Abstract:
Apparatus and methods for securing a first structural member and a second structural member to one another. In one embodiment a two-piece fitting is provided. The fitting includes a variable gap between attachment surfaces, enabling the fitting to accommodate structural members of variable thickness. The fitting also includes overlapping sections that provide increased resistance to pull-off loads and impact loads. The configuration of the fitting advantageously simplifies the assembly process and preserves the integrity of the adhesive to form a strong joint.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to apparatus and methods for securing a first structural member and a second structural member to one another. 
     2. Description of Related Art 
       FIG. 1  illustrates one common method for securing a first structural member  1 I and a second structural member  12  to one another. The first structural member  10  may be, for example and without limitation, a frame, a bulkhead, a cap or a rib. The second structural member  12  may be, for example and without limitation, a wing skin, a floor panel or a fuselage skin panel. First and second attachment angles  14  secure the structural members  10 ,  12  to one another. Each angle  14  comprises a first section  16  and a second section  18 . The sections  16 ,  18  may be substantially planar, or may include some curvature. For simplicity, the sections  16 ,  18  are illustrated herein as being planar. 
     The sections  16 ,  18  are substantially perpendicular to one another and are joined at a radiused corner  20 . The first section  16  of each angle  14  is secured to the first structural member  10 , and the second section  18  of each angle is secured to the second structural member  12 . In  FIG. 1  adhesive  22  secures the mating surfaces to one another. However, other methods of attachment are also used, such as mechanical fasteners, bonding and co-bonding. In  FIG. 1  a gap  23  is left between the first structural member  10  and the second structural member  12 . The gap  23  may be filled with resin (not shown). 
     The method of attachment illustrated in  FIG. 1  creates disadvantageous structural loading issues when pull-off loads are high. For example, pull-off loads create induced moments at the interfaces between the angles  14  and the second structural member  12  adjacent the radiused corners  20 . These induced moments can cause the angles  14  to peel away from the second structural member  12 . Also, impacts to the second structural member  12  in the area between the angles  14  can create fracture zones in the resin filling the gap  23 . Cracks can propagate through the resin to the angles  14  causing de-bonding and bond failure. 
       FIG. 2  illustrates another common method for securing a first structural member  10  and a second structural member  12  to one another. As shown, a single-piece Pi fitting  24  includes a first section  26 . Second and third sections  28 .  30  extend substantially perpendicularly from a first face  32  of the first section  26 , making the Pi fitting  24  resemble the Greek letter “Pi”. A space  34  between the second and third sections  28 ,  30  receives the first structural member  10 . Adhesive  22  secures the first structural member  10  to inner faces of the second and third sections  28 ,  30 . A second face of the first section  26  is secured to the second structural member  12  with adhesive  22 . The space  34  may be filled with resin (not shown). 
     The one-piece Pi fitting  24  illustrated in  FIG. 2  provides some structural advantages over the two-piece attachment angles  14  illustrated in  FIG. 1 . However, the Pi fitting  24  presents installation issues due to the fixed distance between the second and third sections  28 ,  30 . For example, variances in the thickness of the first structural member  10  and/or in the distance between the second and third sections  28 .  30  can cause the gap  34  to be too narrow to receive the first structural member  10 , or cause the gap  34  to be so wide that it is not possible to adhere the first structural member  10  to both of the second and third sections  28 ,  30 . Further, after applying adhesive  22  to the first structural member  10  and/or to the second and third sections  28 ,  30  it can be difficult to position the first structural member  10  between the second and third sections  28 ,  30  without disturbing the adhesive  22  and diminishing the structural integrity of the bonded area. 
     SUMMARY 
     The preferred embodiments of the present apparatus and methods have several features, no single one of which is solely responsible for their desirable attributes. Without limiting the scope of these apparatus and methods as expressed by the claims that follow, their more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description of the Preferred Embodiments”, one will understand how the features of the preferred embodiments provide advantages which include increased resistance to pull-off loads and impact loads, a relatively benign failure mechanism, allowance for thickness variations in the structural members, preservation of adhesive during installation, and reduced complexity in applying adhesive. 
     One embodiment of the present apparatus and methods comprises a fitting configured to secure a first structural member and a second structural member to one another. The fitting comprises a first piece including a first section and a second section. The first and second sections adjoin one another at a substantially perpendicular angle. The fitting further comprises a second piece including a third section, a fourth section and a fifth section. The third and fourth sections adjoin one another at a substantially perpendicular angle. The third and fifth sections adjoin one another at a substantially perpendicular angle. The fourth and fifth sections lie in substantially parallel, noncoinciding planes. 
     Another embodiment of the present apparatus and methods comprises a method of securing a first structural member and a second structural member to one another. The method comprises the steps of securing a first section of a first piece of a fitting to a first face of the first structural member, securing a third section of a second piece of the fitting to a second face of the first structural member opposite the first face securing a second section of the first piece to a fifth section of the second piece, and securing the second section and a fourth section of the second piece to the second structural member. 
     Another embodiment of the present apparatus and methods comprises a method of manufacturing an aircraft, the method including at least a pre-production phase and a production phase. The method comprises the steps of: designing the aircraft including subassemblies, and components therefor; specifying and procuring materials; fabricating the components from the materials; assembling the subassemblies by combining subsets of the components- and assembling the aircraft by combining subsets of the subassemblies. The step of assembling the subassemblies includes the step of securing a first structural member and a second structural member to one another. The step of securing the first structural member and the second structural member to one another includes the steps of: a) securing a first section of a first piece of a fitting to a first face of the first structural member; b) securing a third section of a second piece of the fitting to a second face of the first structural member opposite the first face; c) securing a second section of the first piece to a fifth section of the second piece; and d) securing the second section and a fourth section of the second piece to the second structural member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The preferred embodiments of the present apparatus and methods will now be discussed in detail with an emphasis on highlighting the advantageous features. These embodiments depict the novel and non-obvious apparatus and methods shown in the accompanying drawings, which are for illustrative purposes only. These drawings include the following figures, in which like numerals indicate like parts: 
         FIG. 1  is a front elevation detail view of a prior art method and apparatus for securing a first structural member and a second structural member to one another; 
         FIG. 2  is a front elevation detail view of another prior art method and apparatus for securing a first structural member and a second structural member to one another; 
         FIG. 3  is a flowchart illustrating steps in an integrated aircraft production process; 
         FIG. 4  is a front elevation detail view of one embodiment of the present apparatus for securing a first structural member and a second structural member to one another; 
         FIG. 5  is a front elevation detail view of one embodiment of the present apparatus and methods for securing a first structural member and a second structural member to one another; 
         FIG. 6  is a flowchart illustrating steps in one embodiment of the present methods for securing a first structural member and a second structural member to one another; 
         FIG. 7  is a front elevation detail view of another embodiment of the present apparatus for securing a first structural member and a second structural member to one another: 
         FIG. 8  is a front elevation detail view of another embodiment of the present apparatus for securing a first structural member and a second structural member to one another; and 
         FIG. 9  is a front elevation detail view of one embodiment of the present apparatus and methods for securing a first structural member and a second structural member to one another. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  illustrates an integrated aircraft production process  100 , in accordance with embodiments of the present disclosure. As used herein, the integrated aircraft production process  100  also may include manufacturing, support, or both. Typically, the process  100  includes a pre-production phase (S 102 ), a production phase (S 104 ), and a post-production phase (S 106 ). The pre-production phase S 102  may include aircraft, subassembly, and component design (S 110 ), and material specification and procurement (S 120 ). Material specification and procurement (S 120 ) may include selection and procurement of components fabricated, or subassemblies manufactured, by third parties. Examples of such third parties include, without limitation, vendors, subcontractors, and suppliers. The production phase S 104  may include component fabrication and/or subassembly manufacturing (S 130 ), and aircraft assembly (S 140 ). The pre-production phase (S 102 ) and production phase (S 104 ) can be elements of an integrated manufacturing process (S 105 ) including one or more of aircraft and component design, development, and simulation processes, material, component, and sub-assembly specification and procurement processes; automated production planning processes; fabrication and assembly processes; and quality control processes. 
     Frequently, aspects of a modern aircraft production process, such as the integrated process  100 , do not end with final assembly, but may extend over the service life of an aircraft. These aspects may involve iterative and interactive collaborations between manufacturer, governmental authorities, customers and aircraft operators. Accordingly, the integrated production process  100  can include a post-production phase (S 106 ). The post-production phase (S 106 ) may include aircraft delivery and qualification (S 150 ), and/or aircraft service (S 160 ). The aircraft delivery and qualification (S 150 ) may include providing an aircraft to customer specifications, which may have changed from the time the aircraft was assembled. Thus, delivery and qualification can include repair, modification, and/or revision of one or more elements of the aircraft after delivery to a customer or operator. Also, it may be desirable to perform a modification, a repair, or an upgrade to an aircraft in the service interval between aircraft delivery and retirement. Therefore, aircraft service S 160  can include repair, modification, and/or upgrade of a portion of an airframe, including an airframe manufactured or assembled using traditional, pre-existing materials, components, and/or subassemblies. 
       FIG. 4  illustrates one embodiment of the present apparatus for securing a first structural member and a second structural member to one another. The apparatus comprises a two-piece fitting  36 . A first piece  38  includes a first section  40  and a second section  42 . The sections  40 ,  42  may be substantially planar, or may include some curvature. For simplicity, the sections  40 ,  42  are illustrated herein as being planar. The first and second sections  40 ,  42  adjoin one another at a substantially perpendicular angle, with the second section  42  extending outward from approximately a center of a first face  44  of the first section  40 . In the front elevation view shown in  FIG. 4 , the first piece  38  resembles the letter “T”. In the illustrated embodiment, corners  45  of the junction between the first and second sections  40 ,  42  are not radiused. However, those of ordinary skill in the art will appreciate that the corners  45  may be radiused, similar to the corners  20  of the angles shown in  FIG. 1 . 
     A second piece  46  includes a third section  48 , a fourth section  50  and a fifth section  52 . The sections  46 ,  48 ,  50  may be substantially planar, or may include some curvature. For simplicity, the sections  46 ,  48 ,  50  are illustrated herein as being planar. The third and fourth sections  48 ,  50  adjoin one another at a substantially perpendicular angle, with the fourth section  50  extending outward from an end of a first face  54  of the third section  48 . The third and fifth sections  48 ,  52  similarly adjoin one another at a substantially perpendicular angle, with the fifth section  52  extending outward from near an end of a second face  56  of the third section  48 . The fourth and fifth sections  50 ,  52  extend in opposite directions from the third section  48 , and lie in substantially parallel, noncoinciding planes. In the illustrated embodiment, corners  45  of the junctions between the third, fourth and fifth sections  48 ,  50 ,  52  are not radiused. However, those of ordinary skill in the art will appreciate that the corners  45  may be radiused, similar to the corners  20  of the angles shown in  FIG. 1 . 
     When the first and second pieces  38 ,  46  are arranged as shown in FIG.  4 ? with the fifth section  52  of the second piece  46  overlapping the second section  42  of the first piece  38 , the fitting  36  closely resembles the one-piece Pi fitting  24  illustrated in  FIG. 2  Advantageously, however, the present fitting  36  includes first and second pieces  38 ,  46  so that a distance between the first section  40  of the first piece  38  and the third section  48  of the second piece  46  is variable. The present fitting  36  is thus able to accommodate variations in the thickness of structural members, and can be adhered to a structural member without disturbing adhesive applied to either or both of the structural member and the first and second pieces  38 ,  46 . Further advantages of the present two-piece fitting  36  are described in detail below. 
       FIG. 5  illustrates one embodiment of the present methods for securing a first structural member  10  and a second structural member  12  to one another using the fitting  36  shown in  FIG. 4 . The two pieces  38 ,  46  are arranged as they are in  FIG. 4 , with the fifth section  52  of the second piece  46  overlapping the second section  42  of the first piece  38 . In this arrangement the first, third and fifth sections  40 ,  48 ,  52  form a cavity  58  that receives the first structural member  10 . Adhesive  22  interposed between the fifth section  52  and the second section  42  secures the first and second pieces  38 ,  46  to one another. Adhesive  22  interposed between the second section  42  and the fourth section  50  and the second structural member  12  secures the first and second pieces  38 ,.  46  to the second structural member  12 . Adhesive  22  interposed between the first section  40  and the first structural member  10 , and between the third section  48  and the first structural member  10  secures the first and second pieces  38 ,  46  to the first structural member  10 . In  FIG. 5  a gap  60  is left between the first structural member  10  and the fifth section  52 . The gap  60  may be filled with adhesive, tow and/or chopped fibers in order to increase the strength of the joint. Those of ordinary skill in the art will appreciate that the gap  60  need not be provided. 
     The attachment arrangement illustrated in  FIG. 5  advantageously addresses the structural loading issues of the attachment angles  14  shown in  FIG. 1 . When pull-off loads are applied to the first structural member  10  the resultant forces at the junction of the first and second pieces  38 ,  46  and the second structural member  12  are applied over the whole combined length of the second section  42  and the fourth section  50 . Thus, the tendency of the angles  14  ( FIG. 1 ) to peel away from the second structural member  12  is greatly reduced in the arrangement illustrated in  FIG. 5 . Also, impacts to the second structural member  12  in the area between the first section  40  and the third section  48  are less likely to propagate into and/or through the material filling the gap  60  (if such material is provided). The space between the first structural member  10  and the second structural member  12  includes at least the second section  42 , the fifth section  52  and adhesive  22 . Thus, this area is stronger than the corresponding area of the apparatus of  FIG. 1 . 
     Advantageously, the various components in  FIG. 5  can be secured to one another in any order. Thus, for example and without limitation, in a first step the first and second pieces  38 ,  46  of the fitting  36  may be secured to the first structural member  10  and to each other, and then in a second step the subassembly may be secured to the second structural member  12 . Alternatively, in a first step the first piece  38  may be secured to the second structural member  12 , and then in a second step the first structural member  10  may be secured to the first piece  38 , and in a third and final step the second piece  46  may be secured to the first piece  38 , to the first structural member  10  and to the second structural member  12 . 
       FIG. 6  illustrates in greater detail one non-limiting example of a method for securing the first and second pieces  38 ,  46  and the first and second structural members  10 ,  12  to one another. In step S 600  the first section  40  of the first piece  38  is secured to a first face of the first structural member  10 . In step S 602  the third section  48  of the second piece  46  is secured to a second face of the first structural member  10  opposite the first face. In step S 604  the second section  42  of the first piece  38  is secured to the fifth section  52  of the second piece  46 . In step S 606  the second section  42  and the fourth section  50  of the second piece  46  are secured to the second structural member  12 . 
     In one embodiment the first and second pieces  38 ,  46  of the present fitting  36  are constructed of composite materials. For example and without limitation, the composite material may include at least one layer, and in certain embodiments a plurality of layers, of a reinforcement material having a textile constriction.  FIG. 7  illustrates one embodiment of the present fitting  36  comprising a plurality of layers  62 . In  FIG. 7  the spacing between adjacent layers has been exaggerated for clarity. The reinforcement material may be, but is not limited to, glass, polymer (such as aramid) and/or carbon, for example. Where the reinforcement material comprises multiple layers, different layers may comprise different materials. The matrix material may be a polymer, such as and without limitation, epoxy. The matrix may be either thermoset or thermoplastic. 
     In addition to the assembly options described above, the components may be secured to one another using a variety of different techniques, including without limitation bonding and co-bonding. For example, all of the components may be cured prior to assembly and then bonded together with adhesive. Alternatively, some or all of the components may be bonded to each other in a “B” stage, or partially cured stage. Various subassemblies, or the entire assembly, may then be cured together. Such co-bonding techniques may help to strengthen bonds between components while using less adhesive. 
     With reference to  FIG. 8 , in the multi-layer embodiments of the present fitting  36  stitching  64  and/or Z-pins (not shown) may be provided to reinforce the securement of the layers  62  to one another.  FIG. 8  illustrates stitching  64  reinforcing the layers of the second section  42 , fourth section  50  and fifth section  52 . The stitching  64  may comprise, for example and without limitation, graphite thread, glass thread, polymeric thread or another type of thread. The stitching  64  reinforces the layers  62  by providing additional mechanical support to the adhesive support provided by the matrix material. Those of ordinary skill in the art will appreciate that stitching  64  and/or Z-pins may be provided on fewer or more of the sections than as shown in  FIG. 8 . 
     With reference to  FIG. 9 , stitching  64  and/or Z-pins (not shown) may also reinforce the connections between the first and second pieces  38 ,  46  and the second structural member  12 . Such stitching  64  may be provided in both the single-layer and the multi-layer embodiments of the present fitting  36 . As illustrated, a first quantity of stitching  64  extends through the second section  42  and the second structural member  12 , and a second quantity of stitching  64  extends through the fourth section  50  and the second structural member  12 . The stitching  64  advantageously increases the pull-off strength of the joint by providing additional mechanical support. And in the multi-layer embodiments of the present fitting  36 , the stitching  64  also reinforces the securement of the layers  62  to one another. Although not illustrated herein, stitching  64  and/or Z-pins may also be provided between the first and second pieces  38 ,  46  and the first structural member  10 . Such stitching  64  and/or Z-pins advantageously increases the strength of the connection between the first structural member  10  and the present fitting  36 . Further, although also not illustrated herein, stitching  64  and/or Z-pins may also be provided between the second section  42  and the fifth section  52  and/or between the second section  42 , the fifth section  52  and the second structural member  12 . Such stitching  64  and/or Z-pins advantageously increases the strength of the connection between the second and fifth sections  42 ,  52  and/or between the second section  42 , the fifth section  52  and the second structural member  12 . 
     The above description presents the best mode contemplated for carrying out the present apparatus and methods, and of the maimer and process of making and using them in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains to make these apparatus and use these methods. These apparatus and methods are, however, susceptible to modifications and alternate constructions from that discussed above that are fully equivalent. Consequently, these apparatus and methods are not limited to the particular embodiments disclosed. On the contrary, these apparatus and methods cover all modifications and alternate constructions coming within the spirit and scope of the apparatus and methods as generally expressed by the following claims, which particularly point out and distinctly claim the subject matter of the apparatus and methods.