Patent Description:
Composite stiffeners have a variety of different forms. One well-known form is a so-called "blade" stringer. Blade stringers have a relatively high aspect ratio (depth/width) which can result in various disadvantages: for example the free edge of the blade may be prone to damage, and the blade may be prone to buckling.

Another form is an "omega" or "hat" stiffener. Hat stiffeners have a relatively low aspect ratio (depth/width) which can avoid the disadvantages of blade stringers, but can also introduce different problems: for instance increased weight, and increased pitch between the stiffeners.

An example of a "hat" stiffener is disclosed in <CIT>. In one example, composite material is laid up over foam which is placed into an inside mold line tool. Then fuselage skin is placed or laid up onto the composite material, foam, and inside mold line tool. A problem with this manufacturing process is that the stiffener must be assembled on the inside mold line tool, which can make the process complex and difficult to automate.

An aircraft panel assembly disclosed in <CIT> comprises a panel, and a plurality of stiffeners on the panel. Each stiffener has an attachment part attached to the panel and a structural part spaced apart from the panel. A rib foot beam crosses the stiffeners at a series of intersections. At each intersection the rib foot beam is located between the panel and the structural part of a respective one of the stiffeners. <CIT> describes a stiffener or stringer that comprises a structural part, a core in the structural part, a longitudinal channel in the core which extends in a length direction of the core, and a bladder <NUM> in the longitudinal channel. <CIT> teaches A composite structure is provided including a first fabric and a second fabric.

A substantially elongate and substantially rigid first member is spaced apart from and coupled with the first fabric via the second fabric.

<CIT> teaches a composite structure that is made by placing a stiffener preform and a composite structure reinforcement on a tool.

<CIT> teaches a unitized composite structure comprises a composite member having at least one integrally formed composite stiffener.

A first aspect of the invention provides a stiffener comprising: a core; and a shell which surrounds the core, wherein the shell is formed from a fibre material, and the core comprises: first and second battens arranged side by side; and a spacer between the battens, wherein the stiffener extends in a lengthwise direction, and the battens and the spacer have respective lengths which extend in the lengthwise direction of the stiffener.

According to the invention, the battens are spaced apart by a gap; the spacer is in the gap between the battens; the battens, the spacer and the gap have respective widths; the widths of the battens decrease in the lengthwise direction; and the widths of the spacer and the gap increase in the lengthwise direction.

Preferred embodiments form the subject-matter of dependent claims.

Optionally the core has a width which does not change substantially along the length of the stiffener.

Optionally the spacer is formed from a foam material.

Optionally the battens are formed from a fibre material, which is optionally a fibre-reinforced composite material.

Optionally the fibre material of the shell is a fibre-reinforced composite material.

Optionally the battens have a higher mass per unit volume than the spacer.

Optionally the stiffener further comprises a cap between the battens and the shell.

Optionally the battens have a higher mass per unit volume than the cap, and/or the cap is formed from a foam material.

Optionally the core has no more than two battens.

Optionally the shell has a depth and a width transverse to the length of the stiffener, the battens are spaced apart across the width of the shell, and the depth of the shell is greater than the width of the shell.

Optionally the shell comprises: a foot; a crown opposite the foot; a first side wall; and a second side wall opposite the first side wall.

Optionally each sidewall is longer than the foot, viewed in section transverse to the lengthwise direction of the stiffener.

Optionally the first batten is adjacent to the first side wall, and the second batten is adjacent to the second side wall.

Optionally the first and second side walls are substantially parallel with each other.

Optionally the shell has a substantially rectangular outer profile.

Optionally the battens are not in contact with each other at any point along their respective lengths.

Optionally each batten and each spacer extends continuously along a full length of the stiffener, or at least along a majority of a full length of the stiffener.

A second aspect of the present invention provides a stiffened panel assembly comprising: a panel; and a stiffener according to the first aspect adhered to the panel.

Optionally the shell comprises: a foot; a crown opposite the foot; a first side wall; and a second side wall opposite the foot, wherein the foot of the shell is adhered to the panel.

The shell has a shell depth between the foot and the crown; and a shell width between the side walls. Optionally the shell depth is greater than the shell width.

Optionally each batten comprises an inner edge facing the panel; an outer edge facing away from the panel; an inner side facing the other batten; and an outer side facing away from the other batten.

Each batten has a batten depth between its inner and outer edges; and a batten width between its inner and outer sides. For each batten, the batten depth may be greater than the batten width.

Optionally the panel assembly comprises a beam attached to the panel, the stiffener comprises a bridge which crosses over the beam at an intersection from a first side of the beam to a second side of the beam, the bridge has an outer surface facing away from the panel and an inner surface facing towards the panel, the inner surface of the bridge deviates away from the panel to form a recess at the intersection, and the outer surface of the bridge deviates away from the panel to form a protrusion at the intersection.

Optionally the bridge comprises reinforcement fibres which extend continuously along the bridge and cross over the beam at the intersection from the first side of the beam to the second side of the beam. Optionally at least some of said reinforcement fibres deviate away from the panel at each end of the bridge.

A second aspect of the present invention provides an aircraft comprising a stiffened panel assembly according to the second aspect.

A third aspect of the present invention provides a method of manufacturing a stiffener according to the first aspect, the method comprising: assembling the core with the spacer between the battens; then surrounding the core with the shell.

A fourth aspect of the present invention provides a method of manufacturing a stiffened panel assembly according to the second aspect, the method comprising: manufacturing the stiffener by a method according to the third aspect; then adhering the stiffener to the panel.

Optionally the stiffener is adhered to the panel by co-infusing the panel and the fibre material of the shell with a matrix; then curing the matrix.

<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 cover panels <NUM>, <NUM> and front and rear spars <NUM>, <NUM> shown in cross-section in <FIG>. The cover panels <NUM>, <NUM> and spars <NUM>, <NUM> are each Carbon Fibre Reinforced Polymer (CFRP) laminate components. Each cover panel has a curved aerodynamic surface (the upper surface of the upper cover panel <NUM> and the lower surface of the lower cover panel <NUM>) over which air flows during flight of the aircraft. Each cover panel also has an inner surface carrying a series of stiffeners <NUM> extending in the spanwise direction <NUM>. Each cover panel carries a large number of stiffeners <NUM>, only five of which are shown in <FIG> and only seven of which are shown in <FIG> for purposes of clarity. A much larger number of stiffeners may be applied across the chord of the wing. Each stiffener <NUM> is joined to one cover panel but not the other. In the case of an aircraft wing cover panel, the stiffeners <NUM> are commonly referred to as stringers, but the term "stiffeners" will be used below.

The wing box also has a plurality of transverse ribs, each rib being joined to the cover panels <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 cover panels <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 cover panels <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) form internal baffles within the fuel tanks which divide the fuel tanks into 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 bays. As can be seen in <FIG>, the stiffeners <NUM> 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 panel <NUM> to the lower cover panel <NUM>, and <FIG> shows the upper and lower rib/cover connection arrangements for the rib <NUM> by way of example. A rib foot beam <NUM> is adhered to the inner surface of each cover panel <NUM>, <NUM>, and attached to the rib <NUM> between the stiffeners <NUM> by fasteners <NUM> (such as bolts or rivets) which pass through the rib <NUM> and the rib foot beam <NUM>. The stiffeners <NUM> pass through mouse-hole openings <NUM> in the rib <NUM>.

Each stiffener <NUM> crosses over the rib foot beam <NUM> at an intersection. At each intersection the rib foot beam <NUM> is located between the panel <NUM>, <NUM> and a respective one of the stiffeners <NUM>.

As noted above, the upper and lower cover panels <NUM>, <NUM> provide the upper and lower walls respectively of a fuel tank. If the fuel tank is over-filled, then large fuel pressure forces can be generated which risk detaching the rib foot beam <NUM> from the cover panel. The interlocking rib foot/stringer arrangement enables the stiffeners <NUM> to hold the rib foot beam <NUM> down against the cover panel and prevent fuel pressure forces from separating the rib foot beam <NUM> from the cover panel.

The use of a single rib foot beam <NUM> per rib/cover interface makes the assembly less complex to manufacture. It is also not necessary to align multiple rib feet with each other.

<FIG> show a panel assembly which includes the cover panel <NUM>; a rib foot beam <NUM>; and stiffeners <NUM> carried on the inner surface of the cover panel <NUM>.

<FIG> shows one of the stiffeners <NUM> in cross-section transverse to its length. Each stiffener <NUM> 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 foot <NUM>; a crown <NUM> opposite the foot <NUM>; a first side wall <NUM>; and a second side wall <NUM> opposite the first side wall <NUM>. The shell <NUM> surrounds the core <NUM> on all sides.

The foot <NUM> of the shell is adhered to the cover panel <NUM>. Beads of adhesive <NUM> are applied where the rounded corners of the shell <NUM> meet the cover panel <NUM>.

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

The first and second side walls <NUM>, <NUM> are vertical and substantially parallel with each other. The stiffener <NUM> can be inspected by various non-destructive testing (NDT) techniques. In one example, ultrasound is directed into the stiffener through one of its side walls <NUM>, <NUM>, and the reflections analysed. The vertical orientation of the side walls <NUM>, <NUM> makes the stiffener 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 as in <CIT>. 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> comprises first and second battens <NUM>, <NUM>; a spacer <NUM> between the battens; and a cap <NUM> between the battens <NUM>, <NUM> and the crown <NUM> of the shell <NUM>.

The battens <NUM>, <NUM> are arranged side by side as shown in <FIG>. Each batten comprises an inner edge facing the panel <NUM>; an outer edge which faces away from the panel <NUM> and is covered by the cap <NUM>; an inner side facing the other batten; and an outer side facing away from the other batten.

Each batten has a shoulder where its outer edge meets its inner side. The battens are arranged shoulder to shoulder.

The first batten <NUM> is adjacent to the first side wall <NUM>, and the second batten <NUM> is adjacent to the second side wall <NUM>. In this embodiment the outer sides of the battens are in contact with, and optionally adhered to, the side walls <NUM>, <NUM> of the shell. In other embodiments the outer sides of the battens may be adjacent to the side walls <NUM>, <NUM> of the shell without being in contact with the side walls <NUM>, <NUM> of the shell.

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

Preferably the core <NUM> has no more than two battens <NUM>, <NUM>. This makes NDT testing easy and simplifies the assembly of the core <NUM>.

The battens <NUM>, <NUM> are 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 spacer <NUM> is formed from a material which is sufficiently rigid to control the size of the gap between the battens <NUM>, <NUM>. For example the spacer <NUM> may be formed from a foam material.

The cap <NUM> is adjacent to, and in contact with, the crown <NUM> of the shell <NUM>. The cap <NUM> is formed from a material which is sufficiently rigid to provide impact protection. For example the cap <NUM> may be formed from a foam material, which may or may not be the same material as the spacer <NUM>.

The battens <NUM>, <NUM> and the shell <NUM> are structural components, with a higher mass per unit volume than the spacer <NUM> and a higher mass per unit volume than the cap <NUM>.

Each stiffener <NUM> extends in a lengthwise/spanwise direction indicated by an arrow <NUM> in <FIG>, <FIG>, <FIG> and <FIG>. The lengthwise/spanwise direction <NUM> is the spanwise direction of the wing <NUM>, extending outwardly towards the tip of the wing.

The battens <NUM>, <NUM>, the spacer <NUM> and the cap <NUM> have respective lengths which extend in the lengthwise/spanwise direction <NUM> of the stiffener <NUM> as shown in <FIG> and <FIG>. The battens <NUM>, <NUM>, the spacer <NUM> and the cap <NUM> extend continuously along a full length of the stiffener <NUM>, from its inboard end to its outboard end, or at least along a majority of a full length of the stiffener <NUM>.

The battens <NUM>, <NUM> are spaced apart across the width of the stiffener by a gap, and the spacer <NUM> fills the gap between the battens. The battens <NUM>, <NUM> are not in contact with each other at any point along their respective lengths.

The shell <NUM> has a depth (labelled D in <FIG>) and a width transverse to the length of the stiffener (labelled W in <FIG>). The battens <NUM>, <NUM> are spaced apart across the width of the shell <NUM>, and the depth (D) of the shell is greater than the width (W) of the shell. In this example the aspect ratio (average shell depth/average shell width) is about four, although it may vary. Typically, the aspect ratio of the shell is greater than two, or greater than three.

The relatively high aspect ratio (depth/width), compared with the stiffener in <CIT>, makes the stiffener <NUM> lighter and easier to arrange on the panel with a small pitch between adjacent stiffeners.

The stiffener <NUM> also has a relatively low aspect ratio (depth/width), compared with a conventional blade stiffener, which makes it less prone to buckling and less prone to damage at its free edge.

Each batten <NUM>, <NUM> has a batten depth between its inner and outer edges; and a batten width between its inner and outer edges. For each batten, the batten depth is greater than the batten width. In this example the aspect ratio (average batten depth/average batten width) is about ten, although it may vary. Typically, the aspect ratio of each batten is greater than three, or greater than five.

Sandwiching a spacer <NUM> between the pair of battens <NUM>, <NUM> enables the width of the stiffener <NUM> to be tailored by an appropriate selection of the width of the spacer <NUM>.

Sandwiching a spacer <NUM> between the pair of battens <NUM>, <NUM> also enables the mechanical properties of the stiffener <NUM> to be varied along the length of the stiffener, by varying the relative widths of the spacer and the battens.

An example of this is shown in <FIG>. The battens <NUM>, <NUM>, the spacer <NUM> and the gap between the battens <NUM>, <NUM>, have respective widths shown in <FIG>. The widths of the battens <NUM>, <NUM> decrease in the lengthwise direction <NUM>, i.e. towards the tip of the wing, in a transition region <NUM> shown in <FIG>. The widths of the spacer <NUM>, and the gap between the battens, increase in an opposite sense over the same transition region <NUM>.

Thus the overall width of the stiffener - i.e. the width between the side walls <NUM>, <NUM> - does not change in the transition region <NUM>. This ensures that the core <NUM> has a width and cross-sectional area which does not change substantially along the length of the stiffener, even though the widths and cross-sectional areas of the battens <NUM>, <NUM> change.

The stringer <NUM> is manufactured by assembling the core <NUM> with the spacer <NUM> between the battens <NUM>, <NUM>; then surrounding the core <NUM> with the shell <NUM>, for instance by wrapping or braiding the shell <NUM> around the core <NUM>.

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

The cover panel <NUM> may laid up on a mold tool as a dry-fibre preform, and the stiffeners <NUM> may be placed on the panel on the mold tool. Each stiffener <NUM> may be assembled in prepreg and pre-cured before it is placed on the cover panel <NUM>, or it may be placed on the cover panel <NUM> as a dry-fibre preform.

The cover panel preform on the mold tool is then infused with a matrix material, which is then cured. The curing of the matrix material adheres the stiffeners <NUM> to the panel <NUM>. If each stiffener <NUM> is pre-cured before it is laid onto the panel <NUM>, then the stiffener is adhered to the panel <NUM> by a co-bonded joint. If each stiffener is placed on the cover panel <NUM> as a dry-fibre preform, then the stiffener and panel <NUM> preforms are co-infused by the matrix material, so that each stiffener <NUM> becomes adhered to the panel <NUM> 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 stiffener <NUM> to be easily assembled and handled "off-line" in an automated process, rather than being laid up "on-line" on a mold tool, as in <CIT>.

As shown in <FIG>, each stiffener <NUM> comprises a respective bridge <NUM> which crosses over the rib foot beam <NUM> at an intersection from a first side of the rib foot beam <NUM> (on the left-hand side of <FIG>) to a second side of the rib foot beam <NUM> (on the right-hand side of <FIG>).

Each bridge <NUM> has an outer surface <NUM> facing away from the panel <NUM> and an inner surface <NUM> facing towards the panel <NUM>. The inner surface <NUM> of each bridge deviates away from the panel to form a recess <NUM> at the intersection, and the outer surface <NUM> of each bridge deviates away from the panel to form a protrusion <NUM> at the intersection.

Each protrusion <NUM> comprises a pair of ramps <NUM>, and a flat apex <NUM> between the ramps. Each ramp <NUM> is rounded where it meets the apex <NUM>. Other shapes are possible: for instance the apex <NUM> and/or the ramps <NUM> may be continuously rounded.

The inner surface <NUM> of each bridge follows a curved path <NUM> as it deviates up and away from the panel. Other shapes are possible: for instance the inner surface <NUM> may be continuously rounded.

As shown in <FIG>, each stiffener <NUM> comprises: a first stiffener portion <NUM> which is attached to the panel <NUM> on the first side of the rib foot beam <NUM>, the first stiffener portion <NUM> having a first stiffener portion depth D1; and a second stiffener portion <NUM> which is attached to the panel on the second side of the rib foot beam <NUM>, the second stiffener portion <NUM> having a second stiffener portion depth D2. The bridge <NUM> has a bridge depth D3 between its outer surface <NUM> and its inner surface <NUM>.

The bridge depth D3 at the apex of the protrusion <NUM> is substantially the same as the first and second stiffener portion depths D1, D2.

Each shell comprises a foot <NUM> shown in <FIG> which extends continuously across the bridge <NUM>. As shown in <FIG>, the foot <NUM> has a first foot portion 32a which is adhered to the panel on the first side of the rib foot beam <NUM>, a second foot portion 32b which is adhered to the panel on the second side of the rib foot beam <NUM>, and a bridge foot portion 32c which deviates away from the panel at the bridge <NUM>. An inner surface of the bridge foot portion 32c provides the inner surface <NUM> of the bridge <NUM>.

At each intersection a pair of bridge support structures <NUM>, <NUM> are provided. These comprise a first bridge support structure <NUM> between the bridge and the panel on the first side of the rib foot beam; and a second bridge support structure <NUM> between the bridge and the panel on the second side of the rib foot beam. The support structures <NUM>, <NUM> are not wrapped within the shell <NUM>, and may be added as a part of the rib foot beam <NUM> for the stiffener <NUM> to sit on. The support structures <NUM>, <NUM> may be made from a foam material or a carbon-fibre composite material.

Each foot <NUM> comprises reinforcement fibres which extend continuously along the bridge <NUM> and cross over the beam at the intersection from the first side of the beam to the second side of the beam. Some or all of the reinforcement fibres in the foot <NUM> may deviate away from the panel at each end of the bridge <NUM>. Some or all of the reinforcement fibres in the rest of the shell <NUM> may also deviate away from the panel at each end of the bridge <NUM>.

Each batten <NUM>, <NUM> comprises reinforcement fibres which extend continuously along the bridge <NUM> and cross over the beam at the intersection from the first side of the beam to the second side of the beam. Some or all of the reinforcement fibres in the battens <NUM>, <NUM> may deviate away from the panel at each end of the bridge.

As shown in <FIG>, the rib foot beam <NUM> has a beam recess <NUM> at each intersection. The beam recess <NUM> reduces the height of the rib foot beam <NUM> at the intersection so that the bridge <NUM> does not have to deviate too far away from the panel.

Each beam recess <NUM> has a base <NUM> and a pair of angled side walls <NUM>. The inner surface of each bridge <NUM> is seated in a respective one of the beam recesses <NUM>, in contact with the base <NUM>.

The width of the rib foot beam <NUM> increases at each intersection, so the width of the rib foot beam <NUM> at the intersection (at the base <NUM> of each beam recess <NUM>) is greater than the width of the rib foot beam <NUM> at the protrusions <NUM> between the intersections.

<FIG> shows an alternative embodiment of the rib foot beam <NUM>. The rib foot beam <NUM> in <FIG> is the same as the rib foot beam <NUM> in <FIG>, except the base of each beam recess has a cut-out <NUM> with vertical side walls. The stiffeners <NUM> are received in the cut-outs <NUM>.

Each bridge <NUM> has a pair of side walls <NUM>, <NUM> connecting the outer surface <NUM> to the inner surface <NUM>. In the embodiment of <FIG>, the vertical side walls of the cut-out <NUM> contact the side walls <NUM>, <NUM> of the bridge <NUM>. This provides support for the stiffeners <NUM>, preventing them from tipping over sideways.

Each stiffener <NUM> may be adhered to the panel <NUM> on each side of the rib foot beam <NUM> by either a co-bonded or co-cured joint as described above.

Similarly the rib foot beam <NUM> may be adhered to the panel <NUM> by either a co-bonded or co-cured joint.

Also, the inner surface <NUM> of each bridge may be adhered to the rib foot beam <NUM> by either a co-bonded or co-cured joint.

In summary, the aircraft wing <NUM> comprises an upper cover panel <NUM>; a lower cover panel <NUM>; ribs <NUM>, <NUM>, <NUM>, <NUM> connecting the upper cover panel to the lower cover panel; and a plurality of stiffeners <NUM> attached to the upper and lower cover panels. The ribs <NUM>, <NUM> are joined to each cover panel <NUM>, <NUM> by a respective rib/cover connection arrangement shown in <FIG>. Each rib/cover connection arrangement comprises a rib foot beam <NUM> which crosses the stiffeners <NUM> at a series of intersections and is attached to a respective one of the ribs <NUM>, <NUM> between the intersections. Each stiffener <NUM> deviates away from the panel at each intersection to form a respective protruding bridge <NUM> which crosses over the rib foot beam <NUM> at the intersection as shown in <FIG>. This improves over the arrangement in <CIT> since the bridge <NUM> can extend continuously across the intersection without any change in the cross-section of the stiffener.

Each stiffener <NUM> has two continuous load carrying components (battens <NUM>, <NUM>) that run the length of the stiffener. These battens <NUM>, <NUM> have a foam component <NUM> between them allowing any thickness increases in the battens to be taken up inside the stiffener (foam thickness increases and decrease) to keep the outer profile remaining the same. There is a foam cap <NUM> on top of the stiffener to stop edge impacts to the structural plies and improve damage tolerance. Where the stiffeners <NUM> interlock/cross the rib foot beams <NUM> there is a component (interlocking supports <NUM>, <NUM>) to allow the structural components to sit on top. The whole stiffener <NUM> is then wrapped or braided.

<FIG> shows a stiffened panel assembly according to an alternative embodiment of the invention. The lower cover panel <NUM> in <FIG> is identical to the lower cover panel <NUM> in the previous embodiment. Stiffeners <NUM> are adhered to the panel <NUM>. The stiffeners <NUM> are identical to the stiffener <NUM>, including all of the elements shown in the cross-section of <FIG>. The rib <NUM> is replaced by a rib <NUM> and the rib foot beam <NUM> is omitted. The stiffeners <NUM> pass through mouse holes in the rib <NUM> without deviating to form bridges <NUM>. The rib <NUM> has rib feet <NUM> which are bolted to the panel <NUM> between the stiffeners <NUM>.

The stiffened panels described above are covers for an aircraft wing, but the invention may be applied to other types of stiffened panel assembly for an aircraft. For example the stiffened panel assembly may form a skin of an aircraft fuselage, the fuselage comprising longerons which extend in a fore-aft direction and frames which extend circumferentially around the fuselage. In this case the longerons may have bridges which deviate from the skin and cross over the frames, or the frames may have bridges which deviate from the skin and cross over the longerons. Similarly the longerons may be formed as shown in <FIG>, or the frames may be formed as shown in <FIG>. In the latter case, the lengthwise direction of the stiffener is the circumferential direction of the frame.

In other embodiments, the stiffened panel assembly may be part of a different vehicle, such as a boat or spacecraft; or it may be used in something other than a vehicle.

Claim 1:
A stiffener (<NUM>) comprising: a core (<NUM>); and a shell (<NUM>) which surrounds the core, wherein the shell is formed from a fibre material, and the core comprises: first and second battens (<NUM>, <NUM>) arranged side by side; and a spacer (<NUM>) between the battens, wherein the stiffener extends in a lengthwise direction (<NUM>), and the battens and the spacer have respective lengths which extend in the lengthwise direction of the stiffener, characterised in that the battens are spaced apart by a gap; the spacer is in the gap between the battens; the battens, the spacer and the gap have respective widths; the widths of the battens decrease in the lengthwise direction; and the widths of the spacer and the gap increase in the lengthwise direction.