Abstract:
A method for coupling a first and a second stiffening element for a skin of an aircraft or spacecraft, wherein the first stiffening element has a cross-sectional profile with first and second web portions extending from a flange portion of a predefined width toward the skin includes forming a coupling zone of the first stiffening element wherein the second web portion is at least partially removed to expose an inner surface of the first web portion, and a step of fastening a coupling element, for coupling the stiffening elements, to the coupling zone of the first stiffening element.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a stiffening element or a skin of an aircraft or spacecraft and to a method for coupling such a stiffening element to a second stiffening element. Furthermore, the invention relates to a shell component for an aircraft or spacecraft comprising coupled stiffening elements. 
         [0002]    Although applicable to arbitrary stiffening elements for a skin of an aircraft or spacecraft, the invention and underlying problem will be described with reference to frames used for the fuselage shell of an aircraft. 
       BACKGROUND OF THE INVENTION 
       [0003]    Conventionally, manufacturing of fuselage shell components involve riveting, bonding or welding stringers and frame-mounting clips or angles onto a subshell of the aircraft shell. In a further step, segments of frames having an open cross-section of e.g. C- or Z-shape, are positioned on the subshell and riveted to the clips. In alternative manufacturing processes, integral frame segments, which include a foot portion for contacting the skin, are directly mounted to the skin without intervening clips or angles. In both cases, the frame segments are joined in later manufacturing steps by riveting coupling elements like profiles or straps that to the open cross-section in order to form complete frames. 
         [0004]    A recent trend in the manufacturing of shell components for aircraft fuselages is the use of closed frame profiles, in particular so-called Ω-profiles, that include two web portions extending away from the skin, which at their distal ends are connected by a flange portion of predetermined width that can e.g. be rounded or extend flatly in parallel to the skin. While the use of closed frame profiles enables to reduce the weight of the aircraft through an improved strength-to-weight ratio of the shell component, only blind fasteners such as blind rivets can be used to connect frame segments by attaching coupling elements like profiles or straps to the web and flange portions of a closed-profile frame. This makes assembly easier since only one-side access to the fasteners is required, but increases weight due to the decreased strength and higher weight of blind fasteners as compared to fasteners that can be applied from two sides. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    It therefore is one idea of the present invention to reduce the weight of shell components comprising frame segments with closed profiles, while enabling easy assembly and inspection of the structure. 
         [0006]    A concept underlying the present invention consists in that a stiffening element for a skin of an aircraft or spacecraft comprises a cross-sectional profile with first and second web portions extending from a flange portion of a predefined width toward the skin, and furthermore comprises a coupling zone wherein the second web portion is at least partially removed to expose an inner surface of the first web portion. The term “stiffening element” as used herein refers e.g. to a stringer or a frame, which can be an integral frame or non-integral frame. Due to the presence of the flange portion and the first and second web portions, which are connected by the flange portion and each extend toward the direction of the skin, the profile represents a closed profile, which enables particularly efficient stiffening of the skin, with high strength-to-weight ratio, through connecting each of the web portions to the skin, such as via integral foot portions, clips, or angles. 
         [0007]    Because in the coupling zone of the stiffening element the second web portion is at least partially removed to expose the inner surface of the first web portion, the first web portion is accessible from both sides, i.e. from the exposed inner surface in addition to the conventionally accessible outer surface of the first web portion. This enables the stiffening element to be easily coupled to another stiffening element by fastening a coupling element such as strap or angle part to the stiffening element in the coupling zone, due to being able to use reliable and efficient techniques such as riveting that require access from two sides. This also makes possible a favourably high strength-to-weight ratio of the coupling and a favourably low part count by optimally choosing the connection technique according to such needs without accessibility restraints. Furthermore, a high quality of the coupling can be achieved and maintained through quick and thorough inspection from all sides. 
         [0008]    Because of the presence of the coupling element in the coupling zone, an enhanced stiffening effect is enabled to be provided by the coupling element, to thereby counteract a reduction in the stiffening effect of the stiffening element in the coupling zone that is to be expected due to the at least partial removal of the second web portion. For example, the coupling element is enabled to be chosen such that the overall strength of the stiffening element, if desired including further parts present, is rendered constant throughout the entire length of the stiffening element. 
         [0009]    According to one embodiment, in the coupling zone the flange portion is wider than outside the coupling zone, i.e. in the main part of the stiffening element wherein the stiffening profile has the cross-sectional profile with the first and second web portions extending from the flange portion toward the skin, such that an enhanced stiffening effect is enabled to be provided by the widened flange portion, to furthermore counteract the reduction in the stiffening effect of the stiffening element in the coupling zone that is to be expected due to the at least partial removal of the second web portion. This enables to achieve a particularly favourable strength-to-weight ratio of the shell component by mutually adjusting the strength of the coupling element and the degree of widening of the flange portion in the coupling zone. For example, the enhanced width of the flange portion in the coupling zone and the coupling element can be chosen such that the overall strength of the stiffening element including the coupling element is rendered constant throughout the entire length of the stiffening element. 
         [0010]    For example, a transition zone of the first stiffening element is provided wherein the flange portion transitions from the predetermined width to the greater width, next to the coupling zone on the first stiffening element. The gradual transition enables especially strong mechanical coupling between the coupling zone to and the main part of the stiffening element lying outside the coupling zone. 
         [0011]    According to another embodiment, at least one of the flange portion and the first web portion in the coupling zone comprises a greater thickness than a corresponding thickness outside the coupling zone. This serves to enhance the strength and therefore the stiffening effect of the stiffening element in the coupling zone. As a generalisation, the greater thickness of the flange portion and/or the greater thickness of the first web portion according to the present development as well as the greater width of the flange portion according to the previously described development with which the present development can be freely combined each represent an enlarged profile dimension of the stiffening element in the coupling zone as compared to outside the coupling zone. This enlarged profile dimension enables to counteract the reduction in the stiffening effect of the stiffening element in the coupling zone that is to be expected due to the at least partial removal of the second web portion. It becomes possible to optimise the respective degree to which the width of the flange portion is possibly widened in the coupling zone and to which the thickness of the flange portion and/or first web portion is possibly enhanced in the coupling zone in order to achieve e.g. an optimal strength-to-weight ratio or particularly easy manufacturability. 
         [0012]    According to another embodiment, the cross-sectional profile of the stiffening element further has a second foot portion for contacting the skin, the second web portion extending from the flange portion to the second foot portion. This enables the stiffening element to be provided at least partially as an integral stiffening element, which at least partially does not require clips or angles for attachment to the skin, such that a lower weight of the aircraft and simplification of assembly become possible. Independently of the presence of the second foot portion, the cross-sectional profile of the stiffening element may further have a first foot portion for contacting the skin, the first web portion extending from the flange portion to the first foot portion, with corresponding advantages. Preferably, the second foot portion is removed in the coupling zone, thus enabling reduced weight without significant reduction in strength. 
         [0013]    According to another embodiment, the first web portion extends substantially perpendicular to the skin. This enables installation systems of the aircraft or spacecraft such as air conditioning systems, electric or hydraulic lines etc. to be easily arranged and affixed along the first web portion, without being interfered by e.g. the possibly widened flange portion and/or the removed second web portion in the coupling zone. 
         [0014]    According to another embodiment of the shell component according to the invention, which includes a second stiffening element to which the stiffening is coupled by a coupling element, the coupling element comprises at least one L-profile fastened to the stiffening element and to the second stiffening element. This enables a simple, strong and reliable coupling e.g. by riveting the L-profile to the stiffening element and the second stiffening element. Preferably, the at least one L-profile is fastened to the inner surface of the stiffening element. In this way, the L-profile is protected against mechanical damage, and the L-profile does not increase the outer dimension of the stiffening element, such that e.g. installation systems installed next to the stiffening element are not affected. 
         [0015]    According to another embodiment, the at least one L-profile comprises an upper L-profile fastened to the first web portion and the flange portion of the stiffening element, and a lower L-profile fastened to the first web portion of the stiffening element below the upper L-profile. In this way, the coupling of the stiffening element to the second stiffening element is enabled to be highly effective in the strength it provides while accommodating measurement tolerances originating from manufacturing of the individual parts such as the stiffening element and second stiffening element. 
         [0016]    For example, the upper L-profile and lower L-profile are fastened to opposing surfaces of the first web portion of the stiffening element. This enables especially high coupling strength since forces transmitted by the L-profile act at different locations of the stiffening element. 
         [0017]    According to another embodiment, the upper L-profile and lower L-profile overlap. In this way, especially high coupling strength can be achieved overall, and in particular within the overlap region, which enables to provide particularly strong coupling strength in a desired location. For example, the overlap region may be located close to the skin in a constructional situation where loads on the stiffening element are greater close to the skin, or close to the flange portion in a constructional situation where forces on the stiffening element are greatest close to the flange portion, such as in a case of a frame configured to predominantly withstand internal pressure of the aircraft or spacecraft. 
         [0018]    According to another embodiment, the lower L-profile is furthermore fastened to the skin. In this way, particularly strong coupling between the stiffening element and the skin enables a particularly favourable strength-to-weight-ratio of the shell component. 
         [0019]    According to another embodiment, the second stiffening element has the cross-sectional profile with first and second web portions extending from a flange portion of the predefined width toward the skin, and further comprises a coupling zone of the second stiffening element wherein the web portion has a possibly greater width than the predefined width and the second web portion is at least partially removed to expose an inner surface of the first web portion, the coupling element being fastened furthermore to the coupling zone of the second stiffening element. In this way, the above-described advantages hold effective also on the side of the second stiffening element, resulting in a highly uniform strength of the shell component. For example, the second stiffening may be formed substantially mirror-symmetric to the first stiffening element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Detailed embodiments of the invention will be described below with reference to the drawings, in which: 
           [0021]      FIG. 1  is a cut-out perspective view of a shell component for an aircraft or spacecraft according to an embodiment; 
           [0022]      FIG. 2  is a cut-out perspective view of stiffening element for a skin of an aircraft or spacecraft according to an embodiment; and 
           [0023]      FIGS. 3A-3C  are schematic cross sectional diagrams of different shell components for an aircraft or spacecraft, each according to an embodiment of the invention. 
       
    
    
       [0024]    In the drawings, same reference signs denote same constructional elements, unless explicitly indicated otherwise. 
       DETAILED DESCRIPTION 
       [0025]      FIG. 1  depicts, in a perspective view, a cut-out detail of a shell component  100  for an aircraft or spacecraft, which in the present embodiment is assumed to be a part of the outer shell of an aircraft fuselage. The perspective view is directed outward from a point of view inside the fuselage, such that an outer skin  102  of the aircraft, which forms a part of the shell component  100 , presents its interior side to the observer. Furthermore comprised by the shell component  100  are a first frame segment  110  and a second frame segment  120 , which are arranged on the interior side of the skin  102  to function as stiffening elements for stiffening the skin  102 . The frame segments  110 ,  120  are coupled together by a lower L-profile  104  and an upper L-profile  105 , which form a coupling element  104 ,  105  that is part of the shell component  100 . 
         [0026]      FIG. 2  shows, from substantially the same perspective, a frame segment  110  identical to the first frame segment  110  of the shell component  100  depicted in  FIG. 1 . Moreover, the frame segment  110  is identical to a mirror image of the second frame segment  120  depicted in  FIG. 1 . As is best seen in  FIG. 2 , the frame segment  110  is formed of a thin material of general thickness T 0 , which may e.g. comprise a metal such as aluminium or titanium in pure or alloyed form, a plastic reinforced by carbon, glass, aramid or similar fibres, as well as a laminate of alternating layers of metal and reinforced plastic. 
         [0027]    The thin material of the frame segment  110  follows a curved cross-sectional profile of Ω-shape, with a first foot portion  117 , which flatly rests on the skin  102 , and a first web portion  111 , which continues from the first foot portion  117  in an upward turn of 90° away from the skin  102  to extend perpendicularly to the skin  102  up to the full height of the frame segment  110 . At the upper edge of the first web portion  111 , the cross-sectional profile turns by 90° into the direction opposite to the turn between the first foot portion  117  and the first web portion  111 , to continue in a flange portion  113 , which extends over a predetermined width W 0  in parallel to the skin  102 . Then, the cross-sectional profile again turns in the same direction by an oblique angle of e.g. between 70° to 85°, to continue in a second web portion  122  that extends from the flange portion  113  to the skin  102 . At the skin, the cross-sectional profile turns again by an oblique angle of the same magnitude and in the opposite direction to the turn between the flange portion  113  and the second web portion  112 , to continue in a second foot portion  118 , which flatly rests on the skin  102 . 
         [0028]    The frame segment  110  exhibits the above-described default cross-sectional profile over the major part of its length of which  FIG. 2  shows only a part. However, at an end of the frame segment  110 , which is shown in  FIG. 2 , a coupling zone  114  is formed wherein the shape of the frame segment  110  deviates from this default cross-sectional profile. Within the coupling zone  114 , the second web portion  112  is removed together with the second foot portion  118 , such that only the first foot portion  117 , the first web portion  111  and the flange portion  113  are present. Furthermore, the flange portion  113  comprises a width W 1  within the coupling zone  114  that in the present embodiment is greater than the width W 0  of the flange portion  113  of the default cross-sectional profile outside the coupling zone  114 . Additionally, the thin material of the frame segment  110  comprises a thickness T 1  within the coupling zone  114  that in the present embodiment is greater than the thickness T 0  of the thin material according to the default cross-sectional profile outside the coupling zone  114 . It is noted that in alternative embodiments, the width W 1  of the flange portion  113  in the coupling zone  114  may equal the predetermined width W 0  outside the coupling zone  114 , and/or the thickness T 1  within the coupling zone  114  may be the same as the thickness T 0  outside the coupling zone  114 . 
         [0029]    Adjacent to the coupling zone  114 , a transition zone  116  of the frame segment  110  is formed wherein the flange portion  113  transitions from the predetermined width W 0  according to the default cross-section to the greater width W 1  valid in the coupling zone  114 . Also, the second web portion  112  is removed only partly within the transition zone  116 , thus forming a diagonal contour. Furthermore, the thin material of the frame segment  110  gradually transitions in the transition zone  116  from the thickness T 0  of the thin material according to the default cross-sectional profile outside the coupling zone  114  to the enhanced thickness T 1  valid within the coupling zone  114 . 
         [0030]    In the first foot portion  117 , the first web portion  111 , and the flange portion  113 , rivet holes  107  are formed for fastening a coupling element to the coupling zone  114  in order to couple the frame segment  110  to a further frame segment. Further rivet holes (not shown) for attaching the frame segment  110  to the outer skin of the aircraft may be formed in the foot portions  117 ,  118 . 
         [0031]    In order to couple the frame segment  110  shown in  FIG. 2  to another frame segment, e.g. as shown in  FIG. 1  to a second frame segment  120  of the same cross-section with a coupling zone  124  that is mirror-symmetric to the coupling zone  114  of the first frame segment  110 , various arrangements of coupling elements  104 ,  105  may be chosen. One such arrangement of coupling elements  104 ,  105  is exemplified by the shell component  100  in  FIG. 1  and furthermore schematically illustrated in a cross sectional diagram of the shell component  100  given in  FIG. 3A . Because the second frame segment  120  in  FIG. 1  is mirror-symmetric to the first frame segment  110 , it has an identical cross-sectional shape including a respective first foot portion  127 , first web portion  121 , flange portion  113 , second web portion  122 , and second foot portion  128 . Also, a coupling zone  124  and a transition zone  126  are formed that are mirror-identical to the corresponding zones  114 ,  116  of the first frame segment  110 . 
         [0032]    As can be seen in  FIG. 3A , the lower L-profile  104  of the coupling element  104 - 106  comprises an L-shaped cross section with legs of approximately equal width and is attached from outside the first frame segment  110  to the first foot portion  117  and the lower part of the first web portion  111  of the first frame segment  110 . In the same way, the lower L-profile  104  is also attached from the outside to the first foot portion  127  and the lower part of the first web portion  121  of the second frame segment  120 .  FIG. 1  shows the lower L-profile  104  in a state before installation in its intended position, which is shown in  FIG. 3A . 
         [0033]    The upper L-profile  105  of the coupling element  104 - 106  comprises an L-shaped cross section with one short leg, which is slightly shorter than the width W 1  of the flange portion  113  within the coupling zone  114 , and one long leg, which is somewhat shorter than the height of the first web portion  111 . The upper L-profile  105  is attached from inside the frame segments  110 ,  120  to the respective flange portions  113 ,  123  and first web portions  111 ,  121  of both frame segments  110 ,  120 . As can be seen in  FIG. 3A , the legs of the upper L-profile  105  and the lower L-profile  104  that are attached to the first web portions  111 ,  121  overlap each other in the lower part of the frame segments  110 ,  120 . While in the overlap region the upper L-profile  105  is formed equally wide as the lower L-profile  104 , it is formed less wide than the lower L-profile  104  outside the overlap region. In order to couple the frame segments  110 ,  120  by means of the coupling element  104 - 106 , rivets  106  are deployed throughout the area covered by the L-profiles  104 ,  105 . Each rivet  106  runs through the material of one of the frame segments  110 ,  120  and at least one of the L-profiles  104 ,  105 . In the region of overlap between the L-profiles  104 ,  105 , rivets are deployed through both L-profiles  104 ,  105  and the one of the first web portions  111 ,  121 . At the first foot portions  117 ,  127 , rivets are deployed through the lower L-profile  104 , one of the first foot portions  117 ,  127 , and the skin  102 . 
         [0034]      FIGS. 3B and 3C  show alternative examples for realising a coupling element  104 - 106  by using two L-shaped profiles  104 ,  105  and rivets  106 . In the examples of both  FIG. 3B and 3C , the lower L-profile  104  and the upper L-profile  105  each have one short leg, which is slightly shorter than the width of the first foot portions  117 ,  127  and shorter than the enhanced width W 1  of the flange portions  113 ,  123  of the frame segments  110 ,  120  in the respective coupling zones  114 ,  124 , and one long leg, which is slightly shorter than half of the height of the first web portions  111 ,  121 . (It is noted that  FIGS. 3A-C  do not show the second frame element  120  and its parts  121 - 128  due to the cross-sectional view being located at the border between the first and second frame segments  110 ,  120 .) For example, the lower L-profile  104  and the upper L-profile  105  may be configured identical in shape, which simplifies assembly. 
         [0035]    In the example of  FIG. 3B , the L-profiles  104 ,  105  are fastened to the frame segments  110 ,  120  from the outside. On the other hand, in the example of  FIG. 3C , the L-profiles  104 ,  105  are fastened to the frame segments  110 ,  120  from the inside. Here, the short leg of the lower L-profile  104  is oriented away from the first foot portion  127  and riveted to the skin  102 , which it directly contacts. 
         [0036]    While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.