A vertical tethered aircraft system, method of operating the same for circular launching, transition to flight and recovery of the aircraft and use thereof

Abstract

The present invention relates to a vertical tethered aircraft system for circular launching, transition to flight and recovery of tethered aircraft which uses the vertical plane to initiate launching and to terminate recovery. The vertical tethered aircraft system comprises a tower (04), a fixed-wing aircraft (01) connected via a tether (02) to an anchorage point (03), wherein the anchorage point (03) is positioned at the top of a tower (04). The invention also refers to a method of operating the vertical tethered aircraft system and use of the same. The invention can be applied to airborne wind energy systems for the generation of electrical energy. It can also be used in other applications of tethered aircraft such as agricultural spraying, or aerial surveillance and monitoring.

Description

[0001] DESCRIPTION

[0002] "A VERTICAL TETHERED AIRCRAFT SYSTEM, METHOD OF OPERATING THE SAME FOR CIRCULAR LAUNCHING, TRANSITION TO FLIGHT AND RECOVERY OF THE AIRCRAFT AND USE THEREOF"

[0003] TECHNICAL FIELD

[0004] The present invention relates to a vertical tethered aircraft system and a method of operating the same for circular launching, transition to flight and recovery of the aircraft , operating on a vertical plane . This invention applies on Airborne Wind Energy Systems as well as agricultural spraying, or aerial surveillance and monitoring .

[0005] BACKGROUND

[0006] Airborne Wind Energy Systems (AWES ) are devices that convert wind energy into electricity using autonomous flying wings attached to the ground by a tether . These devices can harvest wind energy at high altitudes , where the wind is stronger and more consistent , being able to generate electricity from a yet unexplored renewable energy resource . AWES use a much lighter infrastructure than the conventional wind turbines , since the tower and foundations are not needed and are replaced by a tether . ( See e . g . [ Chris Vermillion, Mitchell Cobb, Lorenzo Fagiano , Rachel Leuthold, Moritz Diehl , Roy S . Smith, Tony A. Wood, Sebastian Rapp, Roland Schmehl , David Olinger, and Michael Demetriou . "Electricity in the Air : Insights from Two Decades o f Advanced Control Research and Experimental Flight Testing of Airborne Wind Energy Systems" , Annual Reviews in Control , 2021 ; Schmehl , Roland . "Airborne Wind Energy : Advances in Technology Development and Research" , Springer Singapore, 2018] for an overview of the concepts, [KiteGen Research, "High Altitude Wind Energy IP right ownership world overview", 2014; Mendonpa, Anny Key de Souza, et al. "Comparing patent and scientific literature in airborne wind energy." Sustainability 9.6 (2017) : 915] for IP surveys, or [EC-DG for R&I, "Study on Challenges in the commercialization of airborne wind energy systems", European Commission Directorate-General for Research and Innovation, September 2018. ISBN 978-92-79-80282-9, DOI 10.2777 / 87591; United

[0007] States Department of Energy, "Challenges and Opportunities for Airborne Wind Energy in the United States". Report to the Congress, November 2021] for reports on challenges and opportunities) .

[0008] One of the main challenges for AWES to become a viable and competitive technology, as is recognized by the research community (e.g., in the EC report [EC-DG for R&I, "Study on Challenges in the commercialization of airborne wind energy systems", European Commission Directorate-General for Research and Innovation, September 2018. ISBN 978-92-79-80282-9, DOI 10.2777 / 87591] , in the report to the US Congress [United States Department of Energy, "Challenges and Opportunities for Airborne Wind Energy in the United States". Report to the Congress, November 2021] and in the survey [Chris Vermillion, Mitchell Cobb, Lorenzo Fagiano, Rachel Leuthold, Moritz Diehl, Roy S. Smith, Tony A. Wood, Sebastian Rapp, Roland Schmehl, David Olinger, and Michael Demetriou. "Electricity in the Air: Insights from Two Decades of Advanced Control Research and Experimental Flight Testing of Airborne Wind Energy Systems", Annual Reviews in Control, 2021] ) , is the ability to operate safely, reliably, and autonomously for long periods of time in several weather and environmental conditions. To achieve such a goal, it is crucial to develop an Automatic Take-Off and Landing (ATOL) scheme for the Tethered Aircraft (also denoted by kites) . Fixed-wing aircrafts are used in several applications in which it is relevant to have a mechanism to take-off and landing (TOL) in short areas or automatically, using techniques denoted by STOL (Short Take-Off and Landing) or ATOL, respectively. Among the several STOL or ATOL concepts, three types are being investigated and developed in the context Airborne Wind Energy Systems AWES: (i) vertical TOL (VTOL) with multiple rotors, (ii) Catapult Assisted Take-Off with Barrier Arrested Recovery (CATOBAR) , and (iii) circular TOL (CTOL) .

[0009] The VTOL technique is completely different from the one proposed by the present invention, since it requires multiple rotors capable of lifting the aircraft's own weight. The CATOBAR, and other linear TOL techniques rely on the aerodynamic lift force of the wings, which is approximately vertical when the aircraft attains a certain horizontal speed. Therefore, in these techniques, the aircraft must move on a horizontal or near horizontal plane. An exception is the Catapult assisted take-off, which in some situations uses a slight upward slope (for example, in aircraft carriers) . The solutions of the state of the art for circular takeoff and landing (CTOL) are all on a horizontal plane. A CTOL technique on a vertical, or even on a non-horizontal plane, has not been disclosed.

[0010] The use of an AWES with a tower, taking advantage of the existing infrastructure of the tower and of the electrical grid connection, is described in [Bauer F., Hackl C.M., Smedley K. , Kennel R.M. (2018) Crosswind Kite Power with Tower. In: Schmehl R. (eds) Airborne Wind Energy. Green Energy and Technology. Springer, Singapore] .

[0011] There are several recent patents (US20200132048A1,

[0012] US 11002252B2 , US 10914289B2 , US20200378356A1 , US20200116130A1 ) involving AWES and its combination with traditional wind towers, either active or dismantled.

[0013] Comparing with alternative TOL schemes for tethered aircrafts, the present invention has the following advantages:

[0014] • Removes the need to use a landing gear, or any other landing apparatus aboard the aircraft, such as a rotating arm;

[0015] • Consumes less energy and requires less power, when comparing with the vertical take-off and landing (VTOL) technique, which requires a constant and large thrust capable of supporting the aircraft weight when performing any TOL procedures;

[0016] • Allows the take-off and landing on a vertical plane, instead of on a horizontal plane, occupying a much reduced land area. This is especially relevant when the land area is limited, not allowing the horizontal circular TOL or horizontal linear TOL;

[0017] • The fact that we use a pendulum movement in order to accelerate, implies that the thrust needed to accelerate and increase the swinging amplitude only needs to be larger than the dissipative forces, such as friction and drag; this allows us to use less power in the aircraft, not only less powerful motors, but also lower capacity batteries, reducing weight.

[0018] AWES mounted on a tower have some advantages (as is discussed in [Bauer F., Hackl C.M., Smedley K. , Kennel R.M. (2018) Crosswind Kite Power with Tower. In: Schmehl R. (eds) Airborne Wind Energy. Green Energy and Technology. Springer, Singapore] ) , chief among which is the cosine loss reduction with direct implication in efficiency. The maximum power that can be attained by an AWES has a term that involves the cube of the cosine of the angle between the tether and the wind vector, which, predominately, is approximately horizontal. When the anchorage point is on the ground, this angle is higher, reducing the maximum power. Regarding the application on wind turbines, the following advantages are identified:

[0019] • Can be a complement to traditional wind turbines, since it can be placed on wind towers;

[0020] • Can use the same electrical connection to the grid;

[0021] • Can produce electricity in weaker winds, when the wind turbine is stopped;

[0022] • When the wind turbine reaches the end of the life cycle (the blades get destroyed and need replacement) , the towers can be equipped with these systems.

[0023] Therefore, there is the need of a system and a method for circular launching and recovery of tethered aircraft operating on a vertical plane, that will allow to overcome the disadvantages above mentioned for alternative takeoff and landing methods, enabling the application to Airborne Wind Energy Systems and also to other tethered aircraft applications.

[0024] BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The invention will now be described in further detail with reference to the accompanying drawings in which:

[0026] Fig. 1: Illustrates the vertical tethered aircraft system in resting position (front view of the vertical plane) .

[0027] Fig. 2: Illustrates the vertical tethered aircraft system using a conventional wind turbine tower to obtain the elevated anchorage point (side view of the vertical plane) .

[0028] Fig. 3: Illustrates the pendulum movement of the vertical tethered aircraft system on the vertical plane. Fig. 4: Illustrates the sliding mechanism which can move the anchorage point along the tower.

[0029] Fig. 5: Illustrates the rigid rod fixed to the anchorage point guiding the tether.

[0030] Fig. 6: Represents the normalized power as a function of the mass per area of the kite, (taken from [Luchsinger R.H. (2013) Pumping Cycle Kite Power. In: Ahrens U., Diehl M., Schmehl R. (eds) Airborne Wind Energy. Green Energy and Technology. Springer, Berlin, Heidelberg] )

[0031] SUMMARY OF THE INVENTION

[0032] The present invention relates to a vertical tethered aircraft system for circular launching, transition to flight and recovery of tethered aircraft on a vertical plane comprising a tower (04) and a fixed-wing aircraft (01) connected via a tether (02) to an anchorage point (03) , wherein the anchorage point (03) is positioned at the top of the tower (04) .

[0033] In one aspect, the tower (04) is a conventional wind turbine tower .

[0034] In another aspect, the tower (04) is on a moving platform or vehicle .

[0035] In another aspect, the tower (04) is replaced by an elevated place or structure where the anchorage point (03) is located. In another aspect, the vertical tethered aircraft system further comprises a sliding mechanism (05) which can move the anchorage point (03) along the tower (04) .

[0036] In another aspect, the vertical tethered aircraft system further comprises a rigid rod (06) fixed to the anchorage point (03) to be used to guide the tether (02) .

[0037] In another aspect, the vertical tethered aircraft system further comprises a tethered aircraft twisting mechanism and an anchorage point swivel mechanism.

[0038] The invention also refers to a method of operating the vertical tethered aircraft system comprising the following steps for launching and flying:

[0039] - inducing a swing movement on a vertical plane with resonant thrust, between -90 and 90 degrees with respect to a resting position;

[0040] - accelerating the aircraft (01) in a circular motion within the said vertical plane;

[0041] - increasing an altitude of the tethered aircraft (01) by controlling aircraft with its aerodynamic surface and also reel-out the tether (02) , until a tethered flight altitude is reached; for recovery:

[0042] - decreasing altitude of the aircraft (01) , reel-in the tether (02) and also enter the vertical plane;

[0043] - reaching a swing movement of the aircraft (01) , with damping thrust, reducing the velocity until a resting position is reached .

[0044] The invention further relates to the use of the vertical tethered aircraft system on an airborne wind energy system for the generation of electrical energy.

[0045] In a further aspect, the use of the vertical tethered aircraft system is for agricultural spraying, or aerial surveillance and monitoring .

[0046] DETAILED DESCRIPTION OF THE INVENTION

[0047] The present disclosure describes a system and a method to enable automatic circular launching, transition to flight, and recovery of tethered aircrafts, working on a vertical plane. The anchorage point (03) of the tether (02) is at an elevated position, for example the top of a wind turbine tower.

[0048] In the vertical tethered aircraft system of the present disclosure, a fixed-wing aircraft (01) is connected through a tether (02) to an anchorage point (03) , positioned at the top of a tower (04) , e.g. a wind turbine tower (see Figure 1) . The anchorage point (03) allows the movement of the tether ( 02 ) / aircraft (01) in a pendular or circular motion, in a vertical plane. This is done without intersecting the structure itself; in other words, the tower structure does not intersect the vertical plane where the aircraft and tether move, except at the attachment point (see Figure 2) .

[0049] The main purpose of the invention is for the aircraft to be able to move in a circular or pendular controlled motion within a vertical plane, in order to accelerate for launching, or decelerate for retrieving.

[0050] In the context of the present inventions, the term "vertical" should be understood as acting in a vertical plane and the expression "vertical plane" refers to a plane that is perpendicular to the ground plane.

[0051] In the context of the present description, the term "comprising" should be interpreted as "including, among others". As such, said term should not be interpreted as "consisting only of" .

[0052] The wings airfoil profile should be symmetric so that no lift is produced when moving in the vertical plane with the angle of attack selected to be zero.

[0053] Therefore, the system herein disclosed comprises the following components (see Figures 1 and 2) :

[0054] • Aircraft ( 01 ) ,

[0055] • Tether ( 02 ) ,

[0056] • Anchorage point (03) ,

[0057] • Tower ( 04 ) .

[0058] The system might also comprise the following (see Figures 4 and 5) :

[0059] • Sliding mechanism (05) ,

[0060] • Rigid rod (06) .

[0061] • Aircraft twisting mechanism;

[0062] • Anchorage point swivel mechanism.

[0063] The sliding mechanism (05) has the purpose to be able to move the attachment point towards the base of the tower (04) , so that in situations when the tension on the tether is very high (e.g. when in the power generating phase of an airborne wind energy system) the moment on the basis of the tower structure is reduced, while also enabling to place the attachment point at a high point during takeoff and landing maneuveres. The rigid rod ( 06 ) , which is attached to the anchorage point , guarantees that the tether maintains itsel f straight in the part closest to the anchorage point , and further guarantees that the aircraft is always at a minimum distance from the same anchorage point .

[0064] The main purpose of the aircraft twisting mechanism is the inversion of the aircraft orientation when, at the highest point of the pendular motion, the velocity direction inverts . This can be done by al lowing the propeller heading to pitch up . Alternatively, rotating an internal mass within the aircraft might also allow the desired 180 degrees turn . In any case, the twisting rotation should be in the direction that combines with the gyroscopic ef fect due to the propeller axis upward motion .

[0065] The swivel mechanism in the anchorage point has the purpose of orienting the motion in the vertical plane in such a way that the said vertical plane is orthogonal to the hori zontal component of the predominant wind direction .

[0066] The present invention also provides a method of operating the system for circular launching, transition to flight and recovery of tethered aircrafts operating on a vertical plane and can be divided into 7 steps :

[0067] Step 0 : Resting position - The aircraft ( 01 ) is stopped, hanging from the vertical tether ( 02 ) ;

[0068] • Step 1 : Accelerating pendulum motion - swinging aircraft ( 01 ) with resonant thrust , between - 90 and 90 degrees with respect to the resting position;

[0069] • Step 2 : Accelerating circular motion - Keep accelerating the aircraft ( 01 ) , moving in a full 360 degrees circle , within the vertical plane ; • Step 3: Ascent - The motion circle starts tilting away from vertical. Increasing at the same time the tether (02) length, obtaining a helical motion;

[0070] • Step 4: Flight - Usual tethered flight, on the surface of the sphere centered at the attachment point with radius equal to the tether length, possibly as an AWES generating electricity;

[0071] • Step 5: Descent - Decrease altitude, decrease tether (02) length, enter vertical plane;

[0072] • Step 6: Decelerating circular motion - Aircraft

[0073] (01) in the vertical plane, moving in circles, decelerating;

[0074] • Step 7 : Decelerating pendulum motion - Swinging the aircraft (01) with damping thrust.

[0075] In a full operating cycle, the aircraft (01) starts, in step 0, at a resting position and stopped, hanging from the vertical tether (02) (assuming a no wind scenario) . Then, when the system initiates, in step 1, a pendulum swinging movement on a vertical plane is induced from the onboard thrusters of the aircraft (01) , with thrust in the same direction of the velocity to gain altitude and speed. In Figure 3 the pendulum movement is depicted.

[0076] Increasing the speed, step 2, will eventually lead to a complete 360 degrees circular motion, still in the vertical plane. Further increasing the speed allows a complete control of the aircraft (01) using its onboard surfaces, which can then transition to circular flight on a non-vertical plane.

[0077] The motion plane will progressively increase its angle with the vertical, defining the motion circle by intersection with the sphere of radius equal to the tether (02) length, centered at the anchorage point (03) . This is step 3, with altitude gaining with ailerons control, where the length of the tether (02) can be gradually increased.

[0078] In step 4, the aircraft (01) can then carry out a normal flight and its mission, possibly as an AWES generating electricity. In this step, the anchorage point can possibly move downwards along the tower so that during a crosswind flight the moment on the tower can be reduced. Before landing the anchorage point would return to the original elevated position.

[0079] In case there is wind, the vertical plane is defined where the take-off will take place to be orthogonal to the wind horizontal component, with the tower (04) structure being positioned upwind, which might require a swivel tower or swivel anchorage point. When the aircraft (01) is at rest hanging from the anchorage point (03) , the tether (02) might not have a vertical direction. Nevertheless, the aircraft (01) may continue to swing or rotate on a vertical plane, while the tether (02) describes a cone with vertex at the anchorage point (03) .

[0080] A converse procedure can be described for the recovery, "landing", of the aircraft (01) . In step 5, the aircraft (01) starts to decrease velocity and altitude, flying in circular motion, approaching the vertical plane. Meanwhile, the tether (02) is being reeled-in in the anchorage point (03) , synchronized with the movement of the aircraft (01) .

[0081] When the aircraft (01) reaches a safe zone (with the tether (02) length smaller than the anchorage point (03) height) , the plane of circular motion can transition to vertical (step 6) .

[0082] Finally, in step 7, the aircraft (01) enters a swing movement of a pendulum that, with damping thrust, reduces the velocity until reaching a rest position and stops.

[0083] The use of CTOL in the vertical plane as proposed by this invention brings two main technical advantages:

[0084] (i) a reduced mass on-board the aircraft when compared to other TOL schemes; and

[0085] (ii) an anchorage point high above the ground, decreasing the cosine losses.

[0086] Regarding the benefits of reducing the mass on-board, one can read from [Schmehl R., Noom M., van der Vlugt R. (2013) Traction Power Generation with Tethered Wings. In: Ahrens U., Diehl M., Schmehl R. (eds) Airborne Wind Energy. Green Energy and Technology. Springer, Berlin, Heidelberg] "The effect of gravity and inertia on continuous power generation can be significant..." . "...an increasing mass of the airborne components always decreases the available traction power of the wing." Moreover, [Luchsinger R.H. (2013) Pumping Cycle Kite Power. In: Ahrens U., Diehl M., Schmehl R. (eds) Airborne Wind Energy. Green Energy and Technology. Springer, Berlin, Heidelberg] identifies problems with the manoeuvrability of the kite with increased mass and defines lines that relate the maximum power with a mass per wing area ratio (see Figure 4) ; the maximum power decreases with a large mass per area ratio; besides a large mass per area ratio increases the minimum wind speed that maintains the kite airborne, with consequences in the capacity factor.

[0087] Regarding the benefits of having the anchorage point on the top of a tower, it has significant implications on the maximum power extracted. The estimated maximum power that can be extracted from an AWES is given (in [United States Department of Energy, "Challenges and Opportunities for Airborne Wind Energy in the United States". Report to the Congress, November 2021] ) by where 0 is the angle between the tether and the wind direction .

[0088] Since the predominant wind direction is essentially hori zontal , it can be easily seen that i f the anchorage point is on the ground, the tether must be at an angle with the wind for the kite to be airborne and at a safe distance from the ground . Placing the anchorage point higher, might reduce this angle . For example , a 20 degrees to zero angle reduction can increase the maximum power by more than 20% .

[0089] Although the main foreseen application of the system and method of the present invention is for generation o f electrical energy from a renewable energy source ( airborne wind energy) , this technology can also be applied in other areas where tethered aircrafts can be used such as agricultural spraying, aerial surveillance and monitoring . The description herein should be understood as exemplary and not limiting the scope of the present invention, which is defined in the appended claims .

Claims

CLAIMS1. A vertical tethered aircraft system for circular launching, transition to flight and recovery of tethered aircraft on a vertical plane characterized in that it comprises a tower (04) and a fixed-wing aircraft (01) connected via a tether (02) to an anchorage point (03) , wherein the anchorage point (03) is positioned at the top of the tower (04) .

2. The vertical tethered aircraft system, according to claim 1, characterized in that the tower (04) is a conventional wind turbine tower .

3. The vertical tethered aircraft system, according to claim 1, characterized in that the tower (04) is on a moving platform or vehicle .

4. The vertical tethered aircraft system, according to claim 1, characterized in that the tower (04) is replaced by an elevated place or structure where the anchorage point (03) is located.

5. The vertical tethered aircraft system, according to any of the previous claims, characterized in that it further comprises a sliding mechanism (05) which can move the anchorage point (03) along the tower (04) .

6. The vertical tethered aircraft system, according to any of the previous claims, characterized in that it further comprises a rigid rod (06) fixed to the anchorage point (03) guiding the tether (02) .

7. The vertical tethered aircraft system, according to any of the previous claims, characterized in that it further comprises a tethered aircraft twisting mechanism and an anchorage point swivel mechanism.

8. A method of operating the vertical tethered aircraft system of any of the preceding claims 1 to 7, characterized in that it comprises the following steps for launching and flying:- inducing a swing movement on a vertical plane with resonant thrust, between -90 and 90 degrees with respect to a resting position;- accelerating the aircraft (01) in a circular motion within the said vertical plane;- increasing an altitude of the tethered aircraft (01) by controlling aircraft with its aerodynamic surface and also reel-out the tether (02) , until a tethered flight altitude is reached; for recovery:- decreasing altitude of the aircraft (01) , reel-in the tether (02) and also enter the vertical plane;- reaching a swing movement of the aircraft (01) , with damping thrust, reducing the velocity until a resting position is reached .

9. Use of the vertical tethered aircraft system, of any of the preceding claims 1 to 7, on an airborne wind energy system for the generation of electrical energy.

10. Use of the vertical tethered aircraft system, of any of the preceding claims 1 to 7, on agricultural spraying, or aerial surveillance and monitoring.

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