Abstract:
A bonded metal fuselage for aerospace vehicles includes a monocoque structure having an outer metal skin, metal tear straps bonded to the outer skin and metal stringers bonded to the outer skin and to the tear straps. The outer chords of fuselage frames are fastened directly to tear straps and to the skin, obviating the need for clips to fasten the frames to the stringers.

Description:
TECHNICAL FIELD 
     This disclosure generally relates to fuselage structures for aerospace vehicles, and deals more particularly with a fuselage having bonded metal components that result in increased fuselage strength and/or weight reduction. 
     BACKGROUND 
     In metal monocoque fuselage structures for aerospace vehicles such as airplanes, frames and stringers are commonly attached to the metal skin by fasteners such as rivets having heads that are countersunk within the skin. In some cases, circumferential strips of material, commonly known as “tear straps”, are placed between the skin and the frames in order to reduce hoop stress. Also, in some cases, separate clips are used to connect the stringers to the frames in order to transfer a portion of the load on the stringers to the frames. In order to assure that the skin exhibits uniform structural strength, the gauge or thickness of the skin in the area of the fasteners is increased so as to form “lands” or “pad-ups” whose thickness is sufficient to accommodate the countersink depth of the fastener head. The skin lands add weight to the aircraft, and increase aircraft assembly time since it is necessary to machine or otherwise remove a portion of the skin thickness in order to form the lands. 
     It would therefore be desirable to reduce or eliminate the need for skin lands in order to reduce fuselage weight and assembly time. It would also be desirable to reduce the number of fasteners required to attach the skin to the stringers and the frame. 
     SUMMARY 
     The disclosed embodiments provide a metal fuselage construction exhibiting reduced weight and/or increased strength. Through the use of bonded components, the number of fasteners may be reduced and thinner skins may be employed, particularly where fasteners are no longer required. The elimination of clips to structurally connect stringers and frames may reduce the load that may be transferred by the stringers to the frames, resulting in stringers and frames that may possess greater independent stability. 
     According to one disclosed embodiment, an aerospace vehicle fuselage comprises a monocoque structure having an outer skin, tear straps bonded to the outer skin and stringers bonded to the outer skin and to the tear straps; frames; and fasteners for fastening the frames to the tear straps and to the skin. The frames may include openings through which the stringers extend transverse to the frames. The stringers extend transverse to the tear straps and the tear straps are sandwiched between the stringers and the skin. Each of the stringers includes a surface facing the skin, and the stringers include a joggle through which one of the tear straps extends. The tear straps are bonded to the stringers in the area of the joggle. Each of the frames may include a pair of oppositely extending flanges on one end of a web, wherein each of the flanges engages and is fastened to a tear strap. 
     According to another disclosed embodiment, a fuselage for a vehicle having a longitudinal axis comprises: a skin having an inner surface and an outer surface; a plurality of longitudinally extending, circumferentially spaced stringers bonded to the inner surface of the skin; a plurality of longitudinally spaced, circumferentially extending frames; and, a plurality of tear straps respectively sandwiched between the frames and the inner surface of the skin, wherein each of the tear straps is bonded to the inner surface of the skin. Portions of the stringers are bonded to the tear straps in an area of a joggle formed on one side of the stringers. 
     According to a disclosed method embodiment, fabricating a fuselage for an aerospace vehicle comprises: fixing frames to a skin of the fuselage; placing tear straps between each of the frames of the skin; bonding the tear straps to an inner surface of the skin; fixing stringers to the skin; and bonding portions of the stringers to the tear straps. Fixing the frames to the skin may be performed by installing fasteners between each of two flanges on each frame and the skin. The frames are fixed to the skin after the tear straps have been bonded to the skin and after the stringers have been bonded to portions of the tear straps. 
     According to another method embodiment, fabricating a fuselage for an airplane, comprises: bonding metal tear straps to an inner surface of a metal fuselage skin; bonding metal stringers to the inner surface of the skin and to the tear straps; placing frames over the tear straps; and, fastening the frames to the tear straps and to the skin using fasteners. The method may further comprise forming a joggle in one side of the stringers for receiving a tear strap. The method may further comprise forming a pair of flanges on the frame and placing fasteners through the flanges, the tear straps and the skin. 
     The disclosed embodiments satisfy the need for an improved metal fuselage construction making use of bonded components to reduce skin thickness requirements and to reduce the number of fasteners used to join the structure. 
     Other features, benefits and advantages of the disclosed embodiments will become apparent from the following description of embodiments, when viewed in accordance with the attached drawings and appended claims 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         FIG. 1  is a perspective view of a section of bonded metal airplane fuselage according to the disclosed embodiments. 
         FIG. 2  is a perspective view of a portion of the interior of the fuselage shown in  FIG. 1 . 
         FIG. 3  is a cross sectional view taken along the line  3 - 3  in  FIG. 2 . 
         FIG. 4  is a sectional view taken along the line  4 - 4  in  FIG. 2 . 
         FIG. 5  is a sectional view taken along the line  5 - 5  in  FIG. 2 . 
         FIG. 6  is a flow diagram showing the steps of a method for fabricating a bonded metal fuselage. 
         FIG. 7  is a flow diagram of aircraft production and service methodology. 
         FIG. 8  is a block diagram of an aircraft. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIGS. 1-5 , a monocoque fuselage  10  for an aerospace vehicle such as an airplane, includes a longitudinal axis  15 , and may possess any of various cross sectional shapes. In the illustrated embodiment, the fuselage  10  has a circular cross section, however other shapes are possible, including, without limitation, an oval or partial oval shape. The fuselage  10  broadly comprises an outer metal skin  12  formed of one or more sections, a plurality of barrel shaped, longitudinally spaced metal frames  14 , and a plurality of circumferentially spaced, longitudinally extending stringers  16 . The skin  12 , frame  14  and stringers  16  may be formed from a suitable metal, such as, without limitation, aluminum or titanium. 
     Each of the frames  14  includes a single, inner flange  22  connected by a web  24  to an outer chord  23  defined by a pair of substantially coplanar outer flanges  26   a ,  26   b  which extend outwardly in opposite directions from the web  24 . The web  24  may include a reinforcing land  28 , and a plurality of circumferentially spaced, mouse hole-shaped openings  30  formed in the outer part of the web  24  and the double flanges  26   a ,  26   b . In the illustrated embodiment, the frames  14  may be of one piece, unitary construction formed as by machining from a suitable metal such as, without limitation, aluminum, however, in other embodiments, the frames  14  may comprise multiple sections that are joined together using any of various means, such as, without limitation, splice plates (not shown). 
     A circumferentially extending tear strap  18  formed from a suitable sheet of metal such as, without limitation, aluminum is sandwiched between the double flanges  26   a ,  26   b  and the inner surface  25  of the skin  12 . The outer edges  27  of the tear strap  18  extend laterally beyond the outer edges  29  of the double flanges  26   a ,  26   b , as best seen in  FIG. 3 . In one particular fuselage application by way of example and without limitation, the tear straps  18  may be formed of aluminum, measuring three inches wide and 0.040 inch thick. 
     The stringers  16  are hat shaped in cross section as best seen in  FIG. 2 , and comprise a flat inner surface  32  and a pair of outer flanges  34  forming a “brim”. The stringers  16  may comprise a suitable metal, such as, without limitation, aluminum or titanium and may be fabricated using common manufacturing techniques such as forming or extrusion. As best seen in  FIG. 5 , the inner surface  34  of each of the stringers  16  is slightly spaced from the frame  28 . Each of the stringers  16  includes longitudinally spaced joggles  38  formed in the flanges  34  in the area  36  where the flanges  34  and tear strap  18  overlap each other. The joggles  38  form a clearance space within the stringers  16  for receiving the thickness of the tear straps  18 . 
     As will be discussed below in more detail, the tear straps  18  pass through the joggles  38  formed in the stringer  16  and are bonded to both the stringer  16  and the inner surface  25  of the skin  12 . The joggles  38  in the flanges  34  may be formed by any of various fabrication techniques, such as a forming process in which a set of tooling (not shown) crushes portions of the flanges  34 . The tear straps  42  are bonded to the inner surface  25  of the skin  12  by a layer  40  of bonding adhesive suitable for bonding two metals such as aluminum. Similarly, another layer  42  of bonding adhesive is used to bond the stringers  16  to both the tear straps  42  and the inner surface  25  of the skin  12 , on opposite sides of the tear straps  18 . 
     Referring now particularly to  FIGS. 2 and 3 , the frames  14  may be fixed to the skin  12  by fasteners such as rivets  20  which pass through the skin  12 , tear strap  18  and flanges  26   a ,  26   b . Since the frames  14  are not directly connected to the stringers  16 , loads on the skin  12  may be transferred directly through the tear straps  18  to the frames  14 . Similarly, compressive and bending loads on the skin  12  may be transferred to the stringers  16  either directly, or indirectly to the stringers  16  through the tear straps  18 . 
     From the forgoing, it can be appreciated that because the stringers  16  are fixed to the skin  12  and to the tear strap  18  by adhesive bonding, a large number of fasteners otherwise required to fix the stringers  16  to the skin  12  may be eliminated. Moreover, the enhanced stability and load carrying characteristics of the frames  14  and stringers  16  may allow the use of lighter gauge skins  12  and/or the elimination of skin gauge lands to accommodate fastener countersink depths. It should be noted here that although the illustrated bonded metal fuselage  10  has been shown with a uniform construction around its circumference, the construction techniques of the disclosed embodiments may be used in only a portion of the fuselage, such as only in an upper lobe of the fuselage  10 . It should also be pointed out that the disclosed embodiments may be advantageously used in fuselage structures formed partly or completely from materials other than metal, such as composite materials. Thus for example, and without limitation, any or all of the stingers  16 , tear straps  18  and skin  12  may comprise composite materials that may be bonded together using techniques and adhesives well known in the art of composite materials. 
     As shown in  FIG. 6 , the process of making the bonded metal fuselage  10  begins with the fabrication of the tear straps  18  at step  46  and the fabrication of the stringers  16  at step  48 . During fabrication of the stringers  16 , the joggles  38  are created to provide clearances substantially matching the cross section geometry of the tear straps  18 . The frames  14  are fabricated at step  58  using any of various fabrication techniques, depending upon the frame design and the application. 
     At step  49 , the surfaces to be bonded are cleaned and prepared using commonly employed techniques. For example, the surfaces of the skin  12 , stringers  16  and tear straps  18  may be cleaned using a suitable acid etch, following which these same surfaces may be anodized as by phosphoric acid anodizing in order to provide the bonded surfaces with corrosion protection. Additionally, depending upon the application, it may be desirable to apply a corrosion inhibiting primer to the bonded surfaces in order to increase the durability of the bonded joint. One primer suitable for use is BR-127 available from American Cyanamid. Primer thickness may be controlled to ensure design strength allowable and to maximize bond joint durability. Following the application of the primer, oven curing may be required. 
     Next, at step  50 , a suitable bonding agent is applied to the skin  12  and the tear straps  18 , following which, at step  52 , the tear straps  18  are located and bonded to the skin  12 . Any of various suitable bonding agents may be employed, depending upon the metals being bonded, and the application. For example, where the materials to be bonded are aluminum, one suitable bonding agent is AF 126 available from the Minnesota Mining and Manufacturing Company (“3M”). Other bonding agents are possible, however, such as FM-73 available from American Cyanamid. 
     At step  54 , the bonding agent is applied to mating surfaces of the stringers  16 , tear straps  18  and skin  12 , following which at step  56  the stringers  16  are located and bonded to the tear straps  18  and to the skin  12 . As part of the process of locating the stringer  16 , the joggles  38  are aligned over the tear straps  18 . 
     At step  57  the lay-up of the bonded parts may be placed in an autoclave (not shown) where pressure and heat are applied to the lay-up in order to cure the adhesive bonding agent. The autoclave processing times, temperatures and pressures will vary depending upon the application and the particular adhesive bonding agent that is employed. 
     After the autoclave processing is completed at step  57 , the frames  22  are located and installed in the usual manner, which typically may involve drilling countersunk holes (not shown) through the skin  12 , the tear straps  18  and the flanges  26   a ,  26   b  (see  FIG. 2 ) following which fasteners such as rivets  20  are installed and upset. 
     Embodiments of the disclosure may find use in a variety of potential applications, particularly in the transportation industry, including for example, aerospace, marine and automotive applications. Thus, referring now to  FIGS. 7 and 8 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  62  as shown in  FIG. 7  and an aircraft  64  as shown in  FIG. 8 . During pre-production, exemplary method  62  may include specification and design  66  of the aircraft  64  and material procurement  68 . During production, component and subassembly manufacturing  70  and system integration  72  of the aircraft  64  takes place. Thereafter, the aircraft  64  may go through certification and delivery  74  in order to be placed in service  76 . While in service by a customer, the aircraft  64  is scheduled for routine maintenance and service  78  (which may also include modification, reconfiguration, refurbishment, and so on). 
     Each of the processes of method  62  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 8 , the aircraft  92  produced by exemplary method  62  may include an airframe  80  with a plurality of systems  82  and an interior  84 . Examples of high-level systems  82  include one or more of a propulsion system  86 , an electrical system  88 , a hydraulic system  90 , and an environmental system  92 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the disclosure may be applied to other industries, such as the marine and automotive industries. 
     Systems and methods embodied herein may be employed during any one or more of the stages of the production and service method  62 . For example, components or subassemblies corresponding to production process  90  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  92  is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the production stages  70  and  72 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  64 . Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while the aircraft  64  is in service, for example and without limitation, to maintenance and service  62 . 
     Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.