WO2014072692A2

WO2014072692A2 - Continuous band propeller

Info

Publication number: WO2014072692A2
Application number: PCT/GB2013/052886
Authority: WO
Inventors: Ralph-Peter Bailey
Original assignee: Quantada Ltd
Priority date: 2012-11-09
Filing date: 2013-11-05
Publication date: 2014-05-15
Also published as: GB201220183D0; GB2507773A; WO2014072692A3

Abstract

A band with an airfoil profile that follows a continuous path like a wave substantially in the plane of a virtual disc moving around the circumference between it and a common radial offset such that when the band rotates about the disc's axis the airfoil presents a largely common angle of attack to the fluid flow when moving between it's amplitude peaks and troughs made possible by the fact that at both the peaks and the troughs the upper surface and lower surfaces of the airfoil morph through a largely symmetrical transition to swap sides on the band. Or a band with an airfoil profile that follows a continuous path like a garter spring with it's axis moving around the circumference of a virtual disc such when the band rotates about the disc's axis the airfoil presents a largely common angle of attack to the fluid flow even as it moves around it's path between it's radial maxima and minima made possible by the fact that at both the maxima and minima the upper surface and lower surfaces of the airfoil morph through a largely symmetrical transition to swap sides on the band. Modifications to the chord, band path, airfoil section and angle around the path of the band can enhance it's ability to entrain more fluid and expel it more judiciously in order to increase it's effectiveness as a generator or propeller. The band can be retained about a central hub by spokes or cables or pylons or the tops of the waves can be connected to an external ring.

Description

CONTINUOUS BAND PROPELLER

The invention is a low turbulence fluid acceleration or deceleration device.

FIELD OF THE INVENTION

The present invention relates to a device which when rotated accelerates the ambient fluid medium as if it were a propeller, but which exhibits superior fluid dynamics in avoiding tip vortex turbulence, duct wall turbulence and hub spin losses. As such it can also be driven by the movement of surrounding fluid in the manner of a kinetic energy reclaiming turbine.

BACKGROUND OF THE INVENTION

There are a great many embodiments of propellers, impellers and the like performing a myriad of fluid pumping, blowing or energy reclaiming tasks, however all exhibit some disadvantage in particular applications. Generally a rotating device either accelerates the fluid using centripetal force in the manner of a radial blower or uses Bernoulli's law to generate a pressure differential over an airfoil as demonstrated by a conventional propeller. In the first case centripetal blowers tend to be better at generating higher pressures, but because of their commensurately high speed of rotation they create a lot of turbulence and hence noise and inefficiency. Conventional propellers are more efficient, but suffer from tip vortices where high pressure air on the underside of the blade takes a short cut around the tip to the lower pressure air above it. In addition whereas the outer region of the blade sweeps a relatively large area, the inner region is less effective and tends to be inhibited by the large twist and angle of attack required to keep pace with the axial fluid velocity and so is particularly compromised by changes in fluid or rotation speed.

Recently there have been a number of innovations proposing propellers that form loops or twists to avoid the imposition of an external blade tip. These have been particularly developed for the wind turbine industry, exhibited by HAWT embodiments like the "Loopwing", "Gedayc", and "Wind Wandler". All of these continue their wing solution up to the hub or hub axis. As such they still exhibit the high angle of attack stirring effect that causes an axial vortex and can operate efficiently at only one specific wind speed/rotation speed condition. Some also have significant axial depth thereby being unsuitable for mounting in the conventional manner of a propeller supported at one end and instead require a frame with a long axis being supported at both ends.

It is also very desirable that particularly turbines and to a lesser extent propellers entrain as much fluid flow as possible within as small a diameter rotor as possible. The concept of entrainment caused by a forward facing elliptical blade in the case of a generator is taught by US Patent 6302652 that proposes the same with elliptical forward swept blades, however that embodiment is difficult to build as centripetal force tends to flex the blades whereas this invention supports the tips by being part of the continuous band.

OBJECTS OF THE INVENTION

A principal object of this invention, therefore, is to provide an improved means to translate shaft rotation into axial fluid flow and the reverse.

Another object of the invention is to achieve such translation with reduced turbulence and thereby enjoy less noise and improved efficiency.

Another object of the invention is to offer a swap out upgrade to conventional impellers or blade arrays with a more efficient and compact solution.

Still another object of the invention is to enable a fan to achieve a greater movement of air with less energy and with less noise in a smaller diameter.

A further object of the invention is to extract a greater proportion of kinetic energy from a moving fluid stream than currently possible by entraining as much peripheral flow as possible.

Another object of the invention is to provide for an automatic pitch adjustment system for generator embodiments.

A still further object of the invention is to enable such improved efficiency at low cost and with a high degree of durability.

SUMMARY

This invention considers two ways of forming a continuous band that can be profiled like an airfoil and used to force an axial fluid flow when rotated about it's axis. Or similarly be forced to rotate by an axial flow in the manner of a generator. And all is achieved with a smaller diameter band than the equivalent conventional propeller and without otherwise requiring a diffuser with it's size and encumbrance. One follows a wave (15) with a radially inclined amplitude around a circular perimeter as shown in Fig. 3 with peaks (17) and troughs (16) such that most elements of the band cooperate in accelerating a local fluid when the band is caused to rotate about it's axis. This is because the band's airfoil has an angle of attack that is maintained relative to the flow direction even as the band follows it's peripheral wave path. The other as shown in resembles a stretched garter spring as shown in Fig. 8 in that the wave is like a coil who's axis substantially follows the circumference of a circle moving between maxima (20) and minima (21). The difference between the two is that the former is essentially flat as shown in Fig. 5 - although a degree of three dimensional twist can be added to facilitate a range of operating conditions as shown in Fig. 6 and to an even greater extent in Fig. 7 - while the garter spring variant is naturally three dimensional and so has depth that can be employed to enhance fluid entrainment which is particularly relevant when considering turbine rather that propeller applications. This garter spring form similarly sets it's airfoil to maintain a reasonably constant angle of attack to the prevalent axial direction as the band follows it's path.

The maximum viable angle of attack varies in accordance with the airfoil's profile and local Reynolds number. As the chord reduces so does the Reynolds number. When acting as a propeller the angle of attack is an incremental addition to the setting angle required to accommodate the axial speed of the flow through the rotating band, or when acting as a turbine it is subtracted. The setting angle is inversely proportional to the radial offset, increasing as it approaches the spin axis in the manner of a screw thread.

In order to ensure the band accelerates/decelerates all of the fluid within it's spin area, as well as being effected by the net setting angle and the ratio of local band speed to wind speed, the chord must also vary proportional to the degree of angular displacement between adjacent band members of common radial offset. The further band elements have to travel the greater the chord has to be (being equivalent to having fewer blades). With conventional straight blades the speed ratio is constant for a given blade radius just as the angle between successive blades is constant, however with the wavy band this ratio is only constant at the single radial offset (5) where the angular displacement between band elements is equal. At other radial offsets (4) and (6) alternate band blade elements have a different angular offset in the clockwise direction to the anti-clockwise direction even as the total angle between waves or coils remains constant and equal to 360 degrees divided by the integer number of waves or coils in one circumference.

At the peaks and the troughs of the band waves the airfoil morphs as shown in Fig. 3 such that the lower substantially flat surface (1) becomes the upper substantially curved surface (3) and vice versa, transitioning through a point where it's section is symmetrical (2). This ensures that the airfoil always shows it's curved upper surface facing the spin direction with the leading edge towards the flow when acting like a turbine being driven by the flow and conversely facing opposite to the spin direction with the leading edge away from the flow when acting like a propeller in causing the flow as in Fig. 3. Note that a different airfoil set-up is required to optimize performance for each condition as in order to work both as a propeller and a turbine the airfoil would need to be symmetrical about it's chord mid point normal such that the leading edge is the same shape as the trailing edge. With respect to the first 'wave' embodiment the waves have thus far been described as occurring substantially in the radial plane perpendicular to the spin axis, but particularly at high tip speed ratios where the airfoil's combined angle of attack and setting angle is small the transition through the symmetrical section would leave the chord very short, compromising the band's effectiveness at preventing the formation of tip vortices. This condition can be mitigated by introducing an axial offset to the path of the wave ahead of and behind the transition point such that viewed in a radial direction it forms an "S" with it's centre being drawn towards a more axial direction as show in Figs 6 and 7. In order to maintain the flow of the wave's path the axial offset occurs in the same direction ahead of both the wave's peak and trough, and then again in the reverse direction behind the peak and the trough.

When acting as a propeller, if the band is spinning anti-clockwise from the front propelling fluid backwards, and the "S" loop at the peak starts rearward and ends forward then a radial component of the airfoil's lift vector tends to entrain fluid inwards on the trailing side of the loop (furthest forward) rather than just propel it axially. Similarly the flow is entrained on the trailing side of the trough. Conversely when acting as a turbine but with the same spin and flow direction, fluid tends to be entrained inwards ahead of the wave peaks as it does from in front of the troughs. These effects are beneficial as in the case of the propeller they cooperate with the natural reduction in flow cross sectional area as the fluid accelerates, increasing the propellers effective diameter beyond the band's diameter. In the case of a turbine it acts like a virtual venturi and counteracts the tendency for fluid to be deflected out of the turbine's path by the pressure rise in front of the band. These effects are dependent on the axial depth of the band. The second 'spring' style of band with it's natural depth and enhanced flow entrainment improves the effectiveness of the turbine at ingesting additional fluid by entraining external flow as shown in Fig. 1 (7) thereby increasing the reserves of kinetic energy the turbine can tap. In effect the increased flow concentrates ahead of the band increasing the forward pressure.

On the exhaust side of the band (8) the opposite effect is achieved. Just as in front of the band the flow is entrained inwards, behind the band the flow is pushed radially outwards. This is again very helpful as in expanding the flow volume it's pressure is reduced. The extracted power comes from the difference between the high pressure in front of the band and the low pressure behind it, so reducing the exhaust pressure increases this differential and hence also the power.

A forward flow focusing cone and a rear flow expanding diffuser is another method of achieving this result, but by integrating the actions of a diffuser within the blade structure a more compact and efficient solution is achieved. The wavy blades in effect become a dynamic cone and diffuser, operating over a larger flow section because of their sweep area and the improved flow control.

Parameters can be adjusted to vary the degree of entrainment. The chord of the band can be increased such that a greater proportion of the blade is brought into use for radial entrainment as shown on Fig. 1 where the band chord changes between (9) - thin and (10) - wide. The concave part then does more work than the convex part therefore introducing a bias that favors entrainment. If used as a turbine and the transition point is retarded to the trailing side of the peaks and troughs then again the balance of lift vectors favors entrainment, or the airfoil itself can be adjusted such that the angle of attack or general lift effectiveness is increased in the appropriate entrainment region.

The 'spring' band can also be biased in curvature such that the coil is not round but favors in the case of a turbine the maintenance of the concave curvature of the backward facing portion (10) while the forward convex facing portion is flattened (9). The back concave curvature facing into the flow creates the low radial pressure that entrains flow whereas the forward curvature facing into the flow would otherwise serve to dispel flow radially. Consequently favoring the concave over the convex increases the net entrainment. In all cases the converse applies when working as a propeller as shown in Fig. 2 where the flow (11) is entrained by the convex blade portion (13) with the concave blade portion (14) thinned and flattened leaving the flow to be exhausted largely axially (12).

Many alternative embodiments of the band's configuration are viable. Variations in the integer number of waves or coils per rev can be accommodated. The fewer the waves or coils, the greater the amplitude or coil diameter that can be supported without unduly pinching the rate of curvature. Increases in amplitude or coil diameter are appropriate where there is a preference for a higher speed ratio, which can then be balanced by a smaller chord band and vice versa as shown in Fig 11 and from the back in Fig. 12. The band can be retained either internally or externally. Internal would be the most common preference where a series of spokes e.g. (22) can be employed to couple the undersides of the troughs to the hub. These spokes can be rigid like pylons or flexible like cables. In the case of spokes, if 2 spokes are used at the bottom of each trough it is desirable that they are arranged in pairs with a common vertex at the trough low point with the spokes connecting to the hub at an angle such that the torque component of the axial thrust on the turbine is equal and opposite to the torque generated by the band blade.

In the case of a propeller the spin can be powered from a simple core hub or similarly in the case of a turbine, energy can be extracted by a generator at the hub. Fig. 9 shows a 7 lobe band generator supported via spokes to a central hub. Fig. 10 shows two of such turbines being connected together and pivoted at their combined C of G by a suitable tower for service as a wind turbine.

Particularly in the embodiment of a fan it may be desirable for safety and aesthetics for the band to be driven externally, thereby leaving a completely empty core. To facilitate this a hoop (19) is deployed circumferential to the band with pylon like connections connecting the hoop inwards to the peaks of the waves as shown in Fig. 4. This hoop is then pinched between rollers with at least one of them powered thereby acting as a friction drive to spin the band by driving the hoop tangentially. Other embodiments see the hoop attracted magnetically to the driven member, or coupling and being driven through the use of a magnetic gear rather than friction. The chord of the band can be increased as can the hub diameter favoring marine applications as in Figs 17. The band diameter will be smaller than the equivalent propeller being an advantage in shallow water and where the propeller acts below the bottom of the hull. A further advantage for marine applications is the natural weed shedding capability. The 'screw thread' form of the band pushes back any entrained weed or lines that might otherwise tighten around the shaft. This can be enhanced by a further variation introduces a bias to the band such that the wave assumes a saw tooth form as shown in Fig 18, with the peaks displaced to one side so they do not lie centrally between the troughs. The object being to vary the rake of the blade to improve the airfoil effectiveness or weed shedding ability in the case of a marine propeller.

The band has been described as comprising of an airfoil profile following it's path with surface curvature transitions occurring at the peaks and the troughs, but in addition aerodynamic devices can be employed to further reduce the band's drag, increase the lift without flow separation and reduce the trailing edge turbulence.

Biomimetics teaches that the tubercles on the leading edges on humpback whale flippers can be mimicked on aero and hydro dynamic surfaces to reduce drag and increase the angle of attack that can be sustained without the wing stalling. A Canadian company called Whalepower Corporation have developed various embodiments of this concept but their main US Patent 6431498 is limited in it's scope by prior art. A further embodiment is herewith proposed for the leading edges of the band which promises significant benefits. The simulated tubercle features extend passed the point of maximum thickness on the band's upper surface and blend into the trailing edge. DRAWINGS

The present invention can best be understood in conjunction with the accompanying drawing, in which:

Fig.l illustrates the hypothetical flow lines of a fluid through a garter spring type band blade configured as a generator;

Fig. 2 illustrates the hypothetical flow lines of a fluid through a garter spring type band blade configured as a propeller;

Fig. 3 shows a full face view of a band blade with a radially inclined amplitude configured as a propeller and identifies the morphing airfoil section that occurs at a representative peak;

Fig. 4 shows a similar band blade mounted by retaining an externally connected ring to a driving mechanism in the manner of a fan embodiment;

Fig. 5 shows a side view of such a band, revealing it's thin profile;

Fig. 6 shows a variant of such a band with a degree of twist imposed at the peaks and troughs;

Fig. 7 shows a further variant with a higher degree of twist imposed;

Fig. 8 shows a generator configured garter spring type band with 9 orbits;

Fig. 9 shows a generator configured garter spring type band with 7 orbits and retained to a hub by virtue of spoke pairs emanating from the troughs; Fig. 10 shows two of such band blades configured as turbines connected to a common mast to provide service as a wind turbine; Fig. 11 shows a garter spring type band with 5 orbits arranged as a generator connected to a hub;

Fig. 12 shows the same generator configured garter spring type band from the rear;

Fig. 13 shows a 7 orbit propeller configured garter spring type band connected to a hub;

Fig. 14 shows the same 7 orbit band from the rear;

Fig. 15 shows a 5 orbit propeller configured garter spring type band connected to a hub;

Fig. 16 shows a propeller configured wide chord 5 wave band attached to a hub with pylons;

Fig. 17 shows a similar 5 wave band connected to a large hub in the manner of a marine propeller;

Fig. 18 shows a propeller configured asymmetric 5 wave band in the manner of a saw tooth connected to a hub with pylons.

In the drawings, preferred embodiments of the invention are illustrated by way of example, it being expressly understood that the description and drawings are only for the purpose of illustration and preferred designs, and are not intended as a definition of the limits of the invention.

PREFERRED EMBODIMENT(S) OF THE INVENTION Fig. 3 shows a wave band blade configured as a propeller. This has 9 waves, although other numbers are possible, it being preferred to have an odd number of waves in order to mitigate any weight and balance issues. In Fig. 4 an embodiment of this type of band blade is shown (23) with 7 waves with the purpose of acting as a fan.

The band blade is held by short pylons to a circumscribing peripheral ring (19). In this case the ring has small magnets embedded in it with such magnets aligned axially with alternating forward facing polarities. In the drive assembly (18), two disks are contained in a sealed box with a closed slot between them. The discs are driven by an electric motor such that when the ring is inserted into the drive cavity they overlap the ring and consequently can force its rotation when they themselves rotate. By this means the blade is caused to rotate.

Further rollers offset to either side of the drive point constrain the ring to roll about a fixed virtual axis.

The blade chord is set as appropriate for the spacing between successive blade elements of similar radial offset. Consequently as shown on Fig. 3 (4), when spinning clockwise the blade has a greater chord at (1) than at (3). Only at the mid radial offset (5) is the chord the same on both sides of the wave. Similarly at (6) the chord is greater on the left than on the right of the trough.

The angle of attack is calculated to match the anticipated flow speed at a given rpm, the helix angle being set as appropriate for the radial offset. To this setting angle is added the angle of attack for a near optimum lift/drag ratio as suitable for the chosen airfoil such that flow separation does not occur over a significant part of the upper airfoil surface which would result in a stall and a significant drop in the lift to drag ratio. The area in the centre of the band where no blades operate is nevertheless a very useful space. Firstly it avoids that region where the blades angle would be so severe that the blades would act more as paddles and generate a vortex rather than axial flow. Secondly it then provides for air to be entrained through it by the Venturi effect of the circumscribing flow. In accordance with Bernoulli's principle this reduces the flow speed but increases the flow volume, such additional volume not having been subject to any blade interaction is of a particularly smooth flow which is desirable for fan ventilation.

The wave nature of the band significantly reduces turbulence by avoiding the tip vortices that are the product of open ended propellers. The result is again a less turbulent flow with much less noise generation.

The band is lightweight and smooth edged, and with it's peripheral ring safe to the touch. This obviates the need for external protective cages as required for impeller based fans, resulting in a more elegant and efficient product solution.

The band also has limited depth, so in the embodiment of a wall mounted unit as shown in Fig. 4 it can be folded back against the wall when not in use, thereby being no more obtrusive than a picture frame.

In another embodiment as shown in Fig. 10 two garter spring style turbines as shown in close up in Fig. 9 are mounted together by a horizontal member (24) such that they can share a common slewing and tilting pivot point. The advantage of mounting two turbines side be side is that they create a natural slewing torque as the wind veers to either side. The turbine that is most upwind creates a wind shadow over the turbine most down wind, and the resulting higher thrust on the upwind turbine causes it to slew around until both turbines axes are normal to the wind direction. Furthermore if tilt about the horizontal axis connecting the turbines (24) is permitted then the turbines will also be able to slew around this if the wind approaches from a lower or high aspect as may occur if mounted on a hillside or on an escarpment or roof.

Because the spring has depth, it can generate it's own flow shadow when mounted individually rather than in a pair as previously described. The shadow reduces the thrust on the downwind portion of the blade assisting in providing a torque to slew the blade back to the optimum flow normal condition. This effect can be encouraged by permitting the spring axis of rotation to veer relative to the hub axis, however because of the impact of gyroscopic precession it is most beneficially used to augment the usual slewing axis.

The turbines will perform as per the flow simulation shown in Fig. 1 where the larger concave blade member (10) causes additional flow to be entrained from the forward direction (7) thereby reducing the amount of flow that would otherwise blow around the turbine as a result of it acting as a pressure dam as with a conventional wind turbine. The flow being exhausted from the turbine will be turned towards a blade normal direction (8) and as such will be caused to expand by the increasing cross sectional area. This reduces the pressure as per Bernoulli's principle, increasing the pressure differential between the front and the back of the turbine and hence increasing the energy that can be extracted by the blade band.

To increase the effectiveness of this process, the convex part of the band (9) is flattened backwards to reduce the otherwise compensating opposite effect and it's chord is reduced with the same result. The absence of any blade tips which would otherwise cause tip vortices makes the turbine operate more quietly and consequently also more efficiently.

The central area that is not swept by any blades has collateral advantages. It enables more of the entry flow to be entrained by the concave blade format where it does more good by contributing useful additional flow where the blades work at their best efficiency. It avoids the generation of a strong core vortex caused by the excessive pitch angle, large blade chord and low tangential velocity near the hub. It also avoids the stagnation zone that typically occurs behind the hub that compromises the exhaust flow by increasing the rearward pressure reducing the pressure differential from which power can be extracted.

A variation of the spokes arrangement creates an automatic pitch control system powered by the centripetal and axial forces acting on the turbine. The spring blade is moulded using materials with some flexibility, enabling it to both compress and expand.

In use a dynamic balance is effected between the centripetal force tending to increase the garter spring's diameter and axial thrust acting on the whole band blade tending to fold the spokes backwards and reduce it. This balance is translated into a balance in the setting angle as stretching the spring reduces it while compressing it increases it. In addition, when the spring's diameter reduces in order to maintain it's angular momentum its rate of rotation increases, just as increasing the diameter slows the rpm down. This change in rpm in the manner of a figure skater pulling in her arms to increase her spin rate can occur very quickly as the net rotational kinetic energy stays the same. A gust will therefore reduce the spring's diameter so increasing the setting angle and torque and also the rotational speed so that more power can be extracted, just as a lull will increase the diameter, reducing the setting angle and drag and also the speed so enabling more ready freewheeling without unduly loosing rotational momentum. The solution is superior to an auto pitch that merely rotates a conventional blade about it's axis as it adjusts the blade twist as well as the setting angle. It also reacts quickly to increase or slow the rate of spin. Changing the rate of spin is easier to translate into changing the power output than merely changing the torque.

Different means can be employed to effect the compression of the spring with increasing axial thrust, for example a simple crank lever with pivoted pylons arranged such that thrust loads pull the pylons inwards or inclined ramps that move a carriage supporting the spokes or pylons in towards the axis when displaced under increasing thrust load.

A further embodiment employs the torque acting on the band when driven by the flow to effect a similar reduction in the spring band's diameter, either separately or in conjunction with the axial thrust driven means.

More coils to the spring would also increases the ease with which the spring can be compressed or extended.

Other arrangements of wave or orbit numbers and permutations of design bias offsets and mounting options would be obvious to those skilled in the art and are thus considered in the scope of the present invention.

Claims

What is claimed is:

1. A fan or propeller blade comprising:

a. an airfoil with a substantial sinusoidal-like shape along a circular path about a central rotating axis; and:

b. said airfoil having an aerodynamic profile with a common radial offset such that when the band rotates about the disc's axis the airfoil presents a largely common angle of attack to the fluid flow when moving between it's amplitude peaks and troughs made possible by the fact that at both the peaks and the troughs the upper surface and lower surfaces of the airfoil morph through a largely symmetrical transition to swap sides on the band; and:

c. the angle of attack of the chosen airfoil is added to the appropriate airfoil setting with the leading edge and upper surface of the airfoil facing away from the induced flow direction.

2. A wind or water flow driven generator blade comprising

a. an airfoil with an substantial sinusoidal-like shape along a circular path about a central rotating axis and

c. the angle of attack is subtracted from the appropriate airfoil setting angle with the leading edge and lower surface of the airfoil facing the flow direction.

3. A fan or propeller comprising

a. a band with an airfoil profile that follows a continuous torroidal helical path with it's axis moving around the circumference of a virtual disc such when the band rotates about the disc's axis the airfoil presents a largely common angle of attack to the fluid flow even as it moves around it's path between it's radial maxima and minima made possible by the fact that at both the maxima and minima the upper surface and lower surfaces of the airfoil morph through a largely symmetrical transition to swap sides on the band, and:

b. the angle of attack of the chosen airfoil is added to the appropriate airfoil setting angle with the leading edge and upper surface of the airfoil facing away from the induced flow direction.

4. A wind or water flow driven generator blade comprising

a. a band with an airfoil profile that follows a continuous torroidal helical path with it's axis moving around the circumference of a virtual disc such when the band rotates about the disc's axis the airfoil presents a largely common angle of attack to the fluid flow even as it moves around it's path between it's radial maxima and minima made possible by the fact that at both the maxima and minima the upper surface and lower surfaces of the airfoil morph through a largely symmetrical transition to swap sides on the band; and:

b. the angle of attack is subtracted from the appropriate airfoil setting angle with the leading edge and lower surface of the airfoil facing the flow direction.

5. A band as in Claim 1 or 2 where the width of the band representing the chord of the airfoil varies to account for the angular displacement between successive band elements of similar radial offset such that where the displacement is large the chord is commensurately large and vice versa.

6. A band as in Claim 1 or 2 where at the peaks of the waves the band is twisted about a radial direction due to an axial displacement of the band towards the flow direction ahead of the peaks and troughs and an axial displacement away from the flow direction behind the peaks and troughs.

7. A band as in Claim 1 or 2 where the bottoms of the troughs are connected to a central hub by spokes or cables or pylons or the tops of the peaks are connected to an external ring.

8. A band as in Claim 3 or 4 where the radial minima of the coils are connected to a central hub by spokes or cables or pylons or the radial maxima of the coils are connected to an external ring.

9. A band as in Claim 4 where the rear concave part of the coil when facing into the wind has an enhanced chord and the forward convex part a reduced chord and when acting as a propeller the convex part is enhanced and the concave part reduced.

10. A band as in Claim 4 where the rear concave part of the coil when facing into the wind shows a full radial or elliptical curvature while the forward convex part is significantly flattened and when acting as a propeller the convex part retains a high curvature while the concave part is flattened.

11. A band as in Claim 2 where when acting as a generator the forward convex part of the coil when facing into the wind has an airfoil of a form and at an angle set to do less work compared to the concave part and vice versa when acting as a propeller.

12. A band as in Claims 1 and 2 where the peaks of the waves or tops of the coils do not lie centrally between successive waves or loops but are offset to one side generating a 'saw tooth' form where one side has a greater ramp angle than the other.

13. A band as in Claim 3 or 4 where the means by which the radial minima of the torroidal helix are attached to the hub is permitted to extend radially when centripetal force stretches the torroidal helix thereby causing the outside diameter to increase.

14. A band as in Claim 3 or 4 and/or Claim 13 where the means by which the radial minima of the torroidal helix are attached to the hub results in a reduction in the radial length of such means when the axial thrust loading increases thereby pulling the spring inwards causing the outside diameter of the torroidal helix to reduce such reaction overwhelming any unbalanced centripetal forces acting in the opposite direction.

15. A band as in Claim 3 or 4 and/or Claim 13 where the means by which the radial minima of the spring are attached to the hub results in a reduction in the radial length of such means when the torque loading on the spring blade increases thereby pulling the spring inwards causing the outside diameter of the torroidal helix to reduce such reaction overwhelming any unbalanced centripetal forces acting in the opposite direction.

16. A ceiling fan:

a. a ceiling mounting structure,

b. a housing

c. a motor,

d. one or more fan blades having an aerodynamic profile with a common radial offset such that when the band rotates about the disc's axis the airfoil presents a substantially common angle of attack to the air flow when moving between it's amplitude peaks and troughs made possible by the fact that at both the peaks and the troughs the upper surface and lower surfaces of the airfoil smoothly transition through a largely symmetrical transition to change sides on the band; and:

17. The ceiling fan of claim 16 where the angle of attack of the fan blade aerodynamic surface is added to the appropriate airfoil setting with the leading edge and upper surface of the airfoil facing away from the induced flow direction.

18. A tabletop fan:

a. a base structure,

b. a housing

c. a motor,

19. The tabletop fan of claim 16 where the angle of attack of the fan blade aerodynamic surface is added to the appropriate airfoil setting with the leading edge and upper surface of the airfoil facing away from the induced flow direction.