Load Convergence System for Tethered Aerofoil Wind Propulsion

Field of the Invention

The use of wind power in propulsion of watercraft has been subject to some innovation over recent years. In particular, tethered aerofoils, sometimes known generically as kites, have taken the place of conventional sails in a number of designs. This specification relates to technology intended to enable the effective use of aerofoils and other force sources, including hydrofoils, in propulsion systems for small to medium-sized craft, without adversely affecting the stable operation of the craft.

Background to the Invention

Various forces may be exerted upon a waterborne or airborne craft, by aerofoils, by hydrofoils or by other sources, for example towing forces from another vessel taken in tow. If a craft propelled by tethered aerofoils is to be useful at an industrial level, for example as a passenger-carrying ferry or a tug for cargo vessels, these forces are likely to be large relative to the inertia of the craft.

Large forces acting upon a craft in different directions are likely to give rise to significant roll and pitch moments, which may, in extremis, capsize the craft, but more likely will disturb its attitude significantly away from the horizontal, posing a problem for crew operating the vessel, systems which are designed to remain level, passengers whose comfort and safety will be compromised, and the security of cargo, among many other things.

In some cases, yaw moments may also be undesirable. For example, it may be desirable to align the craft with the direction of water or air flow for the sake of reducing drag, which could be achieved by means of a tail fin, if the craft is free to rotate about the yaw axis, relative to the system. In another example, if a tow line is simply attached at the stern of the craft in the conventional manner, it is best that the other forces, such as aerofoil and hydrofoil forces, do not exert a yaw moment because it will attempt to misalign the craft with the tow line, giving rise to unnecessary stresses.

Torque may also act upon the craft directly, as in the case of mechanically attached hydrofoils or other devices which aren’t inherently free to rotate. Summary of the Invention

In this specification the following definitions shall be taken to apply:

‘Craft* shall be taken to mean something intended for motion on water or in the air.

‘Tethered aerofoil’ shall be taken to mean an aerofoil whose physical restraint relies upon tethers, of which there may be one or more.

‘Tether* shall be taken to mean any flexible tension-bearing element and ‘tethered’ shall be interpreted accordingly.

‘Structural body’ shall be taken to mean a constructed object, with form, mass and stiffness, which may form part of a craft as defined previously and may be the major part.

- ‘Incident forces* shall be taken to mean forces acting upon the system described herein which arise from a tethered aerofoil or aerofoils and from one or more other sources, and which are intended to be transmitted to the structural body.

‘Imposed torque’ shall be taken to mean the torque exerted on the structural body by the incident forces.

This specification defines a system which receives and manages incident force vectors arising from tethered aerofoil forces and from other sources so as to minimise the torque exerted on the structural body of the craft by these forces to substantially zero in the roll, pitch and, in some embodiments, yaw axes, while transmitting the forces to the structural body in three dimensions.

This imposed-torque-minimising system allows a high-powered propulsion system based upon tethered aerofoils to propel a relatively small craft without forcibly tilting it to port, to starboard, forward or aft, which is an important prerequisite for safe, controllable operation of such craft in most cases.

The system is subjected to forces from at least one tethered aerofoil and one or more other sources, the resultant of which it transmits to the structural body of the craft.

The incident forces may vary directionally in two or three dimensions, within a range which in most cases is likely to be limited by design or environmental factors; as such the system can be considered to be effective if it can achieve the transmission offerees and minimisation of imposed torque while the incident force vectors are within the anticipated range. The incident forces are likely to be pulling forces but in some embodiments may be pushing. The operation of the system when minimising imposed torque in all axes essentially amounts to alignment of the resultant of the incident forces such that it can be represented entirely as a vector force acting at a common point which is fixed in its position, or limited within a small range of motion, relative to the craft. Often it would be preferable that this point be directly above the centre of mass of the craft in its designed orientation, but other configurations exist, for example where this resultant force vector acts in conjunction with other forces from outside the system, such as buoyancy or lift, to level the craft.

When minimising imposed torque only in the pitch and roll axes, the operation of the system essentially amounts to alignment of the resultant of the incident forces such that it can be represented entirely as the combination of a vector force acting at the aforementioned common point and a torque about that point in the yaw plane. It is possible to represent it in other ways, for example as a vector force not aligned with the common point combined with compensating torques in the pitch and/or roll axis, which may be closer to reality from the perspective of the system, but for clarity the former is the simpler representation because it more accurately represents the forces and torques felt by the structural body.

The system is intended to operate passively, in respect of the minimisation of imposed torque, by allowing components to move in certain defined ways or along certain defined paths in response to changes in the direction of incident force vectors; the forces required to move these components would arise substantially from the incident forces themselves rather than from independent actuation, ‘independent actuation* meaning actuation using energy which is not derived from the incident forces.

It is possible for alternative embodiments of the system to be designed to operate actively, relying upon independent actuation to position components such that the incident forces are aligned through the common point. This may help to reduce the size of the system, amongst other benefits, but relies upon independent sources of energy to operate safely, hence these are not preferred embodiments.

The system could consist of an independent module that connects to the structural body of the craft, directly or via other components, as a unified whole assembly and transmits to it a resultant force and, if applicable, yaw torque. The resultant force and yaw torque could be transmitted to the structural body by the assembly in various ways. Alternatively the system could consist of a set of subsystems which connect to the structural body of the craft separately, directly or via other components, and thus transmit multiple forces and torques into the structural body. Both forms are capable of minimising imposed torque in pitch, roll, and if desired, yaw.

The structural body of the craft is propelled by the system but able to rotate relative to it in pitch and roll, as a minimum. If the resultant force includes a significant vertical component, the craft may be partially or wholly suspended from the system. Due to the pitch and roll freedom it will remain substantially level if the other forces upon it, for example weight, drag, lift, buoyancy or inertial forces from other moving systems, are designed and/or managed in order to keep it so. Installing the system above the centre of mass and forward of the centre of drag is one likely design strategy.

In this state of suspension, however, there may be a need for damping of pendulum motion, especially in the airborne case where the corrective moments arising from a disturbance to the craft’s attitude may be weak. Generally there may be a need for damping of any kind of oscillation. This can be achieved by velocity-damping and/or acceleration-damping of the system components, relative to the structural body of the craft; torque will arise from this but only when the velocity and/or acceleration of the components, relative to the structural body, is non-zero. The relevant velocities and accelerations may be angular or linear.

Damping torque such as this is considered an allowable aspect of the operation of the system; it is envisaged that, in most embodiments, significant changes of direction of incident force vectors relative to the structural body would be infrequent and relatively slow, thus damping torque would not cause significant disturbance to its attitude. It is envisaged that such damping would be primarily by direct mechanical means but it is possible to enact it artificially through control and feedback systems using independent actuation, by measuring the relative motion of components and responding accordingly to minimise oscillation, for example by actuating a brake or a powering a motor.

Some embodiments of the system may use small external inputs or feedback cycles to actively assist the motion of the system or its components. The motive impulse would still be derived substantially from the incident forces, but additional control inputs may be used, for example to influence or initiate a motion in the case where the system could move from one state to another in more than one way, in order to choose the more preferable, or to overcome an undesirable resistance that occurs in operation through environmental factors e.g. salt, ice, water or debris, or through system degradation e.g. corrosion, creep, wear etc..

In an extreme case, the motion of some or all components could be restricted such that independent actuation is required to position them to achieve the minimisation of imposed torque. This is not the intention of the invention but is a plausible embodiment where active control is for some reason desired.

It may also be desirable in some cases to be able to lock or restrict certain motions of the system so as to effect a deliberate torque on the craft in an axis in which it would usually be free to rotate. The system described may also incorporate the ability to pay out and/or haul in tethers, in order to ease out or retract aerofoils, hydrofoils, towed craft or other tethered objects. Incident force sources need not necessarily be tethered; they could, for example, be mechanically attached, possibly with some freedom of motion, or be an integral component of the system. Incident force sources could also include weights being carried by the system or fixed points to which it is restrained by some means, for example a mooring tether.

This system need not necessarily be used as part of a water- or air- craft; it may be of use in other applications, for example land based systems which use wind power to gain altitude and move around but are restrained to the ground, or land-based vehicles with wheels, skis, air cushions or other such moving ground supports. References to the ‘craft’ herein can be taken to mean whatever craft, vehicle or other system may be propelled by this system.

The invention will now be described solely by way of example and with reference to the accompanying drawings, which can be captioned as follows:

Brief Description of the Drawings

Fig. 1 shows the preferred embodiment, which minimises imposed torque in all axes using a turntable and arch arrangement and incorporates haul-in/pay-out winches, motion damping and a motion lock.

Fig. 2 shows an alternative embodiment which minimises imposed torque only in the pitch and roll axes, transmitting it in the yaw axis, by means of two different types of bearings acting on a ball, and shows the common point through which the resultant force acts and the yaw plane in which the resultant torque acts around that point.

Fig. 3a illustrates a non-preferred embodiment which minimises imposed torque in all axes using a longitudinal drum and arch arrangement with a mechanically attached hydrofoil.

Fig. 3b shows a version of the embodiment from Fig. 3a but using a longitudinal shaft and arches, with a tethered hydrofoil arrangement.

Fig. 4a is a simplified two dimensional representation of how the system can be used to encourage the craft to remain level in the airborne case;

Fig. 4b is similar to Fig. 4a but shows how other forces acting on the craft such as buoyancy and fluid dynamic forces can act in conjunction with the system to keep the craft approximately level. Detailed Description

In Fig. 1 , the preferred embodiment can be seen; it comprises a system of two halves designed to operate with a tethered aerofoil or set of aerofoils above and a tethered hydrofoil below. The two subsystems each comprise tether connections by means of winches 20 to travellers 21 moving along arches 30. These arches are each affixed to one of two turntables 31 which are engaged with the structural body 10 by means of tapered roller bearings acting on the four chamfer faces 32, allowing them to tum only in the yaw axis. The aerofoil tethers 22 are connected to the upper subsystem, the hydrofoil tether 23 to the lower.

As the direction of an incident force vector, say from the hydrofoil, changes, the corresponding turntable will rotate in the yaw axis to keep the arch aligned in plane with the vector. At the same time the winch, on its traveller, will move along the arch until the incident force vector is perpendicular to the curve of the arch at that point, notwithstanding the effect of the weight of the winch and traveller assemblies which is expected to be relatively minor. The shape of the arches forms a sector of a circle, thus the traveller moves about a fixed axis relative to the arch; where this axis crosses the turntable axis is an exact point 40 with which the force vector is consequently aligned. The design of the overall system is such that the two subsystems focus upon the same point; therefore the incident forces can exert no torque upon the structural body in any axis because they remain aligned with the common point 40 regardless of their orientation, as long as they act within the anticipated range of the design. The resultant force vector exerted upon the structural body therefore acts through the point 40.

In this depiction two arches are shown on the upper subsystem; this corresponds to the use of a double-tether arrangement for the aerofoil array, one connected to the right of the aerofoils and the other to the left. It is assumed that in normal operation the tension in each of the two tethers will be similar and therefore the incident force vector can be modelled as a single vector acting in the mid-plane, which is aligned effectively as described above.

In the top right is a view on the transom of the craft, showing an optional feature designed for towing. A towing winch 35 is shown here travelling upon an arch 36 which is fixed to the transom and orientated in a plane which passes through the point 40 and is parallel to the yaw and roll axes, i.e. it is usually vertical. The axis of the curve of the arch 36 also passes through the point 40. Therefore although the arch is fixed and consequently the towing forces can exert a yaw torque upon the structural body when the tow tether 37 becomes misaligned with the plane of the arch, it cannot exert a pitch torque. Roll torque within the designed range of tow vector direction will be small and will in fact be minimised indirectly because the craft will continually yaw into alignment with the tow line, thus aligning the vector parallel to the roll axis where it is unable to exert a roll torque.This arrangement will therefore tend to align the craft so that the towed object is astern, without otherwise disturbing its attitude. This feature is not considered part of the system as defined, because it cannot directly minimise roll torque, but may be useful nonetheless.

Should it be desired to tow without torque transmission, the arch 36 could itself be mounted on an arch-like mechanism that is aligned with the point 40 and is parallel to the pitch and roll axes, i.e. it is usually horizontal, with its axis also intersecting the point 40, creating as such a sub-system that maintains the towing force aligned with the point 40 regardless of the orientation of the craft, within a limited range, thus transmitting no torque. Another option would be to use a third turntable as part of the unified system, concentric with and external to the hydrofoil turntable, with a partial arch upon which the towing winch can travel. In both of these cases the towing feature does become an effective subsystem of the whole imposed-torque-minimisation system.

The three damping units 24 resist motion in the arch axes, i.e. roll/pitch, in proportion to the velocity and acceleration of motion of the travellers along the arches. The two damping units 33 resist motion in the turntable axes, i.e. yaw, in proportion to the velocity and acceleration of motion of the turntables in their bearing housings. These generate no torque, therefore, when the system and craft are in an equilibrium state, progressing directly under steady conditions, but do serve to act against oscillatory motion that may arise from a dynamic instability or as a consequence of a change in direction or conditions.

An example of the design of one of these damping units is as follows: the damping unit may be a module comprising a gearbox, a viscous damper and an acceleration-actuated brake;

- the input shaft for the gearbox would be driven by the motion of the system components, i.e. the winch traveller relative to its arch or the turntable relative to its bearing housing, and the gearbox would multiply this motion to a higher rotational speed;

- the output shaft from the gearbox would drive a rotary viscous damper, generating resistive torque as a function of the rotational velocity, and another part of the output shaft would be threaded; upon this thread would be mounted a high-inertia rotor, which would, by virtue of its polar inertia and the helical nature of the thread, progress axially along the output shaft when the shaft is accelerating or decelerating; this axial motion in one or both directions would actuate a brake in order to exert a resistance to the system motion by means of coulomb friction; a torsion spring would return the rotor to the neutral position when the acceleration returns to zero and would ensure that the normal force exerted by the rotor on the brake, and thus the braking torque, is proportional to the acceleration of the system.

The settings of the damping systems may be adjustable, possibly remotely from the bridge, or may adjust automatically to match the desired settings for the current conditions; for example, the damping rates may need to be different when the arches are aligned fore and aft than when they are aligned athwartships.

A motion lock 34, shown here in the form of a locking pad on a ram, allows the yaw rotation of the lower turntable to be coupled to the yaw of the structural body 10. This may also result in pitch and or roll torque but if the hydrofoil tether is close to horizontal it will primarily transmit torque in the body’s yaw axis, which could be useful, for example, in order to maintain a perpendicular relationship between the craft and the hydrofoil tether, to make steering less complex and ensure the craft is facing in the direction of motion. In other embodiments the lock could be on the upper turntable, such that the craft can be kept aligned with the oncoming airflow as felt by the aerofoils, for example.

Fig. 2 shows a single-assembly system that could otherwise be used to achieve three dimensional imposed-torque-minimisation but in this case has features which prevent free rotation in the yaw axis and thus transmit yaw torque. It comprises a ball joint in which the ball 50 is supported by bearings 51 above and below the yaw plane 53 which would allow it to rotate freely in all axes and transmit force in three dimensions, but is also supported by rollers 52 evenly spaced around its circumference in the yaw plane, their axes of rotation aligned parallel to the surface of the ball at the contact point and parallel to the yaw plane; these allow pitch and roll but prevent rotation of the ball in the yaw axis.

The central common point 40, through which the resultant force vector acts, is shown again here, as is the aforementioned yaw plane 53 in which the resultant torque acts about that point.

A single aerofoil tether 22 is shown here, although more than one may still be used with this system. The hydrofoil tether 23 is also shown again. Towing tethers may also be added directly to this system. Incident force sources may be connected to the ball by tethers with simple connections or winches, by mechanical or structural means or by any other engineered method, but it must be taken into account that their positioning will limit the useful range of the system. A craft using this system, or another system which transmits yaw torque, may have the ability to trim the yaw angle, i.e. to actively change the yaw orientation of the craft relative to the system. The system in this case could be connected to a structural body 10 which is not the main part 11 of the craft but is a subassembly, as shown, the yaw orientation of which, relative to the rest of the craft, can be actuated, for example by a circular rack and motorised pinion, or in a limited range by a hydraulic ram 12.Tapered roller bearings could be used to secure the structural body 10 to the craft body 11 while allowing yaw actuation.

In Figs. 3a and 3b, some non-preferred embodiments of the system are shown. They may achieve the minimisation of imposed torque in all axes but are non-preferred because they are likely to be more limiting to the useable range of motion, and for safety and reliability reasons because it is considered that the motion of the turntable system of Fig.1 on the deck of a craft is less likely to cause harm to crew or damage to the craft than large arches which can rotate in the pitch and/or roll axes through a significant portion of the above-deck space.

These embodiments comprise arches 60 on a long drum 61 , as in Fig. 3a, or on a shaft 62, as in Fig. 3b. This system can be orientated such that the shaft/drum axis is aligned parallel with the roll axis, the pitch axis, or indeed any axis in the same plane as those two axes. To ensure independence of the force sources so that no yaw torque can be imparted, the drum may in fact be divided into upper and lower half-drums, one smaller than the other so that it can rotate within it to an extent.

In this figure the drum 61 is shown not with a tethered hydrofoil arrangement but with a hydrofoil mechanically attached directly to the drum. A U-shaped hydrofoil 63 is shown but this could be T-shaped or L-shaped, amongst other options. Its attachment to the system may be another small arch or a similar mechanism, to preserve the 3-axis minimisation of imposed torque. Another way in which it could achieve this is to connect via a hinge which acts in the same sense as an arch, to a small diameter lower half-drum such that the axes of the hinge and half-drum intersect. If yaw torque transmission is desired, the hydrofoil could be fixed structurally or mechanically to the half-drum in order to couple yaw to the hydrofoil attitude, although this could also transmit pitch torque if the hydrofoil were orientated mainly below the craft rather than outboard.

Mechanically attached incident force sources such as this can exert pushing forces as well as pulling forces. Generally this would give rise to an unstable system but there would still be no imposed torque as long as the pushing forces are aligned through the common point If the hydrofoil were precisely controlled, this could be a workable system for providing lateral resistance to motion, deploying it to leeward of the craft rather than to windward and generating hydrodynamic lift towards the system rather than away from it. In Figs. 4a and 4b, diagrammatical illustrations show the system propelling a craft which maintains an approximately level attitude as it progresses. In Fig. 4a the levelling of the craft is achieved by the use of ballast and fluid dynamic design to position the centre of mass 70 and the effective centre of fluid dynamic drag 71 of the craft in desired positions relative to the system, such that weight, drag and the resultant of the incident forces are balanced so as to exert no moments upon the craft. Dashed arrows show the components of the resultant force perpendicular to the yaw plane and the roll plane. The fluid dynamics in this case exert no lift on the craft.

In Fig. 4b the system is offset forward from the centre of mass 70 such that the craft would pitch bow-up due to the corresponding pitch moment, but for the compensating buoyancy and/or lift generated at the stern which balances that moment and maintains the craft in a level attitude.