Abstract:
The present invention provides an aircraft structure comprising a structural component and a set of reinforcing straps for reinforcing the structural component, wherein the set comprises a first reinforcing strap having an inner and outer surface, wherein the inner surface is attached to a first surface of the structural component such that the first reinforcing strap extends longitudinally along the structural component and a second reinforcing strap having an inner and outer surface, wherein the inner surface is attached to the outer surface of the first reinforcing strap such that the second reinforcing strap extends longitudinally along the first reinforcing strap. The invention also provides an aircraft, a method of reinforcing an aircraft structure and a method of inspecting an aircraft structure.

Description:
RELATED APPLICATIONS 
     The present application is a National Phase of International Application Number PCT/GB2012/051914, filed Aug. 7, 2012, and claims priority from United Kingdom Application Number GB1115080.2, filed Sep. 1, 2011. 
     BACKGROUND OF THE INVENTION 
     The present invention concerns the field of aircraft structures. More particularly, but not exclusively, this invention concerns an aircraft structure comprising a structural component, and at least one reinforcing strap for reinforcing the structural component. The invention also concerns a method of reinforcing an aircraft structure, for example during repair of the structure or retrofit of an additional component to the structure. The invention also concerns a method of inspecting an aircraft structure. 
     It is sometimes desired to retrofit an additional component to an existing aircraft structure. For example, a wing tip device, such as a winglet, may be retrofitted to an existing aircraft wing. As part of the retrofit procedure, the aircraft structure needs to be reinforced. This is because the aircraft structure was not originally designed to include the additional component. In the wing tip device example, the wing box structure needs to be reinforced. It is also sometimes required to reinforce an existing aircraft structure as part of a repair procedure. 
     A typical wing box structure comprises an upper and a lower cover, comprising skin and spanwise extending stringers, and a front and a rear spar, extending spanwise along the wing box structure. The wing box structure also comprises ribs which extend chordwise across the wing box structure between the skins. The ribs are provided with mouseholes to allow the stringers to pass through. The lower cover allows access to the interior of the wing box structure via manholes, to allow inspection of the interior of the wing box structure. The stringers adjacent to the manholes are called manhole boundary stringers. 
     When a wing tip device is retrofitted to the outboard end of the wing, it is likely that the wing box structure requires reinforcement, including reinforcement of the manhole boundary stringers. However, due to the fact that the stringers must pass through the mouseholes, there is not much space for the reinforcement, which must be continuous. As a result, the stringers are often reinforced with a reinforcing strap made of high strength steel so that, for the same load, the strap is smaller than it would have been if it were made from aluminium alloy. 
     A side sectional view of part of a prior art aircraft outboard wing box structure, with a reinforced manhole boundary stringer, is shown in  FIG. 1 . 
     The wing box structure includes a lower cover structure  10  with a manhole boundary stringer  30  extending upwards from the cover structure  10  and extending in a spanwise longitudinal direction of the wing box structure. Extending in the chordwise direction of the wing box structure is a rib  20  with a mousehole  21  for allowing the stringer  30  to pass longitudinally through the rib  20 . The stringer  30  comprises a web portion  31  which extends inwards from the cover structure  10  and an end portion which extends from the distal end of the web  31  portion in both chordwise directions to provide a chordwise extending flange  32 . A reinforcing strap  40  is attached to one side of the web portion  31  of the stringer  30  and extends longitudinally along the stringer  30 . The reinforcing strap  40  is made from high strength steel. 
     However, high strength steel has a low toughness compared to other materials. Hence, it has a lower resistance to crack propagation. This means that a small flaw in the material can develop into a crack in a shorter time frame than with other tougher materials. This can lead to a higher maintenance frequency to check for flaws and cracks in the reinforcing strap. Every maintenance operation causes downtime for the aircraft and costs money for the airline operating the aircraft. 
     In addition, it is important that a retrofit or repair procedure is as quick as possible to minimise downtime of the aircraft. 
     The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved aircraft structure that is less vulnerable to crack propagation, especially after reinforcement during a repair or retrofit procedure, for example. 
     SUMMARY OF THE INVENTION 
     The present invention provides, according to a first aspect, an aircraft structure comprising a structural component, and a set of reinforcing straps for reinforcing the structural component, wherein the set comprises a first reinforcing strap having an inner and outer surface, wherein the inner surface is attached to a first surface of the structural component such that the first reinforcing strap extends longitudinally along the structural component, and a second reinforcing strap having an inner and outer surface, wherein the inner surface is attached to the outer surface of the first reinforcing strap such that the second reinforcing strap extends longitudinally along the first reinforcing strap. Providing more than one reinforcing strap on the structural component provides a greater degree of redundancy in the aircraft structure. A crack in one reinforcing strap cannot propagate into a second reinforcing strap thereby minimising the loss of strength associated with a single crack. This provides a reduced maintenance burden on the operator. 
     The reinforcing straps are separately formed from each other and from the structural component. The first reinforcing strap is then attached to the structural component and the second reinforcing strap is attached to the first reinforcing strap. 
     The term “strap” is used to refer to a relatively thin, substantially flat, plate-like component for lying flush against a substantially flat surface. 
     Preferably, over at least a portion of the length of the reinforcing straps, the first reinforcing strap has a height (along the structural component) between a top and bottom of the first reinforcing strap that is larger than the height (along the structural component) of the second reinforcing strap between a top and bottom of the second reinforcing strap. This allows a portion of the first (inner) reinforcing strap to be seen during an inspection. In other words, the second (outer) strap does not cover up the first (inner) strap entirely. This allows for cracks and flaws to be noticed on the first (inner) strap. 
     Preferably, the height of the first reinforcing strap is approximately 115 to 125% of the height of the second reinforcing strap. More preferably, the height of the first reinforcing strap is approximately 120% of the height of the second reinforcing strap. This allows a significant portion of the first (inner) strap to be seen during inspection. 
     Preferably, over at least a portion of the length of the reinforcing straps, the second reinforcing strap has an average thickness (substantially perpendicular to the plane of the structural component) between the inner and outer surfaces of the second reinforcing strap that is larger than the average thickness (substantially perpendicular to the plane of the structural component) of the first reinforcing strap between the inner and outer surfaces of the first reinforcing strap. More preferably, the thickness of the first reinforcing strap varies along its height. Even more preferably, a top portion and/or a bottom portion of the first reinforcing strap is thinner than a middle portion of the first reinforcing strap. 
     Preferably, the thickness of the second reinforcing strap is constant along its height. 
     Preferably, the average thickness of the second reinforcing strap is approximately 115 to 125% of the average thickness of the first reinforcing strap. More preferably, the average thickness of the second reinforcing strap is approximately 120% of the average thickness of the first reinforcing strap. 
     Preferably, the at least a portion of the length of the reinforcing straps is the entire length of the reinforcing straps. This allows the first (inner) strap to be seen along its entire length during inspection. 
     Preferably, the height and average thickness of the reinforcing straps are such that the cross-sectional areas of the first and second reinforcing straps are substantially the same. This provides both straps with the same load capability and therefore, there is no under or over redundancy of the straps. 
     Preferably, the structural component has a second surface opposite its first surface and wherein a second set of reinforcing straps is attached to the second surface of the structural component in a similar way to the first set of reinforcing straps to the first surface of the structural component. This allows four reinforcing straps to be provided on the structural component. This gives a greater amount of redundancy in the aircraft structure. 
     Preferably, the structural component is a stringer. 
     Preferably, the aircraft structure further comprises a skin component to which the stringer is attached and wherein the first surface of the stringer is a surface extending inwardly from an inner surface of the skin component. This allows the reinforcing straps to be provided on a surface of the stringer without increasing the distance by which the stringer extends inwards from the skin. In other words, the reinforcing straps effectively increase the thickness of the inwardly extending surface, without increasing its length. In particular, it is noted that stringers often have an enlarged portion (with a greater thickness) at a distal end of the stringer. Hence, the reinforcing straps do not, in fact, increase the overall largest thickness of the stringer. 
     Preferably, the aircraft structure further comprises a rib extending across the stringer and wherein the rib is provided with a hole for the stringer to pass through and wherein the reinforcing straps also pass through the hole. 
     More preferably, the aircraft structure is a wing box and wherein a wing tip device is fitted to the wing box structure and wherein the reinforcing straps are located towards an outboard end of at least one stringer to provide reinforcement of the stringer. 
     According to the first aspect, the present invention also provides an aircraft comprising an aircraft structure as described above. Preferably, the aircraft is a commercial transport aircraft. The aircraft is preferably of a size equivalent to an aircraft designed to carry more than 75 passengers, and more preferably more than 100 passengers. 
     According to a second aspect of the invention there is also provided a method of reinforcing an aircraft structure, the method including the steps of attaching a first reinforcing strap to a structural component of the aircraft structure, wherein an inner surface of the first reinforcing strap is attached to a first surface of the structural component such that the first reinforcing strap extends longitudinally along the structural component, attaching a second reinforcing strap to the first reinforcing strap, wherein an inner surface of the second reinforcing strap is attached to an outer surface of the first reinforcing strap such that the second reinforcing strap extends longitudinally along the first reinforcing strap. 
     Preferably, over at least a portion of the length of the reinforcing straps, the first reinforcing strap has a height between a top and bottom of the first reinforcing strap that is larger than the height of the second reinforcing strap between a top and bottom of the second reinforcing strap. 
     Preferably, the method is carried out as part of a repair procedure of the aircraft structure or as part of a procedure to retrofit a further component to the aircraft structure. This allows a repair or retrofit procedure to be carried out, whilst minimising the vulnerability of the aircraft structure to crack propagation and/or minimising the required maintenance frequency. 
     More preferably, the method comprises the steps of fitting a wing tip device to a wing structure of an aircraft, and attaching the first and second reinforcing straps to a stringer of the wing structure to reinforce the wing structure. 
     Preferably, the method further comprises the steps of attaching a second set of reinforcing straps to a second, opposite surface of the structural component in a similar way to the first set of reinforcing straps to the first surface of the structural component. 
     According to a third aspect of the invention there is also provided a method of inspecting an aircraft structure, the aircraft structure comprising a structural component, and a set of at least two reinforcing straps, the first reinforcing strap extending longitudinally along the structural component and the second reinforcing strap extending longitudinally along the first reinforcing strap, the method comprising the steps of inspecting the second reinforcing strap, and inspecting a portion of the first reinforcing strap that is not covered by the second reinforcing strap. This allows a portion of the first (inner) strap to be seen during inspection without having to remove the second (outer) strap. 
     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 methods of reinforcing or inspection of the invention may incorporate any of the features described with reference to the aircraft structure of the invention and vice versa. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
         FIG. 1  shows a side sectional view of a prior art stringer with a reinforcing strap; 
         FIG. 2  shows a side sectional view of a stringer with two reinforcing straps, according to a first embodiment of the invention; and 
         FIG. 3  shows a side sectional view of a stringer with four reinforcing straps, according to a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a side sectional view of part of an aircraft outboard wing box structure, with a reinforced manhole boundary stringer, according to a first embodiment of the present invention. 
     The wing box structure includes a lower cover structure  110  with a manhole boundary stringer  130  extending upwards from the cover structure  110  and extending in a spanwise longitudinal direction of the wing box structure. Extending in the chordwise direction of the wing box structure is a rib  120  with a mousehole  121  for allowing the stringer  130  to pass longitudinally through the rib  120 . The stringer  130  comprises a web portion  131  which extends upwards from the cover structure  110  and an end portion which extends from the web  131  in both chordwise directions to provide a chordwise flange  132 . 
     A set of reinforcing straps  140  is attached to one side of the web portion  131  of the stringer  130 . The set of reinforcing straps includes a first reinforcing strap  141  attached to one side of the web portion  131  and extending longitudinally along the stringer  130 . The first reinforcing strap  141  has a middle portion  141   b  with a constant thickness. It has top and bottom portions  141   a  which taper on the inner surface of the first reinforcing strap to a point. This allows the first reinforcing strap  141  to fit against the side of the web portion  131 , allowing for the rounded corners between the web portion  131  and the cover structure  110  and the web portion  131  and the chordwise flange  132 . The outer surface of the first reinforcing strap  141  is substantially flat. A second reinforcing strap  142  is attached to the outer surface of the first reinforcing strap  141  and extends longitudinally along the first reinforcing strap  141 . The second reinforcing strap  142  has a constant thickness. The top of the second reinforcing strap  142  is at the same height as the top of the first reinforcing strap  141 . However, the bottom of the second reinforcing strap  142  is higher than the bottom of the first reinforcing strap  141 . Hence, a lower portion of the first reinforcing strap  141  is left exposed by the second reinforcing strap  142 . 
     Both reinforcing straps  141 ,  142  are made from high strength steel. In order to provide the same load carrying ability, the cross-sectional areas of the straps  141 ,  142  are the same. As the height of the first reinforcing strap is greater than the second reinforcing strap (by about 20%), the thickness of the second reinforcing strap  142  is greater (by about 20%) than the average thickness of the first reinforcing strap  141 . 
     The straps  141 ,  142  are designed with a cross-sectional area to allow for one of the straps to have developed a flaw that has propagated to a crack and therefore the strap has a loss of strength, where the overall section has enough residual strength to carry sufficient load until the next inspection. 
       FIG. 3  shows a side sectional view of part of an aircraft outboard wing box structure, with a reinforced manhole boundary stringer, according to a second embodiment of the present invention. 
     The wing box structure includes a lower cover structure  210  with a manhole boundary stringer  230  extending upwards from the cover structure  210  and extending in a spanwise longitudinal direction of the wing box structure. Extending in the chordwise direction of the wing box structure is a rib  220  with a mousehole  221  for allowing the stringer  230  to pass longitudinally through the rib  220 . The stringer  230  comprises a web portion  231  which extends upwards from the cover structure  210  and an end portion which extends from the web  231  in both chordwise directions to provide a chordwise flange  232 . 
     A first set of reinforcing straps  240  is attached to one side of the web portion  231  of the stringer  230 . The first set of reinforcing straps includes a first reinforcing strap  241  attached to one side of the web portion  231  and extending longitudinally along the stringer  230 . The first reinforcing strap  241  has a middle portion  241   b  with a constant thickness. It has top and bottom portions  241   a  which taper on the inner surface of the first reinforcing strap to a point. This allows the first reinforcing strap  241  to fit against the side of the web portion  231 , allowing for the rounded corners between the web portion  231  and the cover structure  210  and the web portion  231  and the chordwise flange  232 . The outer surface of the first reinforcing strap  241  is substantially flat. A second reinforcing strap  242  is attached to the outer surface of the first reinforcing strap  241  and extends longitudinally along the first reinforcing strap  241 . The second reinforcing strap  242  has a constant thickness. The top of the second reinforcing strap  242  is at the same height as the top of the first reinforcing strap  241 . However, the bottom of the second reinforcing strap  242  is higher than the bottom of the first reinforcing strap  241 . Hence, a lower portion of the first reinforcing strap  241  is left exposed by the second reinforcing strap  242 . 
     Both reinforcing straps  241 ,  242  are made from high strength steel. In order to provide the same load carrying ability, the cross-sectional areas of the straps  241 ,  242  are the same. As the height of the first reinforcing strap is greater than the second reinforcing strap (by about 20%), the thickness of the second reinforcing strap  242  is greater (by about 20%) than the average thickness of the first reinforcing strap  241 . 
     A second set of reinforcing straps  250  is also provided on the other, opposite side of the web portion  232  of the stringer  230 . The second set of reinforcing straps  250  is similar to the first set of reinforcing straps  240 . In the same way, a lower portion of the first reinforcing strap  251  of the second set is left exposed by the second reinforcing strap  252  of the second set. 
     The second reinforcing strap  251  has a middle portion  251   b  with a constant thickness. It has top and bottom portions  251   a  which taper on the inner surface of the first reinforcing strap to a point. The straps  241 ,  242 ,  251 ,  252  are designed with a cross-sectional area to allow for one of the straps to have developed a flaw that has propagated to a crack and therefore the strap has a loss of strength, where the overall section has enough residual strength to carry sufficient load until the next inspection. 
     During a repair or retrofit procedure, the stringers of the wing box structure can be reinforced by attaching first and second reinforcing straps  141 ,  142 ,  241 ,  241 ,  251 ,  252  to the stringers  130 ,  230 . 
     During inspection, the lower portion of all first reinforcing straps  141 ,  241 ,  252  can be seen and inspected through a manhole in the covers  110 ,  210  without removing the second reinforcing straps  142 ,  242 ,  252 . This means that cracks and flaws can be detected in the first reinforcing straps  141 ,  241 ,  252 , as well as the second reinforcing straps  142 ,  242 ,  252 , without deconstruction. 
     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. 
     The reinforcing straps, or at least one of the reinforcing straps, may be made from a different material, other than high strength steel. For example, a different metallic alloy may be used or a composite material may be used. 
     The reinforcing straps may be attached to a different structural component, such as a rib, a spar, a skin, or a stringer that is not a manhole boundary stringer. The reinforcing straps may also be used in a location in an aircraft other than a wing, such as the tailplane or fuselage. 
     The reinforcing straps may be attached to the structural component using any convenient attachment mechanism. For example, rivets, bolts or adhesive may be used. The second reinforcing strap may be attached to the first reinforcing strap prior to the first reinforcing strap being attached to the structural component. The first and second reinforcing straps may be attached to the structural component using rivets or bolts passing through both straps. 
     In relation to the second embodiment of the invention, all four reinforcing straps may be attached using rivets or bolts passing through all four straps. 
     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.