement of the location of the tether attachment mechanism 400 is largely perpendicular to the direction of the tether force (the tension in the tether), there is little energy requirement to achieve the movement.

Using mathematical modelling and experimentation, the inventors have found that there are a small number of factors that are particularly relevant to the forces needed to adjust pitch and roll of the kite system. Figure 14 will be referred to now in

explanation.

The roll neutral point (RNP) is a point in the vertical direction about which the kite is neutrally stable. It can also therefore be said to be the point at which the lift vector rotates due to side slip. The roll neutral point actually exists on a roll neutral axis which extends fore and aft relative to the flight direction, and the terms roll neutral point and roll neutral axis will be understood to be two- and three-dimensional versions of the same thing.

The location of the roll neutral point RNP depends on the aerodynamics of the kite system 1. The roll neutral point RNP is dependent on the shape of the kite, including the degree of curvature, but for an anhedral wing with curvature like that shown in Figure 1 is usually between one third and one half of the wingspan distance down from the centre of the wing.

The location of the roll neutral point RNP moves depending on the flight condition, such as AoA (angle of attack - shown in Figure 6) and roll angle. For a zero roll angle and a given AoA value, the roll neutral point RNP of a kite system can be calculated mathematically, and can be verified experimentally if needed. In the case of a wing that is the shape of an arc, the calculation of the roll neutral point is straightforward since the lift vectors of the different parts of the wing all go through the same point. In other cases, the mathematical calculation is a little complex because the roll neutral point results from a secondary effect (change in side slip). The roll neutral point RNP for a zero roll angle and an AoA value for a powered position of the kite system 1 is shown in Figure 14.

The tether attachment location TAL is the point where the tether 402 is attached to the kite system 1, and is defined on an axis that extends fore and aft (perpendicular to the page for Figure 12). This point defines the axis about which the kite system 1 can roll relative to the tether 402 when there is high tether tension.

The centre of gravity CG is the position about which the kite system rolls when there is no tether tension. The centre of gravity CG is the centre point (on an axis extending fore and aft - perpendicular to the page for Figure 12) of the mass of the wing 100, the frame 300 and the other components of the kite system 1. The inventors have found that placing the tether attachment point close to the roll neutral point reduces the energy required to roll the kite system 1. Since the energy needed is related to the weight of the kite system and since very high acceleration (up to log) can be experienced in a turn, minimising the energy required to roll the kite is an important advantage.

The closer the tether attachment point is to the roll neutral point, the lower the energy required to roll the kite system 1. Particularly satisfactory performance can be found with the tether attachment point being within 10%, or less preferably 20%, and further less preferably 30%, of the distance between the centre point of the wing and the tether attachment point from the roll neutral point.

The closer the centre of gravity is to the roll neutral point, the lower the energy required to roll the kite system 1. Particularly satisfactory performance can be found with the centre of gravity being within 10%, or less preferably 20% and further less preferably 30%, of the distance between the centre point of the wing and the centre of gravity from the roll neutral point. By providing the tether attachment location TAL, the roll neutral point RNP and centre of gravity CG relatively close to one another, the forces required for rolling the kite system 1 are minimised. Also, because most of the forces from the wing are contained within the area defined by the frame 300, the frame 300 in conferred with the requirement to be strong enough to support these forces. Moreover, the frame 300 can be relatively small, and thus relatively light.

The inventors have found that having the centre of gravity CG below the roll neutral point RNP provides good stability for the kite system 1 in low tether tension flight. The inventors have found furthermore that for good kite control it is better if the roll neutral point (in steady flight) is above the tether attachment location TAL. The inventors have found also that the position of the centre of gravity CG relative to the tether attachment location TAL is not so crucial for roll control. However, locating the centre of gravity CG below the tether attachment location TAL means that the kite system 1 naturally flies in the correct vertical orientation. For pitch control, the situation has been found to be slightly different. However, for a highly curved anhedral wing the pitch neutral point PNP is vertically a long way above the roll neutral point RNP and much closer to the wing 100 (typically about one half to one third of the height of the wing down from the centre of the wing), and therefore a different mechanism is provided to minimise the forces needed for pitch control.

The inventors devised an arrangement which does not require the tether 402 to be extended up to the pitch neutral point. In particular, the arrangement involves using an actuator (the pitch actuator 501) to push the tether attachment mechanism 400 fore and aft, for which the moments then resolve to pitch the kite system 1 but requiring nearly no work done. Put another way, the arrangement minimises force requirements for pitching by allowing the kite system 1 to rotate to the desired pitch angle. Moreover, it allows for more control of the pitch of the kite system 1 than can be achieved in conventional arrangements which use deformation of the wing itself or adjustment of bridle line lengths to adjust pitch.

By using a rigid frame 300, there is less deformation of the wing compared to conventional kite steering and pitching techniques. This significantly reduces the wear of the material of the wing 100. It also allows for improved aerodynamic performance, improved controllability and better kite stability. It also avoids the undesirable phenomenon of tip folding, which can occur in conventional kites. The use of the frame also provides more options for bridle line positions, and thus allows bridle line and wing design optimisation. Furthermore, the aerodynamics have become simpler and more predictable through the use of the frame 300. A rigid structure (the frame 300) additionally allows for mounting points of equipment (e.g. lighting, thrusters, propellers, control surfaces) and attachment points for docking of the kite system 1. Supporting the kite system 1 at or near ground level, e.g. in docking, by the frame 300 (e.g. a brace strut 304) allows it to freely rotate into the wind and makes it resistant to folding or tucking

Referring now to Figure 2, a kite system 2 according to a second embodiment will now be described. Only differing features will be described, and features which are substantially the same as the Figure 1 kite system 1 are not discussed here. The same also applies to the later embodiments and figures. Figure 7 shows the wing 100 of the Figure 2 kite system 2.

The wing 100 of the Figure 2 kite system 2 is shown as having a ram air inlet 104 at the leading edge 101. This allows the wing 100 to be inflated by air that is forced into the ram air inlet 104 as the wing 100 flies forward through the atmosphere. The wing 100 here may be a parafoil wing or a hybrid wing.

The kite system 2 of Figure 2 includes many more bridle lines 201 than the kite system 1 of Figure 1. This allows the wing 100 to be more flexible, since the shape of the wing 100 can be maintained effectively by the bridle lines 201 without requiring significant rigidity of the wing itself. The number of connection points of bridle lines 201 to the wing 100 is much greater than the number of connection points of bridle lines 201 to the frame 300. In the kite system 2 of Figure 2, the bridle line arrangement comprises primary bridle lines 201 and secondary bridle lines 202. The primary bridle lines 201 may be rigid or flexible. The secondary bridle lines 202 may be rigid or flexible.

As with the Figure 1 kite system 1, the frame 300 includes left and right spars 311, 312 and a horizontal spar 307.

The Figure 2 kite system 2 is absent of the secondary pitch actuator 502 of the Figure 1 kite system 1. Instead, the tether 402 is located within a slot 403 that is connected to the roll actuator 500. In this way, the tether 402 is free to move in the forward and aft directions, within the limits provided by the ends of the slot, but is constrained in the left and right directions by the side edges of the slot 403. This allows roll control to be provided by the roll actuator 500. This means also that pitch actuation is enabled solely by the pitch actuator 501 that is located at the uppermost part of the frame 300. A third embodied kite system 3 will now be described in reference to Figure 3.

Reference numerals are re-used from Figure 1 for like elements, and only different features will now be discussed.

The main difference between the kite system 3 and the kite system 1 of Figure 1 is the arrangement of the roll actuator 500 and the secondary pitch actuator 502. In the Figure 3 kite system 3, the roll actuator 500 includes a triangular frame. One side of the triangle of the frame extends substantially parallel to, and is in close location with, the horizontal spar 307 of the frame 300. The other two parts of the triangular frame of the roll actuator 500 extend forward from ends of the base of the triangle to join at a location that is forward of the horizontal spar 307. The secondary pitch actuator 502 comprises a member that extends forward from the substantially central location on the roll actuator at the part where it contacts the horizontal spar 307 forwards to a location substantially at the point of the triangle provided by the frame of the roll actuator 500. The tether constraining component 403 is controlled so as to be moveable along the longitudinal axis of the secondary pitch actuator 502, so as to adjust the distance between the tether 402 and the horizontal spar 307. Some detail of this arrangement (although not the triangular frame) is shown in Figure 8. Additionally, the roll actuator 500 is operable to move the tether 402 laterally relative to the wing 100, that is to say in the same direction as the horizontal spar 307 extends, so as to effect control of the roll of the kite system 3. The triangular arrangement of the roll actuator 500 provides improved strength for a given mass, or put another way provides an improved strength to mass ratio, whilst allowing roll and pitch control to be achieved.

Referring to Figure 4, a fourth embodied kite system 4 will now be described.

Reference numerals are retained from Figure 1 for like elements, and only differing elements will now be described.

In the Figure 4 kite system 4, the frame 300 includes a second horizontal spar 307a. The second horizontal spar 307a is located between the first horizontal spar 307 at the lowermost part of the frame 300 and the point of the triangle at the uppermost part of the frame 300.

In general, a secondary roll actuator 502 is provided on one of the horizontal spars 307, 307a and a primary actuator 500 is provided on the other one of the horizontal spars 307, 307a. The secondary roll actuator 502 includes a tether constraining component 405, which takes the form of a slot. The primary actuator 500 includes a gated tether constraining component 403. The gated tether constraining component 403 is structurally similar to the tether constraining component described above, save for comprising an open free end 308, operable to allow the tether to exit out of the gated tether constraining component. It will be appreciated that it may, under certain circumstances, be advantageous or necessary to restrict or prevent movement of the tether out of the tether constraining component 403. To facilitate this, the gated tether constraining component is, in some examples, fitted with a tether gate element 309 operable to selectively restrict or prevent movement of the tether out of the tether constraining component 403. The tether gate element may be formed in a number of suitable ways easily envisaged by the skilled person, all of which are operable to switch between an open configuration in which the tether is allowed to exit out of the gated tether constraining component, and a closed configuration in which the tether is constrained within the gated tether constraining component in a manner substantially similar to that described above. It will be appreciated that, whilst described in connection with the kite system 4, the gated tether constraining component may, in principle, be comprised in any of the embodiments of the kite system as described above, and/ or as described in the following.

In this example, the secondary roll actuator 502 is provided on the second horizontal spar 307a.

The secondary roll actuator 502 is controllable to be moved laterally along the second horizontal spar 307a. This allows for the tether 402 to exit out of the front of the tether constraining component 403 of the primary roll actuator 501 while still being constrained by the tether constraining component 405 of the secondary roll actuator 502. This means that the angle that the tether 402 can take relative to the frame is much higher due to the close location of the roll slider 405. When the wing 100 is in a powered position (in which the tether 402 is pulled close to the frame 300) then the tether 402 slides into the gated tether constraining component 403 where it has a better moment to act against the tether 402.

The two roll actuators 501 and 502 may share a single actuator and utilise a linking mechanism, or they may have separate actuators.

The secondary roll actuator 502 may include a mechanism that allows the location of a second tether constraining component 405 to be moved in the forwards and aft direction, i.e. a secondary pitch actuator. Doing so changes the distance from the second horizontal spar 307a at which the tether 402 is located. As is explained elsewhere, this can be used to control the pitch of the kite system 1 and provides a function of stabilising the tether position. The kite system 4 includes a thruster mechanism. In particular, the kite system 4 includes left and right thrusters, each labelled 600 in the Figure. The thrusters 600 may be configured to provide downwards thrust, so as to enable lifting of the kite system 4. This may be useful if for instance there is insufficient wind speed to enable the wing 100 to lift the kite system 4 from a dock or the ground. The thrusters 600 may alternatively or additionally be configured to provide thrust force in a backwards direction, so as to result in the kite system 4 being propelled forwards.

By causing application of differential vertical thrust on the left and right sides of the kite system 4, the kite can be rolled (and thus steered) through the thrusters 600.

By causing application of differential horizontal thrust on the left and right sides of the kite system 4, the kite can be adjusted in yaw (and thus steered) through the thrusters 600. The thrusters 600 maybe configured to be rotatable such than one provides a forward thrust and the other provides a backwards thrust.

The thrusters 600 may be configured to be rotatable between positions in which they can provide downwards thrusts and positions in which they can provide backwards thrusts, for instance by being pivotable about an axis that extends laterally relative to the wing 100. The thrusters 600 may also be configured to be rotatable to positions in which they can provide forward thrusts.

A fifth embodiment of a kite system 5 will now be described with reference to Figure 5. Reference numerals are retained from Figure 1 for like elements.

Instead of the pitch actuator 501 being located at the uppermost end of the frame 300, the pitch actuator 501 in the kite system 5 is located approximately half way between an uppermost end of the frame 300 and the lowermost end, or more generally between one third of the distance between the top and the bottom and two thirds of the distance between the top and the bottom.

In this embodiment, the pitch actuator 501 is located on a second horizontal spar 304, that extends between the left and right spars 311, 312. This enables a triangular frame to be provided in the substantially horizontal plane. By connecting the forwardmost point of this triangle to the tether brace 406, a tetrahedron shape (which is inherently very strong) is provided. Additionally, the attachment mechanism 200 extends to the second horizontal spar 304, and is attached also at the top of the frame 300. The frame 300 here is taller than the frame 300 of the Figure 1 kite system 1. The horizontal location of the pitch actuator 501 is approximately the same in both kite systems 1, 5. As with the Figure 1 arrangement, the position at which the tether 402 is connected to the pitch actuator 501, in terms of its distance from the vertical plane of the frame 300, is controllable so as to control the pitch of the kite system 5.

In Figure 10, a kite system 10 is shown. Like reference numerals relate to like elements from Figure 1.

The main difference between the kite system 10 of Figure 10 and the kite system 1 is that the wing-frame attachment mechanism 200 comprises two members (e.g. rods or poles), for which there are two interfaces 204 on the wing 100. One or more braces are provided between the forward end of the pitch actuator 501 and the frame 300 or the attachment mechanism 200, but these are omitted from the drawing for simplicity. By providing two interfaces 204, less force is experienced at any interface 204 for a given pitching moment. A further embodied kite system 11 will now be described with reference to Figure 11. Reference numerals are retained from Figure 1 for like elements.

The primary difference between the kite system 11 and the kite system 1 is that the roll actuator 500 is provided above the lowermost part of the frame 300. In particular, it is provided on a horizontal spar 307 that is located between the lowermost and uppermost parts of the frame 300, more specifically between one third of the distance between the top and the bottom and two thirds of the distance between the top and the bottom. Operation is substantially the same as for the Figure 1 system, although less movement and higher force is required of the roll actuator 500 for a given amount of roll.

A further embodied kite system 12 will now be described with reference to Figure 12. Reference numerals are retained from Figure 1 for like elements. In figure 12, control surfaces 601, 602, 603 are provided as part of the kite system 12. The first control surface 601 is provided above the tether attachment mechanism 402, so between the tether attachment mechanism 402 and the wing 100. The first control surface 601 is relatively distant from the tether attachment mechanism 402. The first control surface 601 here is connected to the frame-wing attachment spar component 200. The first control surface 601 extends in a generally vertical plane.

The second control surface 602 is provided below the tether attachment mechanism 402, so on the opposite side of the tether attachment mechanism 402 to the wing 100. The second control surface 602 is connected to the frame 300, in particular to the left spar 311. The second control surface 602 extends in a plane that is off vertical.

The third control surface 603 mirrors the second control surface 602. It is provided below the tether attachment mechanism 402, so on the opposite side of the tether attachment mechanism 402 to the wing 100. The second control surface 602 is relatively distant from the tether attachment mechanism 402. The third control surface 603 is connected to the frame 300, in particular to the right spar 312. The third control surface 603 is relatively distant from the tether attachment mechanism 402. The third control surface 603 extends in a plane that is off vertical, but in a different direction to that of the second control surface (it mirrors the direction).

The control surfaces 601, 602, 603 include actuators (not shown), which can cause deflection about a hinge axis. The hinge axis is co-located with the spar 200, 311, 312 to which it is connected, or alternatively the axis may be behind the spar. One, two or advantageously all of the control surfaces 601, 602, 603 are balanced control surfaces, in that the axis of rotation is behind the front edge. This reduces the force needed to deflect the control surface 601, 602, 603.

Actuation of the first control surface 601 to deflect left or right introduces, by virtue of air passing over the control surface 601, a roll moment. This tends to cause the kite system 12 to roll about the roll neutral axis, or an axis close thereto. The same applies regarding actuation of the second control surface 602, or the third control surface 603. Thus, any of the control surfaces 601, 602, 603 can be used to induce a roll moment in the kite system 12. With sufficient roll force, rolling of the kite system 12 can be executed solely by one or more of the control surfaces 601, 602, 603. Thus, the control surface 601, 602, 603 constitute a roll actuation mechanism. The roll moment or force depends on in particular the amount of deflection of the control surface 601, 602, 603 and the speed at which the kite system 12 travels through the air.

Additionally, by using one or more of the control surfaces 601, 602, 603 to roll the kite system 12, the control surfaces 601, 602, 603 may cause the tether 402 to move the roll slider 500 along the horizontal spar 307. In this way, the roll actuator 500 may be used in reverse to generate electrical power, for storage in a battery (not shown) for later use by electrically-powered components of the kite system 12.

Furthermore, by controlling the second and third control surfaces 602, 603 to operate in different directions, a drag force can be provided. Since this drag force is below the pitch neutral point PNP, this results in the kite system 12 being pitched forwards, or put another way it reduces the angle of attack of the wing 100. Thus, pitch actuation can be achieved using the control surfaces 601, 602, 603. Thus, the control surfaces 601, 602, 603 constitute a pitch actuation mechanism. A pitch effect can be achieved using only one or a different pair of the control surfaces 601, 602, 603, although such would typically also produce yaw and/or roll forces as well as a pitch force.

A drag force above the pitch neutral point PNP can be provided by providing the first control surface 601 with a split and causing different parts of the control surface 601 to deflect in left and right directions simultaneously. This then pitches the kite system 12 backwards.

By actuating the control surfaces 601, 602, 603 such as to provide forward pitching of the kite system 12, the energy required for the pitch actuator 501 to move the tether connection mechanism in the forwards direction is reduced, or even reduced to zero. Thus, the operation of the pitch actuator 501 to move the tether attachment mechanism 400 in the forwards direction can be assisted using the control surfaces 601, 602, 603. A further embodied kite system 13 will now be described with reference to Figure 13. Reference numerals are retained from Figure 1 for like elements. In this embodiment, winglets 311a and 312a extend sideways from the bottom of the triangle of the frame 300. These winglets 311a and 312a reduce the height of the frame 300 for a given width of the triangle of the frame, which is advantageous since it allows an overall smaller frame 300 to be used.

Various modifications and alterations are within the scope of the claims. Some modifications and variations will now be described. As discussed above, the tether constraining component 403 moves along the roll slider 307 to roll the kite. This roll slider 307 can be straight or curved, and in the above- described embodiments it is straight. In embodiments in which a curved roll slider 307 is used, the curve of the roll slider 307 advantageously has an arc with a centre at or nearby the vertical position of the tether attachment location TAL. The advantage of a curved roll slider 307 is that it reduces the wear and friction on the tether 402 when rolling the kite system 1 due the distance between the tether constraining component 403 and the tether attachment 400 being fixed. Also, the angle between the tether 402 and the track can be maintained tangentially for different tether displacements. The wing-frame attachment mechanism 200 may not require a rigid component, as with the embodiments described above, and instead may be replaced with several bridle lines 201 in tension between the (rigid or semi-rigid) wing 100 and the frame 300. The wing-frame attachment mechanism 200 may alternatively be omitted altogether.

The horizontal spar or roll slider bar 307 may not be entirely rigid. Instead, it may be a belt or other such flexible member. Here, the roll actuator 500 can be a unit that moves along the belt or other flexible member. As previously explained, during the power stroke, the wing is flown near the centre of the wind in order to extract the largest amount of energy from the wind. Conversely, during the retract stage, the wing is flown away from the centre of the wind, which reduces the energy needed to retract the wing. During the retract stage, the pitch of the wing is changed so as to maintain a low apparent angle of attack. As will be appreciated, the orientation of the wing with respect to the wind will change the magnitude of the various forces and/or force components acting upon the wing during flight. During the power stroke, the aerodynamic forces (i.e., the resultant or net force of the drag force and the lift force) acting upon the wing are substantially in alignment with the direction of force imparted on the wing by the tether. During this phase, the wing (including the bridle lines) is substantially in alignment with the direction of the aerodynamic forces and the tether forces in a fore-and-aft direction. This ensures that the plan form or shape of the wing is substantially maintained as the bridle lines are maintained under tension.

However, during the retract stage the apparent angle of attack of the wing is changed, at least in part due to the change in wing pitch described above. This causes the wing, and in particular the bridle lines, to be only partially in alignment with the forces imparted on the wing in the fore-and-aft direction. As the wing may not have a rigid structure, but may maintain its structure at least partially by way of the bridle lines, it is necessary for the bridle lines to be under tension for them to assist in maintaining the shape of the wing. When the apparent angle of attack of the wing is reduced, the misalignment of the aerodynamic forces and tether forces with the structural components of the wing (e.g., the bridle lines) may in some instances reduce the tension on the bridle lines enough to cause the wing to be unable to wholly or partially maintain its shape, thereby reducing the overall mechanical stability of the wing. A kite system that addresses the above will now be described with reference to Figure 15. It will be appreciated that, whilst shown in the context of a particular embodiment, the exemplary kite system described in the following is, in principle, applicable to any of the foregoing embodiments. Further, reference numerals are re-used from the previous Figures for like elements, and only different features will be discussed in detail.

In the kite system shown in Figure 15, a tertiary bridle line arrangement connects the pitch actuator 501 and the wing 100. The tertiary bridle line arrangement may comprise any suitable number of individual tertiary bridle lines 203. As the tertiary bridle lines are mainly of use during the retract stage, in which the loads on the wing are lower than during the power stroke, it is typically not necessary to have as many tertiary bridle lines as bridle lines 201. In the example shown in Figure 15, four tertiary bridle lines are used to connect the pitch actuator with the wing.

Each of the tertiary bridle lines 203 comprised in the tertiary bridle line arrangement is attached at one end to a line attachment point 204 on the pitch actuator 501, and at a second end to the wing 100. The line attachment point may be positioned in any suitable location on the pitch actuator. In the kite system shown in Figure 15, line attachment point is positioned substantially at a free end of the pitch actuator. It will be appreciated that this is for exemplary purposes only, and that other specific implementations may easily be envisaged. In one example, the line attachment point is positioned in between the free end and a fixed end of the pitch actuator. In some examples, a plurality of line attachment points may be utilised. In such an example, a first subset of the tertiary bridle line arrangement may be connected to a first line attachment point on the pitch actuator whereas the remainder of the tertiary bridle line arrangement may be connected to a second line attachment point on the pitch actuator. In another example, each of the tertiary bridle lines is connected to a separate line attachment point on the pitch actuator. In yet another example, one or more of the tertiary bridle lines is split into a plurality of sub-bridle lines, each of which is connected to a separate location on the pitch actuator.

At an opposite end thereof, each of the tertiary bridle lines 203 is attached the wing 100 in a suitable location. In the example illustrated in Figure 15, each of the tertiary bridle lines is attached to the wing at substantially the same location as a corresponding one of the bridle lines 102. However, each of the tertiary bridle lines could, in principle, equally well be attached any other suitable location on the wing.

In the kite system shown in Figure 15, the roll actuator 500 is implemented in a fashion substantially identical to the roll actuator described in respect of Figure 4 above, i.e., it comprises a tether constraining component 403 having an open free end 308 and a tether gate element 309.

Figure 15 illustrates a configuration of the kite system such as may be used during the retract stage. In this configuration, the connection point between the tether and the pitch actuator is located at or towards the free end of the pitch actuator. In the kite systems described above, such a configuration would cause the direction of the aerodynamic forces on the wing and the tether forces on the wing to only be partially in alignment. As described above, this may decrease the mechanical stability of the wing.

In the present example, this reduction in mechanical stability is mitigated by way of the tertiary bridle line arrangement. As will be appreciated, when the connection point is located substantially at the free end of the pitch actuator, the tertiary bridle line arrangement is functionally similar to the bridle lines 201 when the tether is in the "innermost" position on the pitch actuator. In other words, the forces from the tether and the aerodynamic forces acting on the wing (which are transmitted at least partially by the tertiary bridle arrangement) are substantially kept in alignment during the retract stage. The mechanical stability of the wing is thereby maintained during this stage.

Other alternatives and modifications will be apparent to the skilled person. The scope of protection is limited by the attached claims and not by the above description.

Claims

1. A kite system (1-5, 10-13), comprising:

an anhedral wing (100) having wing tips (103) and a central part located vertically higher than the wing tips;

a frame (300) coupled to the wing by plural bridle lines (201);

a pitch actuator (501) connected to the frame; and

a tether attachment mechanism (400) for attaching the kite system to a tether (402) at a tether attachment position,

wherein the pitch actuator is configured to move the tether attachment mechanism, and thus the tether attachment position, in the fore and aft direction with respect to a forward flying direction of the kite system,

wherein the kite system has a pitch neutral point (PNP) that is located below the central part of the wing when the kite system is flying in the forward flying direction without any roll, and

wherein the pitch actuator is located on the frame at a vertical position that is below the pitch neutral point of the kite system.

2. A kite system as claimed in claim 1, comprising a roll actuator (500) located on the frame at a position vertically below a roll neutral axis of the kite system and below the tether attachment mechanism.

3. A kite system as claimed in claim 2, wherein the roll actuator is configured to apply a force laterally onto a tether (402) extending from the tether attachment mechanism (400).

4. A kite system as claimed in claim 3, wherein the roll actuator is configured to apply a force laterally onto the tether using a tether constraining component (403).

5. A kite system as claimed in claim 4, wherein the roll actuator is configured to apply a force laterally onto the tether using a tether constraining component (403) by moving the tether constraining component along a generally horizontal spar (307, 307a) of the frame (300).

6. A kite system as claimed in claim 4 or claim 5, wherein the tether constraining component (403) includes an elongate slot for constraining the tether.

7. A kite system as claimed in any of claims 4 to 6, wherein the tether constraining component (403) comprises an open free end (308) operable to allow the tether to exit out of the tether constraining component.

8. A kite system as claimed in claim 7, wherein the tether constraining component (403) comprises a tether gate element (308) operable to selectively prevent or restrict movement of the tether out of the tether constraining component.

9. A kite system as claimed in any preceding claim, wherein the pitch actuator is connected to the top of the frame.

10. A kite system as claimed in any of claims 1 to 8, wherein the pitch actuator is connected to a cross brace forming part of the frame.

11. A kite system as claimed in any preceding claim, wherein the frame is triangular in shape.

12. A kite system as claimed in claim 11, wherein the base of the triangle of the frame is vertically lower than the apex of the triangle and wherein spars extending from the base to the apex are capable of withstanding compression forces.

13. A kite system as claimed in any preceding claim, wherein spars (307, 311, 312) of the frame are thinner at the bottom of the frame than at the top of the frame.

14. A kite system as claimed in any preceding claim, wherein the frame is provided with one or more brace members.

15. A kite system as claimed in claim 14, wherein a brace member connects an end of the pitch actuator to a part of the frame that is above the position of the pitch actuator.

16. A kite system as claimed in any preceding claim, comprising a wing-frame attachment member (200) rigidly coupled to the frame (300) and coupled to the wing (100).

17. A kite system as claimed in any preceding claim, further comprising a tertiary bridle line arrangement, the tertiary bridle line arrangement comprising at least one tertiary bridle line (203), wherein

the tertiary bridle line arrangement is operable to couple the pitch actuator (501) to the wing.

18. A kite system as claimed in claim 17, wherein each of the tertiary bridle lines (203) is attached to the wing in a location identical to that of at least one of the plurality of bridle lines (201).

19. A kite system as claimed in claim 17 or claim 18, wherein the tertiary bridle line arrangement is attached to the pitch actuator (501) at a line attachment point.

20. A kite system as claimed in claim 19, wherein the line attachment point is positioned substantially at a free end of the pitch actuator (501).

21. A kite system as claimed in claim 19, wherein the line attachment point is positioned in between a free end and a fixed end of the pitch actuator (501).

22. A kite system as claimed in claim 17 or claim 18, wherein the tertiary bridle line arrangement is attached to the pitch actuator (501) at a plurality of line attachment points.

23. A kite system (1-5, 10-13), comprising:

a frame (300) coupled to the wing by plural bridle lines (102);

a pitch actuator (501, 600-603) connected to the frame; and

a tether attachment mechanism (400) for attaching the kite system to a tether (402) at a tether attachment position (TAL),

wherein the kite system has a roll neutral axis (RNP) that is located below the central part of the wing when the kite system is flying in the forward flying direction and extends substantially fore and aft with respect to the forward flying direction, and wherein the tether attachment mechanism (400) is located in the vertical location at a position that is substantially coterminous with the roll neutral axis of the kite system.

24. A kite system as claimed in claim 23, wherein the tether attachment mechanism (400) is located in vertically above the roll neutral axis of the kite system.

25. A kite system as claimed in claim 23 or claim 24, wherein the centre of gravity of the kite system is substantially coterminous with the roll neutral axis of the kite system.

26. A kite system as claimed in any of claims 23 to 24, wherein the centre of gravity of the kite system is below the roll neutral axis of the kite system.

27. A kite system as claimed in any of claims 23 to 26, wherein the centre of gravity of the kite system is below the tether attachment location.

28. A kite system as claimed in any of claims 31 to 27, wherein the pitch actuator is configured to move the tether attachment mechanism, and thus the tether attachment position, in the fore and aft direction with respect to the forward flying direction of the kite system.

29. A kite system as claimed in any of claims 23 to 28, comprising a roll actuator (500) located on the frame at a position vertically below a roll neutral axis of the kite system and below the tether attachment mechanism.

30. A kite system as claimed in claim 29, wherein the roll actuator is configured to apply a force laterally onto a tether (402) extending from the tether attachment mechanism (400).

31. A kite system as claimed in claim 30, wherein the roll actuator is configured to apply a force laterally onto the tether using a tether constraining component (403).

32. A kite system as claimed in claim 31, wherein the roll actuator is configured to apply a force laterally onto the tether using a tether constraining component (403) by moving the tether constraining component along a generally horizontal spar (307,

33. A kite system as claimed in claim 31 or claim 32, wherein the tether constraining component (403) includes an elongate slot for constraining the tether.

34. A kite system as claimed in any of claims 31 to 33, wherein the tether constraining component (403) comprises an open free end (308) operable to allow the tether to exit out of the tether constraining component.

35. A kite system as claimed in claim 34, wherein the tether constraining component (403) comprises a tether gate element (308) operable to selectively prevent or restrict movement of the tether out of the tether constraining component.

36. A kite system as claimed in any of claims 23 to 35, wherein the pitch actuator is connected to the top of the frame.

37. A kite system as claimed in any of claims 23 to 35, wherein the pitch actuator is connected to a cross brace forming part of the frame.

38. A kite system as claimed in any of claims 23 to 37, wherein the frame is triangular in shape.

39. A kite system as claimed in claim 38, wherein the base of the triangle of the frame is vertically lower than the apex of the triangle and wherein spars extending from the base to the apex are capable of withstanding compression forces.

40. A kite system as claimed in any of claims 23 to 39, wherein spars (307, 311, 312) of the frame are thinner at the bottom of the frame than at the top of the frame.

41. A kite system as claimed in any of claims 23 to 40, wherein the frame is provided with one or more brace members.

42. A kite system as claimed in claim 41, wherein a brace member connects an end of the pitch actuator to a part of the frame that is above the position of the pitch actuator.

43. A kite system as claimed in any of claims 23 to 42, comprising a wing-frame attachment member (200) rigidly coupled to the frame (300) and coupled to the wing (100).

44. A kite system as claimed in any of claims 23 to 43, further comprising a tertiary bridle line arrangement, the tertiary bridle line arrangement comprising at least one tertiary bridle line (203), wherein

the tertiary bridle line arrangement is operable to couple the pitch actuator (203) to the wing.

45. A kite system as claimed in claim 44, wherein each of the tertiary bridle lines (203) is attached to the wing in a location identical to that of at least one of the plurality of bridle lines (201).

46. A kite system as claimed in claim 44 or claim 45, wherein the tertiary bridle line arrangement is attached to the pitch actuator (501) at a line attachment point.

47. A kite system as claimed in claim 46, wherein the line attachment point is positioned substantially at a free end of the pitch actuator (501).

48. A kite system as claimed in claim 46, wherein the line attachment point is positioned in between a free end and a fixed end of the pitch actuator (501).

49. A kite system as claimed in claim 44 or claim 45, wherein the tertiary bridle line arrangement is attached to the pitch actuator (501) at a plurality of line attachment points.