Abstract:
A load patch for attaching concentrated loads to a flexible inflated structure such as a high-altitude airship is stiff in the direction of the applied load so as to carry the load, and is attached to the inflated structure for a very short distance through T-tape so as to minimize strain concentration in the inflated structure due to the stretching of the inflated structure. Since the load patch must be attached for a long length transverse to the load to distribute the applied load, it is transversely compliant to minimize strain concentration in the inflated structure due to stretching of the inflated structure.

Description:
GOVERNMENT LICENSE RIGHTS 
     The U.S. Government may have a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. HQ0006-04-9-0001 awarded by the Missile Defense Agency—Department of Defense. 
    
    
     TECHNICAL FIELD 
     The present invention is related to a load patch for a high-altitude pressurized airship. More specifically, the present invention is related to a load patch for a high-altitude airship which limits the development of strain concentrations along a fabric skin of the airship. More particularly, the present invention is related to a load patch for a pressurized airship which has limited contact with the fabric skin and is composed of materials which efficiently distribute applied loads to the fabric skin. 
     BACKGROUND ART 
     Load patches serve to provide attachment points for securing various components including moorings, handling lines, and propulsion systems to the hulls of pressurized airships. Pressurized airships use a fabric skin for their hulls, which is expandable to accommodate changes in atmospheric pressure and temperature. Conventionally, the load patches are adhesively bonded directly to the fabric skin. 
     Generally, load patches are used to distribute the concentrated loads from the attachment of the moorings, handling lines, and propulsion systems into the fabric skin. Typically, conventional load patches are configured to disperse the concentrated loads radially outward away from the point of attachment of the respective component. To that end, conventional load patches usually have triangular shapes. 
     However, use of a conventional load patch leads to the development of relatively large strains concentrated along the fabric skin. For example, when attaching a conventional load patch to the fabric skin, the entire surface of one side of the load patch is adhesively bonded to the fabric skin to define a lamination area. During inflation of the pressurized airship, and changes in atmospheric pressure, the lamination area generates resistance to expansion within the fabric skin, and such resistance causes relatively large strains to develop along the fabric skin. 
     Due to the formation of the lamination area, relatively large strains develop during expansion of the fabric skin. These relatively large strains can be concentrated in areas of the fabric skin surrounding the lamination area. The effects of relatively large strains concentrated around the lamination area can be exacerbated when using the conventional load patches on a pressurized airship configured for high-altitude operation. There is a concern that the development of relatively large strains around the lamination area could cause a tear in the fabric skin resulting in catastrophic failure of the airship. 
     To illustrate the development of relatively large strains concentrated around the lamination area,  FIGS. 1A ,  1 B,  1 C, and  1 D are provided.  FIGS. 1B-1D  are color representations of strains applied to a prior art load patch. Although not absolutely required for understanding the product, it is believed that  FIGS. 1B-1D  aid in understanding the prior art. And although dimensions are provided in the drawings, they are for reference only and should not be construed as a limitation. 
       FIG. 1A  depicts a conventional load patch  10  bonded to a fabric skin  12  forming the hull of a pressurized airship. Together, the conventional load patch  10  and fabric skin  12  define a lamination area designated generally by the numeral  13 . The conventional load patch  10  is triangularly shaped with a rear edge  14 , a first angled edge  16  and a second angled edge  17 . The first angled edge  16  and second angled edge  17  intersect at an apex  18 . A load of 2000 lbf. load is applied to the conventional load patch  10  at the apex  18 . The applied load simulates the concentrated loads applied by the moorings, handling lines, propulsion systems or other component secured to the fabric skin  12  through the conventional load patch  10 . 
     The fabric skin  12  is constructed from a material that expands significantly during inflation of the pressurized airship, and changes in atmospheric pressure and temperature.  FIG. 1A  and the background of  FIGS. 1B ,  1 C, and  1 D demonstrate the deformation of the fabric skin  12  during expansion and application of different applied loads utilizing various colors to indicate the relative amounts of strain sustained by the fabric skin  12 . A color gradient is used to represent the relative amounts of strain, which may range from red, indicating high amounts of strain, to purple/blue, indicating low amounts of strain. 
     In addition,  FIGS. 1B ,  1 C, and  1 D were developed using a SHELL finite element model employing classical laminate theory to depict the resistance to expansion that is generated by the lamination area  13 , and to illustrate the strain generated as various colors related to percentages of maximum allowable strain. For example,  FIGS. 1B ,  1 C, and  1 D depict the strain developed in various areas along the fabric skin  12  as percentages from 0-68% of the maximum allowable strain in those areas. The gradations in color, as previously mentioned, indicate which of the various localities along the fabric skin  12  are experiencing the largest relative amounts of strain. 
     Because of the resistance to expansion generated by the lamination area  13  along the fabric skin  12 , and due to the application of the applied load, differing amounts of strain develop along the fabric skin  12  due to expansion.  FIGS. 1B ,  1 C, and  1 D are provided to illustrate the differing amounts of strain developed along the fabric skin  12  in three (3) directions due to the resistance to expansion and the applied load.  FIG. 1B  depicts the amount of strain developed in the axial direction,  FIG. 1C  depicts the relative amount of strain developed in the transverse direction, and  FIG. 1D  depicts the relative amount of strain developed in the negative bias direction (oriented at forty-five degrees (−45°) with respect to the axial direction and transverse direction). It should be appreciated that a high amount of relative stress is sustained by the conventional load patch  10  is found in each of the axial, transverse, and negative bias directions, as indicated by the various red areas shown in  FIGS. 1B-D . 
     As seen in  FIG. 1B , relatively large strains, indicated as red areas, are found in the axial direction that projects outwardly from the rear edge  14 , and in areas along the first angled edge  16  and second angled edge  17  adjacent the apex  18 . Furthermore, as seen in  FIG. 1C , relatively large strains in the transverse direction are concentrated in large areas projecting outwardly from the first angled edge  16  and second angled edge  17  near the rear edge  14 . And, as seen in  FIG. 1D , relatively large strains in the negative bias direction are concentrated in large areas projecting outwardly from the first angled edge  16 , and from the rear edge  14  near its intersection with the second angled edge  17 . 
     As illustrated in  FIG. 1C , relatively large strains are developed around the lamination area  13  in the transverse direction. Because the application of the applied load is in the axial direction, and, hence, in a direction perpendicular to the transverse direction, the applied load has limited responsibility for the relatively large strains developed in  FIG. 1C . The relatively large strains depicted in  FIG. 1C  are developed due to resistance to expansion generated by the lamination area. 
     When relatively large strains are concentrated in particular areas along the fabric skin  12 , those areas are susceptible to catastrophic failure, and can result in tearing of the fabric skin  12 . Because the relative amounts of strain depicted in  FIGS. 1B ,  1 C, and  1 D are shown in proportion to sixty-eight percent (68%) of the allowable strain in those areas, the relatively large strains concentrated in the areas  26 ,  27 , and  28  are within the allowable strain limits established for those areas. However, if the fabric skin  12  undergoes greater expansion, the relatively large strains concentrated around the conventional load patch  10  will be exacerbated, and will likely cause the fabric skin  12  to tear. 
     Therefore, while use of conventional load patches is suitable for pressurized airships configured for low altitude operation, such conventional load patches are unsuitable for high-altitude pressurized airships. Such high-altitude pressurized airships require their fabric skin to expand significantly. As discussed above, the conventional load patches would likely cause the fabric skin of high-altitude pressurized airships to tear because of the strain concentrations developed around the lamination area. Due to significant expansion of the fabric skin, high-altitude pressurized airships require use of load patches which limit development of strain concentrations along the fabric skin. Such load patches should limit contact with the fabric skin of the high-altitude pressurized airships, and be composed of materials which efficiently distribute the applied load to the fabric skin. 
     DISCLOSURE OF THE INVENTION 
     In light of the foregoing, it is a first aspect of the present invention to provide a load patch for airships. 
     It is another aspect of the present invention to provide a load patch comprising a load patch body having a first elastic sheet having a first and a second surface, a second elastic sheet having a first and a second surface, a plurality of filaments interposed between the first surface of the first and second elastic sheets, the plurality of filaments having a first end and a second end wherein the first end of the filaments are collected together to form an apex and the second end of the filaments radiate outwardly from the apex to a rear edge, and an attachment loop assembly attached to the plurality of filaments and a T-shaped attachment tape having a base and an arm section extending from the base wherein the rear edge of the plurality of filaments and the first and second elastic sheets are attached to the arm section of the T-shaped attachment tape. 
     Yet another aspect of the present invention is a load patch assembly adapted to secure load components to a fabric, the load patch assembly comprising at least one substantially elastic sheet having one end adapted to be secured to the fabric and an opposite end adapted to be secured to the load component, a plurality of substantially inelastic filaments secured to the sheet, the plurality of filaments emanating radially from the opposite end toward the one end, and an attachment tape secured to the one end and adapted to be secured to the fabric. 
     Still a further aspect of the present invention is a load patch used to interconnect a load component to a strained elastic surface, the load patch comprising at least one sheet having one end adapted to be secured to the strained elastic surface, the sheet having an opposite end adapted to be secured to the load component and a plurality of filaments secured in a direction generally between the ends, wherein the load patch is stiff in the direction of the load component applied to the load patch as compared to a direction of strain generated by the strained elastic surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The filing of this patent contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       For a complete understanding of the objects, techniques and structure of the invention, reference should be made to the following detailed description and accompanying drawings, wherein: 
         FIG. 1A  is a representational view of a conventional load patch attached to the fabric skin of a high-altitude pressurized airship; 
         FIG. 1B  is a colored map depicting the relative amounts of axial strain which various localities on the fabric skin and conventional load patch of  FIG. 1A  are subjected to; 
         FIG. 1C  is a colored map depicting the relative amounts of hoop strain which various localities on the fabric skin and conventional load patch of  FIG. 1A  are subjected to; 
         FIG. 1D  is a colored map depicting the relative amounts of negative bias strain which various localities on the fabric skin and conventional load patch of  FIG. 1A  are subjected to; 
         FIG. 2A  is a fragmentary plan view of the load patch showing the filaments of the present invention; 
         FIG. 2B  is an exploded perspective view of the load patch of the present invention; 
         FIG. 3  is a plan view of the T-shaped attachment tape of the present invention; 
         FIG. 4  is a side elevational view of the T-shaped attachment tape according to the present invention; 
         FIG. 5  is a partial perspective view, partially cut-away, of the T-shaped attachment tape of the present invention that is attached to the fabric skin, the load patch being secured between a first arm segment and a second arm segment thereof; 
         FIG. 6A  is a representational view of a load patch according to the present invention attached to the fabric skin of a high-altitude pressurized airship; 
         FIG. 6B  is a colored map depicting a cut-away of the load patch revealing the relative amounts of axial strain which various localities on the fabric skin and load patch of  FIG. 6A  are subjected to; 
         FIG. 6C  is a colored map depicting a cut-away of the load patch revealing the relative amounts of hoop strain which various localities on the fabric skin and load patch of  FIG. 6A  are subjected to; 
         FIG. 6D  is a colored map depicting a cut-away of the load patch revealing the relative amounts of negative bias strain which various localities on the fabric skin and load patch of  FIG. 6A  are subjected to; and 
         FIG. 7  is a schematic representation of tailored filaments used to selectively distribute load patch hull stresses. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The load patch of the present invention is generally indicated by the numeral  30  as shown in the accompanying drawings. During its use, the load patch  30  is attached to the hull of a high-altitude pressurized airship (not shown) to secure various components such as moorings, handling lines, propulsion systems and the like thereto. Because the load patch  30  is configured for use with high-altitude pressurized airships, the load patch  30  is configured to efficiently distribute the applied load to the hull of such airships. Furthermore, the load patch  30  is attached to any portion of the hull in a manner that effectively limits contact therewith. The configuration of the load patch  30 , and its manner of attachment to the hull limits development of strain concentrations along the hull of the high-altitude pressurized airship. As shown in the drawings, the load patch  30  comprises a patch body designated generally by the numeral  31  and a T-shaped attachment tape designated generally by the numeral  32 . As will become apparent, the co-action of the body  31  and the tape  32  improve the overall integrity of the associated airship and enable the effective handling of components attached to a hull of the airship by the load patches. Although the load patch  30  is described in use with an airship, it will be appreciated that the load patch could be used in any application where a load is secured to a fabric, a laminate or other planar construction such as a sail, a tent, tarp, or especially any inflated flexible structure, such as an airship, hot-air balloon, or other pressurized body. 
     As shown in  FIG. 2A , the load patch body  31  is triangularly-shaped with a rear edge  33 , a first angled edge  34  and second angled edge  35 . The load patch body  31  is attached to the hull along its rear edge  33  using the T-shaped attachment tape  32  shown in  FIG. 5 , which will be more fully described later. Furthermore, the first angled edge  34  and second angled edge  35  intersect at an apex  36 , and the various components are attached to the load patch  30  using a loop assembly  37  formed at and secured to the apex  36 . As such, the applied load from the various components is transferred to the load patch  30  at the apex  36  using the loop assembly  37 . 
     The load patch body  31  is configured to be compliant in the transverse direction, but non-compliant to strain applied in the lengthwise direction. As shown in  FIG. 2B , the load patch body  31  is formed from various filaments  40  sandwiched between elastic first and second sheets  42  and  43 . The sheets  42  and  43  may be constructed of a polyester film such as Mylar™ which has a thickness of about 0.75 mil. Of course, other polyester films with varying thicknesses exhibiting similar mechanical properties, such as elasticity, strength, weight, flexibility and so on, could be used for the sheets  42  and  43 . Although the sheets  42  and  43  could be of any shape, it is believed that the triangular shape provides for the best distribution of applied forces. The various filaments  40  radiate from the apex  36  toward the rear edge  33 , and the noncompliancy of the load patch  30  is provided thereby. The filaments  40  may be constructed of fibers exhibiting high strength and modulus, and high temperature resistance such as found in liquid crystal polymers such as Vectran™. Also sandwiched between the sheets  42  is the loop assembly  37  which includes an attachment loop  44  that serves as the connection interface with the mooring lines or other components secured to the hull. The loop assembly  37  includes a plurality of loop strands  45  which are typically constructed of a material like the filaments  40  or a material substantially equivalent thereto. The strands  45  may be separated into two groups, wherein one group is splayed or disposed over the filaments  40  and the other group is splayed or disposed underneath. And one of the groups may be longer than the other, or strands within the groups may be of different lengths to prevent stress concentration. In any event, an attachment tie  47 , which is located at the apex  36 , groups the loop strands  45  together and forms them into the attachment loop  44 . The loop strands  45  are layered upon or otherwise intermeshed with the filaments  40 , all of which are sandwiched or received between the sheets  42  and  43 . 
     A reinforcement tab  48  and a reinforcement tab  49  are positioned over respective sheets  42  and  43  and encapsulate the materials assembled therebetween. The tabs  48  and  49  are triangle-shaped to match the ends of the sheets, and they may be sized the same or differently as shown. The tabs may be constructed of a polyester fiber material such as Dacron or the equivalent. The stacked or assembled layers—tab  48 , sheet  42 , strands  45 /filaments  40 , sheet  43 , and tab  49 —may be secured to each other with a compatible adhesive  90 , which may further be reinforced with stitching  92  that interlocks the various strands and filaments to one another. 
     Because the various filaments  40  are somewhat inelastic, they serve to prevent the load patch body  31  from stretching in the lengthwise direction. As such, when the applied load from the various components is applied, the load patch  30  can be pulled taut to transfer the applied load therethrough. Furthermore, the arrangement of the various filaments  40  serve to disperse the applied load radially outward from the loop assembly  37  into the hull of the high-altitude pressurized airship. 
     The compliancy of the load patch body  31  in the transverse direction is provided by the arrangement of various filaments  40  and strands  45  between elastic first and second fabric sheets  42  and  43 . The separation of the various filaments  40  and strands  45  from one another, and the elasticity of the elastic first and second fabric sheets  42  and  43  allow the load patch body  31  to stretch transversely, and, therefore, provides its compliancy in the transverse direction, along the point of attachment to a hull of a pressurized airship. The compliancy of the load patch body  31  in the transverse direction is provided to limit resistance to expansion during the expansion of the hull. As such, the load patch  30  is capable of accommodating expansion of the hull or attached planar material, thus limiting the strain concentrations developed therein. 
     Thus, when the load patch  30  is attached to the high-altitude pressurized airship, the applied load is efficiently distributed therethrough to the hull. Additionally, because the various filaments  40  are inelastic, the force generated by the applied load is efficiently transferred therethrough. Furthermore, the separation of the various filaments  40  and strands  45  disperses the applied load radially outward from the attachment loop assembly  37  into the hull. Simultaneously, the compliancy of the load patch  30  in the transverse direction limits development of strain concentrations along the hull. 
     The load patch body  31  is attached to the hull of the high-altitude pressurized airship using the T-shaped attachment tape  32 . The T-shaped attachment tape  32  effectively limits the contact of the load patch  30  with a fabric skin  50 . As discussed below, the lamination area formed by the T-shaped attachment tape  32  and the material of the hull is significantly smaller than the lamination area formed using a conventional load patch. As such, the attachment of the load patch body  31  to the hull material using the T-shaped attachment tape  32  serves to limit the development of strain concentrations along the hull of the high-altitude pressurized airship. 
     As seen in  FIGS. 3-5 , the T-shaped attachment tape  32  includes a base portion  52  having an interior surface  54  and a hull surface  55  bisected by an arm section  56 . The arm section  56  extends outwardly from the hull surface  55  and includes a first arm segment  58  and a second arm segment  59  having inner surfaces  60 A and  60 B, respectively. The area of the load patch body  31  proximal the rear edge  33  is sandwiched between the arm segments  58  and  59 . That is, the area of the load patch body  31  directly adjacent the rear edge  33  (sheets  42  and  43 ) is secured to the inner surfaces  60 A and  60 B such that the body  31  is trapped between the arm segments  58  and  59 . An adhesive, such as a liquid adhesive, having suitable bonding strength can be used to secure the body  31  to the inner surfaces  60 A and  60 B of the first arm segment  58  and second arm segment  59 , respectively. Of course, stitching may be used to secure the patch body  31  to the tape  32 . 
     Thereafter, the T-shaped attachment tape  32  is secured to the fabric skin  50  which forms the hull of the airship. One exemplary fabric is disclosed in U.S. patent application Ser. No. 10/388,772, published as U.S. Published Application No. 2004/0180161-A1, which is incorporated herein by reference. Of course, other airships or other materials which need load patches could be used. As seen in  FIG. 5 , the fabric skin  50  has an outer surface  62  and an inner surface  63 , and includes a slit  64  therein adapted to receive the arm section  56 . To attach the T-shaped attachment tape  32  to the fabric skin  50 , the base portion  52  is inserted through the slit  64 , and both sections of the hull surface  55  are attached to the inner surface  63  of the fabric skin  50 . The attachment of the T-shaped attachment tape  32  to the fabric skin  50  may be achieved using any suitable adhesive, or the like. Of course, the attachment of the tape  32  to the fabric skin  50  must provide a gas tight connection. 
     When the base portion  52  is attached to the fabric skin  50 , a lamination area  66  is formed. The lamination area  66  ( FIG. 6A ) is effectively defined by the dimensions of the hull surface  55 . Additionally, the length and stiffness of the lamination area  66  formed by use of the T-shaped attachment tape  32  to attach the load patch  30  to the fabric skin  50  determines the amount of strain concentrations along the hull of the high-altitude pressurized airship. 
     To illustrate the relative amounts of strain developed along the hull of the high-altitude pressurized airship,  FIGS. 6A ,  6 B,  6 C, and  6 D are provided.  FIG. 6A  depicts the load patch  30  attached to the fabric skin  50 , and the lamination area  66  formed by the attachment of the load patch body  31  using the T-shaped attachment tape  32 . The fabric skin  50  is configured to expand significantly during inflation of the high-altitude pressurized airship, and during changes in atmospheric pressure. Furthermore, an exemplary load of 2000 lbf. is applied at the apex  36  to simulate the loads applied by the moorings, handling lines, and propulsion systems to the fabric skin  50  through the load patch  30 . 
     Using various colors to represent relative amounts of strain as previously discussed with respect to the conventional load patch  10 ,  FIG. 6A  and the background of  FIGS. 6B ,  6 C, and  6 D demonstrate the deformation of the fabric skin  50  during expansion of the fabric skin  50  and during application of the applied load. 
     In contrast to the conventional load patch  10 , discussed with respect to  FIGS. 1A-D , the present load patch  30  limits the extent to which relatively large strains are concentrated in the fabric skin  50  in response to pressurization at the lamination area  66 . For example, as seen in  FIG. 6B , the strain developed in the axial direction is concentrated in area  76  as a somewhat semi-circular shape projecting outwardly from the rear edge  33 , but unlike with use of the conventional load patch  10 , the strains are not concentrated adjacent the apex  36 . In fact, in the area  80  underneath the load patch, and areas  70 ,  71 ,  72 , and  73 , relatively small amounts of strain develop. Furthermore, as seen in  FIG. 6C , the strain developed in the transverse direction are concentrated in relatively large areas  82  and  83  adjacent the distal ends of the lamination area  66 . Moreover, as seen in  FIG. 6D , the strain developed in the negative bias direction are concentrated in a small area  86  adjacent one distal end of the lamination area  66 , and in a large area  88  adjacent the other distal end of the lamination area  66 . Overall, the strain developed in the load patch  30  is significantly lower than the strain developed in corresponding areas of the conventional load patch  10 . In addition, the size and number of the areas experiencing strain of the load patch  30  are significantly reduced over that of the conventional load patch  10 . 
     Referring now to  FIG. 7 , a load patch with a tailored radiated stiffness configuration is designated generally by the numeral  100 . In this configuration, the filaments  40  have ends  40   a  that are selectively positioned on the load patch body  31 . For example, the ends  40   a  proximal the edges  34  and  35  may be longer than those ends more medially disposed. Accordingly, the stiffness of the patch may be varied continuously through the patch angle by bonding the ends  40   a  to the facing surfaces of at least one of the sheets  42  and  43 . Thus, it will be appreciated that any hull stress distribution may be attained by selectively positioning and distributing the ends  40   a  on either of the sheets  42  and  43 . 
     In conclusion, the load patch  30  is configured to efficiently distribute the applied load to the hull, and is attached to the hull in a manner which effectively limits contact therewith. The configuration of the load patch  30 , and its manner of attachment to the hull fabric limit development of strain concentrations along the hull of the high-altitude pressurized airship. 
     It will, therefore, be appreciated that one advantage of one or more embodiments of the present invention is that the load patch is non-compliant in its lengthwise direction, allowing the load patch to radially disperse forces, generated by an attached load, into the hull of the high-altitude pressurized airship. Still another advantage of the present invention is that the load patch is transversely compliant, and as such, is able to stretch as the hull increases in size as the internal pressure of the high-altitude pressurized airship increases. Yet another advantage of the present invention is that the lamination area defined by the T-shaped attachment tape used to affix the load patch body to the hull of the high-altitude pressurized airship is reduced. The transverse compliance of the load patch body, along with the reduced lamination area of the T-shaped attachment tape, contributes to the reduction in strain concentrations that develop in the high-altitude pressurized airship as the hull expands. And the hull stress distribution associated with the load patch can be tailored or varied to specific types of load application. 
     Thus, it should be evident that the load patch body and the T-shaped attachment tape used in attaching the load patch disclosed herein carries out one or more of the objects of the present invention set forth above and otherwise constitutes an advantageous contribution to the art. As will be apparent to persons skilled in the art, modifications can be made to the preferred embodiment disclosed herein without departing from the spirit of the invention, the scope of the invention herein being limited solely by the scope of the attached claims.