BACKGROUND

US Patent No. 6,781 ,254 (Roberts) discloses a windmill kite that converts the energy of the wind into electrical power. The windmill kite comprises a flying platform that contains a plurality of mill rotors, and at least one tethering line for maintaining the windmill kite at a substantially fixed geographic location. It describes two different embodiments each supporting four rotors, the first embodiment as shown in Fig 1 , using a platform frame comprising a cross when viewed in plan, whilst the second embodiment as shown in Fig 2, utilises a H-shaped platform frame.

US Patent Nos 7,109,598 (Roberts et al) and 7,183,663 (Roberts et al) disclose a method of maintaining a windmill kite of the abovementioned type in a defined airspace by use of global positioning system (GPS) for ascertaining the altitude and attitude of the kite.

International Patent Publication No. WO 2009/126988 (Wongalea Holdings Pty Ltd) is directed to a control system for a windmill kite of the type described in the abovementioned prior art.

One of the disadvantages of the system shown in Fig 1 and Fig 2 of US Patent No. 6,781 ,254 is that at least one rotor is positioned directly ahead of at least one other rearward rotor. As a result under certain wind conditions, and craft pitch attitudes, it is possible that the rotor wake from the "ahead" rotor can interfere with the aerodynamic flow of air into the "rearward" rotor. Such an aerodynamic interference could upset the pitch, roll or yaw attitude of the windmill kite. Another disadvantage is that while it is possible to add more rotors it is difficult to do so in the structural arrangements shown. The present invention seeks to overcome at least one of the disadvantages of the prior art. SUMMARY OF INVENTION According to a first aspect the present invention consists of a windmill kite of the type having a platform tethered by at least one tethering line and supporting at least four mill rotors that generate electrical power, wherein said mill rotors are aligned in two rows, a fore row comprising a first pair of spaced apart rotors and an aft row comprising a second pair of spaced apart rotors, wherein the spacing between each of said first pair of rotors is substantially different to the spacing between each of the second pair of rotors.

Preferably said platform comprises a fuselage supporting a laterally extending structure, and all of said rotors are mounted to said laterally extending structure. Preferably the spacing between each of said first set of rotors is substantially greater than the spacing between each of the second set of rotors.

Preferably said tethering line is attached to said laterally extending structure. Preferably said tethering line comprises a main tether connected to two auxiliary tether lines, each of which are connected to said lateral structure on opposed sides of said fuselage.

Alternatively said two auxiliary tether lines may consist of more than two auxiliary tethers connected to the said lateral structure and joining to one main tether.

Preferably said windmill kite comprises a further row of rotors having a third pair of spaced apart rotors, and the spacing between the third pair of rotors is substantially different to both the spacing between each of said first pair of rotors and the spacing between each of said second pair of rotors.

Preferably said laterally extending structure is a single structure. Alternatively, said laterally extending structure is two separate structures joined to opposed sides of said fuselage. In one preferred embodiment, each of said rotors comprises an electrical motor/generator. Preferably each of said rotors rotates at the same speed.

In another preferred embodiment, the rotation of said rotors is mechanically linked together. Preferably said windmill kite further comprises a central electrical motor/generator mechanically linked to said rotors. Preferably, said rotors are mechanically linked together by rotating shafts. Preferably, said platform comprises at least one tubular bracing and at least one of said rotating shafts is at least partially supported by said tubular bracing.

BRIEF DESCRIPTION OF DRAWINGS

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig 1 is a diagrammatic perspective view of a windmill kite (craft) according to a first embodiment of the present invention. Fig 2 is a diagrammatic reduced plan view of the windmill kite shown in Fig 1.

Fig 3 is a diagrammatic plan view of a windmill kite according to a second embodiment of the present invention. Fig 4 is a diagrammatic plan view of a windmill kite according to a third embodiment of the present invention.

Fig 5 is a diagrammatic reduced front elevation view of the windmill kite shown in Fig. 4. Fig 6 is a diagrammatic plan view of a windmill kite according to a fourth embodiment of the present invention. BEST MODE OF CARRYING OUT INVENTION

Figs 1 and 2 depict a first embodiment of a windmill kite 1 having a central fuselage 2 supporting a laterally extending structure 3. Fuselage 2 has a fore end 5 and opposed aft end 6, and a vertical tail fin with rudder 7. Fuselage 2 has a longitudinal axis L2. For ease of reference windmill kite 1 , will throughout this description be referred to as a "craft".

Laterally extending structure 3 may be a single lateral tube, laced truss or other similar structure. Alternatively, laterally extending structure 3 may be two separate structures joined to opposed sides of fuselage 2. In this embodiment the longitudinal axis L3 of structure 3 is substantially normal to longitudinal axis L2.

Four rotors Rl , R2, R3 and R4 are arranged along laterally extending structure 3. A first pair of rotors Rl and R4 are mounted forward of laterally extending structure 3 in a spaced apart arrangement in an in-line "first fore row", with the second pair of rotors R2 and R3 mounted aft of laterally extending structure 3 in a spaced apart arrangement in an in-line "second aft row".

The first pair of rotors Rl and R4 have a dimensional spacing between them (normal to fuselage 2) that is substantially different to the dimensional spacing (normal to fuselage 2) between the second pair of rotors R2 and R3. In this particular embodiment the dimensional spacing between each of the first pair of rotors Rl and R4 is substantially greater than the spacing between each of the second pair of rotors R2 and R3. The rotors in each pair namely, the first pair Rl and R4 and the second pair R2 and R3, rotate in opposite directions.

Rotors Rl and R4 have their collective pitches (that is thrust levels) varied collectively in opposition to the collective pitch changes applied collectively to rotors R2 and R3. This action controls the pitch attitude of craft 1 relative to the wind vector V, shown in Fig 1 at an angle of Θ degrees to L2. Similarly rotors Rl and R2 have their collective pitch changes applied collectively to rotors R3 and R4. This action controls the roll of craft 1.

Finally, the collective pitches on Rl and R3 can be collectively changed relative to pitches on R2 and R4 collectively to control yaw attitude via differential torque reactions. This action is applied up to the yaw reversal condition. At wind speeds higher than the yaw reversal speed the above yaw control is deleted, and yaw is controlled by vertical fin (or stabiliser) 7, which may or may not have an attached rudder. Fin 7 is acted upon by the oncoming wind V. Two auxiliary attachment tethers 8 are attached to the laterally extending structure at points A and B. These join at the point C, a distance of some one or two rotor diameters below the craft. The points A and B are chosen to have minimum bending stresses in structure 3. However, these points can vary in location depending on the compressive stresses induced by the two auxiliary tethers 8. Whilst two auxiliary tethers 8 are used, it should be understood that more than two auxiliary tethers may be used, all joining at point C into a single tether 4.

The nacelle of each rotor R l , R2, R3 and R4 contains the necessary shafting, any necessary gearbox and the electrical motor/generator(s). This has not been detailed herein, but is as referred to in US patent 6,781 ,254. The RPMs (rotational speeds in revolutions per minute) of rotors Rl , R2, R3 and R4 are identical. In this embodiment having an individual motor/generator for each rotor Rl , R2, R3 and R4, this can be preferably achieved by electronic control of the motor/generators.

One of the improvements in the design of craft 1 is that the wakes from all rotors do not in any way interfere with each other due to the lateral spacing of (ie. clearance between) the rotors along the lateral structure 3. This embodiment is an improvement over the prior art via a simplification of the fuselage 2 and structure 3. It also allows for the easy addition of rotors beyond four, to allow for say six or more rotors, as further described. Figure 3 depicts a plan view of a second embodiment of a windmill kite (craft) 10. It has a fuselage 2 and laterally extending structure 3, in a similar manner to the first embodiment, though structure 3 in this embodiment has a greater span. In this embodiment six rotors Rl , R2, R3, R4, R5 and R6 are arranged along laterally extending structure 3. A first pair of rotors Rl and R6 are mounted forward of laterally extending structure 3 in a spaced apart arrangement in an in-line "first fore row", with a second pair of rotors R2 and R5 mounted aft of laterally extending structure 3 in a spaced apart arrangement in an in-line "second aft row". A third pair of rotors R3 and R4 are mounted along structure 3 in a "third intermediate row". Like the first embodiment shown in Figs 1 and 2, each of the six rotors Rl to R6 has a nacelle that includes an electrical motor/generator.

In similar fashion to the first embodiment, the first pair of rotors Rl and R6 have a dimensional spacing between them (normal to fuselage 2) that is substantially different to the dimensional spacing (normal to fuselage 2) between the second pair of rotors R2 and R5. In this particular embodiment the dimensional spacing between each of the first pair of rotors l and R6 is substantially greater than the spacing between each of the second pair of rotors R2 and R5. Likewise both the first pair of rotors l and R6 and second pair of rotors R2 and R5 each have a dimensional spacing between them (normal to fuselage 2) that is substantially different to the dimensional spacing (normal to fuselage 2) between the third pair of rotors R3 and R4. The rotors in each pair namely, the first pair Rl and R6, second pair R2 and R5 and third pair R3 and R4 rotate in opposite directions. Figs 4 and 5 depict a third embodiment of a windmill kite (craft) l a in accordance with the present invention. Craft l a has four rotors Rl a, R2a, R3a and R4a arranged in the same layout in plan view as craft 1 shown in Figs 1 and 2. The main difference between craft 1 and craft l a is that craft l a has a single electrical motor/generator 1 1 centrally mounted to its fuselage 2a, instead of individual motor/generators in each rotor.

The rotation of rotors Rla, R2a, R3a and R4a and motor/generator 1 1 are all mechanically linked together by rotating shafts 12, 13, 14 and 19 that connect each pair of adjacent rotors R4a- R3a, R3a-R2a, and R2a-R l a respectively. The nacelle of each rotor Rl a, R2a, R3a and R4a has a gear box that drives the ends of the shafts 12, 13, 14 and 19 that connect them. The shafts 13 and 19 that connects the pair of rotors R2a and R3a closest to fuselage 2a passes through a gearbox 15 at the front of motor/generator 1 1 to rotationally connect shafts 13 and 19 to motor/generator 1 1. Like windmill kite 1 , rotors R 1 a and R3a rotate in the opposite direction to rotors R2a and R4a. The outer shafts 12 and 14 that connect the pair of rotors Rl a - R2a and R3a - R4a each pass externally above tubular bracing 16 that brace the mounting of forward rotors R 1 a and R4a to the laterally extending structure 3a. External mounting of shafts 12 and 14 to bracing 16 and laterally extending structure 3a, allows for easy visual inspection for any defects and any required repair work during maintenance checks.

Alternatively, shafts 12 and 14 may pass substantially below or to the side of tubular bracing 16, or even possibly through tubular bracing 16.

An advantage of bracings 16 is that they strengthen the structure and protect and/or support shafts 12 and 14. Shafts 12, 13, 14 and 19 may for example, be constructed from a suitable diameter tube that can flex but still have adequate torsional strength.

An advantage of the single centrally mounted motor/generator 1 1 and the mechanical linking of rotors Rla, R2a, R3a and R4a to each other and motor/generator 1 1 is that rotors R l a, R2a, R3a and R4a are synchronised to always rotate together at the same speed (ie. same RPM). In contrast, the rotors Rl , R2, R3 and R4 of windmill kite 1 must be individually electronically controlled to rotate at the same RPM, which can add complexity to the system. A further advantage of the single centrally mounted motor/generator 1 1 and its associated controller is that it reduces the weight of craft l a when compared with craft 1 of the first embodiment.

It is also important to note that the mechanical linking of rotors Rla, R2a, R3a and R4a to each other and motor/generator 1 1 via shafts 12, 13, 14 and 19 so that rotors R 1 a, R2a, R3a and R4a are synchronised is particularly advantageous, as this allows for emergency landing of kite 1 a should there be an electrical failure.

Fig 6 depicts a fourth embodiment of a windmill kite (craft) 10a in accordance with the present invention. Craft 1 Oa has six rotors Rla, R2a, R3a, R4a, R5a and R6a in a similar arrangement to that of craft 10 shown in Fig 3. Like craft l a shown in Figs 4 and 5, craft 10a has a single electrical motor/generator 1 1 centrally mounted to its fuselage 2a, instead of individual motor/generators in each rotor, and the rotation of rotors Rl a, R2a, R3a, R4a, R5a and R6a and motor/generator 1 1 are mechanically linked together by rotating shafts 12, 13, 14, 19, 17 and 18 that connect each pair adjacent rotors of the rotors Rl a, R2a, R3a, R4a, R5a and R6a. In other not shown embodiments of the invention, rotors may be mechanically linked together by other means, such as belt drives for example. Also, there may be more than one electrical motor/generator driven by the mechanically linked rotors. Points A and B in Figures 1 , 2 and 3, as well as similar unlabelled points in Figures 4, 5 and 6, are chosen to minimise the combined stress in the lateral cross-boom 3,3a for the most severe "thrust loads" developed by the rotors.

As previously indicated, points A and B are connection points for auxiliary attachment tethers 8, which connect to main tether 4 at point C. Whilst in Fig 1 , point C is shown on "one-side" of laterally extending structure 3, it should be understood that in alternative arrangement for all the above mentioned embodiments, point C would be on the "opposite- side" of laterally extending structure 3 to that shown in Fig 1. In these alternative arrangements where point C is on the opposite side, it should be understood that the wind vector V would be reversed in direction to that shown in Fig 1 . In the alternative arrangements the location of vertical tail fin with rudder 7 also needs to be reversed.

In Figs 4, 5 and 6 preferred relative sizes of the components are shown. However, it should be understood that these sizes can be scaled up or down to suit a variety of constructions.

In all of the abovementioned embodiments each of the rotors are shown with two blades, for ease of reference. However, it should be understood that the present invention is not limited to such arrangement, and each of the rotors could have more than two blades or a

counterweighted single blade.

In an alternative arrangement the outmost rotors of the abovementioned embodiments, namely rotors Rl & R4 in Fig 1 , rotors Rl & R6 in Fig 3, rotors Rla & R4a in Fig 4, and rotors Rla & R6a in Fig 6, may be advanced or retreated by 90 degrees in the arrangements as presently depicted. This is so that all outmost rotor tips operate in the void space formed by their opposite neighbours.

It should be understood that all directions of rotations on all rotors in each of the

abovementioned embodiments can be reversed, if desired. Preferred rotation direction is with the two centremost rotors having their blade tips, when adjacent, organised so that these blades are moving against the wind direction.

In other not shown embodiments the rotor arrangements fore and aft, and centrally for six, eight or more rotors can be arranged in a range of different manners to achieve the desired results.

The terms "comprising" and "including" (and their grammatical variations) as used herein are used in an inclusive sense and not in the exclusive sense of "consisting only of.