Patent Publication Number: US-10773785-B2

Title: Unmanned aerial vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 16/089,305, filed Sep. 27, 2018, which is a national stage application from International Patent Application No. PCT/JP2017/015108, filed on Apr. 13, 2017, and claims benefit of the earlier filing date of foreign application JP 2016-083760, filed on Apr. 19, 2016. The disclosure of the foregoing applications is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an unmanned aerial vehicle. 
     BACKGROUND ART 
     Conventional small-size unmanned aerial vehicles represented by industrial unmanned helicopters have had airframes too expensive to be affordable. Also, these vehicles used to require skillful pilotage for stable flight. In recent years, however, there have been considerable improvements in sensors and software used to control posture of unmanned aerial vehicles and to implement autonomous flight of unmanned aerial vehicles. This has led to considerable improvement in manipulability of unmanned aerial vehicles and availability of high-end airframes at lower prices. Under the circumstances, multi-copters, especially small size multi-copters, are currently not only used for hobbyist purposes but also applied to various missions in a wide range of fields. 
     CITATION LIST 
     Patent Literature 
     PTL1: JP 2010-120641 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     As the use of unmanned aerial vehicles for freight purposes becomes more and more common, strict demands are likely to emerge, such as: a demand for space efficiency with which a large number of airframes are stored in an area of space; and a demand for efficiency of the work of loading and unloading of freight, in addition to those for freight performance and safety. 
     In light of the above-described problems, a problem to be solved by the present invention is to provide an unmanned aerial vehicle that enables a large number of airframes to be stored with improved space efficiency, that carries freight safely and stably, and that performs the work of loading and unloading of freight efficiently. 
     Solution to Problem 
     In order to solve the above-described problem, the present invention provides an unmanned aerial vehicle that includes a plurality of propellers. An airframe of the unmanned aerial vehicle includes: a body having a freight chamber that is a hollow portion and that is integral with the body; and a plurality of arms supporting each of the plurality of propellers. A combination of the one arm and the one propeller or plurality of propellers supported by the one arm constitute a retractable propeller. The retractable propeller is partially or entirely storable in the freight chamber. 
     In a typical multi-copter, a plurality of arms radially extend from the center of the airframe, and a propeller is mounted on the leading end of each arm. When such multi-copter is not in use and stored, the arms and the propellers take up much space. The unmanned aerial vehicle according to the above-described configuration includes a freight chamber integral with the body, and the arms and the propellers are storable in the freight chamber. This increases the number of unmanned aerial vehicles storable in a storage space. 
     Also in the unmanned aerial vehicle according to the above-described configuration, the freight chamber is provided in the airframe in advance. This eliminates the need for providing an additional attachment dedicated to carrying freight. Also, the freight chamber is spacious and thus capable of carrying freight that vary in number, size, and kind. Further, the above-described configuration prevents the airframe from keeping the center of gravity at an excessively low position, as compared with a configuration in which a piece of freight is attached to a lower portion of the airframe. This enables the airframe to more easily maintain a balance. More specifically, an unmanned aerial vehicle having a plurality of propellers performs position control in horizontal directions, movement control, and posture control by inclining the airframe as a whole. Generally, the airframe becomes more stable as the center of gravity is kept at a lower position. However, this causes such an adverse effect that the airframe becomes more difficult to incline. As the airframe becomes more and more difficult to incline, it becomes difficult to control the airframe to move quickly. In light of the circumstances, it is important from an airframe stability standpoint to keep the center of gravity at a position high enough to enable the airframe to incline. In the unmanned aerial vehicle according to the above-described configuration, freight is contained in the airframe of the unmanned aerial vehicle. This minimizes the difficulty with which the airframe maintains a balance. 
     On the airframe, another airframe may preferably be placeable. 
     In the unmanned aerial vehicle according to the above-described configuration, airframes are stackable on top of one another. This enables the storage space of the unmanned aerial vehicle to be used three-dimensionally, that is, makes the storage space an area of volume, with height added to the area of space of the storage space. Further, by storing the arms and the propellers in the freight chamber, the number of unmanned aerial vehicles storable in the storage space is maximized. 
     Also, each of the plurality of arms may preferably have a base end portion that is located at the side of the body in a longitudinal direction of the each arm and that is turnably supported by the body. The retractable propeller may preferably be storable in the freight chamber or expandable from the freight chamber by turning the retractable propeller about the base end portion serving as the turning center. 
     With such a configuration in which the retractable propeller is storable in the freight chamber by turning the retractable propeller about its base end, the retractable propeller is stored and unfolded in a simple manner. 
     Also, the base end portion may be turnable in a horizontal direction relative to the body, and the retractable propeller may include a plurality of units of retractable propellers in a circumferential direction of the body, each one unit of the plurality of units comprising two retractable propellers having a common turning center. 
     Two retractable propellers having a common arm turning center are grouped as one unit, and the two retractable propellers are displaced from each other in the vertical direction (axial direction of the turning center). This increases the number of propellers mountable and increases the maximum lift force of the airframe, ensuring that heavy objects can be carried more stably. 
     Also, the two retractable propellers constituting the one unit may preferably be expandable to positions at which the two retractable propellers do not overlap each other in the circumferential direction of the body. 
     The arrangement in which two propellers overlap each other in the vertical direction leads to such a problem that there is a large amount of loss of lift force, especially when the gap between the propellers is small. In the unmanned aerial vehicle according to the above-described configuration, two retractable propellers constituting one unit are expandable to positions at which the two retractable propellers do not overlap each other. This minimizes the loss of lift force. 
     Also, the freight chamber may preferably have an opening at an upper portion of the freight chamber. 
     In such an application that a piece of freight is attached to a typical multi-copter, it is necessary to temporarily lift the airframe up and fix the freight to a lower portion of the airframe. Thus, the freight mounting work is complicated. The same applies in the freight removal work. In such an application that a piece of freight is suspended from the airframe using a string member such as a wi it is necessary to perform, at a side of the airframe on land, the work of connecting the freight to a lower portion of the airframe. In order to connect freight in a simple manner, it is necessary to devise a suspending structure for this purpose. Also, in such an application that a piece of freight is carried by being suspended from the airframe, the center of gravity of the airframe is kept at a low position, as described above. This causes such a problem that the stability of the airframe at take off time and flight time is undermined. In the unmanned aerial vehicle according to the above-described configuration, the freight chamber has an opening at an upper portion of the freight chamber. This ensures that freight can be inserted and retrieved into and from the freight chamber through the opening of the freight chamber. This makes the work of loading and unloading of freight efficient. 
     Also, the unmanned aerial vehicle may preferably further include one container or a plurality of containers attachable and detachable to and from the freight chamber. A piece of freight may preferably be put in the container, and the piece of freight in the container may preferably be contained in the freight chamber. 
     Also, the container may preferably have a standardized shape. 
     In such an application that pieces of freight are directly contained in a freight chamber, the work of filling the gaps between the pieces of freight with shock-absorbing material is difficult to stylize. Thus, this work is not an efficient part of the distribution process as a whole. By using a container of a predetermined size as a unit container and packaging a piece of freight in the container, “containerization” of freight can be implemented, resulting in streamlining and automation of packaging work. 
     Also, at least one of a plurality of retractable propellers and the container may be containable simultaneously in the freight chamber. 
     Making the number of propellers adjustable according to the load of freight increases energy efficiency at the time of transportation. 
     Also, in the unmanned aerial vehicle according to the present invention, a plurality of through holes connected to the freight chamber may be formed on an outer peripheral surface of the body. Each of the plurality of arms may include a base end portion that is located at a side of the body in a longitudinal direction of the each arm and that is located in the through hole and supported by the body. The each arm may be storable in the freight chamber by being inserted further into the through hole. The retractable propeller may be expandable by pulling the each arm out of the body. 
     By sliding each arm in its longitudinal direction, each arm is inserted and pulled into and out of the freight chamber. Thus, each retractable propeller (arm) is stored and unfolded in a simple manner. 
     Also, each of the plurality of propellers may preferably include a blade foldable in a circumferential direction of the each propeller. 
     By storing each arm into the freight chamber and folding the blade along the outer shape of the body, the unmanned aerial vehicle can be stored in a more compact form. 
     Also, the retractable propeller may include two propellers aligned in a vertical direction in a coaxial manner. The retractable propeller may preferably include a plurality of retractable propellers arranged along the circumferential direction of the body. 
     Each retractable propeller includes two retractable propellers. This increases the lift force of the unmanned aerial vehicle, resulting in increased transportable weight on the unmanned aerial vehicle. 
     Advantageous Effects of Invention 
     Thus, the unmanned aerial vehicle according to the present invention stores a large number of airframes with improved space efficiency, carries freight safely and stably, and performs the work of loading and unloading of freight efficiently. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of an exterior appearance of an unmanned aerial vehicle according to an embodiment. 
         FIG. 2  is a block diagram illustrating a functional configuration of the unmanned aerial vehicle according to the embodiment. 
         FIG. 3  is an enlarged view of the portion indicated by broken line A in  FIG. 1 . 
         FIG. 4  is a perspective view of an exterior appearance illustrating retractable propellers contained in a freight chamber. 
         FIG. 5  is a plan view of the unmanned aerial vehicle illustrated in  FIG. 1 . 
         FIG. 6  is a perspective view of an exterior appearance of an unmanned aerial vehicle according to another embodiment. 
         FIG. 7  is a plan view illustrating an accommodation structure in which the arms are contained. 
         FIG. 8  is a perspective view of an exterior appearance of an unmanned aerial vehicle according to an embodiment having thin-type containers. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described in detail below by referring to the drawings. A multi-copter  90  according to this embodiment is an example unmanned aerial vehicle having a plurality of propellers. As used herein, the terms “upward”, “upper”, “downward”, and “lower” refer to vertical directions in  FIG. 1 , which are parallel to the z axis direction of the coordinate axes illustrated in some of the drawings. Also as used herein, the term “horizontal” refers to xy plane directions in the coordinate axes. Also as used herein, “circumferential direction” of the multi-copter (and a multi-copter  96  according to another embodiment) refers to a circumferential direction in the yawing direction of the multi-copter. 
     [Container Configuration] 
       FIG. 1  is aperspective view of an exterior appearance of the multi-copter  90 . An airframe  10  of the multi-copter  90  mainly includes: a box-shaped body  12 , which has an approximately rectangular parallelepiped shape; and a plurality of arms  11 , which radially extend in horizontal directions from portions of the body  12  corresponding to the apices of the body  12  in the circumferential direction. Each of the arms  11  supports a rotor that includes a blade  42 , which constitute a fixed pitch propeller. In the body  12 , a freight chamber  13  is located. The freight chamber  13  has an opening  131  at an upper portion of the freight chamber  13 . 
     In the multi-copter  90 , the freight chamber  13  is integral with the airframe  10 , that is, the freight chamber  13  is undetachable from the airframe  10 . This eliminates the need for providing an additional attachment or equipment dedicated to transportation of freight  91 . As used herein, the term “piece of freight” refers to a “piece of freight” and an article carried and transported by the unmanned aerial vehicle. 
     In such an application that a piece of freight is attached to a typical multi-copter, it is necessary to temporarily lift the airframe up and fix the freight to a lower portion of the airframe. Thus, the freight mounting work is complicated. The same applies in the freight removal work. In such an application that a piece of freight is suspended from the airframe using a string member such as a wi it is necessary to perform, at a side of the airframe on land, the work of connecting the freight to a lower portion of the airframe. In order to connect freight in a simple manner, it is necessary to devise a suspending structure for this purpose. In the multi-copter  90 , the freight chamber  13  has the opening  131  at an upper portion of the freight chamber  13 . This enables a worker to access the freight chamber  13  from above the freight chamber  13 . This makes the work of loading and unloading of the freight  91  efficient. 
     Also in the multi-copter  90 , the freight chamber  13  is integral with the airframe  10 . This minimizes the influence that the freight has on the airframe balance, as compared with a configuration in which a piece of freight is attached to a lower portion of the airframe. Generally, the airframe becomes more stable as the center of gravity is kept at a lower position. However, this causes such an adverse effect that the airframe becomes more difficult to incline. As the airframe becomes more and more difficult to incline, it becomes difficult to control the airframe to move quickly. In light of the circumstances, it is important from an airframe stability standpoint to keep the center of gravity at a position high enough to enable the airframe to incline. In the multi-copter  90  according to this embodiment, the freight  91  is contained in the airframe  10 . This minimizes the difficulty with which the airframe maintains a balance. 
     The freight chamber  13  includes four containers  14 , which are containers mountable in the freight chamber  13 . The freight  91 , which is a piece of freight carried by the multi-copter  90 , is collected on a one-container- 14  basis and contained in the freight chamber  13  on a one-container- 14  basis. The containers  14  according to this embodiment have standardized shapes for the sake of efficiency and simplicity of packaging work. This form of the containers  14 , however, will not limit the form of the containers according to the present invention. Each container according to the present invention may have any of other shapes insofar as combined containers fit in the freight chamber without gaps. For example, it is possible to prepare a plurality of containers of different sizes so as to secure a range of sizes of freight that can be carried in the freight chamber. Further, it is possible to prepare one container that occupies the entire volume of the freight chamber. 
     In the multi-copter  90 , a container  14  of a predetermined size is regarded as a packaging unit, and the freight  91  is packaged in the container  14 . This promotes “containerization”. 
     While the freight chamber according to the present invention preferably includes a container, a container is not an essential configuration. There may be a case where it is more convenient to directly contain freight in the freight chamber, without using a container. There also may be a case where there is no particular work efficiency problem in directly containing freight in the freight chamber. In such cases, it is possible to provide a lid on the upper surface opening  131  so that freight can be directly contained in the freight chamber. 
     [Flight Functions] 
       FIG. 2  is a block diagram illustrating a functional configuration of the multi-copter  90 . The airframe of the multi-copter  90  includes: a flight the controller  20 ; a plurality of rotors R; an ESC  43  (Electric Speed Controller), which controls rotation of the rotors R; a wireless transmitter-receiver  33 , which wirelessly communicates with a handling terminal  95  of an operator; and a battery  51 , which supplies power to the foregoing elements. 
     Each of the rotors R includes: a motor  41 , which is a DC motor; and the blade  42 , which is mounted on the output shaft of the motor  41 . The ESC  43  is connected to the motor  41  of the rotor R and causes the motor  41  to rotate at a speed specified by the flight controller FC. While the multi-copter  11  according to this embodiment is an octocopter equipped with eight rotors R, the number of the rotors R will not be limited to eight; the multi-copter  11  may be a quadcopter that includes four rotors R. The multi-copter  11  may have any other number of rotors R, which may be determined considering required flight stability, cost tolerated, and other considerations. Examples include a hexacopter that includes six rotors R, and even a multi-copter that includes more than eight rotors. 
     The flight controller FC includes a controller  20 , which is a micro-controller. The controller  20  includes: a CPU  21 , which is a central processing unit; a memory  22 , which is a storage device such as ROM and RAM; and a PWM controller  23 , which controls the number of rotations of each motor  41  and the rotation speed of each motor  41  through the ESC  43 . 
     The flight controller FC further includes a flight control sensor group  31  and a GPS receiver  32  (these will be hereinafter occasionally referred to as “sensors”). The flight control sensor group  31  and the GPS receiver  32  are connected to the controller  20 . The flight control sensor group  31  of the multi-copter  90  according to this embodiment includes an acceleration sensor, an angular velocity sensor, a pneumatic sensor (altitude sensor), and a geomagnetic sensor (direction sensor). The controller  20  is capable of obtaining, from these sensors, how much the airframe is inclined or turning, latitude and longitude of the airframe on flight, flight altitude, and position information of the airframe including nose azimuth. 
     The memory  22  of the controller  20  stores a flight control program FCP, in which a flight control algorithm for controlling the posture of the multi-copter  90  during flight and controlling basic flight operations is described. In response to an instruction from an operator (handling terminal  95 ), the flight control program FCP adjusts the number of rotations of each rotor R based on the current position of the multi-copter  90  obtained from the sensors so as to correct the posture and/or position of the airframe while the multi-copter  90  is making a flight. 
     The multi-copter  90  may be manipulated by the operator using the handling terminal  95 . Another possible example is to register parameters, such as latitude longitude, flight altitude, and flight route, in the flight control program FCP in advance and to cause the multi-copter  90  to fly autonomously to the destination (this kind of autonomous flight will be hereinafter referred to as “autopilot”). This embodiment is basically under the assumption that the multi-copter  90  is caused to make an autonomous flight to a predetermined destination by autopilot. 
     Thus, the multi-copter  90  according to this embodiment has high-level flight control functions. It is to be noted, however, that insofar as the unmanned aerial vehicle according to the present invention includes a freight chamber and is capable of flying using a plurality of propellers, the unmanned aerial vehicle according to the present invention encompasses an unmanned aerial vehicle with some of the sensors omitted, an unmanned aerial vehicle without autopilot function, and an unmanned aerial vehicle capable of flying only by manual manipulation. Also, while the multi-copter  90  according to this embodiment is suitable for outdoor freight transportation because of the employment of the GPS receiver  32  to detect latitude and longitude of the multi-copter  90  during flight, the unmanned aerial vehicle according to the present invention may be used for indoor freight transportation as well. Specifically, the unmanned aerial vehicle may be provided with a near-field wireless communication module that identifies the current flight position by communicating with devices dispersed in a facility, examples of the module including Wi-Fi (registered trademark) access points and Bluetooth (registered trademark) low-energy beacons. 
     [Space-Saving Configuration] 
       FIG. 2  is an enlarged view of the portion indicated by broken line A in  FIG. 1 . A combination of each arm  11  of the multi-copter  90  and a corresponding blade  42  supported by the arm  11  constitutes a retractable propeller SR (SR 1 , SR 2 ). Each arm  11  has a base end portion  11   a , which is the body  12  side end of the arm  11  in its longitudinal direction. The base end portion  11   a  is turnably supported by the body  12 . At the leading end of the arm  11  opposite to the base end portion  11   a , a rotor guard  11   b  is located. The rotor guard  11   b  protects the blade  42 . The rotor guard  11   b  is an optional configuration and may be omitted. In this embodiment, the rotor guard  11   b  is regarded as part of the arm  11 . 
     Each retractable propeller SR according to this embodiment, includes, as one unit, two retractable propellers SR 1 , SR 2 , which have a common turning center. As illustrated in  FIG. 1 , four units of retractable propellers SR 1 , SR 2  in the circumferential direction of the body  12 . As described earlier, the number of the rotors R, that is, the number of the retractable propellers SR may be subject to change insofar as a plurality of retractable propellers SR are provided. In this embodiment, two retractable propellers SR 1 , SR 2  having a common turning center are regarded as one unit and arranged such that the retractable propellers SR 1 , SR 2  are displaced from each other in the vertical direction. This increases the number of the retractable propellers SR mountable and increases the maximum lift force of the airframe. This ensures that even heavy weight freight can be carried stably. 
       FIG. 4  is a perspective view of an exterior appearance illustrating the retractable propellers SR of the multi-copter  90  contained in the freight chamber  13 . Slidable plates  1  are mounted on two side surfaces of the freight chamber  13  of the multi-copter  90  in the x direction of the coordinate axes illustrated in  FIG. 4 . The slidable plates  133  are planar members slidable upward and downward. While this embodiment is described under the assumption that the slidable plates  133  are openable and closable manually, the slidable plates  133  may be openable and closable using an actuator such as a servo mechanism. 
     As illustrated in  FIG. 4 , the slidable plates  133  slide upward, leaving side openings  132  of the freight chamber  13  open. Then, the retractable propellers SR 1 , SR 2  are turned about the base end portion  11   a  serving as a turning center toward the side openings  132 , causing the retractable propellers SR 1 , SR 2  to be stored in the freight chamber  13 . With this configuration, the multi-copter  90  according to this embodiment ensures that the retractable propellers SR are stored and unfolded in a simple manner. 
       FIG. 5  is a plan view of the multi-copter  90  illustrated in  FIG. 1 . As illustrated in  FIG. 5 , the retractable propellers SR 1 , SR 2  are expandable to positions at which the retractable propellers SR 1 , SR 2  do not overlap each other in the circumferential direction of the body  12 . More specifically, there is a 90° difference of spread angle between the retractable propellers SR 1 , SR 2 ; the spread angle of the retractable propeller SR 1  is 90° larger than the spread angle of the retractable propeller SR 2 . This ensures that the blades  42  of the retractable propellers SR 1 , SR 2  are arranged at positions at which the blades  42  do not overlap each other in the circumferential direction of the body  12 . The arrangement in which two propellers overlap each other in the vertical direction leads to such a problem that there is a large amount of loss of lift force, especially when the gap between the propellers is small. In the multi-copter  90  according to this embodiment, the retractable propellers SR 1 , SR 2 , which constitute one unit, are expandable to positions at which the blades  42  of the retractable propellers SR 1 , SR 2  do not overlap each other. This minimizes the loss of lift force. 
     The retractable propeller SR according to this embodiment is turnable in a horizontal direction to be stored in the freight chamber  13  through the side opening  132 . This configuration, however, is not intended as limiting the method of storing the retractable propeller SR in the freight chamber  13 . For example, in such an application that the retractable propeller SR 1  alone constitutes one unit of retractable propeller SR, the retractable propeller SR 1  may be turned in a vertical direction and stored in the freight chamber  13  through the upper surface opening  131 . For further example, the retractable propeller SR may be removed and stored in the freight chamber  13 . Also, while this embodiment is described under the assumption that the retractable propeller SR is turned manually to be stored in the freight chamber  13 , it is possible to use an actuator such as a servo mechanism for this purpose. 
     In a typical multi-copter, a plurality of arms radially extend from the center of the airframe, and a propeller is mounted on the leading end of each arm, similarly to the multi-copter  90 . When the airframe is not in use and stored, the arms and the propellers take up much space. The multi-copter  90  includes the freight chamber  13 , and the arms  11  and the blades  42  are storable in the freight chamber  13 . This increases the number of unmanned aerial vehicles storable in a storage space of a predetermined area. 
     While the freight chamber  13  of the multi-copter  90  according to this embodiment is capable of containing the freight  91 , the freight chamber  13  may be a storage space dedicated to the retractable propellers SR. In this case, the upper surface opening  131  may not necessarily be provided. 
     As illustrated in  FIGS. 1 and 3 , the body  12  of the multi-copter  90  includes skid joint portions  15  on the upper surface of the body  12 . The skid joint portions  15  are located at four corners of the upper surface of the body  12 . The positions of the skid joint portions  15  respectively correspond to the positions of the legs (skids), not illustrated, of the multi-copter  90 . Each of the skid joint portions  15  has a depression engageable with a leg. Providing the multi-copter  90  according to this embodiment with the skid joint portions  15  ensures that when the airframe  10  of another multi-copter  90  is placed on the airframe  10  of the multi-copter  90 , the positioning of the airframes  10  is facilitated. Providing the skid joint portions  15  also enhances the stability of a stack of a plurality of airframes  10 . 
     In the multi-copter  90  according to this embodiment, a plurality of airframes  10  can be stacked on top of each other. This enables the storage space to be used three-dimensionally, that is, makes the storage space an area of volume, with height added to the area of space of the storage space. Further, by storing the retractable propellers SR in the freight chamber  13 , the number of retractable propellers SR storable in the storage space is maximized. 
     In this embodiment, when the multi-copter  90  is stored, the multi-copter  90  is without all of the containers  14 , with only the retractable propellers SR stored in the freight chamber  13 . This configuration is implemented under the assumption that the containers  14  are attached to the freight chamber  13  only while the freight  91  are carried. This enables the storage space to be used more efficiently. For this reason, the freight chamber  13  is used exclusively for the containers  14  or the retractable propeller SR. It is also possible, however, to contain the containers  14  and the retractable propeller SR simultaneously in the freight chamber  13 . As illustrated in  FIG. 8 , a possible example is to: prepare a thin-type container  14 ′, which is capable of containing a small piece of freight  91 ; attach the container  14 ′ to the freight chamber  13 ; and store the lower retractable propeller SR 2 , among the retractable propellers SR 1 , SR 2 , in the freight chamber  13 . Thus, the number of the rotors R to be driven is adjusted based on the load of the freight  91 . This increases energy efficiency at the time of transportation of the freight  91 . 
     Another Embodiment 
     Another embodiment of the present invention will be described below by referring to  FIGS. 6 and 7 .  FIG. 6  is a perspective view of an exterior appearance of the multi-copter  96 , which is an unmanned aerial vehicle according to another embodiment of the present invention.  FIG. 7  is a plan view of an accommodation structure in which the arms  17  of the multi-copter  96  are contained. It is to be noted that in the following description, those elements identical or similar to each other in this embodiment and the previous embodiment have the same or identical reference numerals and will not be elaborated further upon here. Also, the flight functions of the multi-copter  96  are basically the same as the flight functions of the multi-copter  90  according to the previous embodiment and will not be elaborated further upon here. 
     An airframe  16  of the multi-copter  96  mainly includes: a box-shaped body  18 , which has an approximately rectangular parallelepiped shape; and a plurality of arms  17 , which radially extend in horizontal directions from portions of the body  18  corresponding to the apices of the body  18  in the circumferential direction. Each of the arms  17  is made up of two pipe materials parallel to each other in the vertical direction. At the leading end of each arm  17 , two rotors R 1 , R 2  are mounted. The rotors R 1 , R 2  are aligned in the vertical direction in a coaxial manner. Each of the rotors R 1 , R 2  includes a blade  45 , which is foldable in circumferential directions. A combination of each arm  17  according to this embodiment and a corresponding blade  45  supported by the arm  17  constitutes a retractable propeller ER. 
     In the body  18 , a freight chamber  13  is located. The freight chamber  13  has an upper surface opening  131  at an upper portion of the freight chamber  13 . In the multi-copter  96  illustrated in  FIG. 6 , a single container  18  occupies the entire volume of the freight chamber  13 . 
     Through holes  181  are formed on the outer peripheral surface of the body  18  at portions corresponding to the apices of the body  18 . The through holes  181  are connected to the freight chamber  13 . Each arm  17  illustrated in  FIG. 6  has a base end portion  17   a  (see  FIG. 7 ), which is the body  18  side end of the arm  17  in its longitudinal direction. The base end portion  17   a  is inserted through the through hole  181  and supported by the body  18  using a fixing structure, not illustrated. 
     As illustrated in  FIG. 7 , in the multi-copter  96  according to this embodiment, the arms  17  in the state illustrated in  FIG. 6  can be further inserted into the through holes  181  so that the arms  17  are contained in the freight chamber  13  after the container  14  has been taken out of the freight chamber  13 . Additionally, the blade  45  can be folded along the outer shape of the body  18 . This ensures that the multi-copter  96  can be stored in a compact form. While the freight chamber  13  of the multi-copter  96  according to this embodiment is capable of containing the freight  91 , the freight chamber  13  may be a storage space dedicated to the arms  17 . In this case, the upper surface opening  131  may not necessarily be provided. 
     When the retractable propeller ER is expanded, the arms  17  are pulled out of the body  18 , and the blades  45  are opened. Thus, in the multi-copter  96  according to this embodiment, the arms  17  are inserted and pulled into and out of the freight chamber  13  by sliding the arms  17  in their longitudinal directions. This ensures that the retractable propeller ER is stored and unfolded in a simple manner. 
     Embodiments of the present invention have been described hereinbefore. The present invention, however, will not be limited to the above-described embodiments but may have various modifications without departing from the scope of the present invention.