Patent Publication Number: US-2023142588-A1

Title: Stiffener with core and shell

Description:
CROSS RELATED APPLICATION 
     This application claims priority to United Kingdom Patent Application GB 2115992.6 filed Nov. 8, 2021, the entire contents of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a stiffener, and a panel assembly incorporating such a stiffener. 
     BACKGROUND OF THE INVENTION 
     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 US2010129589. 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 WO2020/229501 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. 
     SUMMARY OF THE INVENTION 
     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. 
     Optionally 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. 
     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 stringer 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG.  1    shows an aircraft; 
         FIG.  2    shows a starboard wing of the aircraft; 
         FIG.  3    is a sectional view of a wingbox; 
         FIG.  4    is an isometric view of a stiffened panel assembly; 
         FIG.  5    is a plan view of the assembly of  FIG.  4   ; 
         FIG.  6    is a side view of the assembly of  FIG.  4   ; 
         FIG.  7    is a sectional view taken along line C-C in  FIG.  6   ; 
         FIG.  8    is a sectional view taken along line B-B in  FIG.  6   ; 
         FIG.  9    is a sectional view taken along line A-A in  FIG.  5   ; 
         FIG.  10    is a sectional view taken along line D-D in  FIG.  5   ; 
         FIG.  11 A  shows a first embodiment of the rib foot beam; 
         FIG.  11 B  shows a second embodiment of the rib foot beam; and 
         FIG.  12    is an isometric view of a stiffened panel assembly according to a further embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
       FIG.  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  42  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  FIGS.  2  and  3   . 
     The main structural element of the wing  3  is a wing box formed by upper and lower cover panels  21 ,  22  and front and rear spars  6 ,  7  shown in cross-section in  FIG.  3   . The cover panels  21 ,  22  and spars  6 ,  7  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  21  and the lower surface of the lower cover panel  22 ) over which air flows during flight of the aircraft. Each cover panel also has an inner surface carrying a series of stiffeners  8  extending in the spanwise direction  42 . Each cover panel carries a large number of stiffeners  8 , only five of which are shown in  FIG.  2    and only seven of which are shown in  FIG.  3    for purposes of clarity. A much larger number of stiffeners may be applied across the chord of the wing. Each stiffener  8  is joined to one cover panel but not the other. In the case of an aircraft wing cover panel, the stiffeners  8  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  21 ,  22  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 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  10 , a mid-span rib  11 , the cover panels  21 ,  22  and the spars  6 ,  7 ; and an outboard wing fuel tank bounded by the mid-span rib  11 , an outboard rib  12  at the tip of the wing box, the cover panels  21 ,  22  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  FIG.  2   , the stiffeners  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 . 
     Each rib  10 ,  11 ,  12 ,  13  connects the upper cover panel  21  to the lower cover panel  22 , and  FIG.  3    shows the upper and lower rib/cover connection arrangements for the rib  11  by way of example. A rib foot beam  18  is adhered to the inner surface of each cover panel  21 ,  22 , and attached to the rib  11  between the stiffeners  8  by fasteners  14  (such as bolts or rivets) which pass through the rib  11  and the rib foot beam  18 . The stiffeners  8  pass through mouse-hole openings  20  in the rib  11 . 
     Each stiffener  8  crosses over the rib foot beam  18  at an intersection. At each intersection the rib foot beam  18  is located between the panel  21 ,  22  and a respective one of the stiffeners  8 . 
     As noted above, the upper and lower cover panels  21 ,  22  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  18  from the cover panel. The interlocking rib foot/stringer arrangement enables the stiffeners  8  to hold the rib foot beam  18  down against the cover panel and prevent fuel pressure forces from separating the rib foot beam  18  from the cover panel. 
     The use of a single rib foot beam  18  per rib/cover interface makes the assembly less complex to manufacture. It is also not necessary to align multiple rib feet with each other. 
       FIGS.  4  and  5    show a panel assembly which includes the cover panel  22 ; a rib foot beam  18 ; and stiffeners  8  carried on the inner surface of the cover panel  22 . 
       FIG.  7    shows one of the stiffeners  8  in cross-section transverse to its length. Each stiffener  8  comprises a core  30  and a shell  31 . The shell  31  has a closed cross-section and fully surrounds the core  30  on all sides. In this example the shell  31  has a substantially rectangular outer profile, with rounded corners, although other shapes are possible. 
     The shell  31  is formed from a fibre-reinforced composite material, such as a carbon-fibre reinforced polymer. For example the shell  31  may comprise a layer of woven fabric which is wrapped around the core  30 , or it may be formed by braiding. 
     The shell  31  comprises a foot  32 ; a crown  33  opposite the foot  32 ; a first side wall  34 ; and a second side wall  35  opposite the first side wall  34 . The shell  31  surrounds the core  30  on all sides. 
     The foot  32  of the shell is adhered to the cover panel  22 . Beads of adhesive  36  are applied where the rounded corners of the shell  31  meet the cover panel  22 . 
     Each side wall  34 ,  35  is longer than the foot  32 , viewed in section transverse to the lengthwise direction of the stiffener, as in  FIG.  7   . Each side wall  34 ,  35  is also longer than the crown  33 , viewed in section transverse to the lengthwise direction of the stiffener, as in  FIG.  7   . 
     The first and second side walls  34 ,  35  are vertical and substantially parallel with each other. The stiffener  8  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  34 ,  35 , and the reflections analysed. The vertical orientation of the side walls  34 ,  35  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 US2010129589. However, in other embodiments the shell  31  may have a trapezoidal section so that the first and second side walls  34 ,  35  are not parallel with each other. 
     The core  30  comprises first and second battens  40 ,  41 ; a spacer  50  between the battens; and a cap  51  between the battens  40 ,  41  and the crown  33  of the shell  31 . 
     The battens  40 ,  41  are arranged side by side as shown in  FIG.  7   . Each batten comprises an inner edge facing the panel  22 ; an outer edge which faces away from the panel  22  and is covered by the cap  51 ; 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  40  is adjacent to the first side wall  34 , and the second batten  41  is adjacent to the second side wall  35 . In this embodiment the outer sides of the battens are in contact with, and optionally adhered to, the side walls  34 ,  35  of the shell. In other embodiments the outer sides of the battens may be adjacent to the side walls  34 ,  35  of the shell without being in contact with the side walls  34 ,  35  of the shell. 
     In this embodiment the battens  40 ,  41  have a rectangular cross-section, but this is not essential and other cross-sectional shapes are possible. 
     Preferably the core  30  has no more than two battens  40 ,  41 . This makes NDT testing easy and simplifies the assembly of the core  30 . 
     The battens  40 ,  41  are formed from a fibre-reinforced composite material, which may be a carbon-fibre reinforced polymer like the shell  31 , or another type of fibre-reinforced composite material. 
     The spacer  50  is formed from a material which is sufficiently rigid to control the size of the gap between the battens  40 ,  41 . For example the spacer  50  may be formed from a foam material. 
     The cap  51  is adjacent to, and in contact with, the crown  33  of the shell  31 . The cap  51  is formed from a material which is sufficiently rigid to provide impact protection. For example the cap  51  may be formed from a foam material, which may or may not be the same material as the spacer  50 . 
     The battens  40 ,  41  and the shell  31  are structural components, with a higher mass per unit volume than the spacer  50  and a higher mass per unit volume than the cap  51 . 
     Each stiffener  8  extends in a lengthwise/spanwise direction indicated by an arrow  42  in  FIGS.  2 ,  5 ,  6 ,  8  and  9   . The lengthwise/spanwise direction  42  is the spanwise direction of the wing  3 , extending outwardly towards the tip of the wing. 
     The battens  40 ,  41 , the spacer  50  and the cap  51  have respective lengths which extend in the lengthwise/spanwise direction  42  of the stiffener  8  as shown in  FIGS.  8  and  9   . The battens  40 ,  41 , the spacer  50  and the cap  51  extend continuously along a full length of the stiffener  8 , from its inboard end to its outboard end, or at least along a majority of a full length of the stiffener  8 . 
     The battens  40 ,  41  are spaced apart across the width of the stiffener by a gap, and the spacer  50  fills the gap between the battens. The battens  40 ,  41  are not in contact with each other at any point along their respective lengths. 
     The shell  31  has a depth (labelled D in  FIG.  7   ) and a width transverse to the length of the stiffener (labelled Win  FIG.  7   ). The battens  40 ,  41  are spaced apart across the width of the shell  31 , 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 US2010129589, makes the stiffener  8  lighter and easier to arrange on the panel with a small pitch between adjacent stiffeners. 
     The stiffener  8  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  40 ,  41  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  50  between the pair of battens  40 ,  41  enables the width of the stiffener  8  to be tailored by an appropriate selection of the width of the spacer  50 . 
     Sandwiching a spacer  50  between the pair of battens  40 ,  41  also enables the mechanical properties of the stiffener  8  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.  8   . The battens  40 ,  41 , the spacer  50  and the gap between the battens  40 ,  41 , have respective widths shown in  FIG.  8   . The widths of the battens  40 ,  41  decrease in the lengthwise direction  42 , i.e. towards the tip of the wing, in a transition region  52  shown in  FIG.  8   . The widths of the spacer  50 , and the gap between the battens, increase in an opposite sense over the same transition region  52 . 
     Thus the overall width of the stiffener—i.e. the width between the side walls  34 ,  35 —does not change in the transition region  52 . This ensures that the core  30  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  40 ,  41  change. 
     The stringer  8  is manufactured by assembling the core  30  with the spacer  50  between the battens  40 ,  41 ; then surrounding the core  30  with the shell  31 , for instance by wrapping or braiding the shell  31  around the core  30 . 
     The stiffener  8  may be assembled as a dry-fibre preform, i.e. with the shell  31  and the battens  40 ,  41  formed from porous dry-fibre material. Alternatively, the stiffener  8  may be assembled as a prepreg, i.e. with the shell  31  and the battens  40 ,  41  assembled from “prepreg” fibre-reinforced composite material. 
     The cover panel  22  may laid up on a mold tool as a dry-fibre preform, and the stiffeners  8  may be placed on the panel on the mold tool. Each stiffener  8  may be assembled in prepreg and pre-cured before it is placed on the cover panel  22 , or it may be placed on the cover panel  22  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  8  to the panel  22 . If each stiffener  8  is pre-cured before it is laid onto the panel  22 , then the stiffener is adhered to the panel  22  by a co-bonded joint. If each stiffener is placed on the cover panel  22  as a dry-fibre preform, then the stiffener and panel  22  preforms are co-infused by the matrix material, so that each stiffener  8  becomes adhered to the panel  22  by a co-cured joint. 
     The use of a shell  31  with a closed cross-section which fully surrounds the core  30  is advantageous because it enables the stiffener  8  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 US2010129589. 
     As shown in  FIGS.  9  and  10   , each stiffener  8  comprises a respective bridge  60  which crosses over the rib foot beam  18  at an intersection from a first side of the rib foot beam  18  (on the left-hand side of  FIGS.  9  and  10   ) to a second side of the rib foot beam  18  (on the right-hand side of  FIGS.  9  and  10   ). 
     Each bridge  60  has an outer surface  61  facing away from the panel  22  and an inner surface  62  facing towards the panel  22 . The inner surface  62  of each bridge deviates away from the panel to form a recess  63  at the intersection, and the outer surface  61  of each bridge deviates away from the panel to form a protrusion  64  at the intersection. 
     Each protrusion  64  comprises a pair of ramps  70 , and a flat apex  71  between the ramps. Each ramp  70  is rounded where it meets the apex  71 . Other shapes are possible: for instance the apex  71  and/or the ramps  70  may be continuously rounded. 
     The inner surface  62  of each bridge follows a curved path  65  as it deviates up and away from the panel. Other shapes are possible: for instance the inner surface  62  may be continuously rounded. 
     As shown in  FIG.  10   , each stiffener  8  comprises: a first stiffener portion  66  which is attached to the panel  22  on the first side of the rib foot beam  18 , the first stiffener portion  66  having a first stiffener portion depth D 1 ; and a second stiffener portion  67  which is attached to the panel on the second side of the rib foot beam  18 , the second stiffener portion  67  having a second stiffener portion depth D 2 . The bridge  60  has a bridge depth D 3  between its outer surface  61  and its inner surface  62 . 
     The bridge depth D 3  at the apex of the protrusion  64  is substantially the same as the first and second stiffener portion depths D 1 , D 2 . 
     Each shell comprises a foot  32  shown in  FIG.  7    which extends continuously across the bridge  60 . As shown in  FIG.  9   , the foot  32  has a first foot portion  32   a  which is adhered to the panel on the first side of the rib foot beam  18 , a second foot portion  32   b  which is adhered to the panel on the second side of the rib foot beam  18 , and a bridge foot portion  32   c  which deviates away from the panel at the bridge  60 . An inner surface of the bridge foot portion  32   c  provides the inner surface  62  of the bridge  60 . 
     At each intersection a pair of bridge support structures  80 ,  81  are provided. These comprise a first bridge support structure  80  between the bridge and the panel on the first side of the rib foot beam; and a second bridge support structure  81  between the bridge and the panel on the second side of the rib foot beam. The support structures  80 ,  81  are not wrapped within the shell  31 , and may be added as a part of the rib foot beam  18  for the stiffener  8  to sit on. The support structures  80 ,  81  may be made from a foam material or a carbon-fibre composite material. 
     Each foot  32  comprises reinforcement fibres which extend continuously along the bridge  60  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  32  may deviate away from the panel at each end of the bridge  60 . Some or all of the reinforcement fibres in the rest of the shell  31  may also deviate away from the panel at each end of the bridge  60 . 
     Each batten  40 ,  41  comprises reinforcement fibres which extend continuously along the bridge  60  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  40 ,  41  may deviate away from the panel at each end of the bridge. 
     As shown in  FIG.  11 A , the rib foot beam  18  has a beam recess  90  at each intersection. The beam recess  90  reduces the height of the rib foot beam  18  at the intersection so that the bridge  60  does not have to deviate too far away from the panel. 
     Each beam recess  90  has a base  91  and a pair of angled side walls  92 . The inner surface of each bridge  60  is seated in a respective one of the beam recesses  90 , in contact with the base  91 . 
     The width of the rib foot beam  18  increases at each intersection, so the width of the rib foot beam  18  at the intersection (at the base  91  of each beam recess  90 ) is greater than the width of the rib foot beam  18  at the protrusions  93  between the intersections. 
       FIG.  11 B  shows an alternative embodiment of the rib foot beam  18 . The rib foot beam  18  in  FIG.  11 B  is the same as the rib foot beam  18  in  FIG.  11 A , except the base of each beam recess has a cut-out  95  with vertical side walls. The stiffeners  8  are received in the cut-outs  95 . 
     Each bridge  60  has a pair of side walls  34 ,  35  connecting the outer surface  61  to the inner surface  62 . In the embodiment of  FIG.  11 B , the vertical side walls of the cut-out  95  contact the side walls  34 ,  35  of the bridge  60 . This provides support for the stiffeners  8 , preventing them from tipping over sideways. 
     Each stiffener  8  may be adhered to the panel  22  on each side of the rib foot beam  18  by either a co-bonded or co-cured joint as described above. 
     Similarly the rib foot beam  18  may be adhered to the panel  22  by either a co-bonded or co-cured joint. 
     Also, the inner surface  62  of each bridge may be adhered to the rib foot beam  18  by either a co-bonded or co-cured joint. 
     In summary, the aircraft wing  3  comprises an upper cover panel  21 ; a lower cover panel  22 ; ribs  10 ,  11 ,  12 ,  13  connecting the upper cover panel to the lower cover panel; and a plurality of stiffeners  8  attached to the upper and lower cover panels. The ribs  11 ,  13  are joined to each cover panel  21 ,  22  by a respective rib/cover connection arrangement shown in  FIG.  3   . Each rib/cover connection arrangement comprises a rib foot beam  18  which crosses the stiffeners  8  at a series of intersections and is attached to a respective one of the ribs  11 ,  13  between the intersections. Each stiffener  8  deviates away from the panel at each intersection to form a respective protruding bridge  60  which crosses over the rib foot beam  18  at the intersection as shown in  FIG.  4   . This improves over the arrangement in WO2020/229501 since the bridge  60  can extend continuously across the intersection without any change in the cross-section of the stiffener. 
     Each stiffener  8  has two continuous load carrying components (battens  40 ,  41 ) that run the length of the stiffener. These battens  40 ,  41  have a foam component  50  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  51  on top of the stiffener to stop edge impacts to the structural plies and improve damage tolerance. Where the stiffeners  8  interlock/cross the rib foot beams  18  there is a component (interlocking supports  80 ,  81 ) to allow the structural components to sit on top. The whole stiffener  8  is then wrapped or braided. 
       FIG.  12    shows a stiffened panel assembly according to an alternative embodiment of the invention. The lower cover panel  22  in  FIG.  12    is identical to the lower cover panel  22  in the previous embodiment. Stiffeners  108  are adhered to the panel  22 . The stiffeners  108  are identical to the stiffener  8 , including all of the elements shown in the cross-section of  FIG.  7   . The rib  11  is replaced by a rib  111  and the rib foot beam  18  is omitted. The stiffeners  108  pass through mouse holes in the rib  111  without deviating to form bridges  60 . The rib  111  has rib feet  118  which are bolted to the panel  22  between the stiffeners  108 . 
     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.  7   , or the frames may be formed as shown in  FIG.  7   . 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. 
     Where the word ‘or’ appears this is to be construed to mean ‘and/or’ such that items referred to are not necessarily mutually exclusive and may be used in any appropriate combination. 
     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.