Abstract:
An aerodynamic drag reducing apparatus for use with vehicles having downstream surfaces that are not streamlined. The apparatus consists of folding panels that extend rearward for use in a drag reducing configuration and collapse for use in a space saving configuration.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/741,155, filed Dec. 1, 2005, which application is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to an aerodynamic drag reducing apparatus for use with vehicles having downstream surfaces that are not streamlined. Examples include: over-the-road trucks and truck trailers, vans and minivans, motor homes and travel trailers, and pickup trucks. In vehicles such as pickup trucks, one non-streamlined surface is near the center of the vehicle. The present invention is suitable for use with vehicles having rear doors.  
       BACKGROUND  
       [0003]     It is known that a significant amount of aerodynamic drag is created when a vehicle travels at velocities typical on a modern roadway. This is due, in large part, to areas of low pressure that are induced on the rear surfaces of the vehicle. The low pressure becomes more pronounced as airflow over the vehicle separates from the vehicle surfaces. The phenomenon of airflow separation is also well known in the design of aircraft wings and, in this case, causes the wing to stall.  
         [0004]     Vehicles having blunt rear ends are especially affected by airflow separation starting at the abrupt transition to the rear—near vertical surface. The low pressure that the airflow separation induces is compounded by the relatively large area on which the low air pressure acts in comparison with more streamlined vehicles.  
         [0005]     The low air pressure acting on the rear surfaces of a moving vehicle produces a force that resists the forward motion of the vehicle. This force is opposed by the vehicle&#39;s engine and requires power that is typically produced by burning fuel. Any reduction in aerodynamic drag results in a reduction in fuel consumption.  
         [0006]     In the current period of high fuel prices and increasing environmental consciousness, fuel efficiency improvements are a growing concern. Aerodynamic improvements are especially valuable since they can be combined with other improvements such as engine efficiency and reduced chassis weight. Increasing fuel efficiency also provides the valuable benefit of increasing the range a given vehicle can travel between refueling.  
         [0007]     The present invention employs the technique of adding tapered rear surfaces to a vehicle. The same streamlining principle is practiced with other vehicles such as high-speed cars and airplanes. It has also been applied to over-the-road trucks where it is known as a “boat-tail”.  
       SUMMARY  
       [0008]     The present invention is concerned with providing an aerodynamic drag reducing apparatus for vehicles with the purpose of reducing energy consumption. More specifically, this is achieved by adding gently sloping surfaces downstream of rear facing surfaces of the vehicle with the goal of reducing airflow separation and aerodynamic drag. This, in turn, reduces fuel consumption.  
         [0009]     On certain vehicles, simply adding the required additional surfaces would result in a substantial increase to the vehicle length. This length would be acceptable, in many cases, on the open road in uncongested traffic, but would be impractical on crowded urban roadways, in parking lots, in campgrounds, and by loading docks. To address this, the present invention has two primary configurations. The first is an extended configuration that reduces drag and fuel consumption, especially at highway speeds. The second is a retracted configuration that provides much less, if any, drag reduction, but results in a more compact vehicle that is practical in crowded areas. This combination of configurations in the same apparatus is especially useful since zones of higher speed traffic are often not congested. These high-speed zones are also where the drag reducing potential is the highest. Likewise, congested areas often have reduced traffic speed with less drag reducing potential, but in these cases, the retracted configuration may be required for maneuvering.  
         [0010]     In order to easily and conveniently convert between the retracted configuration and the extended configuration, the present invention employs panels that fold as part of an apparatus attached to the rear of the vehicle.  
         [0011]     To accommodate vehicles where access to the rear end is required, certain embodiments of the present invention allow the apparatus to be temporarily moved without removal from the vehicle. In certain embodiments, a single whole apparatus is mounted on a support panel, door, or framework that in turn is mounted on a hinge, linkage, or linear slide. Other embodiments of the present invention have the overall aerodynamic shape split into two halves. These halves can be mounted on hinges and opened, providing access to the rear of the vehicle. Optionally, the halves can be integrated with the rear doors of the vehicle. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The benefits and advantages of the present invention will become more readily apparent to those of ordinary skill in the relevant art after reviewing the following detailed description and accompanying drawings, wherein:  
         [0013]      FIGS. 1A through 1M  show a first embodiment of the present invention mounted on the rear of an over-the-road trailer in various configurations. In this embodiment, the aerodynamic drag reducing apparatus is in two halves, split right and left. The right and left halves can be opened for access to the rear of the trailer. In particular:  
         [0014]      FIGS. 1A through 1F  are all views sharing the same perspective from the left rear corner. In particular:  
         [0015]      FIG. 1A  shows a fully extended configuration with the trailer doors closed.  
         [0016]      FIG. 1B  shows a fully retracted configuration with the trailer doors closed.  
         [0017]      FIGS. 1C through 1F  are enlarged partial views of the same scale. In particular:  
         [0018]      FIG. 1C  shows a fully retracted configuration with the trailer doors closed.  
         [0019]      FIG. 1D  shows the right apparatus fully extended with its trailer door closed and the left apparatus fully retracted with its trailer door open.  
         [0020]      FIG. 1E  shows a fully retracted configuration with the trailer doors opened.  
         [0021]      FIG. 1F  shows a partially retracted configuration with the trailer doors closed.  
         [0022]      FIGS. 1G through 1I  are all enlarged partial left elevation views of the same scale shown with the trailer doors closed. In particular:  
         [0023]      FIG. 1G  shows a fully extended configuration.  
         [0024]      FIG. 1H  shows a partially retracted configuration.  
         [0025]      FIG. 1I  shows a fully retracted configuration.  
         [0026]      FIGS. 1J  through IM are all enlarged partial top plan views of the same scale. In particular:  
         [0027]      FIG. 1J  shows a fully extended configuration with the trailer doors closed.  
         [0028]      FIG. 1K  shows a partially retracted configuration with the trailer doors closed. The right apparatus has its non-vertical panels removed for the purpose of illustration.  
         [0029]      FIG. 1L  shows a fully retracted configuration with the trailer doors closed.  
         [0030]      FIG. 1M  shows a fully retracted configuration with the trailer doors opened.  
         [0031]      FIGS. 2A through 2D  are all perspective views showing the right apparatus of  FIGS. 1A through 1M  by itself in various configurations. In this embodiment of the present invention, two panel groups are shown. The larger group is attached to the right door of the trailer on its large end, and the smaller group is attached to the panel of the larger group that is farthest from the trailer door. In particular:  
         [0032]      FIGS. 2A and 2B  share the same scale and view perspective from the right rear corner. In particular:  
         [0033]      FIG. 2A  shows both panel groups in a fully retracted configuration.  
         [0034]      FIG. 2B  shows both panel groups in a fully extended configuration.  
         [0035]      FIGS. 2C and 2D  share the same scale and partly show the interior of the apparatus with both panel groups in a partially retracted configuration. In particular:  
         [0036]      FIG. 2C  is a view from the left rear corner.  
         [0037]      FIG. 2D  is a view from the left front corner.  
         [0038]      FIGS. 3A through 3D  are all perspective views showing one of the two panel groups of  FIGS. 2A through 2D  in a fully extended configuration. In particular:  
         [0039]      FIGS. 3A and 3B  share the same view perspective from the right rear corner. In particular:  
         [0040]      FIG. 3A  shows the larger panel group of  FIGS. 2A through 2D .  
         [0041]      FIG. 3B  shows the smaller panel group of  FIGS. 2A through 2D .  
         [0042]      FIGS. 3C and 3D  share the same scale and view perspective from the left rear corner and partly show the interior of the panel group. In particular:  
         [0043]      FIG. 3C  shows the smaller panel group of  FIGS. 2A through 2D .  
         [0044]      FIG. 3D  shows the larger panel group of  FIGS. 2A through 2D .  
         [0045]      FIGS. 4A through 4F  are all perspective views of the same scale, showing several panels of the panel group of  FIGS. 3A and 3D . The panels that are not near the large end of the panel group have been removed for the purpose of illustration. In particular:  
         [0046]      FIGS. 4A through 4C  share the same view perspective from the left rear corner. In particular:  
         [0047]      FIG. 4A  shows the panels in a fully extended configuration.  
         [0048]      FIG. 4B  shows the panels in a partially retracted configuration.  
         [0049]      FIG. 4C  shows the panels in a fully retracted configuration.  
         [0050]      FIGS. 4D through 4F  share the same view perspective from the right rear corner. In particular:  
         [0051]      FIG. 4D  shows the panels in a fully extended configuration.  
         [0052]      FIG. 4E  shows the panels in a partially retracted configuration.  
         [0053]      FIG. 4F  shows the panels in a fully retracted configuration.  
         [0054]      FIGS. 5A through 5D  are all perspective views of the same scale as  FIGS. 4A through 4F , showing several panels of the panel group of  FIGS. 3A and 3D . The side panels have been removed for the purpose of illustration. In particular:  
         [0055]      FIGS. 5A through 5C  have the same view perspective as  FIGS. 4D through 4F . In particular:  
         [0056]      FIG. 5A  shows the panels in a fully extended configuration.  
         [0057]      FIG. 5B  shows the panels in a partially retracted configuration.  
         [0058]      FIG. 5C  shows the panels in a fully retracted configuration.  
         [0059]      FIG. 5D  has the same view perspective as  FIGS. 4A through 4C  and shows the panels in a fully extended configuration.  
         [0060]      FIGS. 6A through 6D  are all perspective views of the same scale as  FIGS. 4A through 4F , showing several panels of the panel group of  FIGS. 3A and 3D . The top and bottom panels have been removed for the purpose of illustration. In particular:  
         [0061]      FIGS. 6A and 6B  show the panels in a fully extended configuration. In particular:  
         [0062]      FIG. 6A  has the same view perspective as  FIGS. 4A through 4C .  
         [0063]      FIG. 6B  has the same view perspective as  FIGS. 4D through 4F .  
         [0064]      FIGS. 6C and 6D  show the panels in a partially retracted configuration. In particular:  
         [0065]      FIG. 6C  has the same view perspective as  FIGS. 4A through 4C .  
         [0066]      FIG. 6D  has the same view perspective as  FIGS. 4D through 4F .  
         [0067]      FIG. 7  has the same view perspective and scale as  FIGS. 4D through 4F  and shows several panels of the panel group of  FIGS. 3A and 3D  in a fully retracted configuration. The top, bottom, and rear panels have been removed for the purpose of illustration.  
         [0068]     FIGS. SA and  8 B are enlarged views of the same scale, showing the panel group of  FIGS. 3A and 3D  in a fully retracted configuration. In particular:  
         [0069]      FIG. 8A  has the same view perspective as  FIGS. 4D through 4F .  
         [0070]      FIG. 8B  is a right elevation view.  
         [0071]      FIGS. 9A through 9N  are all views with the same perspective and scale as  FIG. 8A , showing the individual panels of the panel group of  FIGS. 3A, 3D ,  8 A, and  8 B. The configuration specific orientation of each panel is from the panel group in a fully retracted configuration (as shown in  FIG. 8A ). In particular:  
         [0072]      FIG. 9A  is the front panel.  
         [0073]      FIG. 9B  is the forward-most top panel.  
         [0074]      FIG. 9C  is the forward-most bottom panel.  
         [0075]      FIG. 9D  is the forward-most upper triangular panel.  
         [0076]      FIG. 9E  is the forward-most lower triangular panel.  
         [0077]      FIG. 9F  is the forward-most side panel.  
         [0078]      FIG. 9G  is the middle top panel.  
         [0079]      FIG. 9H  is the middle bottom panel.  
         [0080]      FIG. 91  is the rearmost upper triangular panel.  
         [0081]      FIG. 9J  is the rearmost lower triangular panel.  
         [0082]      FIG. 9K  is the rearmost side panel.  
         [0083]      FIG. 9L  is the rear panel.  
         [0084]      FIG. 9M  is the rearmost top panel.  
         [0085]      FIG. 9N  is the rearmost bottom panel.  
         [0086]      FIGS. 10A through 10C  are all perspective views sharing the same scale and perspective from the left rear corner, showing a panel group from a second embodiment of the present invention in various configurations. In this embodiment, the aerodynamic drag reducing apparatus is not split in two halves, but is fanned of panel groups that span the vehicle. In particular:  
         [0087]      FIG. 10A  shows the panel group in a fully extended configuration.  
         [0088]      FIG. 10B  shows the panel group in a partially retracted configuration.  
         [0089]      FIG. 10C  shows the panel group in a fully retracted configuration.  
         [0090]      FIG. 11  is a perspective view from the left rear corner, showing a pair of panel groups from a third embodiment of the present invention in a fully extended configuration. In this embodiment, the aerodynamic drag reducing apparatus is split in two halves, one upper and the other lower.  
         [0091]      FIGS. 12A through 12C  are all perspective views sharing the same scale and perspective from the left rear corner, showing the lower panel group of  FIG. 11 . In particular:  
         [0092]      FIG. 12A  shows the panel group in a fully extended configuration.  
         [0093]      FIG. 12B  shows the panel group in a partially retracted configuration.  
         [0094]      FIG. 12C  shows the panel group in a fully retracted configuration.  
         [0095]      FIGS. 13A through 13C  are all perspective views sharing the same scale and perspective as  FIGS. 12A through 12C , showing the upper panel group of  FIG. 11 . In particular:  
         [0096]      FIG. 13A  shows the panel group in a fully extended configuration.  
         [0097]      FIG. 13B  shows the panel group in a partially retracted configuration.  
         [0098]      FIG. 13C  shows the panel group in a fully retracted configuration.  
         [0099]      FIGS. 14A through 14D  are all perspective views sharing the same scale, showing a framed panel group from a forth embodiment of the present invention. The covering of the framed panel group is not shown for the purpose of illustration. In particular:  
         [0100]      FIGS. 14A through 14C  have the same scale and view perspective as their counterparts in  FIGS. 5A through 5C . In particular:  
         [0101]      FIG. 14A  shows the framed panel group in a fully extended configuration.  
         [0102]      FIG. 14B  shows the framed panel group in a partially retracted configuration.  
         [0103]      FIG. 14C  shows the framed panel group in a fully retracted configuration.  
         [0104]      FIG. 14D  has the same scale and perspective as its counterpart in  FIG. 5D  and shows the framed panel group in a fully extended configuration.  
         [0105]      FIGS. 15A and 15B  are enlarged perspective views showing the three top framed panels and their covering from the framed panel group of  FIGS. 14A through 14D  in a partially retracted configuration. In particular:  
         [0106]      FIG. 15A  is a view from the upper right corner primarily showing the exterior of the framed panels.  
         [0107]      FIG. 15B  is a view from the lower right corner primarily showing the interior of the framed panels.  
         [0108]      FIGS. 16A through 16E  are enlarged partial views illustrating typical folding fabric panel joints. In particular:  
         [0109]      FIG. 16A  is a perspective view illustrating a fully closed joint with the fabric wrapping around the joined panels.  
         [0110]      FIG. 16B through 16E  illustrates a joint with the fabric folding between the joined panels with a joint-stop and bias spring as optional features. In particular:  
         [0111]      FIG. 16B  is a perspective view illustrating a partially opened joint.  
         [0112]      FIGS. 16C through 16E  are views perpendicular to the folding axis. In particular:  
         [0113]      FIG. 16C  illustrates a fully opened joint held by the joint-stop and compressing the bias spring.  
         [0114]      FIG. 16D  illustrates a partially opened joint contacting the bias spring.  
         [0115]      FIG. 16E  illustrates a fully closed joint.  
         [0116]      FIG. 17  is a perspective view from the left rear corner, showing a set of four panel groups from a fifth embodiment of the present invention in a fully extended configuration. In this embodiment, the aerodynamic drag reducing apparatus is split into four quarters.  
         [0117]      FIGS. 18A through 18D  are all perspective views sharing the same scale of  FIG. 17  and showing the lower left panel group of  FIG. 17 . In particular:  
         [0118]      FIGS. 18A and 18B  share the same view perspective from the left rear corner and primarily show the exterior of the panel groups. In particular:  
         [0119]      FIG. 18A  shows the panel group in a fully retracted configuration.  
         [0120]      FIG. 18B  shows the panel group in a partially retracted configuration.  
         [0121]     FIGS. l 8 C and  18 D share the same view perspective from the right front corner and primarily show the interior of the panel groups. In particular:  
         [0122]      FIG. 18C  shows the panel group in the same configuration as  FIG. 18B .  
         [0123]      FIG. 18D  shows the panel group in a fully extended configuration. 
     
    
     DETAILED DESCRIPTION  
       [0124]     While the present invention is susceptible of embodiment in various forms, there are shown in the drawings and will hereinafter be described presently preferred embodiments with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiments illustrated.  
         [0125]     The embodiments presented are also shown in various forms and shapes and with various optional features in various combinations. These variations also are exemplifications of the invention and are not intended to limit the combinations of fonts, shapes, and optional features.  
         [0126]     As described above, the present invention is suited for use behind vehicles with vertical or near vertical rear facing surfaces and serves to streamline the vehicle when in the extended configuration. When necessary, the present invention can transform into a retracted configuration to save space. The ability to form two configurations and transform from one to the other without disassembly is made possible by the strategic use of folding panels in an apparatus as described below.  
         [0127]     In the present invention, the folding panels are arranged into panel groups. Multiple panel groups may be attached to each other in series from front to back within the same apparatus. The panel groups may be individually extended or retracted. When all panel groups are fully extended, the apparatus itself is fully extended and is in the first of its primary configurations. Likewise, when all the panel groups are fully retracted, the apparatus itself is fully retracted and is in the second of its primary configurations. When at least one panel group is extended and at least one panel group is retracted, the apparatus is in a secondary configuration. The apparatus may be comprised of a single panel group. In this case, the concept of a “secondary configuration” does not apply. Likewise, the definition of “primary configuration” extends to a panel group but “secondary configuration” does not.  
         [0128]     The panels and the folding joints between them support loads imposed on the apparatus by gravity, airflow, uneven roads, and other operational causes. The panels may depend on a framework or multiple frameworks for structural support, or they may serve as their own structural support. The folding joint and structural connection between the panels may take the form of a simple or complex hinge; a linkage; a fibrous material, such as fabric or cord; or a solid deformable material, such as plastic. The same apparatus may use both framed and non-framed panels as well as a mixture of joint types.  
         [0129]     In all of the embodiments of the present invention, folding panels are employed to allow the apparatus to transform between an extended and retracted configuration. The joints may take various forms as discussed above in any of the embodiments. Furthermore, a given apparatus may employ any of the joint forms in any combination. To simplify the discussion in this disclosure, the term “fold-line” is used to represent the folding action of any of the various joint types.  
         [0130]     In certain embodiments of the present invention, the joints between certain panel pairs may both rotate along a hinge-line and linearly slide along the same hinge-line. This hinge-line may also be considered a “fold-line”. In other embodiments, particularly those that use a fibrous or solid deformable material as a structural connection between the panels, multiple rotational and linear movements may occur between the panels, approximated by a fold-line and dominated by the folding rotation.  
         [0131]     In certain embodiments of the present invention, flexible material, such as fabric, may be used to cover framework, forming a framed panel as illustrated in  FIGS. 15A and 15B . The flexible material may extend from panel to panel and serve to keep dirt and debris from entering the interior of the apparatus. This is also illustrated in  FIGS. 15A and 15B . In certain embodiments, the entire apparatus may be covered by a single continuous piece of flexible material. Optionally, the flexible material may also serve as a structural connection between certain panel pairs as mentioned in the preceding paragraphs as illustrated in  FIGS. 16A through 16E . Flexible material may be located inside and/or outside the framework and may fully or partially form the interior and/or exterior of the apparatus. The flexible material may serve to seal the joints, making the apparatus substantially airtight.  
         [0132]     An airtight apparatus may be filled with air as a method of extending the apparatus. Likewise, the air may be evacuated to retract the apparatus.  
         [0133]     In certain embodiments of the present invention, the joints between panels may be spring-loaded, as illustrated in  FIGS. 16B through 16E . The springs may assist in the extension and/or retraction of the panels. Bi-stable springs may also be used that serve both purposes. The joints may also have detents that aid in maintaining the apparatus in a primary configuration.  
         [0134]     In certain embodiments of the present invention, removable parts may be attached to the panels or panel framework to keep them in a primary configuration. These parts may be fastened to the panels or panel framework by threaded fasteners, latches, hooks, or other means. In certain embodiments, the same removable parts may be used to keep both the extended and retracted configurations by attaching them in a different sequence and joining the features of the panels and the removable parts in different combinations.  
         [0135]     Other optional features that may be used separately or together include joint-stops that keep the panels from reaching an undesired configuration, as illustrated in  FIGS. 16B through 16E ; latches that keep the apparatus in the primary configurations; and retraction and extension devices.  
         [0136]     In certain embodiments of the present invention, pulleys and cables and/or other optional components may be used in a retraction and extension device either separately or together. These include pneumatic and hydraulic cylinders, linear drives, electric motors, gear sets, cord, chain, webbing, cams, and springs.  
         [0137]     In certain embodiments of the present invention, certain panels or panel frameworks may deform from one configuration to the next or while transitioning between configurations. In certain cases, this is necessary to avoid kinematic lockup. This deformation will cause forces and moments to develop within and between the panels. These forces and moments may be employed to keep the panel groups stable in one or both primary configurations.  
         [0138]     Referring now to the figures, in particular to  FIGS. 1A through 1M , there is shown a first embodiment of the present invention mounted on the rear of an exemplary over-the-road trailer  1 . More specifically, a left hand apparatus  302  and a right hand apparatus  303  are mounted and shown in various configurations.  FIGS. 2A through 2D  show the right hand apparatus  303  in detail and in various configurations. In the present embodiment, the right rear trailer door serves as a mounting platform for the right hand apparatus  303  and may also be integrated to become part of the apparatus  303 . The exterior shape of the left hand apparatus  302  is a mirror image of the right hand apparatus  303 . The same relationships between the left hand door and the left hand apparatus  302  exist as mentioned above for the right. To gain access to the cargo holding area  1 A of the trailer  1 , the apparatuses  302  and  303  are opened as typical trailer doors or with the trailer doors.  
         [0139]     If the apparatuses  302  and  303  are symmetric in a vertical direction, as they are shown in  FIGS. 1A through 1M , they can be identical with each other. In this case, the apparatuses  302  and  303  are simply assembled to their respective trailer doors upside down and rotated one-half turn from each other.  
         [0140]     As mentioned above, the apparatuses  302  and  303  are normally used in one of two primary configurations, fully extended and fully retracted.  FIG. 1A  illustrates the fully extended configuration and  FIG. 1B  illustrates the fully retracted.  
         [0141]     In certain panels of certain embodiments of the present invention, clearance cuts  339  may be required to avoid interference with the vehicle, hinges, latches, and even other panels. For example,  FIG. 9F  shows clearance cuts  339  made on a panel  323  to avoid interference with the trailer door hinge.  
         [0142]      FIGS. 2A through 2D  and  3 A through  3 D illustrate that the panels are arranged in panel groups  311  and  312 . The panel groups  311  and  312  combine to form the apparatus  303 . The panel group farthest upstream  311  attaches to the rear door of the trailer  1  or the rear of the vehicle as appropriate. The following panel group  312  attaches to the rear of the panel group  311  ahead of it. In these illustrations, only two panel groups  311  and  312  are shown. In other embodiments of the present invention, one panel group may be used alone in an apparatus, or more than two panel groups may be assembled in succession within an apparatus. The mating panels between panel groups  311  and  312 , specifically the front panel of the reward group and the rear panel of the forward group, may be integrated into a single structure serving the requirements of both groups.  FIGS. 2A through 2D  show two panel groups  311  and  312  connected to each other, as they would be in operation.  FIGS. 3A and 3D  show the larger panel group  311  by itself. Likewise,  FIGS. 3B and 3C  show the smaller panel group  312  by itself.  
         [0143]     The panel groups  311  and  312  are similar to each other in regards to the function of their corresponding top and side panels  321 ,  322 ,  323 ,  324 ,  325 ,  326 ,  327 ,  328 ,  329 ,  330 ,  331 , and  333 . The fold-lines  340 ,  341 ,  342 ,  343 ,  344 ,  345 ,  346 ,  347 ,  348 ,  349 ,  350 ,  351 ,  352 ,  353 ,  354 ,  355 ,  356 ,  357 ,  358 , and  359 , illustrated in  FIGS. 9A through 9N , are also similar in function to their corresponding fold-lines from other panel groups  311  and  312 . Furthermore, the front panel  320  is similar in function between the panel groups  311  and  312  in that it connects to or is integrated with what comes immediately in front of it. This could be either the trailer door or the rear panel  332  from the panel group  311  that precedes it. Likewise, the rear panel  332  is similar in function between the panel groups  311  and  312  in that it connects to or is integrated with the front panel  320  of the panel group  312  immediately behind it. In the case of the last panel group  312 , the rear panel  332  is an exterior panel of the apparatus  302  and  303 . The panels belonging to panel groups  311  farther forward are typically larger than their corresponding panels belonging to more rearward panel groups  312 .  
         [0144]      FIGS. 4A through 4F ,  5 A through  5 D,  6 A through  6 D, and  7  illustrate the workings of a panel group  311  of a typical embodiment of the present invention. This is done by removing certain panels of the panel group  311  from each illustration. Even though different panels are hidden from each illustration, the shape and size of all panels, including the hidden panels, along with the location of their respective fold-lines are used to calculate the kinematic position of each panel throughout its range of motion in each illustration. Isolating and studying the relationships between certain sub-groups of panels clarifies the function each individual panel and its respective fold-line(s) has in the panel group  311 . These descriptions and figures are based on a specific panel group  311  of a specific embodiment of the present invention for the purposes of illustration only. Other embodiments of the present invention and even other panel groups within the present embodiment will have different proportions than those shown. Panel groups of other embodiments of the present invention will have different relationships between the panels and may have different quantities of panels.  
         [0145]      FIGS. 4A through 4F  illustrate the relationships between the front panel  320 , the forward-most top panel  321 , the forward-most upper triangular panel  322 , the forward-most side panel  323 , the forward-most lower triangular panel  324 , and the forward-most bottom panel  325 . These panels  320 ,  321 ,  322 ,  323 ,  324 , and  325  are also shown individually with their respective fold-lines in  FIGS. 9A through 9F . The panels are connected to each other by fold-lines as follows: Fold-line  340  connects the front panel  320  ( FIG. 9A ) to the forward-most top panel  321  ( FIG. 9B ). Likewise, fold-line  342  connects the front panel  320  ( FIG. 9A ) to the forward-most bottom panel  325  ( FIG. 9C ). Fold-line  341  connects the front panel  320  ( FIG. 9A ) to the forward-most side panel  323  ( FIG. 9F ). Fold-line  347  connects the forward-most side panel  323  ( FIG. 9F ) to the forward-most upper triangular panel  322  ( FIG. 9D ). Likewise, fold-line  348  connects the forward-most side panel  323  ( FIG. 9F ) to the forward-most lower triangular panel  324  ( FIG. 9E ). Fold-line  343  connects the forward-most top panel  321  ( FIG. 9B ) to the forward-most upper triangular panel  322  ( FIG. 9D ). Likewise, fold-line  344  connects the forward-most bottom panel  325  ( FIG. 9C ) to the forward-most lower triangular panel  324  ( FIG. 9E ). This arrangement of fold-lines and panels results in a coordinated deployment of the panels from the fully retracted to fully extended configurations and a coordinated stowage of the panels from the fully extended to fully retracted configurations. The configuration of panels  320 ,  321 ,  322 ,  323 ,  324 , and  325  within this sub-group is determined by a single variable. Thus, by controlling the angle across any fold-line  340 ,  341 ,  342 ,  343 ,  344 ,  347 , or  348 , the angles across the remaining fold-lines are determined and the configuration of the panels  320 ,  321 ,  322 ,  323 ,  324 , and  325  is known.  
         [0146]      FIGS. 5A through 5D  illustrate the relationships between the front panel  320 , the forward-most top panel  321 , the forward-most bottom panel  325 , the middle top panel  326 , the middle bottom panel  330 , the rearmost top panel  331 , the rear panel  332 , and the rearmost bottom panel  333 . These panels  320 ,  321 ,  325 ,  326 ,  330 ,  331 ,  332 , and  333  are also shown individually with their respective fold-lines in  FIGS. 9A through 9C ,  9 G,  9 H, and  9 L through  9 N. The panels are connected to each other by fold-lines as follows: Fold-line  340  connects the front panel  320  ( FIG. 9A ) to the forward-most top panel  321  ( FIG. 9B ). Likewise, fold-line  342  connects the front panel  320  ( FIG. 9A ) to the forward-most bottom panel  325  ( FIG. 9C ). Fold-line  345  connects the forward-most top panel  321  ( FIG. 9B ) to the middle top panel  326  ( FIG. 9G ). Likewise, fold-line  346  connects the forward-most bottom panel  325  ( FIG. 9C ) to the middle bottom panel  330  ( FIG. 9H ). Fold-line  350  connects the middle top panel  326  ( FIG. 9G ) to the rearmost top panel  331  ( FIG. 9M ). Likewise, fold-line  351  connects the middle bottom panel  330  ( FIG. 9H ) to the rearmost bottom panel  333  ( FIG. 9N ). Fold-line  355  connects the rearmost top panel  331  ( FIG. 9M ) to the rear panel  332  ( FIG. 9L ). Likewise, fold-line  356  connects the rearmost bottom panel  333  ( FIG. 9N ) to the rear panel  332  ( FIG. 9L ). Unlike the preceding sub-group, this sub-group of panels  320 ,  321 ,  325 ,  326 ,  330 ,  331 ,  332 , and  333  by themselves does not form a configuration that can be determined by a single variable. It relies on the panel group as a whole to determine its configuration.  
         [0147]      FIGS. 6A through 6D  illustrate the relationships between the front panel  320 , the forward-most upper triangular panel  322 , the forward-most side panel  323 , the forward-most lower triangular panel  324 , the rearmost upper triangular panel  327 , the rearmost side panel  328 , the rearmost lower triangular panel  329 , and the rear panel  332 . These panels  320 ,  322 ,  323 ,  324 ,  327 ,  328 ,  329 , and  332  are also shown individually with their respective fold-lines in  FIGS. 9A, 9D  through  9 F, and  91  through  9 L. The panels are connected to each other by fold-lines as follows: Fold-line  341  connects the front panel  320  ( FIG. 9A ) to the forward-most side panel  323  ( FIG. 9F ). Fold-line  347  connects the forward-most side panel  323  ( FIG. 9F ) to the forward-most upper triangular panel  322  ( FIG. 9D ). Likewise, fold-line  348  connects the forward-most side panel  323  ( FIG. 9F ) to the forward-most lower triangular panel  324  ( FIG. 9E ). Fold-line  349  connects forward-most side panel  323  ( FIG. 9F ) to the rearmost side panel  328  ( FIG. 9K ). Fold-line  352  connects the rearmost side panel  328  ( FIG. 9K ) to the rearmost upper triangular panel  327  ( FIG. 91 ). Likewise, fold-line  353  connects the rearmost side panel  328  ( FIG. 9K ) to the rearmost lower triangular panel  329  ( FIG. 9J ). Fold-line  354  connects the rearmost side panel  328  ( FIG. 9K ) to the rear panel  332  ( FIG. 9L ). As with the preceding sub-group, this sub-group of panels  320 ,  322 ,  323 ,  324 ,  327 ,  328 ,  329 , and  332  by themselves does not form a configuration that can be determined by a single variable. It relies on the panel group as a whole to determine its configuration. The rearmost upper and lower triangular panels  327  and  329  must fold to prevent interference between themselves and the rearmost top and bottom panels  331  and  333  respectively. This folding is needed during a portion of the extension-retraction process and can be most clearly observed in  FIGS. 2C and 2D . The orientation of the rearmost upper and lower triangular panels  327  and  329  is determined by their contact with the rearmost top and bottom panels  331  and  333  respectively or by a joint-stop keeping them parallel with the rearmost side panel  328 . The related fold-lines  352  and  353  are spring-loaded, keeping the panels  327  and  329  against their respective stops until contact is made with the rearmost top and bottom panels  331  and  333  respectively.  
         [0148]     In the case that the front panel  320  of a rearward panel group  312  and the rear panel  332  of the preceding panel group  311  are integrated, additional fold-lines will be found on the combined panel  320 / 332  to serve their corresponding purpose for both panel groups  311  and  312 . This is illustrated in  FIG. 9L  where fold-lines  357 ,  358 , and  359  are present. These fold-lines serve the same purpose as fold-lines  340 ,  341 , and  342  as shown in  FIG. 9A , but for the following panel group  312 .  
         [0149]     The panel group  311  with all panels present forms an assembly whose configuration is determined by a single variable. This is of great benefit whether the apparatuses  302  and  303  are deployed manually or by automated means, as each panel group can be managed with a single variable rather than as multiple individual panels. Furthermore, it is possible to coordinate panel groups with each other so that the configuration of the entire apparatus or pair of apparatuses can be managed by a single variable.  
         [0150]     The discussions above focus primarily on an embodiment of the present invention suited for vehicles with rear doors having vertical hinge-lines served by two apparatuses  302  and  303 . Vehicles and trailers with neither rear doors nor need for access to the rear of the vehicle are better served by a second embodiment of the present invention that spans the width of the vehicle. This embodiment is formed by combining the two apparatuses  302  and  303 , at their common edges, into a single apparatus, as illustrated by the panel group  411  in  FIGS. 10A through 10C . More specifically, the right hand and left hand versions of the panel group  311  are combined by joining the common edges of the front, rear, top, and bottom panels  320 ,  321 ,  325 ,  326 ,  330 ,  331 ,  332 , and  333 . As in the previous embodiment, multiple panel groups can be arranged one behind the other to form an extended apparatus. As in the preceding panel groups  311  and  312 , the configuration of the combined panel group  411  is also controlled by a single variable.  
         [0151]     The embodiment of the preceding paragraph may be adapted for use with vehicles having rear doors by mounting the combined apparatus on a support panel, door, or framework that in turn is mounted on a hinge, linkage, or linear slide. The whole apparatus can thus be temporarily moved when access to the rear of the vehicle is required.  
         [0152]     A third embodiment of the present invention is arrived at by splitting the apparatus in the preceding paragraph, exemplified by panel group  411 , about a horizontal plane near its center, creating two halves: one upper apparatus and one lower apparatus. This embodiment is illustrated by the panel groups  511  and  611  shown in  FIGS. 11, 12A  through  12 C, and  13 A through  13 C. This configuration is suited for use on vehicle doors with horizontal hinges. This embodiment could also be rotated ninety degrees and used on vehicles whose doors have vertical hinges. As in the previous embodiments, panel groups in successively smaller sizes can be connected one behind the other to extend the gently sloping surfaces and reduce the rear-facing area in the fully extended configuration. This embodiment retains the desirable characteristics of stability and configuration by a single variable. Panel groups of this embodiment can be used individually. For example, the panel group  611  in  FIGS. 13A through 13C  could be used behind the cab of a pickup truck.  
         [0153]     A fourth embodiment of the present invention is comprised of panels and/or framed panels  721 ,  722 ,  724 ,  726 , and  731  combined to form a panel group  711  as illustrated in  FIGS. 14A through 14D ,  15 A and  15 B. Coverings  750 ,  751 , and  752  (not shown in  FIGS. 14A through 14D ) are formed of flexible material and attached to certain areas of certain panels  721 ,  722 ,  724 ,  726 , and  731 . Certain coverings  750  and  752  may attach to and/or cover two or more panels or framed panels  721 ,  722 ,  724 ,  726 , and  731 . Certain forms of the current embodiment have a single covering substantially forming the exterior surface of the apparatus. Certain panels and framed panels may not attach to the covering but serve to provide a shape and structural support for it. Certain panels, such as counterparts to panels  327  and  329  (illustrated in  FIGS. 3A and 3D ), are not required by this embodiment. Other panels  722  and  724  (counterparts to panels  322  and  324  also illustrated in  FIGS. 3A and 3D ) may take a shape that does not fully fill the apparatus&#39; exterior when it is fully extended but still provide the required structural and kinematic support. In these cases, the exterior shape of the extended apparatus in these areas is determined by the panels and/or framed panels adjoining the removed or reduced panels. A covering spans any void created by the removed or reduced panels in the extended configuration and attaches to other panels of the apparatus. The present embodiment may be adapted to and combined with the other embodiments of the present invention.  
         [0154]     A fifth embodiment of the present invention can be arrived at by splitting the third embodiment about a vertical plane near its center as illustrated by  FIGS. 17 and 18 A through  18 D. This embodiment retains the desirable characteristics of stability and configuration by a single variable. In contrast to certain embodiments above which combine a pair of apparatuses to cover the rear surface of a vehicle, this embodiment requires four apparatuses  512 ,  513 ,  612 , and  613  arranged in quarters. In the example of the pickup truck, only the upper two quarters  612  and  613  would be required. Just as in the embodiments above, multiple panel groups can be arranged one behind the other. This embodiment may be combined with the embodiment of the preceding paragraph. In this case, panel  629  is not required. The hole in the extended configuration crated by the removal of panel  629  is spanned by the covering. The shape of the covering over this hole when the apparatus is in the extended configuration is determined by panel edges adjacent to the hole.  
         [0155]      FIGS. 16A through 16E  further illustrate the fabric joint type. More specifically, and first panel or framed panel  810  is joined fabric  820  which in turn is also joined to a second panel  830 . Unlike a common hinge, this joint type does not have a precisely defined hinge-line. Nonetheless, a folding action is made possible with sufficient precision for the present invention.  
         [0156]     Optional joint-stops  840  and bias springs  850 , as illustrated in  FIGS. 16B through 16E , maybe added to certain joints of any of the various joint types.  
         [0157]     In all of the embodiments discussed above, the beneficial features of the present invention are the gently sloping exterior surfaces when extended, the compact space it occupies when retracted, the relative simplicity of the design, the simple and low cost components of which it consists, the imnate ability to control all panels within a panel group with one variable, and the ability to control all the panels within an apparatus or pair of apparatuses with one variable.  
         [0158]     In the present disclosure, the words “a” or “an” are to be taken to include both the singular and the plural. Conversely, any references to plural items shall, where appropriate, include the singular.  
         [0159]     It is desirable for drag reducing devices in accordance with the principles of the present disclosure to be shaped to reduce the effects of air flow separation. In certain example embodiments, drag reducing devices in accordance with the principles of the present disclosure may define angles α,β, and γ(see  FIGS. 1G and 1J ) relative to the rear surface of the vehicle that is less than 18 degrees, or in the range of  10  to  18  degrees.  
         [0160]     When extended, drag reducing devices in accordance with the present disclosure are typically truncated (see  FIG. 1A ). In certain truncated embodiments, the extended lengths of the drag reducing devices are such that the effects of drag caused by air flow separation at the truncated ends are minimal. In example embodiments suitable for tractor trailers, the drag reducing devices may have extended lengths greater than 4 feet, or in the range of 6 to 22 feet, or in the range of 8-14 feet. Vehicles having smaller heights and widths could be equipped with proportionally smaller drag reduction devices. In certain embodiments, the ratio of the extended length of the drag reduction device to a reference dimension of the vehicle is at least 1 to 1. The reference dimension is typically the smaller of the width or the height of the rear of the vehicle body. In the embodiment of  FIG. 1F , the width w is the reference dimension since it is smaller than the height h. In other embodiments, this ratio is at least 1.5 to 1, or at least 2 to 1, or at least 3 to 1.  
         [0161]     In certain embodiments, drag reducing devices in accordance with the present disclosure may be automatically extended or retracted. A control system may be used to automatically control extension and retraction. In certain embodiments, vehicle speed, crosswind speed, or other vehicle parameters may be used to automatically control retraction extension. For example, a controller may sense vehicle speed, and automatically cause retraction of the drag reducing device if the vehicle speed moves below a given speed value (e.g., 45 miles per hour). In another example, a controller may sense crosswind speed, and automatically cause retraction if crosswinds exceed a given value (e.g., 25 miles per hour).  
         [0162]     Retractable drag reducing devices in accordance with the present disclosure can have relatively long extended lengths as compared to retracted lengths. Certain embodiments have an extended length to retracted length ratio of at least 6 to 1. Other embodiments have extended length to retracted length ratios of at least 10 to 1 or at least 20 to 1.  
         [0163]     While specific angles and lengths have been specified for certain embodiments of the present disclosure, it will be appreciated that the broad aspects of the present disclosure are not limited to these values.  
         [0164]     From the foregoing it will be observed that numerous modifications and variations can be effectuated without departing from the true spirit and scope of the novel concepts of the present invention. It is to be understood that no limitation with respect to the specific embodiments illustrated is intended or should be inferred.