Abstract:
An aircraft structure, for example a wing ( 20 ), includes a structural skeleton having structural elements (for example, spars ( 60 ) and ribs ( 70 )) enclosed within a skin ( 200 ). A fastener ( 110   a,b ) is fastened to the skeleton and wholly within the skin ( 200 ). The fastener ( 110   a,b ) fastens to the structure ( 20 ) a bracket ( 120 ) for carrying stores ( 80 ). The fastener ( 110   a,b ) includes a body ( 300 ) defining an internal bore ( 230 ). The body ( 300 ) includes an internal threaded region ( 330 ), within the internal bore ( 230 ), for fastening the bracket ( 120 ) to the aircraft structure ( 20 ).

Description:
RELATED APPLICATIONS 
     The present application is based on, and claims priority from, British Application Number 0526084.9, filed Dec. 21, 2005, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to an improved aircraft structure and also to an improved fastener for attachment of stores to an aircraft structure. More particularly, although not exclusively, the present invention relates to an improved fastener for fastening to an aircraft structure a bracket for carrying stores. 
     BACKGROUND ART 
     It is well known to provide, on aircraft structure, brackets (for example pylons) for the attachment of engines, ordinance, drop tanks or other stores. Some stores, for example ordinance and drop tanks, are releasably attached to the bracket. 
     There is increasing demand for aircraft able to perform a range of different roles. Different roles or missions often require different stores and, in some cases, no stores will be carried. For example, an aircraft may be required to carry air-to-air refueling pods for one mission but not for another. A bracket provided for the air-to-air refueling pod may be redundant when the aircraft is performing other duties. 
     DISCLOSURE OF THE INVENTION 
     According to a first embodiment, the present invention provides an aircraft panel fastener for securing a skin panel to an underlying internal structural member, the fastener comprising: a head arranged to engage a skin panel; and an elongate body arranged to extend through a skin panel for engagement with an underlying structural member; wherein the fastener further comprises: an internal bore extending from the head into a portion of the body, the bore being provided with an internally threaded region adapted to receive a fastener for fastening to the aircraft structure external stores. For example the bore may receive a fastener for fastening a bracket for carrying stores. 
     Accordingly, a fastener according to embodiments of the invention may provide a single fastener for the dual function of providing a hard point for carrying external stores and securing a skin panel to the underlying structure. For example, when external stores are required the fastener may perform both functions and when external stores are not required it will still function as a conventional skin panel fastener. The fastener may help protect the skin from damage, for example damage caused by the repeated installation and removal of external stores. A fastener according to embodiments of the invention may prevent the skin from springing away from the underlying structural member and negate the need for any additional bolting to protect the skin, thereby helping to reduce structural weight. 
     The fastener may be arranged such that the internal bore does not extend completely through the length of the body. For example the internal bore may be a blind bore. This arrangement may be advantageous since the tail of the bolt is solid. When the fastener is attached to an aircraft wing the tail may extend into the fuel tank. A solid tail portion may, for example, help to prevent sparks occurring in the event of a lightening strike. 
     According to another aspect, the invention provides a fastener assembly comprising: an aircraft panel fastener according to the first embodiment; and a fastener for fastening to the aircraft structure a bracket for carrying stores, said fastener comprising an externally threaded portion adapted to engage the internally threaded region of the aircraft panel fastener such that the fasteners may be removably connected. 
     In some embodiments the at least the internally threaded region of the aircraft panel fastener is formed of a tougher material which is tougher than at least the externally threaded portion of the fastener for fastening to the aircraft structure a bracket for carrying stores. In the event of damage, for example thread damage, this arrangement may ensure that only the fastener for fastening to the aircraft structure a bracket for carrying stores would need replacing and that the aircraft panel fastener can remain in the aircraft. 
     From another aspect the invention provides an aircraft structure comprising: a skin panel; an internal structural member; and an aircraft panel fastener according to the first embodiment, securing the skin panel to the internal structure. 
     The aircraft panel fastener may be directly or indirectly fastened to a structural element of the aircraft, for example a wing rib or spar. For example the internal structure may comprise an internal bracket attached to a structural element and specifically adapted for receiving the fastener. 
     The head of the fastener may be substantially flush with the skin panel. For example, the head may be countersunk into the skin panel. This arrangement may help to ensure that the skin retains a smooth aerodynamic profile when the fastener is not being used for securing external stores. 
     The skin panel may comprise a carbon fibre composite panel. The internal structural member may be a metallic internal structure. Embodiments of the invention may be particularly suitable for use with composite structures. Embodiments of the invention may help to protect to protect holes in the carbon fibre skin panels. For example, embodiments may help to prevent local delamination of composite material due to regular installation and removal of external stores. 
     From another aspect, the invention provides an aircraft structure comprising: (1) a structural skeleton comprising structural components enclosed within a skin; (2) a fastener, fastened to the skeleton, for fastening to the aircraft structure a bracket for carrying stores, the fastener comprising a body defining an internal bore, the body including an internal threaded region, within the internal bore, for fastening the bracket to the structure, the fastener being wholly within the structure skin. 
     The invention thus provides a hard point for attachment of an item external to the aircraft structure, whilst allowing the structure to have a clean external surface when the item is not fitted. The bracket is thus removably fastenable. 
     The fastener may also be arranged to fasten the structure skin to the structure components. 
     The aircraft structure may be an aircraft wing or fuselage. 
     The structure may further comprise a bracket external to the structure skin and fastened to the fastener. The bracket may be fastened to the fastener by one or more bolts that engage with the internal threaded region of the bore. It will be understood that the bracket is an element, external to the wing, for attaching underwing stores. The bracket may be, for example, a pylon. 
     The fastener may comprise a substantially cylindrical body having a first end and a second end. The fastener may be fastened to the skeleton at an external threaded region, at the body&#39;s first end. The body may comprise a flange at the body&#39;s second end for engaging with an external surface of the skin. 
     The fastener may be fastened directly to the structural components. The skeleton may include an internal bracket and the fastener may be fastened to the internal bracket, which in turn is fastened to the structural components. The structural components may be a wing rib or a wing spar. 
     The body of the fastener may include a flat region for locking with a locking collar or plate. The skeleton may include a collar for receiving the fastener, the collar being within the structure skin and including a flange, the skeleton defining a slot for receiving the flange, the slot being arranged to prevent rotation of the collar when the flange is received in the slot. 
     The aircraft structure may comprise a plurality of the fasteners, wherein the collar receives a first of said fasteners and wherein the aircraft structure further comprises a flange defining said slot and defining a hole through which a second of said fasteners passes. 
     The aircraft structure may comprise a plurality of the fasteners and at least two of said plurality of fasteners may be locked together with a locking plate. The locking plate may be arcuate, with a fastener passing through a hole at an end of the arc. There may be a fastener at each end of the arc. 
     In a further aspect, the invention provides an aircraft structure comprising: (1) a structural skeleton comprising structural components enclosed within a skin; (2) a fastener, fastening the skin to the skeleton and for fastening to the structure a bracket for carrying stores, the fastener comprising a body defining an internal bore, the body including means for fastening within the internal bore the bracket to the structure, the fastener being wholly within the skin. 
     In a further aspect, the invention provides a fastener for fastening to an aircraft structure a bracket for carrying stores, the aircraft structure comprising an internal skeleton and an external skin, the fastener comprising a substantially cylindrical body having a first end and a second end and defining an internal bore, the body comprising: (1) an internal threaded region, within the internal bore, for fastening the bracket to the aircraft structure, and an external threaded region, at the body&#39;s first end, for fastening the fastener to the aircraft structure&#39;s skeleton; (ii) a flange at the body&#39;s second end for engaging with an external surface of the skin; (iii) a flat region for locking with a locking collar or plate; and wherein the body is also arranged to provide a load path between the external skin and the internal skeleton when not fastening a bracket for carrying stores. 
     It will be appreciated that features of the present invention described above in relation to an aspect of the present invention are equally applicable to other aspects of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain illustrative embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying schematic drawings, in which: 
         FIG. 1  is an aircraft including wings according to the an embodiment of the invention, shown in (a) front elevation, (b) side elevation, (c) plan view; 
         FIG. 2  shows skeletal structure from the wing of the aircraft of  FIG. 1 , together with an air-to-air refueling pod for fastening to the skeletal structure; 
         FIG. 3  shows a detail of two fasteners on a rib forming part of the skeletal structure of  FIG. 2 ; 
         FIG. 4  shows in more detail the two fasteners of  FIG. 3 ; 
         FIG. 5  shows another fastening arrangement from a wing according to an embodiment of the invention. 
         FIG. 6  is a cross-sectional view of a fastener of  FIG. 5  fastening a pylon to a rib of a wing according to the invention; 
         FIG. 7  shows another fastening arrangement from a wing according to an embodiment of the invention; 
         FIG. 8  shows another fastening arrangement from a wing according to an embodiment of the invention; 
         FIG. 9  is two isometric views of an element of one of the fasteners of  FIGS. 7 and 8 ; and 
         FIG. 10  shows the fastener element of  FIG. 9  in (a) side view and (b) cross-section. 
     
    
    
     DETAILED DESCRIPTION 
     An aircraft  10  has wings  20  that are examples of aircraft structure according to the invention ( FIG. 1 ). The wings  20  are arranged to carry a variety of items. Engines  30  are permanently attached to the wings  20  via brackets in the form of pylons. Further pylons  40  are provided for attachment of other underwing stores, in a manner well known in the art. The wing  20  according to the invention includes in this example a further hard point  50  for attaching underwing stores. When hard point  50  is not in use, as in  FIG. 1 , it does extend beyond the surface of wing  20 , so that wing  20  has a substantially smooth outer surface in this area. 
     Hard point  50  is used for attachment of underwing stores such as air-to-air refueling pod  80  ( FIG. 2 ) which is used to refuel other aircraft in mid-air. When aircraft  10  is on a mission that does not include air-to-air refueling, pod  80  is not required and so is not installed, leaving hard point  50  smooth for improved aerodynamic performance. When it is attached to aircraft  10 , pod  80  is attached to the skeletal structure of wing  20 , in particular to ribs  70  and spars  60 , to which it is attached by wingbox interface brackets  90  and  100 , respectively. 
     The attachment arrangement at bracket  90  is shown in more detail in  FIG. 3  (the arrangement at bracket  100  is similar). Pod  80  is attached to pylon bracket  120 , which sits outside the skin of wing  20 . Pylon  120  is attached to bracket  90  by external bolts  130   a,b  and hollow bolts  110   a,b . External bolts  130   a,b  are bolts of conventional design. Hollow bolts  110   a,b  have a threaded internal bore, into which external bolts  130   a,b  are screwed. Thus external bolts  130   a,b  extend through pylon  120  and the wing skin (not shown in  FIG. 3 ) and are secured inside hollow bolts  110   a,b , which in turn extend through and are bolted to bracket  90 . Hollow bolts  110   a,b  and bracket  90  are wholly within the skin of wing  20 ; pylon  120  is wholly outside the skin; and external bolts  130   a,b  pass through the skin. When pod  80  is not in use, pylon  120  and external bolts  130   a,b  are not present and none of the attachment equipment extends beyond the surface of wing  20 . There are holes in the wing skin, which in use receive external bolts  130   a,b , but those are blanked off when the hard point is not in use, so this part of wing  20  presents a substantially smooth aerodynamic surface. That bolt-in-bolt fastening arrangement allows repeated on/off fittings of items external to wing  20 . Hollow bolts  110   a,b  protect the wing skin from damage during repeated installation and removal of external bolts  130   a,b.    
     The arrangement by which bracket  90  is attached to rib  70  is shown in more detail in  FIGS. 4 and 5 . Hollow bolts  110   a,b  extend through rib  70  (not shown in  FIG. 4 ) and collar  150   a,b , which bears against rib  70 . Hollow bolts  110   a,b  are locked in place against collars  150   a,b  by bolt-securing nuts  160   a,b . In order to prevent rotation of hollow bolts  110   a,b  when external bracket attachment bolt  130   a,b  is torqued to a required preload, a locking plate  170  is installed to lock hollow bolts  110   a  and  110   b  to each other. Plate  170  is in turn held in place on hollow bolts  110  by plate-securing nuts  190   a,b.    
     In another example arrangement ( FIG. 5 ), hollow bolts  110   c,d  are attached directly to rib  70 , without use of a bracket  90 . 
     The attachment arrangements are shown in more detail in  FIG. 6 ; pylon  120  is also shown in situ in  FIG. 6 . Hollow bolt  110  includes at its head flange  210 , which is seated in a countersunk region of a hole passing through wing skin  200 . Hollow bolt  110  passes through the skin  200 , rib  70  and also collar  150 , to which it is fastened by bolt-securing nut  160 , as described above. Locking plate  170  co-operates with a flattened region at the distal end of hollow bolt  110 , to prevent its rotation, and is held in place by plate-securing nut  190 , as also described above. Hexagonal socket  235  is provided to allow tightening of nuts  160  and  190 . 
     Countersunk head  210  lies flush with skin  200 . Pylon  120  is fastened to skin  200  by external bolt  130 , which bears on washer  220 . External bolt  130  passes through a hole in pylon  120  and is screwed at its distal end into a threaded region of internal bore  230  within hollow bolt  110 . 
     Wing  20  will usually be used to store fuel for the aircraft  10 . Because external bolt  130  is received within the closed bore  230  of hollow bolt  110 , there is no path for fuel to leak from the tank. 
     In another embodiment of the invention ( FIG. 7 ), hollow bolt  110 ′ passes through flanged collar  250  rather than collar  150 . Flanged collar  250  includes internal flat regions (not shown) which co-operate with flat regions on the exterior of hollow bolt  110 ′ to prevent relative rotation of flanged collar  250  and hollow bolt  110 ′. Flanged collar  250  includes at its base flange  260 , which is seated in slot  270  in bracket  240 . Co-operation between flange  260  and slot  270  prevents rotation of flanged collar  250  and hence of hollow bolt  110 ′. 
     Where a thinner bracket  240 ′ is used ( FIG. 8 ), flange  260  is seated in a slot in packer  290 , which acts to prevent rotation in a similar way to slot  270 . A second hollow bolt  110 ′ passes through packer  290 , which prevents rotation of packer  290 . 
     Hollow bolt  110 ′ is shown in more detail in  FIGS. 9 and 10 . It comprises a cylindrical body  300 , which defines interior bore  230 . Body  300  has at a first end flange  210 , which is seated in use in a countersunk hole in the wing skin  200 , as described above. About two-thirds of the way along its length from flange  210 , two flats  280  are provided, which co-operate with flanged collar  250  to prevent rotation, as described above. Beyond flats  280  there is a head region  310 , which carries an external thread (not shown) to receive collar-locking nut  160 . At the distal end of hollow bolt  110 ′ there is a smooth region  320  around hexagonal socket  235 . 
     Internally ( FIG. 10(   b )), bore  230  extends for about half the length of hollow bolt  110 ′. An internal thread  330  is provided along about half of bore  330  (the deeper half, furthest from flange  210 ). In use, external bolt  130  is received in bore  230  and fastened at thread  330 . 
     Hollow bolt  110  of the first embodiment described above is identical to hollow bolt  110 ′ of the second embodiment, save that it does not have flats  280  but instead has flats in its distal end region  320 , which co-operate with plate  170 , and an external thread at head  320 &#39;s distal end (corresponding to regions  320 ), which receives plate-locking nut  190 . 
     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. Some examples of such variations and alternatives have been described above. 
     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.