FIELD OF THE INVENTION

The present invention relates to an aircraft structure, for instance a wing, horizontal stabiliser or vertical stabiliser.

BACKGROUND OF THE INVENTION

A traditional aircraft wing comprises a wing box formed by upper and lower aerodynamic covers, front and rear spars, and a series of transverse ribs spaced apart along the span- wise length of the wing box. Each rib is bolted to the upper and lower covers and reacts fuel pressure loads between them.

Assembly of such a wing box can be very time-consuming and complicated due to the need to manufacture, drill, shim, and then bolt many components together. A traditional wing box is also very heavy and does not always make the most efficient use of modern aerospace materials such as composites. It is desirable to design an aircraft wing box or similar structure which has a reduced part count, simplified manufacturing process and improved mechanical performance.

SUMMARY OF THE INVENTION

A first aspect of the invention provides an aircraft structure extending from a root to a tip, the root being adapted to be joined to an aircraft fuselage. The structure comprises a cover with an aerodynamic outer surface and an inner surface, a plurality of stiffeners, and a plurality of ribs. The ribs comprise an inboard rib and a plurality of further ribs between the inboard rib and the tip. Each of the stiffeners comprises a stiffener flange joined to the inner surface of the cover and a stiffener web extending away from the stiffener flange. At least one of the further ribs comprises a rib web and a plurality of rib feet, each rib foot comprising a first rib foot flange joined to the web of a respective one of the stiffeners, a second rib foot flange joined to the inner surface of the cover or to the flange of the respective one of the stiffeners, and a rib foot web joined to the rib web.

The joint between the first rib foot flange and the stiffener web provides a new and efficient path for transmitting load from the cover into the rib web. Typically each first rib foot flange is joined to the stiffener web by a joint which can transmit load in shear from the stiffener web to the first rib foot flange.

The structure may be provided with only a single cover, a second cover being added during final assembly of the aircraft. Alternatively the structure may further comprise a second cover with an aerodynamic outer surface and an inner surface. The rib web is arranged to transmit load (such as fuel pressure load) between the first cover and the second cover. Typically the rib web is joined to the second cover, either directly or via additional rib feet.

Typically the structure comprises a front spar and a rear spar, wherein each spar is joined to the first and second covers and to the rib web. These spars may constitute the stiffeners to which the rib feet are attached, but more preferably each of the stiffeners comprises a stringer which is joined to the first cover but not to the second cover.

Preferably each stiffener is a stringer, and the stiffener web is a stringer blade which extends away from the stiffener flange to an elongate edge. The stringer may have a variety of cross-sectional shapes, including a T-shape or a top-hat (omega) shape.

Preferably the rib web of the least one of the further ribs is formed with cut-outs through which the stiffener webs pass.

The rib may form a liquid-tight seal with the cover, but more preferably the rib permits liquid to flow across it. For instance a gap may be provided between the rib web and the inner surface of the cover, the gap being arranged to permit liquid to flow through the gap.

Typically the rib web has an edge with a cut-out through which the stiffener web passes. In the case of a sealed rib then the stiffener web may form a liquid-tight seal with the rib web. However more preferably a gap is provided between the rib web and the stiffener web, the gap being arranged to permit liquid to flow through the gap.

The second rib foot flange may be joined to both the inner surface of the cover and the flange of the respective one of the stiff eners. In this case it may include a ramp or step to account for a difference in height between the inner surface of the cover and the surface of the stiffener web to which the rib foot is joined. Alternatively the second rib foot flange may be joined only to the flange of the respective one of the stiffeners (optionally via a grow-out region of the stiffener flange).

Each rib foot may have only two flanges. Alternatively each rib foot may have a further rib foot flange joined to the stiffener web, the further rib foot flange and the first rib foot flange extending away on opposite sides of the rib foot web (forming a T- section). Also, each rib foot may have a further rib foot flange joined to the inner surface of the cover or to the stiffener flange, the further rib foot flange and the second rib foot flange extending on opposite sides of the rib foot web (forming a T-section). The rib foot may comprise a composite material including reinforcement elements embedded in a matrix material. The reinforcement elements may, for example, comprise carbon fibre and/or glass fibre and/or Kevlar and/or metallic reinforcement in a polymer matrix. The cover and/or stiffener and/or rib web may also comprise a composite material of the same or different construction to the rib foot. The composite material may comprise multiple ply layers of reinforcement material arranged in a stack. For example, the reinforcement may be provided in the form of a series of ply layers. The rib foot may be formed by arranging a stack of dry reinforcement plies together and subsequently adding matrix material to the stack before curing to form a consolidated component, or alternatively by arranging a stack of pre-preg plies comprising reinforcement material before curing to form a consolidated component. Alternatively (or in addition), the composite material may comprise randomly distributed reinforcement. For example, the rib foot may be formed as an injection moulded component with chopped strands or nano-tubes or particles of reinforcement material distributed through at least a portion of the composite material. The rib foot may be joined to the rib web and/or to the cover and/or to the stiffener flange and/or to the stiffener web by an adhesive bonded joint - for instance: a co- cured joint (where the parts are cured together to form the adhesive bond); a co- bonded joint (where one of the parts is cured in contact with a pre-cured other part to form the adhesive bond); or a secondary bonded joint (in which the parts are bonded together by an adhesive, where the only chemical or thermal reaction occurring to form the bond is the curing of the adhesive).

Alternatively (or in addition) the rib foot may be joined to the cover and/or to the rib web and/or to the stiffener flange and/or to the stiffener web by one or more mechanical fasteners.

The structure may be adapted to carry fuel. In this case the cover typically forms part of a sealed wall of a fuel tank which is arranged such that, when the fuel tank contains fuel, fuel pressure load acts on the cover.

The further rib web may form a sealed fuel tank wall (that is a boundary wall adapted to retain fuel on one side of the wall with substantially no movement of fuel past or through the wall to an opposite side of the wall) or alternatively it may be an internal baffle (that is an internal element within a fuel tank adapted to allow fuel to pass from one side to the other via one or more orifices or holes).

The structure is typically a cantilevered aerodynamic structure which extends into an airflow during flight of the aircraft. For instance the structure may be a main wing, horizontal stabiliser or vertical stabiliser.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a plan view of an aircraft;

Figure 2 is a schematic plan view of a starboard wing box and centre wing box;

Figure 3 is a schematic chord-wise sectional view of the starboard wing box; Figure 4 is an isometric view of a cover panel assembly of the starboard wing box viewed from an inboard direction;

Figure 5 is a plan view of the cover panel of Figure 4;

Figure 6 is a cross-section taken along a line A-A in Figure 5; Figure 7 is a cross-section taken along a line B-B in Figure 5;

Figure 8 is an isometric view of part of a rib foot;

Figure 9 is an isometric view of a noodle filler;

Figure 10 is an isometric view of the cover panel assembly viewed from an inboard direction with a baffle rib web attached; Figure 11 is an isometric view of the cover panel assembly of Figure 10 viewed from an outboard direction;

Figure 12 shows the transmission of load from the cover to the rib in the installation of Figure 10;

Figure 13 is an isometric view of an alternative cover panel assembly with stringer grow outs;

Figure 14 is an isometric view of an alternative cover panel assembly with rib feet extending between adjacent stringers;

Figure 15 is an isometric view of one part of a stringer foot;

Figure 16 is an isometric view of a noodle filler; Figure 17 is an isometric view of one part of a stringer foot; and

Figure 18 is an isometric view of a noodle filler. DETAILED DESCRIPTION OF EMBODIMENT(S)

Figure 1 shows an aircraft 1 with port and starboard wings 2, 3. Each wing has a cantilevered structure with a length extending in a spanwise direction from a root to a tip, the root being joined to an aircraft fuselage 4. The wings 2, 3 are similar in construction so only the starboard wing 3 will be described in detail with reference to Figures 2 and 3.

The main structural element of the wing is a wing box formed by upper and lower covers 4, 5 and front and rear spars 6, 7 shown in cross-section in Figure 3. The covers 4, 5 and spars 6, 7 are each Carbon Fibre Reinforced Polymer (CFRP) laminate components. Each cover has an aerodynamic surface (the upper surface of the upper cover 4 and the lower surface of the lower cover 5) over which air flows during flight of the aircraft. Each cover also has an inner surface carrying a series of stringers 8 extending in the spanwise direction. Each cover carries a large number of stringers 8, only five of which are shown in Figures 2 and 3 for purposes of clarity. Each stringer 8 is joined to one cover but not the other.

As shown in Figure 3, each spar has a C-shaped cross-section with upper and lower spar flanges each joined to the inner surface of a respective one of the covers, and a spar web extending between the spar flanges.

The wing box also has a plurality of transverse ribs, each rib being joined to the covers 4, 5 and the spars 6, 7. The ribs include an inner-most inboard rib 10 located at the root of the wing box, and a number of further ribs spaced apart from the innermost rib along the length of the wing box. The wing box is divided into two fuel tanks: an inboard fuel tank bounded by the inboard rib 10, a mid- span rib 11, the covers 4, 5 and the spars 6, 7; and an outboard fuel tank bounded by the mid-span rib 11, an outboard rib 12 at the tip of the wing box, the covers 4, 5 and the spars 6, 7.

The inboard rib 10 is an attachment rib which forms the root of the wing box and is joined to a centre wing box 20 within the body of the fuselage 4. Baffle ribs 13 (shown in dashed lines) form internal baffles within the fuel tanks which divide the fuel tanks into bays. The ribs 10,11,12 are sealed to prevent the flow of fuel out of the two fuel tanks, but the baffle ribs 13 are not sealed so that fuel can flow across them between the bays. As can be seen in Figure 2, the stringers 8 stop short of the inboard rib 10 and the outboard rib 12, but pass through the baffle ribs 13 and the mid-span rib 11.

Figures 4-7 show part of the lower cover 5 including three stringers 8. The stringers 8 are CFRP laminate components. Referring to Figure 7, each stringer has a T-shaped cross-section with a pair of flanges 8a co-cured to the cover 5, and a web or blade 8b extending upwardly from the flanges 8a away from the cover 5 to a free upper edge. Each flange 8a has a tapering lateral edge 8c. The stringers have a "roll-form" structure in which the flanges 8a and web 8b are formed from a single folded sheet with a noodle filler (not shown) between the radius portions where the sheet is folded up.

The baffle rib 13 comprises a planar metallic web 14 (shown in Figures 10-12 but omitted from Figures 4-7) connected to the upper and lower covers by a plurality of CFRP laminate rib feet 30 - six of such rib feet being shown in Figure 4. Each rib foot 30 is formed by two mirror-symmetrical parts positioned back-to-back, one of these parts being shown in Figure 8. Each part has a generally horizontal first flange 31a-c; an upstanding second flange 32; and an upstanding web 33 positioned back-to-back with the web of the other part (these webs 33 being joined together back-to-back by a co-cured joint). The first flange has upper and lower portions 31a, 31b connected by an angled step 31c. A noodle filler 28 shown in Figures 5, 6 and 9 (formed from a glass-fibre filled epoxy paste) fills the gap at the radius where the flanges 31 meet the webs 33 and then runs vertically to fill the gap at the radius where the flanges 32 meet the webs 33 (as shown in Figure 5).

The upper flange portion 31a of the rib foot is co-cured to the stringer flange 8a, and the lower flange portion 31b of the rib foot is co-cured to the inner surface of the cover 5. The angled step 31c follows the profile of the tapering edge 8c of the stringer flange to which it is co-cured. This co-cured joint (without bolts) between the rib foot flange 31a-c and the cover 5 means that no drilled bolt holes need to be provided in the cover. This enables the thickness (and hence weight) of the cover 5 to be reduced compared with a bolted arrangement. The lack of external bolts in the cover 5 also provides protection against lightning strike and improved fuel tank sealing. The second flange 32 is co-cured to the stringer web 8b, and the web 33 is joined to the rib web 14 by bolts 21 shown in Figures 10-12.

The rib web 14 has a lower edge which is separated from the inner surface of the cover 5 by a gap 22 shown in Figures 10 and 11. The opposed edges of the rib feet are separated by a gap 23 shown in Figure 4 which provides a channel permitting fuel to flow across the rib web 14 between the bays of the fuel tank through the gap 22. The lower edge of the rib web 14 is also formed with cut-outs through which the stringer webs 8b pass. Fuel can also flow between the bays through the arched upper part 26 of each cut-out. Holes (not shown) may also be provided in the rib web 14 to minimise its weight and provide further routes for fuel to flow between the bays.

Only the lower part of the rib web 14 is shown in Figures 10-12. The upper part of the rib web is connected to the upper cover 4 by rib feet 30 in a similar fashion. Also the rib web 14 has fore and aft edges not shown in Figures 10-12 which are secured to the spars 6, 7. In use the fuel tanks are filled with fuel which exerts fuel pressure on the wing box. This fuel pressure exerts a vertical load on the covers as indicated at 40 in Figure 12 which is reacted as tension in the rib web 14. This vertical load tracks from the lower cover 5 to the rib web 14 as shown at 41 via the stringer flange 8 a, the stringer web 8b, the first rib foot flange 32 and the rib foot web 33. This is the dominant load path for transmission of the vertical load from the cover to the rib web (transmitting about 85% of the load) because the stringer web 8b has a higher out-of-plane bending stiffness than the cover 5. The shear interface between the stringer web 8b and the rib foot flange 32 thus provides a path for load to track into the rib web which is not present in other conventional arrangements. A secondary load path shown at 42 in Figure 12 transmits load from the cover to the rib web via the lower portion 3 lb of the second rib foot flange.

In an alternative arrangement shown in Figure 13 the stringer flange has a grow-out portion 50 which projects laterally under the rib foot so that the first flange of the rib foot is co-cured to the stringer flange only, without contacting the cover 5. One of the rib feet 51 is formed as shown in detail in Figures 15 and 16. The rib foot comprises a pair of back-to-back parts 52, one of these parts 52 being shown in Figure 15. The part 52 is laid up as a flat stack of prepreg composite plies, each ply comprising unidirectional carbon fibres impregnated with an epoxy resin matrix. Then the flanges 53,54 are bent away from the web 55 so that they meet at a line 56, leaving a gap 57 at the corner. This gap 57 is filled by a projecting part 58 of a noodle filler (formed from a glass-fibre filled epoxy paste) shown in Figure 16.

Another one of the rib feet 70 is formed as shown in detail in Figures 17 and 18. The rib foot comprises a pair of back-to-back parts 71, one of these parts 71 being shown in Figure 17. The part 71 is laid up as a flat stack of prepreg composite plies, each ply comprising unidirectional carbon fibres impregnated with an epoxy resin matrix. Then the flanges 72,73 are bent away from the web 74, leaving a gap 75 between the flanges 72, 73. This gap 75 is filled by an arm 76 of a noodle filler (formed from a glass-fibre filled epoxy paste) shown in Figure 18. When the flat stack of prepreg plies is laid up, at least one of the plies runs into the second rib foot flange, the first rib foot flange and the rib foot web. Other plies may extend only within one of the second rib foot flange, the first rib foot flange and the rib foot web. Other ply layers may extend within any two of the second rib foot flange, the first rib foot flange and the rib foot web. In the examples above the rib feet 30, 51, 70 are formed by prepreg composite parts. Alternatively the rib foot 30, 51, 70 may be manufactured by injection moulding of epoxy resin (or other liquid matrix material) containing short fibre reinforcement elements.

In the embodiments described above the rib feet and stringers are formed as separate components which are joined together by co-curing opposed mating faces. In an alternative embodiment (not shown) some of the ply layers forming the rib foot 30 may be laid up so that they are interleaved with some of the ply layers forming the stringer 8.

In a further alternative arrangement shown in Figure 14, rib feet 60 are provided which span the gap between adjacent stringers. Each rib foot 60 has a first pair of rib foot flanges 61 joined to the web of one of the stringers, a second pair of rib foot flanges 62 joined to the stringer flanges and the inner surface of the cover 6 (only one of these flanges being visible in Figure 14), a third pair of rib foot flanges 63 joined to the web of an adjacent one of the stiffeners, and a rib foot web 64 extending between the two pairs of flanges 61,63. The rib foot web 64 is bolted to a rib web (not shown).

The cover assemblies of Figures 4, 13 and 14 are formed by placing the various components on a mould in an uncured or partially cured state. A vacuum bag is laid over the components on the mould, the space between the vacuum bag and the mould is evacuated to apply pressure, and the assembly is heated to cure the components. As the components cure, the various co-cured joints mentioned above are formed. The mould may be made from a rigid material, or more preferably from a semi-rigid material. A suitable semi-rigid material is a synthetic rubber such as Airpad (an uncured non-silicone rubber available from Airtech Europe Sari), reinforced with open weave dry carbon such as Cristex 170-100, with additional local reinforcement and therefore stiffness added with Toolmaster (R) Prepreg TMGGP4000 and TMGP4100.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.