COMPOSITE STRUCTURE AND METHOD OF MANUFACTURE

The present invention relates to a composite structural member and its method of manufacture. One area in which such extra structural member may be used is in the manufacture of commercial aircraft, in which many structural beams are required, such as wing ribs and flap track beams (a flap track beam being an elongate structural member on which aircraft control surfaces, such as wing flaps, move during their deployment.

Such structural beams have to typically be manufactured in metal and particularly take the form of a u-shaped trough with one or more internal features in the trough, such as rib posts or diaphragms. One or more cappings are fixed over the trough to form a closed cross section. This mode of manufacture requires each structural beam to be made from a number of separate parts, each of which requires separate mouldings or a tooling process to individually manufacture, and multiple fixings to secure individual parts together to form the completed structural beam. As a consequence the complete structural beam is both expensive to manufacture and/or tool and is complex and time consuming to assemble. The complexity and scale of the structural beam also prohibits the beam from being either cast or machined in a single piece.

The structural beams can also be manufactured using carbon fibre composites. This method of manufacture requires a female mould to be produced for each individual part, for example the u-shaped trough, to allow the layers of carbon fibre to be laid up in a manner known to the person skilled in the art and subsequently cured using an autoclave. However, the moulds themselves are expensive to manufacture due to the required accuracy and the need for the moulds to be able to withstand repeated cycles of high temperature and pressure from repeated autoclaving. Since it is likely that only a relatively small number of structural beams will be produced from any single set of moulds, relative to other "mass production" techniques, the cost of manufacturing the moulds themselves is

very high in relation to the overall cost of manufacture of the structural beam. Additionally, it is still necessary to produce the beam in at least two separate parts, such as the u-shaped trough and capping, and for these parts to be subsequently fixed together, since it is not possible to produce the required box structure using conventional carbon fibre lay up techniques. Therefore the final process of assembly of the structural beam can still be relatively complex and the use of multiple fixings, as with the all metal beams, results in areas of potentially high stress around the fixings that must be taken into account in the design of the individual elements of the structural beam and can reduce the theoretical maximum strength of the completed assembly.

The present invention seeks to mitigate these problems and disadvantages by providing an alternative design of structural member and a corresponding alternative method of manufacture.

According to a first aspect of the present invention there is provided a structural member comprising a number of internal elements, each internal element having a longitudinal axis aligned with the longitudinal axis of the remaining internal elements, and a continuous outer skin formed over the internal elements.

Preferably, a first sub-set of the internal elements may comprise a number of internal diaphragms and a second sub-set of the internal elements may comprise a number of spacers, the internal diaphragms and spacers being alternatively arranged to one another. Additionally, the internal diaphragms may be load bearing elements.

Additionally or alternatively, the internal diaphragms may comprise one from the list of metal, metal alloy or carbon fibre composite.

Additionally, the spacers may comprise a non-load bearing material. Equally preferably, the outer skin may comprise the carbon fibre composite.

Advantageously, the structural member may further comprise a longitudinal member passing through each of the internal elements.

The structural member may comprise an aircraft component, such as a flap track beam.

According to a second aspect of the present invention there is provided a method of manufacture of a structural member comprising providing a number of internal elements, each having an aperture formed therein, sequentially assembling the internal elements onto a longitudinal element such that the longitudinal element passes through the aperture of each internal element, and wrapping an outer skin around the assembled internal elements and longitudinal element.

Preferably the outer skin may comprise a carbon fibre composite and the method may further comprise curing the carbon fibre composite.

Additionally or alternatively, the step of wrapping the outer skin may include rotating the assembled internal elements about the axis of the longitudinal member.

Additionally alternatively, the internal elements may be assembled onto the longitudinal element in a predetermined order.

Preferably, a first sub-set of the internal elements may comprise a number of internal diaphragms and second sub-set of the internal elements may comprise a number of spacers, whereby the step of sequentially assembling the internal members comprises sequentially assembling the alternate internal diaphragms and spacers onto the longitudinal element.

Optionally, the method may further comprise removing the spacers from the structural member subsequent to the step of wrapping the outer skin. The step of removing the spacers may comprise heating the structural member to a temperature in excess of the

melting point of the spacers and draining away the molten spacers, or may comprise dissolving the spacers away by applying an appropriate solvent.

Additionally or alternatively, the aperture formed in each of the internal elements may have a matching shape and size to that of the cross sectional profile of the longitudinal element. Additionally, the cross-sectional profile of the longitudinal element may be non-cylindrical.

Embodiments of the present invention will now be described by way of illustrative example only with reference to the accompanying drawings, of which:

Figure 1 schematically illustrates a conventional metal structural beam;

Figure 2 schematically illustrates a partly manufactured structural member according to an embodiment of the present invention, in which internal elements can be seen;

Figure 3 illustrates the partly manufactured structural member of Figure 2 as a solid body; and

Figure 4 illustrates a fully manufactured structural member according to an embodiment of the present invention.

A structural member of a conventional metal construction is schematically illustrated in Figure 1. The structural member comprises a generally u-shaped trough 2 having closed opposite end faces 4. The trough 2 has a number of internal diaphragms 6 that are secured to the side walls of the trough 2 by suitable fixings 8, such as rivets or bolts. The internal diaphragms 6 provide increased rigidity and strength to the structural member. The trough 2 is closed off by means of one or more capping plates 10, each capping plate having a side flange that overlaps with the edge of the side walls of the trough 2 and each capping 10 is secured to the trough 2 multiple fixings passing through the flange and side plate of the trough. In the particular structural member illustrated in Figure 1, the cappings may be

provided as three separate elements or may be provided as a single element. As previously noted, this kind of structural member requires a relatively large number of individual parts and a large number of fixings and is thus expensive to both manufacture and assemble.

Figures 2 and 3 schematically illustrate a partially manufactured structural member according to embodiments of the present invention, with Figure 2 illustrating internal features of the structural member. The structural member 12 is assembled over a central rod 14 (as shown in Figure 2) with the rod 14 being coincident with a longitudinal axis of the completed structural member 12. The partially manufactured member illustrated in Figures 2 and 3 is made up of a number of individual elements through which the central rod 14 passes. The individual elements are assembled on to the rod in a sequential manner. The individual elements include a number of internal diaphragms 16, end plates 18 and a number of core elements 20, which in Figure 2 are illustrated as being transparent for the ease of understanding of the construction of the structural member only.

To construct the structural member the first end plate 18 is threaded on to the central rod 14, the end plate 18 and all the diaphragms 16 and core elements 20 preferably being formed with an aperture corresponding to the cross section of the central core 14. An end nut (not illustrated) may be secured to the central rod 14 to provide a stop for the first end plate. Subsequently, alternate core elements 20 and diaphragms 16 are threaded on to the central rod 14. Each core member 20 and diaphragm 16 is formed such that when assembled together on the central rod 14 they define the required outer surface of the structural member 12. Assembly is completed with a further end plate 18. The internal diaphragms 16 are included to provide the required structural rigidity and overall strength characteristics of the final structural member and are analogous to the internal diaphragms

6 of the conventional metal structural member illustrated in Figure 1. The internal diaphragms 16 of embodiments of the present invention can either be simply machined from a suitable metal, such as aluminium or titanium alloy, or can be simply moulded in carbon fibre composite, the moulds required for the carbon fibre composite internal

diaphragms being simple and thus relatively inexpensive to produce. The internal core members 20 are simply provided to locate the internal diaphragms 16 in their correct positions relative to one another along the central rod 14 and to also define the outer surface of the structural member 12. The internal members 20 therefore do not need to add any structural rigidity or strength to the final structural member and can thus be made of a conveniently machined material, such as for example a closed cell rigid foam plastic such as Rohacell™. Such closed cell rigid foam plastic is easily machined to the desired shape, yet has sufficient strength to endure the further manufacturing steps required to complete the assembly of the structural member 12 according to embodiments of the present invention. To assist in the assembly of the end plates, internal diaphragms and core members, each individual element may be secured to respective adjacent elements by suitable adhesive. As a further alternative locating pins or dimples may be formed on adjacent faces of the core members 20, internal diaphragm 16 and end plates 18 to assist in their correct spatial relation with respect to one another. The central rod 14 may also have a cross section that facilitates the assembly of the individual elements. For example, the central rod 14 can have a T-shaped cross section with each of the individual elements having a corresponding T-shaped aperture through which the central rod 14 passes. This ensures that each individual element is correctly aligned with respect to the central rod 14 and adjacent central elements and also prevents the central elements from rotating with respect to one another during the assembly process.

Subsequent to the individual elements being assembled onto the central rod 14, the whole assembly is over wrapped with carbon fibre reinforced plastic (CFRP) which is provided in long lengths and can thus be wrapped around the completed assembly illustrated in Figures 2 and 3, the CFRP conforming to the outer surface defined by the core elements 20 and the internal diaphragms 16. The wrapped assembly is then cured to form the final composite structural member, as illustrated in Figure 4. In preferred embodiments the CFRP wrapped around the outer surface of the assembly is of the kind that is pre-impregnated with resin, e.g. epoxy resin, typically known as "prepreg", such that the wrapped assembly may be

placed in a vacuum bag and cured either in an autoclave or the prepreg may be formulated to cure in an oven with the vacuum bag only (so-called out-of-autoclave approach). In further embodiments the assembly can be over wrapped with dry fibre (with or without binder) and then placed in a vacuum bag which is then infused with a liquid resin prior to curing.

In preferred embodiments of the present invention the assembly is wrapped by placing it in a suitable jig such that the assembly can be rotated on the central rod 14. This allows the over wrapping to be achieved in a number of ways, e.g. hand wrapping, by the use of an automatic tape laying machine or the use of a fibre placement machine.

The central rod 14 may be removed either immediately prior to the curing process or as a final step after curing or may be designed so as to remain as part of the completed structural member. Equally, since the core members' 20, which in preferred embodiments are formed from Rohacell™ foam, primary purpose is to define the outer surface of the structural member prior to wrapping with CFRP, once the CFRP has been applied and cured the core members 20 may be removed if desired. This can be achieved by, for example, placing the completed assembly in an oven raising the temperature above the melting point of the Rohacell™ foam and thus allowing the molten foam to flow out of the structural member, for example along the central core 14, or by dissolving the Rohacell™ foam using suitable solvent, such as an alkaline solvent. In both instances removal of the Rohacell™ foam is done by reducing it to a liquid state and is thus facilitated if the central rod 14 has been previously removed. The internal diaphragms 16 are held in place after the outer skin has been cured since the curing process also bonds the outer skin to the internal diaphragms.

The composite structural member produced according to embodiments of the present invention is much simpler to manufacture previously discussed prior art structural members, since not only do the individual elements only require simple machining

processes or simple moulds for their individual manufacture, but there is no requirement for any fixings to assemble the individual elements together, since the individual elements are completely enclosed by the outer skin material. The complete absence of fixings not only significantly simplifies the final assembly of the structural member it more importantly completely avoids the stress concentration that occurs around conventional structural members using fixings and thus achieves a greater overall strength.