Abstract:
A system and method for internally supporting a flexible envelope such as an airship is described, comprising a truss structure and supporting struts attached to a central support member. The internal structure may be foldable so as to allow the envelope to be collapsed when not in use.

Description:
[0001]    This application claims priority from co-pending U.S. patent application Ser. No. 11/135,555. 
     
     TECHNICAL FIELD 
       [0002]    This invention relates to stresses induced on a covered vessel moving through a gas or fluid, and more particularly to an improved internal support structure for such a vessel. 
       BACKGROUND 
       [0003]    The stresses on an airship envelope, both on the supporting members and on the envelope, come in two varieties: hoop stress and suspension stress. 
         [0004]    Hoop stress is the stress on the walls of a container, typically having a generally cylindrical cross section, when the pressure inside the container exceeds the pressure outside. This stress tends to cause the container to burst. The name comes from the hoops found on wooden barrels that are used to hold in the staves and withstand this type of stress. 
         [0005]    It is a well established fact of mechanics that the magnitude of hoop stress increases linearly with the diameter of the cylinder as well as linearly with the pressure difference between the inside and outside. 
         [0006]    Suspension stresses are essentially the inverse of hoop stresses. These stresses arise when the pressure outside the container exceed the pressure inside, with the pressure tending to make the container collapse or implode. As with hoop stress, the magnitude of suspension stresses increases linearly with the pressure difference between the inside and outside. 
         [0007]    For a conventional pressurized airship at rest, all of the stresses on the envelope are the hoop stress. Further, these hoop stresses are greatest at the point of maximum diameter—typically at a point generally midway between the nose and tail of the ship. 
         [0008]    At rest, the pressure distribution along the outside of an airship is at a uniform, ambient level. This pressure distribution changes as the airship starts to move through the air. However, the changes in external pressure are not straightforward. In fact, the pressure along the surface varies continuously along the direction of motion, typically parallel with the longitudinal axis of the envelope. At some points, pressure increases to a level greater than the ambient pressure. These are called “positive” aerodynamic forces. At other points, the pressure decreases to levels lower than the ambient pressure. These are called “negative” aerodynamic pressures. They are also sometimes referred to as “suction.” 
         [0009]    As with all aerodynamic forces, the pressures created (both positive and negative) increase in magnitude as the square of the velocity of the airship. So, when the speed of the craft is doubled, the resulting stresses (both hoop and suspension) increase by a factor of 4. 
         [0010]    At the forward-most location, typically the very nose of the ship (called the forward stagnation point,) the outside pressure increases well above the ambient pressure and thus produces more of an inward (positive) force. In a conventional pressurized envelope, this positive pressure actually reduces hoop stress on the nose portion of the envelope. In fact, at a high enough airspeed, the positive pressure will exceed the internal gas pressure making the nose of the envelope want to buckle inward. At such a high speed, the nose ceases to sustain hoop stress and starts to sustain suspension stress. 
         [0011]    The positive (inward) pressure created by the airflow is at its greatest at the nose of the ship. As the air flows back along the outside of the envelope, the magnitude of the inward force rapidly decreases. In fact, by the time the airflow is typically about one tenth of the way back towards the tail (i.e. 10% of the way along the direction of motion, typically the longitudinal axis,) the relatively positive pressure completely disappears. This creates a zero-crossing point where the external pressure remains essentially unchanged at the initial, ambient atmospheric level. 
         [0012]    As the airflow continues along the outside of the envelope, the pressure continues to decrease, and can reach a level below the ambient level and thus create a negative (outward) aerodynamic stress which increases the hoop stress on the envelope material. 
         [0013]    At the widest part of the ship (typically about halfway between nose and tail) the magnitude of the change pressure is between one half and one third of that found the nose of the ship—but obviously in the opposite direction. 
         [0014]    After the midpoint of the ship, the airflow start to re-converge and likewise the external pressure starts to return to ambient (the magnitude of the suction decreases.) Depending upon the exact shape of the tail of the ship and other factors, the air pressure may remain slightly on the side of suction, drop to essentially ambient pressure, or cross back over to a positive pressure at the tail. The stresses along the tail are much smaller in magnitude (and thus much easier to support structurally) than the stresses on the nose and around the middle of the ship. 
         [0015]    It is most inconvenient that in a typical lighter-than-air airship reduction in ambient pressure (with respect to the internal pressure) created by the airflow is greatest around the middle of the ship, just where the hoop stress is already relatively highest due to the larger diameter of the ship at that point. 
         [0016]    The problem of hoop stress is quite severe for designs that use the conventional method of deliberately increasing the internal pressure of the envelope (so-called pressure ships) in order to provide structural support for the envelope. Since the hoop stress is linearly related to the difference in pressure between the inside and the outside of the envelope, increasing the internal pressure must necessarily increase the hoop stress by a comparable amount. By using a structurally reinforced envelope, hoop stresses may be reduced compared to a pressure ship design since the pressure inside the envelope doesn&#39;t need to be artificially increased above the ambient level in order to have the envelope retain its shape. Adding structural elements to stiffen the envelope also, unfortunately, adds weight to the airship. 
         [0017]    What is needed is a lightweight yet strong structural element so as to reduce hoop stress by lessening the need for a positive internal pressure to keep the shape of the vessel. 
       BRIEF SUMMARY 
       [0018]    The invention described has, in one embodiment, a system and method for the support of a flexible envelope such as an airship comprising one or more internal trusses attached to a central structural member. The trusses are supported by one or more struts slidingly attached to the central structural member so as to be able to slide along the central member and allow the trusses to collapse when the airship is not in use. Struts may be located in the nose, centrally and/or the tail of the envelope. In another embodiment, hard points are provided centrally and/or on the tail supported by one or more internal struts, to which hard points landing gear may be attached. Struts may be provided so as to support the flexible envelope at the aerodynamic zero crossing point in the fore portion of the airship. 
         [0019]    The details of one or more embodiments of the invention are set forth in the accompanying drawing and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
     
       DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  is cutaway cross sectional view along the longitudinal axis of a vessel. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    For purposes of description, we will use a generally cigar shaped cylindrical vessel with a flexible skin or envelope, such as a blimp or dirigible, but it may be seen by one skilled in the art that the invention may be beneficially used with other shaped vessels, including irregularly shaped vessels. 
         [0022]    Referring to  FIG. 1 , a longitudinal cross sectional view of a vessel  10  is shown. An envelope  20  is used to retain the internal gas, such as a lifting gas. A central structural member ( 30 ) runs along the central axis of the airship connecting the front end  40  and tail end  50 . A propulsion means  320  is provided on the tail end  50 . Tail fins  60  are supported by tail fin struts  70 . The tail fin struts  70  attach to the central structural member  30  at the tail fin joint  80 . In a foldable design, the tail fin joint  80  would be hinged and/or capable of sliding longitudinally along the central structural member  30 , and be able to be locked in place when unfolded. Additionally, it may be locked in place in intermediate positions to aid in the initial set up and to adjust for varying wind conditions. 
         [0023]    Radial members  90  attach to the central structural member  30  at a radial member joint  100 . In a foldable design, the radial member joint  100  is hinged and/or capable of sliding longitudinally along the central structural member  30 , and to be locked in place when unfolded. Such sliding can be accomplished by a ring structure about the central member  30  with a locking mechanism or a rail and slide or other mechanism well known in the mechanical arts. The radial members  90  optionally obtain lateral support by attaching to the envelope  20  at their outer ends  110 . In a foldable design, the joints at the outer ends  110  of the radial members  90  are hinged and/or capable of sliding longitudinally along the surface of the envelope  20 , and to be able to be locked in place when unfolded. The struts may be made so as to be able to telescope so as to alter their length whilst folding and unfolding. 
         [0024]    In one embodiment, the joint at the outer ends  110  of the radial members  90  attaches to the envelope  20  along adjacent to the flexible members of a foldable ribbed envelope as described in Nachbar U.S. Pat. No. 6,793,180. Outer members  120  connect the outer ends  110  of the radial members  90  to the central structural member  30  at the front end  40  and tail end  50 . 
         [0025]    In one embodiment, the outer members  120  are flexible rope, cable or line. In a foldable design, the lengths of the outer members  120  may be lengthwise adjustable. 
         [0026]    An alternative approach to obtaining lateral support for radial members  90  and outer members  120  is to attach the outer members  120  to the nose members  280  at the outer member nose intersections  410  and  420 . Similarly, the outer members  120  can obtain lateral support by attaching to the tail fin struts  70  at the outer member tail intersections  430  and  440 . 
         [0027]    Nose struts  280  attach to the central structural member  30  at the nose strut joint  290 . In a foldable design, the nose strut joint  290  will be hinged and/or capable of sliding longitudinally along the central structural member. The nose struts  280  may attach to the envelope  20  at the aerodynamic pressure zero-crossing point  250 . This has the effect of relieving the envelope components of the longitudinal oriented stresses at the nose. In a foldable design, the attachment between the nose strut  280  and the envelope  20  may be hinged and/or capable of sliding longitudinally along the envelope and to be locked in place when unfolded. In one embodiment, the nose struts  280  will attach to a foldable, ribbed envelope as described in U.S. Pat. No. 6,793,180 along one of the ribs of the envelope. Each nose strut  280  may be limited from spreading outward beyond a certain point by a flexible tensioning member  300 , such as a wire rope, connected at one end to the outer end of the nose strut  280 . The other end of the flexible tensioning member  300  may be attached to the central structural member  30 . The nose radial member  300  can attach to the central structural member  30  at any number of longitudinal points along the central structural member  30  including a point  310  as to make it oriented perpendicularly to the central structural member  30  the very nose of the ship  40 , or it can attach somewhere between those two points. 
         [0028]    Nose radial members  300  attach to the central structural member at the nose radial joint  310 . In a foldable design, the nose radial joint will be hinged and/or capable of sliding longitudinally along the central structural member and to be locked in place when unfolded. The nose radial members  300  will attach to the envelope  20  at the aerodynamic pressure zero-crossing point  250 . In a foldable design the attachment point between the nose radial members  300  and the envelope  20  will be hinged and/or capable of sliding longitudinally along the envelope  20  and to be locked in place when unfolded. In one embodiment, the nose radial members will only be under tension and may thus be realized as a rope, cord or cable or other flexible means known in the art. 
         [0029]    The central structural member  30  carries the aerodynamic forces from the nose  40  in column to the thrusting force located on the tail  50 . Forward thrust is provided by a propulsion means  320  located at the tail of the ship. Thus the primary main thrust and drag loads are carried longitudinally by the central structural member  30  rather than by the envelope material. 
         [0030]    The central structural member  30  is stiffened against buckling and/or bending by one or more sets of radially oriented struts  90  located longitudinally along the central structural member. These radially oriented struts are themselves supported longitudinally by one or more guys  120  running to the nose and tail. This structure is very similar to the spreaders and guys that are routinely used to stiffen sailboat masts. The radial struts  90  are supported laterally by attaching them to the envelope materials preferably linking them to the ribs of a foldable envelope structure. (Note: The lateral loads that the radial struts  90  place on the envelope  20  are very small in contrast to the longitudinal loads the support.) Alternatively, the radial struts  90  and associated guys  120  can be supported laterally by attaching the guys to the nose struts  280  and/or tail fin struts  70 . 
         [0031]    The truss structure formed by the central structural member  30 , radial struts  90  and guys  120  not only supports the compressive longitudinal compressive loads but also carries the bending moments and associated stresses placed on the envelope. When a bending moment is placed on the entire ship, it is transferred to the central structural member  30  where it attaches to the nose  40  and/or tail  50  of the envelope  20  and/or by the nose struts  280  and tail fin struts  70 . The bending moment is then is carried as tension in the guys  120  and as compression in the radial struts  90  and the central structural member  30 . 
         [0032]    In addition to the nose struts  280  located at the aerodynamic zero-crossing point  250 , one or more additional sets of struts  300  may be located at the longitudinal points near the nose  40  to transfer even more loads to the central structural member  30 . Similarly, one or more additional sets of struts (not shown), comparable to the nose struts, can be added to the tail end of the vessel in order to carry loads from an even larger portion of the envelope to the central structural member  30 . 
         [0033]    An additional benefit of the internal support structure herein described is that hard points (external points supported by the internal structure)  500  and/or  510  can be provided to support landing gear. 
         [0034]    It is beneficial to provide structural support of the envelope  20  other than merely conventional internal air pressure. Otherwise, the airflow could wildly distort the envelope  20  and destroy the pressure gradient described above. The forces on this underlying support structure may be reduced by matching the internal and external pressures. 
         [0035]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the principles of the invention are applicable to vessels other than lighter-than-air airships. Accordingly, other embodiments are within the scope of the following claims.