Abstract:
A method for forming a blended wing body aircraft with a modular body. In one embodiment, the configuration or cargo capacity of the aircraft can be varied by adding or subtracting intermediate body structures rather than by adding or subtracting segments from the lateral sides of the aircraft body. Configuration in this manner preserves key aerodynamic parameters and permits several major components to be used in several aircraft configurations, each of which having a different cargo capacity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 10/437,763 filed May 14, 2003, which is a continuation of U.S. application Ser. No. 09/826,031 filed Apr. 4, 2001 entitled “Variable Size Blended Wing Body Aircraft”, which issued as U.S. Pat. No. 6,568,632 on May 27, 2003. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention generally relates to aircraft and more particularly to an aircraft having a body that is at least partially constructed from a plurality of longitudinally or laterally extending body structures to provide a family of aircraft with each family member having a different cargo capacity.  
         BACKGROUND OF THE INVENTION  
         [0003]    Purchasers of modern airline and cargo aircraft typically require a wide range of capacities to suit the diverse needs of their markets. One option for satisfying these needs is to provide several uniquely designed aircraft, with each design being optimized for an equally unique market. This approach, however, is extremely costly, not only in terms of development and manufacture, but also in terms of their operation, maintenance and servicing. Instead, capacity variations in conventional airline and cargo aircraft are achieved by adding or removing pieces of a constant diameter tubular fuselage section.  
           [0004]    In contrast, blended wing body aircraft designs, also known as flying wing designs, lack a “constant” section whose length can be varied so as to provide the desired variation in capacity. Furthermore, the careful shaping of these designs to permit efficient flight at transonic Mach speeds and adequate internal volume negates the incorporation of simplistic changes to the design that add capacity yet unacceptably effect the balance of the aircraft due to the wing sweep that is required at high transonic cruise speeds. Such changes also tend to negatively effect the lofting and aerodynamics of alternate capacity versions.  
           [0005]    U.S. Pat. No. 5,909,858 to Hawley discloses on solution for varying the capacity of a blended wing body aircraft. The Hawley design employs a variable size body, a pair of wing assemblies and a pair of transition sections. The body is broken up into a plurality of longitudinally extending segments that are based on a single design but which are mirrored across the centerline of the aircraft. Variable capacity is achieved by simply adding or subtracting pairs the segments from the body. The transition section has a negative sweep angle and couples each side of the body of the aircraft to one of the wing assemblies. While the identically configured segments permit the preservation of a common wing assembly and greatly reduce costs associated with the design, tooling, manufacture and maintenance of the aircraft, several drawbacks of this configuration have been noted.  
           [0006]    One such drawback concerns the sharp discontinuity in the wing chord between the wing and the body of the aircraft. This results in a lift distribution that can be unfavorable to drag and stall characteristics. Furthermore, the balance of the aircraft is unlikely to be retained over several differently sized configurations. Accordingly, there remains a need in the art for a blended wing body aircraft having a modular construction that permits easy and cost-effective variations in the size of the aircraft yet which retains balance, aerodynamic loft and aerodynamic performance.  
         SUMMARY OF THE INVENTION  
         [0007]    In one preferred form, the present invention provides a blended wing body aircraft having a pair of aircraft wings and an aircraft body. The aircraft body has a centerbody cabin formed from a plurality of body structures that include a pair of end body structures and at least one intermediate body structure. Each of the plurality of body structures defines a cargo compartment and are arranged about a centerline of the centerbody cabin such that each of the end body structures are juxtaposed between the at least one intermediate body structure and one of the aircraft wings.  
           [0008]    In another preferred form, the present invention provides a blended wing body aircraft having a pair of aircraft wings and an aircraft body that is at least partially formed from a plurality of longitudinally-extending body structures. Each of the body structures on a first side of a centerline of the aircraft body is uniquely configured and arranged generally parallel to the centerline of the aircraft body. Each of the body structures on a second side of the centerline of the aircraft body is a mirror-image of an associated one of the body structures on the first side of the centerline of the aircraft body.  
           [0009]    In another preferred form, the present invention provides a blended wing body aircraft having a pair of aircraft wings, an aircraft body and a propulsion unit. The aircraft body has a centerbody cabin, which is constructed from a plurality of body structures, and an aft centerbody cabin, which is coupled to a rearward end of the centerbody cabin. The propulsion unit is coupled to the aft centerbody cabin.  
           [0010]    In another preferred form, the present invention provides a blended wing body aircraft having a pair of aircraft wings and an aircraft body that is at least partially formed from a plurality of longitudinally-extending body structures. Each of the body structures that are positioned such that their body structure centerline is not coincident with a centerline of the aircraft body includes a tip portion that forms a portion of the leading edge of the blended wing body aircraft. The tip portion has an inner edge with a first portion that extends outwardly away from a centerline of an associated one of the body structures and a second portion that is coupled to the first portion and extends forwardly and outwardly toward the leading edge of the blended wing body aircraft.  
           [0011]    In yet another preferred form, the present invention provides a blended wing body aircraft having a pair of aircraft wings and an aircraft body that is at least partially formed from a plurality of longitudinally-extending body structures and a pair of leading edge panels. Each of the body structures that are positioned such that their body structure centerline is not coincident with a centerline of the aircraft body includes a tip portion that terminates rearwardly of a leading edge of the blended wing body aircraft. The tip portion has an inner edge with a first portion that extends outwardly away from a centerline of the aircraft body. Each leading edge panel is coupled to a portion of the body structures and one of the aircraft wings and forms at least a portion of a leading edge of the blended wing body aircraft.  
           [0012]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein:  
         [0014]    [0014]FIG. 1 is a perspective view of a blended wing body aircraft constructed in accordance with the teachings of the present invention;  
         [0015]    [0015]FIG. 2 is an exploded perspective view of the blended wing body aircraft of FIG. 1;  
         [0016]    [0016]FIG. 3 is a top view of a portion of the blended wing body aircraft of FIG. 1;  
         [0017]    [0017]FIG. 4 is a side view of a portion of the blended wing body aircraft of FIG. 1 illustrating the landing gear in an extended position and a retracted position;  
         [0018]    [0018]FIGS. 5A through 5D illustrate a family of differently sized blended wing body aircraft that are based upon the design of the aircraft of FIG. 1;  
         [0019]    [0019]FIG. 6 is a top view of a portion of a blended wing body aircraft constructed in accordance with the teachings of another aspect of the present invention;  
         [0020]    [0020]FIGS. 7A through 7B are top views of a portion of a family of differently sized blended wing body aircraft that are based upon the design of the aircraft of FIG. 6;  
         [0021]    [0021]FIG. 8 is a perspective view of a blended wing body aircraft constructed in accordance with the teachings of another aspect of the present invention;  
         [0022]    [0022]FIGS. 9A and 9B are perspective views of a portion of a pair of blended wing body aircraft constructed in accordance with the teachings of yet another aspect of the present invention;  
         [0023]    [0023]FIG. 10 is an exploded perspective view of a blended wing body aircraft constructed in accordance with the teachings of another aspect of the present invention;  
         [0024]    [0024]FIG. 11 is a top view of a blended wing body aircraft constructed in accordance with the teachings of another aspect of the present invention;  
         [0025]    [0025]FIG. 12 is a top view of an aircraft that is similar in construction to that of FIG. 11 but smaller in capacity;  
         [0026]    [0026]FIG. 13 is an exploded perspective view of an aircraft that is similar in construction to that of FIG. 11 but larger in capacity; and  
         [0027]    [0027]FIG. 14 is an exploded perspective view of an aircraft constructed in accordance with the teachings of yet another aspect of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    With reference to FIGS. 1 through 3 of the drawings, a blended wing body aircraft constructed in accordance with the teachings of the present invention is generally indicated by reference numeral  10 . Aircraft  10  is illustrated to include an aircraft body  12 , a pair of aircraft wings  14 , a propulsion system  16  which is shown to include a plurality of jet engines, a leading edge  18 , which is cooperatively defined by the aircraft-wings  14  and the aircraft body  12  and a centerline  20 . The aircraft body  12  and aircraft wings  14  are each illustrated to have positive sweep angles.  
         [0029]    In the particular embodiment illustrated, the aircraft body  12  is shown to include a cockpit  30 , a transition section  32 , a centerbody cabin  34  and an aft centerbody cabin  36 . The transition section  32  is unique to the configuration of the aircraft  10 , thereby ensuring that the aircraft  10  will be provided with a smooth aerodynamic and structural transition between the cockpit  30  and the centerbody cabin  34 . The transition section  32  is also employed for housing the front aircraft landing gear  38  as illustrated in FIG. 4 and as such, its construction may also be tailored as necessary in response to the space and structural demands imposed by the front aircraft landing gear  38 .  
         [0030]    The centerbody cabin  34  is formed from a plurality of longitudinally-extending body structures  50 , with each of the body structures  50  on a first side of the centerline  20  of the aircraft  10  being uniquely configured and oriented generally parallel to the centerline  20  of the aircraft  10  and each of the body structures  50  on the other side of the centerline  20  of the aircraft  10  being a mirror-image its associated body structure  50  on the first side of the centerline  20  of the aircraft  10 . Each of the body structures  50  defines a cargo compartment  52  that may be used to transport passengers or cargo.  
         [0031]    More specifically, the plurality of body structures  50  includes a pair of end body structures  60  and  60 ′ and a plurality of intermediate body structures  62   a ,  62   a ′ and  62   d . Those skilled in the art will understand that the body structures  50  having a primed reference numeral (e.g.,  60 ′) are mirror images of an associated body structure that is located on an opposite side of the centerline  20  of the aircraft  10  and that is designated by a similar but unprimed reference numeral (e.g.,  60 ). As illustrated, the centerbody cabin  34  is configured such that each of the end body structures (e.g., end body structure  60 ′) is juxtaposed between one of the intermediate body structures (e.g., intermediate body structure  60   a ′) and one of the aircraft wings  14 . Accordingly, the aircraft  10  does not include a transition section having a negative sweep angle for interconnecting the aircraft body  12  and aircraft wing  14  and as such, there is no sharp discontinuity in the wing chord between the aircraft wing  14  and the centerbody cabin  34 . Preferably, each of the body structures  50  terminates at its rearward point at the rear spar  70  of the aircraft  10 , thereby forming the aft pressure bulkhead for the aircraft  10  at the intersection between the centerbody cabin  34  and the aft centerbody cabin  36 .  
         [0032]    In the particular embodiment illustrated, the outer edge  72  of the end body structures  60  and  60 ′ includes a first portion  74  that is generally parallel to the centerline  20  of the aircraft  10  and a second portion  76  that is angled outwardly toward the aircraft wing  14  and forwardly toward the leading edge  18  of the aircraft  10 . The outwardly and forwardly depending second portion  76  of the outer edge  72  reduces the length of the joint  78  between the end body sections  60  and  60 ′ and its mating aircraft wing  14  and improves the strength of the leading edge  18  of the aircraft  10 . As shown, the second portion  76  of the outer edge  72  and the leading edge  18  of the aircraft  10  are substantially perpendicular to one another.  
         [0033]    In the particular embodiment illustrated, each of the body structures  50  that is positioned such that its centerline  80  is not coincident with the centerline  20  of the aircraft  10  includes a tip portion  82 . The tip portion  82  includes a forward portion  84  that forms a portion of the leading edge  18  of the aircraft  10 . The tip portion  82  includes an inner edge  86  that is configured to mate with a forwardly positioned portion of the aircraft body  12 . For example, the inner edge  86  of the tip portion  82  of end body structure  60 ′ is configured to mate with the forwardly positioned portion of intermediate body structure  62   a ′. Similarly, the inner edge  86  of the tip portion  82  of the intermediate body structure  62   a  is configured to mate with the forwardly positioned transition section  32  to create a smooth loft at the intersection therebetween.  
         [0034]    The aft centerbody cabin  36  fairs out the body of the aircraft  10  and into the aircraft wings  14  and forms a portion of the trailing edge  88  of the aircraft  10 . In the particular embodiment illustrated, the aft centerbody cabin  36  serves at the mounting point for the propulsion system  16 . Preferably, the aft centerbody cabin  36  is unique to the configuration of the aircraft  10 , thereby permitting the structural and aerodynamic characteristics of the aft centerbody cabin  36  to be tailored in a desired manner so as to better integrate the propulsion system  16  into the aircraft  10 .  
         [0035]    In the embodiment illustrated, a portion  90  of each of the outboard sides  92  of the aft centerbody cabin  36  is shown to be skewed to the centerline  20  of the aircraft body  12 , angling forwardly and outwardly from the trailing edge  88  of the aircraft  10 . Another portion  94  of each of the outboard sides  92  is shown to be skewed to the forwardly and outwardly angled portion  90  by an angle that is less than 180 degrees. The forwardly and outwardly angled portion  90  of the aft centerbody cabin  36  is complementary to the inwardly and rearwardly angled portion  96  of the aircraft wings  14  and aids in locking the aft centerbody cabin  36  and aircraft wings  14  together.  
         [0036]    With additional reference to FIGS. 5A through 5D, the modular design of the aircraft  10  is employed to provide a family of differently configured aircraft which includes the aircraft  10  and the aircraft designated by reference numerals  100 ,  110 ,  120  and  130 . The aircraft  100 , for example, includes the aircraft wings  14 , the end body structures  60  and  60 ′ and the cockpit  30  that are common to aircraft  10 . The aircraft  100 , however, includes an intermediate body structure  62   c , a transition section  32   a , an aft centerbody cabin  36   a  and a propulsion system  16   a  that are unique to this configuration.  
         [0037]    A relatively higher degree of similarity is found between the aircraft  110 ,  120  and  130  and the aircraft  10  as the aircraft wings  14 , the end body structures  60  and  60 ′, the intermediate body structures  62   a  and  62   a ′ and the cockpit  30  that are common to all of these aircraft. The aircraft  120  includes a pair of intermediate body structures  62   b  and  62   b ′ that are not employed by any of the smaller aircraft but which are employed by the larger aircraft  130 . The aircraft  110  and  120  each include unique transition sections  32   b  and  32   c , respectively, unique aft centerbody cabins  36   b  and  36   c , respectively, unique propulsion systems  16   b  and  16   c , respectively. The aircraft  130  includes a unique intermediate body structure  62   e , a unique transition section  32   d , a unique aft centerbody cabin  36   d  and a unique propulsion system  16   d . Those skilled in the art will understand that although the transition sections are described herein as being unique to each aircraft configuration, it is within the scope of the present invention to employ a transition section that is common to each of the aircraft configurations. Unique transition sections are presently preferred to provide the aircraft configuration with a smooth aerodynamic and structural transition between the cockpit  30  and the centerbody cabin  34  since in some instances it may be necessary to shift the cockpit  30  up or down relative to the centerbody cabin  34 .  
         [0038]    From the foregoing, it should be apparent to those skilled in the art that the capacity of the aircraft  10  is varied by adding or subtracting body structures  50  from the middle of the centerbody cabin  34 , rather than by adding or subtracting body structures  50  from the lateral sides of the centerbody cabin  34 . Construction in this manner permits the intermediate body structures  62   a ,  62   a ′,  62   b ,  62   b ′,  62   c ,  62   d  and  62   e  to be tailored to the handle the progressively larger wing bending loads that are associated with the larger aircraft configurations. In this regard, the construction of each of the body structures  50  on a first side of the centerline  20  of the aircraft  10  varies-such that the maximum wing bending load of a body structure  50  located closer to the centerline  20  (e.g., intermediate body structure  62   a ) is greater than the maximum wing bending load of a body structure  50  located further from the centerline  20  (e.g., end body structure  60 ). The aircraft wings  14 , being common across the entire family of aircraft  10 ,  100 ,  110 ,  120  and  130 , are designed to handle the highest wing bending loads. The smallest aircraft in the family (i.e., aircraft  100 ) would then suffer a modest weight penalty, but this could be reduced or eliminated by re-gauging the aircraft wings  14  in the manner in which conventional wings are sometimes re-gauged today.  
         [0039]    While the aircraft  10  has been described thus far as having a plurality of body structures  50  that cooperate to define both a centerbody cabin and a portion of the leading edge  18  of the aircraft  10 , those skilled in the art will appreciate that the invention, in its broader aspects, may be constructed somewhat differently. For example, the body structures  50  may be configured to terminate rearwardly of the leading edge of the aircraft  10   a  as illustrated in FIG. 6. In this arrangement, the portion of the leading edge  18  that had formerly been associated with the centerbody cabin  34  is defined by a pair of leading edge panels  150  and  150 ′ that are coupled to the aircraft body  12   a  and the aircraft wings  14 . With additional reference to FIGS. 7A through 7B, the length of the leading edge panels  150  and  150 ′ is illustrated to change in accordance with variations in the capacity of the aircraft.  
         [0040]    Another example is illustrated in FIG. 8 wherein the aircraft  10   b  is shown to be substantially similar to aircraft  10  except for the construction of the aft end of the aircraft body  12   b . In this regard, the body structures  50   b  that form a portion of the aircraft body  12   b  are lengthened to form the trailing edge  88  of the aircraft, eliminating the need for the aft centerbody cabin  36 .  
         [0041]    Yet another example is illustrated in FIGS. 8A and 8B wherein the aircraft  10   c  and  10   d  is shown to be substantially similar to aircraft  10  except with regard to the connection of the centerbody cabin  34  to the cockpit  30 . As discussed above, the aircraft  10  includes a transition section  32  that is disposed between the centerbody cabin  34  and the cockpit  30 . The aircraft  10   c , however, includes an intermediate body structure  160  having a cockpit transition portion  162 , which permits the cockpit  30  to be coupled directly to the centerbody cabin  34 . Similarly, the aircraft  10   d  includes intermediate body structures  170  and  170 ′, each of which having a cockpit transition portion  172 , which, after the intermediate body structures  170  and  170 ′ are coupled to one another, permits the cockpit  30  to be coupled directly to the centerbody cabin  34 .  
         [0042]    A further example is illustrated in FIG. 10 wherein an aircraft constructed in accordance with the teachings of another alternate embodiment is generally indicated by reference numeral  210 . Aircraft  210  is generally similar to aircraft  10  in that it includes an aircraft body  212 , a pair of aircraft wings  14 , a propulsion system  16 , a leading edge  218  and a centerline  20 . The aircraft body  212  includes a cockpit  30 , a transition section  232 , a centerbody cabin  234  and an aft centerbody cabin  36 . The centerbody cabin  234  differs from the centerbody cabin  34  in that it is formed from a plurality of longitudinally extending body structures  250   a , an upper body skin assembly  250   b  and a lower body skin assembly  250   c , as opposed to the set of body structures  50  wherein each of the body structures  50  includes both its structural framing and its upper and lower skins. The body structures  250   a  essentially form a frame or skeleton onto which the upper and lower body skins  250   b  and  250   c  are attached.  
         [0043]    The transition section  232  likewise differs from the transition section  32  in that it is formed from a transition frame assembly  232   a , an upper skin assembly  232   b  and a lower skin assembly  232   c . A pair of leading edge skins  218   a  cover the leading edge ribs  218   b  that are formed into the body structures  250   a  and the transition frame assembly  232   a . In the particular embodiment illustrated, upper skin assembly  232   b  is shown to be coupled to upper body skin assembly  250   b  to form a portion of the upper outer body surface  212   b . Similarly, the lower skin assembly  232   c  is shown to be coupled to lower body skin assembly  250   c  to form a portion of the lower outer body surface  212   c . Those skilled in the art will understand, however, that while the portions of the upper and lower outer body surfaces  212   b  and  212   c  are shown as a complete assembly that is exploded from the body structures  250   a  and the transition frame assembly  232   a , the upper and lower outer body surfaces  212   b  and  212   c  are preferably coupled to the body structures  250   a  and the transition frame assembly  232   a  in segments  212   d , rather than as a complete assembly.  
         [0044]    Construction in this manner permits the segments  212   d  to run span-wise (i.e., perpendicular to the centerline  20 ) to thereby reinforce the structure of the aircraft body  212  while permitting a relatively high degree of flexure. Accordingly, the aircraft body  212  is relatively stronger and lighter in weight than aircraft body  12 .  
         [0045]    Another example is illustrated in FIG. 11, wherein an aircraft constructed in accordance with the teachings of another alternate embodiment is generally indicated by reference numeral  310 . Aircraft  310  is generally similar to aircraft  10  in that it includes an aircraft body  312 , a pair of aircraft wings  14 , a propulsion system  16 , a leading edge  18  and a centerline  20 . The aircraft body  312  includes a cockpit  30 , a transition section  32 , a centerbody cabin  334  and an aft centerbody cabin  36 . The centerbody cabin  334  differs from the centerbody cabin  34  in that it is formed from a plurality of laterally extending (i.e., spanwise) body structures  350 , as opposed to a set of longitudinally extending body structures  50  (FIG. 1). The body structures  350   a  and  350   b  are shown to abut the lateral sides  14   a  of the aircraft wings  14  and do not form any portion of the leading edge  18  of the aircraft  310 . The body structure  350   c  abuts the forward surface  14   b  of the aircraft wings  14 , as well as the forward surface  350   bf  of the body structure  350   b  and defines a portion of the leading edge  18 .  
         [0046]    With additional reference to FIGS. 12 and 13, the capacity of the aircraft  310  is varied by altering the configuration of the body structures  350 . FIG. 12, for example, illustrates an aircraft  310 ′ having a relatively smaller capacity than that of aircraft  310 . As shown, the width of the body structures  350   a ′ and  350   b ′ is relatively smaller than that of the body structures  350   a  and  350   b , and both the length and width of the body structure  350   c ′ are relatively smaller than the length and width of the body structure  350   c . FIG. 13 similarly illustrates an aircraft  310 ″ having a relatively larger capacity than that of aircraft  310 . As shown, the width of the body structures  350   a ″ and  350   b ″ is relatively larger than that of the body structures  350   a  and  350   b , and both the length and width of the body structure  350   c ″ are relatively larger than the length and width of the body structure  350   c . While the configuration of the aircraft  310  in this manner prevents multiple the body structures  350  from being used across a family of aircraft, the use of spanwise extending body structures  350  is structurally efficient, permitting the overall weight of the aircraft  310  to be reduced as compared to that of aircraft  10 .  
         [0047]    In FIG. 14, an aircraft constructed in accordance with the teachings of yet another alternate embodiment is generally indicated by reference numeral  410 . Aircraft  410  is generally similar to aircraft  10  in that it includes an aircraft body  412 , a pair of aircraft wings  14 , a propulsion system  16 , a leading edge  18 . The aircraft body  412  includes a cockpit  30 , a transition section  432 , a centerbody cabin  434  and an aft centerbody cabin  36 . Unlike the previously described embodiments, the transition section  432  and the centerbody cabin  434  are not modular in their form, but rather are formed from a plurality of components that are collectively assembled when the aircraft  410  is being assembled. The centerbody cabin  434  is shown to include a lower cover panel  500 , a cargo floor  502 , a plurality of lower ribs  504 , an intermediate spar  506 , a rear spar  508 , a floor  510 , a plurality of upper ribs  512 , an upper cover panel  514 , a pair of leading edge frames  516  and a pair of leading edge skins  518 . The transition section  432  includes a transition frame  520  and lower and upper cover panels  522  and  524 , respectively.  
         [0048]    The construction of the lower ribs  504  is such that their cross-sections are identical, and that the overall length of each of the lower ribs  504  is adjusted according to its placement within the centerbody cabin  434  as well as the overall capacity of the aircraft  410 . Similarly, the construction of the upper ribs  512  is such that their cross-sections are identical, and that the overall length of each of the upper ribs  512  is adjusted according to its placement within the centerbody cabin  434  as well as the overall capacity of the aircraft  410 .  
         [0049]    In varying the capacity of the aircraft  410 , the length and width of the centerbody cabin  434  will grow or shrink as necessary, in much the same manner as the centerbody cabin  34  grows or shrinks when the capacity of aircraft  10  is changed. To accommodate these changes, the spars  506  and  508 , the ribs  504  and  512 , the leading edge frames  516  and the leading edge skins  518  will grow or shrink in length as necessary, and the length and width of the panels  500  and  514  and the floors  502  and  510  will grow or shrink in length and width as necessary. Additionally, it may also be necessary to add or delete one or more spars  506  and ribs  504  and  512 .  
         [0050]    While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.