Abstract:
The invention concerns an airborne device comprising at least three supporting wings and a linking device, the wings being linked to each other by first flexible cables, each wing being further linked to the linking device by a second flexible cable, the linking device being linked to a third flexible cable intended to be linked to a base, the first second, and third cables being tensioned when the airborne device is carried in the wind.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of International Application Application No. PCT/FR2015/051936, filed Jul. 15, 2015 and claims the priority benefit of French patent application FR14/57001, filed on Jul. 21, 2014 and incorporates the disclosures of each application by reference. 
     
    
     BACKGROUND 
       [0002]    The present application relates to an airborne device for the conversion of the kinetic energy of wind into mechanical energy. Airborne devices used to convert the kinetic energy of wind into mechanical energy generally comprise a kite or an aerostat An advantage is that such airborne devices may be used at high altitudes where winds are generally stronger and/or more regular than at lower altitudes. 
         [0003]    The airborne device may be used to pull a vehicle, for example, a boat. The airborne device may be used to drive an electric power generator. The electric power generator may be supported by the airborne device or be located on ground. The airborne device then forms an airborne wind turbine which enables to convert the kinetic energy of wind into electric power. 
         [0004]    A disadvantage of airborne devices, particularly when they are used as an airborne wind turbine, is their low efficiency, in particular as compared with a conventional wind turbine. Further, the structure of airborne devices may be complex and the control of the trajectory followed by the airborne device may be difficult. 
       SUMMARY 
       [0005]    An object of an embodiment aims at overcoming all or part of the disadvantages of the previously-described airborne devices used to convert the kinetic energy of wind into mechanical energy. 
         [0006]    Another object of an embodiment is to increase the efficiency of the airborne device. 
         [0007]    Another object of an embodiment is for the airborne device to have a simple structure. 
         [0008]    Another object of an embodiment is for the trajectory followed by the airborne device to be simply controlled. 
         [0009]    For this purpose, an embodiment provides an airborne device comprising at least three airfoil wings and a connecting device, the wings being connected together by first flexible cables, each wing being, further, connected to the connecting device by a second flexible cable, the connecting device being connected to a third flexible cable intended to be connected to a base, the first, second, and third cables being stretched when the airborne device is placed in wind. 
         [0010]    According to an embodiment, the device comprises no rigid frame connecting the wings together. 
         [0011]    According to an embodiment, the connecting device comprises a first portion connected to a second portion, the second cables being attached to the first portion and the third cable being attached to the second portion, the first portion being capable of pivoting with respect to the second portion. 
         [0012]    According to an embodiment, at least one of the wings comprises at least one first actuator capable of modifying the length of the portion of one of the first cables stretched between said wing and one of the other wings. 
         [0013]    According to an embodiment, at least one of the wings comprises a second actuator capable of modifying the length of the portion of the second cable stretched between said wing and the connecting device. 
         [0014]    According to an embodiment, each wing is connected to at least two other wings by at least two first cables. 
         [0015]    According to an embodiment, each wing comprises first actuators capable of independently modifying the lengths of the portions of said at least two first cables stretched between said wing and the two other wings. 
         [0016]    According to an embodiment, the device comprises at least two pairs of wings, the two wings of each pair being connected together by one of the first cables, each wing of each pair being connected to at least one of the wings of the other pair by another one of the first cables. 
         [0017]    According to an embodiment, the span of each wing is in the range from 5 m to 50 m. 
         [0018]    According to an embodiment, at least one of the wings comprises an upper surface connected to a lower surface by a leading edge, a trailing edge, and first and second lateral edges, the wing chord increasing and then decreasing from the first lateral edge to the second lateral edge. 
         [0019]    According to an embodiment, for each wing, at least one of the first cables penetrates into the wing through the lateral edge of the wing which is most inside of the airborne device when the airborne device is placed in wind. 
         [0020]    According to an embodiment, for each wing, the second cable penetrates into the wing through the lower surface of the wing. 
         [0021]    An embodiment also provides an electric power generation system, comprising an airborne device such as previously defined and an electric power generator connected to the third cable of the airborne device. 
         [0022]    An embodiment also provides a transport system, comprising an airborne device such as previously defined and a vehicle, particularly a boat, connected to the third cable of the airborne device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0024]      FIG. 1  is a partial simplified perspective view of an embodiment of an airborne device; 
           [0025]      FIG. 2  is a partial simplified perspective view of an electric power generation system comprising the airborne device shown in  FIG. 1 ; 
           [0026]      FIG. 3  is a partial simplified perspective view of a transport system comprising the airborne device shown in  FIG. 1 ; 
           [0027]      FIG. 4  is a partial simplified top view of an embodiment of a wing of the airborne device shown in  FIG. 1 ; 
           [0028]      FIGS. 5 and 6  respectively are a perspective view and a front view, partial and simplified, of another embodiment of a wing of the airborne device shown in  FIG. 1 ; 
           [0029]      FIG. 7  is a partial simplified top view of another embodiment of a wing of the airborne device shown in  FIG. 1 ; and 
           [0030]      FIGS. 8 and 9  are partial simplified cross-section views of embodiments of a cable of the airborne device shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    For clarity, the same elements have been designated with the same reference numerals in the various drawings and, further, the various drawings are not to scale. In the following description, unless otherwise indicated, terms “substantially,” “about,” “approximately,” and “in the order of” mean “to within 10%.” 
         [0032]      FIG. 1  shows an embodiment of an airborne device  10 . Airborne device  10  comprises at least three wings, for example, from three to eight wings  12 . Preferably, airborne device  10  comprises an even number of wings  12 . Wings  12  are connected together by flexible cables. A flexible cable is a cable which may, under the action of an external force, deform, and particularly bend, without breaking or tearing. There is no rigid frame connecting wings  12  together. As an example, in the case where airborne device  10  comprises four wings  12 , each wing  12  is connected to each adjacent wing by a flexible cable  14  and is connected to the opposite wing by a flexible cable  16 . Further, each wing  12  is connected to a connecting device  18  by a flexible cable  20 . Connecting device  18  is connected to an anchoring circuit, not shown, by a flexible cable  22 . According to the envisaged application, the anchoring system may be on ground, on a buoy, or on a ship. According to an embodiment, connecting device  18  comprises a first portion  24  having cables  20  attached thereto and connected to a second portion  26  having cable  22  attached thereto. First portion  24  is capable of pivoting with respect to second portion  26  around the axis of cable  22 . Connecting device  18  may correspond to a swivel. 
         [0033]    Each wing  12  corresponds to an airfoil comprising a lower surface  30  connected to an upper surface  32  by a leading edge  34 , a trailing edge  36 , an outer lateral edge  38 , directed towards the outside of device  10 , and an inner lateral edge  40 , directed towards the inside of device  10 . Each wing  12  may correspond to a profiled wing, for example, having a NACA profile. 
         [0034]    According to an embodiment, for each wing  12 , cable(s)  14  and  16  are substantially connected to the same point of inner lateral edge  40 . According to an embodiment, for each wing  12 , cable  20  is connected to wing  12  at a point of lower surface  30  at a distance from leading edge  34 , from the trailing edge, from outer lateral edge  38 , and from inner lateral edge  40 . As a variation, cable  20  may be connected to inner lateral edge  40 . 
         [0035]    Airborne device  10  operates as follows. Under the action of wind, schematically shown by arrow  42 , wings  12  displace under the effect of lift forces. Centrifugal forces tend to radially separate wings  12 , so that cables  14  and  16  are permanently stretched. A rotating motion of wings  12  is then obtained, which is shown in  FIG. 1  by arrow  44 . The lift forces exerted on each wing  12  result in pulling cables  20 , and thus in pulling cable  22 . A conversion of the kinetic energy of wind  42  into mechanical energy for pulling cable  22  is thus obtained. Preferably, cable  20  is connected to lower surface  30  so that the longitudinal axis of wing  12  is aligned with cable  16 . 
         [0036]    Wings  12  of airborne device  10  rotate as the blades of a wind turbine on ground. The present embodiment is based on the fact that, for a conventional wind turbine on ground, the blade portions which are, in operation, the most efficient to capture the kinetic energy of wind, are located close to the free ends of the blades, where the drive torque due to wind is the highest. Wings  12  are thus located in useful areas where the drive torque due to wind  42  is the highest and cables  14 ,  16 ,  20  are located in areas where the drive torque due to wind  42  is low. Thereby, the surface area described by wings  12  during their motion may be large while the airborne device has a simple structure and a small weight. 
         [0037]    Preferably, the maximum diameter in operation of airborne device  10  is in the range from 20 m to 200 m, preferably from 100 m to 150 m. The weight of airborne device  10 , without counting cable  22 , may be in the range from 20 kg to 20 tons. The rotation speed in operation of the wings may be in the range from  1 . 5  to  200  revolutions per minute. 
         [0038]      FIG. 2  shows an embodiment of an electric power generation system  45  where cable  22  of airborne device  10  is connected to an electric power generator  46 . As a variation, each wing  12  may comprise an electric power generator comprising a turbine driven during the displacement of wing  12 . The generated electric power may then be transmitted to ground by cables  20  and  22 . 
         [0039]      FIG. 3  shows an embodiment of a transport system  47  where cable  22  of airborne device  10  is connected to a vehicle  48 , in the present example, a ship. Airborne device  10  is then used as means for pulling vehicle  48 . 
         [0040]      FIG. 4  is a simplified view of an embodiment of one of wings  12  of airborne device  10  shown in  FIG. 1 . Each wing  12  of airborne device  10  may have substantially the structure shown in  FIG. 4 . Wing  12  forms a partially hollow enclosure and a plurality of elements arranged in the internal volume of wing  12  have been schematically shown in  FIG. 4 . Wing  12  is for example made of composite materials. Cables  14 ,  16 ,  20  may be made of synthetic fibers, particularly the product commercialized under trade name Kevlar. 
         [0041]    In the following description, longitudinal axis D of the wings designates an axis perpendicular to the two most distant parallel planes, one of them being tangent to the outer lateral edge  38  and the other one being tangent to inner lateral edge  40 . Span E of wing  12  is the distance between these planes. Span E is in the range from 5 m to 50 m, preferably from 25 m to 35 m. The chord of wing  12 , measured in a plane perpendicular to longitudinal axis D, is not constant along axis D. The chord increases from inner lateral edge  40  to a maximum chord and then decreases towards outer lateral edge  38 . The maximum chord is in the range from 0.25 m to 5 m, preferably, from 1.25 m to 3.5 m. The maximum chord is substantially located between 10% and 45%, preferably between 15% and 30%, of the span away from inner lateral edge  40 . 50% of the span away from inner lateral edge  40 , the ratio of the chord to the maximum chord is in the range from 60% to 100%, preferably from 70% to 90%. The maximum thickness between the upper surface and the lower surface is in the range from 7% to 25% of the value of the chord at this location, preferably from 8% to 15% of the value of the chord at this location. Each cable  14 ,  16 ,  20  has an average diameter in the range from 5 mm to 10 cm. 
         [0042]    Leading edge sweep F A  designates the angle between axis D and a plane tangent to leading edge  34 . The sweep is positive when the angle, directed from axis D to the tangent plane, is in the counterclockwise direction when looking at the upper surface of the wing, and negative in the opposite case. According to an embodiment, leading edge sweep F A  varies along axis D from inner lateral edge  40  to outer lateral edge  38 . According to an embodiment, leading edge sweep F A  is successively, as the distance from inner lateral edge  40  increases along axis D, negative and decreases in absolute value as the distance from inner lateral edge  40  increases along axis D, cancels and is then positive and increases towards outer lateral edge  38 . According to an embodiment, 20% of the span away from inner lateral edge  40 , the sweep of leading edge  34  is in the range from −20 degrees to 5 degrees, and 60% of the span away from inner lateral edge  40 , the sweep of leading edge  34  is in the range from 0 degrees to 10 degrees. Trailing edge sweep F F  designates the angle between a plane tangent to trailing edge and axis D. According to an embodiment, trailing edge sweep F F  varies along axis D from inner lateral edge  40  to outer lateral edge  38 . According to an embodiment, trailing edge sweep F F  is successively, as the distance from inner lateral edge  40  increases along axis D, positive, zero, negative, zero, and positive. According to an embodiment, 20% of the span away from inner lateral edge  40 , trailing edge sweep F F  is in the range from 30 degrees to 0 degrees, and 60% of the span away from inner lateral edge  40 , trailing edge sweep F F  is in the range from −10 degrees to 10 degrees. 
         [0043]    Wing  12  may comprise a twist, that is, the angle between the chord and a reference plane, or pitch angle, may vary along axis D. 
         [0044]    Wing  12  comprises: 
         [0045]    a control unit  50 , for example comprising a processor; 
         [0046]    sensors  52 , connected to control unit  50 , for example, a speed sensor, a wing position sensor, for example, a GPS (Global Positioning System), gyroscopes, accelerometers, a pitot tube, magnetometers, and a barometer; 
         [0047]    actuators  53 ,  54 ,  55 ,  56 , each actuator  53 ,  54 ,  55 ,  56  being controlled by control unit  50  and being connected to one of cables  14 ,  16 ,  20 ; 
         [0048]    at least one mobile trailing edge spoiler, two mobile spoilers  57 ,  58  being shown in  FIG. 4 ; 
         [0049]    a remote communication unit  59  connected to control unit  50 ; and 
         [0050]    a battery of accumulators  60  for powering control unit  50 , actuators  53 ,  54 ,  55 ,  56 , and the drive motors of spoilers  57 ,  58 . 
         [0051]    As a variation, batteiy  60  may be replaced with an electric power generator. As a variation, the electric power for supplying control unit  50 , drive motors  54  of cables  14 ,  16 ,  20 , and drive motors of spoilers  57 ,  58  may be conveyed to each wing via cables  20  and  22 . 
         [0052]    Each actuator  53 ,  54 ,  55 ,  56  is capable of modifying the length of the stretched portion of cable  14 ,  16 , or  20  outside of wing  12 . As an example, each actuator  53 ,  54 ,  55 ,  56  is capable of unwinding or of winding the cable  14 ,  16 ,  20  to which it is connected. The length of the portion of each cable  14 ,  16  extending between two wings  12  and the length of the portion of each cable  20  extending between a wing  12  and connecting device  18  may thus be modified. 
         [0053]    The control unit  50  of each wing  12  is capable of remotely exchanging signals, via communication unit  59 , with the control units  50  of the other wings  12 , for example, according to a high-frequency type remote data transmission method. The control unit  50  of each wing  12  may further be capable of remotely exchanging signals, via communication unit  59 , with a ground station. 
         [0054]    The control of the incidence of each wing  12  is performed by control unit  50  by modifying the inclination of spoilers  57 ,  58  and by modifying the length of the portions of cables  14 ,  16 , and  20  stretched in operation between wings  12  or between wings  12  and connecting device  18 . According to an embodiment, the incidence of each wing  12  may be cyclically modified during a revolution of wing  12 . According to another embodiment, in the case where airborne device  10  is connected to an electric power generator  46 , the operation of electric power generator  46  may comprise an alternation of first and second phases. In each first phase, the incidences of wings  12  are controlled to increase the pulling efforts exerted by airborne device  10 , airborne device  10  moving away from electric power generator  46 . In each second phase, the incidences of wings  12  are controlled to decrease the pulling efforts exerted by airborne device  10  on cable  22  to be able to bring airborne device  10  closer to generator  46  while spending a minimum quantity of energy. 
         [0055]    Further, when airborne device  10  is lifted from the ground up to an operating altitude, the stretched portions of  14 ,  16 ,  20  between wings  12  or between wings  12  and connecting device  18  may be initially decreased to decrease the bulk of airborne device  10 . 
         [0056]      FIGS. 5 and 6  show another embodiment of wing  12  where wing  12  further comprises two stabilizers  62  which may each comprise a mobile flap  64 . First stabilizer  62  protrudes from upper surface  32  and second stabilizer  62  projects from lower surface  30 . The actuation of the mobile flap  64  of each stabilizer  62  is controlled by control unit  50 . The actuation of mobile flap  64  especially enables to control the lateral position of airborne device  10  relative to wind  42 . 
         [0057]    Each wing  12  may be provided with a propulsion system. Before the launching of airborne device  10 , wings  12  may be arranged on a support and the lengths of cables  14 ,  16 , and  20  may be decreased. The propulsion system of each wing  12  may be actuated. This causes the stretching of cables  14 ,  16 , and the rotating of wings  12 . Under the action of lift efforts, airborne  10  rises in the air. 
         [0058]    The lengths of cables  14 ,  16 ,  20  may be progressively increased as the altitude of airborne device  10  increases, until airborne device  10  reaches the desired altitude. As soon as airborne device  10  is exposed to a sufficient wind to maintain the altitude and the rotation of airborne device  10 , the propulsion systems of wings  12  may be deactivated. Propulsion systems may further be actuated in flight, while airborne device  10  is at its operating altitude, when the power of wind  42  is not sufficient to maintain airborne device  10  at this altitude. 
         [0059]      FIG. 7  shows an embodiment of wing  12  where the wing propulsion system comprises a motor-driven helix  70  which projects from leading edge  34  of the wing to the front of the wing according to the rotation direction of wing  12  in operation. Motor-driven helix  70  may be controlled by control unit  50  or may be remotely controlled from a ground station. An advantage of the use of a motor-driven helix is that it further enables to displace the center of gravity of wing  12  towards the front according to the rotation of wing  12  in operation. This may be advantageous to improve the wing stability. According to an embodiment, helix  70  may be removable and at least partly folded into wing  12  when it is not being used. As a variation, the propulsion system may comprise a jet engine, particularly a rocket engine or a compressed air propulsion system. 
         [0060]    Each wing  12  may further comprise a landing gear, not shown, which allows displacements of wing  12  on the ground. The landing gear may be removable to be at least partly folded into wing  12  when it is not being used. 
         [0061]      FIG. 8  shows an embodiment where each cable  14 ,  16 ,  20 , or  22  or at least one of cables  14 ,  16 ,  20 , or  22  has a thinned profiled section comprising a leading edge  72  and a trailing edge  74 . This enables, in particular, to decrease the cable drag. 
         [0062]      FIG. 9  shows an embodiment where each cable  14 ,  16 ,  20 , or  22  or at least one of cables  14 ,  16 , or  30  further comprises a core  76  contained within a profiled casing  78 . Core  76  may be made of a first material and casing  78  may be made of a second material, the density of the first material being greater than the density of the second material. This enables to draw the center of gravity of the cable towards the leading edge and to thus improve the aerodynamic stability of the cable. 
         [0063]    Various embodiments with different variations have been described hereabove. It should be noted that those skilled in the art may combine various elements of these various embodiments and variations without showing any inventive step. In particular, airborne device  10  may both comprise a propulsion system, such as helix  70  shown in  FIG. 7 , profiled cables  14 ,  16 ,  20  as shown in  FIGS. 8 and 9 , and a landing gear.