WO2023118830A1

WO2023118830A1 - Tethered aerostat

Info

Publication number: WO2023118830A1
Application number: PCT/GB2022/053290
Authority: WO
Inventors: Peter Davidson; Christopher BURGOYNE
Original assignee: Tethercells Limited
Priority date: 2021-12-24
Filing date: 2022-12-19
Publication date: 2023-06-29

Abstract

A tethered aerostat (101, 201, 203, 205, 401, 601, 708) positioned at an elevated location in the atmosphere, the aerostat (101, 201, 203, 205, 401, 601, 708) having an elongate body (403) having a length extending between an upper end (207, 407) and a lower end (408) in use, the aerostat (101, 201, 203, 205, 401, 601, 708) comprising sufficient lighter-than-air gas such that the aerostat (101, 201, 203, 205, 401, 601, 708) is substantially buoyant in air, and defining a buoyancy force (108, 203, 431) acting through a centre of buoyancy (107, 108, 202), the aerostat (101, 201, 203, 205, 401, 601, 708) having a gravitational force (433) acting through a centre of mass (105, 204, 413), wherein the centre of mass (105, 204, 413) is closer to the lower end (408) than is the centre of buoyancy (107, 108, 202); the tethered aerostat (101, 201, 203, 205, 401, 601, 708) comprising a tether (113, 213, 215, 437, 607) connecting the aerostat (101, 201, 203, 205, 401, 601, 708) to a substantially ground level location, wherein the tether (113, 213, 215, 437, 607) defines a tether (113, 213, 215, 437, 607) line of action (114, 218), wherein the tether (113, 213, 215, 437, 607) line of action (114, 218) is a straight line that is coaxial to the tether (113, 213, 215, 437, 607) proximate the aerostat (101, 201, 203, 205, 401, 601, 708), and wherein, when the upper end (207, 407) is substantially vertically positioned above the lower end (408) in use: in the absence of any wind (104, 703), the moment about any point (109) on the tether (113, 213, 215, 437, 607) line of action (114, 218) produced by the gravitational force (433) and produced by the buoyancy force (108, 203, 431), are substantially equal and opposite to each other, thereby providing a stable vertical orientation in a no-wind condition in the atmosphere; and when wind (104, 703) is passing over the aerostat (101, 201, 203, 205, 401, 601, 708), the sum of the moments about any point (109) on the tether (113, 213, 215, 437, 607) line of action (114, 218) produced by the drag (209, 503) forces and any resulting additional forces induced by the wind (104, 703), also cancel out to provide a stable vertical orientation in a wind (104, 703) condition in the atmosphere.

Description

Tethered Aerostat

Technical Field

The invention relates to tethered aerostats operable in low and high winds, for providing a stable platform for a variety of uses, including delivering information services and providing astronomical information. Depending on the application, the aerostat is operated at a variety of altitudes from a few hundred metres to 20 km.

Background to the invention

Low latency access to information services is becoming vital for economic and social wellbeing. The Covid Pandemic has accelerated this process. New applications (for example, autonomous driving, remote medical support) and more generally the Metaverse, require reliable, low latency and high bandwidth mobile communications.

Compared with satellites, aerostat-supported platforms have several advantages, primarily because the distance from a transmitter to a receiver on Earth can be much less, with geostationary satellites typically at 36,000 km altitude and around 1000 km altitude for a “Low Earth Orbit” or LEO satellite.

This relative nearness of tethered aerostat platforms can result in much stronger signals relayed to Earth and avoid the expense of rocket launches as well as providing shorter development times, and allows power and backhaul connection via the tether.

Recently-developed, lightweight, very large-capacity phased array antennas have the potential to transform global mobile and fixed line connectivity e.g. by delivering cellular telephone services, including linking to the internet. This is dependent upon them being positioned appropriately, and at typically between 200m and 2500m over most geographies. Suitable tethered aerostats are therefore needed to support such antennas, that are reliable in all weathers at moderate elevations, with power and fibre optic cables as well as lighting and lightning protection being part of the tether. Such aerostats need to be situated below commercial aviation traffic, but sufficiently elevated to provide links of up to 80 km range for low population densities, up to 30 km for moderate rural densities and 5 km in urban areas.

Similarly, there are a range of earth observation, meteorological data collection, and astronomical data collection systems which could substantially benefit from being supported by a suitable tethered aerostat system that is reliable in all weathers at elevations at typically between 200m and 22000m.

A tethered aerostat has the potential to be far cheaper than satellite systems with similar functionality: power supply and fibre optic cable links can be supported by or be an integral part of the aerostat tether, avoiding the expensive backhaul systems and power systems required for satellites or aircraft. Furthermore data latency effects are important to many applications (including but not exclusively, augmented reality, autonomous driving, health care, interactive video games, video conferencing, remote control of UAVs etc.); there are significant problems with latencies offered by satellites, even low earth orbit satellites.

The need for improved mobile connectivity has prompted a resurgence of interest in using alternative delivery technologies rather than ever large numbers of mobile communication masts e.g. low earth orbit satellites, stratospheric platforms and tethered aerostats. All of these solutions have issues; respectively: data capacity and latency, technology readiness level, and wind stability.

Current tethered aerostat systems include tethered rigid and non-rigid airships (blimps) and hybrid balloon/kite systems. Airship-type designs are usually inclined to the horizontal to provide both aerodynamic lift as well as buoyancy. The principle of a hybrid balloon/kite system is that the balloon provides lift in low wind speed conditions and the kite provides lift in high wind speed conditions. Hybrid balloon/kite systems are usually less expensive than airships. However, for such balloon/kite systems in high wind speed conditions the horizontal drag on the balloon is high and the kite therefore needs to be large, incurring considerable drag forces to provide sufficient lift to maintain altitude. These large drag forces then require a very strong and hence, heavy, tether, which has its own associated drag that requires a still larger balloon/kite system to support it, thus reducing payload carrying capacity.

Current tethered airship designs and balloon/kite systems do not survive strong winds of more than 50 knots for smaller systems and for very large systems winds of more than 70 knots or exceptionally 100 knots. For continuous operation at useful elevations of typically over 400m, high wind speeds of over 100 knots will have to be sustained. With high winds, existing systems become unstable, moving uncontrollably and are ultimately blown over.

Because of these effects it has not been possible hitherto to design and build a reliable tethered aerostat system to deliver a high availability service because very high wind conditions will be encountered almost everywhere from time to time, particularly at suitable altitudes needed for many different applications. That, in turn, has meant that any potential use of a tethered kite/balloon aerostat carrying a system that requires high availability and reliability has not been possible.

Lighter than air balloon/kite systems are described in US 2011/0222077A1 with the following examples: US 2,398,745 and US 2,431,938 to Jalbert, US 4,029,273 to Cristofel Jr., in US 6,016,998 to R. Allsopp, and in U.S.Pat.No.6,499,695 to Talamo. In US 6,555,932 to Mizzi, there is described a combined buoyant aerofoil for use in generating electricity with wind power or for aerial advertising. Such combination balloon/kite systems are also available commercially for surveillance and advertising use, such as the SkyDoc TM Aerostat, supplied by Floatograph Technologies LLC, of SilverSpring Md., or the Helikite, supplied by Allsopp Helikites Ltd of Fordingbridge, Hampshire, UK. Such systems have been used for military and civilian use, and the data generated has been described as being conveyed to and from the ground station by means of wireless, cable or optical fiber. US 2018/0050797 Al discloses a conceptual tethered lighter-than-air unmanned aerial vehicle that has a wing and a fuselage. However, no consideration is given to how to maintain such an arrangement so that it is stable in the atmosphere.

KR 10-2016-0081328 discloses a buoyant aerostat that is tethered to ground level. However, no consideration for how such an apparatus can be kept stable in the atmosphere is given. US 2016/0122014 Al discloses a conventional blimp attached to the ground by two tethers. US 3,620,292 discloses a known type of wing balloon.

An aerostat system highly stable in extreme weathers for a wide variety of locations with the ability to withstand meteorological challenges including: extreme gusts, lightning strikes; prevention of snow and ice build-up; and safety features to prevent accidents with low flying aircraft, is therefore likely to be valuable in a whole variety of applications including but not limited to those described above.

Of particular importance is maintaining highly stable orientation for many payloads, including but not limited to the use of phased array antennas for linking to ground based user equipment, and earth observation payloads. Continuous orientation to within 5 degrees of the nominal vertical orientation can be required to avoid interference and reduced performance due to artefacts in beam formation. Gimballed payloads can mitigate this problem but have associated weight, space around the payload and power requirements, reducing payload performance.

Therefore, further improvements in aerostats that can operate at high wind and low wind or even no wind, for improved positional stability would be highly desirable.

Summary of the Invention

In a first aspect, the invention relates to a tethered aerostat positioned at an elevated location in the atmosphere, the aerostat having an elongate body having a length extending between an upper end and a lower end in use, the aerostat comprising sufficient lighter-than-air gas such that the aerostat is buoyant in air, and defining a buoyancy force acting through a centre of buoyancy, the aerostat having a gravitational force acting through a centre of mass, wherein the centre of mass is closer to the lower end than is the centre of buoyancy; the tethered aerostat comprising a tether connecting the aerostat to a substantially ground level location, wherein the tether defines a tether line of action, wherein the tether line of action is a straight line that is coaxial to the tether proximate the aerostat, and wherein, when the upper end is substantially vertically positioned above the lower end in use: in the absence of any wind, the moment about any point on the tether line of action produced by the gravitational force and produced by the buoyancy force, are substantially equal and opposite to each other, thereby providing a stable vertical orientation in a no-wind condition in the atmosphere; and when wind is passing over the aerostat, the sum of the moments about any point on the tether line of action produced by the drag forces and any resulting additional forces induced by the wind, also cancel out to provide a stable vertical orientation in a wind condition in the atmosphere.

By “buoyant in air” is meant that the aerostat is lighter- than-air, e.g. wherein the buoyancy force is greater than the weight of the aerostat.

As used herein, the term ‘aerostat’ includes and pay load and associated equipment attached to the aerostat other than the tether.

It will be appreciated to a person skilled in the art that, due to the changing forces that such an aerostat experiences when in the atmosphere, force are to be considered over an inertially significant period of time. An inertially significant period of time means a time interval that is sufficiently long such that the forces applied to the aerostat have sufficient time to induce noticeable movement of the aerostat by overcoming inertia, and being a time interval greater than the “time constant” calculable by those skilled in the art.

It has been found that such a tethered aerostat having an elongate body can be positioned at an elevated location in a stable vertically-oriented manner, regardless of the wind speeds encountered, by ensuring that the moments about the tether line of action due to the buoyancy and weight cancel each other out when so vertically oriented.

The centre of mass of the aerostat refers to the centre of mass of the aerostat skin and internal components and any equipment attached to or supported by the aerostat except the tether.

The centre of buoyancy refers to the centre of lift of the aerostat resulting from the difference in density between the lighter than air gas contained in the aerostat envelope and the surrounding air.

When discussing the orientation of the aerostat in a wind, a conventional system of reference will be used: pitch refers to the aerostat rotating about an axis which is horizontal and at right angles to the wind, a positive pitch being rotation so as to move the top of the aerostat into the wind, yaw refers to rotation of the aerostat around a vertical axis at right angles to the wind, a positive yaw being a rotation around this axis in an anticlockwise direction when viewed from above the aerostat, roll being a rotation around a horizontal axis parallel to the direction of the wind a positive roll being an anticlockwise rotation when viewed along the axis upstream of the aerostat.

It will be appreciated that the tension in the tether line of action produces no moment about any point on the tether line of action. Thus, by defining the vertically stable condition in this way, only the gravitational forces and buoyancy forces need to be considered for vertical stability in a no wind condition. How wind affects this is discussed below.

The tether will adopt a curved profile in any wind, however in the region that is proximate to the aerostat, the tether will become substantially linear. The aerostat tether defines a “tether line of action,” wherein the tether line of action is a straight line that is coaxial to the tether proximate the aerostat, prior to any bifurcation or multiple splitting of the tether to multiple points of attachment on the aerostat. Preferably, the centre of buoyancy is not just above but is vertically above the centre of mass (i.e. they are vertically colinear) when the upper end is vertically positioned above the lower end in use. This has been found to be an optimal condition to provide vertical stability regardless of wind speed.

When the aerostat is in the atmosphere and air is passing over it, e.g wind impinging on the aerostat horizontally in use, the aerostat will experience a substantially horizontal drag force, which will induce an equal but opposite horizontal force in the tension of the tether. This therefore results in a change of angle of the tether line of action as the wind speed varies. Other forces may also arise such as a lifting force, produced by any lifting surfaces present on the aerostat. The design of the aerostat, in particular the location of the centre of mass, centre of buoyancy and the location or locations of the point of physical attachment of the tether to the aerostat, can therefore be arranged to satisfy the requirements of vertical stability regardless of wind speed mentioned above.

Preferably, the tethered aerostat has a tether pivot point, defined as the point on the tether line of action that the tether line of action passes through in the absence of wind and also passes through when wind is passing over the aerostat, and the tether pivot point is the point on the tether line of action about which the moments are calculated. The tether pivot point is therefore the point on the tether line of action (and for practical purposes also on the tether itself) about which the tether appears to pivot as wind speed changes.

Thus, in a second aspect the invention relates to a tethered aerostat positioned at an elevated location in the atmosphere, the aerostat having an elongate body having a length extending between an upper end and a lower end in use, the aerostat comprising sufficient lighter- than-air gas such that the aerostat is substantially buoyant in air, the tethered aerostat comprising a tether connecting the aerostat to a substantially ground level location, wherein the tether defines a tether line of action, wherein the tether line of action is a straight line that is coaxial to the tether proximate the aerostat, and wherein, when the upper end is positioned above the lower end in use, the tether line of action passes between the upper end and lower end of the elongate body at a tether pivot point, the aerostat having a centre of mass positioned below the tether pivot point.

If the centre of mass is not closer to the lower end than is the centre of buoyancy then the aerostat may become unstable and could roll over: having the centre of mass below the aerostat centre of buoyancy promotes a stable vertical orientation, in view of the fact that deviations from a vertical orientation induce a counter-rotational moment due to the horizontal movement of the centre of mass from the centre of buoyancy. Thus, preferably the centre of mass of the aerostat is preferably also at least 10%, preferably 20%, more preferably 30% of the height of the aerostat, i.e. the distance between the upper end and lower end, below the aerostat centre of buoyancy.

A particularly stable arrangement is wherein the centre of buoyancy is vertically above the centre of mass and wherein the tether pivot point is vertically above the centre of mass and vertically below the centre of buoyancy, when the upper end is vertically positioned above the lower end in use. In other words, they are vertically colinear.

Tethered aerostats according to the present invention are therefore vertically stable in a wide range of wind speeds. It has therefore been found to be particularly advantageous when the aerostat is shaped as an aerofoil. Therefore, preferably the elongate body, when the upper end is substantially vertically positioned above the lower end in use, has a horizontal cross-section at each point throughout substantially the entire length that is an aerofoil, providing a leading edge and a trailing edge extending between the upper end and lower end, and defining between them, for each horizontal cross-section, a chord line between the leading edge and the trailing edge of the cross-section, having a chord length. Such an arrangement has the appearance of a “vertical wing”, as discussed in more detail below, and can be described as a vertical buoyant aerofoil or aerostat.

Preferably the elongate body is substantially symmetrical about a vertical plane (when the upper end is vertically positioned above the lower end) the centre of mass, the centre of buoyancy and the tether line of action all lying on the vertical plane. Furthermore, if the aerostat has an aerofoil shape, during moderate or high winds, a horizontal aerodynamic force arises that is substantially perpendicular to the wind direction supplied by the flow of air over the aerostat, when the aerostat is substantially pointed into the wind and the flow is not substantially separated over the majority of the trailing edge surface of the aerostat. For aerodynamic stability, the action of this horizontal force must be closer to the trailing edge than is the centre of mass of the aerostat. In other words, the centre of mass of the aerostat (i.e. including payload but not the tether) should be forward (i.e. into the wind) of the centre of aerodynamic sideways pressure. This is similar to that encountered in conventional aircraft with horizontal wings, well known to those skilled in the art, where for stability in pitch, an aircraft whose wing axis is mainly horizontal needs to have the system centre of lift behind or at the centre of mass. This is to prevent yaw instabilities that can increase drag and/or provide oscillations that promote wear and tear of the aerostat and shorten the life of the aerostat.

As will be appreciated, the magnitude of the buoyancy force exceeds the magnitude of the gravitational force, so that the aerostat remains buoyant in a zero wind condition. In practice for such aerostats the magnitude of the gravitational force is typically between 50 and 90% of the magnitude of the buoyancy force. Additionally, in a zero wind condition, the tether (and therefore the tether line of action) will tend to become essentially vertical (due to no side winds). Therefore, in the limit, the three forces acting on the aerostat in a zero wind condition will all be acting vertically, with the force applied by the tether being equal to the buoyancy force minus the gravitational force.

Therefore, in order to ensure that the moments due to the gravitational force and buoyancy force cancel each other out (i.e. the aerostat is rotationally stable) in a zero wind condition, when the upper end is substantially vertically positioned above the lower end in use, the tether line of action must have a horizontal position such that the horizontal position of the centre of buoyancy is between the horizontal position of the centre of mass and the horizontal position of the tether pivot point. This is because, in this arrangement, the buoyancy force will exert a moment about the tether line of action and the gravitational force will exert an moment about the tether line of action in the reverse sense, which are then capable of cancelling each other out, being equal but opposite, with appropriate spacings.

As the buoyancy force has a greater magnitude than the gravitational force, this arrangement requires that the buoyancy force acts at a point closer to the tether line of action than does the gravitational force. Thus, due to the lever rule, the moments arising from the two forces having different magnitudes, can be arranged to be of the same magnitude, cancelling each other out, and providing a rotationally stable vertical zero wind arrangement.

As already discussed, in the special case that the buoyancy force and gravitational force have the same horizontal position (i.e. that they are vertically colinear), then the tether line of action, passes through both the centre of mass and the centre of buoyancy, i.e. that all three forces, i.e. tension in the tether, buoyancy and gravitational, are vertically aligned.

A substantive range of wind speeds, over which the tethered aerostat can remain vertically oriented includes from 0 m/s to max operating wind strength typically between 40 m/s and 100 m/s, preferably as much as 120 m/s.

In a third aspect, the invention relates to a low drag, tethered, buoyant in air aerostat, the aerostat being substantially tall vertically compared with its horizontal dimensions, the aerostat having a horizontal cross-section at each point throughout substantially the entire vertical height that is an aerofoil, providing a leading edge and a trailing edge, and having a centre of mass and a centre of buoyancy, and wherein the tether defines a tether line of action, wherein the tether line of action is a straight line that is coaxial to the tether proximate the aerostat, the aerostat being substantially symmetrical about a vertical plane of symmetry, the vertical plane of symmetry including the centre of mass, the centre of buoyancy, the leading edge, the trailing edge and the tether line of action; wherein the aerostat is arranged such that it does not change its pitch or roll significantly over a substantive range of wind strengths by the centre of mass of the aerostat being at a lower height than the centre of buoyancy and wherein the sum of the moments in any plane caused by the mass and buoyancy forces and the aerodynamic forces acting on the aerostat over a substantive range of wind strengths being close to zero when taken about any point on the tether line of action over an inertially significant period of time.

A low drag aerostat is highly preferred for such an aerostat to operate in high winds without an excessively strong and therefore heavy tether. The weight of the tether will be experienced by the aerostat as tension in the tether. Therefore, the buoyancy force has to exceed not only the gravitational force but the excess must be at least as great as the vertical component of tension in the tether.

If the aerostat has a higher drag then the aerostat needs to be larger to carry the weight of the heavy tether and payload at high winds. A larger aerostat is more costly and less economic. It has been discovered that for practical utility in being able to carry substantive pay loads in high winds of more than 30 metres per second, a drag coefficient of less than 0.2, preferably less than 0.06 , most preferably less than 0.03 is required.

In this application, the drag coefficient of a tethered aerostat is defined as CD = F / (’A u² A) , where F is the horizontal aerodynamic drag, p is the air density , u is the horizontal component of wind velocity and - as is conventional in aerofoil theory, well known to those skilled in the art, A is the plan area when seen looking at the aerostat horizontally perpendicular to the wind. The leading edge at any vertical level, is defined by the point which first meets oncoming air, and the trailing edge at any level, the point which last meets oncoming air.

In order to provide a suitably large ratio of internal volume to external envelope surface area, the aerofoil cross sections are preferably “thick” in aerodynamic terminology, i.e. the width of the aerofoil is substantial in comparison with the chord length, the thickness being defined as the width (maximum dimension normal to the airflow) divided by the chord length (maximum dimension in the direction of the airflow). This reduces the envelope skin mass for a given volume of aerostat. Preferably, the maximum thickness of the aerofoil is at least 20% of the chord length, more preferably at least 30% and most preferably above 35% averaged over each aerofoil cross-section. However, it is preferably no greater than 55%.

Although the tether may be attached directly to the aerostat at a single location, it has been found to be preferable that it bifurcates into multiple sub-tethers proximate to the aerostat, in order to facilitate attachment of the tether to the aerostat. Such bifurcation can be split between horizontal bifurcation and vertical bifurcation. Horizontal bifurcation is where the sub-tethers have the same vertical position, and vertical bifurcation is where the sub tethers have a different vertical position. The location of attachment of the tether or the sub-tethers to the aerostat is a primary means of governing the location of the tether line of action and the tether pivot point, and thus is of high importance.

In general, if a tether is directly attached to the aerostat then the point of attachment will also provide the tether pivot point. However, when the tether bifurcates vertically, the tether pivot point will be at the point of bifurcation. When the tether bifurcates horizontally, e.g. into two sub-tethers and are attached to the sides of the aerostat, the tether pivot point will be at a point on the tether line of action within the aerostat and between the points of attachment of the sub-tethers.

It will be appreciated that the tether may therefore have multiple points of attachment to the aerostat. However, it will be appreciated that the tensions in the sub-tethers will have the same static effect as a single tension force acting through the tether along the tether line of action, due to Newton’s third law of motion. Therefore the arrangement of sub-tethers has no effect on the action of the tether line of action, however their arrangement may determine whether the tethered aerostat has a single tether pivot point or not. Usually it is important to ensure that all the sub-tethers are in tension at all wind strengths the aerostat operates in to avoid sub-tethers becoming slack and potentially causing aerostat skin damage. Preferably the tether pivot point is aft of the leading edge of the aerostat, more preferably aft of the centre of buoyancy of the aerostat. This allows the forward pitch at low winds described above to be greatly reduced and that if this axis of attachment is at a vertical height where the drag moment is zero with the aerostat vertical then the pitch variation can be minimised with a substantially vertical aerostat at both very low and very high wind speeds.

In a preferred embodiment, such a positioning of the tether pivot point can be achieved by horizontally bifurcating the tether into two sub-tethers, each sub-tether having a “point of attachment” at the same height on opposing positions on the exterior of the aerostat - when viewed from above e.g. along a vertical plane of symmetry. A convenient construction is by the placement of a horizontal spar or bridle of at least the width of the aerostat below the aerostat point of attachment shortly after the point of bifurcation, to keep the two subtethers in an approximately parallel relationship to each other and a sufficient distance apart to avoid impinging on the aerostat when close to or at a vertical orientation in light winds.

The two sub-tethers are preferably attached to support points attached to the aerostat on either side of the substantially symmetrical aerostat that will act to define the tether pivot point at the appropriate position as defined above.

Such sub-tethers can be attached to the aerostat at two points aft of the leading edge of the aerostat can be by means of multiple sub - sub tethers or projections, but these sub tethers must end in two points which are proud of the aerostat skin symmetrically placed on opposite sides of a vertical axis of symmetry normally parallel to the aerostat apparent wind direction, to ensure that at low wind strengths when the tether line of action is close to vertical, the sub tethers do not rub or interfere with the aerostat skin.

According to the present invention, such an aerostat can have a variation in pitch or roll of less than 15 degrees, preferably less than 10 degrees and most preferably less than 5 degrees for a variation in wind strength of 15 m/s, more preferably 40 m/s, and most preferably 80 m/s. Preferably the aerostat is located at an elevated location at an altitude of from 100m to 20,000m, more preferably from 200m to 10,000m, most preferably from 250m to 3000m.

Preferably the elongate or tall body has an aspect ratio, defined as the ratio of the length of the elongate body divided by the average width of the body in a direction perpendicular to the length, wherein the aspect ratio is greater than 1.5, preferably greater than 3.0, more preferably greater than 5.0.

The elongate or tall body may be of any desirable length according to needs of the aerostat, that can be suitably manufactured. Preferably the distance between the upper end and the lower end (i.e. its vertical height) is from 5 to 500m.

The lighter-than-air gas may be helium or hydrogen, or a mixture of these gases and other gases. In the prior art, lighter-than-air aerostats inevitably suffer leakage of the buoyant gas and typically need to be refilled every 4 weeks or so, or even more frequently. It has been discovered that an effective means of achieving improved containment of such gases is by the addition of one or more layers of lightweight metal foil (e.g. having a thickness of from 6 to 50 microns), such as, but not limited to, aluminium. That protection if applied externally also provides UV protection for the materials used in the elongate body, as known to those skilled in the art.

Although the foregoing provides excellent positional stability, it has furthermore been recognised that additional lifting surfaces e.g. substantially horizontal wings can assist the aerostat buoyancy especially in high winds and prevent large tether inclinations to the vertical.

These wings may be like the wings of a conventional aircraft, optionally with a dihedral arrangement. It will be understood by those skilled in the art that the horizontal drag of the aerostat is approximately proportional to the square of the windspeed, as is the vertical lift of lifting surfaces or substantially horizontal wings. Thus, as wind speeds increase, the horizontal drag and lifting forces tend to increase to the same extent as each other. This results in the highly desirable property in having a relatively constant tether inclination with increasing windspeed once the aerodynamic forces on the lifting surfaces are substantially greater than the net buoyancy (i.e. the buoyancy of the aerostat minus its weight). Such a property implies less movement of the aerostat when wind speeds are changing which is important to reduce fatigue and wear and tear in the components of the aerostat system as well as being desirable for many aerostat payloads.

Thus, preferably the aerostat comprises at least one lifting surface adapted to provide upwards lift from the movement of air over the lifting surface, to the aerostat in use, the lifting force supplied by the lifting surface typically acting at a point on the aerostat that is positioned nearer to the upper end than to the lower end.

It has been discovered that it is advantageous for the maintenance of a substantially vertical orientation to have a horizontal lifting surface which generates lift, arranged so that the lift vector is used to control pitch, where preferentially the line of action of the lift vector when the lift of the horizontal surface is used to maintain a near constant angle of the tether line of action to the vertical and goes through or near the tether line of action, and if present the tether pivot point, so that no net moment is exerted by the lift from the lifting surface. Thus, preferably the aerodynamic lift vector from the lifting surfaces passes the tether line of action by a distance less than 5% of the aerostat height.

As may be apparent, the greater the distance that the centre of lift of the horizontal lifting surfaces is from the tether line of action, the more roll stability will be provided, if the lifting surfaces are free to move relative to the aerostat to allow a steady orientation to the vertical despite movement of the aerostat.

Preferably the at least one lifting surface comprises a wing located at the upper end. Preferably the wing comprises a wing with a dihedral angle to the horizontal when in use, and preferably also attached to a fuselage and a tail. In some embodiments, the need to avoid snow or ice build-up may necessitate a reverse dihedral angle - the wing surface elevation being lower further away from the horizontal axis of the aerostat parallel to the wind direction.

Preferably the lift is adjusted by control of wing surface pitch or by additional flaps to control pitch of the aerostat.

If the vertically elongated aerostat provides a net rotation or added vorticity of the air passing over the aerostat along a horizontal axis parallel to the wind direction, the aerostat will have variable roll dependent on the wind speed. This phenomenon can give rise to unacceptable vertical orientation variations. The aerostat may comprise tapes or ribbons implemented as a helix around the aerostat used to adjust vorticity affecting roll generated by the aerostat. The aerostat may use a vertical surface attached to the aerostat inclined to the axis of symmetry to adjust vorticity affecting roll generated by the aerostat.

Any symmetry of the aerostat must be such as not to cause substantive vorticity or local spinning motion of the wind as would be seen by an observer looking along the direction of the wind towards the aerostat, a concept well-known to those skilled in the art. Substantive vorticity being defined as that vorticity which would lead under the maximum wind strength in which the aerostat is designed to operate, providing a roll greater than 10 degrees. Substantively symmetrical shapes providing suitably low vorticity with a roll angle of less than 5 degrees are to be preferred.

It has been discovered that some degree of asymmetry in the aerostat can be accommodated, such as manufacturing defects that cause a degree of mis-alignment of different levels of the aerostat, for example a twist of the aerostat when viewed from above where the longest sections of the different horizontal sections do not align. It has been discovered that the net vorticity as previously described can be minimised by the addition of a small vertical aerofoil as a flap or tail or the twist reduced by “warping” aerostat with spiral torsion members located on or close to the skin of the aerostat. A discussed, the aerostats according to the present invention are particularly suitable for providing telecommunications services to a wide area. Thus, preferably the aerostat comprises an antenna, preferably located in the vicinity of the lower end, and even below the lower end. This is because such antennas usually have a substantial mass, and therefore a low position contributes to the lowering of the centre of mass, which provides increased vertical stability, as discussed above. Likewise, such antennas should be positioned forwards as far as possible, close to the leading edge, as this moves the centre of mass forwards, which assists with stability also as discussed above. A preferred design of antenna is a phased array antenna. Such antennas may be free to rotate relative to the aerostat to allow vertical orientation to be held independently of that of the aerostat (e.g. to within 10°).

Preferably the antenna is located below the lowest point of substantive portions of the aerostat envelope that has a metal surface. The antenna is preferably free to rotate relative to the aerostat to allow vertical orientation to be held independently of that of the aerostat.

Optionally, the elongate or tall body may take a swept-back or swept-forwards arrangement. Thus, preferably the aerostat comprises regions that are oriented at an angle to the vertical. A swept-back or swept-forwards arrangement can be provided for when the leading edge and trailing edge, in the regions that are oriented at an angle to the vertical, are located within the single vertical plane of symmetry, parallel to the wind direction in use.

As discussed, given the mass or weight implications of a payload, its location has to be carefully considered in a swept-back or swept-forwards arrangement. For many payloads it is important that the payload is not obscured by an envelope skin. This is particularly relevant for communication purposes and when the envelope skin may have a metal barrier. In that context, it is advantageous or essential to have the payload either inside a projection below the main envelope or to have different barrier arrangements local to the payload if the payload is mounted inside the envelope. In a swept-back arrangement, in order for the payload to contribute to the centre of mass being forwards, it is preferable for the payload to be placed at the “nose” at or near the leading edge. However, such a position does not have the advantage of having a low vertical position, and thus is not ideal for vertical stability considerations. However, a swept-forwards arrangement could have the payload at or near the lower end, as this location is naturally both low down and forwards in use, providing excellent stability. In a swept-forwards arrangement the trailing edge is further from a vertical line extending between the upper end and lower end than is the leading edge.

Fibre reinforced plastics are composite materials with a polymer matrix reinforced by fibres. When the aerostat is suitably pressurised they are always in tension, and the aerostat skins can advantageously have certain layers that are mainly fibre with only a partial fill of polymer to save weight. Such skins typically need to be cured in an oven or autoclave at temperatures of typically over 90 centigrade. It can be desirable to cure the systems in autoclaves at pressure to ensure good fatigue life with a minimum of internal voids.

Additionally, the tether, or sub-tethers, may have a coating in the vicinity of the aerostat, to protect them and the aerostat in the event of any rubbing. Alternatively, the sub- tethers may be encased in tubes which have a low coefficient of friction - minimising frictional forces if contact is reached. Such low friction materials include a variety of plastics such as Teflon or Nylon, and also graphite and similar materials. Such studs may be proud of the surface of the aerostat by at least 0.1%, preferably more than 0.5% of the distance between the top and bottom of the aerostat

For all-weather, continuous operation, it is essential to cater for adverse weather conditions, which includes freezing rain and heavy ‘sticky snow’ fall, which on conventional aerostats can rapidly lead to a build-up of ice or snow on the top or sides of the aerostat and cause the aerostat to lose lift and descend. As discussed, in many instances the aerostat of the present invention will comprises an antenna, such as a phased array antenna. Such devices generate a substantial amount of heat.

Thus, in a fourth aspect, the invention relates to an aerostat comprising a heat-generating payload, the aerostat comprising sufficient lighter-than-air gas such that the aerostat is substantially buoyant in air, and contained within an enclosed skin, and a gas-circulation arrangement, arranged to provide a flow of lighter-than-air gas to the heat-generating payload, whereupon heat is transferred to the gas from the heat-generating payload, whereupon the heated gas is directed to the inside surface of the skin, whereupon heat is transferred from the gas to the skin.

Preferably the gas then cycles through the pay load e.g. by natural or forced convention, and towards the skin, in a continuous process of heating and cooling.

This has two major advantages, firstly the skin external surface becomes significantly warmer (e.g. by as much as 10 or 20°C in low wind conditions) than the surrounding air, which prevents snow or ice accumulating, and secondly, the payload is protected from moisture, dust and particulate material.

In high wind conditions the lifting surfaces previously described provide significant extra lift which allows some of the weight of any build-up of snow or ice to be readily accommodated. Additional protection against the build-up of snow and ice can be provided by electrically heating the leading edge and portions of the top of the aerostat as well as any lifting surfaces or instrumentation.

The invention will now be illustrated, by way of example with reference to the following figures.

Figure 1 is a side view of a known conventional tethered aerostat with payload, not according to the invention. Figures 2a and 2c are side elevations of a tethered aerostat according to the present invention, in respectively a high wind and a low wind, figure 2b and 2c show the force balances in the high wind and low wind case. In this aerostat the centre of buoyancy, the centre of mass and tether pivot point are vertically colinear, i.e. directly above or below one another.

Figures 3a and 3b are a plan view and side view respectively of the tethered vertical buoyant aerostat shown in figures 2a and 2b in a light wind condition with the colinear property mentioned above.

Figures 4a and 4b are a plan view and a side view respectively of a second tethered vertical buoyant aerostat according to the present invention in a moderate to high wind with the centre of buoyancy, the centre of mass and the tether pivot point being at different horizontal positions relative to any wind and are not vertically co-linear, so as to provide a vertical aerostat orientation and an aerostat that has a modest “swept forward” angle making construction and transport easier. Such a non - vertical colinear arrangement requires a point of attachment well aft of the centre of buoyancy if the aerostat is not to have a substantial forward pitch.

Figure 5 shows the same aerostat as described in figures 4a and 4b but with a suitable horizontal wing

Figure 6 is a perspective view of a third tethered vertical buoyant aerostat according to the present invention.

Figures 7a and 7b are a side sectional view and plan sectional view respectively through the lower portion of an aerostat according to the present invention, showing some of the internal structure of an antenna housing showing buoyant gas flow cooling the antenna and heating the skin of the buoyant section constituting part of a vertical buoyant aerofoil. Turning to the figures, figure 1 shows a known aerostat 101 having a body with tail 102 and a payload compartment 103 in a wind 104, with a centre of mass 105 through which the weight of the aerostat 101 and payload 103 acts shown as 106. The centre of buoyancy 107 of the aerostat 101 through which the buoyancy force 108 acts vertically is close to the aerostat centreline 112. The centre of aerodynamic lift is at point 109, through which the aerodynamic drag force 110 and the aerodynamic lift 111 act due to the air motion over the aerostat 101. If the aerostat is not pointed into the wind, there will be a sideways aerodynamic lift (not shown) that acts at or close to this point 109.

The aerostat 101 is attached to a tether 113 by a number of sub-tethers (not all shown) 117. The sub-tethers 117 meet the tether 113 at the tether confluence point 120. Tether 113 defines a tether line of action 114 that passes vertically above the centre of lift 109 by a distance 115, and the centre of mass 105 is vertically below the tether line of action 114 by a distance 116. The fact that the centre of lift 109 (being an upwards acting force) is below the tether line of action 114 is inherently destabilising. However, in this instance this is more than compensated for by the position of the centre of mass 105.

If the stability to rotation about the axis of the tether line of action 114 is considered, the greater the vertical distance beneath the tether line of action 114 the centre of mass 105 is, the less the propensity the aerostat 101 has to rotate around this axis 112. Similarly, the stability to rotation about the tether line of action 114 is also improved if the centre of buoyancy 107 is as little as possible below and preferably situated above the tether line of action 114.

It has long been recognised that for aerostat stability the centre of sideways aerodynamic lift of the system 109 needs to be aft of the centre of mass, otherwise yaw instability results.

The rotational axes are defined respectively as yaw (1) around axis Z, 11, pitch (2) around axis Y,12, and roll (3) around axis X ,13, with the conventional “right hand rule.” The tether line extension 114 is an imaginary line extending the line of the tether at the point it is attached to the aerostat or is attached to multiple lines 117 which in turn are attached to the tether 113.

In practice for such conventional known aerostats 101 the weight of the system 106 is always less than the buoyancy 108, otherwise the aerostat 101 would not fly when wind strengths are low. It can therefore be seen that when wind strengths are high the aerostat 101 is likely to become unstable and have a propensity to rotate around the axis 114 if the vertical distance between the centre of buoyancy 107 and centre of mass 105 is small.

Furthermore it can be seen at high winds, when the tether line of action 114 is at a greater angle to the vertical, the centre of sideways aerodynamic lift 109 can be well forward of the tether line of action and any small disturbance to the wind direction can result in the moment of the sideways aerodynamic lift around the tether line of action being greater than the restraining moment provided by the tails 102 and the aerostat then turning away from the wind and diving into the ground. It is this effect, coupled with the modest righting moment around the axis 112 that results in difficulties in designing aerostats of conventional shape to be able to operate in high winds.

Figures 2a and 2c are side elevations of a tethered aerostat according to the present invention, in a swept-forwards arrangement and in a vertical orientation. Figure 2b and 2c show the force balances in the high wind and moderate wind case respectively. In both cases, a symmetrical elongate or tall low drag aerostat 201 is shown in side elevation, with the same centre of buoyancy 202 and buoyancy force 203, the same centre of mass 204, with the gravitational force due to the aerostat and payload weight 205 acting from there, and the same payload 206, which brings the centre of drag 209 marginally below the centreline of the aerostat. The same upper end 207 is identified, but the aerostat in figure 2a is shown in a strong wind 208, and the aerostat in figure 2c is shown in a lighter wind 200, which is in the same direction. A tether 215 is attached to two sub tethers 216, and 217 in both cases, and these tethers and sub-tethers meet at a point of confluence 214 and the tether 215 has a tether line of action 218 in both cases. It will be noted also that the centre of buoyancy 202 is vertically above the centre of mass 204 when the upper end 207 is vertically positioned above the lower end in use.

In the high wind case the aerodynamic drag on the systems 211 is much greater than the aerodynamic drag 210 in the low wind case. That drag results in a change of angle of the tether 215 and tether line of extension 218. However, all forces, the weight of the aerostat 205, the buoyancy of the aerostat 203 the aerodynamic drag 211 or 210 and the tension in the tether 213 provide no net moments about any point on the tether line of action 218.

Furthermore, it can be seen that a tether pivot point 214 is provided, about which the tether appears to rotate as wind speed changes, and that this is also the point of bifurcation of the tether. It will also be noted that as well as the centre of buoyancy 202 being vertically above the centre of mass 204, the tether pivot point 214 is also vertically above the centre of mass 204 and vertically below the centre of buoyancy 202, when the upper end is vertically positioned above the lower end in use. The aerostat is therefore stable in a vertical orientation in spite of a pronounced change in wind strength.

Therefore, the inherent stability provided in a no wind condition, due the balance of the moments due to buoyancy and gravitational forces, survives the addition of wind from low to high speeds.

Figure 3a shows a front elevation and figure 3b shows a side elevation of the same aerostat 201 as shown in figures 2a and 2c but in a very low wind condition and with slightly different tether arrangements. As a result of the low wind, the tether 215 has become close to vertical, and there is a much smaller drag force 310. For this aerostat to satisfactorily operate in such low winds to avoid the tether 215 impacting on the aerostat, the tether bifurcates horizontally into two sub-tethers 319 and 320 which are kept apart by a horizontal spar or bridle 321, which is connected to a further horizontal spar 324, which is in line with the tether pivot point 325 around which the aerostat can pitch with respect to the tether 215. The ends of the bridle 321 bifurcate vertically and are connected by further sub-tethers 2161 and 2162 to “points of attachment” 2163 and 2164 which are arranged at the same vertical position but symmetrically on either side of the aerostat 201. The spar 324 is also connected to one point on the leading edge of the aerostat via additional sub-tethers 2171 and 2172. The horizontal separation of the “points of attachment” 2161 and 2163, keep the spars 324 and 321 essentially horizontal when the aerostat 201 is vertical in roll - that is to say the centre of buoyancy 202 is directly above the centre of mass 204 when viewed from the direction of the wind. Provided the lengths of sub- tethers 322 and 323 are sufficiently long and the spars or “bridles” 324 and 321 are substantially wider than the width of the aerostat beneath its centreline when viewed from the direction of the wind, the tether and sub-tethers never impact on the aerostat no matter what the wind strength is.

Figure 4a shows a plan view and figure 4b shows a side view of a further example of an aerostat 401 according to the present invention in a moderate to high wind 400. It can be seen that the elongate body 403 of the aerostat 401 comprises two sections 402, 406 that are oriented at an angle to the vertical when the upper end 407 is vertically positioned above the lower end 408, resulting in a forward swept arrangement.

The presence of a payload 404 of substantive mass at the lower end of the elongate body 403 at or close to the leading edge of the elongate body 403, provides for yaw stability with the swept forward arrangement: the swept forward arrangement moves the centre of horizontal aerodynamic lift 418 aft compared to the position of the centre of mass 413, if the pay load has significant mass compared to the rest of the aerostat and is positioned near the leading edge of the aerostat.

The sweep angle 405 is typically at least 6 degrees, preferably 8 to 25 degrees.

The aerostat tether 437 bifurcates into two sub-tethers which pass over a horizontal spar or bridle 435 which is connected by two further sub-tethers 4341 and 4342 to attachments 438 proud of the aerostat surface. The attachments 438 are situated well aft of the leading edge of the aerostat to ensure that the net moment of the aerostat weight and buoyancy around the tether line of action is zero when the aerostat is in a substantially vertical position. The line between the ends of the rigid attachments proud of the aerostat surface provides the tether pivot point 439 around which the tether 437 effectively rotates as wind changes. The bridle and bifurcation arrangement prevents the lines 434 from rubbing or fouling the surface of the aerostat in high or low winds when the tether angle can vary from being steeply inclined to the vertical (as shown) or more vertical in low winds.

This arrangement of the points of attachment 439 far back from the leading edge of the aerostat allows the aerostat to be orientated vertically than if the tether is attached close to the leading edge. The drag force and the horizontal component of the tether force is not shown.

It will also be appreciated that the tether pivot point is also located at the same location as the attachments 438, as this is the point on the tether line of action about which the tether will appear to pivot as wind speed changes.

However, it can be seen that the buoyancy force 431 produces a clockwise moment about the tether pivot point, and that the gravitational force 433 produces an anti-clockwise moment about the tether pivot point. Although the buoyancy force 431 has a greater magnitude than the gravitational force 433, as the gravitational force 433 acts from a further distance to the tether pivot point 438 than does the buoyancy force 431, the moments produced by the two forces are equal and opposite, and therefore cancel each other out, producing no net rotational effects. The moments of the buoyancy force 431, and the weight 433 around the tether pivot point, the attachment points 438 cancel each out with suitable horizontal positioning of the attachment points 438 such that the aerostat orientation is vertical at all wind strengths - the horizontal component of the drag and tether forces balancing each other out.

Figure 5 shows the same aerostat as described in figures 4a and 4b but with a suitable horizontal wing 501 positioned just above the aerostat on a spar 502 such that the direction 504 of the aerodynamic forces (lift and drag) 503 goes through the tether pivot point 439, and so with variable wind strengths there is no net moment of these aerodynamic forces around the tether pivot point and therefore no inclination for the aerostat to change its pitch and deviate from vertical.

Figure 6 shows a perspective view of a further example of an aerostat 601 according to the present invention. It can be seen that the aerostat 601 has an aerofoil shape and is swept forwards, as in figure 4.

There two substantially horizontal lifting surfaces positioned near the top of the aerostat, to provide additional lift in strong winds to ensure the tether 607, is relatively vertical. These lifting surfaces are shown as 602, a lightweight CFRP aerofoil fixed at constant pitch, and a gimballed aircraft 603 with autonomous or radio control to provide variable direction aerodynamic forces to stabilise the aerostat orientation.

The tether 607 bifurcates into two sub-tethers 608, 609 proximate to the aerostat, each subtether having a point of attachment 610, 611 on opposing positions on the exterior of the aerostat 601 that oppose each other and have the same horizontal and vertical positions as each other. The tether 607 comprises a bridle 612 positioned shortly after the point of bifurcation to help prevent the sub-tethers from rubbing against the aerostat. Additionally, studs are provided at the points of attachment 610, 611 to further prevent any rubbing of the sub-tethers 608, 609 against the aerostat 601.

A surface ribbon 614 is shown spiralling around the aerostat from a fastening close to the base of the aerostat to a fastening close to the top of the aerostat. By suitable tensioning of this ribbon the aerostat can be subjected to twist around a substantially vertical aerostat. Such a single ribbon or tape “helix” or multiple “helices” can be used to modify the aerostat shape to allow minimal vorticity to be generated by the aerostat when taken around an axis parallel to the wind direction if the aerostat shape is not quite symmetrical around a vertical plane in the direction of the wind. More detail is provided in figure 7a of the internal cross section of an antenna 705 mounted at the base of an aerostat 708 in wind 703, and a plan view of the horizontal cross section shown in figure 7b.

The antenna 705, consists of a number of phased array elements arranged in a vertical cylinder, supported by a tube 704, in turn supported by gimbal 707, so as to be able to be orientated with the tube axis vertical with significant pitch of the aerostat 708 and modest roll. It is desirable for the antenna 705 to maintain a vertical orientation within +/- 10 degrees, preferable +/- 5 degrees to enable a simplified, lighter, antenna to form satisfactory beams on the ground.

The gaseous envelope 706 which can be streamlined as shown into an aerofoil shape to reduce drag and provide space for the antenna to accommodate significant pitching movement, is made from a thicker plastic skin but without metal foils so as to provide a medium moderately impervious to buoyant gas diffusion but transparent to relevant radio frequencies. The relatively small size of the antenna surface compared to the main aerostat provides for acceptable total gas diffusion rates despite the relatively increased diffusion rate for the material.

Lighter-than-air gas (helium or hydrogen or mixtures thereof) 709, within the main aerostat 708, is drawn by natural convection or forced convection by a fan (not shown) from inside the aerostat into an annular space between the antenna tube where it moves downwards 710, before a change of direction 711, where it is drawn through an entry section 712 designed to minimise pressure drop, before rising inside the tube 713, and exiting through a diffuser 714 and entering the main aerostat 708 at modest velocity, to minimise pressure drop. The jet can be directed at the leading edge or conveyed substantially vertically along the leading edge in a pipe or conduit with directed jets to provide greater heating on the leading edge of the aerostat 708 and if need be on the top surface of the aerostat 708 to mitigate the effects of snow or freezing rain causing build up or weight on the aerostat 708. Some measure of humidity control can be provided by an absorbent to ensure that the partial pressure of water vapour in the circulating gases is not negligible but has a dew point below -40C but provides a more benign atmosphere for electronics inside the aerostat.

Claims

29 Claims

1. A tethered aerostat positioned at an elevated location in the atmosphere, the aerostat having an elongate body having a length extending between an upper end and a lower end in use, the aerostat comprising sufficient lighter-than-air gas such that the aerostat is substantially buoyant in air, and defining a buoyancy force acting through a centre of buoyancy, the aerostat having a gravitational force acting through a centre of mass, wherein the centre of mass is closer to the lower end than is the centre of buoyancy; the tethered aerostat comprising a tether connecting the aerostat to a substantially ground level location, wherein the tether defines a tether line of action, wherein the tether line of action is a straight line that is coaxial to the tether proximate the aerostat, and wherein, when the upper end is substantially vertically positioned above the lower end in use: in the absence of any wind, the moment about any point on the tether line of action produced by the gravitational force and produced by the buoyancy force, are substantially equal and opposite to each other, thereby providing a stable vertical orientation in a no- wind condition in the atmosphere; and when wind is passing over the aerostat, the sum of the moments about any point on the tether line of action produced by the drag forces and any resulting additional forces induced by the wind, also cancel out to provide a stable vertical orientation in a wind condition in the atmosphere.

2. A tethered aerostat according to claim 1, wherein the centre of buoyancy is vertically above the centre of mass when the upper end is vertically positioned above the lower end in use. 30 A tethered aerostat according to claim 1 or claim 2, which has a tether pivot point, defined as the point on the tether line of action that the tether line of action passes through in the absence of wind and also passes through when wind is passing over the aerostat, and the tether pivot point is the point on the tether line of action about which the moments are calculated. A tethered aerostat according to claim 2 and claim 3, wherein the tether pivot point is vertically above the centre of mass and vertically below the centre of buoyancy when the upper end is vertically positioned above the lower end in use. A tethered aerostat according to any one of the preceding claims, wherein the elongate body, when the upper end is substantially vertically positioned above the lower end in use, has a horizontal cross-section at each point throughout substantially the entire length that is an aerofoil, providing a leading edge and a trailing edge extending between the upper end and lower end, and defining between them, for each horizontal cross-section, a chord line, between the leading edge and the trailing edge of the cross-section, having a chord length. A tethered aerostat according to claim 5, wherein the horizontal aerodynamic force, perpendicular to the wind direction supplied by a flow of air over the aerofoil, at a point on the aerostat, is positioned closer to the trailing edge than is the centre of mass of the aerostat. A tethered aerostat according to any one of the preceding claims, wherein the position of the centre of mass of the aerostat is at least 10%, preferably 20%, more preferably 30% of the height of the aerostat below the aerostat centre of buoyancy. A tethered aerostat according to any one of the preceding claims, wherein the magnitude of the gravitational force is between 50 and 90% of the magnitude of the buoyancy force. A tethered aerostat according to any one of the preceding claims, wherein the drag coefficient as defined by the ratio of the aerodynamic drag compared to the dynamic pressure over the aerostat plan area when viewed horizontally normally to the wind direction is less than 0.2, preferably less than 0.06, most preferably less than 0.03. A tethered aerostat according to any of the preceding claims, wherein the aerofoil cross sections over a majority of the height of the aerostat are thick, and have a chord width to length ratio of at least 20%, preferably 30% and most preferably greater than 35%. A tethered aerostat according to any one of the preceding claims, wherein the tether bifurcates to link to one or more essentially horizontal spars then in turn link to two points of attachment on or proud of the aerostat surface. A tethered aerostat according to any of the previous claims, wherein the net vorticity generated by the aerostat causes a roll of less than 15 degrees, preferably less than 10 degrees, most preferably less than 5 degrees. A tethered aerostat according to any one of the preceding claims, comprising tapes or ribbons implemented as a helix around the aerostat used to adjust vorticity affecting roll generated by the aerostat. A tethered aerostat according to any one of the preceding claims, that uses a vertical surface attached to the aerostat to adjust vorticity affecting roll generated by the aerostat. A tethered aerostat according to any of the preceding claims, located at an elevated location at an altitude of from 100 m to 20,000 m, preferably from 200m to 10,000 m, more preferably from 250 m to 3000 m. A tethered aerostat according to any one of the preceding claims, wherein the tether, or sub-tethers, are attached to studs that stand proud of the external surface of the aerostat. A tethered aerostat according to claim 16, wherein the studs project by at least 0.1%, preferably more than 0.5% of the height of the aerostat. A tethered aerostat according to any one of the preceding claims, wherein the aerostat’s vertical height is from 5 to 500m. A tethered aerostat according to any one of the preceding claims, which comprises at least one lifting surface adapted to provide upwards lift from the movement of air over the lifting surface, to the aerostat in use, the lifting force supplied by the lifting surface acting at a point on the aerostat that is positioned above the tether pivot point and preferably also the axis of attachment. A tethered aerostat according to claim 19, wherein the at least one lifting surface comprises a wing located at or near the upper end of the aerostat. A tethered aerostat according to any one of claims 18 to 20, wherein the lift is adjusted by control of wing surface pitch or by additional flaps to control pitch of the aerostat. A tethered aerostat according to any one of the preceding claims, which comprises an antenna, preferably located in the vicinity of the lowest point on the aerostat. A tethered aerostat according to claim 22, wherein the antenna is a phased array antenna. A tethered aerostat according to claim 22 or claim 23, wherein the antenna is located below the lowest point of substantive portions of the aerostat envelope that has a metal surface. 33 A tethered aerostat according to any one of claims 22 to 24, wherein the antenna is free to rotate relative to the aerostat to allow vertical orientation to be held independently of that of the aerostat. A tethered aerostat according to any one of the preceding claims, wherein the elongate body, when the upper end is vertically positioned above the lower end in use, comprises regions that are oriented at an angle to vertical. A tethered aerostat according to claim 26, wherein the aerostat body is an aerofoil and wherein the leading edge and trailing edge in the regions that are oriented at an angle to the elongate axis are located within a single vertical plane. A tethered aerostat according to claim 27, wherein the trailing edge is further from a vertical line extending between the highest and lowest point than is the leading edge. A tethered aerostat comprising a heat-generating payload, the aerostat comprising sufficient lighter-than-air gas such that the aerostat is substantially buoyant in air, and contained within an enclosed skin, and a gas-circulation arrangement, arranged to provide a flow of lighter-than-air gas to the heat-generating pay load, whereupon heat is transferred to the gas from the heat-generating payload, whereupon the heated gas is directed to the inside surface of the skin, whereupon heat is transferred from the gas to the skin. A tethered aerostat according to claim 29 which comprises the features of an aerostat according to any one of claims 1 to 28.