A DEVICE FOR REDUCING THE DRAG OF STRUCTURES IN A FLUID FLOW FIELD OF THE INVENTION This invention is directed to a device which can reduce parasitic drag of an elongate member which passes through a fluid flow, and particularly relates to a device which can be attached to a drag creating elongate member (e. g. a wire, rod, bar or tube) and which can reduce the drag of that member as the member passes through a fluid flow (for instance an air flow). The invention will be described with reference to a stay or strut, but it should be appreciated that the device can be used to reduce drag of other members.

BACKGROUND ART Stays and struts are used for supporting structural members such as boat masts, a plane wing (for smaller planes), transmitting masts and towers, and a host of other devices. The stays and struts are usually steel or metal wire, rods, bars or tubes and function to prevent undesirable movement of the mast, wing and the like. The stays or struts allow the mast or wing to be lighter and less structural while still providing rigidity and strength.

Surprisingly, stays and struts, even of fairly small diameter, provide significant drag when passing through a fluid flow such as air. In particular, it is found that circular wires, rods, bars or tubes have quite large drag even when the sections are fairly thin. Non circular members can have even larger drag.

For moving objects such as vessels or aircraft, drag must be kept to a minimum in order to maximise the efficiency of operation. Racing yachts are particular examples of vessels which require minimal drag.

Even in the case of ground based fixed installations such as radio masts, the drag on the supporting wire stays can induce significant additional load onto the structure during periods of high wind loadings.

It is known to reduce the drag coefficient of an elongate member such as a stay or a strut by streamlining the elongate member. It is found that streamlining greatly reduces the drag coefficient. Streamlining is achieved by

manufacturing the elongate member into an elongate cross-section which is "teardrop"-shaped. This type of streamlining is well-known. However, streamlining boat masts and stays and wing struts and stays adds to the expense of the article. As well, the article still needs to be load supporting which places limitations on the streamlining which can be achieved.

A substantial disadvantage with streamlining is that it converts the normally round stay or strut into an elongate cross-section. While a round stay or strut has a higher drag coefficient, the drag coefficient is approximately the same irrespective of the direction that the stay or strut passes through the fluid flow. However, with streamlined stays or struts, the drag coefficient is reduced only when the streamlined stay or strut is substantially in line with the fluid flow. If the streamlined stay or strut is not in line with the fluid flow, the drag coefficient increases enormously and is larger than if the stay or strut was circular.

It can be appreciated that for vessels and aircraft, it is not always possible to ensure that the streamlined stay or strut passes through the airflow at the correct angle. It is also not convenient and sometimes it is not practical to pivot the entire stay or strut in order to keep it in line with the fluid flow. To pivot large yacht mast rigging would require heavy and complicated machinery. Additionally, if the elongate member (for instance yacht mast rigging) is rather long, it is not uncommon for the wind direction to be different at different portions of the mast.

In another manner of speaking, in a dynamic situation such as on a vessel or an aircraft, airflow conditions and directions can vary considerably along the length of the strut or stay. Thus, any streamlining of the strut or stay may provide a reduction in drag along part of the stay but an increase in drag along another part of the stay. It could very well be that overall drag is increased on a streamlined strut or stay as opposed to one which is circular.

The persistence with streamlining such members is because the coefficient of drag of a streamlined member can be up to 100 times less than the coefficient of drag for a round section member. Therefore, there is a

significant advantage in streamlining such elongate members.

OBJECT OF THE INVENTION It is an object of the invention to provide a device which may overcome the abovementioned disadvantages or provide the public with a useful or commercial choice.

In one form, the invention resides in an elongate device for reducing the drag of an elongate member, the device having an outer drag- reducing surface and able to be rotatably attached at least partially about the member to present the drag-reducing surface in line with a fluid flow over the member.

By rotatably, hingedly or pivotally attaching the device to the elongate member such as a strut or stay, the device can present a drag- reducing surface irrespective of the direction of fluid flow over the elongate member.

In one form, the drag-reducing surface is of an air foil shape.

The device can be moulded or otherwise formed to a length to suit, and can be attached over the entire length of the elongate member where it is desired to reduce the drag. If the elongate member is of sufficient length, and if there is a possibility that the fluid direction will vary along the length of the elongate member, it is possible to attach more than one elongate device to the elongate member, with at least some and preferably all of the devices being independently rotatable.

In this manner, the elongate member can be formed from normal materials such as circular cross-section wires, rods, bars or tubes, and the drag-reducing device can be rotatably attached to form a drag-reducing surface which can track or pivot to stay in line with the fluid flow.

It is preferred that the device itself is non-structural and lightweight which means that it can be formed fairly inexpensively and can be then attached to a structural member such as a wire, rod, bar, tube and the like.

The size of the drag-reducing surface can be varied to suit and it is considered that a person skilled in the art would be able to develop

suitable drag-reducing surfaces.

The device can be attached to the elongate member in any suitable manner providing that a degree of rotation is provided for. If the elongate member itself can twist or rotate to a sufficient amount (for instance a swivel wire), then it may be possible for the device to be rigidly attached to the member as it will still be able to move to be in line with the fluid flow.

If the elongate member is rigid and non-pivoting, the elongate device is rotatably attached by any suitable method to the member. In one form, the elongate device can have an internal clip such as a C-clip, which can clip about the wire or other elongate member. For more heavy-duty applications, the elongate device may have a bearing arrangement to allow it to smoothly pivot and rotate relative to the elongate member.

It may be necessary to attach the device such that while it may rotate relative to the elongate member, it is restricted from sliding along the elongate member. This may be necessary if the elongate device is substantially shorter than the elongate member or if a number of devices are to be fitted. In these situations, a fixing means can be provided either on the device or on the elongate member which functions as a stop to prevent the device from undesirable sliding or other linear movement relative to the elongate member.

In another form, the invention resides in an assembly for reducing the drag of an elongate member, the assembly comprising a plurality of elongate devices, each device having an outer drag-reducing surface and able to be rotatably attached at partially around the member to present the drag-reducing surface in line with a fluid flow over the member, at least some of the plurality of devices able to rotate independently relative to each other.

The elongate devices may be separated from each other by a spacing means. The spacing means may be in the form of a collar disposed at one or both ends of the foil for holding foils mounted on a stay or strut in spaced apart relationship to one another. The collar may be formed integrally with the foil assembly or separately thereto.

The spacing means suitably includes part-spherical bearing

means centered on the stay or strut axis and disposed at each of the opposite ends of the length of foil whereby the foil assembly may engage with complementary bearing means adjacent each end thereof. The complementary bearing means may be separate bearings attached to the stay or complementary bearing means may be associated with adjacent foil assemblies. This latter arrangement provides a ball-socket type engagement between adjacent foil assemblies enabling the foils to maintain free relative rotation about the supporting stay even though the axis of the stay may be curved between its ends.

The part-spherical bearing means may each comprise a part spherical spigot portion adapted for engagement with complementary part- spherical socket bearings on adjacent foil assemblies. Alternatively, the part- spherical bearing means may comprise a spigot bearing at one end and a complementary socket bearing at the other end whereby substantially identical foil assemblies supported along a common stay may engage rotatably with one another.

The device may be attached to the member via a support means. The support means may be associated with the bearing means or be separate therefrom. Suitably, the support means includes a support adjacent each end of the length of foil and further the spherical bearing means may be formed at the outer end of each support. The support means may provide a cylindrical bearing at each end of the foil assembly through which the stay or strut to be streamlined may be fed. These could be plain or rolling element bearings.

Alternatively, the support means may be formed as open part- cylindrical bearings adapted for captive mounting about a stay and suitably the bearings are formed from resilient material for this purpose, such as polyurethane polymer. In such arrangement, it is preferred that the foil be formed with its opposed trailing walls not connected to one another such that the trailing walls may be separated for introducing a stay or strut therebetween and into the open bearings.

The drag-reducing surface may comprise an air foil or foil. The

foil assembly may be specifically formed for use on stays which are incline to the perpendicular. In such applications the foil is formed as a lightweight foil and the support means supports the foil with a balance of between 10% and 14%.

In one form, the foil has a section which conforms with a NACA (U. S. A. National Advisory Committee for Aeronautics) section and the support means supports the length of foil on a stay with a balance which does not exceed 12.5%. This balance factor will ensure that the foil weathercocks without undue fluttering. Such fluttering is friction damped to some extent by the spherical bearing engagement between the ends of the foil assemblies.

It is also preferred that foil assemblies for use on incline stays such as on yachts and hang gliders be formed in a lightweight manner and with foil lengths in the order of 300mm and suitably within the range of between 200mm and 1m.

The foil is suitably extruded from plastics material. A suitable material includes a styrene-acrylonitrile copolymer impact modified with acrylate rubber and manufactured under the trademark LURAN S by BASF Aktiengesellschaft. The foil can be formed as a thin walled section formed of plastics material and can be reinforced with carbon fibre for minimising section thickness while maintaining sufficient rigidity for prevention distortion of the air foil section. In such application, light weight is required so that the foil does not hang inward from the foil with a sufficient bias to inhibit its weathercocking ability in average or light wind strengths.

In one form, the foil section is formed from pre-impregnated epoxy resin/carbon fibre cloth about a male mould inside a split female mould and suitably a UV shielding film is bonded to the outside of the foil either during or after the moulding process so as to minimise long term degradation by exposure to the sun. However, other laminates could be used such as high modulus fibreglass cloth and polyester or vinylester resin. Alternatively, the foil assembly could be formed in a female mould from a self-skinning foam or the like. The section can be extruded from suitable material such as plastics.

In another aspect, this invention resides in an elongate member assembly which has a plurality of streamlining foil assemblies supported on the elongate member with the streamlining foils thereof supported in a non- overlapping relationship along the member, each foil assembly being supported on the member so as to weathercock to the wind passing thereacross. The foil assembly may be of any suitable form or as variously described above.

In yet a further aspect, this invention resides broadly in a method of converting a drag creating member such as a round-section elongate member having end fittings thereon to a streamlined member, including providing foil assemblies each with an open support bearing at the leading end thereof and non-connected converging rear walls through which the member may pass for captive entry to said open bearing, and spreading the rear converging walls and passing the member therebetween into captive engagement with the open bearing.

The member may be a standing member whereupon the method further includes providing the foil assemblies as relative short foil assemblies; sequentially engaging the foil assemblies over a lower accessible portion of the standing member, and elevating the foil assemblies engaged over the member to clear the accessible lower portion of the member for engaging a further or further foil assemblies with the accessible portion of the member.

BRIEF DESCRIPTION OF THE DRAWINGS In order that this invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate typical embodiments of this invention, wherein Figure 1 is a perspective view of a foil assembly according to an embodiment of the invention.

Figure 2 is a transverse cross-section through the foil assembly of Figure 1.

Figure 3 is a longitudinal cross-section through the foil assembly of Figure 1, Figure 4A is a section view of an extruded foil according to another embodiment of the invention, Figure 4B is a plan view of a leading bearing and a trailing locking cap according to another embodiment of the invention, Figure 4C is a perspective view of the bearing and cap of Figure 4B, Figure 4D is a plan view of the bearing and cap in the fastened position in the top of the foil, Figure 4E is a perspective view of an upper part of the foil of Figure 4A with the fitted bearing and cap, Figure 4F is a longitudinal cross-section view of the foil assembly of Figure 4E, Figure 5A is a section view of an injection moulded foil identical to Figure 4A, Figure 5B is a plan view of a leading bearing and a trailing locking cap and including an additional component being a split adapter to accept wires of non-circular cross section, Figure 5C is a perspective view of the bearing, adapter, and cap of Figure 5B, Figure 5D is a plan view of the bearing, adapter, and cap in the fastened position in the top of the foil, Figure 5E is a perspective view of an upper part of the foil of Figure 4A with the fitted bearing, adapter, and cap, Figure 5F is a longitudinal cross-section view of the foil assembly of Figure 5E, Figure 6 illustrates the drag-reducing qualities of a streamlined wire or rod.

BEST MODE The foil assembly 10 illustrated in Figures 1 to 3 is adapted for fitting to a stay such as illustrated at 11. For this purpose, the foil assembly

includes a streamlining thin walled foil 12, which in the illustrated embodiment conforms to NACA Foil No. 0024. This foil has a drag coefficient of about 0.008 at a 0° angle of attack, depending on the Reynolds Number.

The thin walled foil 12 is slotted at each end at its opposite sides at 13 to receive a locating protrusion 19 extending from the bearing assembly 14 which is closely accommodated within the leading portion of the foil 12. A spring wire clip 15 extends about the leading edge of the foil 12 and passes through aligned apertures in the foil. walls and the bearing assembly 14 and is located at the leading edge through the foil and into the bearing assembly 14 so as to axially locate the bearing assembly 14 relative to the foil 12.

The foil 12 is formed about a male mandrel or mould inside a split female mould from pre-impregnated epoxy resin/carbon fibre cloth such that the trailing edges 16 of the converging rear walls 17 are not bonded to one another. Thus they may be separated so as to enable the stay 11 to pass therebetween into engagement with the open bearing assembly 14.

It will be seen that the open bearing assembly 14 has a part cylindrical bearing surface 20 which extends about a major portion of the stay so as to hold the stay captive but freely rotatable therein. The open bearing assembly 14 has lead-in portions 18 which taper rearwardly from the bearing surface 20 so as to enable the trailing ends of the bearing to be forced apart to permit the stay 11 to pass into captive engagement therewith.

In this embodiment, the open bearing assembly 14 is formed from a polyurethane polymer.

Referring to Figure 3 in particular, it will be seen that the open bearing assemblies 14 at the opposite ends of the foil are substantially identical and that each has an underside face provided with a part-spherical socket 21 and a top face provided with a complementary part-spherical spigot 22. End stops 25 adapted for clamping to the stay 11 at the top and bottom thereof are also provided with similar complementary socket and spigot formations 21 and 22 as illustrated.

It will be seen that the socket 21 extends into the underside face of the foil 12 whereas the spigot 22 extends upwardly beyond the upper edge

of the foil 12. Thus, engagement between the upper protruding spigot 22 of one foil assembly 12 into the socket 21 of an adjacent foil assembly 10 supported on the common stay 11, spaces the adjacent edge upper and lower edges 31 and 32 of the foils 12 apart such that they will not foul one another during independent rotation about the stay 11, even with a slight bowing of the stay 11 which will occur in use.

The spherical complementary bearings 21 and 22 permit automatic realignment of the adjacent foil assemblies 10 without adversely affecting the ability of one foil assembly to rotate freely on the upper bearing of a lower foil assembly. Accordingly, each foil assembly 10 is able to weathercock to the breeze passing across that particular foil assembly independent of the adjacent foil assembly 10.

In use, the direction of the wind passing across the foil assemblies 10 will vary considerably throughout the length of the stay 11 as some foils 12 are close to the deck of a vessel whereas other foils may be further from the deck but closer to the sail. These position variations will be accompanied by changes in wind strength and direction. Thus, all the foils will weathercock independently so that at any instant they will have a different inclination to the fore and aft axis of the vessel.

This arrangement will reduce the drag over the length of the stay. The foils will also automatically weathercock when the vessel changes tack or its heading so as to provide a reduction in the drag caused by the stays.

Figures 4A-4E illustrate an arrangement according to an alternative embodiment, which conforms to NACA foil No. 0030. This foil has a drag coefficient of about 0.009 at 0° angle of attack, depending on the Reynolds Number. In this arrangement, foil 40 is extruded from Luran S (trade mark of BASF) and which is a styrene-acrylinitrile copolymer impact modified with acrylate rubber. Foil 40 can be extruded and can then be cut to length. The foil is extruded with four internal longitudinal channels 41 the function of which are to accept fasteners 48 (see Figures 4C and 4D). Foil 40 has two free ends 42,43 which can be prised apart to open the foil thereby

allowing it to be fitted about a wire stay 44 (see Figure 4F). However, the foil is extruded in a manner such that ends 42,43 are normally against each other or even biased against each other.

A bearing 45 is similar to that describes above in the sense that it has a semi-circular channel 46 which is dimensioned to accommodate stay 44 and to allow the bearing to pivot about the stay. Bearing 45 has two fastening apertures 47 which allow fasteners 48 to screw the bearing into the end of foil 40.

To lock the two ends 42,43 together after the foil has been placed about a stay 44, a locking cap 49 is provided. Locking cap 49 has two apertures 47 which accept fasteners 48 to screw the locking cap into the end of foil 40 with fasteners 48 engaging into channels 41. Once the bearing and locking cap have been screwed in place, the ends 42,43 of the foils are locked in the manner illustrated in Figures 4A.

The channel 46 forming part of bearing 45 is not entirely straight. Instead, the channel slightly curves inwardly at the upper and the lower open ends of the channel. This is better illustrated in Figure 4F where bearing 45 has a channel which curves at 50 and 51. This curvature prevents locking of the bearing against the stay (and therefore loss of pivotal movement) if the stay is flexible (i. e. a wire).

Figure 4F illustrates an assembly of two foils 40A, 40B each foil being pivoted to a common stay 44 and each stay being pivotable relative to the other foil. The longitudinal channels 41 in each foil is illustrated as is that end cap 49 which is fastened to each open end of the foil. In Figure 4F, three bearings 45 are illustrated. The bearings consist broadly of two different types being a top bearing having a convex upper surface 52 and a bottom bearing (fixed to the bottom of each foil) and which has a concave surface 53.

This allows the bearings to engage and rotate with respect to each other. By having the top bearing on each foil convex, any moisture that passes between the foils will run off the convex surface of the bearing and will not pool.

The foils are fixed in position by end stops 54 which can be

clamped to the stay.

Figures 5A-5F are similar to Figures 4A-4F except that the design of the bearing is changed slightly, and an additional split adaptor 56A and 56B is provided. Bearing 57 is similar to the bearing described above except that its channel 58 is not curved and its top is not concave or convex.

A split adaptor comprising two parts 56A, 56B has an internal configuration which is square when the two parts are fitted together. This allows the bearing 57 to pivot about a stay 59 which is of square cross-section (see Figure 5E). The adaptor attaches to the square cross-section stay and the adaptor does not pivot relative to the stay. However, the adaptor has a rounded outer body which when fitted to bearing 57 allows the bearing to pivot about the fixed adaptor. The fixed adaptor is able to slide along the stay 59. This allows pivoting to be achieved around stays which are non-circular, and various different types of adaptors can be made to accommodate various different stay or structure cross-sections. The foil is unchanged as is the locking cap 49.

The end stops 54 have a split nut design (illustrated in 4D and 5D) comprising a larger overcentre nut portion 60 and a smaller wedge- shaped nut portion 61 which are screwed together through fasteners 62 to clamp the stops to the stay.

Figure 6 shows the great advantage in streamlining a drag creating elongate member. Figure 4 is a particular example showing drag (Y axis) and air flow velocity (X axis) for a bare wire (drag coefficient 1.45), a bare rod (drag coefficient 1.2), a bare square cross-section stay (drag coefficient 2.0), a fixed non-feathering foil (NACA-0030 foil drag coefficient approx 0.2 with an angle of attack of 24°), and a feathering foil over the wire, rod, or square (NACA-0030 foil drag coefficient of approximately 0.009 with an angle of attack of 0°). The wire or rod has a diameter of 11 mm which is typical for use to support a large yacht mast. Figure 6 shows that the drag per foot length of the bare stay becomes quite large as the air flow velocity rises to over 30 or 40 knots. A fixed non-feathering foil does not substantially reduce the drag at normal angles of attack of the wind. In contrast, the solid

line illustrates the same wire, square, or rod which now has a pivoting air foil attached to it. The distinct advantage of having a drag-reducing foil on a wire or rod is clearly illustrated in Figure 6.

It will of course be realised that the above has been given only by way of illustrative example of the invention and that all such modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as is herein set forth.