Patent Description:
In an aircraft fuel tank lower cover, fuel can be trapped in stringer bays and prevented from flowing to a low point of the fuel tank where a fuel pump will be located.

In an aircraft fuel tank upper cover, air can become trapped in a similar way in stringer bays and thus prevented from flowing to a high point of the fuel tank where it can be released by a venting system. <CIT>, <CIT>, <CIT> disclose prior art documents.

A first aspect of the invention provides a fuel tank comprising a lower cover and an upper cover; wherein the lower cover or the upper cover comprises a panel assembly; the panel assembly comprises a panel and a stringer attached to the panel; the stringer comprises a pair of portions which are spaced apart in a lengthwise direction of the stringer and connected to each other by a bridge; each portion is connected to a respective end of the bridge; each end of the bridge deviates away from the panel; the stringer comprises reinforcement fibres which extend between the portions via the bridge; each reinforcement fibre deviates away from the panel at each end of the bridge; and the stringer comprises a stringer recess between the bridge and the panel, and a flow passage in the stringer recess through which fluid can flow across the stringer.

Optionally the stringer comprises a foot and a stiffening part; and the foot is attached to the panel.

Optionally the portions comprise attachment portions of the foot; and the attachment portions of the foot are attached to the panel.

Optionally the portions comprise stiffening portions of the stiffening part.

Optionally the bridge comprises a core, and a shell surrounding the core.

Optionally the bridge comprises a core and a shell surrounding the core; the shell comprises a shell foot which is attached to the panel; the portions comprise portions of the core; and the shell foot is between the bridge and the panel.

Optionally the stringer comprises a core and a shell surrounding the core; the shell comprises a shell foot; and the portions comprise attachment portions of the shell foot which are attached to the panel.

Optionally the bridge comprises a fibre-reinforced composite material, and the reinforcement fibres are fibres of the fibre-reinforced composite material.

Optionally the reinforcement fibres follow curved paths as they deviate away from the panel at each end of the bridge.

Optionally the stringer is adhered to the panel.

The stringer is attached to one cover but not the other. That is, the stringer may be attached to the upper cover and not attached to the lower cover; or the stringer may be attached to the lower cover and not attached to the upper cover.

Optionally the fuel tank further comprises a pair of further stringers attached to the panel on opposites sides of the stringer; a first stringer bay; and a second stringer bay, wherein each stringer bay is between the stringer and a respective one of the further stringers, and the flow passage enables fluid to flow from the first stringer bay to the second stringer bay.

Each further stringer is attached to one cover but not the other. That is, each further stringer may be attached to the upper cover and not attached to the lower cover; or each further stringer may be attached to the lower cover and not attached to the upper cover.

Optionally the lower cover comprises the panel assembly; and the flow passage enables liquid fuel to flow across the stringer.

Optionally the fuel tank further comprises a rib attached to the panel; wherein the rib comprising a rib recess, the stringer passes through the rib recess between the rib and the panel, and the rib recess is configured to provide a flow passage through which fluid can flow across the rib.

Optionally the fuel tank further comprises a fitting in the stringer recess, wherein the flow passage is in the fitting.

Optionally the bridge comprises an outer surface which deviates away from the panel at each end of the bridge to form a protrusion.

Optionally the bridge has a depth at an apex of the protrusion; and each portion has a depth which is substantially the same as the depth of the bridge at the apex of the protrusion.

Optionally the protrusion comprises a pair of ramps, and an apex between the ramps.

Optionally the bridge and/or each reinforcement fibre deviates away from the panel to form a hump.

Optionally the fuel tank is an aircraft fuel tank.

Optionally the reinforcement fibres are Odegree fibres which extend parallel with the lengthwise direction of the stringer. Alternatively the reinforcement fibres may follow spiral paths along the length of the stringer, wherein a central axis of each spiral path deviates away from the panel at each end of the bridge.

A further aspect of the invention provides an aircraft wing comprising an aircraft fuel tank according to the first aspect of the invention, wherein the aircraft wing extends in a spanwise direction from a wing root to a wing tip; and the lengthwise direction of the stringer extends in the spanwise direction.

A further aspect of the invention provides a fuel tank comprising a lower cover and an upper cover; wherein the lower cover or the upper cover comprises a panel assembly; the panel assembly comprises a panel and a stringer attached to the panel; the stringer comprises a pair of portions which are spaced apart in a lengthwise direction of the stringer and connected to each other by a bridge; each portion is connected to a respective end of the bridge; each end of the bridge deviates away from the panel; the bridge comprises an outer surface which deviates away from the panel at each end of the bridge to form a protrusion; the stringer comprises reinforcement fibres which extend between the portions via the bridge; and the stringer comprises a stringer recess between the bridge and the panel, and a flow passage in the stringer recess through which fluid can flow across the stringer.

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

The main structural element of the wing <NUM> is a wing box formed by upper and lower covers <NUM>, <NUM> and front and rear spars <NUM>, <NUM> shown in cross-section in <FIG>. The covers <NUM>, <NUM> and spars <NUM>, <NUM> are each Carbon Fibre Reinforced Polymer (CFRP) laminate components. Each cover comprises a panel assembly with a curved aerodynamic outer surface (the upper surface of the upper cover <NUM> and the lower surface of the lower cover <NUM>) over which air flows during flight of the aircraft. The panel of each panel assembly also has an inner surface carrying a series of stringers extending in the spanwise direction <NUM>. Each cover carries a large number of stringers, only five of which are shown in <FIG> (labelled 8a-e) and only seven of which are shown in <FIG> for purposes of clarity. One of the stringers 8c is labelled in <FIG> and <FIG>. A much larger number of stringers may be applied across the chord of the wing. Each stringer is joined to one cover but not the other.

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

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

Each rib <NUM>, <NUM>, <NUM>, <NUM> connects the upper cover <NUM> to the lower cover <NUM>, and <FIG> shows the upper and lower rib/cover connection arrangements for the rib <NUM> by way of example. The stringers 8c etc. pass through rib recesses <NUM> in the rib <NUM>.

As noted above, the upper and lower covers <NUM>, <NUM> provide the upper and lower walls respectively of the fuel tanks of the wing.

<FIG> shows part of the lower cover <NUM>, an adjacent pair of ribs <NUM>, <NUM>, and five of the stringers-. The lower cover <NUM> comprises a panel assembly 22a which is shown in <FIG> without the ribs <NUM>, <NUM>. The panel assembly of <FIG> comprises a panel 22a and the stringers, which are numbered individually as 8a-8e.

As shown in <FIG>, the ribs <NUM>, <NUM> are attached to the panel 22a by fasteners which pass through rib feet. The rib recesses <NUM> (or "mouseholes") are between the rib feet, and the stringers passes through the rib recesses <NUM> between the rib and the panel 22a. The ribs <NUM>, <NUM> are attached to the upper cover <NUM> in a similar way.

The rib recesses in the baffle rib <NUM> are not sealed, so they provide flow passages through which liquid can flow across the baffle rib <NUM>. The rib recesses in the mid-span rib <NUM> may be sealed to prevent such flow across the rib <NUM>.

One of the stringers 8c is formed with a stringer recess, which will be explained further below. A pair of further stringers 8b, 8d are attached to the panel 22a on opposites of the stringer 8c. The panel assembly comprises a first stringer bay <NUM> and a second stringer bay <NUM>. Each stringer bay is between the stringer 8c and a respective one of the further stringers 8b, 8d.

A flow passage is provided in the stringer recess through which liquid can flow across the stringer 8c, from the first stringer bay <NUM> to the second stringer bay <NUM>. The second stringer bay <NUM> may be at a low point of the wing, and may contain a fuel pump or other fuel pickup (not shown).

The stringer 8c is shown in cross-section in <FIG>. The stringer 8c comprises a pair of feet and a stiffening part. In this case the stiffening part is a blade <NUM> which extends away from the panel 22a to a free crown <NUM>, and each foot comprises a flange <NUM> which extends away from the blade <NUM> to a flange edge <NUM>. The stringer 8c in this case has a conventional "T-section" cross-section. The stringer 8c may be manufactured by forming a pair of L-section parts and joining them back-to-back. A noodle <NUM> fills the gap where the corners of the L-section parts meet.

Each L-section part may comprise a carbon-fibre reinforced epoxy resin composite material, or any other suitable fibre-reinforced composite material. The L-section parts may be formed by tape laying, or any other suitable manufacturing technique.

The stringer 8c is symmetrical, so only one of the feet will be described. As shown in <FIG>, the foot comprises a pair of attachment portions 52a, 52b and a bridge 52c. The pair of attachment portions 52a, 52b are spaced apart in a lengthwise direction of the stringer 8c, which extends in the spanwise direction <NUM> of the wing.

The pair of attachment portions 52a, 52b are connected to each other by the bridge 52c. Each attachment portion 52a, 52b of the foot comprises an inner (lower) surface adhered to the panel 22a, and an outer (upper) surface facing away from the panel 22a.

Each attachment portion 52a, 52b is connected to a respective end of the bridge 52c. Each end of the bridge deviates away from the panel to form a hump which makes space for a concave stringer recess <NUM> in the underside of the stringer 8c. The stringer recess <NUM> is positioned between the bridge 52c and the panel 22a.

The bridge 52c follows a curved path as it deviates away from the panel 22a, then it follows a planar ramp <NUM> up to a flat apex <NUM> of the bridge 52c.

The bridge 52c has an inner (lower) surface which deviates away from the panel at each end of the bridge to form the stringer recess <NUM>. The bridge also has an outer (upper) surface which deviates away from the panel at each end of the bridge to form a convex protrusion.

A fitting <NUM> in the stringer recess <NUM> has a through-hole <NUM> which provides a flow passage through which liquid fuel can flow across the stringer 8c. The fitting <NUM> may be formed from a foam material, a fibre-reinforced epoxy resin composite material made from recycled short carbon-fibres, or any other suitable lightweight material. The inner (lower) surface of the bridge 52c is adhered to the insert <NUM>.

The flow passage <NUM> stops liquid fuel being trapped in the first or second stringer bay. For example if the second stringer bay <NUM> is lower than the first stringer bay <NUM>, then the flow passage <NUM> enables the fuel to flow under gravity from the first stringer bay <NUM> to the second stringer bay <NUM>. Such flow becomes important when the fuel level drops (through use or defuel) so that only a small slug of fuel remains. The flow passage <NUM> allows the fuel to flow to the next stringer bay (usually running forward to aft due to the angle of incidence of the wing) until it gets to the lowest tank point, the sump, where the fuel pump pick up is located. Thus the amount of trapped fuel is kept relatively low.

In this example, only one of the stringers 8c has a stringer recess, but optionally all or some of the stringers 8a-e may have a similar stringer recess.

As noted above, the pair of L-section parts may be formed by tape laying, or any other suitable manufacturing technique. During this process, carbon reinforcement fibres are laid up which extend parallel to the lengthwise direction of the stringer, which is aligned with the spanwise direction <NUM> of the wing. These reinforcement fibres are known as Odegree fibres, and preferably they run continuously along the full length of the stringer or at least along a majority of the length of the stringer. The continuity of these Odegree fibres in the stringer foot is maintained by the undulating shape of the bridge 52c, which ensures that such Odegree fibres do not need to be cut or otherwise terminated at the stringer recess <NUM>. Thus the Odegree fibres extend continuously between the attachment portions 52a, 52b of the foot via the bridge 52c.

In order to maintain continuity of the Odegree fibres through the full thickness of the foot, each Odegree fibre deviates away from the panel at each end of the bridge to form a hump following the same undulating contour as the bridge. An exemplary pair of Odegree fibres <NUM>, <NUM> is indicated in <FIG>. The bridge 52c comprises a large number of such undulating Odegree fibres.

The Odegree fibres <NUM>, <NUM> follow the undulating shape of the bridge 52c. Thus the Odegree fibres follow curved paths as they deviate away from the panel at each end of the bridge 52c, then they follow ramps up to a flat apex between the ramps. Each ramp is rounded where it meets the apex.

The stringer foot may also comprise fibres running at different angles, for instance they may be 45degree fibres running at +/-45degrees to the length of the stringer.

In this example all of the Odegree fibres in the bridge 52c deviate away from the panel at each end of the bridge 52c. However this is not essential, and optionally further reinforcement fibres may be provided in the bridge 52c which either do not deviate away from the panel or do not extend along the full length of the bridge 52c.

In this example, the crown of the stringer (which in this case is the crown <NUM> of the blade <NUM>) has no protrusion at the recess, as can be seen most clearly in <FIG>. Thus the Odegree fibres in the blade <NUM> follow straight paths rather than following the undulating shape of the bridge 52c.

<FIG> show an alternative embodiment. The ribs <NUM>, <NUM> and cover panel 22a are given the same reference number and will not be described again. The stringers 8a-d are replaced by stringers 108a-d, one of which (stringer 108c) is formed with a stringer recess.

<FIG> show the stringer 108c in cross-section transverse to its length. The other stringers have the same construction.

The stringer 108c comprises a core <NUM> and a shell <NUM>. The shell <NUM> has a closed cross-section and fully surrounds the core <NUM> on all sides. In this example the shell <NUM> has a substantially rectangular outer profile, with rounded corners, although other shapes are possible.

The shell <NUM> is formed from a fibre-reinforced composite material, such as a carbon-fibre reinforced polymer. For example the shell <NUM> may comprise a layer of woven fabric which is wrapped around the core <NUM>, or it may be formed by braiding.

The shell <NUM> comprises a shell foot <NUM>; a shell crown <NUM> opposite the shell foot <NUM>; a first side wall <NUM>; and a second side wall <NUM> opposite the first side wall <NUM>. The shell foot <NUM> provides a foot of the stringer 108c and is adhered to the panel 22a. Beads of adhesive (not shown) may be applied where the rounded corners of the shell <NUM> meet the panel 22a, to help reinforce the skin to stringer connection. These beads of adhesive could be applied pre-infusion and pre-cure.

The core <NUM> and the vertical side walls <NUM>, <NUM> of the shell provide a stiffening part of the stringer.

Each side wall <NUM>, <NUM> is longer than the shell foot <NUM>, viewed in section transverse to the lengthwise direction of the stringer, as in <FIG>. Each side wall <NUM>, <NUM> is also longer than the shell crown <NUM>, viewed in section transverse to the lengthwise direction of the stringer, as in <FIG>.

The first and second side walls <NUM>, <NUM> are vertical and substantially parallel with each other. The stringer 108c can be inspected by various non-destructive testing (NDT) techniques. In one example, ultrasound is directed into the stringer through one of its side walls <NUM>, <NUM>, and the reflections analysed. The vertical orientation of the side walls <NUM>, <NUM> makes the stringer easy to inspect in this way, because the ultrasound is directed back to the NDT probe rather than being directed up at an angle by an oblique sidewall. However, in other embodiments the shell <NUM> may have a trapezoidal section so that the first and second side walls <NUM>, <NUM> are not parallel with each other.

The core <NUM> is formed from a fibre-reinforced composite material, which may be a carbon-fibre reinforced polymer like the shell <NUM>, or another type of fibre-reinforced composite material.

The core <NUM> may be formed from a single piece as shown in <FIG>, or from multiple battens of fibre-reinforced composite material. Foam fillers, caps, or other elements may also form part of the core <NUM>.

In this embodiment the core <NUM> has a rectangular cross-section, but this is not essential and other cross-sectional shapes are possible.

The shell <NUM> has a depth (labelled D1 in <FIG>) and a width transverse to the length of the stringer (labelled W in <FIG>). The depth (D1) of the shell is greater than the width (W) of the shell. In this example the aspect ratio (depth/width) is about four, although it may vary.

The relatively high aspect ratio (depth/width), compared with the T-section stringer 8c of <FIG> makes the stringer 108c lighter and easier to arrange on the panel 22a with a small pitch between adjacent stringers, less prone to buckling, and less prone to damage at its free edge.

The stringer 108c is manufactured by surrounding the core <NUM> with the shell <NUM>, for instance by wrapping or braiding the shell <NUM> around the core <NUM>.

The stringer 108c may be assembled as a dry-fibre preform, i.e. with the shell <NUM> and the core <NUM> formed from porous dry-fibre material. Alternatively, the stringer 108c may be assembled as a prepreg, i.e. with the shell <NUM> and the core <NUM> assembled from "prepreg" fibre-reinforced composite material.

The panel 22a may be laid up on a mold tool as a dry-fibre preform, and the stringers 108a-d may be placed on the panel on the mold tool. Each stringer 108a-d may be assembled in prepreg and pre-cured before it is placed on the panel 22a, or it may be placed on the panel 22a as a dry-fibre preform.

The cover preform on the mold tool is then infused with a matrix material, which is then cured. The curing of the matrix material adheres the stringers 108a-d to the panel 22a. If each stringer 108a-d is pre-cured before it is laid onto the panel 22a, then the stringer is adhered to the panel 22a by a co-bonded joint. If each stringer is placed on the panel 22a as a dry-fibre preform, then the stringer and panel 22a preforms are co-infused by the matrix material, so that each stringer 108a-d becomes adhered to the panel 22a by a co-cured joint.

The use of a shell <NUM> with a closed cross-section which fully surrounds the core <NUM> is advantageous because it enables the stringer 108a-d to be easily assembled and handled "off-line" in an automated process, rather than being laid up "on-line" on a mold tool.

As shown in <FIG>, the core <NUM> comprises inboard and outboard portions 30a, 30b and a bridge 30c. The portions 30a, 30b of the core are spaced apart in the lengthwise direction of the stringer 108c and connected to each other by the bridge 30c. Each portion 30a, 30b of the core is connected to a respective end of the bridge 30c. Each end of the bridge 30c deviates away from the panel 22a to form a hump with a concave stringer recess <NUM> under the bridge. As in the previous embodiment of <FIG>, this stringer recess <NUM> is positioned between the bridge 30c and the panel 22a. Unlike the previous embodiment, the shell foot <NUM> is positioned between the stringer recess <NUM> and the panel 22a.

The bridge 30c follows a curved path as it deviates away from the panel 22a, then it follows a planar ramp up to a flat apex. Other humped shapes are possible: for instance the apex and/or the ramps may be continuously rounded.

The bridge 30c has an inner (lower) surface which deviates away from the panel at each end of the bridge 30c to form the stringer recess <NUM>. The bridge 30c also has an outer (upper) surface which deviates away from the panel 22a at each end of the bridge to form a convex protrusion.

As shown in <FIG>, the inboard portion 30a of the core has a depth D1 between its inner and outer surfaces; the outboard portion 30b of the core has a depth D2 between its inner and outer surfaces, and the bridge 30c has a bridge depth D3 between its inner and outer surfaces at the apex of the protrusion. The bridge depth D3 at the apex of the protrusion is substantially the same as the depths D1, D2 of the inboard and outboard portions. In other words, the depth of the bridge 30c does not vary along the length of the bridge 30c.

The core <NUM> may be formed by tape laying, or any other suitable manufacturing technique. During this process, carbon reinforcement fibres are laid up which extend in the lengthwise direction of the stringer, which is aligned with the spanwise direction <NUM> of the wing. These reinforcement fibres are known as Odegree fibres, and preferably they run continuously along the full length of the stringer or at least a majority of its length. The continuity of these Odegree fibres is maintained by the undulating shape of the bridge 30c, which ensures that such Odegree fibres do not need to be cut or otherwise terminated at the stringer recess. Thus the Odegree fibres extend continuously between the inboard and outboard portions 30a, 30b of the core via the bridge 30c.

In order to maintain continuity of the Odegree fibres through the full thickness of the core <NUM>, each Odegree fibre deviates away from the panel at each end of the bridge to form a hump following the same undulating contour as the inner and outer surfaces of the bridge. An exemplary pair of Odegree fibres <NUM>, <NUM> is indicated in <FIG>. The bridge 30c comprises a large number of such undulating Odegree fibres.

The Odegree fibres follow the undulating shape of the bridge. Thus the Odegree fibres follow curved paths as they deviate away from the panel at each end of the bridge, then they follow ramps up to a flat apex between the ramps. Each ramp is rounded where it meets the apex. Other humped shapes are possible: for instance the apex and/or the ramps may be continuously rounded.

The fibres may be laid up as tape layers, and optionally the Odegree fibres <NUM>, <NUM> may be in the same vertically oriented tape layer. In this case the Odegree fibres <NUM>, <NUM> deviate in the vertical plane of the tape layer, rather than deviating out of the plane of the tape layer.

Optionally a continuous tow shearing technique may be used to lay up the undulating tape layers of the bridge 30c, for example as described in: <NPL>.

The core <NUM> may also comprise fibres running at different angles, for instance they may be 45degree fibres running at +/-45degrees to the length of the stringer.

The side walls <NUM>, <NUM> of the shell and the shell crown <NUM> may also have Odegree carbon reinforcement fibres which follow an undulating path like the Odegree fibres in the bridge 30c, ensuring fibre continuity in the shell <NUM> as well as the core <NUM>. Alternatively, the shell <NUM> may have no Odegree carbon fibres. For instance the shell <NUM> may consist of braided fibres running at +/-45degrees to the length of the stringer, which follow spiral paths along the length of the stringer. In this case, the central axis of each spiral path will follow an undulating path like the Odegree fibres in the bridge 30c, so the central axis of the spiral path deviates away from the panel at each end of the bridge.

A fitting <NUM> in the stringer recess <NUM> has a through-hole which receives a tube <NUM>. The tube <NUM> provides a flow passage <NUM> in the stringer recess <NUM> through which liquid fuel can flow across the stringer 108c. The fitting <NUM> may be formed from a foam material, a fibre-reinforced epoxy resin composite material made from recycled short carbon-fibres, or any other suitable lightweight material. The inner (lower) surface of the bridge 30c and the outer (upper) surface of the shell foot <NUM> are adhered to the fitting <NUM>.

The tube <NUM> is formed from a non-porous and gas-tight material, such as a polymer, which does not fill up with resin during the resin infusion process. The tube <NUM> helps the fitting <NUM> keep its shape during the resin infusion process and prevents the flow passage <NUM> from filling with resin. Optionally, the tube <NUM> may be replaced by a solid Polytetrafluoroethylene (PTFE) plug, which prevents the through-hole in the fitting <NUM> from filling with resin and is then removed after cure.

An advantage of the stringer 108c is that it can be manufactured "off-line" as a single part, with the shell <NUM> enclosing not only the core <NUM> but also the fitting <NUM> and the tube/plug <NUM>. The single part can then be laid onto the panel 22a and adhered to the panel 22a.

<FIG> show an alternative embodiment. The ribs <NUM>, <NUM> and cover panel 22a are given the same reference number and will not be described again. The stringers 8a-d, 108a-d are replaced by stringers 208c etc., one of which (stringer 208c) is formed with a stringer recess.

<FIG> show the stringer 208c in cross-section transverse to its length. The other stringers in <FIG> have the same cross-section.

The stringer 208c has a similar construction to the stringer 108c, with a core <NUM> and a shell <NUM>. The same reference numbers are given for the various elements of the core and shell, and these elements will not be described again.

The stringer 208c has a stringer recess <NUM> which receives the fitting <NUM>. In this case no tube <NUM> is provided, so a through-hole <NUM> of the fitting <NUM> provides the flow passage in the stringer recess <NUM> through which liquid fuel can flow across the stringer 208c.

In the stringer 108c of <FIG>, the shell foot <NUM> is adhered to the panel 22a along its full length, so that the shell foot <NUM> is positioned between the stringer recess <NUM> and the panel 22a. In the stringer 208c of <FIG>, the shell foot <NUM> deviates away from the panel at the stringer recess <NUM> as shown most clearly in <FIG>. Thus in the embodiment of <FIG>, the shell foot <NUM> is not positioned between the stringer recess <NUM> and the panel 22a. An advantage of this embodiment is that the flow passage <NUM> of the stringer 208c is slightly closer to the panel 22a than the flow passage <NUM> of the stringer 108c.

The shell foot <NUM>, the side walls <NUM>, <NUM> of the shell and the shell crown <NUM> may have Odegree carbon reinforcement fibres which follow an undulating path like the Odegree fibres in the bridge 30c, ensuring fibre continuity in the shell <NUM> as well as the core <NUM>. Alternatively, the shell <NUM> may have no Odegree carbon fibres. For instance the shell <NUM> may consist of braided fibres running at +/-45degrees to the length of the stringer, which follow spiral paths along the length of the stringer. In this case, the axis of each spiral path (i.e. its geometric centre) will follow an undulating path like the Odegree fibres in the bridge 30c, so the axis of the spiral path deviates away from the panel at each end of the bridge.

<FIG> show an alternative embodiment which is identical to the embodiment of <FIG>, except the fitting <NUM> is omitted. The same reference numbers are used for identical components, which will not be described again.

Since no fitting <NUM> or tube <NUM> is provided, the full area of the stringer recess <NUM> provides the flow passage through which liquid fuel can flow across the stringer 208c.

<FIG> schematically illustrates the undulating paths of the Odegree fibres <NUM>, <NUM> as they deviate away from the panel at each end of the bridge 30c. The reinforcement fibres of the shell foot <NUM>, the shell side walls <NUM>, <NUM> and the shell crown <NUM> may follow similar undulating paths. The Odegree fibres <NUM>, <NUM> in the stringer 8c may follow undulating paths with a similar profile, in line with the upper and lower surfaces of the bridge 52c.

In this example, all of the Odegree fibres in the bridge 30c deviate away from the panel at each end of the bridge 30c to form a humped or undulating shape. However this is not essential and optionally further reinforcement fibres may be provided in the bridge 30c which either do not deviate away from the panel 22a or do not extend along the full length of the bridge 30c.

The stiffened panel assemblies described above are covers for an aircraft wing, but the invention may be applied to other types of stiffened panel assembly for an aircraft fuel tank which may be located in some other part of the aircraft, such as the fuselage.

The invention may also be applied to fuel tanks other than aircraft fuel tanks - for example fuel tanks for other vehicles, or static fuel tanks.

An alternative method of providing a drain hole in a stringer, not part of the present invention, would be to provide a conventional "T-section" stringer and drill a hole in the stringer blade. This would create fibre discontinuities and resulting stress concentrations which affect the static strength of the stringer. The present invention provides reinforcement fibres which deviate away from the panel at each end of the bridge, resulting in a more continuous fibre arrangement.

In the embodiments described above, flow passages are provided in stringers of the lower cover <NUM> to enable liquid fuel to migrate between stringer bays. In an alternative embodiment, similar flow passages may be provided in one or more stringers of the upper cover <NUM>, to enable air to migrate between stringer bays.

As the fuel tank is filled with liquid fuel, air can become trapped between stringer blades and the outboard boundary of the fuel tank. Flow passages in the stringers allow the air to migrate between stringer bays to the top of the tank where a vent system is provided. The reverse applies when the tank is emptied. Thus flow passages in the stringers of the upper cover <NUM> enable the fuel tank to be filled to almost maximum capacity.

In summary, the embodiments of the invention described above provide a panel assembly comprises a panel 22a and stringers attached to the panel. At least one of the stringers 8c, 108c, 208c has a flow passage through which fluid (liquid or gas) can flow across the stringer. The stringer comprises a pair of portions 52a/52b, 30a/30b which are spaced apart in a lengthwise direction of the stringer and connected to each other by a bridge 52c, 30c. Each portion is connected to a respective end of the bridge and each end of the bridge deviates away from the panel. The stringer comprises reinforcement fibres which extend between the portions via the bridge and deviate away from the panel at each end of the bridge. These reinforcement fibres may be Odegree fibres extending parallel to the lengthwise direction of the stringer, or they may follow spiral paths as they run along the length of the stringer. The stringer comprises a stringer recess between the bridge and the panel, and the flow passage is located in the stringer recess.

Optionally not all of the reinforcement fibres of the stringer extend between the portions via the bridge and deviate away from the panel at each end of the bridge. For example, further reinforcement fibres may be provided in the stringer which do not extend between the portions via the bridge, such as 90degree fibres extending transverse to the lengthwise direction of the stringer or 45degree fibres extending at 45degrees to the lengthwise direction of the stringer. Alternatively, further Odegree reinforcement fibres may be provided which extend between the portions via the bridge but do not deviate away from the panel as they do so.

Claim 1:
A fuel tank comprising a lower cover (<NUM>) and an upper cover (<NUM>); wherein the lower cover or the upper cover comprises a panel assembly; the panel assembly comprises a panel (22a) and a stringer (8c) attached to the panel; the stringer comprises a pair of portions (52a, 52b) which are spaced apart in a lengthwise direction of the stringer and connected to each other by a bridge (52c); each portion is connected to a respective end of the bridge; each end of the bridge deviates away from the panel; and the stringer comprises a stringer recess (<NUM>) between the bridge and the panel, and a flow passage (<NUM>) in the stringer recess through which fluid can flow across the stringer, characterized in that the stringer comprises reinforcement fibres which extend between the portions via the bridge; each reinforcement fibre deviates away from the panel at each end of the bridge.