Abstract:
A motor pylon system adapted for use with an airborne power generations system is disclosed. The pylons may support turbine driven generators for wind based electrical power generation which also function as electric motors in some aspects. The pylons may be designed to provide side force useful for turning a tethered flying wing flying in a circular cross wind flight path. The pylons may be designed to minimize air flow disruptions over the main wing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/582,408 to Vander Lind et al., filed Jan. 2, 2012, which is hereby incorporated by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under Contract Numbers DE-AR0000122/AR0000243 awarded by Advanced Research Projects Agency-Energy (ARPA-E). The government has certain rights in the invention. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a system and method of flying tethered flying vehicles. 
     2. Description of Related Art 
     Crosswind kite systems comprising tethered wings (kites) can extract useful power from the wind for purposes such as, for example, generating electricity, lifting or towing objects or vehicles, etc. To provide or use consistent power, it may be desired to fly the kite in repeating trajectories (i.e., a limit cycle). It may also be desired to maintain the kite aloft and flying consistent trajectories during a large range of environmental conditions such as high wind speeds, large gusts, turbulent air, or variable wind conditions. However, take-off and landing of such kites can present difficulties, as the kites may not be well adapted for landings similar to that of an aircraft. Therefore, a mode of operation is desired so that a kite system can take-off, land, and operate safely in high and changing winds. 
     SUMMARY OF THE INVENTION 
     A motor pylon system adapted for use with an airborne power generations system is disclosed. The pylons may support turbine driven generators for wind based electrical power generation which also function as electric motors in some aspects. The pylons may be designed to provide side force useful for turning a tethered flying wing flying in a circular cross wind flight path. The pylons may be designed to minimize air flow disruptions over the main wing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a kite system flying according to some embodiments of the present invention. 
         FIG. 2  illustrates a flying kite according to some embodiments of the present invention. 
         FIG. 3  illustrates a wing mounted pylon with turbine driven rotors mounted thereon according to some embodiments of the present invention. 
         FIG. 4  illustrates a pylon with turbine driven rotors mounted thereon according to some embodiments of the present invention. 
         FIG. 5  illustrates the relative locations of rotor axes and rotor mounting according to some embodiments of the present invention. 
         FIG. 6  illustrates pylon airfoil cross-sections according to some embodiments of the present invention. 
         FIG. 7  illustrates pylon and wing spar structures according to some embodiments of the present invention. 
         FIG. 8  illustrates lower pylon geometries according to some embodiments of the present invention. 
         FIG. 9  illustrates a wing spar according to some embodiments of the present invention. 
         FIG. 10  is a side view of a pylon according to some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A motor pylon design for a kite system is disclosed. A kite system comprising motors or motor/generators may be used for a number of purposes. For example, a kite system comprising rotors and motor/generators might be used for extraction of power from the wind, might be used for towing of a vehicle, might be used for surveillance, or might be used as a communications relay. A kite system of this type can be launched and landed from a ground station in a hovering mode of flight, in which the kite hovers under thrust from on-board rotors, while the tether attaching the kite is reeled in or out from the ground station. The on-board rotors, as referred to herein, may be adapted for dual function. When providing thrust, the rotors are viewed as motor driven propellers. When being used to convert wind energy into electrical energy, the rotors are viewed as turbine driven generators. When operating, the kite either flies stationary in the wind, in the matter of a traditional kite, or flies in a crosswind flight pattern along a flightpath, generally at a speed which is a high multiple of the ambient wind speed. When flying in a crosswind flight pattern, the stability, controllability, and balance of forces on the kite system are substantially improved by use of a tail, much in the manner of a traditional aircraft. In some embodiments, the kite system is used to generate power in the crosswind mode of flight, and onboard rotors which are used to provide thrust during takeoff and landing, or during lulls in wind are operated at a lower RPM and used to turn the motor/generators to produce power. There is a strong incentive to make the main wing of such systems operate at a high coefficient of lift and have a high aspect ratio, as the performance of the system is described by the simplified performance metric P: 
     P is proportional to CL^3/CD^2. (where Cl is the coefficient of lift and CD is the coefficient of drag) 
     In order to have pitch control in hover through changes in thrust on the various rotors, and in order to keep the wake of the rotors off the main wing, thereby reducing disturbances to flow over the main wing which may reduce coefficient of lift, the rotors and motor/generators are placed substantially above and below the main wing. In addition, to counter the mass of the tail, which is desired for stability in flight, and to reduce the impact of flow over the main wing on the pressure distribution across the swept area of the rotors, the rotors and motor/generators are located substantially in front of the main wing. Additionally, it is desirable to generate a variable level of side (lateral or along-span) aerodynamic force with the kite in order to counter the gravitational, centripetal, and aerodynamic forces causing the kite to deviate from the desired flightpath. It may be desirable to use the area of the vertical sides of the pylons in order to generate a component of the force required to turn the wing when flying in substantially circular flight paths, and thus to allow the wing to operate at variable levels of sideslip. Aspects of this type of operation are seen in U.S. patent application Ser. No. 13/288,527 to Vander Lind, which is hereby incorporated by reference in its entirety. In some embodiments of the present invention is a design for a motor pylon which does not interfere with the lift-generating capacity of the main wing of the kite, which integrates structurally with the main wing of the kite, which generates significant aerodynamic side-force, and which has these properties at varying angles of sideslip (e.g. from −5 to +5 degrees or from −10 to +10 degrees of sideslip). 
     In some aspects, the present invention comprises lower and upper pylon segments, connected by a blended joint. The lower pylon attaches to the lower surface of the main wing, while the upper pylon attaches to the upper surface of the lower pylon. The foremost attachment point of the upper pylon is near the leading edge stagnation point of the main wing, such that there is little interference between the pylon and the upper surface of the main wing. Furthermore, the blend from lower to upper pylon tapers to a narrow width, and the chord of the upper pylon is substantially smaller than the maximum chord of the lower pylon. References to the chord of the pylon refer to the length of the section in the yaw axis. In some embodiments, the blend between lower and upper pylons is contoured such that the mean streamlines about the main wing, when projected onto the surface of the pylon, follow smooth contours with decreasing curvature far from the leading edge of the pylon blend. In some embodiments, the blend between lower and upper pylons may be contoured such that the streamlines passing over the upper part of the lower pylon segment cross over to the upper surface of the main wing and flow at a small angle to streamlines passing over the upper pylon segment about the stagnation point on the main wing proximal to the pylon. 
       FIG. 1  is a diagram depicting a kite system according to some embodiments of the present invention. A kite system  100  comprises a tether  103  that connects the kite  101  to the ground station  102 . The kite  101  flies along the flightpath  104  at a high multiple of the speed of the wind  122  during normal operation in a cross wind flight path  104 . In some embodiments, to launch and land, the kite  101  hovers under thrust from the rotors  109 , which are controlled by an automatic control system. Aspects of the take-off and landing of the kite system may be seen in U.S. patent application Ser. No. 13/070,157 to Vander Lind, which is hereby incorporated by reference in its entirety. To provide pitch control authority, some of the rotors  109  are distributed above the center of gravity of the kite  101  and some are distributed below the center of gravity of the kite  101 , as viewed in the typical aircraft build reference frame. In some embodiments, the tail  106  comprises a horizontal element which is located substantially above the kite center of gravity, and which rotates 90 degrees pitch down during hover, to both reduce the pitching moment on the kite  101  due to the wind  122  on the tail  106 , and to stabilize the kite  101  in pitch. The kite  101  further comprises a main wing  105 . In embodiments in which kite system  100  is used to generate power, the main wing  105  is used to generate substantial lift, such that the kinetic energy available in the wind is transferred into the kite  101  in the same manner as the tip of a wind turbine blade. 
       FIG. 2  is a diagram depicting an embodiment of a kite  201 , such as a kite  101  comprised by the kite system  100  depicted in  FIG. 1 . The kite  201  comprises a main wing  205 , which generates substantial lift during operation of the kite  201  along its flightpath (e.g. flightpath  104 ). In some embodiments, the main wing  205  comprises a trailing element  207  which increases the maximum coefficient of lift which may be generated by the main wing  205 . The kite  201  further comprises a tail  206 , which both counters the pitching moment generated by the main wing  205  and trailing element  207 , and increases the pitch stability, yaw stability, and coupled stability of the kite  201 . However, the tail  206  has mass, which suggests that, to locate the kite center of mass  223  at its target location near the quarter chord of the main wing  205 , a countering mass must be located forward of the wing  205 . The rotors  209 , along with the motor/generators which drive the rotors  209 , are located in front of the main wing  205  such that their mass counters that of the tail  206 . The rotors  209  and the associated motor/generators are attached to the wing through pylons  212 . The pylons  212  comprise upper pylons  211  and lower pylons  213 . As the main wing  205  may be operated at a high coefficient of lift, it is designed with a specific distribution of pressure along the chord of the wing  205  such that lift remains high, drag remains low, and the wing  205  does not prematurely stall. As the coefficient of pressure over the surface of the wing  205  derives from the shape of the wing  205 , modifications of the wing  205  have the potential to change the pressure distribution and affect stall. Furthermore, as the coefficient of lift of the wing  205  may be high in some embodiments (e.g. above 0.7 as referenced to the wing area of the wing  205 ), air on the bottom surface of the wing  205  is moving slowly and has a lesser effect on the pressure distribution about the wing  205  due to shape changes. Thus, the lower pylon  213  attaches to the bottom surface of the wing  205  to take advantage of this lower sensitivity of the bottom surface of the wing to disturbance, and the upper pylon  211  attaches to the top of the lower pylon  213 , as opposed to the top surface of the wing  205 . 
     The kite  201  may operate for a long period of time. As such, the rotors  209 , wing  205 , tail  206 , and motor pylons  212  must be subjected to low time varying aerodynamic stresses to reduce structural fatigue. The rotors  209  are located substantially in front of the wing  205  such that the change in pressure about the wing  205  as the wing  205  generates lift does not substantially impact the flow entering any portion of the swept area of the rotors  209 . The rotors  209  are located substantially above and below the main wing  205  such that the wakes of the rotors  209  do not impinge on the main wing  205  during normal operation of the kite  201 , thereby avoiding an increase in turbulence impinging on the wing  205 . 
       FIG. 3  is a diagram a motor pylon  312  and the manner in which it is affixed to a main wing  305  according to some embodiments of the present invention. A lower pylon  313  is affixed to the lower surface  314  of the main wing  305 , and an upper pylon  311  is affixed to the leading portion of the lower pylon  313 . A junction  316  joining the upper pylon  311  and the lower pylon  313  is shaped to follow the mean streamlines over the wing  305  in the normal operating condition of the wing  305 . In some embodiments, sharp corners along junction blends, such as the junction blend  317 , are incorporated such that in normal operating conditions the defining contours of the junction blend  317  runs along the direction of mean flow during normal operating conditions, but at low angles of attack, flow crosses the junction blend  317  at an angle and, due to the sharpness of the junction blend  317 , detaches or separates. In some embodiments, a sharp junction blend is incorporated to increase drag at low angles of attack, such that, in cases where the flight speed of the kite and tension on the tether  102  must be limited, the kite may produce increased drag at low angles of attack. Increased drag may be a desired characteristic when flying at high wind speeds. 
     In some embodiments, the pylon  312  comprises asymmetric, cambered airfoils oriented at some angle of inclination about the pylon span, relative to the mean oncoming flow direction. In such cases, the aerodynamic force generated by the pylons  312  produce a side force on the kite (e.g. kite  101 ), which accelerates the kite around its flightpath (e.g. flightpath  104 ). In some embodiments, the trailing edge of the pylon  312  extends behind the trailing edge of the main wing  305 , and attaches to and supports the trailing elements  307 . In some embodiments, just the lower pylon  313  extends beyond the trailing edge of the main wing  305 . In the depicted embodiment, the pylon  313  comprises an airfoil with thickness of 15 percent of local chord, and camber of 4 percent of local chord, and is rotated six degrees relative to the design oncoming flow case. Thus, the pylon  312  generates a side force during the normal flight condition of the kite. The upper pylon  311  is also cambered 4 percent of chord, but is only inclined 3 degrees from the typical oncoming flow. As a strong wake from the upper pylon  311  would have the potential to impact the attachment of flow on the upper surface of the wing  307 , the upper pylon is both smaller in chord and inclined at a lower angle than the lower pylon  313 , such that the upper pylon  311  contributes a smaller portion of the side force than the lower pylon  313 . 
     Junction  316  tapers from a wider cross-section at the top of the lower pylon  313  to a smaller cross-section with a pointed trailing tip  315  at the bottom of the upper pylon  311 . The pointed tip  315  is located near the leading edge of the main wing  305 , and the upper pylon  311  does not attach to the wing  305  over a large portion of the upper surface of the wing  305 . In some embodiments, the pointed tip  315  is located at or near the stagnation point of the wing  305  during normal flight conditions. The stagnation point, of the point of flow stagnation on the leading edge of the wing  305  moves as the angle of attack of the wing  305  changes. 
     In some embodiments, a landing gear extension  319  extends below the bottom of the lower pylon  313 , such that landing gear may attach to the bottom of the pylon  312  and clear the rotors for landing, with tether detached, in the manner of an aircraft. Aircraft type landing gear of this type are used in some embodiments which land in the manner of an aircraft in the event of a rotor, motor/generator, or power systems failure. 
       FIG. 4  is a diagram depicting a motor pylon  312 , the manner in which the motor pylon  312  is affixed to the main wing  305 , and the manner in which the motors and/or generators  318  are attached to the upper pylon  311  and the lower pylon  313  according to some embodiments of the present invention. The lower pylon  313  is affixed to the lower surface  314  of the main wing  305 , and the upper pylon  311  is affixed to the leading portion of the lower pylon  313 . The junction  316  is shaped to follow the mean streamlines  420  under the wing  305  in the normal operating condition of the wing  305 , which are seen below the stagnation point  421 . 
     In some embodiments, the pylon  312  comprises an asymmetric, cambered airfoil mounted at some angle of inclination relative to the pylon span (angle of sideslip relative to the main wing, such that the pylon  312  is, in normal operation, generating lift primarily in the same direction in at all pointe around the flightpath. In such cases, the aerodynamic force generated by the pylons  312  produces a side force on the kite (e.g. kite  101 ), which accelerates the kite around its flightpath (e.g. flightpath  104 ) when flying in a circular flight path, for example. In the depicted embodiment, the upper pylon  313  comprises an airfoil with thickness of 15 percent of local chord, and camber of four percent of local chord, and is rotated six degrees relative to the design oncoming flow case. Thus, the pylon  312  generates a side force during the normal flight condition of the kite. In such embodiments, air on the pressure surface of the lower pylon  313  is moving slowly and has a lesser effect on the pressure distribution about the lower pylon  313  due to shape changes. The pressure surface is what would be viewed as the bottom of an airfoil in a horizontal configuration. Thus, the motor/generator  318  attaches to the pressure surface of the pylon  313  and may protrude onto the other (suction) surface of the pylon  312  to a lesser extent. The motor/generator  318  is oriented such that its rotation axis is parallel to the oncoming airflow and the disc of any propeller  309  attached to the motor/generator  318  is roughly normal to the oncoming airflow. 
     In some embodiments, the upper pylon  311  is also cambered 4 percent of chord, but is only inclined 3 degrees from the typical oncoming flow. As a strong wake from the upper pylon  311  would have the potential to impact the attachment of flow on the upper surface of the wing  305 , the upper pylon  311  is both smaller in chord and inclined at a lower angle than the lower pylon  313 , such that the upper pylon  311  contributes a smaller portion of the side force than the lower pylon  313 , and the pylon junction  316  tapers from a wider cross-section at the top of the lower pylon  313  to a smaller cross-section with a pointed tip at the trailing edge attachment point  315 , at the bottom of the upper pylon  311 . In such embodiments, air on the pressure surface (the surface toward the direction of the sideforce) of the upper pylon  311  is moving slowly and has a lesser effect on the pressure distribution about the upper pylon  311  due to shape changes. However, the upper pylon  311  is much thinner and has a significantly smaller chord than the lower pylon  313 , thus has less space for attachment of the motor/generator  318  on any pressure or suction surface. Thus, the motor/generator  318  attaches to the top surface of the pylon  311 , thereby reducing airflow interference and potentially increasing sideforce capability. The trailing edge attachment point  315  is located near the leading edge of the main wing  305 , and the upper pylon  311  does not attach to the wing  305  over a large portion of the upper surface of the wing  305 . In some embodiments, the trailing edge attachment point  315  is located at or near the stagnation point of wing  305  during normal flight conditions. The motor/generator  318  is oriented such that its rotation axis is parallel to the oncoming airflow and the disc of any propeller  309  attached to the motor/generator  318  is roughly normal to the oncoming airflow. 
     In some embodiments, the pylon  312  comprises symmetric, uncambered airfoils mounted at zero angle of sideslip relative to the mean oncoming flow direction. In such cases, no aerodynamic side force is generated by the pylons  312  on the kite (e.g. kite  101 ). In such embodiments, the motor/generator  318  may be mounted such that its cowling may protrude on both surfaces of the upper pylon  311  and/or the lower pylon  313  equally or unequally. 
       FIG. 5  is a diagram depicting an embodiment of a motor pylon  312 , the manner in which it is affixed to a main wing  305 , the manner in which the motors and/or generators  318  are attached to the upper pylon  311  and the lower pylon  313 , and the manner in which the upper pylon  311  is attached to the lower pylon  313 . The lower pylon  313  is affixed to the lower surface  314  of the main wing  305 , and the upper pylon  311  is affixed to the leading portion of the lower pylon  313 . The upper-to lower pylon junction  316  is shaped to follow the mean streamlines (e.g. streamlines  420 ) over the wing  305  in the normal operating condition of the wing  305 . In the depicted embodiment, the upper pylon  311  is thinner than the lower pylon  313  and the junction  316  tapers to a smaller width from the lower pylon to the upper pylon. In such embodiments, the upper pylon may be laterally offset from the centerline of the lower pylon  313 , such that the suction surfaces on the upper pylon  311  and the lower pylon  313  are closely aligned to follow the natural streamlines (e.g. streamlines  420 ) of oncoming airflow, or such that any spar or structural element may be correctly positioned. 
     In some embodiments, the motor/generator  318  mounted on the upper pylon  311  may be offset towards the pressure surface potentially minimizing any loss of sideforce due to interference with higher speed airflow on the suction surface which is more susceptible to separation and loss of attachment than the lower speed airflow over the pressure surface. In some embodiments, the motor/generator  318  mounted on the lower pylon  313  may be offset towards the pressure surface potentially minimizing any loss of sideforce due to interference with higher speed airflow on the suction surface which is more susceptible to separation and loss of attachment than the lower speed airflow over the pressure surface. 
     In the depicted embodiment, the majority of the sideforce exerted on the kite (e.g. kite  101 ) by the pylons  312  is provided by the lower pylon  313 . In such embodiments, the placement of the motor/generator  318  is more critical for the lower pylon  313 . In such cases, the lateral placement of the motor/generator  318  on the upper pylon  311  may be such that it is at the same spanwise location along the main wing  305  as the motor/generator  318  on the lower pylon  313 . This reduces angular accelerations of the kite (e.g.  101 ) along axes other than those specifically intended, in case the motor/generators  318  are used for attitude control by changing the thrust or drag or torque to different extents for the motor/generators  318  at various positions relative to the kite&#39;s center of mass. 
       FIG. 6  is a diagram depicting the cross-sections and alignment of components of an embodiment of a motor pylon. The chord line  624  of the lower pylon, and the chord line  625  of the upper pylon are depicted. The chord line  624  of the lower pylon is inclined 6 degrees from the mean chord direction  629  or x body axis of main wing  605 , and the chord line  625  of the upper pylon is inclined 3 degrees from the mean chord direction  629  or x body axis of the main wing  305 . This has the effect of reducing aerodynamic loading on the upper pylon such that the wake from the upper pylon incident on the top surface of the main wing  305  is weaker, and has less effect on the maximum coefficient of lift of the main wing  305 . The lower pylon chord line  624  is at a greater angle relative to the main wing  605  such that the lower pylon generates greater aerodynamic loads creating a larger side-force on the kite (e.g. kite  101 ), providing acceleration to turn the kite along its flightpath (e.g. flightpath  104 ). The trailing edge of the bottom profile  626  of the upper pylon attaches to the main wing  605  at a trailing edge attachment point  615 , which is also the foremost attachment point of the motor pylon to the main wing  305 . 
     In some embodiments, the upper pylon profile  626  and the lower pylon profile  628  are symmetric sections. For example, in some embodiments, symmetric profiles might be used if the target flightpath is a figure eight for which the required direction of side force changes through the flightpath, or if the target flightpath is a large circle requiring a small sideforce to turn. 
       FIG. 7  is a diagram depicting the internal structure of an embodiment of a motor pylon. In some embodiments, the main wing is constructed using a box type spar, in which at least two shear walls  732  connect two spar caps  733 . In these embodiments, it is desirable to attach the motor pylon  312  to the main spar  730  of the wing by way of bonded, riveted, or bolted shear interface plates  731 . The shear interface plates  731  attach to the front and back of the shear walls  732 , as well as to the spars or internal structure of the pylon  712 . In some embodiments, the main wing spar  730  is formed of carbon fiber, fiberglass, Kevlar, or another composite material through a bladder molding process, as a single component. In some embodiments, the shear interface plates  731  are co-molded of composite materials with the main spars of the lower pylon  313 . In some embodiments, the shear interface plates  731  are made of aluminum or some other metal, and are bonded or riveted to the shear walls of the main wing spar  730 . In a preferred embodiment, the main wing spar  730  is formed by a bladder molding process as a single completed piece and the shear wall interface plates  731  comprise the top ends of spars reinforcing the lower pylon  313 . 
       FIG. 8  is a diagram depicting an embodiment of a motor pylon  812 , the manner in which it is affixed to a main wing  805  and the attachment of the upper pylon  811  to the lower pylon  813 . As the main wing  805  is operated a at a high coefficient of lift, it is designed with a specific distribution of pressure along the chord of the wing  805  such that lift remains high, drag remains low, and the wing  805  does not prematurely stall. As the coefficient of pressure over the surface of the wing  805  derives from the shape of the wing  805 , modifications of the wing  805  have the potential to change the pressure distribution and affect stall. Furthermore, as the coefficient of lift of the wing  805  is high (e.g. above 0.7 as referenced to the planform area of the wing  805 ), air on the bottom surface of the wing  805  is moving slowly and has a lesser effect on the pressure distribution about the wing  805  due to shape changes. Thus, lower pylon  813  is affixed to the lower surface  814  of the main wing  805 , and the upper pylon  811  is affixed to the leading portion of the lower pylon  813 , such that the trailing edge of the upper pylon  811  ends ahead of or within close proximity to the stagnation point of the main wing  821 . 
     In the depicted embodiment, the attachment of the lower pylon  813  to the main wing  805  is smoothly filleted through the pylon to wing attachment  814 . As air on the bottom surface of the main wing  805  is already moving slowly due to the high lift design, the presence of sharp corners can lead to the creation of thickened boundary layers due to the retarding effect of two walls in close proximity. This thick boundary layer and accompanying slow airflow are highly susceptible to separation when exposed to even a small adverse pressure gradient as can be experienced over the trailing edge of high lift devices  807 . Thus the smooth pylon  813  to main wing  805  junction  814  reduces the chances of flow separation over the trailing edge devices  807 . A smooth attachment point for the lower pylon  813  to the lower surface of the main wing  805  also smooths pressure distribution and thus delays tripping the airflow from laminar to turbulent, thereby reduces drag and delays the loss of attachment of flow over the trailing edge high-lift device  807  of the main wing  805 . 
     In the depicted embodiment, the trailing edge of the lower pylon  813  curves outwards toward the trailing edge high-lift devices  807  before curving back inwards to the attachment point  814  on the lower surface of the main wing  805 . In such embodiments, the extent of the curve back is sized such that the streamlines of airflow in the wake of the lower pylon  813  has a minimal lateral component at the stagnation point on the leading edge of the high lift device  807 , thereby reducing spanwise flow on the trailing edge device  807  and associated drag and loss of capacity to generate lift. In some embodiments, the trailing edge of the lower pylon  813  may be extended even further back such that the trailing edge high lift device  807  may be supported directly by the lower pylon  813 . 
     In some embodiments, the pylon  812  comprises asymmetric, cambered airfoils mounted at some angle of sideslip relative to the mean oncoming flow direction. In such cases, the aerodynamic force generated by pylons  812  produces a side force on the kite (e.g. kite  101 ), which accelerates the kite around its flightpath (e.g. flightpath  104 ). In the depicted embodiment, pylon  813  comprises an airfoil with thickness of 22 percent of local chord, and camber of four percent of local chord, and is rotated six degrees relative to the design oncoming flow case at the bottom of lower pylon  813 , with thickness decreasing to 18 percent of local chord and rotation decreasing to 3 degrees relative to the design oncoming flow case near the attachment point of the lower pylon to the main wing  814 . In such embodiments, the decrease in thickness to chord ratio of upper sections of the lower pylon  813  reduces profile drag while maintaining a uniform minimum thickness throughout the pylon for structural or other purposes (e.g. for electrical conduits). The upper pylon  811  is also cambered 4 percent of chord, but is only inclined 3 degrees from the typical oncoming flow. As a strong wake from the upper pylon  811  would impinge upon the upper surface of the wing  805  and effect flow attachment at that location, the upper pylon is both smaller in chord and inclined at a lower angle than the lower pylon  813 , such that the upper pylon  811  contributes a smaller portion of the side force than the lower pylon  813 . The upper part of the lower pylon  813  is also rotated to only 3 degrees, resulting in a weaker wake close to the main wing thus reducing the potential to impact the attachment of flow on the main wing  805 . 
     In some embodiments, the upper pylon  811  and the lower pylon  813  may be swept forward, with the leading edge of the top of the upper pylon  811  and the leading edge of the bottom of the lower pylon  813  located further forward toward the normal flight direction than the leading edge of the sections of the respective pylons vertically proximal to the main wing  805 . In such embodiments, the sweep allows the placement of motor/generators (e.g.  418 ) near the top of pylon  811  and the bottom of the lower pylon  813  while allowing their respective rotors to be well clear of the main wing  805  as well as allowing the kite (e.g.  101 ) center of gravity to rest further forward, aiding longitudinal static stability. By locating the rotors well above and below the main wing  805 , and also in front of the main wing  805 , the wake of the rotors does not interact with the boundary layer on the main wing, and the decreased pressure on the upper surface of the main wing does not significantly increase the flow velocity through the lower half of the upper rotors as compared to the upper half of the upper rotors. In the depicted and similar embodiments where the pylons generate aerodynamic sideforce, the sweep further promotes spanwise flow along the pylons from the top of the upper pylon  811  and the bottom of the lower pylon  813  towards the main wing  805 , thus increasing local static pressure near the main wing stagnation point  821 , thereby reducing adverse pressure gradient of the airflow over the top surface of the main wing, which reduces likelihood of flow separation, in turn increasing maximum lift capability of the main wing  805 . 
     In the depicted embodiment, the lower pylon  813  is longer than the upper pylon  811 . In such embodiments, a landing gear may be attached to the bottom of the lower pylon  813 . In such embodiments, the lower pylon  813  may be swept in sections, with the lower section of the lower pylon  813  swept forward to a much lower degree than the upper section of the lower pylon  813 , or not swept at all, or swept back relative to the rest of the lower pylon  813  to enable clearance for rotors associated with any motor/generators (e.g.  418 ) that may be mounted to the lower pylon  813 . 
     In the depicted embodiment, the lines defining the leading and trailing edges of the pylon  812  are smooth and continuous curves, reducing occurrence of sharp shape transitions thereby reducing formation of regions of localized flow separation and reducing drag. In some embodiments, the top of the upper pylon  811  and/or the bottom of the lower pylon  813  may be capped by dome-shaped structures to prevent separated flow at the tips of the pylon  812 . In some embodiments, other devices such as winglets or raked wingtips or wingtip fences may be used to cap the tips of the pylon  812 . 
     In some embodiments, the junction between the lower pylon  813  and the upper pylon  811  near the main wing  805  is constructed with the high pressure surfaces of the lower pylon  813  and the upper pylon  811  more closely aligned than their respective low pressure or suction surfaces such that a portion of the airflow that has passed over the lower pylon  813  crosses upwards to flow over the top surface of the main wing  805 . In the depicted embodiment, the junction between the lower pylon  813  and the upper pylon  811  further causes the portion of the wake of the lower pylon  813  that is flowing over the top surface of the main wing to flow at a small angle relative to the wake from the upper pylon  811 . The forward swept angle of the pylons further sheds vorticity into the air behind the pylon  812  on the top surface of the main wing  805 , re-energizing the boundary layer of the upper surface of the main wing  805  behind the pylon-wing junction, thus delaying onset of flow separation behind the pylon  812  to such time when the main wing  805  is at a higher angle of attack relative to the oncoming flow, thereby increasing maximum lift capability of the main wing  805 . 
     In some embodiments the entire pylon  812  may be angled such that the junction between the upper pylon  811  and the lower pylon  813  is no longer horizontal with respect to the ground or the gravity vector of the Earth. In such embodiments, the angle between the spanwise vector of the pylon  812  and the spanwise vector of the main wing  805  is such that the pylon causes the least drag from the oncoming airflow at an angle of attack for the main wing  805  that the kite (e.g.  101 ) is most likely to employ during normal flight. 
       FIG. 9  is a diagram depicting an embodiment of the structural connection of a motor pylon for an airborne wind turbine main wing spar  930 . In this embodiment, the motor pylon is attached to a glove  937  which is bonded to the shear walls of the main wing  932 , and to a portion of main wing spar caps  933 . The main wing spar  930  sees both large fore and aft bending moments, as well as large up and down bending moments. These combined yield large stresses on each face of the main wing spar caps  933  and the main wing shear walls  932  of varying amounts over time. The glove  937  bonds to the main wing spar spar  930  over a surface comprising a tapered laminate, with ply drops  935  reducing the thickness of the laminate over the length of the bond. The glove  937  further comprises a protrusion  934  which extends below the spar caps  933  in a planar manner from the shear walls  933  so as to transfer load in shear from pylon attachment points  935  in shear, through the shear walls of the glove  937 , and further through the bond of the glove  937  to the shear walls  933  of the main wing spar  930 . The pylon is attached by bolts or shear pins to the attachment points  935 . This allows the pylon to be quickly attached or detached for transport, maintenance, or replacement. The glove  937  further bonds to the lower spar cap of the main wing spar  930 . This connection is made through a curved segment  938 , which prevents significant in-plane compression of extension from creating a peel force in the bond area, but does not prevent shear loads from being transferred, thereby preventing peeling forces from being transmitted into the shear interface places  931  from thrust or drag on the rotors on the pylons. 
       FIG. 10  is a diagram depicting an embodiment of a pylon substantially similar to that depicted in  FIGS. 3 ,  4 ,  5 ,  6 , and  7 . The rotors  1009  are depicted by the disc or plane through which they rotate. A number of iso-pressure lines  1034  of the main wing  305  are depicted, along with the tangents  1035  of those iso-pressure lines most closely aligned with the rotors  1009 . In many embodiments, the rotors  1009  are located such that the rotor discs are near tangent to the iso pressure lines in front of and above and below the main wing, such that the rotor discs do not intersect a significant number of iso-pressure lines or cross a significant range in incident flow velocities. The location of the rotors in front of and substantially above and below the main wing results in not only lower differential inflow velocity over various portions of the rotor disc, but also reduced impact of the rotor wake and expansion field on the main wing  305 . Note that, in the drawing as depicted, the rotors would intersect five or more iso-pressure lines (equally spaced in differences of pressure) if located directly above or below the wing, while they intersect roughly 0.1-0.5 iso-pressure lines in their depicted configuration. 
     In some embodiments of the kite, the lifting surfaces are comprised of horizontal surfaces and vertical surfaces. In the presence of relative airflow, the horizontal surfaces produce lift on the pitch plane and the vertical surfaces produce a lifting force on the yaw plane, i.e., aerodynamic side-force. In various embodiments, a component of the lift generated by the horizontal surfaces is the primary motive force of kite. In some embodiments, the kite is rolled relative to the tether such that a component of the lift generated by the horizontal surfaces contributes to the turning force of the kite. In various embodiments, the lift generated by the vertical surfaces is the primary component of turning force of the kite. In high wind flight, the vertical surfaces are used instead of horizontal surfaces to generate the primary turning force, while the orientation of the kite is changed such that the coefficient of lift due to the horizontal surfaces is reduced. In this manner larger deviations in angle of attack may be tolerated on the horizontal surfaces prior to stall or spar failure. In some embodiments, the lifting surfaces are comprised of lifting surfaces in a number of different orientations that serve the same combined purpose of the vertical surfaces and the horizontal surfaces. 
     In some embodiments, the parasitic and induced drag of the horizontal surfaces and the vertical surfaces is determined by the trim angles of attack and side-slip of the kite and by the deflections of the control surfaces. In some embodiments, the drag from the horizontal surfaces and the vertical surfaces increases significantly at a range of side-slip angles that are large, which may be seen in high wind conditions, compared to the side-slip angles observed when the crosswind kite system operates in normal wind conditions. In some embodiments, the coefficient of lift of the horizontal surfaces decreases at a range of side-slip angles that are large compared to the side-slip angles observed when crosswind kite systems operate in normal wind conditions. In some embodiments, the aspect ratios of the vertical surfaces are small such that the vertical surfaces generate a large amount of induced drag when generating side-force. In some embodiments, the vertical surfaces are shaped to have a low span efficiency by comprising an irregular chord, span-wise gaps, span-wise slots, or alternating trailing edge deflections. In some embodiments, the vertical surfaces of the motor pylons have asymmetric airfoils such that the vertical surface is adapted for lift in one direction, which may be the center of a circular flight path in some aspects. In some embodiments, a subset of lifting surfaces comprise side-slip dependent lift modifiers, which modify the lift and drag of the surfaces which comprise them. In various embodiments, side-slip dependent lift modifiers comprise vortilators, fences, or any other appropriate lift modifiers. In some embodiments, the lift modifiers modify the stall characteristics of a subset of the lifting surfaces as a function of side-slip. In some embodiments, the vertical surfaces comprise through-wing vents or leading edge slats which see little airflow in normal operation but which exhibit a large through flow and a large drag coefficient at large side-slip angles. In some embodiments, the vertical surfaces comprise a subset of control surfaces that, when deflected or actuated, increase the side-force of vertical surfaces at a given angle of side-slip. 
     In some embodiments of the present invention, a pylon might comprise a NACA 2415 airfoil and have zero angle of incidence in normal power generating flight, producing a pylon coefficient of lift of 0.25. At an aspect ratio of 4 and span efficiency of 1, this results in a coefficient of induced drag, referenced to the pylon area alone, of 0.005. If, in high wind flight, the kite is flown at an average sideslip of 7 degrees, the pylons then generate a pylon-referenced coefficient of induced drag of 0.08. In some embodiments, the pylons have about 0.25 of the area of the main wing, resulting in an increase in coefficient of drag of 0.02 referenced to wing area. In some embodiments, the pylons are shaped in a manner which has a very low span efficiency. For instance, the pylons may incorporate large changes in chord over short pylon-spanwise distances, or may incorporate sharp edges near the pylon tips, oriented to be aligned with the flow at kite sideslips, but to be misaligned with the flow at high sideslips. For example, the tip of the pylon may be cut off with a square end. A pylon with a vertical pylon as described above offers an advantage in that induced drag is significantly increased when the kite is flown in sideslip. As the side slip angle is increased in flight in high winds, induced drag increases, moderating the increase in structural loading on the system due to the increase in wind speed. 
     The pylon airfoil profile may also be modified to produce greater profile drag above a critical angle of sideslip. For example, the pylon profile  1301  may incorporate a leading edge cuff over a portion of the span of the pylon, as depicted in  FIG. 13 . A cross-sectional profile  1301  of a segment of the pylon, may cover, for example, 20% of total pylon span. The pylon may have a leading edge cuff  1302  with a sharp curvature discontinuity, causing a separation bubble over a segment of the top surface of the pylon cross-section above a critical kite sideslip or critical angle of attack of the pylon cross-section relative to the apparent wind. As the majority of the pylon still utilizes a conventional airfoil cross-section, the added separation and parasitic drag due to the cuffed pylon segments does not dramatically affect stall angle of attack or kite handling. A pylon with profile features as described above offers an advantage in that profile drag is increased when the kite is flown at a significant sideslip angle. As the side slip angle is increased in flight in high winds, profile drag increases, moderating the increase in structural loading on the system due to the increase in wind speed. 
     The main airfoil, in some embodiments, has an aspect ratio of 25, and operates at a coefficient of lift of 2 in normal power generating flight, and a coefficient of lift of at or above 0.7 in high wind flight. To provide an example, this results in a coefficient of induced drag of between 0.085 and 0.05 referenced to wing area during normal crosswind flight, and a coefficient of induced drag of 0.006 at the low coefficient of lift used in high wind flight. In this example, assume the tether has a coefficient of drag referenced to wing area of 0.05, and a parasitic and profile drag of 0.04 referenced to wing area. This results in a lift to drag of 14 for the airframe, and a performance metric (C_L^3/C_D^2) of 400. 
     In high winds, again neglecting the effects of flightpath geometry, the resulting lift to drag ratio at a coefficient of lift of 0.7 on the main wing is 7, and the performance metric is 40. If, however the added pylon drag due to sideslip previously listed (0.01) and due to change in profile (0.01) are included, the lift to drag becomes 6, and the performance metric becomes 25. In the example given, continued flight of circles becomes difficult at a coefficient of lift of 1.5, due to the requirement for excessive tether roll angle in order to complete the turn (in turn due to the lower aerodynamic force available to counteract centripetal forces). If this is taken as the minimum coefficient of lift of a kite system not incorporating aspects of the present invention in its flight, including turning with side slip, the lift to drag and performance metric of the system are, respectively, 12.6 and 240. Aerodynamic forces increase roughly as the square on incoming windspeed. Thus, if the nominal flight example above uses full allowable flight-loads (20000 Newtons for an 4 square meter wing, for example) in winds of 10 m/s, the example with a minimum coefficient of lift of 1.5 is able to fly in winds no higher than 13 m/s, and the example incorporating multiple aspects of the present invention, with a minimum coefficient of lift of 0.7, is able to flight in winds no higher than 39 m/s. Although in practice embodiments of the present invention may utilize additional features to moderate loads in high wind conditions, one can see that just this aspect allows for a 290% increase in wind capability versus just 30% without this aspect in this exemplary embodiment. 
     As evident from the above description, a wide variety of embodiments may be configured from the description given herein and additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader aspects is, therefore, not limited to the specific details and illustrative examples shown and described. Accordingly, departures from such details may be made without departing from the spirit or scope of the applicant&#39;s general invention.