FIELD OF THE INVENTION

This invention relates to a variable contouring foil for use in a fluid environment. More specifically, this invention relates to a foil whose tip and trailing edge are flexible to allow twist of the tip and trailing edge in response to pressure differences induced by fluid flow along the surface of the foil.

BACKGROUND OF THE INVENTION As a foil passes through a fluid environment, various forces exert an influence upon the foil. In order to improve the performance of a foil, either these various forces must be eliminated or at least reduced and/or the foil must be able to quickly adapt to the changing forces. In particular, the performance of a foil is reduced by non-laminar flow of the fluid over the exterior surface of the foil. In addition trailing edge vortices, which arise from the passage of a foil through a fluid environment, also result in decreased performance of the foil. Several designs have been developed in the past which seek to increase the performance of a foil by allowing for a temporary structural deformation of the foil. Typically, the foil is provided with a flexible outer skin. The outer skin of each side of the foil is movable from a first position outwardly to a second position whereby the curvature of the side of the foil is increased. Various means have been utilized to affect such a change in profile. For example, United States Patent No. 4,074,646 discloses a double action hydraulic cylinder to affect such a change. Alternately, United States Patent No. 4,538,539 discloses an air bladder internal of the exterior skin which may be inflated by suitable hydraulic means.

United States Patent No. 4,341,176 discloses an air foil that is designed to produce lift which is automatically reversed by changing the angle of incidence

of the wind upon the foil thus resulting in lift in the opposite direction. According to this disclosure, an air foil is provided which has a solid internal structure which extends the entire width of the foil. A flexible moveable skin is provided on each side of the foil. A series of moveable bars is utilized to pivotally connect a portion of the flexible skin on one side of the frame with the respective portion of the flexible skin on the other side of the frame. United States Patent No. 4,325,154 discloses a surfboard fin. The fin is a hollow body comprising two opposed shaped thermoplastic members. The thermoplastic members are shaped so as to provide a low drag profile. United States Patent No. 4,537,143 discloses a fin comprising two flexible side walls which surround a rigid centre wall. According to a preferred embodiment of this design, vacuum pumps are provided along the trailing edge of the fin. As the fin travels through water, the vacuum pumps affect a change in the profile of the sidewalls of the fin.

According to some of these designs, the structural deformation of the foil is affected by human, servo-mechanical or computer controlled infrastructures. Further, all of the fins have sidewalls which deform to change the profile of the fin as it passes through a fluid environment or are designed to provide reduced drag.

It is an object of the present invention to provide a foil which encourages the maintenance of laminar flow of fluid as it passes over the surface of the foil. It is also an object of the present invention to eliminate, or reduce, trailing edge vortices. It is a further object of the present invention to provide a foil which has increased off-design performance and reduced incidence of stall. Further, it is also an object of this invention to provide a foil having reduced flow noise which is produced by the fluid flowing by the foil.

SUMMARY OF THE INVENTION

According to this invention, a variable contouring foil is provided for use in a fluid environment. The foil has a leading edge and a trailing edge. Both of these edges extends longitudinally to join at a tip. The foil comprises a rigid internal spar which is adapted to inhibit permanent deformation when acted upon by forces induced by fluid flow along the surface of the foil and a di ensionally stable flexible outer skin. The spar is located adjacent the leading edge of the foil and extends towards the tip and trailing edge of the foil. The spar terminates a sufficient distance from the tip and trailing edge to allow the trailing edge and tip to twist in response to pressure differences induced by fluid flow along the surface of the foil.

It has surprisingly been found that a foil having significantly improved performance characteristics may be obtained by allowing the trailing edge and the tip of the foil to temporarily deform due to pressures applied during the passage of the foil through the fluid environment. This is achieved by designing the spar so that it does not extend all the way to the tip and the trailing edge of the foil but is recessed from these points. The spar provides sufficient internal rigidity to the foil to prevent permanent deformation of the foil due to pressures applied during the passage of the foil through the fluid environment. Without being limited by theory, it appears that the recessing of the internal spar from the tip and the trailing edge permits the tip and trailing edge to twist and camber to provide a variable geometry which improves the laminar flow of fluid past the foil and reduces trailing edge vortices.

According to an alternate embodiment of this invention, the foil is not generally triangular in shape but has first and second ends each of which extends between the leading edge and the trailing edge. Each of these ends is adapted to be secured to a housing so that

both the first end and the second end are fixed in position. According to this embodiment, the spar is located adjacent the leading edge of the foil and extends towards the trailing edge of the foil. The spar terminates a sufficient distance from the trailing edge to allow the trailing edge to twist in response to pressure differences induced by fluid flow along the surface of the foil.

The spar may be made of a solid material such as aluminum. According to a further embodiment of this invention, the aluminum spar may contain a plurality of longitudinally extending titanium rods. The spar may be prepared by placing preformed titanium rods in a mould and then pouring the molten aluminum into the mould. Surprisingly, it has been found that a strong bond is' obtained between the aluminum and the titanium according to this process. When prepared in this manner, it has been found that the titanium is not embrittled by contact with molten aluminum. Further, it has also been found that stress cracking from the thermal shock is not evident in the titanium. Further, no blows are formed. These and other advantages of the instant invention will be better understood in reference to the description of the preferred embodiment and the following drawings of the said invention in which:

Figure 1 is a front view of a foil according to the present invention in which the exterior surface of the spar is shown in a dashed outline.

Figure 2 is a cross-section of the foil of Figure 1 along line 2-2.

Figure 3 is a side view of a foil according to a second embodiment of this invention in which the spar is shown in dashed outline.

Figure 4 is a longitudinal cross section of a spar of a foil according to a third embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A foil 10 is provided comprising an internal spar 12 and a flexible exterior skin 14.

The foil may be of any particular design and may be for use in any fluid environment. In particular the foil of this invention may be used either in a gaseous environment or in a liquid environment. As will be appreciated, the shape and size of the foil will vary depending upon the fluid dynamics and requirements of the device in association with which the foil is to be utilized. The foil which is disclosed herein may be readily adapted to any desired size or shape. Accordingly, the device of the instant invention may be used to replace any existing foil used in a marine environment, such as a foil, centreboard, keel or rudder for a boat or other marine vessel. Alternately, the foil of the instant invention may also be utilized as an air foil.

Preferably, the foil is generally triangular in shape having a leading edge and a trailing edge which meet at a tip. At the ends of the leading and trailing edges distal from the tip, the foil is attached to the device in association with which it is to be used. Preferably, the foil is for use in an aqueous environment and, more preferably, it is for use as a foil on a marine vessel such a sailboard foil.

In particular, the foil shown in Figure 1 is designed as a fin for use with a sailboard. The foil includes a head 16 which is designed to ensure mating engagement with the sailboard. The specific shape of the head is determined by the requirements of the sailboard to which the foil is to be attached and does not form part of the instant invention. Leading edge 20 of the foil is arcuate in shape. The leading edge curves downwardly and rearwardly towards the tip 24 of the foil. Trailing edge 22 of the foil is slightly convex in shape. As is known by those skilled in the art, the specific shape of the leading and trailing edges may vary greatly and, in fact.

the tip may be located substantially rearwardly of the head.

Internal spar 12 is located adjacent leading edge 20. Spar 12 extends towards trailing edge 22 and tip 24 of the foil and has a leading edge 30, a trailing edge 32 and a tip 36.

Spar 12 is a rigid member which is adapted to inhibit permanent deformation of the spar when the spar is acted upon by transient forces induced by the flow of fluid past the foil. As such, the material from which the spar is manufactured and the thickness thereof will vary depending upon the strength and nature of the transient forces which act upon the foil and the desired thickness of the foil. The spar of Figure 1 is used with a sailboard and, for this particular use, the spar is also preferably adapted to withstand forces that are not caused by fluid flow but which may, in any event, act on the foil as it passes through the fluid environment such as the impact from the foil striking a rock or the force of an impact when the sailboard runs aground. In view of these forces which a sailboard fin should withstand, the sailboard industry typically requires sailboard fins to pass impact test DIN 7873 and vibration test DIN 7873, Teil 4. In the case of a foil for use in a marine environment, the spar is preferably made of a suitable material which has high tensile strength, high tensile elongation, high flexural strength and a high flexural modulus. Preferably, the spar will also be made of a light weight material.

Depending upon the application, the spar may be made from a variety of materials. In addition, the spar may also be made from multiple layers of material or different materials. For example, the spar may be made from metal or metal alloys such as aluminum, titanium or stainless steel, high molecular weight plastics such as polycarbonate, polybutylene terephthalate or polyphenylene

sulphide or fibre reinforced plastic materials such as glass or carbon reinforced plastic or epoxy resins. The polybutylene terephthalate and the polyphenylene sulphide may optionally be glass filled. If the foil is to be used in air, then the spar may be made of wood in some applications.

The spar provides rigidity to the leading edge of the foil. Further, by making spar 12 and head 16 as a one piece unit, the mounting of the foil to a sailboard is facilitated.

As mentioned above, the spar extends towards tip 24 and trailing edge 22 of the foil. The spar is located adjacent leading edge 20 so as to reinforce the leading edge. Accordingly, the leading edge is rigid so as to decrease friction as the foil passes through the fluid environment. While the specific distance will vary with different applications, in the case of a sailboard foil, the leading edge of the spar is preferably no more than about 5 mm from the leading edge of the foil. Preferably, the leading edge of the spar parallels the leading edge of the foil.

The spar terminates a sufficient distance from the tip and the trailing edge of the foil to allow the tip and trailing edge to twist or flex in response to transient pressure differences induced by fluid flow along the surface of the foil. The extent to which this spar will extend towards the tip and towards the trailing edge will vary depending upon a number of factors including the strength of the transient forces acting on the foil, the physical properties and thickness of the flexible skin, and the geography of the spar. If the spar does not extend sufficiently towards the trailing edge, then the tip and trailing edge will be too flexible. This will result in increased turbulence as the foil passes through the fluid environment and, accordingly detracts from the benefits arising from the instant invention. In the case of a sailboard foil, the distance along the leading edge of the

foil from the tip to a point opposite the termination of the leading edge of the spar is preferably about 2.5 cm. In addition, the distance between the leading edge of the spar 30 and the leading edge of the foil 20 is less than the distance between the trailing edge of the spar 32 and the trailing edge of the foil 22.

In the preferred embodiment shown in Figure 1, the leading edge of the spar, 30, is arcuate in shape and is generally parallel to the leading edge of the foil. The trailing edge of the spar, 32, may be generally concave in shape such that (1) the distance between the trailing edge of the foil 22 and the trailing edge of the spar 32 is less adjacent head 16 or tip 24 then the distance between trailing edge 22 of the foil and trailing edge 32 of the spar at the centre portion of the foil, and (2), the distance between the leading edge of the spar 30 and the leading edge of the foil 20 is less than the distance between the trailing edge of the spar 32 and the trailing edge of the foil 22. The material from which the flexible skin is made will depend upon the environment in which the foil will be used as well as the specific application for which the foil is designed. The skin may also be made from layers of one or more materials. The material must be flexible. Otherwise, after repeated use, the skin will crack and flake away. Further, that flexibility must be maintained over the temperature range in which the foil may be used. For example, in the case of a sailboard foil, the skin must be flexible at least over the range 0-30°C. The skin is in contact with the spar over the entire length of the spar. Further, the skin should be substantially dimensionally stable when the foil is in use. In other words, under dynamic loading conditions, the skin should remain substantially in the same position relative to the part of the spar with which it is in contact when the foil is not subjected to loading. Thus, the skin will not substantially slip over the spar under

loading since this would result in a deterioration of the performance characteristics of the foil.

The spar may be encased in the flexible skin by any suitable means which will provide intimate contact between the skin and the spar. The foil may be prepared by placing the spar in a mould to which a suitable plastic is added. The foil may then be prepared by injection moulding. Further, as discussed below, the dimensional stability of the foil may be improved if the surface of the spar is rough, such as is obtained when the spar is prepared by sand casting. Alternately, an adhesive could be used. The dimensional stability of the foil may also be increased by using a material having increased internal strength. As will be appreciated, since the skin is dimensionally stable, the skin must be flexible to absorb compressive and tensional stresses to which it is subjected.

The flexible skin is preferably made from a material having high strength and stiffness and high abrasion resistance. Preferably, the skin is made from an energy absorbing plastic material. The foil may be made from an ultra high molecular weight polyethylene or a modified polycarbonate. Further, certain nylons, epoxies and thermoset and thermoplastic polyesters may also be utilized.

According to the preferred embodiment described herein, the foil is for use with a sailboard. Accordingly, the flexible skin must be water resistant and also scratch resistant. When used as a sailboard foil, it is preferred that the foil is made of a polyurethane and, in particular, a urethane elastomer. Preferably, the urethane has a hardness from about 70 to about 90 Shore D, and more preferably, from about 70 to about 75 Shore D. A suitable material is ADIPRENE L-325™ which has the following properties:

Hardness, Durometer D 72

100% Modulus", psi [MPa] 3750 [25.8]

Tensile Strength, psi [MPa] 8800 [60.6]

Elongation at Break ^b , % 260 Bashore Rebound, % 58

D-470 Tear, pli [MPa] 112 [19.5]

NBS Abrasion Index 425

Compression Set, %

Method A, 1350 psi [9.3 MPa] load After 22 hours at 158°F [70°C] 10

Flexural Modulus (AST D-790), psi [MPa] 85000 [586]

Heat Distortion Temperature, F [C°] 293 [145]

Izod Impact Strength ⁸ , Notched,

N-m/m[lbf-ft/in] 640 [12] Solenoid Brittleness Temperature Lower than

-94°F [-70°C]

Specific Gravity 1.20 a: ASTM D256 b: obtained at an extension rate of 2.54 cm/min.

The thickness of the skin may be about 1 mm and, in the case of a sailboard foil, preferably about 1-2 mm. The thickness will vary depending upon the material which is used. The thickness of the flexible skin is preferably increased where internal spar 12 meets head 16. Flexible skin 14 may be thickened at this point so that head 16 and the exterior surface of flexible skin 14 forms a smooth surface as indicated at position 34 in Figure 2.

The construction of the foil according to this invention allows the foil to react to variations in fluid flow along the exterior surface of the foil. The fluid flow induces tip twist and the cambering or warping of the foil along its cord length. The combination of these two abilities effectively enables twist of the trailing edge along the length of the foil. A foil constructed according to this teaching demonstrates improved performance. The construction results in increased laminar flow of fluid along the surface of the foil and a reduction in trailing edge vortices. The foil has increased off-design performance, reduced incidence of stall and a reduction in flow noise.

As discussed above, the flexible coating may be affixed to the spar by any suitable means. When the spar is made by sand casting, it has been surprisingly found that the surface is sufficiently rough so that the flexible coating will adhere to the spar and remain in intimate contact therewith when the foil is subjected to dynamic loading.

In a further alternate embodiment, the spar may be internally reinforced. For example, the spar may itself have an insert. This insert may have a plurality of perforations. The spar may be prepared by placing the insert into a mould and then moulding the spar. Alternately, as shown in Figure 4, spar 12 may contain a plurality of longitudinally extending titanium rods 38. A variety of rod sizes and rod spacings may be employed. The specific diameter and spacing which is employed will vary depending upon the degree of reinforcement which is required. The titanium rods may be spaced approximately a half inch apart and may have a nominal diameter of about three millimetres.

The spar may be prepared by placing titanium or titanium alloy rods in a mould. Typically the titanium will extend outside the mould. The molten aluminium is then poured into the mould where it cools. Once the aluminium has cooled sufficiently, the spar is removed from the mould and the ends of the titanium rods may be cut and sanded as needed. Contrary to expectations, it has surprisingly been found that when prepared in this manner, there is a strong bond which forms between the titanium and the aluminum. The contact of the cold titanium with the molten aluminum does not result in embrittlement of the titanium, the formation of blows or stress cracking. The titanium is preferably not preheated prior to its placement in the mould. Further, the titanium should be cleaned prior to placement in the mould such as by solvent wiping.

In a further embodiment, the foil may have first

and second ends. In such an embodiment, the foil is not generally triangular in shape. Each of the first and second ends extends between the trailing edge and the leading edge of the foil and is adapted to be secured to a housing so that the first and second ends are fixed in position. In such an embodiment, the spar would be located adjacent the leading edge of the foil so as to reinforce the leading edge of the foil. The spar would extend towards the trailing edge of the foil and terminate a sufficient distance therefrom to allow the trailing edge to twist in response to transient pressures induced by fluid flow along the surface of the foil.

While specific embodiments have been described, it will be appreciated by one skilled in the art that many modifications may be made without departing from the true spirit and scope of the invention. In particular, it is noted that the specific profile of the foil, both the curvature of the leading and trailing edges as well as the leading and trailing edges of the spar, may be adapted depending upon the particular use for which the foil is designed. The foil of the instant invention may be used to replace any existing foil. When used in such a manner, the specific shape and thickness of the spar as well as that of the flexible coating may be modified depending upon the forces which act upon the spar and the foil.