Abstract:
A wing structure is provided, wherein the wing structure comprises; an upper covering, a lower covering, and a spar, the spar comprising; a spar web, an upper spar cap attached to the upper covering, and a lower spar cap attached to the lower covering, the wing structure also comprising a face spaced apart from the spar web extending between the upper and lower spar caps. The wing structure is arranged to contain fuel in a fuel containment area between the upper and lower coverings on one side of the spar web. The upper and lower spar caps extend from the spar web only on the other side of the spar web to the fuel containment area.

Description:
RELATED APPLICATIONS 
     The present application is a National Phase of International Application Number PCT/GB2010/050555, filed Mar. 31, 2010 and claims priority from, British Application Number 0906157.3, filed Apr. 9, 2009. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of wing structures, and in particular to aircraft wing structures. The present invention also relates to a method of assembly of a wing structure and to a wing spar. 
     An aircraft wing box is a wing structure and generally includes the structural members of the wing such as front and rear spars, ribs, braces, struts etc. The upper and lower skins of the wing enclose the wing box. Generally, the roots of the front and the rear spars are connected to the fuselage. 
     The spars and other structural members of a wing structure are generally fastened together using metal fasteners, such as rivets or metal nuts and bolts. These metal fasteners extend through the thickness of the members to be fastened. For example, a spar is usually fastened to a wing skin by fastening a metal fastener through a hole in a spar cap of the spar and a corresponding hole in the wing skin. Hence, the metal fastener extends from the outside surface of the wing to an underside of the spar cap. 
     Most large aircrafts have fuel tanks inside the wings. The fuel tanks inside the wings are generally contained between the front and rear spars. Therefore, the metal fasteners used to fasten the spars to the wing skins extend from the outside wing surface into the fuel tank. 
     Since aluminium alloy, commonly used in aircraft, is highly electrically conductive, a transient charge from lightning striking a metal fastener discharges into the rest of the aircraft structure with current from the discharge being distributed relatively evenly over the aircraft. Therefore, a typical lightning strike to a metal aircraft causes either no or only minor damage to aircraft components. 
     Composite materials (such as carbon fibre) are increasingly being used in aircraft manufacture due to their higher strength-to-weight ratio compared to aluminium alloy. However, typical composite materials are much more electrically resistive than aluminium alloy. For example, Carbon Fibre Reinforced Plastic (CFRP) is at least 2000 times more resistive than aluminium. 
     As the composite material is an electrical insulator, any lightning that hits the metal fastener is not easily dissipated within a composite wing skin. This can increase the risk of sparks or ignition of fuel in the region of the fastener. Furthermore, metal fasteners on an exposed composite surface are most susceptible to a direct lightning strike. 
     Accordingly, composite structures on an aircraft typically have some protection where a metal fastener extends into a fuel tank region. This is because any ignition events in a region of the metal fastener could risk ignition of fuel in the fuel tank. Unfortunately, typical lightning strike protection approaches are complicated and difficult to implement. For example, complex sealing or electrical bonding techniques are often used. 
     The present invention seeks to provide a wing structure, especially a composite wing structure that provides improved lightning protection. Additionally, or alternatively, the invention seeks to provide a wing structure, especially a composite wing structure that provides improved bird strike protection. Additionally, or alternatively, the invention seeks to provide an improved wing structure. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a wing structure wherein the wing structure comprises an upper covering, a lower covering, and a spar, the spar comprising a spar web, an upper spar cap attached to the upper covering, and a lower spar cap attached to the lower covering, and wherein the wing structure is arranged to contain fuel in a fuel containment area between the upper and lower coverings on one side of the spar web, and wherein the upper and lower spar caps extend from the spar web only on the other side of the spar web to the fuel containment area. 
     In the context of the present invention, the spar caps may be defined as the elements of the spar that provide for attachment of the spar to the wing coverings. In general, the spar caps extend approximately perpendicularly to the spar web. 
     In the present invention, the spar caps extend from the spar web only on one side of the spar web (the side opposite the fuel containment area). This does not preclude other parts of the spar extending away from the spar web on the fuel containment side. For example, there may be non-structural elements of the spar located on the fuel containment side of the spar web, such as spar brackets. There may also be structural elements, such as ribs, or rib posts, on the fuel containment side of the spar web. In addition, there may be elements of the spar that appear to be spar caps on the fuel containment side but that do not, in fact, provide for attaching of the spar to the wing coverings. 
     The wing structure of an embodiment of the invention described below has a spar that is only attached to the upper and lower spar coverings by the spar caps in a region outside of the fuel containment area. Hence, any metal fasteners used to fasten the wing coverings and spars together do not extend into the fuel tank. In addition, it has been found that such a wing structure with an outward facing C-shaped section spar has an increased capacity for withstanding fuel pressures over one with traditional inward facing C-shaped section spars. 
     Preferably, the wing structure comprises a face, the face being spaced apart from the spar web extending between the upper and lower spar caps. 
     In the illustrated embodiments, the face is located on the other side of the spar web to the fuel containment area and faces outwards from the spar web. 
     Such a face provides additional structure in front of the front spar or behind the rear spar. This is particularly beneficial in relation to the front spar as it provides additional structure to protect the front spar and absorb damage caused by impacts such as bird strikes. 
     Preferably, systems cables are run along a length of the spar in between the spar web and the face. This provides improved segregation of the system cables and provides additional protection of the system cables from bird strike as the cables are contained behind the face. 
     Preferably, a leading or trailing edge structure is mounted on a plate or joining structure before the plate/joining structure is mounted to the rest of the wing structure. This allows the leading or trailing edge structures to be fully assembled and equipped before they are attached to the wing structure. This simplifies assembly. 
     Preferably, the leading or trailing edge structure is mounted on the plate/joining structure using an angled bracket, the bracket having slotted attachment holes for fixing to the plate. This allows the covers to be adjusted to suit the airflow profile steps caused by the variation in the cover thickness tolerances. Using slotted holes and angled attachments allows this adjustment without the need for using buttstraps and spreader plates. These require fettling, which is time consuming and costly. 
     Preferably, the face, plate and/or joining structure has a recessed section. This allows system cables to be run along a length the spar in the recessed section. This simplifies the integration of the leading and trailing edge structures. 
     The present invention also provides an aircraft comprising such a wing structure. 
     The present invention also provides a method of assembly of a wing structure, the method comprising the steps of fully assembling a leading or trailing edge structure, attaching the leading/trailing edge structure to a mounting structure, and then attaching the mounting structure to a spar of the wing structure. Preferably, the mounting structure is a plate. Such a method allows complete fully equipped leading and trailing edge structures to be built away from the main assembly jig and then attached as complete assemblies to the wing box. This can improve build rates. In addition, disassembly for repair and maintenance of the leading and trailing edges structures could be carried out while still leaving the fuel containment area undisturbed. 
     The present invention also provides a wing spar wherein the upper spar cap of the wing spar comprises a mounting flange and the lower spar cap comprises a mounting flange, the mounting flanges extending away from the respective spar cap towards the opposite spar cap. A joining structure is attached to the mounting flanges of the upper and lower spar caps to form a box-section spar. 
     Preferably, the spar web, upper spar cap and lower spar cap are made from composite material. 
     The present invention is of greater application to relatively large commercial aircraft. The aircraft is preferably heavier than 30 tonnes dry weight, and more preferably heavier than 100 tonnes, or even 200 tonnes dry weight. The aircraft is preferably of a size equivalent to an aircraft designed to carry more than 75 passengers, and more preferably more than 200 passengers. The aircraft preferably has a wing span of at least 20 meters, and more preferably at least 40 m. The spar of the wing could typically have a height of 150 mm or more at the outboard end. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       An aircraft wing structure, embodying the present invention will now be described, by way of example only, with reference to the accompanying Figures, in which: 
         FIG. 1  is a perspective view of a prior art wing box with a top wing cover omitted for clarity, 
         FIG. 2  is a perspective view of part of a wing box according to the present invention, showing a front spar and upper and lower wing covers, 
         FIG. 3  is a side view of the part of the wing box of  FIG. 2 , and 
         FIG. 4  is a side view of a part of a wing box according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A wing box structure  10 ′ is shown generally in  FIG. 1 . It has a generally trapezoidal shape in plan and a generally rectangular cross-section. The wing box  10 ′ extends from a root section  16 ′ where the wing box  10 ′ cross-section is biggest to a tip section  17 ′ at an opposite end, where the wing box  10 ′ cross-section is smallest. 
     The wing box  10 ′ is made up of various structural members. A front spar  12 ′ runs along a front face of the wing box  10 ′ from the root  16 ′ to the tip  17 ′. A rear spar  11 ′ runs along a rear face of the wing box  10 ′ from the root  16  to the tip  17 ′. 
     The wing box  10 ′ has a lower cover  14 ′ with a generally trapezoidal shape. The lower cover  14 ′ extends between the front  12 ′and rear  11 ′ spars and from the root section  16 ′ to the tip section  17 ′. The lower cover  14 ′ has sixteen apertures  18 ′ along a longitudinal axis of the lower cover  14 ′. The upper cover has been omitted from  FIG. 1  for clarity. 
     The wing box shown partially in  FIG. 2  is similar, in terms of general layout, to the prior art wing box  10 ′ of  FIG. 1 . Reference numerals referring to features in  FIG. 2  that correspond to those in  FIG. 1  will take the reference numeral used in  FIG. 1  without the ′ suffix. 
     The wing box  10  shown in  FIG. 2  differs from the prior art wing box  10 ′ of  FIG. 1  in that the front  12  and rear  11  spars have a different construction and are fastened to the upper  13  and lower  14  covers differently. 
       FIGS. 2 and 3  show a front spar  12  attached at one edge of upper  13  and lower  14  covers of the wing box  10 . 
     The front spar  12  is made up of two parts; a main C-shaped section  20  and a front plate  30 . The main C-shaped section  20  and front plate  30  form a box-section. 
     The main section  20  has an approximately vertical spar web  21 . An upper spar cap  22  extends approximately horizontally outwards from the top of the spar web  21 . A lower spar cap  23  extends approximately horizontally outwards from the bottom of the spar web  21 . The lower spar cap  23  is slightly inclined upwards in order to follow an upwards slope of the inside of the lower cover  14 . The upper  22  and lower  23  spar caps have similar lengths. 
     At the end of the upper spar cap  22  is an approximately vertical upper mounting flange  24 . The upper mounting flange  24  extends downwardly from the upper spar cap  22 . The upper mounting flange  24  has a length approximately equal to the length of the upper spar cap  22 . Similarly, an approximately vertical lower mounting flange  25  extends upwards from the lower spar cap  23 . The lower mounting flange  25  has a length approximately equal to the length of the lower spar cap  23 . The upper  24  and lower  25  mounting flanges are at the same angle and in the same plane as each other so as to form part of a notional plane surface approximately parallel to the spar web  21 . 
     The main section  20  is attached to the upper cover  13  by metal fasteners through holes  15  along the length of the upper cover  13  and corresponding holes in the upper spar cap  22 . Similarly, the main section  20  is attached to the lower cover  14  by metal fasteners through holes (not shown) along the length of the lower cover  14  and corresponding holes in the lower spar cap  23 . 
     The main section  20  is made of composite materials and can be manufactured using filament winding or fibre placement technology, which are known in the art. The main section  20  has rounded corners where the mounting flanges  24 ,  25  meet the spar caps  22 ,  23  and where the spar caps  22 ,  23  meet the spar web  21 . 
     The front plate  30  is made up of three parts; a flat upper section  31 , a lower flat section  32  and a recessed section  33  joining the two flat sections. The flat upper section  31  is at the same angle and in the same plane as the flat lower section  32  so they form part of a notional plane surface. 
     The recessed section  33  is also a flat section of the front plate  30  and is parallel to the flat upper  31  and lower  32  sections. It is joined to the flat sections  31 ,  32  by upper  33   a  and lower  33   b  joggle sections. The upper joggle section  33   a  extends back and slightly downwards from the lower end of the flat upper section  31  and the lower joggle section  33   b  extends back and slightly upwards from the upper end of the lower flat section  32 . 
     The flat upper surface  31  is mounted against the upper mounting flange  24 . It is attached using metal fasteners through holes  34  along the length of the flat upper surface  31  and corresponding holes in the upper mounting flange  24 . The lower flat section  32  is mounted against the lower mounting flange  25 . It is attached using metal fasteners through holes  35  along the length of the flat lower surface  32  and corresponding holes in the lower mounting flange  25 . Hence, the recessed section  33  is approximately parallel to and in between the spar web  21  and the notional plane surface of the two flat sections  31 ,  32 . 
     The main section  20  is attached to the upper  13  and lower  14  covers slightly inwards from the edge of the covers such that when the front plate  30  is attached to the mounting flanges  24 ,  25  of the main section  20 , the outer sides of the flat upper  31  and lower  32  section are flush with the edge of the upper  13  and lower  14  covers. 
     The front plate  30  can be made from composite materials. It could also be metallic or made of fibre metal laminate. 
     A leading edge structure  60  is mounted to the wing box  10  in front of the front plate  30  and upper  13  and lower  14  covers. The leading edge structure  60  is attached to the wing box  10  by an upper leading edge structure  61  and a lower leading edge structure  62 . The upper leading edge structure  61  has holes  63  along its length. Similarly, the lower leading edge structure  62  also has holes (not shown) along its length. 
     The upper leading edge structure  61  is attached at one edge to an upper angled bracket  40 . The bracket  40  has two flange sections  41 ,  42  mounted at an angle of slightly less than 90 degrees to each other. The first bracket flange section  41  is attached underneath the upper leading edge structure  61  using metal fasteners through holes  63  and corresponding holes (not shown) in the first bracket flange section  41 . The second bracket flange section  42  extends downwardly from the underside of the upper leading edge structure  61  at one edge. 
     The angle between the first  41  and second  42  bracket flange sections reflects the angle of the upper cover  13  at its front edge. Hence, if the upper cover  13  is designed to be at an angle of 5 degrees to the horizontal at its front edge, the angled bracket  40  will have an angle of 85 degrees. Hence, when the second bracket flange section  42  is attached to the front plate  30 , the upper leading edge structure  61  follows the line of the upper cover  13 . 
     The second bracket flange section  42  has slotted holes  43  along its length and is attached to the front plate  30  using metal fasteners through the slotted holes  43  and corresponding (normal) holes in the flat upper section  31  of the front plate  30 . 
     Similarly, the lower leading edge structure  62  is attached at one edge to a lower angled bracket  50 . The bracket  50  has two flange sections  51 ,  52  mounted at an angle of approximately 75 degrees to each other. The first bracket flange section  51  is attached on an upper side of the lower leading edge structure  62  using metal fasteners through holes in the lower leading edge structure  62  and corresponding holes (not shown) in the first bracket flange section  51 . The second bracket flange section  52  extends upwardly from the underside of the lower leading edge structure  62  at one edge. 
     The angle between the first  51  and second  52  bracket flange sections reflects the angle of the lower cover  14  at its front edge. Here, the lower cover  14  is designed to be at an angle of 15 degrees to the horizontal at its front edge, so the angled bracket  50  has an angle of 75 degrees. Hence, when the second bracket flange section  52  is attached to the front plate  30 , the lower leading edge structure  62  follows the line of the lower cover  14 . 
     The second bracket flange section  52  has slotted holes  53  along its length and is attached to the front plate  30  using metal fasteners through the slotted holes  53  and corresponding (normal) holes in the flat lower section  32  of the front plate  30 . 
     The slotted holes  43 ,  53  allow the leading edge structure  60  to be attached to the wing box  10  such that the outer surfaces of the leading edge structure  60  can follow the surface profile line and shape of the covers  13 ,  14 . I.e. The metal fasteners (and therefore the corresponding holes in the flat sections  31 ,  32  of the front plate  30 ) can be located at different places along the length of the slotted holes  43 ,  53  so that the outer surfaces of the covers  13 ,  14  and the leading edge structure  60  line up. This allows for tolerance differences in the thickness of the covers  13 ,  14 . 
     The rear spar  11  (not shown) has a similar construction to front spar  12 . The rear spar  11  is also attached to upper  13  and lower  14  covers of the wing box  10  with the spar caps  22 ,  23  extending outwards. A trailing edge structure (not shown) can be mounted at the rear edge of the wing box  10  in a similar way to the mounting of the leading edge structure  60  at the front edge of the wing box  10 . 
     The wing box  10  is assembled by firstly attaching the main section  20  of the front spar  12  to the upper  13  and lower  14  covers and attaching the leading edge structure  60  to the front plate  30  using angled brackets  40 ,  50 . Then, the front plate  30  is attached to the main section  20  of the front spar  12 . Similarly, the trailing edge structure is attached to a rear plate (equivalent to the front plate  30 ) of the rear spar  11  before the rear plate of the rear spar  11  is attached to the main section of the rear spar  11 . 
     Systems cables can be included in the spars  11 ,  12  by containing them in the main C-shaped sections  20  before the front (or rear) plates  30  are attached. 
     Systems cables can also be run along outside of the recessed sections  33  of the front (or rear) plates  30  such that the cables can be contained on one side of the notional plane of the flat upper  31  and lower  32  sections of the front (or rear) plates  30 . 
     In use, fuel is contained in a fuel area  70  (shown in  FIG. 3 ) located inwards from the front  12  and rear  11  spars. In other words, the fuel is located on the inside of the spar webs  21 . The spar caps  22 ,  23  extend outwards from the spar webs  21 . 
     Alternatively, as shown in  FIG. 4 , the front  12  and rear  11  spars can be made such that the spars have a mounting face  26  extending between the upper and lower spar caps. The spars could be manufactured as complete box-sections including the mounting face  26 . The mounting face  26  has window apertures  27   a ,  27   b  and  27   c  in it. 
     Alternatively, the front  12  and rear  11  spars can be made in a box shape without the front (or rear) plates  30 . In other words, a box-section spar is made using a single piece construction. If made from composite materials, the box-section spar can be manufactured using a bottle core moulding process. Such a process is known in the art and involves using an inflatable sacrificial mandrel tool. 
     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. 
     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.