FIELD OF THE INVENTION

The invention relates to tethered aircraft and a method of steering it.

BACKGROUOND OF THE INVENTION Tethered aircrafts (kites) are used in sports, lifting of things, energy production with airborne generators and motors or converting the tractive force in the tether into energy on a ground based generator, or for pulling various crafts. Tethered aircrafts can have a rigid self-supporting structure with or without propulsion engines. Tethered aircraft can also be have a non-rigid structure such as kites or even be collapsible such as a parafoil. The aircraft can be attached to multiple tethers to the ground, or a single tether in a single point or spread into multiple points often referred to as «bridles». It can be beneficial to use bridles to increase the strength to weight ratio to improve the excessive lift from the aircraft. Many utilizations of tethered aircrafts utilizes the pull of the aircraft in the tether, the so-called excessive lift, to perform a function (sports, lifting and pulling of things, energy production with ground based generators), whereas some utilizes the airspeed (energy production with airborne generators). As the excessive lift is a function of the airspeed of the aircraft, and the airspeed is a function of the aircraft's effective lift-to-drag ration, L/D, the tethered aircraft is often flown in various patterns crosswind to maximize airspeed/excessive lift. It is therefore of importance to be able to steer a tethered aircraft in a controlled manner, and a number of methods are applicable depending on the configuration of the tethered aircraft:

Warping and rolling to turn : Multiple tethers or control lines are used to warp or roll the aircraft to turn it by making one tether shorter than the other. Several Airborne Wind Energy (AWE) companies have adopted this method, such as SkySails, EMPA / Swiss Kite, Delft University, KiteGen, Enerkite, etc. Bank / roll of the aircraft: the turn is caused by the horizontal component of lift. The lifting force, perpendicular to the wings of the aircraft, is tilted in the direction of the intended turn by rolling the aircraft into the turn. As the bank angle is increased, the lifting force, which was previously acting only in the vertical, is split into two components: One acting vertically and one acting horizontally. Single tether configuration where the aircraft is operated similar to how aircrafts are flown in free flight.

Sideforce turn using vertical stabilizer, such as pylons in combination with a rudder (on a canard or tail) to turn the aircraft: When a single tether spread into multiple bridles close to the aircraft, the freedom of roll is reduced for the aircraft. The idea is to maximize the projected area, by keeping the wingspan as level as possible to the perpendicular tether, and using the rudder to control it and the pylons to reduce sideslip. For this method, To turn the aircraft, a vertical control surface (typically a rudder on a tail) is deflected to rotate the aircraft around a vertical axis and a longitudinally displaced vertical stabilizer creates the sideforce turning the aircraft due to the change in angle of attack of the stabilizer caused by the rotation.

These methods of steering a tethered aircraft involves the following

disadvantages:

Warping, rolling or banking the aircraft reduces the projected (effective) area used to generate lift in a tethered configuration.

Only using a rudder, keeping the wings level to the perpendicular aligned tether, will create sideslip, reduce lift, and increase drag.

· Using a rudder in combination with a longitudinally displaced vertical stabilizer will increase the projected area. But, more non-effective lifting surface in the tether direction is added to the wing configuration, increasing complexity, adding drag, and adding weight. SUMMARY OF THE INVENTION

The invention presents a new method for controlling a tethered aircraft, which method allows the aircraft to take full advantage of bridles used to increase its structural integrity and to increase the excessive lift during turns. In a first aspect, the invention provides a method for controlling yaw of a tethered aircraft, where the aircraft has rigid wings, and is connected to an object by a sing le tether attached to the aircraft via multiple brid les of constant length that spread out spanwise to make the aircraft roll-limited, wherein yaw is controlled by creating a difference in lift-to-drag ratio, L/D, between wing sections on different sides of a center chord of the aircraft while maintaining L/D > 0 for both sides, thereby causing the aircraft to yaw in the direction of the side with lower L/D . In an alternative formulation, the aerodynamic properties are adjusted to adjust L/D differently on different sides of a center chord of the aircraft while maintaining L/D > 0 for both sides, thereby causing the aircraft to yaw or turn in the d irection of the side with lower L/D, or to fly straig ht if the adjustment resulted in the same overall L/D on each side.

In a second aspect, the invention provides a tethered, rigid-wing aircraft comprising two or more attachment points d istributed along a span-direction of the aircraft connecting the aircraft to a single tether via two or more brid les, the aircraft comprising wing control surfaces on each side of a center chord capable of adjusting the lift-to-drag ratio, L/D, characterized in that the aircraft does not have a rudder.

In the following, a number of advantages, preferred and/or optional features, elements, examples and implementations will be summarized . Features or elements described in relation to one embod iment or aspect may be combined with or applied to the other embod iments or aspects where applicable. Also, explanations of underlying mechanisms of the invention as realized by the inventors are presented for explanatory purposes, and should not be used in ex post facto analysis for deducing the invention .

A tethered aircraft is connected to an object via one or more tethers (sometimes referred to as cables or control lines) . In the invention, the single tether spreads into at least two bridles (sometimes also referred to as struts or force members) at a point under the aircraft, the brid le point. The bridles attach to the aircraft at bridle attachment points distributed along a span-direction of the aircraft. For aircrafts with symmetric wings, the bridle attachment points are preferably distributed symmetrically with relation to a center chord . The aircraft preferably comprises a set of bridles made from a substantially inelastic material, each bridle being of a fixed length and having one end attached to an attachment point on the aircraft and the other ends of the bridles being attachable to the tether. In a preferred embodiment, the aircraft has at east three bridles, one of which has an attachment point at or near the center of the wingspan to prevent the wings from breaking downwards.

The center chord is used to refer to the line in the chord-direction in the center of the aircraft in the span-direction, this would be the fuselage if the aircraft has a fuselage (some tethered aircrafts doesn't, for example kites) and be the rotational axis for rolls performed by the aircraft (if it wasn't roll-limited). Taking aircrafts with asymmetrical wing length or asymmetric configurations into consideration, the center chord may not be the appropriate definition, and an alternative definition in this case may be the rotational axis for yaws or the line in the chord directions which intersects with this. The definition of the "center in the span direction" discussed here is used for specifying that a difference in L/D between wing sections on different sides of the center in the span-direction is created. As many different aircraft configurations exist and will exist, finding a definition that applies equivalently to all of these may not be possible as the person skilled in the art of controlling tethered aircrafts will appreciate.

The "span-direction" or "spanwise" is a direction parallel to the wingspan of the aircraft, the wingspan is measured in a straight line, from wingtip to wingtip, independently of wing shape or sweep. Thus, the span direction is perpendicular to the chord direction (nose-to-tail direction or leading edge to trailing edge direction). It maybe preferred that the aircraft is self-supported when not flying/on the ground.

A wing section is a section of a wing, and each wing may be seen as composed of many separate sections, each generating a lift and a drag. Lift over drag may be considered for each separate section, for each wing or side of the center chord (local L/D, sum of L/D for all sections of this wing or side), or for the entire aircraft (global L/D). Creating a difference in L/D between wing sections on different sides can be done by increasing or reducing lift and/or drag for one or both sides. Almost all changes in the aerodynamic properties of the airplane will have some effect on in particular the drag, leaving a wide range of adjustment options such as spoilers, ailerons, aerobrakes, propellers (generators and motor configurations). Preferably the difference in L/D is created through changing L/D for a particular wing section compared to another wing section positioned on the opposite side of the center chord. A number of preferred ways of adjusting L/D will be presented later. Maintaining L/D > 0 for both sides ensures that there is a tensile force in bridles on both sides of the center chord. A negative lift in one side would result in roll motion and in a slacking of the bridles (or tethers) attached to the wing that is lowered. Sind this again results in the aircraft being asymmetrically hinged in the bridles/tethers on the other side, such situation would typically result in an unstable aircraft out of control. Preferably, L/D > 0 is maintained for all wing sections that has a bridle attached to it, in order to maintain a tensile force in all bridles, this criteria is particularly relevant for non-rigid wings and wings where the bridles provide a dominant structural integrity. As presented above, the invention specifies bridles being spread out in the span- direction and being of constant length. This setup provides two advantageous effects that are essential for tethered aircrafts according to the invention :

• Roll-limitation : The bridles being spread out spanwise results in the aircraft being unable to roll, bank, or sideslip (roll counteracted by opposite rudder) while bridles are under tension. This is advantageous in that it ensures the maximum excessive lift at all times, also during turns. For a tethered aircraft for power generation with ground-based generator, maximizing excessive lift is crucial. If the bridle(s) on one side is slacked, the aircraft could hypothetically roll. But this would mean an aircraft completely out of control since it would be tethered to one wing only with all normal controls being out of play.

• Structural integrity: The bridles carry the load on the wings and thus provide the structural strength of the spar in an untethered fixed-wing aircraft. This means that the aircraft can be made lighter with a larger effective wing area. In Figures 1 through 5, the following considerations to the bridle setup are illustrated, where :

- The pull vector of the bridle : B

- B's projection of the wing : BH

- B's projection perpendicular to the wing : Bv

- Bridle angle to wing : β

The fig ures are not to scale. Looking at Fig ure 1, BH does not contribute to roll- limitation nor structural integ rity, only Bv does. So, maximizing Bv provides larger excessive lift and allows for stronger and/or lighter designs, and maximizing Bv is the same as maximizing β. However, it does matter where Bv is applied, i .e. the position of the bridle attachment point along the wing . β can be increased by making BS smaller like in Fig ure 2. Minimizing BS to maximize β will not work since it approximates a no-bridle setup with a single tether attached at the center with no roll-restriction and since the bridles provide no structural integrity to the outer parts of the wings. On the other hand, having BS equal to the wingspan as in Fig ure 3, even with one or more brid les in the wing sections in between, will not be an effective design since the BPD must be large to obtain a large β and since the excessive length and number of bridles increase drag .

The number of brid les and the optimal position of the outermost brid les naturally also depends of the form and construction of the wings. In the present invention, the bridlespan is preferably at least 1/3 of the overall wingspan, preferably 2/5 of the overall wingspan, such as at least Vi of the overall wingspan . β can also be increased by increasing BPD as illustrated in Figure 4, making the angles to the wings less acute to increase B _v . However, maximizing BPD to maximize β will not work due to the increased drag from the very long bridles.

From the above considerations, it is clear that the optimal brid le setup is in the simplest approach a balance between providing the largest effect of roll-limitation and structural integrity while minimizing the total d rag of the bridles by red ucing the number and length of bridles. In a more complete approach, the optimal brid le setup is a complex relation between a large number of parameters and will not be treated here.

In a preferred embodiment, the aircraft comprises two or more bridle attachment points distributed spanwise along the wings for connecting the aircraft to the tether via the bridles, the aircraft being roll-limited in that :

1. a distance between the outermost bridle attachment points, the bridlespan BS, being at least one third of an overall wingspan ; and

2. a shortest distance between the bridlespan and where the bridles connect to the tether, the bridle point d istance (BPD), is equal to or larger than half the bridlespan ;

The bridle setup of this embodiment thus involves a minimum bridlespan and a minimum bridle point distance (or bridle length) . In order to maximize the effects of roll-limitation and increased structural integrity in accordance with the above embodiment, the bridle point distance is equal to or larger than half the

bridlespan, such as preferably 3/5 of the bridlespan, 3/4 of the bridlespan, or even preferably equal to or larger than the bridlespan . As illustrated in Fig ure 5, the outermost bridles and the bridlespan forms a triangle with a height of BPD and a base of BS . The ang le b between bridle and wingspan (or bridlespan) is then :

, DPD DPD

tan b — = 2 .

-BS BS

β eq uals b for aircrafts with straight, horizontal wings, for others such as anhedral, dihedral, polyhedral etc., the ang le of the wing at the attachment point must be taken into account. For the relations between BS and BPD listed above, one gets :

- BPD = Vi BS gives b = 45 deg rees

- BPD = 3/5 BS g ives b = 50 deg rees

- BPD = 3/4 BS g ives b = 56 degrees

- BPD = BS gives b = 63 deg ree

The manoeuvres of a traditional, untethered aircraft is well understood and described with a well-defined terminology. For tethered aircrafts without a roll- limiting bridle setup, the freedom of movement is upwards constrained but it is otherwise possible to perform most of the standard manoeuvres as long as the weight and d rag from the tether does not disturb . For tethered, roll-limited aircrafts, the situation is markedly d ifferent, and all manoeuvres involving rolling or banking cannot be made without slacking the brid les in one side. If the bridles in only one side are slacked, it compares to flying an airplane with a tether attached to only the opposite wing, a difficult feat. Controlling yaw by creating a difference in L/D in accordance to the first aspect of the invention results in counterintuitive performance of the roll-limited aircraft. As will be explained in more detail later, if the aircraft is flown with a global L/D below maximum and L/D is increased by increasing lift for wing sections of only one wing, then the roll- limited aircraft will have proverse yaw.

For tethered aircrafts, "up" and "down" are not constant d irections, but depends on the flying mode. For a kite pulling in its power window, the force of gravity is insig nificant in comparison to the pull from the tether, and thus "up" and "down" would be towards and away from the object to which the tether attaches, respectively, and "flat" would be perpendicular to this d irection rather than horizontal . Hence, when the aircraft flies to the side of the wind window, the wingspan will not be level - this, however, is a rotation around the object where "down" is towards the object, and not a roll .

In a preferred embodiment, the aircraft is flown at L/D below maximum and the difference in L/D is created by increasing L/D for a wing section by increasing or decreasing a lift coefficient of an airfoil of the wing section to cause proverse yaw of the aircraft.

It is preferred that the controlling of yaw in accordance with the first aspect of the invention is performed while hold ing the aircraft flat, i .e. without rolling or banking the aircraft, which is limited by the bridle setup. This is an advantage since this means a smaller deviation from the ideal kite to tether angle (and thus a larger projected area), which again increases the effective L/D .

The aircraft will only turn as long as there is a difference in L/D between the wings. This is d ifferent from banking an aircraft to turn . A banking turn is initiated by rolling the aircraft (and balancing adverse yaw with the rudder), but the rolling is stopped when the desired roll angle is obtained and the wings then have equal L/D during the remainder of the turn until the turn is ended by rolling back to horizontal. In the present invention, it is preferred that the difference in L/D between wing sections on different sides of a center chord are upheld at all times during a turn. In order to finish the turn/yawing, the method may further comprise subsequently adjusting the aerodynamic properties to create the same L/D for wing sections on each side of the center chord to stop the yawing of the aircraft.

Since no vertical control surfaces are adjusted to cause the aircraft to yaw or turn, the invention allows for the absence of a rudder, vertical stabilizers, and other vertical surfaces such pylons normally used in turning the aircraft. Thus, according to a preferred embodiment of the invention, the aircraft does not have or incorporate a rudder, i.e. a movable vertical control surface on a tail assembly. In a preferred embodiment, the aircraft does not comprise any movable vertical or non-horizontal control surfaces in tail- or canard configuration, such as a rudder or ruddervators in a V-tail configuration. In another preferred embodiment, the aircraft does not comprise any adjustable vertical control surfaces. In yet another embodiment, the aircraft neither comprises a fixed vertical stabilizer on a tail, such as one on which a rudder is normally hinged . These embodiments may each be combined with the others.

The above embodiments provide a number of advantages:

· the weight of the aircraft can be reduced

• the overall drag of the aircraft can be reduced

• wingloads and shearloads can be reduced

• the aircraft can have an overall simpler design with less parts influencing each other aerodynamically, e.g. where flow or turbulence from one part interacts with another part., all of which create interference drag, reduce the

performance, and make it more complex to understand the aerodynamics.

• The aircraft can be designed with less moving parts and less parts altogether, leading to less parts to disassemble and assemble during production, checkup, and maintenance. These advantages may all add to increase the excessive lift of the aircraft. For aircrafts for airborne wind energy production in particular, a just a few percentage increase in excessive lift, and thus in the potential energy harvest, will be important in the competition with other sources of renewable energy.

The aircraft may comprise winglets (also referred to as wingtip devices or wingtip fences) that involve fixed vertical or non-horizontal surfaces at the extreme end of the wings. Wing lets are not vertical control surfaces as such, but devices intended to improve the efficiency of fixed-wing aircrafts by red ucing the aircraft's d rag by partial recovery of the tip vortex energy.

In a preferred embod iment, the wings of the aircraft have a bell-shaped lift distribution first described by Ludwig Prandtl in 1932 or the modified bell-shape described Klein und Viswanathan in 1975, see "On the Minimum Ind uced Drag of Wings - or - Thinking Outside the Box" by Albion H . Bowers, NASA Dryden Flight Research Center, September 2011, and references therein .

In another preferred embod iment, the aircraft does not have a tail or an empennage. In another preferred embod iment, the aircraft is a flying wing, a tailless fixed-wing aircraft with no definite fuselage extend ing before or abaft the wing . These embod iments provides the advantages of reducing overall d rag and weight even further.

To increase projected area of the aircraft, with minimal sideslip when staying in the powerzone of the wind window, the aircraft may be designed with a lift distribution that allows for as flat as possible flig ht while turning with a very short turning radius - basically an aircraft optimized for flat turning and not for straig ht forward flight. In one embodiment, the aircraft therefore has asymmetrical wings with one wing being desig ned with a permanent higher L/D than the other wing . The set of brid les would then also be asymmetric configured in order to

accommodate the asymmetric load .

As mentioned previously, the use of bridles in the span direction allows the designer to increase the strength to weight ratio, or red uce the weight for the same power the aircraft is capable of provid ing, thereby increasing the excessive lift available for energy e.g. production. The higher strength to weight ratio provides the further advantage that the aircraft can be used in a larger wind range. With lower weight the stall speed goes down and the aircraft can take off in lighter winds, and is more safe. The aircraft can also fly in stronger winds when using more bridles, as the force is more evenly distributed into the bridles acting as force members.

The higher structural integrity combined with less complex design also enables reduced levelized cost of energy through, less cost, less maintenance, longer expected lifespan, increased capacity factor

The basic idea of the invention according to the first and second aspect is to have a tethered aircraft with bridles spread out in the span-direction making the aircraft roll-limited, and to control the yaw of this aircraft using a difference in L/D on different sides of a center chord. Thereby, the rudder becomes excessive and only adds drag and weight to the aircraft, and can therefore be omitted, making the aircraft lighter, simpler and with less drag . Having the aircraft roll-limited and controlling it using difference in L/D also increases the lift since it forces the wings to be as perpendicular to the tether as possible and thus maintaining maximum lift parallel to the tether. Having bridles spread out along the span-direction reduces the weight as the bridles provide structural integrity allowing a lighter beam/wing spar, normally being one of the heaviest components in the wing . Thus less lift is required (less loss of excessive lift) to keep the aircraft airborne and more can be converted into tractive force. Controlling yaw using difference in L/D reduces the drag compared to wings that either rolls (lift converted into "sideforce" lift to turn the aircraft), or uses large vertical surfaces to yaw the aircraft.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1-5 illustrate the variation of parameters in a bridle setup. Figure 6A is a side view of a tethered aircraft stationary in the air; Figure 6B shows the aircraft in figure 6A seen from above; Figure 7 illustrates the lift-to-drag ratio, L/D as a function of the angle of attack with arbitrary numbers on the axis as well as the variations in Ci_ and CD. Figure 8A and B illustrate some of the forces on a wing section before (8A) and after (8B) increasing L/D for the wing section . Figure 8C shows a comparison between the force components in 8A and 8B. Figure 8D illustrates the increase in L/D in a d ifferent scenario, and 8E shows a comparison of the horizontal force components here.

Figure 9A is a side view of a tethered aircraft flying in a closed loop;

Figure 9B shows the aircraft in fig ure 7A seen from above; Figures 10- 14 illustrate a number of different embodiments for adjusting aerodynamic properties to create the d ifference in L/D.

DETAILED DESCRIPTION OF THE INVENTION Fig ures 6A and 6B show an aircraft 10 with a tether 11. Waved arrows indicate the wind d irection . In fig ure 6B is shown control surfaces, here in the form of flaps 12, 13 arranged on both wings. The flaps 12, 13 have equal positions whereby the lift-to-drag ratio, L/D, is equal on both sides of the aircraft. In this config uration the aircraft is stationary above the g round and airspeed is eq ual to wind speed .

As the amount of lift varies with the ang le of attack, so too does the drag . Thus, althoug h it is desirable to obtain as much lift as possible from a wing, this cannot be done without increasing the drag . The lift and drag of an airfoil depend not only on the angle of attack, but also upon at least the shape of the airfoil, the wing area, the true wind speed and the density of the air. Figure 7 is a graph that schematically illustrates L/D (sometimes also referred to as the glide ratio) as a function of the angle of attack, a . The numbers on the axes are arbitrary values for illustration purposed only, and such graphs are typically empirically

determined . The graph reaches its maximum at an angle of attack of 4,5, meaning that at this angle we obtain the most lift for the least amount of drag . The variation of the lift and d rag coefficients Ci_ and CD, are also shown .

The dynamics of the yawing or turning of a tethered, roll-limited aircraft according to the present invention is described in the following in relation to Figures 8A-E .

Firstly, when air flows past the surface of a wing section, it exerts a force on the airfoil, the so-called aerodynamic force, denoted FAD in Fig ure 8A. Lift (L) is the component of this force that is perpendicular to the oncoming flow direction 14 and drag (D) is the component of the force parallel to the flow direction . For a free-flying aircraft or a tethered aircraft that is not roll-limited, creating a difference in L/D between the wings creates a difference in lift between wings that makes the aircraft roll . When the aircraft is rolled, the lift L not only points upwards but also inwards towards the turn, and the horizontal component of the lift can be seen as the resulting force turning the aircraft.

On a tethered, roll-limited aircraft accord ing to the invention, the brid les also exert a force on the wing, denoted FT in Figure 8B as its direction is ultimately determined by the pull of the tether, which direction is again dependent upon the relative positions of the aircraft and the object to which the tether is attached, but also on the slack of the tether (the deviation from a straig ht line) which is a function of among others the wind speed, the drag and weig ht of the tether, and the excessive lift of the aircraft. FT has a horizontal and a vertical component, TH and Tv respectively.

Looking at a wing section during flight, FT and FAD will not balance out since other parts of the aircraft (attached to the wing segment) provide drag but no lift and all parts are affected by gravity. But all of these other forces exerted on the wing can with a good approximation be considered constant in the following .

In Fig ure 8B, L/D and the angle of attack is increased for the wing section . If we are to the left of the maximum in the L/D curve in Figure 7, FAD becomes larger, F'AD, and L increases while D increases proportionally less to L' and D'

respectively. The old FAD is shown in grey (the changes are exaggerated in the fig ures) . Looking at FT, the increased lift means that the wing section pulls harder in the bridle(s) to which it is attached and in the tether. If the tether is attached to a fixed object on the g round, which is the case for AWE aircrafts, it holds the wing section down (ignoring the change in slack in the tether) and the increased lift is counteracted by an increase in FT to F'T where the change in Tv approximately equals the change in L, the old FT is shown in grey. As the d irection of FT will not momentarily change, TH also increases. If the change in slack in the tether is not ignored, the d irection of FT might change a little as slack is taken up due to the increased pull, which would lead to a larger increase in TH . The wing of the wing section in q uestion might move up little, not because of the taken up slack in the tether (which would cause the entire aircraft to move up), but due to the brid le point under the aircraft moving somewhat sideways towards the wing due to the increased pull in a bridle to that wing . Also, the bridle will stretch a little bit d u to the increased pull as unstretchable wires only exist in theory. Regardless of these considerations, D for the wing section increases proportionally less than the other components, TH in particular. The various components before and after the increase in L/D are compared in Figure 8C.

In the moment of the change in L/D for the wing sections, the forces exerted on the wing section from the rest of the aircraft and gravity are constant, and while AL ¾ ΔΤν so that the changes in L and Tv balances out, the changes in D and TH do not. Since Δϋ < ΔΤΗ, the manoeuvre results in an external force of magnitude (ΔΤΗ - AD) acting on the wing section in the forward d irection . Thus, when L/D is increased for wing sections on one side of the aircraft only, the roll-limiting bridle setup and the tether pull that side forward, thereby turning the aircraft to the side with the lower L/D .

If comparing the reaction to that of a not roll-limited aircraft, the increased L/D of only one wing would cause the aircraft to roll . But since we increased L/D from the left side of the maximum in the L/D curve in Figure 7, we see from the CD curve that the drag also increases. This increased drag of that wing would make that wing slower, resulting in adverse yaw, thus the nose of the aircraft would yaw the aircraft to the side with the increased L/D, which is the opposite direction that the roll limited aircraft turns with the same manoeuvre. Adverse yaw is normally counteracted by means of rudder and vertical stabilizers to yaw the aircraft in the direction of the roll . With a roll-limited aircraft, the same

manoeuvre automatically turns the airplane in the direction of the (non-existing) roll, thus creating an equivalent to proverse yaw. This renders the rudder and other vertical control surfaces normally used to fight adverse yaw unnecessary in turning and controlling yaw of the aircraft. These can therefore be omitted with the advantages listed earlier.

If L/D is instead increased from the rig ht side of the maximum in the L/D curve in Figure 7, both L and D will decrease. But, since the CD curve is steeper than the Ci_ curve in this reg ion, D will decrease proportionally more that L. This is illustrated in Figure 8D, with the horizontal components being compared in Fig ure 8E . This time, Δϋ > ΔΤΗ but both changes have occurred with a different sig n, and again the manoeuvre results in an external force of magnitude (Δϋ - ΔΤΗ) acting on the wing section in the forward direction . Thus, when L/D is increased on one wing from the rig ht side of the L/D maximum (so that L and D both decreases), the aircraft will again turn to the side with the lower L/D . Hence, it does not matter from which side of the maximum of the L/D curve in Figure 7 the angle of attack is changed to increase L/D . The control of the aircraft in accordance with the invention may also be explained with a different approach . The speed of the kite in largely proportional to L/D, and in a simplified formula, the speed of the aircraft is L/D x TWS, where TWS is the true wind speed, when d isregard ing the inclination of the kite, i .e. the projected area of the kite exposed to the true wind direction, TWD . By adjusting

aerodynamic properties of the aircraft, typically wing control surfaces, the local L/D can be controlled individually on each side the aircraft (each side of the center chord) . Remembering the relations illustrated in the graph of Fig ure 7, if for example the global L/D of the aircraft is "4" and the control surface on one wing is deflected to produce a local L/D of "6" while the other wing is not altered, then there will be a difference in L/D between wing sections on different sides.

Normally, this would result in the aircraft rolling, with the wing with the higher L/D going up. But, with a tethered, roll-limited aircraft the aircraft cannot roll, and as is also apparent from the simplified formula above, the increased lift will translate into thrust and the side with the higher L/D will fly faster than the other side. The resulting movement is therefore the aircraft yawing instead of rolling . If the angle of attack of that wing is further increased while still keeping the other wing on L/D = 4, the D (drag) component will start to "grow" faster than the L (lift) component, and at some point the same L/D is obtained albeit with a larger angle of attack.. If the angle of attack is increased further, the L/D for that wing will become smaller that L/D for the other wing, and the wing will become slower whereby the aircraft will begin rotating in the opposite direction .

Figures 9A and 9B illustrates a tethered, roll-limited aircraft flying in a loop using the present invention to steer. Instead of banking and using side forces to turn, the d ifference in L/D across the wingspan changes the forces in the d irection of travel as indicated by arrows of different length at the leading edge of the wings, causing the aircraft to yaw and turn .

The method becomes increasingly more effective the more perpendicular the tether is to the chord/airflow direction, "locking" the roll of the aircraft.

Depending on where in the graph of Figure 7 we are, the aircraft may be controlled in a number of d ifferent ways. T aircraft can be flown at maximum global L/D, and the aerodynamic properties can be adjusted to decrease L/D for the wing sections on one side of the center chord, while maintaining L/D for the wing sections on the other side. This adjustment may also be used when the aircraft is flown with a global L/D below maximum .

If the aircraft is flown with a g lobal L/D below maximum, the aerodynamic properties can be adjusted to increase L/D for the wing sections on one side of the center chord, while maintaining L/D for the wing sections on the other side.

As previously mentioned, creating a d ifference in L/D between wing sections on different sides can be done by increasing or reducing lift and/or drag . In the following, a number of difference embodiments adjusting d ifferent aerodynamic properties to create the difference in L/D will be described in relation to Fig ures 10- 14.

In order to optimize excessive lift and/or TWS, the adjustment can be carried out to increase or decrease lift, while the d rag is kept at a minimum to increase the overall efficiency. In a preferred embodiment, the difference in L/D is created using control surfaces, preferably horizontal wing control surfaces such as slats or flaps that can increase the lift. In Figure 10A, a horizontal wing control surface such as a slat or flap is adjusted using mechanics and/or electronic internal in the aircraft. Figure 10B illustrates a specific exemplary solution where the control surface is actuated using a servo placed in the aircraft wing to push/pull on a horn attached to the control surface.

Control surfaces could be actuated using individual bridles attached to movable, such as hinged or flexible, control surfaces, by adjusting the length of such bridle. The bridle can be kept in tension during such actuation - the control surface can be designed move upwards if the bridle length in increased, thereby keeping the bridle tight instead of the bridle becoming slack. Alternatively, if there are fixed- length bridles on either side and the bridle used to actuate are not also used to support the structural integrity, such bridle may be allowed to slack. In Figure 11, a control surface is actuated using motors or winches hanging underneath the aircraft. This can actuated a hinged surface or can change the angle-of-attack of the whole wing section. The aerodynamic properties can also be adjusted by displacing at least one bridle attachment point in the chord direction to pitch a specific wing section forward or aft. This is particularly relevant for rigid wings where whole wings or wing-sections can be individually rotated around an axis in the span directions and for non-rigid wings.

A difference in drag on different sides of the center chord may be created using adjustable vertical surfaces on different sides of the center chord. For example, wingtips or winglets may have control surfaces or being all moving wings to adjust the drag separately. In another example, a canard wing is mounted separately or in combination with a tail, where the canard and tail is coupled to yaw the aircraft. In Figure 12, vertical surfaces attached any place along the wingspan are rotated to create additional drag, for example using a servo inside the wing. By rotating them differently on different sides of the center chord, the aircraft will be forced to turn around the side creating the most drag. Since vertical surfaces near the end of the wings (winglets) will not only induce drag, but may also directly influence the lift, a good understanding of the involved aerodynamics is

prerequisite.

In yet another embodiment, the aerodynamic properties are adjusted using rotary wings such as rotors, propellers, turbines, fans, etc. to create a d ifference in drag on different sides of the center chord . This may in particular be relevant for energy prod uction with airborne generators, where simply increasing the load on a generator in one side only increases the d rag for that side, while harvesting part of the lost speed as energy in the generator. Such rotary wings can be mounted on pylon or in slots of the aircraft.

The rotary wings such as rotors, propellers, turbines, fans may be used to take in air from one section of a wing and send it out on another section of the wing to change the boundary layer or air flow around the wing . This is also referred to as NACA Ducts, Blow Holes, Boundary Layer Suction (BLS) or active turbulators.

Figure 13 illustrates an aircraft with propellers in "turbine-mode", where the propellers drive airborne generators. Inducing a higher load on the generator on only one side will force this one propeller to rotate slower, thereby increasing the drag . The aircraft will be forced to yaw around the side where the air is slowed down the most due to increased drag .

Figure 14 illustrates an aircraft with propellers in "motor-mode", where the propellers are driven by an airborne motor to create thrust. Adjusting the RPM differently on the motors will create a difference in thrust between the propellers, forcing the aircraft to yaw around the side where the propeller creates the least thrust.

In a preferred application the invention is used for energy prod uction where the excessive lift of the tethered aircraft is used to turn a generator, airborne or on the ground . In this application, it is the excessive lift that should be maximized to optimize energy production .

It is the wings that produce the lift, and the more of that lift that is parallel to the direction of the tether between the generator on g round and the aircraft, the more of that lift can be converted into energy. The problem with banking aircrafts is that some of that lift that was parallel to the tether direction when the aircraft flew straight is used to bank the aircraft and create side forces to turn the aircraft. Our method allows the aircrafts wingspan to be perpendicular to the tether direction, and thus most of the lift becomes parallel to the tether and can be transmitted to as a tractive force via the tether to the generator.

The above considerations are also applicable to other applications utilizing the excessive lift such as sports, lifting and pulling of things.