Patent Publication Number: US-2021163120-A1

Title: Unmanned aerial vehicle and associated method for reducing drag during flight of an unmanned aerial vehicle

Description:
PRIORITY 
     This application is a non-provisional of U.S. Ser. No. 62/942,976 filed on Dec. 3, 2019. 
    
    
     FIELD 
     This application relates to unmanned aerial vehicles and, more particularly, to the use of fairings to reduce drag during flight of unmanned aerial vehicles. 
     BACKGROUND 
     Unmanned aerial vehicles (UAVs) are increasingly used for performing a variety of functions in civilian, commercial, and military applications. For example, UAVs may be implemented for delivering payloads, performing emergency services such as firefighting management, locating schools of fish, and other functions. 
     Unmanned aerial vehicles are typically powered by one or more batteries. Therefore, unmanned aerial vehicles remain in service for only a limited amounted of time (i.e., the service duration), as dictated by battery life. Once a battery on an unmanned aerial vehicle is exhausted, the unmanned aerial vehicle must be taken out of service while the exhausted battery is charged or while the exhausted battery is swapped with a charged battery. 
     Various factors contribute to battery life and, thus, the service duration of an unmanned aerial vehicle. One particular factor is drag. The greater the drag, the more energy is consumed by the unmanned aerial vehicle per a given flight path, and the shorter the service duration. 
     Accordingly, those skilled in the art continue with research and development efforts in the field of unmanned aerial vehicles. 
     SUMMARY 
     Disclosed are various unmanned aerial vehicles. 
     In one example, the disclosed unmanned aerial vehicle includes a frame elongated along a frame axis, the frame has a leading side and an aft side, the frame further includes a left end portion and a right end portion, a first forward rotor assembly connected to the left end portion of the frame, a second forward rotor assembly connected to the right end portion of the frame, the first forward rotor assembly and the second forward rotor assembly being positioned on the leading side of the frame, a compartment connected to the frame, the compartment having a leading side, and a curved leading-edge fairing disposed on the frame, wherein a portion of the curved leading-edge fairing extends to cover the leading side of the compartment, and wherein the curved leading-edge fairing reduces drag during flight in a forward direction to enable a substantially level flight profile. 
     In another example, the disclosed unmanned aerial vehicle includes a frame elongated along a frame axis, the frame having a leading side and an aft side, the frame further including a left end portion and a right end portion axially opposed from the left end portion relative to the frame axis, a first forward rotor assembly connected to the left end portion of the frame by way of a first forward boom, a first aft rotor assembly connected to the left end portion of the frame by way of a first aft boom, a first outboard rotor assembly connected to the left end portion of the frame by way of a first outboard boom, the first outboard boom having a leading side and an aft side, a first forward boom fairing positioned over the leading side of the first outboard boom, a second forward rotor assembly connected to the right end portion of the frame by way of a second forward boom, a second aft rotor assembly connected to the right end portion of the frame by way of a second aft boom, a second outboard rotor assembly connected to the right end portion of the frame by way of a second outboard boom, the second outboard boom having a leading side and an aft side, a second forward boom fairing positioned over the leading side of the second outboard boom, a compartment connected to the frame, the compartment having a leading side, and a curved leading-edge fairing disposed on the frame, wherein a portion of the curved leading-edge fairing extends to cover the leading side of the compartment. 
     Also disclosed are methods for reducing drag during flight of an unmanned aerial vehicle. The unmanned aerial vehicle includes a frame elongated along a frame axis, the frame having a leading side and an aft side, as well as a left end portion and a right end portion, at least two rotor assemblies connected to the left end portion of the frame by way of at least two first booms, and at least two rotor assemblies connected to the right end portion of the frame by way of at least two second booms. 
     In one example, the disclosed method for reducing drag during flight of such unmanned aerial vehicle includes steps of (1) positioning a curved leading-edge fairing over at least a portion of the leading side of the frame; (2) positioning a first forward boom fairing over a boom of the at least two first booms; and (3) positioning a second forward boom fairing over a boom of the at least two second booms. 
     Other examples of the disclosed unmanned aerial vehicles and methods for reducing drag during flight of an unmanned aerial vehicle will become apparent from the following detailed description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front perspective view of one example of the disclosed unmanned aerial vehicle; 
         FIG. 2  is a front elevational view of the unmanned aerial vehicle of  FIG. 1 ; 
         FIG. 3  is a side elevational view of the unmanned aerial vehicle of  FIG. 1 ; 
         FIG. 4  is a top plan view of the unmanned aerial vehicle of  FIG. 1 ; 
         FIG. 5  is a front elevational view of the unmanned aerial vehicle of  FIG. 1 , but without the curved leading-edge fairing and the boom fairings, thereby showing internal structure; 
         FIG. 6  is a front elevational view of the unmanned aerial vehicle of  FIG. 5 ; 
         FIG. 7  is a front perspective view of the curved leading-edge fairing of the unmanned aerial vehicle of  FIG. 1 ; 
         FIGS. 8 and 9  are front perspective view of the boom fairings of the unmanned aerial vehicle of  FIG. 1 ; 
         FIG. 10  is a flow diagram depicting one example of the disclosed method for reducing drag during flight of an unmanned aerial vehicle; 
         FIG. 11  is a flow diagram of an aircraft manufacturing and service methodology; and 
         FIG. 12  is a block diagram of an aircraft. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed are unmanned aerial vehicles that have been designed and configured for forward flight with a significantly reduced pitch attitude. For example, the disclosed unmanned aerial vehicles may have a pitch angle about the frame axis of less than 13 degrees (e.g., about 10 degrees or less) when traveling in a forward direction at airspeeds up to about 50 knots. As such, the disclosed unmanned aerial vehicles may experience significantly less drag, thereby significantly increasing service duration of the unmanned aerial vehicles. 
     Referring to  FIGS. 1-6 , one example of the disclosed unmanned aerial vehicle, generally designated  10 , includes a frame  12 , a first forward rotor assembly  30 , a second forward rotor assembly  40 , a first aft rotor assembly  50 , a second aft rotor assembly  60 , a first outboard rotor assembly  70 , a second outboard rotor assembly  80 , a compartment  90 , a curved leading-edge fairing  100 , a first forward boom fairing  120  and a second forward boom fairing  150 . The unmanned aerial vehicle  10  may further include a first aft boom fairing  130  and a second aft boom fairing  160 . In other words, the unmanned aerial vehicle  10  includes a pair of forward rotor assemblies  30 ,  40 , a pair of aft rotor assemblies  50 ,  60 , a pair of outboard rotor assemblies  70 ,  80 , and a pair of forward boom fairings  120 ,  150 , and a pair of aft boom fairings  130 ,  160 . Various additional components and features may be included in the disclosed unmanned aerial vehicle  10  without departing from the scope of the present disclosure. 
     As best shown in  FIGS. 5 and 6 , the frame  12  of the disclosed unmanned aerial vehicle  10  may include a truss  22  (e.g., a Brown-type truss) and may be elongated along a frame axis A F , which may be orthogonal to (e.g., lateral relative to) the forward direction F the unmanned aerial vehicle  10 . Therefore, the frame  12  may have a left end portion  18  and a right end portion  20  that is axially opposed from the left end portion  18  relative to the frame axis A F . The frame  12  may further have a leading side  14  and an aft side  16 , as well as an underside  24  and a topside  26 . 
     Still referring to  FIGS. 1-6 , the first forward rotor assembly  30  of the disclosed unmanned aerial vehicle  10  is connected to the left end portion  18  of the frame  12  on the leading side  14  of the frame  12 . For example, the unmanned aerial vehicle  10  may include a first forward boom  32  having a proximal end portion  31  ( FIG. 4 ) connected to the left end portion  18  of the frame  12  and a distal end portion  33  ( FIG. 4 ) connected to the first forward rotor assembly  30 . 
     The first forward rotor assembly  30  may include a rotor shaft  34 , a rotor  36 , and a motor  38 , as best shown in  FIGS. 1-3 . The rotor shaft  34  may define a rotor shaft axis A R1 , which may be slightly tilted inward (i.e., toward the right end portion  20  of the frame  12 ) and slightly tilted in the forward direction F. The rotor shaft  34  may be rotatable about the rotor shaft A R1 . The rotor  36  may be fixedly connected to the rotor shaft  34  such that the rotor  36  rotates with the rotor shaft  34 . The motor  38  (e.g., a battery-powered electric motor) may be operatively connected to the rotor shaft  34  to cause the rotor shaft  34  and, thus, the rotor  36 , to rotate about the rotor shaft axis A R1 . 
     In one particular implementation, the first forward rotor assembly  30  may include a pair or rotors  36 ,  37  (e.g., a pair of co-rotating rotors (same direction)). The second rotor  37  of the first forward rotor assembly  30  may be fixedly connected to a second rotor shaft  35  and may be powered by a second motor  39 . The motors  38 ,  39  of the first forward rotor assembly  30  may be configured as a coaxial motor stack. 
     The second forward rotor assembly  40  of the disclosed unmanned aerial vehicle  10  may be connected to the right end portion  20  of the frame  12  on the leading side  14  of the frame  12 . For example, the unmanned aerial vehicle  10  may include a second forward boom  42  having a proximal end portion  41  ( FIG. 4 ) connected to the right end portion  20  of the frame  12  and a distal end portion  43  ( FIG. 4 ) connected to the second forward rotor assembly  40 . Therefore, the unmanned aerial vehicle  10  may have a pair of forward booms  32 ,  42 . 
     The second forward rotor assembly  40  may include a rotor shaft  44 , a rotor  46 , and a motor  48 . The rotor shaft  44  may define a rotor shaft axis A R2 , which may be slightly tilted inward (i.e., toward the left end portion  18  of the frame  12 ) and slightly tilted in the forward direction F. The rotor shaft  44  may be rotatable about the rotor shaft A R2 . The rotor  46  may be fixedly connected to the rotor shaft  44  such that the rotor  46  rotates with the rotor shaft  44 . The motor  48  (e.g., a battery-powered electric motor) may be operatively connected to the rotor shaft  44  to cause the rotor shaft  44  and, thus, the rotor  46 , to rotate about the rotor shaft axis A R2 . 
     In one particular implementation, the second forward rotor assembly  40  may include a pair or rotors  46 ,  47  (e.g., a pair of co-rotating rotors (same direction)). The second rotor  47  of the second forward rotor assembly  40  may be fixedly connected to a second rotor shaft  45  and may be powered by a second motor  49 . The motors  48 ,  49  of the second forward rotor assembly  40  may be configured as a coaxial motor stack. 
     The first aft rotor assembly  50  of the disclosed unmanned aerial vehicle  10  may be connected to the left end portion  18  of the frame  12  on the aft side  16  of the frame  12 . For example, the unmanned aerial vehicle  10  may include a first aft boom  52  having a proximal end portion  51  ( FIG. 4 ) connected to the left end portion  18  of the frame  12  and a distal end portion  53  ( FIG. 4 ) connected to the first aft rotor assembly  50 . 
     The first aft rotor assembly  50  may include a rotor shaft  54 , a rotor  56 , and a motor  58 . The rotor shaft  54  may define a rotor shaft axis A R3 , which may be slightly tilted inward (i.e., toward the right end portion  20  of the frame  12 ). The rotor shaft  54  may be rotatable about the rotor shaft A R3 . The rotor  56  may be fixedly connected to the rotor shaft  54  such that the rotor  56  rotates with the rotor shaft  54 . The motor  58  (e.g., a battery-powered electric motor) may be operatively connected to the rotor shaft  54  to cause the rotor shaft  54  and, thus, the rotor  56 , to rotate about the rotor shaft axis A R3 . 
     In one particular implementation, the first aft rotor assembly  50  may include a pair or rotors  56 ,  57  (e.g., a pair of co-rotating rotors (same direction)). The second rotor  57  of the first aft rotor assembly  50  may be fixedly connected to a second rotor shaft  55  and may be powered by a second motor  59 . The motors  58 ,  59  of the first aft rotor assembly  50  may be configured as a coaxial motor stack. 
     The second aft rotor assembly  60  of the disclosed unmanned aerial vehicle  10  may be connected to the right end portion  20  of the frame  12  on the aft side  16  of the frame  12 . For example, the unmanned aerial vehicle  10  may include a second aft boom  62  having a proximal end portion  61  ( FIG. 4 ) connected to the right end portion  20  of the frame  12  and a distal end portion  63  ( FIG. 4 ) connected to the second aft rotor assembly  60 . Therefore, the unmanned aerial vehicle  10  may have a pair of aft booms  52 ,  62 . 
     The second aft rotor assembly  60  may include a rotor shaft  64 , a rotor  66 , and a motor  68 . The rotor shaft  64  may define a rotor shaft axis A R4 , which may be slightly tilted inward (i.e., toward the left end portion  18  of the frame  12 ). The rotor shaft  64  may be rotatable about the rotor shaft A R4 . The rotor  66  may be fixedly connected to the rotor shaft  64  such that the rotor  66  rotates with the rotor shaft  64 . The motor  68  (e.g., a battery-powered electric motor) may be operatively connected to the rotor shaft  64  to cause the rotor shaft  64  and, thus, the rotor  66 , to rotate about the rotor shaft axis A R4 . 
     In one particular implementation, the second aft rotor assembly  60  may include a pair or rotors  66 ,  67  (e.g., a pair of co-rotating rotors (same direction)). The second rotor  67  of the second aft rotor assembly  60  may be fixedly connected to a second rotor shaft  65  and may be powered by a second motor  69 . The motors  68 ,  69  of the second aft rotor assembly  60  may be configured as a coaxial motor stack. 
     The first outboard rotor assembly  70  of the disclosed unmanned aerial vehicle  10  may be connected to the left end portion  18  of the frame  12  and may protrude laterally outward from the frame  12  (e.g., along the frame axis A F ). For example, the unmanned aerial vehicle  10  may include a first outboard boom  72  having a leading side  110  and an aft side  112 , as well as a proximal end portion  114  and a distal end portion  116  opposed from the proximal end portion  114 . The proximal end portion  114  of the first outboard boom  72  may be connected to the left end portion  18  of the frame  12  and the distal end portion  116  of the first outboard boom  72  may be connected to the first outboard rotor assembly  70 . 
     The first outboard rotor assembly  70  may include a rotor shaft  74 , a rotor  76 , and a motor  78 . The rotor shaft  74  may define a rotor shaft axis A R5 , which may be slightly tilted in the forward direction F. The rotor shaft  74  may be rotatable about the rotor shaft A R5 . The rotor  76  may be fixedly connected to the rotor shaft  74  such that the rotor  76  rotates with the rotor shaft  74 . The motor  78  (e.g., a battery-powered electric motor) may be operatively connected to the rotor shaft  74  to cause the rotor shaft  74  and, thus, the rotor  76 , to rotate about the rotor shaft axis A R5 . 
     In one particular implementation, the first outboard rotor assembly  70  may include a pair or rotors  76 ,  77  (e.g., a pair of contra-rotating rotors). The second rotor  77  of the first outboard rotor assembly  70  may be fixedly connected to a second rotor shaft  75  and may be powered by a second motor  79 . The motors  78 ,  79  of the first outboard rotor assembly  70  may be configured as a coaxial motor stack. 
     The second outboard rotor assembly  80  of the disclosed unmanned aerial vehicle  10  may be connected to the right end portion  20  of the frame  12  and may protrude laterally outward from the frame  12  (e.g., along the frame axis A F ). For example, the unmanned aerial vehicle  10  may include a second outboard boom  82  having a leading side  140  and an aft side  142 , as well as a proximal end portion  144  and a distal end portion  146  opposed from the proximal end portion  144 . The proximal end portion  144  of the second outboard boom  82  may be connected to the right end portion  20  of the frame  12  and the distal end portion  146  of the second outboard boom  82  may be connected to the second outboard rotor assembly  80 . Therefore, the unmanned aerial vehicle  10  may have a pair of outboard booms  72 ,  82 . 
     The second outboard rotor assembly  80  may include a rotor shaft  84 , a rotor  86 , and a motor  88 . The rotor shaft  84  may define a rotor shaft axis A R6 , which may be slightly tilted in the forward direction F. The rotor shaft  84  may be rotatable about the rotor shaft A R6 . The rotor  86  may be fixedly connected to the rotor shaft  84  such that the rotor  86  rotates with the rotor shaft  84 . The motor  88  (e.g., a battery-powered electric motor) may be operatively connected to the rotor shaft  84  to cause the rotor shaft  84  and, thus, the rotor  86 , to rotate about the rotor shaft axis A R6 . 
     In one particular implementation, the second outboard rotor assembly  80  may include a pair or rotors  86 ,  87  (e.g., a pair of contra-rotating rotors). The second rotor  87  of the second outboard rotor assembly  80  may be fixedly connected to a second rotor shaft  85  and may be powered by a second motor  89 . The motors  88 ,  89  of the second outboard rotor assembly  80  may be configured as a coaxial motor stack. 
     The compartment  90  of the disclosed unmanned aerial vehicle  10  has a leading side  92  and an aft side opposed from the leading side  92 , and is connected to the frame  12 . As one example, the compartment  90  may be connected to the underside  24  of the frame  12 , as shown in  FIGS. 5 and 6 . As another example, the compartment  90  may be connected to the topside  26  of the frame  12 . 
     The compartment  90  may define a partially enclosed or a fully enclosed space for storing various items on the unmanned aerial vehicle  10 . As one example, the compartment  90  may be a battery compartment, and a battery  94  may be received (e.g., housed) in the compartment  90 . The battery  94  may be used to power the various motors  38 ,  39 ,  48 ,  49 ,  58 ,  59 ,  68 ,  68 ,  78 ,  79 ,  88 ,  89  of the unmanned aerial vehicle  10 . As another example, the compartment  90  may be a payload compartment. As yet another example, the compartment  90  may be configured for receiving both a battery  94  and a payload (not shown). 
     The unmanned aerial vehicle  10  may further include landing gear  96 . The landing gear  96  may be connected to the frame  12 , such as to the underside  24  of the frame  12 . With the landing gear  96  positioned on the underside  24  of the frame  12 , the landing gear  96  may elevate the frame  12  above any structure (e.g., landing pad (not shown)) upon which the unmanned aerial vehicle  10  is resting. 
     The curved leading-edge fairing  100  of the disclosed unmanned aerial vehicle  10  is positioned over at least a portion of the leading side  14  of the frame  12  and at least a portion of the leading side  92  of the compartment  90 . In one particular construction, the curved leading-edge fairing  100  substantially completely covers (e.g., covers at least  90  percent of) the leading side  14  of the frame  12  and the leading side  92  of the compartment  90 . For example, the curved leading-edge fairing  100  may be disposed relative to the frame  12  such that a first portion  101  of the curved leading-edge fairing  100  extends over the leading side  14  of the frame  12  and a second portion  103  extends over the leading side  92  of the compartment  90 . 
     As shown in  FIG. 7 , the curved leading-edge fairing  100  may include an upper edge  102 , a lower edge  104 A associated with the first portion  101  of the curved leading-edge fairing  100 , and a lower edge  104 B associated with the second portion  103  of the curved leading-edge fairing  100 , and may have a continuous curvature  106  from the upper edge  102  to the lower edge  104 A and a continuous curvature  106  from the upper edge  102  to the lower edge  104 B. For example, from the upper edge  102  to the lower edge  104 A, the curved leading-edge fairing  100  has a minimum radius of curvature of at least about 3 inches. Therefore, the curved leading-edge fairing  100  may have an aerodynamic shape that reduces drag during flight of the unmanned aerial vehicle  10  in a forward direction F ( FIG. 1 ) to enable a substantially level flight profile. Within examples, the substantially level flight profile is a pitch angle Θ of the frame  12  about the frame axis A F , during flight in a forward direction F, of 0 degrees up to about 13 degrees, and more preferably a pitch angle Θ of 0 to about 10 degrees. 
     The curved leading-edge fairing  100  may be constructed from various materials without departing from the scope of the present disclosure. Appropriate material selection may ensure that the outer surface  108  of the curved leading-edge fairing  100  is sufficiently smooth and aerodynamic. Examples of suitable materials include, without limitation, metals and metal alloys (e.g., titanium alloys), polymeric materials, and composite materials (e.g., carbon fiber-reinforced epoxy composites). 
     The first forward boom fairing  120  of the disclosed unmanned aerial vehicle  10  is positioned over at least a portion of the leading side  110  of the first outboard boom  72 . In one particular construction, the first forward boom fairing  120  substantially completely covers the leading side  110  of the first outboard boom  72 . 
     As shown in  FIG. 8 , the first forward boom fairing  120  may include an upper edge  122  and a lower edge  124 , and may have a continuous curvature  126  from the upper edge  122  to the lower edge  124 . In one particular construction, the first forward boom fairing  120  may have a first radius of curvature R L1  proximate (i.e., at or near) the proximal end portion  114  ( FIG. 6 ) of the first outboard boom  72  and a second radius of curvature R L2  proximate the distal end portion  116  ( FIG. 6 ) of the first outboard boom  72 , and wherein the first radius of curvature R L1  is substantially greater (e.g., at least 20 percent greater, such as at least 50 percent greater) than the second radius of curvature R L2 . For example, the first radius of curvature R L1  may be about 5.75 inches and the second radius of curvature R L2  may be about 3 inches. Therefore, the first forward boom fairing  120  may have an aerodynamic shape that reduces drag during flight of the unmanned aerial vehicle  10  in a forward direction F ( FIG. 1 ) to enable a substantially level flight profile. 
     The second forward boom fairing  150  of the disclosed unmanned aerial vehicle  10  is positioned over at least a portion of the leading side  110  of the second outboard boom  82 . In one particular construction, the second forward boom fairing  150  substantially completely covers the leading side  140  of the second outboard boom  82 . 
     As shown in  FIG. 9 , the second forward boom fairing  150  may include an upper edge  152  and a lower edge  154 , and may have a continuous curvature  156  from the upper edge  152  to the lower edge  154 . In one particular construction, the second forward boom fairing  150  may have a first radius of curvature R R1  proximate (i.e., at or near) the proximal end portion  144  ( FIG. 6 ) of the second forward boom fairing  150  and a second radius of curvature R R2  proximate the distal end portion  146  ( FIG. 6 ) of the second forward boom fairing  150 , and wherein the first radius of curvature R R1  is substantially greater (e.g., at least 20 percent greater, such as at least 50 percent greater) than the second radius of curvature R R2 . For example, the first radius of curvature R R1  may be about 5.75 inches and the second radius of curvature R R2  may be about 3 inches. Therefore, the second forward boom fairing  150  may have an aerodynamic shape that reduces drag during flight of the unmanned aerial vehicle  10  in a forward direction F ( FIG. 1 ) to enable a substantially level flight profile. 
     The first aft boom fairing  130  of the disclosed unmanned aerial vehicle  10  is positioned over at least a portion of the aft side  112  of the first outboard boom  72 . The first aft boom fairing  130  may be sized, shaped and configured in a manner similar to the first forward boom fairing  120  on the forward side  110  of the first outboard boom  72 . 
     The second aft boom fairing  160  of the disclosed unmanned aerial vehicle  10  is positioned over at least a portion of the aft side  142  of the second outboard boom  82 . The second aft boom fairing  160  may be sized, shaped and configured in a manner similar to the second forward boom fairing  150  on the forward side  140  of the second outboard boom  82 . 
     The first aft boom fairing  130  and the second aft boom fairing  160  may facilitate drag reduction. Additionally, the first aft boom fairing  130  and the second aft boom fairing  160  may enhance handling qualities in hover. For example, the first forward boom fairing  120  and the second forward boom fairing  150  may be identical to but opposite of the first aft boom fairing  130  and the second aft boom fairing  160 . Therefore, in hover, the first forward boom fairing  120 , the second forward boom fairing  150 , the first aft boom fairing  130 , and the second aft boom fairing  160  may passively have equal and opposite forces in the horizontal direction, helping the flight control system to hold position above a point on the ground. 
     The presence on the unmanned aerial vehicle  10  of the curved leading-edge fairing  100 , the first forward boom fairing  120 , the second forward boom fairing  150 , the first aft boom fairing  130 , and/or the second aft boom fairing  160  may significantly reduce drag when the unmanned aerial vehicle  10  moves in a forward direction F, thereby reducing power consumption (e.g., up to about 20 percent) and increasing service duration of the unmanned aerial vehicle  10 . Therefore, the pitch angle Θ (i.e., the angle between the forward direction F and the pitch attitude P of the frame  12  about the frame axis A F ) of the unmanned aerial vehicle  10  during flight in the forward direction F can be significantly reduced (e.g., a predetermined reduction in pitch angle Θ can be achieved). Within examples, the predetermined reduction in pitch angle Θ is a reduction (compared to a pitch angle of a UAV configured without the presence of the disclosed fairings) in pitch angle Θ of about 2 to 5 degrees. For example, a reduction in pitch angle Θ of up to about 5 degrees may be achieved, such as a 4 degree reduction in pitch angle Θ or a 3 degree reduction in pitch angle Θ or a 2 degree reduction in pitch angle Θ. The reduced drag enables the predetermined reduction in the pitch angle Θ that is effective to yield a reduced pitch angle Θ of the frame  12  about the frame axis A F  that is at most 13 degrees. Within examples, the reduced pitch angle Θ is between about 10 and 13 degrees. 
     In one expression, the pitch angle Θ of the frame  12  about the frame axis A F  is at most about 13 degrees when the unmanned aerial vehicle  10  is traveling in a forward direction F at airspeeds up to about 50 knots. In another expression, the pitch angle Θ of the frame  12  about the frame axis A F  is at most about 12 degrees when the unmanned aerial vehicle  10  is traveling in a forward direction F at airspeeds up to about 50 knots. In another expression, the pitch angle Θ of the frame  12  about the frame axis A F  is at most about 11 degrees when the unmanned aerial vehicle  10  is traveling in a forward direction F at airspeeds up to about 50 knots. In another expression, the pitch angle Θ of the frame  12  about the frame axis A F  is at most about 10 degrees when the unmanned aerial vehicle  10  is traveling in a forward direction F at airspeeds up to about 50 knots. 
     As mentioned above, the presence on the unmanned aerial vehicle  10  of the curved leading-edge fairing  100 , the first forward boom fairing  120 , the second forward boom fairing  150 , the first aft boom fairing  130 , and the second aft boom fairing  160  helps to reduce power consumption and increase service duration of the unmanned aerial vehicle  10 . Furthermore, the presence on the unmanned aerial vehicle  10  of the curved leading-edge fairing  100 , the first forward boom fairing  120 , the second forward boom fairing  150 , the first aft boom fairing  130 , and the second aft boom fairing  160  may significantly reduce noise when the unmanned aerial vehicle  10  moves in a forward direction F. Without being limited to any particular theory, it is presently believed that the presence of the curved leading-edge fairing  100 , the first forward boom fairing  120 , the second forward boom fairing  150 , the first aft boom fairing  130 , and the second aft boom fairing  160  enhances the stability (e.g., closer to neutral stability) of the unmanned aerial vehicle  10 , thereby contributing to UAV/rotor noise reduction. 
     Also disclosed are methods for reducing drag during flight of an unmanned aerial vehicle that includes a frame elongated along a frame axis, the frame comprising a leading side and an aft side, the frame further comprising a left end portion and a right end portion, at least two rotor assemblies connected to the left end portion of the frame by way of at least two first booms, and at least two rotor assemblies connected to the right end portion of the frame by way of at least two second booms. 
     Referring to  FIG. 10 , one example of the disclosed method for reducing drag during flight of an unmanned aerial vehicle, generally designated  500 , begins at Block  510  with the step of positioning a curved leading-edge fairing  100  over at least a portion of the leading side  14  of the frame  12 . At Block  520 , a first forward boom fairing  120  is positioned over the first outboard boom  72 . At Block  530 , a second forward boom fairing  150  is positioned over the second outboard boom  82 . At Block  540 , a first aft boom fairing  130  may be positioned over the first outboard boom  72 . At block  550 , a second aft boom fairing  160  may be positioned over the second outboard boom  82 . 
     Examples of the disclosure may be described in the context of an aircraft manufacturing and service method  1000 , as shown in  FIG. 11 , and an aircraft  1002 , as shown in  FIG. 12 . During pre-production, the aircraft manufacturing and service method  1000  may include specification and design  1004  of the aircraft  1002  and material procurement  1006 . During production, component/subassembly manufacturing  1008  and system integration  1010  of the aircraft  1002  takes place. Thereafter, the aircraft  1002  may go through certification and delivery  1012  in order to be placed in service  1014 . While in service by a customer, the aircraft  1002  is scheduled for routine maintenance and service  1016 , which may also include modification, reconfiguration, refurbishment and the like. 
     Each of the processes of method  1000  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of venders, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 12 , the aircraft  1002  produced by example method  1000  may include an airframe  1018  with a plurality of systems  1020  and an interior  1022 . Examples of the plurality of systems  1020  may include one or more of a propulsion system  1024 , an electrical system  1026 , a hydraulic system  1028 , and an environmental system  1030 . Any number of other systems may be included. 
     The disclosed unmanned aerial vehicle and/or method for reducing drag during flight of an unmanned aerial vehicle may be employed during any one or more of the stages of the aircraft manufacturing and service method  1000 . As one example, the disclosed unmanned aerial vehicle and/or method for reducing drag during flight of an unmanned aerial vehicle may be employed during material procurement  1006 . As another example, components or subassemblies corresponding to component/subassembly manufacturing  1008 , system integration  1010 , and or maintenance and service  1016  may be fabricated or manufactured using the disclosed unmanned aerial vehicle and/or method for reducing drag during flight of an unmanned aerial vehicle. As another example, the airframe  1018  and the interior  1022  may be constructed using the disclosed unmanned aerial vehicle and/or method for reducing drag during flight of an unmanned aerial vehicle. Also, one or more apparatus examples, method examples, or a combination thereof may be utilized during component/subassembly manufacturing  1008  and/or system integration  1010 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  1002 , such as the airframe  1018  and/or the interior  1022 . Similarly, one or more of system examples, method examples, or a combination thereof may be utilized while the aircraft  1002  is in service, for example and without limitation, to maintenance and service  1016 . 
     The disclosed unmanned aerial vehicle and/or method for reducing drag during flight of an unmanned aerial vehicle are described in the context of an aircraft; however, one of ordinary skill in the art will readily recognize that the disclosed unmanned aerial vehicle and/or method for reducing drag during flight of an unmanned aerial vehicle may be utilized for a variety of applications. For example, the disclosed unmanned aerial vehicle and/or method for reducing drag during flight of an unmanned aerial vehicle may be implemented in various types of vehicles, including, for example, helicopters, passenger ships, automobiles and the like. 
     Although various examples of the disclosed unmanned aerial vehicle and/or method for reducing drag during flight of an unmanned aerial vehicle have been shown and described, modifications may occur to those skilled in the art upon reading the specification. The present application includes such modifications and is limited only by the scope of the claims.