Patent Description:
Rotor sails of this type, as for example disclosed in <CIT> are also known as Flettner rotors. Known such rotor sails are typically made as a cylindrical sleeve which forms the rotor. This sleeve is adapted to spin on a static tower. Upper and lower bearings locate the rotor on the tower. Wind loads act on the rotor, typically seen as a reduction in air pressure on one side of the rotor known as the suction side. The air pressure distribution has two main effects structurally:.

In the life of a rotor sail, major stresses on the rotor will fluctuate or even reverse with each revolution. The number of revolutions in the life of a rotor is very large, of the order of billions. This means that known sail rotors are made from a material that is resistant to fatigue failure. Laminated composite materials of continuous glass fibres or carbon fibres in a polymer resin are suitable for this application.

In order to provide strength to the rotor, the fibres of the composite material used to make the cylindrical sleeve should be aligned with the principal stresses on the sleeve during use of the rotor. This is because it is the fibres of the composite material that will provide the most strength to the rotor as a whole.

In known rotor sails, around <NUM>% of the total strength will need to be longitudinal, in other words aligned with the rotor axis, and around <NUM>% of the total strength will need to come from approximately circumferentially orientated fibres, i.e. fibres which are perpendicular to the rotor axis.

The remaining material provides resistance to in-plane shear stresses although the in-plane shear stresses in a rotor sail are relatively small due to the inherent shear and torsional resistance of a large diameter tube of the type forming part of a rotor sail.

A further requirement is that the circumferentially orientated fibres should be as far as possible from the midplane or neutral axis of the laminate material forming the cylindrical sleeve. Such an arrangement will provide optimum bending strength in the circumferential direction. This helps to resist the local bending moments tending to distort the circular cross-section.

Bending stiffness in the circumferential direction is also beneficial in order to resist buckling of the cylinder.

The cylindrical sleeve is more liable to buckle in the circumferential direction than in the axial direction because cylinders are naturally more resistant to buckling axially due to the curvature of the surface of the cylinder.

In known rotor sails it is known to add a foam core to the laminate material forming the cylindrical sleeve in order to achieve appropriate distancing of the circumferential fibres from the neutral axis of the laminate forming the cylinder. Such a foam core may be added to the middle the laminate to form a sandwich type configuration.

Both the in-plane axial stresses and the circumferential bending stresses on laminate forming a known rotor will fluctuate with every revolution of the rotor. A large rotor sail will spin at typically up to <NUM> rpm. This means that throughout the life of such a rotor sail, which is typically <NUM> to <NUM> years, the laminate forming the rotor may be subject to the order of <NUM> to <NUM> billion fatigue cycles.

For glass fibre / epoxy laminates, the fatigue strength at a billion cycles is only around <NUM>% of the static stress. For carbon fibre / epoxy laminates, the fatigue strength may be around <NUM>% of the static strength.

The demands on the laminate of a rotor sail are therefore considerably more onerous than the demands on other large composite structures such as boat hulls, wind turbine blades, aircraft wings, pressure vessels, pipes or water tanks.

Because the demands on a laminate when used to make a rotor sail are considerably different and more onerous than the demands on other large composite structures, known methods of making suitable composite structures may not therefore be appropriate with respect to rotor sails.

One known method of making composite materials is known as pultrusion. This method is suitable for making straight tubes of circular or any other hollow or solid cross section in a single operation. Pultrusion is a low-cost process because it is automated and the raw materials are in their simplest form (liquid polymer resins and tows of glass or carbon fibre) which are used straight from a bobbin on which the tows are wound.

Tension is used to pull a profile through a dye. This means that fibres are straight, which maximises the compressive strength of a cured material forming a straight tube.

For many uses, compressive strength is often a main consideration when designing a structural part from composite materials and therefore pultrusion is often an appropriate method to use. However, pultrusion is most practical for small cross sections or long production runs. Pultrusion becomes more expensive for larger diameter tubes because it is necessary to use a larger dye and to exert greater pulling forces.

Still further, pultrusion is not suitable for a forming a composite material that is required to withstand circumferential stresses, unless off-axis fibres are incorporated, which adds cost to the process.

Another known method involves winding pre-impregnated tape or fibre ("prepreg") around a mandrel. This process can be automated to minimise labour costs but is nevertheless expensive due the high cost of the prepreg material and the high temperatures required for curing the material.

Another known method for forming composite materials is resin infusion, also known as VARTM (Vacuum Assisted Resin Transfer Moulding). This method is reasonably economical, with material costs driven down by the high-volume wind turbine blade industry.

However, the process of laying up the material into a mould and applying the vacuum consumables is labour intensive and difficult to automate. Furthermore, the vacuum consumables represent a wasted cost as they are typically not reusable or recyclable.

In addition, resin infusion is not suited to the formation of a complete tubular structure. Because of this a tubular structure formed using resin infusion will typically be formed from two or more pieces. These pieces then need to be bonded together after the material has been cured. The bonding operation adds a process step, and therefore increases the time required and the costs.

Another known method for forming hollow tubes from a composite material is filament winding. This method is economical, particularly because it can be almost entirely automated thus reducing labour costs. In addition, similarly to the pultrusion method, the materials used are in their simplest form. The materials are potentially in the form of liquid polymer resin and tows of glass or carbon fibre are used straight from the bobbin storing such fibres.

Filament winding is not suitable for making composite structures in which the majority of fibres are orientated axially along the tube formed from the winding process.

For these reasons, it is known to manufacture rotor sails using the resin infusion method. A typical known rotor sail made using the resin infusion method will use a composite material having a sandwich construction with a foam core in the middle of the composite structure. The foam core will separate the outer layers to provide the required bending strength in the circumferential direction. Disadvantages of this method include the following:.

With rotor sails, the typical operating strains need to be kept to around <NUM>% or less in both the axial and circumferential directions in order to achieve sufficient fatigue life.

There is therefore a need for an economical method of forming a composite material to form a rotor sail which has the required axial and circumferential strength.

According to a first aspect of the present invention there is provided a method of manufacturing a rotor body forming part of a rotor sail, the method comprising the steps of: winding first fibres around a mandrel to form a tubular first skin forming a rotor tube and having a tube axis; forming a plurality of strips from second fibres; attaching the strips to a surface of the first skin such that at least some of the second fibres extend axially along the rotor body.

The tubular first skin forms the outer cylindrical sleeve of a rotor sail. Because the method of filament winding is used, the cylindrical sleeve can be manufactured in one piece circumferentially. This means that it is not necessary to use additional steps to bond separate portions together to form the tubular shape.

The method of filament winding is economical, not least of all because it can be automated.

The method comprises the additional step of forming a plurality of strips made from second fibres. The fibres in the plurality of strips extend axially along a rotor body.

The strips are attached to a surface of the first skin.

The strips therefore provide the axial strength required for rotor sails.

The inventors have realised that by forming the first skin using a filament winding method and then attaching strips to a surface of the first skin, wherein the strips are formed from fibres at least some of which extend axially along the rotor body, both circumferential and axial strength is provided to the rotor body.

By means of embodiments of the invention, therefore, a cylindrical sleeve for a rotor body for a rotor sail may be made at relatively low cost.

In embodiments of the invention, the method comprises the further step of impregnating the fibres with resin before winding the fibres around the mandrel.

In embodiments of the invention, the first fibres are wound around the mandrel such that the orientation of the first fibres is between <NUM>° and <NUM>° to the tube axis. In other embodiments of the invention, the fibres are wound such that their orientation is between <NUM>° and <NUM>° to the tube axis.

Because the first skin is formed by winding fibres or material around the mandrel, the first skin may be made to have any desirable thickness.

In another embodiment of the invention, the step of winding the first fibres around the mandrel comprises the step of winding a fabric formed from the first fibres around the mandrel.

In such embodiments of the invention rather than using a filament winding method, a material formed from warps and wefts, which the warps and wefts are appropriately orientated may be wound around the mandrel rather than winding tows of fibres around the mandrel.

In embodiments of the invention the first skin has a thickness of between <NUM> and <NUM>, and a diameter of between <NUM> and <NUM>.

The strips may be formed by any desirable method and in the embodiments of the invention, the strips are formed using a pultrusion process.

In such embodiments of the invention, the rotor body may be made particularly efficiently, since both the filament winding method and the pultrusion method may be automated.

As is well known in the art, in a pultrusion method, material is pulled through a dye as it is extruded through the dye. This has the advantage of orientating the fibres axially along the strip. A strip formed from such a method is known as a pultrusion.

Using a pultrusion method means that the strips may be formed to have any desirable dimensions, and in some embodiments of the invention the strips have a thickness of between <NUM> and <NUM>.

Both the first skin and the pultrusions may be made to any desired length.

In embodiments of the invention, the strips are attached to an outer surface of the first skin.

The strips may be attached by any convenient method. In embodiments of the invention, the strips are attached to the outer surface of the first skin by pressing the strips onto the first fibres before the resin around the first fibres has been cured.

In such embodiments of the invention, the uncured resin will flow around the strips and will bond the strips to the outer surface of the first skin when cured.

In some embodiments of the invention, the strips may be attached to the outer surface of the first skin such that they are spaced apart from one another. In other embodiments of the invention, the strips may be placed on the outer surface of the first skin such that adjacent strips abut one another axially when in position.

The first fibres may comprise glass fibres, and the second fibres may comprise glass fibres or carbon fibres.

In embodiments of the invention, the method comprises the further step of forming a second skin by winding third fibres around the strips.

In such embodiments of the invention, the first and second skins together form the rotor tube.

In embodiments of the invention a method of filament winding is used to wind the third fibres around the strips in a similar manner to the method used to wind the first fibres around the mandrel to form the first skin.

In embodiments of the invention, the third fibres are wound around the strips such that the orientation of the third fibres is between <NUM>° and <NUM>° to the tube axis. In other embodiments of the invention, at least some of the third fibres are wound such that their orientation is between <NUM>° and <NUM>° to the tube axis. In some embodiments of the invention, one or more outer layers of the third fibres is wound such that the fibres are oriented at close to <NUM>° to the tube axis, preferably between <NUM>° and <NUM>°; this provides a smoother surface and greater consolidation pressure on the fibres beneath while the resin cures.

In embodiments of the invention, the method comprises the further step of impregnating the third fibres with resin before winding the third fibres around the strips.

In embodiments of the invention, the method comprises the further step of holding the strips in place until the third fibres have been wound around the strips.

In some embodiments of the invention, the step of holding the strips in place comprises the steps of applying one or more straps around the strips, which one or more straps are unwound as the third fibres are wound around the strips.

In such embodiments of the invention, the strips may be attached to an outer surface of the first skin, and an inner surface of the second skin to form a composite material in which the strips are sandwiched between the first and second skins.

In some embodiments of the invention, the strips are attached to both the first and second skins by applying pressure to the first and/or second skins before the skins have cured.

In embodiments of the invention, the resin will flow round the strips and once cured will attach the strips to the first and second skins.

The third fibres may be formed from any convenient material, and in embodiments of the invention the third fibres comprise glass fibres.

In embodiments of the invention, the step of forming the strip comprises the step of forming hollow strips. This increases the thickness of the strips without adding weight, hence enabling the skins to be spaced further apart.

In other embodiments of the invention, the method may comprise the step of attaching the strips to an inner surface of the first skin.

In such embodiments of the invention, the rotor body comprises the first skin and the strips only, and the rotor body does not comprise a second skin.

An advantage of such embodiments of the invention is that fewer process steps may be required in order to make the rotor body.

In such embodiments of the invention, the number of strips attached to the inner surface of the first skin may vary axially. In other words, there may be more or fewer strips along parts of the rotor body. This allows for a variation in the strength of the rotor to suit the variation in bending moment along the length of the rotor body.

In embodiments of the invention, the rotor body may have a length of between <NUM> and <NUM>, although it may have any desirable length.

In such embodiments, the rotor body may be formed from a plurality of rotor tubes, each of which rotor tubes having a length of between <NUM> and <NUM>.

In such embodiments of the invention where the rotor body comprises a plurality of rotor tubes, the rotor tubes may be joined together by any desirable method to provide a rotor body having the desired length.

In some embodiments of the invention the strips may be of the same length as the rotor tubes, whilst in other embodiments of the invention, such as embodiments where the strips are attached to an inner surface of the skin, and there is no second skin, the strips may be longer than the rotor tubes. In such embodiments of the invention, the strips will span across joints between adjacent rotor tubes, thus adding strength to the rotor body.

According to a second aspect of the present invention there is provided a rotor body forming part of a rotor sail and comprising: a tubular first skin forming a rotor tube and having a tube axis; a plurality of strips extending axially along a surface of the skin, wherein: the first skin is integrally formed from a first fibrous material formed from first fibres, and the strips are formed from a second fibrous material formed from second fibres, at least some of which second fibres extend axially along the rotor body.

In embodiments of the invention the first fibres are orientated at between <NUM>° and <NUM>° to the tube axis, and preferably at between <NUM>° and <NUM>° to the tube axis.

In embodiments of the invention, the strips may have a thickness of between <NUM> and <NUM>.

In embodiments of the invention the strips extend along an outer surface of the first skin.

In embodiments of the invention, the first and second fibres are glass fibres.

In other embodiments of the invention the first fibres may be glass fibres and the second fibres may be carbon fibres.

In embodiments of the invention the rotor body comprises a second, integrally formed skin formed from a third fibrous material, at least some of which third fibres are orientated at between <NUM> and <NUM> degrees to the tube axis, and optionally at between <NUM> and <NUM> degrees to the tube axis, wherein the first and second skins together form the rotor tube.

In embodiments of the invention, third fibres in one or more outer layers of third fibres are orientated at close to <NUM> degrees to the tube axis, optionally between <NUM> degrees and <NUM> degrees to the tube axis.

In embodiments of the invention, the third fibres may comprise glass fibres.

In embodiments of the invention, the strips comprise hollow strips. This increases the thickness of the strips without adding weight, hence enabling the skins to be spaced further apart.

In embodiments of the invention, the strips may extend along an inner surface of the first skin. In such embodiments of the invention, the rotor body comprises the first skin and the strips only, and the rotor body does not comprise the second skin.

In embodiments of the invention, the number of strips may vary along the length of the rotor body. This allows for a variation in the strength of the rotor to suit the variation in the bending moment along the length of the rotor body.

In embodiments of the invention the rotor body comprises a plurality of rotor tubes, which rotor tubes are joined together to form the rotor body.

In such embodiments, the rotor body may be made with any desirable length by joining together an appropriate number of rotor tubes.

In embodiments of the invention in which the strips extend along an inner surface of the first skin, and there is no second skin at least some of the strips may span joints between adjacent rotor tubes.

According to a third aspect of the present invention there is provided a rotor body according to embodiments of the second aspect of the invention formed using a method according to embodiments of the first aspect of the invention.

According to a fourth aspect of the present invention there is provided a vessel, which vessel comprises a rotor sail attached to a portion of the vessel, which rotor sail comprises a rotor body according to embodiments of the first and third aspect of the invention.

The invention will now be described by way of example only with reference to the accompanying drawings in which:.

Referring initially to <FIG>, an example of a rotor sail, or Flettner rotor, is shown wherein the rotor sail is defined generally by the reference numeral <NUM>. The rotor sail comprises a rotor body <NUM> rotatably mounted to a static cylinder <NUM> via upper bearings <NUM> and lower bearings <NUM>. The rotor body <NUM> comprises a plurality of circumferential ribs <NUM> which reinforce the rotor body <NUM> to provide circumferential bending strength and stiffness. In this example the circumferential ribs <NUM> are attached to the inside of the rotor body <NUM>, although circumferential ribs may also be attached to the outside of a rotor body.

<FIG> shows the step of winding, or filament winding, first fibres around a mandrel <NUM> to form a tubular first skin <NUM> forming a rotor tube <NUM> having a tube axis <NUM>. A rotor body may be formed of a single rotor tube <NUM> or of a plurality of rotor tubes <NUM> joined together coaxially.

In this embodiment of the invention, the first fibres are bundled to form a first fibre tow <NUM> which is dispensed from a first fibre tow spool <NUM>, passed through a resin bath <NUM> to be coated in resin <NUM> and then wound around the mandrel <NUM>. The first fibre tow <NUM> is guided up and down the length of the mandrel <NUM>, parallel to the tube axis <NUM>, as it is wound onto the mandrel, thereby gradually forming the tubular first skin as layers of the first fibre stack on top of one another. The winding process may be continued until the first skin is formed with a desired thickness, between <NUM> and <NUM> for example. To reduce the time required to form the first skin <NUM>, multiple first fibre tows <NUM> are dispensed, coated and wound simultaneously.

The first fibres may be any suitable material such as carbon, aramid, basalt, E-glass, S-glass or ECR-glass for example. Similarly, the resin may be any suitable type of resin, such as epoxy resin, vinylester resin, polyester resin, polyurethane resin or acrylic resin, for example. Resins may be thermoset or thermoplastic and may be cured at ambient temperature or at elevated temperature to suit the required speed of the process and the eventual strength and temperature resistance required of a rotor body in use forming part of a rotor sail. For example, the resin may be an epoxy resin which cures to become a solid after several minutes or hours at ambient temperature, allowing the winding process to be conducted conveniently at ambient temperature and the rotor tube to be later removed from the mandrel after the resin has solidified. The resin and any adhesive used in the manufacture of the rotor body may then be further cured (post-cured) by elevating the temperature of the complete rotor body to increase the degree of cure of the resin and further improve its strength and temperature resistance.

The orientation of the first fibres in the first skin <NUM> is determined by the orientation at which the first fibre tows <NUM> are wound on to the mandrel <NUM> relative to the tube axis <NUM>. This orientation may be varied by varying the speed at which the first fibre tows are guided up and down the length of the mandrel <NUM> relative to the speed at which the mandrel <NUM> is rotated to wind the first fibre tows <NUM> on to it.

The mandrel <NUM> may be tapered to facilitate removal of the first skin <NUM> from the mandrel <NUM>, after the resin <NUM> has cured. The first skin <NUM> may therefore be frustrum shaped rather than cylindrical.

In embodiments of the invention, the first fibre tows <NUM> are orientated at between <NUM> and <NUM> degrees to the tube axis <NUM>. For example, the first fibre tows <NUM> could be orientated at a mixture of orientations such as some being wound at +/- <NUM> degrees and some being wound at close to <NUM> degrees (e.g., +/- <NUM> degrees).

In embodiments of the invention, the first fibre tows are orientated at between <NUM> and <NUM> degrees. For example, the first fibre tows <NUM> could all be wound at around +/- <NUM> degrees. This would allow the first fibre tows <NUM> to be guided up and down the length of the mandrel <NUM> at a constant speed throughout the process of forming the first skin <NUM>, facilitate a greater degree of automation and also allow the mandrel <NUM> to rotate faster than if the fibre tows <NUM> were wound at +/-<NUM> degrees.

<FIG> shows the step of attaching a plurality of strips <NUM> to the first skin <NUM>. The strips <NUM> are formed from second fibres and are attached to the first skin <NUM> such that at least some of the second fibres extend axially along the first skin <NUM>, i.e. parallel to the tube axis <NUM>. The second fibres may be any suitable material such as carbon, aramid, basalt, E-glass, S-glass or ECR-glass for example. Each strip <NUM> may be formed using a pultrusion process.

In this embodiment of the invention, the strips <NUM> are positioned on the outer surface of the first skin <NUM> while the resin forming part of the first skin <NUM> is still curing so that the strips <NUM> may bond to the first skin <NUM> as the resin cures. In order to ensure that the plurality of strips <NUM> remain in position until they are attached to the first skin <NUM>, a temporary strap assembly <NUM> may be used to retain the plurality of strips <NUM> in contact with the first skin <NUM> as it rotates around the mandrel <NUM> to allow further strips <NUM> to be positioned.

In other embodiments of the invention the plurality of strips may be pre-bonded to a backing scrim <NUM> as shown in <FIG>. The plurality of strips <NUM> may therefore be attached to the first skin <NUM> as a single assembly rather than as individual strips. This is analogous to mosaic tiles for bathrooms and kitchens, for example, which are mounted to a backing layer so that the tiles can be applied to a wall in large sheets rather than individual tiles.

Using a backing scrim <NUM> may simplify the attachment of strips <NUM> to the first skin <NUM>. However, the process of pre-bonding the strips <NUM> to the backing scrim <NUM> requires an additional process step which is not required in the method demonstrated in <FIG>.

In further embodiments of the invention the strips may be held in place on the first skin with rigid or flexible jigs which are adapted to ensure desired spacing of the strips around the circumference of the first skin (either on its internal or external surface).

In <FIG>, a second skin <NUM> is formed by winding third fibres around the plurality of strips <NUM> once they are all positioned on the first skin <NUM>. The second skin <NUM> forms the rotor tube <NUM> together with the first skin <NUM>. Similarly to the first fibres, the third fibres are bundled to form a third fibre tow <NUM> which is dispensed from a third fibre tow spool <NUM>, passed through a resin bath <NUM> to be coated in resin <NUM> and then wound around the plurality of strips <NUM>. The third fibres may be any suitable material such as carbon, aramid, basalt, E-glass, S-glass or ECR-glass for example.

Initially, a temporary strap assembly (such as the temporary strap assembly <NUM> shown in <FIG>) may hold the strips <NUM> in place until a sufficient quantity of the second skin <NUM> is formed to hold the strips <NUM> without requiring additional support. If a plurality of straps is used along the length of the body in the temporary strap assembly the straps may be removed one by one as the second skin <NUM> is formed.

The winding process may be continued until the second skin <NUM> is formed with a desired thickness, between <NUM> and <NUM> for example. To reduce the time required to form the second skin <NUM>, multiple third fibre tows <NUM> may be dispensed, coated and wound simultaneously, similarly to the first fibre tows <NUM> (as shown in <FIG>).

Although not included in this embodiment of the invention, in other embodiments of the invention external circumferential ribs may be incorporated into the second skin <NUM> by winding a pile of third fibres at <NUM> degrees to the tube axis <NUM> and at intervals along the length of the rotor tube <NUM>. In further embodiments of the invention, circumferential ribs may be formed separately to the rotor tube <NUM> and bonded to either the first <NUM> or second skin <NUM>.

In order to provide a smooth outer surface to the second skin <NUM> and maximise consolidation pressure on the layers of fibres beneath, the outermost layers of third fibres (forming the outermost <NUM> of the second skin, for example) may be formed wherein the third fibre tows <NUM> are orientated at close to <NUM> degrees to the tube axis <NUM> (e.g. +/-<NUM> degrees).

The resin <NUM> coating the third fibre tows <NUM> may also soak onto the strips <NUM> as it cures, thereby bonding the second skin <NUM> to the plurality of strips <NUM>, similarly to the first skin <NUM>.

Once the resin <NUM> throughout the different layers of first, second and third fibres has cured, the rotor tube <NUM> may be removed from the mandrel <NUM>.

Referring now to <FIG>, a rotor tube <NUM> is shown comprising a first skin <NUM>, formed as shown in <FIG>; a layer of strips <NUM>, attached to the first skin <NUM> as shown in <FIG>; and a second skin <NUM>, formed as shown in <FIG>. The rotor tube <NUM> may form part of a rotor body according to an embodiment of the second aspect of the invention which may, in turn, form part of a rotor sail such as the rotor sail shown in <FIG>.

The first and second skins <NUM>, <NUM>, with first and third fibres orientated at between <NUM> and <NUM> degrees to the tube axis, provide the rotor tube <NUM> with strength in a circumferential/hoop direction, normal to the tube axis, so that a rotor body comprising one or more of the rotor tubes <NUM> may maintain its circular cross-sectional shape when in use forming part of a rotor sail. Meanwhile, the strips <NUM> comprising second fibres, at least some of which are orientated parallel to the tube axis, provide the rotor tube <NUM> with strength in the axial direction, parallel to the tube axis, so that in use the rotor body comprising the rotor tube <NUM> may withstand bending forces caused by pressure from the wind.

Referring now to <FIG>, the rotor tube <NUM> is shown in cross-section. Each strip <NUM> is substantially rectangular in cross-sectional shape with edges abutting against the edges of adjacent strips <NUM>. Resin <NUM> fills any gaps between adjacent strips <NUM> and between the first skin <NUM>, plurality of strips <NUM> and second skin <NUM> and acts to bond the first skin <NUM>, plurality of strips <NUM> and second skin <NUM> together.

Although the resin <NUM> bonds the various components of the rotor tube together, reducing the amount of resin used may advantageously reduce material costs and the resultant weight of the rotor tube. It may therefore be preferable to avoid using resin in excess of that which is needed for bonding the layers of the rotor tube together.

Accordingly, <FIG> shows a rotor tube <NUM> similar to the rotor tube <NUM> shown in <FIG> except that each of a plurality of strips <NUM> is arcuate in cross-sectional shape so that the strips <NUM> may fit more closely to the first and second skins <NUM>, <NUM>. Also, the edges of the strips <NUM> are angled so that each strip may fit more closely to the adjacent strips. The spaces between adjacent strips <NUM> and between the first skin <NUM>, plurality of strips <NUM> and second skin <NUM> is therefore reduced in comparison to the rotor tube <NUM> shown in <FIG> so less resin <NUM> may be required to fill the spaces and bond the parts together. The rotor tube <NUM> may therefore be manufactured with lower material cost and with a lower weight.

The strips may also be shaped to improve the ease with which they are attached to the first skin <NUM> and held there while the second skin is formed. For example, in <FIG> a rotor tube <NUM> which may form part of a rotor body according to another embodiment of the second aspect of the invention is shown that comprises a plurality of strips <NUM>. Each strip <NUM> has profiled edges <NUM> shaped to nest against the profiled edges <NUM> of adjacent strips <NUM>. Each strip <NUM> thereby encourages its adjacent strips <NUM> to stay in position which may reduce the burden on temporary straps <NUM> (shown in <FIG>) or the second skin <NUM> (shown in <FIG>) to hold the strips <NUM> in place.

Similarly, in <FIG> a rotor tube <NUM> is shown that comprises a plurality of strips <NUM> comprising profiled edges <NUM> that interlock with the profiled edges <NUM> of adjacent strips <NUM>.

A rotor sail may require its rotor body to be between <NUM> and <NUM> in length whereas the winding process may be limited to forming rotor tubes that are between <NUM> and <NUM> in length. Therefore, in embodiments of the invention, two or more rotor tubes may be joined together coaxially with one another to form a single rotor body suitable for forming part of a rotor sail.

Two rotor tubes which may form part of a rotor body according to an embodiment of the second aspect of the invention may be joined by any suitable means. For example, in <FIG>, an end of a first rotor tube 402a and an end of a second rotor tube 402b, which is equal in diameter to the end of the first rotor tube 402a, are abutted against one another. The abutted edges of the first and second rotor tubes 402a and 402b are bonded together with resin or adhesive <NUM>.

To reduce stress concentrations in the resin <NUM> connecting the parts together, the ends of the of the rotor tubes 402a, 402b are tapered in laminate thickness, laminate thickness being the combined thickness of the first skin, layer of strips and second skin. In this embodiment of the invention each rotor tube 402a, 402b comprises a tapered edge <NUM> wherein the laminate thickness of each rotor tube 402a, 402b is tapered from the outer second skin <NUM> towards the inner first skin <NUM>. A wedge-shaped first joining part 442a fills a groove formed by the tapered edges <NUM> while a flat second joining part 442b covers the join and adjacent portions of the first skin <NUM> of each rotor tube 402a, 402b.

The joining pieces 442a, 442b may be laminated and cured directly onto the rotor tubes 402a, 402b being joined or laminated and cured separately before being bonded onto the rotor tubes 402a, 402b with structural adhesive.

In <FIG>, two rotor tubes 502a, 502b are joined similarly to the rotor tubes 402a, 402b shown in <FIG> except the rotor tubes 502a, 502b comprise tapered edges <NUM> wherein the laminate thickness of each rotor tube 502a, 502b is tapered from the inner first skin <NUM> towards the outer second skin <NUM>. The joining pieces 442a, 442b are accordingly reversed so that the wedge-shaped first joining part 442a is positioned on the inner surfaces of the rotor tubes 502a, 502b and the flat second joining piece 442b is positioned on the outer surfaces of the rotor tubes 502a, 502b.

In <FIG>, the first rotor tube 502a shown in <FIG> is joined to the second rotor tube 402a shown in <FIG>. The tapered edges <NUM>, <NUM> therefore abut against one another, obviating the need for a wedge-shaped joining piece and allowing two flat joining pieces <NUM> to be used.

Tapering of the laminate thickness from the outer surface, to form the tapered edge <NUM> shown in <FIG>, may be achieved by grinding the rotor tube <NUM> to a taper after curing but while it is still supported internally by the mandrel <NUM>, as shown in <FIG>. Hence, an outer section <NUM> is removed from the rotor tube <NUM>.

Where an internal taper is required, to form the tapered edge <NUM> shown in <FIG> for example, this can be achieved by winding the rotor tube <NUM> over a wedge-shaped part <NUM> on the mandrel <NUM>, as shown in <FIG>. The rotor tube <NUM> may then be ground from the outside to remove an outer section <NUM> and leave the tapered edge <NUM>.

In <FIG> a rotor body <NUM> suitable for forming part of a rotor sail comprises six rotor tubes <NUM>, <NUM> joined to one another as shown in <FIG>. Each rotor tube <NUM>, <NUM> is frustrum shaped due to the taper of the mandrel on which they were each formed. To ensure that ends of equal diameter are joined together to facilitate the joining means shown in <FIG>, some of the rotor tubes <NUM>, <NUM> are orientated with the taper in the opposite direction to the taper of other rotor tubes <NUM>, <NUM>.

However, in other embodiments of the invention, rotor tubes may be joined together such that they overlap with one another.

In <FIG>, an end of a first rotor tube 402a is received within the end of a second rotor tube 402b which has a slightly larger diameter than the end of the first rotor tube 402a. The ends are bonded with resin or adhesive <NUM>, similarly to the joined ends in <FIG>, and first and second joining pieces 742a and 742b cover the inner and outer surfaces of the join.

In <FIG>, a first rotor tube 502a is joined to a second rotor tube 402b similarly to that shown in <FIG> except that the first rotor body comprises a tapered edge <NUM> which is tapered to avoid a large space existing between the first and second rotor tubes 502a, 402b that requires filling with resin or adhesive <NUM>. Hence the rotor tubes 502a, 402b may be joined with lower material cost and requiring less additional weight to be added.

Rotor tubes <NUM>, <NUM> joined as shown in <FIG> may form a rotor body <NUM> such as that shown in <FIG>.

Referring now to <FIG>, a rotor tube <NUM>, which may form part of a rotor body according to another embodiment of the second aspect of the invention, comprises a plurality of strips <NUM>. The rotor tube <NUM> is similar to the rotor tube <NUM> shown in <FIG> except that each strip <NUM> is a hollow strip comprising a void space <NUM>.

Due to the void space in each strip, the strips may be formed with a greater cross-sectional area despite using the same quantity of material and hence having the same weight. The first and second skins <NUM>, <NUM> may therefore be spaced further apart in rotor tube <NUM> compared to the skins of rotor tube <NUM> shown in <FIG>. The increased spacing of the first and second skins <NUM>, <NUM> may increase the circumferential bending strength and stiffness of the rotor tube <NUM> without adding the parasitic cost or weight of a foam core nor increasing the material cost or weight of the strips. With this increased bending strength and stiffness the need for circumferential ribs can be avoided.

In <FIG>, a rotor tube <NUM> is similar to the rotor tube <NUM> except that it comprises a plurality of strips <NUM> which are trapezoidal in cross section. In other words the edges of the strips <NUM> which abut against the edges of other strips are angled so that the spacing between strips may be reduced and the amount of resin <NUM> required to fill the spaces is also reduced.

In <FIG>, a rotor tube <NUM> is similar to the rotor tubes <NUM> and <NUM> shown in <FIG> except that each strip is wider and comprises a plurality of void spaces <NUM>. This further increases the efficiency of material required to form the strips <NUM> without sacrificing the shear strength of the strips <NUM>. Each strip may be formed with angled edges similarly to the strips <NUM> shown in <FIG> and may further be formed with an arcuate cross-section similarly <NUM> shown in <FIG>, thereby reducing spacing present in the rotor tube <NUM> and reducing the amount of resin <NUM> required.

Two rotor tubes with hollow strips, such as those shown in <FIG> and <FIG> may be joined similarly to the ways shown in <FIG> for rotor tubes with solid strips. However, when joining rotor tubes with hollow strips it may be preferable that the void spaces are sealed to ensure stability of the rotor body comprising the joined rotor tubes when it is in use forming part of a rotor sail.

In <FIG>, two rotor tubes <NUM> are joined (although similar means may be used for joining rotor tubes <NUM> and <NUM>). Due to the increased laminate thickness of the rotor tubes <NUM>, a taper that extends substantially from the first skin <NUM> to the second skin <NUM> would need to be long and require the removal of a large quantity of material. Rather than doing this, each rotor tube comprises two tapered edges <NUM>, one tapering from the first skin <NUM> and one from the second skin <NUM>. None of the tapered edges <NUM> extend into the void space <NUM> to ensure that the void space may be sealed with minimal amounts of resin <NUM>. Wedge-shaped first and second joining pieces 842a, 842b similar to those shown in <FIG> are applied to each side of the join.

Referring now to <FIG>, a rotor tube <NUM> which may form part of a rotor body according to another embodiment of the second aspect of the invention is shown. The rotor tube <NUM> comprises a first skin 12and no second skin.

In this embodiment of the invention, the first skin <NUM> may be made according to the method shown in <FIG> and removed from the mandrel. A plurality of strips <NUM> are added afterwards, onto the internal surface of the first skin <NUM> rather than the outer surface as in previously described embodiments of the invention. As the resin in first skin <NUM> is required to cure in order to remove it from the mandrel, the strips <NUM> are bonded in place with a structural adhesive <NUM>.

Advantageously, the winding can be done in a single operation, removing the need to hold strips in place before a second skin has been wound. Further, the quantity of axial material can be more easily varied along the length of the rotor (by adding more strips locally) to match variations in the bending moment, thereby minimising the total weight and cost of the axial material.

Also, more expensive carbon fibre strips of practical thickness can be used cost effectively instead of glass fibre, because they can be spread apart rather than abutted in a continuous layer. In a continuous layer only around <NUM>-<NUM> thickness of carbon fibre is required on a <NUM> diameter rotor body, which means previously described embodiments of the invention would barely benefit from having the pultrusions to separate the first and second skins for good bending strength. In this embodiment of the invention, narrower strips can be used, for example, with dimensions of <NUM> width, <NUM> thick, and with <NUM> gaps in between. Carbon fibre is advantageous because it is stronger and lighter and in particular carbon fibre has a better resistance to fatigue than glass fibre. The strength advantage of carbon fibre is particularly significant when it is pultruded as the fibre straightness is beneficial. Thus, using carbon fibre in the axial direction of a rotor sail according to embodiments of the invention can be more cost effective than using glass fibre, even though carbon fibre material is more expensive per kg.

Additionally, filament winding machines typically have a maximum mandrel length which is less than the desired length of a rotor body for use forming part of a rotor sail. Therefore several rotor bodies according to embodiments of the second aspect of the invention may need to be joined together, and the joints between them need to carry the full axial load. In the embodiment of the invention shown in <FIG>, a plurality of rotor tubes <NUM> can be joined before the strips <NUM> are bonded on. The strips <NUM> are then continuous across each joint, as shown in <FIG>, providing the necessary axial strength while the first skins <NUM> and joining piece <NUM> applied across the joints only needs to transmit the relatively lower shear forces easily accommodated by a few millimetres thickness of biaxial (+/-<NUM> degree) material.

However, without the pultrusion in the middle of two skins, the required thickness of the first skin <NUM> for circumferential bending strength must be made up using more first fibre. In other words, the first skin <NUM> must be thicker. Further, a second step of bonding the pultrusions to the first skin <NUM> is necessary and a large amount of adhesive <NUM> is required which adds cost and weight.

Referring now to <FIG>, a further means of joining two rotor tubes is shown. This joining means may be applied to any of the tubes shown in <FIG>, although rotor tubes <NUM> shown in <FIG> are used as an example. A joining piece <NUM> is bonded to each of the rotor tubes <NUM> to be joined using adhesive (not shown). Each joining piece <NUM> comprises a radial surface <NUM> adapted to abut against the radial surface of the other joining piece <NUM>. The two joining pieces <NUM> are then bolted together using a bolt assembly <NUM>.

Claim 1:
A method of manufacturing a rotor body (<NUM>) forming part of a rotor sail (<NUM>), the method being characterised by comprising the steps of:
winding first fibres around a mandrel (<NUM>) to form a tubular first skin (<NUM>) forming a rotor tube (<NUM>) having a tube axis (<NUM>);
forming a plurality of strips (<NUM>) from second fibres;
attaching the strips to a surface of the first skin such that at least some of the second fibres extend axially along the rotor body.