Abstract:
Disclosed are structures and features of a space frame aircraft. In particular, this disclosure relates to a space frame aircraft with a pressure membrane.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application, under 35 U.S.C. §119, claims the benefit of U.S. Provisional Patent Application Ser. No. 62/365,266 filed on Jul. 21, 2016, and entitled “Space Frame Aircraft Structures,” the contents of which is hereby incorporated by reference herein. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    This disclosure relates generally to structures and features of a space frame aircraft. In particular, this disclosure relates to a space frame aircraft with a pressure membrane. 
       BACKGROUND 
       [0003]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. Space frame aircraft are known. For example, U.S. Pat. No. 7,891,608, titled “Space Frame Fuselage Structure And Related Methods,” discloses embodiments of space frame aircraft and is hereby incorporated by reference in its entirety. In general, space frame aircraft may be used for, among other things, carrying cargo in one or more containers, such as an International Organization for Standardization (“ISO”) shipping container.  FIGS. 1-2  are schematic examples of portions of a space frame aircraft carrying a plurality of ISO containers. 
         [0004]    In various configurations throughout this disclosure, a fuselage structure may accommodate inter-modal containers conforming to ISO specification 1496. ISO specification 1496 describes a family of inter-modal containers. Containers conforming to the foregoing specification have been commonly accepted throughout the world for surface vehicle use, e.g., to transport cargo on large ships, trucks and trains. A related specification, ISO specification 8323, describes an air-compatible, lightweight container. Throughout this disclosure all of the family of containers meeting either specification are collectively referred to as “ISO containers.” 
         [0005]    Typically, a space frame fuselage structure of the aircraft may include a plurality of nodes and a plurality of elements connecting the nodes to form a space frame in which to carry cargo. As disclosed in U.S. Pat. No. 7,891,608, a space frame may generally include longitudinal elements (e.g., longerons), lateral elements, vertical elements, or other elements that are joined together at nodes. Diagonal elements (also referred to herein as trusses, braces, or bracing) may also be included and connected between nodes. 
         [0006]    One implementation of a fuselage space frame is indicated generally in  FIG. 1  by reference number  20 . The space frame  20  has a front, rear, and right and left sides indicated generally by reference numbers  22 ,  24 ,  26  and  28  respectively. The space frame  20  includes a plurality of longitudinal elements  30 , lateral elements  32  and vertical elements  34  joined at a plurality of nodes  36 . A plurality of diagonal elements  40  are connected between some of the nodes  36 . Also included, though not shown on  FIG. 1 , may be a number of pins or mechanisms that support ISO containers  68 . In some embodiments, the pins or support mechanisms for the ISO containers  68  may be slightly displaced relative to the space frame  20  structural nodes  36  to allow for a more simple integration of the support mechanisms in the space frame structure  20 . This offset feature may result in some amount of bending moment being sustained by the longitudinal members  30 . For the purposes of this disclosure, the locations of the support pins will be shown as being coincident with the structural nodes  36 . In some places on the figures, some nodes  36  are depicted with a larger dot which represents the nodes  36  that are connected or close to the pins that carry the ISO containers  68 , and are thus places where loads (mostly vertical) are introduced into the truss. Both the depiction of coincident location of pins and nodes, and differing size dots, are for simplicity and are inconsequential to the concepts of the current disclosure. 
         [0007]    The space frame fuselage structure  20  is included in a space frame aircraft  44  parts of which are shown schematically in  FIG. 2 . External struts  48  (shown in phantom) may optionally be used to link wings  52  of the aircraft  44  with a portion  54  of the fuselage in the vicinity of landing gear  55 . In this disclosure, the terms “wing” and “wings” may be used interchangeably. Other portions of the space frame  20  include a cargo hold  56  and an aft fuselage portion  60 . Of course, other features of aircraft  44  are also possible. 
         [0008]    The cargo hold  56  is configured to hold one or more ISO containers  68  in one or more generally rectangular bays  72  defined by one or more decks  76   a ,  76   b , a plurality of longitudinal columns  80 , and a plurality of transverse rows  84 . For example, as shown in  FIG. 1 , a two-high stack or block  88  of 20-foot long ISO containers are in the left-most row  84  in the third 20-foot long column  80  of a deck  76   a  of the space frame  20 . It should be noted that a space frame  20  may have rows  84  of different lengths. For example, as shown in  FIG. 1 , the space frame  20  has four rows  84 : two outer rows and two center rows which are longer than the outer rows by the length of two bays  72 . Other row  84  configurations are also possible. Likewise, columns  80  may be of differing widths and sizes. 
         [0009]    It also should be noted that the term “deck” as used herein does not necessarily denote the presence of a “floor” on which one may walk. In the  FIG. 1  embodiment, the decks  76   a ,  76   b  do not include floor surfaces (except, e.g., for such surface areas as may be provided by longitudinal and lateral elements  30  and  32 .) Rather, “deck” refers to a level of the aircraft  44  that supports the cargo containers  68  from below. Thus, e.g., in the aircraft  44  of  FIG. 1 , the deck  76   a  is an upper deck on which the containers  68  are supported above a lower deck  76   b . Likewise in  FIG. 1 , the space frame  20  is open at the front end  22  to permit full-width loading of the cargo hold  56  as further described below. Other configurations are possible. It should be noted that the open nature of the space frame allows it to typically be non-pressurized during flight. 
         [0010]    The word “bay” has two meanings in this document. The first meaning is the open volume within the fuselage for carrying cargo—the “cargo bay.” The second meaning refers to the approximately rectangular shape formed by coplanar, approximately orthogonal primary space frame elements. 
         [0011]    In general, a space frame  20  fuselage is designed to carry ISO containers  68  in an efficient manner. This type of fuselage is typically rectangular in cross section. If the fuselage is unpressurized, this rectangular profile is not a disadvantage. However, if the interior of the space frame  20  fuselage is required to be pressurized, the rectangular profile presents a disadvantage because pressure vessels with flat sides are inefficient from a weight standpoint compared to circular pressure vessels. 
         [0012]    In addition, some cargo must be pressurized. In order to accommodate a wide variety of cargo types it may be desirable to have at least a portion of the fuselage to be pressurized, or partially pressurized. Therefore, for some applications there may be a need to efficiently pressurize a space frame structure  20 . 
         [0013]    One solution to pressurizing a fuselage, is present in the commercial freighters in operation today, such as the 777F and the 747F made by The Boeing Company. These freighters have the advantage that, being derivatives of a passenger commercial aircraft, pressurization is included in the original design for the benefit of the passengers. However, these aircraft are typically heavier for a comparable payload compared to a freighter purpose-built to carry ISO containers  6 . The higher operating empty weight (OEW) results in higher cargo rates compared to the freighter with a space frame  20  fuselage. In addition, being originally designed for passengers, these aircraft are not the ideal shape and size for carrying one or more ISO containers in a pressurized envelope, and it would require excessive work to reconfigure the size of the pressurized portion of the aircraft. 
         [0014]    Another solution is to use ISO containers  68  that are designed to be individually pressurized. These ISO containers  68  are typically significantly heavier than the standard ISO containers  68 , which would increase the cargo rate for those containers. In addition, some operators may greatly prefer a pressurized fuselage because it may simplify their operations. For example, they may also be risk-adverse, and not want to consider problems that may occur if some cargo which requires pressurization is inadvertently shipped on an aircraft that is unpressurized. Other drawbacks also exist. 
       SUMMARY 
       [0015]    Accordingly, the disclosed systems and methods address the above noted drawbacks and issues with existing systems and methods. Disclosed embodiments include methods and apparatus for pressurizing some or all of the cargo hold bays. 
         [0016]    Disclosed embodiments include a space frame fuselage including a cargo hold comprising a plurality of substantially rectangular bays configured to house one or more ISO containers, and a pressure membrane substantially enclosing the cargo hold. 
         [0017]    In some embodiments, the plurality of substantially rectangular bays are arranged in one or more decks having external perimeter bays, and the pressure membrane further includes a curved surface covering the external perimeter bays. In some embodiments, the curved surface covering an external perimeter bay in one deck has a substantially eighteen inch offset spacing from the curved surface covering an external perimeter bay in an adjacent deck. 
         [0018]    In some embodiments, the plurality of substantially rectangular bays connect at nodes, and the pressure membrane also connects to the spaceframe fuselage at the nodes. In some embodiments, an endcap membrane substantially covers a terminal end of the cargo hold. 
         [0019]    In some embodiments, the pressure membrane includes a plurality of segments attached to the space frame. In some embodiments, tension wires span the cargo hold and counteracting a tension load caused by the pressure membrane. 
         [0020]    Disclosed embodiments also include a space frame fuselage including a cargo hold comprising a plurality of substantially rectangular bays configured to house one or more ISO containers, and a pressure membrane substantially enclosing at least one of the plurality of substantially rectangular bays. 
         [0021]    In some embodiments, the enclosed at least one of the plurality of substantially rectangular bays has a perimeter, and the pressure membrane further includes a curved surface extending around the perimeter of the enclosed at least one of the plurality of substantially rectangular bays. In some embodiments, the curved surface extending around the perimeter of one of the enclosed at least one of the plurality of substantially rectangular bays has an offset spacing of substantially eighteen inches from the curved surface extending around the perimeter of a vertically adjacent one of the enclosed at least one of the plurality of substantially rectangular bays. 
         [0022]    In some embodiments, the enclosed at least one of the plurality of substantially rectangular bays connects at nodes, and the pressure membrane also connects to the spaceframe fuselage at the nodes. In some embodiments, an endcap membrane substantially covers a terminal end of the enclosed at least one of the plurality of substantially rectangular bays. 
         [0023]    In some embodiments, the pressure membrane comprises a plurality of segments attached to the space frame. In some embodiments, tension wires span the cargo hold and counteracting a tension load caused by the pressure membrane. 
         [0024]    Also disclosed is a method for selectively pressurizing at least a portion of an aircraft cargo hold including attaching a pressure membrane to at least one of a plurality of substantially rectangular bays configured to support one or more ISO containers therein, wherein the pressure membrane forms a substantially enclosed chamber when attached to the at least one of a plurality of substantially rectangular bays, and selectively changing an environmental condition within the substantially enclosed chamber. In some embodiments, the environmental condition comprises a pressure within the enclosed chamber and the method further includes pressurizing the enclosed chamber to a desired pressure. 
         [0025]    In some embodiments, the plurality of substantially rectangular bays are arranged in one or more decks having internal bays and external perimeter bays; and the method further includes attaching the pressure membrane to the external perimeter bays to form an enclosed chamber that substantially encloses both the internal bays and the external perimeter bays. In some embodiments, the method further includes spacing the pressure membrane with substantially eighteen inches of offset between vertically adjacent external perimeter bays. In some embodiments, the method includes providing tension wires spanning the cargo hold to counteract a tension load caused by the pressure membrane. Other embodiments and modifications are also disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a perspective view of a space frame aircraft fuselage in accordance with embodiments of the disclosure. 
           [0027]      FIG. 2  is a schematic side view of a space frame aircraft in accordance with embodiments of the disclosure. 
           [0028]      FIG. 3  is a schematic illustration of a space frame fuselage that has pressure membranes that enclose the entire cargo hold in accordance with disclosed embodiments. 
           [0029]      FIG. 4  is a schematic cross-section of a cargo hold and pressure membranes in accordance with disclosed embodiments. 
           [0030]      FIG. 5  is a schematic illustration showing enclosure of the top, bottom, and side bays in accordance with disclosed embodiments. 
           [0031]      FIG. 6  is schematic, exploded-view, showing an embodiment for attaching the membrane edges to the space frame. 
           [0032]      FIG. 7  is a schematic illustration of forces applied by the membranes in accordance with disclosed embodiments. 
           [0033]      FIG. 8  is a schematic illustration showing cross sections of two space frame fuselage structures that have pressure membranes that enclose a single deck in accordance with disclosed embodiments. 
           [0034]      FIG. 9  is a schematic illustration of tension wires in accordance with disclosed embodiments. 
       
    
    
       [0035]    While the disclosure is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the disclosure is not intended to be limited to the particular forms disclosed. Rather, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0036]    It should be noted that, although implementations are described with reference to ISO containers  68  and/or reference to containers having specific dimensions, the disclosure is not so limited. The disclosure may be implemented in relation to many different types and/or sizes of containers. 
         [0037]      FIG. 3  is a schematic illustration of a space frame  20  fuselage that has pressure membranes  136  that enclose the entire cargo hold  56 . The membranes  136  around the perimeter of the frontal cross section are straight in the longitudinal direction, and have circular curvature in the other direction (in the YZ plane). Pressure membranes  136  may be made of any suitable material. For example, pressure membranes  136  may be constructed as monocoque, semi-monocoque, sandwich panels, corrugated panels, or the like. The membranes  136  may be made from aluminum, carbon fiber reinforced polymers (CFRP), steel, polymers, Mylar, plastic impregnated cloth, rubber, vinyl, or the like. The membranes  136  may be formed as a continuous membrane over part, or all, of the fuselage cross section, or they may be segments fastened to the space frame  20 . Because they are shielded from the sun&#39;s ultraviolet rays by the skin of the aircraft, the membranes  136  need not be resistant to ultraviolet radiation. Exemplary thickness for pressure membranes  136  are 0.044 inches for top and bottom portions, 0.063 inches for side portions, and 0.028 for end portions (e.g., end cap membrane  136   a  as shown in  FIGS. 5-6 ). Other configurations and thicknesses are also possible. 
         [0038]    Sample cross sections of the cargo hold  56  are depicted in  FIG. 4  in which the pressure membranes  136  are also shown. The cross section on the left in  FIG. 4  shows a 3×2 arrangement of ISO containers  68 , and the cross section on the right of  FIG. 4  shows a 2×2 arrangement of ISO containers  68 . As shown on the 3×2 arrangement, an 18 inch offset between the space frame and the outer mold line (“OML”) of the pressure membrane  136  may be used. In addition, the membrane  136  may be curved with a radius of curvature of 99.25 inches from the center of bay  72 , and a radius of curvature of 141.25 inches from a center of inner vertical member  34  as indicated in  FIG. 4 . 
         [0039]      FIG. 5  is a schematic illustration that shows how the top, bottom, and side bays  72  may be closed out. The ends of a sixteen twenty-foot-equivalent units (“TEU”) freighter may utilize a single end-cap membrane  136   a  as shown in  FIG. 5 . This allows for simpler more cost effective repairs. Alternatively, each of the four bays  72  could have a separate pressure membrane  136 . In some embodiments, these pressure membranes  136  may be covered by a light non-pressurized aero surface. 
         [0040]      FIG. 6  is a schematic, exploded-view, showing an embodiment for attaching the membrane  136  edges to the space frame  20 . Appropriate seals are used with a bracket  138 , the space frame  20 , and the membrane  136  to maintain pressure. 
         [0041]    In some embodiments, the pressure membranes  136  are connected to the node locations  36  on the space frame  20 . Since the required thickness of a circular pressure membrane  136  for a given pressure is proportional to the inverse of the radius of the circle, it is advantageous to approximately minimize the radius of curvature of the membrane  136 . For a given distance between the space frame  20  and the most distant part of the pressure membrane  136  (the eighteen inches offset shown at  140  in  FIG. 4 ), it is advantageous to minimize the span of the pressure membrane  136 , and thus it is advantageous to have the membrane  136  connect at the interior nodes  36   a ,  36   b , of the side of the fuselage compared to connecting the membranes  136  only at the corner nodes  36   c.    
         [0042]    The outward pressure applied to the membranes  136  results in compression forces being applied to the space frame  20  members. Thus, the membranes  136  on the side of the fuselage will apply a compression load to the vertical elements  34  of the space frame  20  on the side of the fuselage, and the top and bottom membranes  136  will apply compression to the lateral elements  32  on the top and bottom of the fuselage. 
         [0043]    The pressure membranes  136  also apply loading to the space frame  20  substantially normal to the face on which they are situated. Thus, the side membranes  136  apply a tension force to the all the lateral elements  32 , both the lateral elements  32  on the top and bottom of the fuselage, and also the center interior lateral elements  32 . The top and bottom membranes  136  apply a tension force to all the vertical elements  34  of the space frame  20 , including both the right and left side vertical elements  34 , and also the interior vertical elements  34 . These forces tend to counteract the compression forces applied to the space frame  20  described above. 
         [0044]    The forces applied by the membranes  136  are also partially counteracted not just by the outer space frame  20  members (indicated at A in  FIG. 7 ), but also by the membranes in the substantially normal direction (indicated at B in  FIG. 7 ). Thus, the normal load in the horizontal direction applied by the side membrane  136  upon the surrounding structure is counteracted partially by the horizontal component of the tension force in the top or bottom membrane  136 . Similarly, the normal force upward on the top membrane  136  or downward on the lower membrane  136  is at least partially counteracted by the vertical component of the side membrane  136  attached to the same corner. 
         [0045]    The optimum geometry for the pressure membranes  136  in some embodiments may involve varying the radius of curvature of the various membranes  136 . Of course, the geometry is limited by the constraints of the location of the nodes  36  and the maximum distance the membrane  136  is allowed to be from the space frame  20 . 
         [0046]    The weight impact upon an operator of the pressurized space frame  20  fuselage for the pressurized membrane  136  is an increase on the order of six percent of the cargo rate. 
         [0047]      FIG. 8  schematically shows cross sections of two space frame  20  fuselage structures that have pressure membranes  136  that enclose just a single “stick” of ISO containers  68  in lower bay  72   b . They are shown by the thicker lines  136  in  FIG. 8 . The structural behavior of this  FIG. 8  embodiment is similar to that described for the membranes  136  that enclose the entire cargo hold  56 , with some differences. 
         [0048]    One difference is that there are no internal members. Another difference is that there is an additional constraint upon the distance between the membrane  136  and the space frame  20  (i.e., the eighteen inches indicated at  140  in  FIG. 4 ) because of the adjoining ISO containers  68 . As shown on the right side of  FIG. 8 , the interior columns  142  have been designed so that there is ample space for the membrane  136 . 
         [0049]    The weight impact of this  FIG. 8  configuration upon an operator of the pressurized space frame  20  fuselage for this concept is an increase approximately one percent of the cargo rate. This is less impact compared to the six percent increase incurred with the membranes  136  enclosing the whole cargo hold  56 . For that reason, this configuration may be attractive to an operator that greatly desires to have a pressurized fuselage. 
         [0050]    As the fuselage is pressurized, the membranes  136  will transfer tension load to the space frame  20 . For the membranes  136  on the side of the fuselage, for example, the horizontal component of that load will tend to bend the longitudinal elements  30  so that they bow outward. This bending moment that the longitudinal element  30  experiences will result in additional weight to the space frame  20 . One way to substantially eliminate these bending moments is to provide additional horizontal tension wires  144  (shown in  FIG. 9 ) running cross-ship to the opposite side. The tension load then is transferred directly to the opposite side of the space frame  20  instead of being carried by bending moments along the longitudinal element  30  until it can be transferred by one of the lateral elements  32  of space frame  20 . The tension wires  144  thus serve the same function as the lateral space frame elements  32  discussed above. 
         [0051]    Horizontal tension wires  144  can also be added on the upper and lower horizontal faces of the space frame  20 , but they are not as effective because the membranes  136  on the top and bottom sides of the space frame  20  also carry tension loads, and thus help reduce the bending moments on the corner longitudinal elements  30 . The same arrangement can be provided in the vertical direction, especially for the interior longitudinal elements  30  on the top and bottom faces of the space frame  20 . These interior wires  144  thus substantially eliminate the bending moment in the longitudinal elements  30  that would make them bow outward. 
         [0052]    Although various embodiments have been shown and described, the present disclosure is not so limited and will be understood to include all such modifications and variations are would be apparent to one skilled in the art.