Abstract:
The present invention provides an aircraft structure comprising a first sub-structure, having a first box cross-section, a second sub-structure, having a second box cross-section, and a joint joining a first end region of the first sub-structure to second end region of the second sub-structure, wherein the joint comprises an overlap region where the end regions of the first and the second sub-structures overlap such that the first sub-structure is inside the second sub-structure. The present invention also provides an aircraft comprising such an aircraft structure and a method of manufacturing an aircraft structure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention concerns aircraft structures. 
         [0002]    More particularly, but not exclusively, this invention concerns an aircraft structure comprising a joint between two sub-structures, in particular between two sections of wing. The invention also concerns an aircraft comprising such an aircraft structure and a method of manufacturing an aircraft structure. 
         [0003]    There are many different known ways of connecting two wing sections together. For example, a full tension fitting arrangement can be used to connect two wing box sections together or to attach a wing tip to a wing box. Such an arrangement is a quick way of connecting the two sub-structures together but has a weight penalty, as the tensioning arrangement is relatively heavy. 
         [0004]    Another way of attaching two wing sections together involves having a cruciform structure in between an outer wing box and a centre wing box on the upper cover and having a triform structure in between the two wing boxes on the lower cover. These are shown in  FIGS. 1   a  and  1   b , respectively. 
         [0005]    In  FIG. 1   a , it can be seen that a generally upright portion  11  of the cruciform structure  10  is connected to a shared rib  20  of both wing boxes  30 ,  40 . A first generally horizontal portion  12  of the cruciform structure is attached to the upper cover  37  of the outer wing box  30  and to a stringer flange  31   a  of a stringer  31  of the outer wing box  30  by bolts  33  (a to d, although not individually labelled). A second generally horizontal portion  13  of the cruciform structure  10  is attached to a stringer flange  41   a  of a stringer  41  of the centre wing box  40  again by bolts  43  (a to d, although not individually labelled). There are also additional crown fittings  32 ,  42  used to spread the load between the cruciform structure  10  and the stringers  31 ,  41  of both wing boxes  30 ,  40 . The crown fittings  32 ,  42  are attached to the shared rib  20  and cruciform structure  10  by a tension bolt  21  extending through the shared rib  20 , generally upright portion  11  of the cruciform structure  10  and crown fitting flanges  32   a,    42   a  of the crown fittings  32 ,  42 . The crown fittings  32 ,  42  are attached to the stringers  31 ,  41  of both wing boxes by fasteners  34 ,  44  through blades  32   b,    42   b  of the crown fittings  32 ,  42  and stringer blades  31   b,    41   b  of the stringers  31 ,  41 . 
         [0006]    In  FIG. 1   b  it can be seen that a generally upright portion  51  of the triform structure  50  is attached to the shared rib  20  by a bolt  22 . Two generally horizontal portions  52 ,  53  of the triform structure  50  are also attached to stringer flanges  35   a,    45   a  of stringers  35 ,  45  of both wing boxes  30 ,  40  by bolts  36   a,    36   b,    36   c,    36   d,    46   a ,  46   b,    46   c,    46   d.  In addition, a splice plate  23  (also known as a buttstrap) connects the two stringers  35 .  45 . It is also held in place by some of the same bolts  36   b,    36   c,    36   d,    46   a ,  46   b,    46   c  attaching the stringer flanges  35   a,    45   a  to the triform structure  50 . 
         [0007]    Having a cruciform and triform structure to connect two wig boxes allows the two wing boxes to be able to be completed separately. However, it also means that the joining procedure is relatively complex and requires significant fettling and shimming to meet the required tolerances. This takes a significant amount of time to do. In addition, a large number of parts are involved, which adds to the costs and weight of the aircraft. 
         [0008]    The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved aircraft structure, an improved aircraft and an improved method of manufacturing an aircraft structure. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides, according to a first aspect, an aircraft structure comprising a first sub-structure, having a first box cross-section, a second sub-structure, having a second box cross-section, and a joint joining a first end region of the first sub-structure to second end region of the second sub-structure, wherein the joint comprises an overlap region where the end regions of the first and the second sub-structures overlap such that the first sub-structure is inside the second sub-structure. 
         [0010]    Such an arrangement allows the two sub-structures to be joined together quickly and easily. The joint is also low in weight. In addition, bolts used to connect the two sub-structures are loaded under shear force, especially when the joint is chordwise, rather than under tension. 
         [0011]    Preferably, in the overlap region of the joint, the first box cross-section of the first sub-structure is contained within the second box cross-section of the second sub-structure. 
         [0012]    More preferably, in the overlap region of the joint, outer sides of the first box cross-section of the first sub-structure are in contact with inner sides of the second box cross-section of the second sub-structure. Even more preferably, all outer sides of the first box cross-section of the first sub-structure are in contact with inner sides of the second box cross-section of the second sub-structure. 
         [0013]    Even more preferably, all inner sides of the second box cross-section of the second sub-structure are in contact with outer sides of the first box cross-section of the first sub-structure. 
         [0014]    Preferably, each box cross-section of the sub-structures has four sides formed from one or more structural elements, such as spars and covers. Alternatively, one or more of the box cross-sections may have only three sides, therefore giving a partial box structure. 
         [0015]    More preferably, one or both of the box cross-sections of the sub-structures are formed by an n-shape or top-hat-shape spar on three sides and a separate cover on a fourth side. 
         [0016]    Preferably, one or both of the box cross-sections of the sub-structures are formed by a u-shape or upside-down-top-hat-shape spar on three sides and a separate cover on a fourth side. 
         [0017]    Preferably, one or both of the box cross-sections of the sub-structures are formed by a four sided box spar. Here, one or more of the sub-structures may be formed from a single structural element. 
         [0018]    Preferably, one or both of the box cross-sections of the sub-structures are formed by two spars on two opposite sides and two covers on the remaining sides. 
         [0019]    Preferably, on one or both of the sub-structures, extended portions of one or more structural elements extend further than the box cross-sections. 
         [0020]    More preferably, one or more of the structural elements of the second sub-structure are separated from other structural elements of the second sub-structure by one or more extended portions of one or more structural elements of the first sub-structure. Hence, the second sub-structure is in the form of a separated box structure. 
         [0021]    Preferably, the first and second sub-structures are attached to each other using fasteners, such as bolts, fastened through structural elements of both sub-structures. Alternatively, they could be attached together using bonding or welding. 
         [0022]    Preferably, one of the sub-structures comprises a structural element with an offset portion in the overlap region that is offset from a portion of that structural element adjacent the overlap region, such that a portion of a structural element of the other sub-structure can be located adjacent to the offset portion in the overlap region. This allows the structural element of the other sub-structure to be aligned with the non-offset portion of the one sub-structure. 
         [0023]    More preferably, the portion of the structural element of the other sub-structure is substantially in the same plane as the portion adjacent the overlap region of the structural element of the one sub-structure. 
         [0024]    Preferably, one or more sides of the second box-section of the second sub-structure are angled outwards. This allows the first sub-structure to be easily placed inside the second sub-structure. For example, one or more of the sides of the second sub-structure may be angled by an angle of approximately 3 degrees. 
         [0025]    Preferably, the first sub-structure has a cross-section that decreases away from its first end region and wherein the second sub-structure has a cross-section that increases away from its second end region. This allows the first sub-structure to be slid inside the second sub-structure from a far end towards the joint/overlap region. Hence, the second sub-structure may be a complete box cross-section prior to joining with the first sub-structure. 
         [0026]    Also according to a first aspect of the invention there is provided an aircraft comprising the aircraft structure as described above. 
         [0027]    Also according to a first aspect of the invention there is provided a method of manufacturing an aircraft structure comprising the following steps: providing a first sub-structure, providing a second sub-structure, placing a first end region of the first sub-structure adjacent the second end region of the second sub-structure, such that the end regions of the first and the second sub-structures overlap, providing any necessary additional structural elements so that both sub-structures have box cross-sections and so that the box cross-section of the first sub-structure is inside the box cross-section of the second sub-structure, and joining the first and second sub-structures together in the overlap region. 
         [0028]    Preferably, the first and second sub-structures are joined using fasteners, such as bolts, fastened through structural elements of both sub-structures. Alternatively, they could be attached together using bonding or welding. 
         [0029]    Preferably, the first sub-structure has a cross-section that decreases away from its first end region and wherein the second sub-structure has a cross-section that increases away from its second end region and wherein the second sub structure is moved inside the first sub-structure towards the second end region of the second sub-structure. This allows the first sub-structure to be slid inside the second sub-structure from a far end towards the joint/overlap region. Hence, the second sub-structure may be a complete box cross-section prior to joining with the first sub-structure. 
         [0030]    It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0031]    Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
           [0032]      FIG. 1   a  shows a chordwise cross-sectional view of a prior art joint between a centre wing box and an outer wing box at an upper cover; 
           [0033]      FIG. 1   b  shows a chordwise cross-sectional view of a prior art joint between a centre wing box and an outer wing box at a lower cover; 
           [0034]      FIG. 2   a  shows a spanwise cross-sectional view of a joint/overlap region of two wing boxes according to a first embodiment of the invention; 
           [0035]      FIG. 2   b  shows a perspective view of the two wing boxes in  FIG. 2   a;    
           [0036]      FIG. 3  shows a chordwise cross-sectional view of a joint/overlap region of two wing boxes according to a second embodiment of the invention; 
           [0037]      FIG. 4  shows a chordwise cross-sectional view of a wing box, for use in a third embodiment of the invention; and 
           [0038]      FIG. 5  shows a plan view of two wing boxes, ready to be joined together according to a fourth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]      FIG. 2   a  shows a spanwise cross-sectional view of a joint/overlap region  100  of two wing boxes  110 ,  120  according to a first embodiment of the invention. 
         [0040]    The first, inner wing box  110  comprises an n-shaped spar  111  with a generally horizontal top portion  112  and two generally vertical side portions  113 ,  114 . The side portions  113 ,  114  are each formed with an outwardly extending generally horizontal flange portion  115 ,  116  (giving the spar  111  an omega-shape). The inner wing box  110  also comprises a lower cover  117  extending underneath and between the outer edges of the flange portions  115 ,  116 . 
         [0041]    The second, outer wing box  120  comprises a generally horizontal upper cover  122  on top of the top portion  112  of the first wing box  110 . The second wing box  120  also comprises two generally vertical outwardly facing side spars  123 ,  124 . Each side spar  123 ,  124  is angled outwards at a shallow draft angle of 3 degrees. Each side spar  123 ,  124  comprises an upper  125   a,    126   a  and lower  125   b,    126   b  flange portion extending outwards. The spar flange portions  125   a ,  125   b,    126   a,    126   b  extend outwards the same distance as the flange portions  115 ,  116  of the n-shape spar  111  of the first wing box  110 . The upper cover  122  extends between from slightly beyond the upper flange portions  125   a,    126   a.  The second wing box  120  also comprises a generally horizontal lower cover  127  beneath the lower cover  117  of the first wing box  110 . The lower cover  127  extends between from slightly beyond the lower flange portions  125   b,    126   b.    
         [0042]      FIG. 2   b  shows a perspective view of the two wing boxes  110 ,  120  in  FIG. 2   a . Here, it can be seen that shear bolts  130  are used to form the two wing boxes  110 ,  120  and attach them together. For example, shear bolts  130  are bolted through the following portions:
       Upper cover  122  of second wing box  120  and upper flange portions  125   a,    126   a  side spars  123 ,  124  of second wing box  120 ,   Upper cover  122  of second wing box  120  and top portion  112  of n-shape spar  111  of first wing box  110 ,   Spars  123 ,  124  of second wing box  120  and sides  113 ,  114  of first wing box  110 ,   Lower flange portions  125   b,    126   b  of side spars  123 ,  124  of second wing box  120 , flange portions  115 ,  116  of n-shape spar  111  of first wing box  110 , lower cover  117  of first wing box  110  and lower cover  127  of second wing box  120 , and   Lower flange portions  125   b,    126   b  of side spars  123 ,  124  of second wing box  120  and lower cover  127  of second wing box  120 .       
 
         [0048]    This figure also shows an offset portion  128  of the lower flange  125   b  of the second wing box  120  in the overlap region. The offset portion  128  is raised above the plane of the rest of the flange  125   b  so as to give a space  129  for the lower cover  117  of the first wing box  110  and the flange portion  115  of the first wing box  110 . A similar offset portion and space is present for the other lower flange  126   b  of the second wing box  120 . 
         [0049]      FIG. 3  shows a chordwise cross-sectional view of a joint/overlap region  200  of two wing boxes  210 ,  220  according to a second embodiment of the invention. 
         [0050]    The first, inner wing box  210  comprises four-sided box spar  211  with a generally horizontal top portion  211 , two generally vertical side portions  212 ,  214  and a generally horizontal bottom portion  213 . 
         [0051]    The second, outer wing box  220  comprises a generally horizontal upper cover  227  on top of the top portion  211  of the first wing box  210  and extending outwards beyond the edges of the top portion  211 . The second wing box  220  also comprises a u-shaped spar  221  with a generally horizontal bottom portion  222  beneath the lower portion  213  of the first wing box  210  and two generally vertical side portions  223 ,  224  adjacent the side portions  212 ,  214  of the first wing box  210 . Each side portion  223 ,  224  is angled outwards at a shallow draft angle of 3 degrees. Each side portion  223 ,  224  comprises an upper flange portion  225 ,  226  extending outwards. The flange portions  225 ,  226  extend outwards the same distance as the flange portions upper cover  227  of the second wing box  220 . 
         [0052]      FIG. 4  shows a chordwise cross-sectional view of a wing box  310 , for use in a third embodiment of the invention. The wing box  310  is especially suited for use as an inner wing box, but could also be used as an outer wing box. 
         [0053]    The wing box  310  comprises a generally horizontal upper cover  312  and two generally vertical inwardly facing side spars  313 ,  314 . Each side spar  313 ,  314  comprises a generally horizontal upper  315   a,    316   a  and a generally horizontal lower flange portion  315   b,    316   b  extending inwards. The outer vertical portions of the side spars  313 ,  314  are in line with the edges of the upper cover  312 . The upper flange portions  315   a,    316   a  are underneath the upper cover  312 . A generally horizontal lower cover  317  is beneath the lower flange portions  315   b,    316   b.  The edges of the lower cover  317  may be in line with outer vertical portions of the side spars  313 ,  314 . As an alternative, as shown by the dashed lines, the lower cover  127  may have “grow outs”  319  such that the lower cover  317  may extend between from slightly beyond the side spars  313 ,  314  and upper cover  312 . 
         [0054]      FIG. 5  shows a plan view of two wing boxes  410 ,  420 , ready to be joined together according to a fourth embodiment of the invention. As can be seen, the inner wing box  410  has a cross section that decreases as the wing box extends away from the joining region  400 . It is noted that dashed line  430   a  represents one of the largest cross-sections of wing box  410  at a root end of the wing box  410 . Conversely, the outer wing box  420  has a cross section that increases as the wing box extends away from the joining region  400 . It is noted that dashed line  430   b  represents one of the smallest cross-sections of wing box  420  at a tip end of the wing box  420 . 
         [0055]    In  FIG. 5 , the inner wing box  410  is being slid into position with respect to wing box  420  by sliding it inside the wing box  420  so that cross-section  430   a  abuts against corresponding cross-section  430   b.    
         [0056]    This arrangement removes the need for the side portions (e.g. side spars or side portions of u- or n-shaped spars) to be at an angle to the upper/lower covers. This is because the inner wing box can be slid along through the outer wing box instead of being placed up/down into it. 
         [0057]    In the above embodiments, references to inner wing boxes and outer wing boxes refers to which wing box is located inside the other wing box in the joining/overlap region. Inner/outer wing boxes do not refer to the relative position of the wing boxes in relation to the root/tip of the wing itself. 
         [0058]    During manufacture of the above embodiments, the first, inner wing box is fully assembled with its ribs and systems installed. If the inner wing box has grow outs, as in  FIG. 4 , these may be removed. The second, outer wing box is also assembled, however, it is not fully assembled and has its upper cover missing at this stage. The shim/fettle allowance of the two wing boxes is scanned so that one of the wing boxes (generally the first, inner wing box) is machined to match the interface with the other wing box (generally the second, outer wing box). 
         [0059]    Except for the  FIG. 5  embodiment, to join the two wing boxes together, the fist, inner wing box is lowered into position with respect to the second, outer wing box. This process is aided by the shallow draft angle in the side portions/spars of the second, outer wing box. 
         [0060]    Once positioned in place, shear bolts are used to secure the two wing boxes together. Stringers (not shown) may also be connected together through simple splice plates (not shown) between the two wing boxes. These splice plates provide a similar function to the crown fittings in the prior art. However, these splice plates are loaded in shear, rather than tension (as the crown fittings generally are). 
         [0061]    Once, this has been done, the missing cover of the second, outer wing box, which has been machined to match the profile of the resulting assembly, is installed and further shear bolts are used to attach the cover. 
         [0062]    In all of the above embodiments, the join region is intended to be placed at a point on the wing away from areas where required aerodynamic contours are required. This makes assembly simplified. 
         [0063]    Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. 
         [0064]    In the above embodiments, it is envisaged that the two wing boxes are assembled as much as possible separately prior to being joined together. In particular, it is envisaged that at least one of the wing boxes is assembled with all of its ribs prior to being joined to the other wing box. In addition, systems may also be installed on one or more of the wing boxes. However, different elements of the wing boxes may be assembled into the wing boxes after they have been joined together. For example, ribs in the first, inner wing box may be installed after the first wing box has been joined to the second, outer wing box. 
         [0065]    If the first, inner wing box has external facing spar flanges, allowance needs to be made for these through joggling or the use of a separate component to ensure adequate load transfer across the joint. 
         [0066]    As an alternative to the embodiment of  FIG. 5 , the inner wing box  410  may instead by slid into the outer wing box  410  from the tip end to the root end, depending on the shape of the final wing required and the required angles of the leading edge and trailing edge of different sections of the wing provided by the wing boxes  410 ,  420 . 
         [0067]    The use of overlapping structures can be used in other places on the aircraft, rather than just joining one wing box to another at a chordwise point along the wing. For example, such a joint could be used between a root wing box and the fuselage, between a wing tip and a wing box structure (although this is unlikely as the wing tip joint is traditionally a temporary joint), between a horizontal tailplane and the fuselage, between a horizontal tailplane and the other horizontal tailplane or between a vertical tailplane and the fuselage (although this is unlikely as vertical tail planes are not traditionally made from box structures). 
         [0068]    Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.