Abstract:
An unmanned aerial vehicle selectively formed of high strength composite structural part portions and lightweight aerodynamic foam portions to provide a low-cost and lightweight UAV that comports with export, civil airspace, and safety regulations. To further to reduce an overall weight of the UAV, mechanical elements are designed to provide multiple functionalities. Structural elements may be manufactured in same or similar non-specialized processes, and non-structural elements manufactured in same or similar non-specialized processes, reducing overall manufacturing costs. Materials and bonding elements are selected to provide frangibility and yet maintain normal flight structural integrity.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application also claims priority to provisional U.S. Application No. 61/120,681, filed on Dec. 8, 2008, the entire contents of which are herein incorporated by reference. 
    
    
     BACKGROUND 
     1. Field Of The Invention 
     The present invention relates, in general, to the field of unmanned aerial vehicle (UAVs). More specifically, it is directed to the field of UAVs capable of vertical take-offs and landings (VTOLs) with one or more ducted fans as the propulsion system. 
     2. Description of the Related Art 
     An unmanned aerial vehicle (UAV) is an unpiloted and/or remotely controlled aircraft. UAVs can be either remotely controlled or flown autonomously based on pre-programmed flight plans or more complex dynamic automation and vision systems. UAVs are currently used in a number of military roles, including reconnaissance and attack scenarios. An armed UAV is known as an unmanned combat air vehicle (UCAV). 
     UAVs are also used in a limited number of civil applications such as firefighting when a human observer would be at risk, police observation of civil disturbances and crime scenes, and reconnaissance support in natural disasters. UAVs are often preferred for missions that are too dull, dirty, dangerous, or expensive for manned aircraft. 
     There are a wide variety of UAV shapes, sizes, configurations, and characteristics. Modern UAVs are capable of controlled, sustained, level flight and are powered by one or more jets, reciprocating engines, or ducted fans. 
     Payloads carried by UAVs in civil applications normally include an optical sensor (which may capture image or video in the visible or infrared spectrums, for example) and/or a radar system. A UAV&#39;s sophisticated sensors can provide photographic-like images through clouds, rain or fog, and in daytime or nighttime conditions; all in real-time. A concept of coherent change detection in synthetic aperture radar images, for example, allows for search and rescue abilities by determining how terrain has changed over time. 
     Providing a vertical takeoff and landing (VTOL) capability allows improved portability and allows a UAV to maneuver into situations and be utilized in areas that a fixed-wing aircraft may not. 
     While UAV&#39;s have been utilized extensively in military roles, their use in civil applications has been limited due to cost concerns, export regulations (such as International Traffic in Arms Regulations—ITAR), civil airspace regulations, and safety regulations, for example. Additionally, various regulations related to autonomous flying objects having a weight that could pose a hazard to life and/or property may have limited further penetration of UAV&#39;s into civil applications. 
     SUMMARY 
     The present application is directed to a vertical take off and landing (VTOL) unmanned aerial vehicle (UAV) that is selectively constructed out of composites, metals, and foams in a manner so as to reduce weight and cost, allow for controlled disintegration by selective implementation of frangible elements, and provide an overall endurance at least approximately equivalent to larger liquid fueled UAV&#39;s, for example. A UAV design is provided that separates aerodynamic elements from structural elements, and chooses corresponding materials for each class of elements so as to reduce weight, maintain high structural integrity, and provides a frangible airframe structure. For example, aerodynamic elements previously formed of complex composite layups or metal materials may be replaced with lighter foam injection molded materials. Structural components may then be selectively formed of composite materials having varying strengths, weights, and costs depending on the applied forces that each structural component is expected to encounter. Individual selected items in the airframe structure may themselves be constructed of frangible materials that disintegrate upon impact. By design, parts of the airframe separate when stress levels are exceeded in order to, for example to avoid damage to an object with which it comes into contact with, such as for example, a small manned aircraft. 
     By selectively reducing component weight, the overall weight of a completed UAV may fall below the limit of many civil regulations as posing a hazard to life and property, thus allowing purchase and operation of the UAV without the additional expenses of obtaining permits and/or special insurance, for example. 
     A UAV may have several inter-linked components, including a motor, a fan blade assembly, duct rings, stator mounts, motor mounts, stator slipstreams, inter-duct slipstreams, inter-duct interconnects, avionics mounting tubes, landing gear mount, avionics mount, avionics interconnects, vane shaft, tail cone lid, tail cone hinge, tail cone latch, servo gear, landing legs, duct lip, vanes, tail cone, and tail bumper, for example. 
     Other embodiments may include additional components, substituted components, or a subset of these components. 
     Structural support elements such as the motor, fan blade assembly, duct rings, stator mounts, motor mounts, stator slipstreams, inter-duct slipstreams, inter-duct interconnects, avionics mounting tubes, landing gear mount, avionics mount, avionics interconnects, vane shaft, tail cone lid, tail cone hinge, tail cone latch, servo gear, and landing legs may be manufactured of wood, metal, or composite materials in order to maintain high structural integrity, for example. Aerodynamic elements such as the duct lip, vanes, tail cone, and tail bumper may be manufactured of a foam material, for example. 
     Structural interconnect components such as inter-duct interconnects, landing gear mounts, avionics mount, and motor mounts may be designed with composite materials of a specific fiber length to provide sufficient structural integrity but that will separate or disintegrate with the application of sufficiently high stress levels so as to provide structural frangibility. The selective implementation of frangible interconnect components can provide further or additional control over the size and mass of the disintegrated fragments. An objective is balance the normal maneuvering stress and strain against impact and tear strength properties of selected joining points in the airframe so as to fragment the airframe into smaller, lower impact masses. Materials are selected, such as carbon fiber and foams, for key structural points that do not show plastic deformation but fail while the deformation is elastic. For a mid-air collision, the impact and tear strength properties are exceeded causing the airframe to disintegrate into desired mass fragments. Management of elastic, shear modulus, tensile strength, impact strength, and tear strength of the materials at key joining points in the airframe provides the desired frangibility on an application by application basis. 
     Bonding and attachment mechanisms between duct lip, and tail cones may be accomplished by separable bonding materials that enable replacement of damaged parts and/or support impact frangibility. This may be accomplished by bonding agents such as an RTV compound or by an O-ring mechanism that when stretched releases the attachment. 
     The one or more motors for powering the one or more ducted fans can be electric motors powered by an on-board electrical power source, such as a battery. The use of an electric motor can minimize vibration and decrease maintenance costs. 
     In another embodiment, mechanical elements can be designed to provide multiple functionalities. For example, a duct ring supporting a light-weight aerodynamic air duct may also serve as an engine mount, connecting tubes that provide resonance rejection for vibration may also serve as shielded wiring channels, and inter-duct attach points may also serve as landing leg mounts. 
     Structural elements such as a duct ring, an interduct slip-stream, and/or a duct lip could provide dual roles including an serving as an energy storage device such as a battery or fuel cell component integrated into the airframe structure. 
     Preferably, a UAV constructed according to the instant disclosure should have a total system weight of equal to or less than 4 pounds. 
     In example embodiments, mechanical elements are provided with two functions, which may reduce total component cost and overall cost of a UAV structure. The structural elements may be manufactured in a same or similar non-specialized process, and non-structural elements may be manufactured in same or similar non-specialized process, reducing overall manufacturing costs. 
     Other features and further scope of applicability of disclosed embodiments are set forth in the detailed description to follow, taken in conjunction with the accompanying drawings, and will become apparent to those skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an Unmanned Aerial Vehicle (UAV) according to an embodiment. 
         FIG. 2  is a cut-away perspective view of the UAV of  FIG. 1 . 
         FIG. 3  is a perspective view of a UAV with the landing legs in the stowed position. 
         FIG. 4  illustrates an example expanded duct assembly having a duct lip and duct ring. 
         FIG. 5(   a ) is a top perspective view of an example motor mount. 
         FIG. 5(   b ) is a bottom perspective view of the example motor mount. 
         FIG. 6  illustrates an example stator slipstream mount. 
         FIG. 7  illustrates an example combined stator, leg, and connecter tube mount structure. 
         FIG. 8  illustrates an example connector tube for use as a stator slipstream and/or a duct linkage tube. 
         FIG. 9  illustrates an example payload mounted on example payload support tubes. 
         FIG. 10  illustrates a tail cone and servo structure. 
         FIG. 11  is a perspective view of a fan assembly including a motor mount and tail cone in which the tail cone is rotated to a stowed position. 
         FIG. 12  is a perspective view of an Unmanned Aerial Vehicle (UAV) according to an alternate embodiment. 
         FIG. 13  is a close-up view of an example stator slipstream, leg, duct linkage, and connector of  FIG. 12 . 
         FIG. 14  is a close-up view of the example connector of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aspects of the present application describe a system and method for construction of a light-weight unmanned aerial vehicle (UAV). Aerodynamic and structural elements are selected corresponding to required performance characteristics. Mechanical elements are designed to provide dual functionality and decrease a part count and cost of a corresponding UAV. 
     Although the following disclosure references a double ducted hovering air-vehicle, it should be appreciated that the present embodiments have a broader applicability in the field of air-borne vehicles. Particular configurations discussed in examples can be varied and are cited to illustrate example embodiments. 
     As set forth in  FIG. 1 , a UAV  100  according to one embodiment includes ducted fan assemblies  102  interconnected via duct linkage assembly  104 . The UAV  100  includes legs  120  to keep the fan assemblies  102  and duct linkage assembly  104  from touching the ground. Although  FIG. 1  sets forth two ducted fan assemblies, any number of ducted fan assemblies could be used consistent with this disclosure. For example, a single ducted fan assembly could be used without the need for duct linkage assembly. Alternately, three or more ducted fan assemblies could be interconnected via one or more duct linkage assemblies. 
     Each ducted fan assembly  102  may include a light-weight aerodynamic duct lip  106  structurally supported by a structurally rigid duct ring  108  extending around an outer circumference of the duct lip  106 . The duct lip  106  has a cylindrically shaped lower wall  105  that extends slightly outward in a radial direction towards an upper end of the wall  105  and then bends outwardly and downwardly over itself to form an upper shoulder portion  107 . 
     Housed within the duct lip  106  and duct ring  108  is a motor  109  mounted onto a motor mount  110 . The motor mount  110  is in turn secured to the duct ring  108  via stator slipstreams  112 . The stator slipstreams  112  are secured to the duct ring  108  via stator mounts  113 . While  FIG. 1  discloses three stator slipstreams  112 , more than three could also be provided. As shown in  FIG. 1 , a first two of the stator slipstreams  112  are placed at an angle A of less than about 60° apart in order to connect with the duct linkage assembly  104  and provide rigid support for the UAV  100 . The third stator slipstream is placed at an angle B of greater than about 60° from either of the first two of the stator slipstreams  112 . The angles A and B are set forth as examples only. Other stator slipstream arrangements could also be used. 
     The stator slipstreams  112  may pass through holes formed in the duct lip  106  and duct ring  108  to reach the stator mounts  113 . The stator mounts  113  may be provided on an outer surface of the duct ring  108  to receive and secure one end of the stator slipstreams  112 . 
     Two or more propeller blades  111  may be provided in each ducted fan assembly  102  connected to the motor  109  to provide lift to the UAV. The number of propeller blades  111  is variable and is preferably designed to match motor loading requirements and rpm efficiencies. The motor  109  is preferably an electric motor, for example, a brushless direct current (DC) motor powered by a separately provided battery. 
     As shown in  FIG. 2 , a tail cone assembly  202  is provided including a hollow tail cone  203 , a control vane assembly  114 , and a servo  204  for tilting vanes  206  relative to a general air flow direction C. The tilt of the vanes  206  relative to the general air flow direction C generates a change in outgoing thrust direction, causing the UAV  100  to move in a corresponding desired direction. A control vane assembly  114  disposed at a lower end of the tail cone  203  includes two oppositely opposed vanes  206  connected via a shaft  208 . The shaft  208  is preferably connected to the vanes  206  at a point offset forward from the center of lift of each vane  206 . The servo  206  functions to rotate the shaft  208 , and therefore the vanes  206 , relative to a control signal received from a control circuit. Connection of the molded foam vane  206  to the shaft  208  may be accomplished by a split shaft in a tuning fork configuration, or a shaft extending a flattened perforated surface within the vane  206  to give it rigidity. 
     As set forth in  FIGS. 1 and 2 , the UAV  100  includes a duct linkage assembly  104  for rigidly connecting the two ducted fan assemblies  102 . The duct linkage assembly  104  includes a plurality of inter-duct slipstreams  116  and a plurality of inter-duct interconnects  118 . Each of the inter-duct interconnects  118  connects at one end to a stator mount  113  that is receiving one of two closely spaced stator slipstreams  212 , and at the other end to an inter-duct slipstream  116 . The inter-duct interconnects  118  of the UAV  100  in one alternative may include a vertical displacement characteristic such that the inter-duct slipstreams  116  are vertically offset in a horizontal plane from a horizontal plane in which the stator slipstreams  212  are disposed. 
     Each inter-duct interconnect  118  may also serve as a landing leg interconnect and hold a corresponding landing leg  120  in place. Two or more inter-duct slipstreams  116  may be used to connect the two ducted fan assemblies  102 . 
     As shown in  FIG. 3 , each landing leg  120  may be attached to the inter-duct interconnect  118  in a spring loaded notched fashion so that the landing gear may be quickly rotated 90 degrees to place the UAV  100  in a stowed position. A shaft portion  121  of the landing gear  120  may be pinned to mate to notches (not shown) on the inter-duct interconnect  118 . A spring on an opposing side of the inter-duct interconnect  118  may hold the landing gear in position and may provide some landing shock absorption. Landing leg ball elements  123  attached to a lower end of each shaft portion  121  may be formed of a soft deformable rubber, for example, in order to provide for additional landing shock absorption. 
     Returning to  FIG. 2 , the inter-duct slipstreams  116  may also support one or more avionics mounting tubes  122  via avionics interconnects  124 . The avionics mounting tubes  122  may provide an avionics mounting area in which an avionics payload  126  may be mounted. An avionics payload  126  may include, for example, control systems, wireless remote control interfaces, a battery source, and/or other flight-enabling systems. Alternatively, and as set forth in  FIG. 3 , an avionics mounting plate  302  that integrates avionics interconnects into one assembly may be directly mounted to the inter-duct slipstreams  116 . 
       FIG. 4  sets forth a detailed view of the duct lip  106  and duct ring  108 . As set forth earlier, the duct lip has a cylindrically shaped lower wall  105  that projects slightly outward in a radial direction towards an upper end of the wall  105 . As best shown in  FIG. 2 , an upper portion of the wall  105  extends into an outwardly and downwardly bending shoulder portion  107 . The lower edge  105  of the duct lip  106  mates with the inside surface of the duct ring  108  and the flange  404  formed on an outside surface of the duct ring  108  to position and secure the duct lip  106  to the duct ring  108 . Alternatively the duct lip  106  may have a flange molded into its surface that mates with the duct ring  108  so as to eliminate the need for the protruding flange  404  on the duct ring  108 . 
     The duct lip  106  may instead be attached to the duct ring  108  by application of removable bonding agents to the lower portion of the duct lip  106  and upper outside portion of the duct ring  108 . Alternatively, a mechanical snapping or O-ring mechanism could be placed at regular intervals around the ducted fan assemblies  102 . Of course, other methods of securing the duct lip  106  to the duct ring  108  could also be used. 
     The duct ring  108  includes holes  402  patterned in accordance with the distribution pattern of the stator slipstreams  112 . The outwardly protruding flange  404  is formed along an outer circumference of the duct ring  108  near an upper edge of the duct ring  108 . The flange  404  aids in the placement of the stator mounts  113  on the outer cylindrical surface wall of the duct ring  108  and adds circular rigidity to the duct ring  108 . 
       FIGS. 5(   a ) and  5 ( b ) set forth a top and bottom perspective view, respectively, of the motor mount  110 . As shown in  FIG. 5(   b ), the motor mount  110  includes an upper surface  514 , a first outer cylindrically-shaped wall  510  extending perpendicularly from an outer circumference of the upper surface  514 , and a second inner cylindrically-shaped wall  512  disposed within the outer wall  510  and extending from the upper surface  514  in a same direction as the outer wall  510 . Through holes  502  and  518  are correspondingly formed in the first and second walls in a pattern corresponding to the distribution of the stator slipstreams  112 . 
     A wiring hole  506  formed in the upper surface  514  and a notch  516  formed in the inner cylindrical wall  512  allows a wiring (not shown) to be fed into and out of the motor mount  510 . Wiring is fed from the motor through the wiring hole  506 , across the notch  516  and into one or more of the stator slipstreams  212 , and then through the inter-duct slipstreams  116  to the avionics payload  126 . A centrally disposed motor drive hole  504  is formed in the upper surface  514  to allow a drive shaft of the motor  109  to extend through the motor mount  110  so as to drive one or more propellers  111 . Additional holes  508  are provided in the upper surface  414  for securely mounting a motor  109  to the motor mount  110 . O-ring posts  520  extend radially outward from the outer wall  510  and provide an attachment mechanism for the tail cone assembly  202 . Alignment tabs  132  extend axially from a bottom surface of the outer wall  510  and mate with corresponding alignment slots formed in the tail cone assembly  202  to properly orient the tail cone assembly  202  to the motor mount  110  and thus to the UAV  100 . 
       FIG. 6  sets forth a perspective view of a stator mount  113 . The stator mount  113  has a rectangular-shaped outer-band frame  602  that is curved slightly inward to match a curvature of the duct ring  108 . A thicker central section  604  is formed integrally with the frame  602  and includes holes  606  and  608 . The through hole  606  is formed in a tear-drop shape substantially the same as the shape of the stator slipstreams  112  so as to rigidly secure the stator mount  113  to a stator slipstream  112 . Other shapes could also be used. Alternatively, and as illustrated in  FIG. 11 , the stator slipstreams  112  could be directly bonded to the duct ring  108  by, for example, applying an adhesive agent or by a type of soldering process. 
     Returning to  FIG. 6 , the holes  608  of the stator mount  113  may serve a dual purpose depending upon which stator slipstream  112  a stator mount  113  is secured to. As further set forth in  FIG. 2 , the stator mounts  113  securing the two closely-angled stator slipstreams  212  may be connected to the inter-duct interconnects  118  via parallel, closely spaced pegs  214 . The holes  608  of the stator mounts  112  are sized accordingly to receive and secure the pegs  214 . The pegs  214  may also be used to secure the stator mount  113  to the duct ring  108 . 
     Alternately, for the stator mount  113  positioned relative to the stator slipstream  112  spaced at a large angular distance relative to the other two stator slipstreams  212 , the holes  608  may be used to secure the stator mount  113  to the duct ring  108  via a fastener such as a rivet or screw, for example. 
       FIG. 7  sets forth a perspective view of an inter-duct interconnect  118  while connected to a stator mount  113 . The inter-duct interconnect  118  includes two parallel, closely spaced pegs  214 , as mentioned earlier, for securedly connecting the inter-duct interconnect  118  with the correspondingly sized holes  608  in the stator mount  113 . The pegs  214  may also secure a landing gear mount  701  and an avionics mount  706 . The landing gear mount  701  includes a hole  702  for securing a landing gear  120 , as shown in  FIG. 2 . The avionic mount  706  includes a hole  708  formed in an upper surface for receiving a similarly shaped projection formed on an avionics attachment  130 , as seen in  FIG. 2 . By providing an interconnect structure  118  that connects a ducted fan assembly  102  to duct linkage assembly  104 , and also includes a landing gear mount  701 , the number of components required by a UAV  100  may be reduced, and a total weight and cost of the UAV  100  can be correspondingly reduced. 
       FIG. 8  sets forth a perspective view of a stator slipstream  112 . The stator slipstream  112  of  FIG. 8  could also be used as the stator slipstreams  212  of  FIG. 2 . The stator slipstream  112  performs several functions, including securing the motor  109  and motor mount  110  within the ducted fan assembly  102  to the rigid duct ring  108  and to a stator mount  113 , routing power and control cables from the avionics payload  126  to the motor  109  and servo  204  in the ducted fan assembly  102 , providing electromagnetic shielding of any and all control and power cables passing there through, and providing an aerodynamic slipstream surface to air flowing through the ducted fan assembly  102  to improve performance and handling of the UAV  100 . 
     The stator slipstream  112  may be formed in a tear-drop shape to match the shapes of the holes  502  formed in the motor mount  110 , the holes  402  formed in the duct ring  108 , and the holes  606  formed in the stator mount  113 . As shown in  FIG. 8 , the tear-drop shape is comprised of a smooth rounded top-end  802  and a more pointed bottom-end  806 . The tear-drop shape of the stator slipstream  112  minimizes air-resistance created by the stator slipstream  112  within the ducted fan assembly  102 . Of course, other shapes could also be used. 
     The stator slipstream  112  is formed to be hollow, providing a wiring path  704  within the stator slipstream  112 . By routing wiring from the avionics payload  126  to the motor  109  and/or servo  204  through the stator slipstreams  112 , drag can be eliminated compared to routing the wiring along an outside surface of the stator slipstreams  112 , and the walls of the stator slipstream and function to electromagnetically shield the cables passing there through. 
       FIG. 9  sets forth a perspective view of the avionics payload  126  and mounting tubes  122 . The mounting tubes  122  are spaced in a parallel manner with a pre-determined distance there between. An outer casing  902  is sized at a width equal to or slightly larger than the width of the tubes  122  and electronics contained therein so as to be able to slide over the mounting tubes  122  and cover the electronics. Inside the outer casing  902  may be housed a battery  904 , one or more motor controllers  906 , and avionics control systems  908 . The avionics control systems  908  may include, among other things, a radio RF transceiver for receiving commands and sending information, translation control system for translating received control commands into signals to control motors and/or tail cone servos, stability control systems for maintaining flight stability, an autonomous flight control system, a global positioning system (GPS), and/or a video image encoding system. Additionally, a subset of, and/or alternative systems may also be included in the avionics control systems  908 . 
       FIG. 10  sets forth a perspective view of a tail cone assembly  202 . The tail cone assembly  202  comprises a hollow tail cone  203 , an end bumper  1012 , a vane shaft  1002 , vanes  114 , servo  1004 , and mounting plate  1006 . The hollow tail cone  203  has an inverted cone shape having a linear taper in which a narrow end of the cone is cut-off before reaching its peak. At the lower end of the hollow tail cone  203 , a solid half-sphere bumper  1012  is provided to close off a lower end of the hollow open-ended cone  203 . 
     On an upper end of the tail cone  203 , a tail cone mounting plate  1006  is provided having a hinge  1008  on one end thereof and a latch  1010  on the other. As shown in  FIG. 11 , a corresponding motor mounting plate  1102  is attached to a lower surface of the outer wall  510  of the motor mount  110 . The latch  1010  allows the tail cone mounting plate  1006  to attach and detach from the motor mounting plate  1102 . The hinge  1008  allows the tail cone mounting plate  1006  to hingedly attach the tail cone  202  to the motor mount  110  and allows the tail cone  202  to rotate between an open position in which the tail cone is stowed within the ducted fan assembly between the stator slipstream  112  and one of the state slipstreams  212 , and a closed position in which the tail cone is rigidly attached to the motor mount  110 . A servo mount  1013  is provided on a bottom surface of the tail cone mounting plate  1006  for holding the servo  1004 . 
     In accordance with one embodiment, compositions of respective aerodynamic and structural support elements noted above are selectively chosen to reduce an overall weight of the UAV  100  while maintaining a structural integrity of the UAV  100 . 
     Structural support elements such as the duct rings  108 , stator mounts  113 , motor mount  110 , stator slipstreams  112 , inter-duct slipstreams  116 , inter-duct interconnects  118 , avionics mounting tubes  122 , landing gear mount  602 , avionics mount  606 , avionics interconnects  124 , vane shaft  902 , tail cone lid  906 , tail cone hinge  908 , tail cone latch  910 , servo gear  914 , and landing legs  120  may be manufactured of wood, metal, or composite materials in order to maintain high structural integrity. The wood may be, for example, bent red oak dowel. Other woods could also be used. The metal may be, for example, machined or extruded aluminum. Other metals or metal alloys could also be used. The composite may be, for example, thermoplastics, including polyetheretherketone (PEEK), polyamide-imide (Torlon), amorphous polyetherimide (Ultem), Nylon 6, Nylon 12, or Nylon 66; or may be, for example, thermosets including Lytex, epoxy, or bismaleimide (BMI). Other composites could also be used. The composites may be formed via an injection molding processing, a compression molding process, a selective laser sintering process, a thermoforming process, or an autoclave/over cure process, for example. 
     Specifically, for example, the duct ring  108  may be formed of a prepeg plain weave epoxy fabric. The avionics mounting tubes  122 , vane shaft  902 , tail cone lid  906 , stator slipstreams  112 , and inter-duct slipstreams  116  may be formed of aluminum. The landing leg  120  may be formed of oak, composite, or aluminum. The tail cone hinge  908 , tail cone latch  910 , and servo gear  914  may be formed of nylon. The stator mounts  113 , motor mounts  110 , inter-duct interconnects  118 , avionics interconnects  124 , landing gear mount  602 , and avionics mount  606  may be formed of an injection molded composite. The injection molded composite could be nylon similar to that used for the tail cone hinge  908 , for example. The selection of the materials in key frangibility locations such as the stator mount  113  or inter-duct interconnect  1018  and motor mounts  110  are made based on expected impact stress levels that define fiber lengths and resin materials necessary to maintain structural integrity. Semi-rigid foam materials, that disintegrate upon impact (having tensile strengths below 100) can be applied to aerodynamic surfaces such as duct lip, control vanes and tail cones. Short fiber composites that exhibit a brittle nature (shear modulus of approximately 5.9e+05, modulus of elasticity of approximately 6.5e+05, ultimate tensile strength of approximately 7250) on impact can be applied to the structural interconnecting joints and leg mounts and motor mounts that separate on high speed impacts. However, it should be understood that these are just examples, and that structural components may be formed of any one or more of the high rigidity materials noted earlier. 
     By providing rigid structural materials, an increased rigidity of a light weight UAV  100  can be provided, and the length of service and durability of the light weight UAV  100  may be improved. 
     Aerodynamic elements such as the duct lip  106 , vanes  114 , tail cone  203 , and tail bumper  912  may be manufactured of a foam material. The foam material may be, for example, a soft or rigid foam including expanded polyethylene (EPE), Low Density Polyethylene (LDPE), expanded polystyrene (EPS), or expanded polypropylene (EPP). The foam materials may be formed via a foam injection molding process. For example, the duct lip  106  may be formed of a molded EPE foam material. The aerodynamic elements can be formed separately from (e.g. not integrally with) and in a manner to be detachably connected to the structural components so as to allow for easy replacement of the light-weight aerodynamic parts as necessary. 
     Additional or other UAV structural or aerodynamic elements may be comprised of a corresponding structural or aerodynamic material. 
     A UAV  100  comprised of a mix of lighter weight aerodynamic elements and more rigid supporting elements preferably has a total system weight of about or under 4 pounds. More preferably, a UAV  100  comprised of a mix of lighter weight aerodynamic elements and more rigid supporting elements preferably has a total system weight of about or under 2 pounds. Importantly, however the disclosed low weight and frangibility techniques can also be applied to larger airframes. 
     For example, each ducted fan assembly  102  preferably has a total weight of equal to or less than 0.786 lbs. Each tail cone assembly  202  preferably has a total weight of equal to or less than 0.114 lbs. Each duct linkage assembly  104  preferably has a total weight of equal to or less than 0.217 lbs. A total weight of the UAV  100 , including battery  804 , is preferably equal to or less than 2.04 lbs. 
     The motor  109  may be an electric motor powered by the battery  804  contained in the avionics payload  126 . The battery  804  could be, for example, a lithium-based power source including, lithium, lithium-ion, lithium-polymer, and/or lithium aluminum hydride batteries. Of course, any other type of battery, including a fuel cell, could be used, as long as it provides sufficient power to drive the motor and sufficient power density to provide an extended operating time period. The use of an electric motor also minimizes vibration and decreases maintenance costs in the UAV  100 . 
       FIG. 12  sets forth an alternate embodiment of a UAV  1200 . Each ducted fan assembly  1202  is the same or similar to that of the ducted fan assemblies  102  of UAV  100 . However, UAV  1200  includes a duct linkage assembly  1204  with a parallel set of inter-duct slipstreams  1210  that are displaced in a same horizontal plane as stator slipstreams  1216 . In the embodiment of  FIG. 12 , the stator slipstreams  1216  of UAV  1200  extend beyond the duct ring  1208  and connect with inter-duct interconnects  1218 . 
     As shown in  FIG. 13 , the inter-duct interconnects  1218  include an angular offset between a stator slipstream interface side  1302  and an inter-duct slipstream interface side  1304 . The angular offset may be, for example, in the range of 15-45°. Most preferably, the angle is approximately 30°.  FIGS. 14(   a ) and  14 ( b ) show alternate views of the inter-duct interconnect  1218 . Each inter-duct interconnect  1218  includes a landing leg through hole  1406  and  1408  formed respectively on each of the stator slipstream interface side  1302  and inter-duct slipstream interface side  1304 . One of the two landing leg through holes  1406  and  1408  is utilized in each of the four positions of the duct linkage assembly  1004 . By forming two leg through holes  1406 / 1408  in each inter-duct interconnect, the part becomes common for all four positions. In this manner, the same manufactured inter-duct interconnect  1218  design can be used on either side of the duct linkage assembly  1204  to connect a landing leg  1220  to the UAV  1200 . A shaft of each landing gear  1220  is pinned to mate to notches the through holes  1206  and  1208  of the inter-duct interconnect  1218 . A spring (not shown) on the opposite side of the inter-duct interconnect holds the landing gear  1220  in position along with a washer and screw. 
     As shown in  FIG. 14(   b ), each inter-duct interconnect  1218  includes tear-shaped holes  1402  and  1404  for interfacing with similarly-shaped inter-duct slipstreams  1210  and similarly-shaped stator slipstreams  1216 . 
     The disclosure above regarding UAV  100  can be equally applied to UAV  1200 , including choices of structural and aerodynamic elements, structural element composition, aerodynamic element composition, and overall weight. The inter-duct interconnects  1218  are structural elements and may be made of an injection molded composite, for example, nylon. Furthermore, although not shown in  FIG. 12 , an avionics payload and mounting tubes could also be attached to the UAV  1200  of  FIG. 12  to provide a centrally-located power source and flight control circuitry. 
     Note that while examples have been described in conjunction with present embodiments of the application, persons of skill in the art will appreciate that variations may be made without departure from the scope and spirit of the application. The true scope and spirit of the application is defined by the appended claims, which may be interpreted in light of the foregoing.