Patent Publication Number: US-11390369-B2

Title: Aircraft

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. continuation application of PCT International Patent Application Number PCT/JP2017/005207 filed on Feb. 14, 2017, claiming the benefit of priority of Japanese Patent. Application Number 2016-047673 filed on Mar. 10, 2016, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an aircraft including a plurality of rotor units. 
     2. Description of the Related Art 
     Japanese Unexamined Patent Application Publication No. 2011-046355 discloses an aircraft including a plurality of rotor units that each include a propeller. This type of aircraft is referred to as, for example, a multicopter or drone. 
     Japanese Unexamined Patent Application Publication No. H04-022386 discloses an aircraft including: a single rotor unit including a propeller; and a buoyant body filled with helium gas. In the aircraft disclosed in Japanese Unexamined Patent Application Publication No. H04-022386, the donut-shaped buoyant body is disposed so as to surround the surrounding area of the single rotor unit. 
     SUMMARY 
     In the aircraft disclosed in Japanese Unexamined Patent Application Publication No. 2011-046355, the plurality of rotor units and the fuselage, which is equipped with on-board devices such as a camera, are exposed. The aircraft disclosed in Japanese Unexamined Patent. Application Publication No. H04-022386 achieves flight with a single rotor unit including a large propeller. The aircraft therefore includes, protruding beyond the buoyant body, legs for supporting the weight of rotor unit when landing and fins for controlling flight direction. Accordingly, if either of these aircrafts experience an unexpected flight situation, descend and contact an object, the object contacted by the aircraft may be damaged and the on-board devices in the aircraft may be damaged. 
     The present disclosure has been conceived in view of the above points, and provides an aircraft that achieves flight via a plurality of rotor units and has improved safety. 
     An aircraft according to the present disclosure includes: a plurality of rotor units each including a propeller and a motor that drives the propeller; a shock absorber that laterally covers the plurality of rotor units, across the height of the plurality of rotor units in the up-and-down direction; an on-board device that protrudes, along a predetermined axis, beyond the shock absorber; and a holding component that holds the on-board device and whose overall length can be shortened along the predetermined axis. 
     With the aircraft according to the present disclosure, it is possible to cause an aircraft that contains gas that is less dense than air to quickly descend when the aircraft becomes uncontrollable. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other objects, advantages and features of the disclosure will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the present disclosure. 
         FIG. 1  is a perspective view of an aircraft according to Embodiment 1 from below; 
         FIG. 2  is a plan view of the aircraft according to Embodiment 1; 
         FIG. 3  is a cross-sectional view of the aircraft taken at line III-III in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of the aircraft taken at line IV-IV in  FIG. 2 ; 
         FIG. 5  is a plan view of the balloon according to Embodiment 1; 
         FIG. 6  is a cross-sectional view of the balloon taken at line VI-VI in  FIG. 5 ; 
         FIG. 7  is a block diagram illustrating a configuration of the aircraft according to Embodiment 1; 
         FIG. 8A  is a flow chart of one example of gas release control in the aircraft according to Embodiment 1; 
         FIG. 8B  is a flow chart of another example of the gas release control in the aircraft according to Embodiment 1; 
         FIG. 9  illustrates the aircraft according to Embodiment 1 when the gas release control is performed; 
         FIG. 10  illustrates an aircraft according to Variation 1 of Embodiment 1 when the gas release control is performed; 
         FIG. 11  illustrates an aircraft according to Variation 2 of Embodiment 1 when the gas release control is performed; 
         FIG. 12  illustrates the aircraft according to Embodiment 1 having fallen to the ground as a result of the gas release control being performed; 
         FIG. 13  is an enlarged view of holding components and camera in the aircraft; 
         FIG. 14  is an enlarged view of holding components and camera in an aircraft according to Variation 1 of Embodiment 2; 
         FIG. 15  is a block diagram illustrating a configuration of an aircraft according to Variation 2 of Embodiment 2; 
         FIG. 16A  is a flow chart of one example of contraction control for the holding components in the aircraft according to Variation 2 of Embodiment 2; 
         FIG. 16B  is a flow chart of one example of the contraction control for the holding components in the aircraft according to Variation 2 of Embodiment 2; 
         FIG. 17  is an enlarged view of the holding components and camera in the aircraft; 
         FIG. 18  is an enlarged view of the holding components and camera in the aircraft; 
         FIG. 19  illustrates an aircraft according to Embodiment 3 when the shape-change control is performed; 
         FIG. 20  is a block diagram illustrating a configuration of the aircraft according to Embodiment 3; 
         FIG. 21A  is a flow chart of one example of the shape-change control performed by the drive unit in the aircraft according to Embodiment 3; 
         FIG. 21B  is a flow chart of another example of the shape-change control performed by the drive unit in the aircraft according to Embodiment 3: 
         FIG. 22  illustrates an aircraft according to Variation 1 of Embodiment 3 when the shape-change control is performed; 
         FIG. 23  illustrates an aircraft according to Variation 2 of Embodiment 3 when the shape-change control is performed; 
         FIG. 24  illustrates an aircraft according to Variation 3 of Embodiment 3 when the shape-change control is performed; and 
         FIG. 25  illustrates an aircraft according to Variation 4 of Embodiment 3 when the shape-change control is performed. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments will be described in detail with reference to the drawings when appropriate. However, unnecessarily detailed description may be omitted. For example, detailed descriptions of well-known matters or descriptions of components that are substantially the same as components described previous thereto may be omitted. This is to avoid unnecessary redundancy and provide easy-to-read descriptions for those skilled in the art. 
     Note that the accompanying drawings and subsequent description are provided by the inventors to facilitate sufficient understanding of the present disclosure by those skilled in the art, and are thus not intended to limit the scope of the subject matter recited in the claims. 
     Embodiment 1 
     (Outline of Aircraft Configuration) 
     Next, aircraft  10  according to Embodiment 1 will be described. 
       FIG. 1  is a perspective view of an aircraft according to Embodiment 1 from below.  FIG. 2  is a plan view of the aircraft according to Embodiment 1.  FIG. 3  is a cross-sectional view of the aircraft taken at line Ill-III in  FIG. 2 .  FIG. 4  is a cross-sectional view of the aircraft taken at line IV-IV in  FIG. 2 . 
     As illustrated in  FIG. 1  and  FIG. 2 , aircraft  10  according to this embodiment includes balloon  20  as a shock absorber, four rotor units  30 , and fixing component  50 . Aircraft  10  further includes release unit  26  that is disposed on the upper part of first shock absorber  20   a  of balloon  20  and releases a first gas contained in balloon  20  at a predetermined timing. 
     As illustrated in  FIG. 3  and  FIG. 4 , aircraft  10  is provided with, as on-board devices, controller  41 , battery  42 , projector  43 , and camera  44 . Aircraft  10  is further provided with light emitter  46  (a lighting apparatus). 
     (Balloon) 
     Next, balloon  20  will be described. 
       FIG. 5  is a plan view of the balloon according to Embodiment 1.  FIG. 6  is a cross-sectional view of the balloon taken at line VI-VI in  FIG. 5 . 
     As illustrated in  FIG. 3 ,  FIG. 4 , and  FIG. 6 , balloon  20  is made of a flexible sheet material (for example, vinyl chloride), and includes gas chamber  21 , which is a space enclosed by the sheet material. In  FIG. 3 ,  FIG. 4 , and  FIG. 6 , the bold lines indicate the cross section of the sheet material that forms balloon  20 . The sheet material that forms the outer surface of balloon  20  is white in color and semitransparent so as to allow light to pass through. 
     Gas chamber  21  includes first gas chamber  21   a  in the upper portion of aircraft  10  and second gas chamber  21   b  in the lower portion of aircraft  10 . Stated differently, balloon  20  includes first shock absorber  20   a  functioning as a first chamber that defines first gas chamber  21   a , and second shock absorber  20   b  functioning as a second chamber that defines second gas chamber  21   b . First shock absorber  20   a  and second shock absorber  20   b  are mutually different shock absorbers. First gas chamber  21   a  defined by first shock absorber  20   a  and second gas chamber  21   b  defined by second shock absorber  20   b  are mutually independent from one another, and are not in fluid communication with one another. More specifically, partition  27  is disposed between and separates first gas chamber  21   a  and second gas chamber  21   b . Partition  27  is made of the same sheet material used for balloon  20 . 
     In gas chamber  21  made of the sheet material, first gas chamber  21   a  contains a first gas that is less dense than air, and second gas chamber  21   b  contains a second gas that is more dense than the first gas. In this embodiment, for example, the first gas is helium and the second gas is air. 
     As illustrated in  FIG. 5 , balloon  20  has rotational symmetry about an axis of symmetry extending in the up-and-down direction (line extending vertically out from the drawing in  FIG. 5 ). This axis of symmetry is central axis P of balloon  20 . Balloon  20  illustrated in  FIG. 5  has a rotational symmetry of 90 degrees. In other words, balloon  20  has the same shape after each 90 degree rotation about central axis P. 
     As illustrated in  FIG. 6 , balloon  20  has a flattened shape. More specifically, the height axis of balloon  20  is flattened. Moreover, when viewed from a lateral side, balloon  20  has a streamline shape. Balloon  20  gradually decreases in height from its central region toward its peripheral edge. More specifically, in a cross section of balloon  20  taken along central axis P illustrated in  FIG. 6 , balloon  20  has an elliptical shape whose major axis extends horizontally and minor axis extends vertically. Stated differently, balloon  20  has a cross sectional shape that is approximately symmetrical about its horizontal axis. Note that the cross sectional shape of balloon  20  need not be a precise ellipse; it may be a shape that would be recognized as an ellipse at a glance. 
     Balloon  20  includes as many ventilation holes  22  as it does rotor units  30  (four in this embodiment). As illustrated in  FIG. 6 , each ventilation hole  22  is a passageway having an approximately circular cross section, and passes through balloon  20  in the up-and-down direction. Central axis Q of each ventilation hole  22  is approximately parallel to central axis P of balloon  20 . Each ventilation hole  22  extends across first shock absorber  20   a  and second shock absorber  20   b.    
     As illustrated in  FIG. 6 , central axis Q of each ventilation hole  22  is located between (i) a center point between central axis P of balloon  20  and the peripheral edge of balloon  20  and (ii) the peripheral edge of balloon  20 . More specifically, the distance S between central axis P of balloon  20  and central axis Q of ventilation hole  22  is longer than half of the distance R between central axis P of balloon  20  and the peripheral edge of balloon  20  (S&gt;R/2). Thus, rotor units  30  are located closer to the peripheral edge of balloon  20  than central axis P of balloon  20 . Arranging rotor units  30  in this manner makes it possible to secure enough space between rotor units  30  and stably fly aircraft  10 . 
     The cross sectional area of each ventilation hole  22  is smallest in the central region, in the up-and-down direction, of ventilation hole  22  (i.e., the area of a cross section taken perpendicular to central axis Q is smallest in the central region of ventilation hole  22 ). Each ventilation hole  22  has a shape that gradually expands in cross sectional area from the central region in the up-and-down direction toward the top end portion and from the vertical central region toward the bottom end portion. Stated differently, each ventilation hole  22  has the shape of a pillar with a pinched midsection. As described above, balloon  20  gradually decreases in height from its central region toward its peripheral edge. As such, with respect to each ventilation hole  22 , the height h measured closer to the peripheral edge of balloon  20  is less than the height H measured closer to the central region of balloon  20 . 
     As illustrated in  FIG. 5 , four ventilation holes  22  are arranged at 90 degree intervals around central axis P of balloon  20 . Central axes Q of ventilation holes  22  are equidistant from central axis P of balloon  20 . Stated differently, central axis Q of each ventilation hole  22  is approximately orthogonal to one pitch circle PC centered on central axis P of balloon  20 . 
     As illustrated in  FIG. 5 , in a top view, the peripheral edge of balloon  20  includes reference curve sections  23  and small curvature radius sections  24 . There are the same number of reference curve sections  23  as there are ventilation holes  22  and the same number of small curvature radius sections  24  as there are ventilation holes  22  (four in this embodiment). Reference curve sections  23  and small curvature radius sections  24  are alternately arranged around the peripheral edge of balloon  20  in a top view. Each small curvature radius section  24  is arranged outward of a different one of ventilation holes  22  (here, “outward” means on a side opposite central axis P of balloon  20 ). Each reference curve section  23  is disposed between two adjacent small curvature radius sections  24 . 
     Reference curve sections  23  and small curvature radius sections  24  are both curved lines. The midpoint of the length (in the circumferential direction) of each small curvature radius section  24  is located on line L that intersects central axis Q of the closest ventilation hole  22  as well as central axis P of balloon  20  at right angles. 
     The radius of curvature of each small curvature radius section  24  is shorter than the radius of curvature of each reference curve section  23 . However, the radius of curvature of each reference curve section  23  need not be constant throughout the length of reference curve section  23 . The radius of curvature of each small curvature radius section  24  also need not be constant throughout the length of small curvature radius section  24 . When the radius of curvature of reference curve sections  23  and small curvature radius sections  24  is not constant, the maximum radius of curvature of each small curvature radius section  24  may be less than the minimum radius of curvature of each reference curve section  23 . 
     As illustrated in  FIG. 6 , balloon  20  includes a tubular coupler  25 . Coupler  25  is made of a transparent sheet material shaped into a cylinder (or round tube) whose top and bottom end portions have a slightly increased diameter. Coupler  25  is disposed such that its central axis is approximately coaxial with central axis P of balloon  20 . Inside balloon  20 , the top end of coupler  25  is connected to the upper portion of balloon  20  and the bottom end of coupler  25  is connected to the lower portion of balloon  20 . 
     The top end of the tubular coupler  25  is sealed, whereas the bottom end is open. The space inside coupler  25  is therefore in fluid communication with the space outside balloon  20 . Air is present in the space inside coupler  25 , and the pressure inside the space is essentially the same as the atmospheric pressure. 
     As described above, balloon  20  has rotational symmetry about central axis P extending in the up-and-down direction. The first gas, such as helium, that fills first gas chamber  21   a  of balloon  20  is evenly distributed throughout the entire first gas chamber  21   a . Similarly, the second gas, such as air, that fills second gas chamber  21   b  of balloon  20  is evenly distributed throughout the entire second gas chamber  21   b . Accordingly, the working point of the buoyant force (center of buoyancy) imparted by the first gas in balloon  20  is located approximately on central axis P of balloon  20 . 
     As illustrated in  FIG. 2 ,  FIG. 4 , and  FIG. 5 , aircraft  10  further includes release unit  26  that is disposed on the upper portion of first shock absorber  20   a  of balloon  20  and releases the first gas contained in first gas chamber  21   a  at a predetermined timing (for example, upon receipt of an instruction from controller  41  to release the first gas). Release unit  26  includes a valve that selectively places first gas chamber  21   a  in fluid communication with the outside space. The valve is, for example, an electromagnetic valve. In other words, release unit  26  releases the first gas contained in first gas chamber  21   a  to the outside space by opening the valve at a predetermined timing. 
     Balloon  20  includes a plurality of ducts  28  that communicatively connect the plurality of ventilation holes  22 . More specifically, the plurality of ducts  28  communicatively connect the space inside coupler  25 , which is located in the center of balloon  20  in a top view, with each of the plurality of ventilation holes  22 . A part of fixing component  50  is disposed inside each of the plurality of ducts  28 . In other words, the plurality of ducts  28  define spaces for accommodating part of fixing component  50 . 
     Balloon  20  includes recess  29  of a size capable of housing one or more on-board devices including camera  44 . The opening of recess  29  is located on the bottom end of coupler  25 . 
     Moreover, protective nets  62  and  63  are provided at the top and bottom portions of ventilation holes  22  of balloon  20  to inhibit contact with rotor units  30  disposed inside ventilation holes  22  in the event that an object contacts the upper or lower portion of ventilation holes  22 . 
     In this embodiment, the inner volumetric capacity of balloon  20  (i.e., the volumetric capacity of gas chamber  21 ) is determined such that the buoyant force of the gas filling balloon  20  is slightly more than the gross weight of aircraft  10 . Thus, aircraft  10  slowly ascends even if the plurality of rotor units  30  stop mid-air. 
     (Rotor Units) 
     Next, rotor units  30  will be described. 
     As illustrated in  FIG. 2  and  FIG. 3 , each rotor unit  30  includes propeller  32  and motor  33 . 
     Motors  33  are attached to arms  52  of fixing component  50  (to be described later). Propellers  32  are attached to the output shafts of motors  33 . Note that each rotor unit  30  may include two propellers  32  that coaxially rotate in opposite directions. In other words, each rotor unit  30  may include contra-rotating propellers. 
     One rotor unit  30  is disposed in each ventilation hole  22 . Rotor units  30  are oriented such that the axes of rotation of propellers  32  are approximately vertical. The axes of rotation of propellers  32  are approximately coaxial with central axes Q of ventilation holes  22 . Rotor units  30  are disposed in the central regions of ventilation holes  22  in the up-and-down direction. In other words, as illustrated in  FIG. 3 , rotor units  30  are disposed so as to overlap central plane M in the up-and-down direction of balloon  20 . This central plane M is located in the central region of balloon  20  in the up-and-down direction, and is orthogonal to central axis P of balloon  20 . The outer diameter of each rotor unit  30  is roughly equal to the inner diameter of the central region of ventilation hole  22  in the up-and-down direction. 
     Each rotor unit  30  is disposed such that the entire height fits within ventilation hole  22 . In other words, each rotor unit  30  is laterally covered by balloon  20 , across the height of rotor unit  30  in the up-and-down direction. In particular, each rotor unit  30  is laterally covered by second shock absorber  20   b  of balloon  20 , across the height of rotor unit  30  in the up-and-down direction. Note that the “up-and-down direction” refers to the up and down directions when aircraft  10  is horizontally level and not tilted. In other words, the up-and-down direction is approximately parallel to the axis of rotation of each rotor unit  30 . 
     Each ventilation hole  22  preferably has a height such that distances from the center in the up-and-down direct of rotor unit  30  to the top and to the bottom are each greater than or equal to the radius of rotor unit  30 . With this, when rotor unit  30  receives an impact or breaks, for example, even if the axis of rotation of propeller  32  of rotor unit  30  were to rotate 90 degrees relative to aircraft  10 , rotor unit  30  can be inhibited from projecting out of ventilation hole  22 . Accordingly, balloon  20  can laterally cover rotor unit  30  to a degree such that rotor unit  30  is not likely to contact an object. 
     (On-Board Devices, Light Emitter) 
     As described above, aircraft  10  is provided with, as on-board devices, controller  41 , battery  42 , projector  43 , and camera  44 . Aircraft  10  is further provided with light emitter  46 . 
     As illustrated in  FIG. 3 , aircraft  10  includes disc  40 . Disc  40  is a disc-shaped component whose diameter is substantially equal to the bottom end of coupler  25 . Disc  40  is disposed so as to cover the bottom end surface of coupler  25 . Disc  40  may be made of a resin material such as polypropylene (PP), polycarbonate (PC), polybutylene terephthalate (PBT), or ABS resin, and may be made of a metal material such as aluminum, copper, or stainless steel. 
     Camera  44 , which is used for capturing images, is attached to the bottom surface of disc  40  via gimbal  45 . Camera  44  is for capturing aerial video, and is angled diagonally downward. As illustrated in  FIG. 3  and  FIG. 4 , camera  44  protrudes beyond balloon  20  in a downward direction along a predetermined axis. Gimbal  45  is for holding camera  44  at a steady angle, even if the orientation of aircraft  10  changes. 
     Controller  41 , battery  42 , and projector  43  are disposed on top of disc  40 . Controller  41  is a device that controls operation of plurality of rotor units  30 . In this embodiment, controller  41  includes receiver  41   a  that receives an instruction signal transmitted from a radio control device functioning as a control terminal operated by an operator. Controller  41  controls rotor units  30 , camera  44 , projector  43 , and LEDs based on the instruction signal received by receiver  41   a . Controller  41  also transmits video captured by camera  44 . 
     Note that controller  41  having the functions described above is implemented as a computer including, for example, a central processing unit (CPU), random access memory (RAM), read only memory (ROM), communications interface, and an I/O port. 
     Battery  42  supplies power to rotor units  30 , controller  41 , projector  43 , and light emitter  46 . Projector  43  projects video onto the inner surface of balloon  20 , which is made of a semi-transparent material. 
     Light emitter  46  is an LED light strip including an elongated flexible printed substrate and multiple light-emitting elements (such as LEDs) aligned in the lengthwise direction of the elongated flexible printed substrate 
     Note that aircraft  10  may include only one device among projector  43 , camera  44 , and light emitter  46 , and, alternatively, may include all of the devices. Aircraft  10  may also include other types of devices such as a speaker and/or a display panel. In other words, aircraft  10  need only include devices for achieving basic flight, such as rotor units  30 ; aircraft  10  may include devices that do not essentially contribute to flight, such as projector  43  and camera  44 , on an as-needed basis according to the needs of the user. 
     (Fixing Component) 
     Fixing component  50  fixes the plurality of rotor units  30  in predetermined positions in a plan view and fixes the plurality of rotor units  30  such that the axes of rotation of the plurality of rotor units  30  are substantially parallel to the up-and-down direction. More specifically, fixing component  50  includes main body  51 , four arms  52 , and two holding components  54 . 
     Main body  51  is a cylindrical component having a bottom in the upper portion. Main body  51  is disposed inside the space on the inner side of coupler  25 . Stated differently, main body  51  defines a space therein. 
     The four arms  52  are tubular components that are fixed to the side surface of main body  51 , and extend in four different directions from the side surface of main body  51 . Here, four different directions are directions toward each of the plurality of ventilation holes  22  from the space on the inner side of coupler  25 . The four arms  52  are disposed inside ducts  28 . 
     The four arms  52  include distal end sections  52   a  that fix the four rotor units  30  such that the axes of rotation of the four rotor units  30  are substantially parallel to the up-and-down direction. More specifically, the lower portion of motors  33  of rotor units  30  are fixed to distal end sections  52   a.    
     The two holding components  54  are fixed to the lower portion of main body  51 , and extend downward from main body  51 . The bottom ends of the two holding components  54  hold disc  40  that supports on-board devices. 
     Note that main body  51  includes, where the four arms  52  are fixed thereto, through-holes for fluid communication with the inner space of main body  51 . With these, the inner space of main body  51  is in fluid communication with spaces inside the four arms  52 . The spaces inside the four arms  52  house electrical wiring (not illustrated) for supplying power from battery  42  to the plurality of rotor units  30 . That is, the four arms  52  also function as conduit for housing electrical wiring. 
     Note that fixing component  50  may be supported by balloon  20  as a result of the plurality of arms  52  being held in the plurality of ducts  28  under pressure from second gas chamber  21 B, and, alternatively, main body  51  may be fixed in a predetermined position on coupler  25 . In other words, fixing component  50  may be fixed by any means so long as it is fixed in a predetermined position relative to balloon  20 . 
     Disc  40  supports housing  55  that houses a weight in addition to the on-board devices. Stated differently, aircraft  10  includes housing  55 . Housing  55  is a box-shaped component that defines a space that can house a metal (e.g., lead, copper, alloy) weight. Note that the weight is not limited to a metal weight; the weight may be a non-metal weight (such as sand). The weight is capable of adjusting the gross weight of aircraft  10  in predetermined units of weight (for example, 1 to 10 grams). 
     Since balloon  20  is made of a stretchable material, it is difficult to set the amount of gas to be filled in gas chamber  21  (stated differently, to set the volumetric area of gas chamber  21 ). Accordingly, it is difficult to estimate the magnitude of the buoyant force imparted by the gas without error before the gas is filled in gas chamber  21  of balloon  20 . 
     Accordingly, by providing housing  55 , after gas is filled into gas chamber  21  of balloon  20 , the gross weight of aircraft  10  can be adjusted by adding or removing weights to or from housing  55 . With this, as described in Embodiment 1, it is easy to adjust the gross weight of aircraft  10  such that the magnitude of the buoyant force imparted by the gas filling balloon  20  is slightly greater than the gross weight of aircraft  10 . Note that the magnitude of the buoyant force imparted by the gas is set so as to always be greater than the gross weight of aircraft  10  when no weights are housed in housing  55 , even when there are slight differences in volumetric area between gas chambers  21  of balloons  20 . 
     Voltage regulators  53  are provided to the four arms  52 . Voltage regulators  53  are amplifiers that adjust the voltage of the power that drives the respective motors  33  included in rotor units  30  disposed on arms  52 . Voltage regulators  53  are disposed in ventilation holes  22 . 
     (Flying Orientation of Aircraft) 
     As described above, in aircraft  10 , on-board devices such as controller  41  and battery  42  are disposed in the lower end portion of the space inside coupler  25 . In other words, the relatively heavy on-board devices are clustered in the lower portion of aircraft  10 . Accordingly, the overall center of gravity of aircraft  10  is lower than the working point of the buoyant force imparted by the gas filling balloon  20 . With this, even when rotor units  30  are stopped, aircraft  10  can maintain an orientation in which camera  44  is oriented downward, without rotating horizontally or flipping top over bottom, for example. 
     Moreover, relatively heavy on-board devices are disposed below rotor units  30 . As a result, the overall center of gravity of aircraft  10  is lower than the working point of the buoyant force imparted by rotor units  30  operating. With this, even when rotor units  30  are operating, aircraft  10  can maintain an orientation in which camera  44  is oriented downward. 
     Aircraft  10  includes a plurality of rotor units  30 . When moving aircraft  10  in a substantially horizontal direction, increasing the rotational speed of a rotor unit  30  located further in the opposite direction of travel to a speed greater than the rotational speed of a rotor unit  30  located further in the direction of travel allows aircraft  10  to increase propulsion in a horizontal direction. 
     Note that the “rotational speed of a rotor unit  30 ” means the rotational speed of propeller  32  included in the rotor unit  30  (revolutions of propeller  32  per unit time). 
     (Aircraft Operation Control Example) 
     With aircraft  10  according to this embodiment, gas release control is performed at a predetermined timing. That is to say, release unit  26  is controlled so as to release the first gas contained in first gas chamber  21   a  at a predetermined timing. 
     Next, this gas release control according to aircraft  10  will be described with reference to  FIG. 7  through  FIG. 9 . 
       FIG. 7  is a block diagram illustrating a configuration of aircraft  10  according to Embodiment 1. Note that illustration of some elements, such as battery  42  and camera  44 , are omitted in  FIG. 7 .  FIG. 8A  is a flow chart of one example of the gas release control in aircraft  10  according to Embodiment 1.  FIG. 8B  is a flow chart of another example of the gas release control in aircraft  10  according to Embodiment 1.  FIG. 9  illustrates aircraft  10  according to Embodiment 1 when gas release control is performed. Note that  FIG. 9  is the same cross section as illustrated in  FIG. 4 . 
     As illustrated in  FIG. 7 , aircraft  10  includes a plurality (in this embodiment, four) rotor units  30 , controller  41 , release unit  26 , and detector  80 . The plurality of rotor units  30  generate thrust for flying aircraft  10 . 
     Controller  41  controls the rotation of propellers  32  of the plurality of rotor units  30 . Controller  41  includes receiver  41   a  that receives an instruction signal transmitted from a radio control device. Controller  41  controls the rotation of propellers  32  of the plurality of rotor units  30  in accordance with a flight instruction signal transmitted from the radio control device. Receiver  41   a  may also receive signals other than the above-described instruction signal. 
     Detector  80  detects an abnormal state of aircraft  10 , and transmits an abnormal state signal indicating the result of the detection to controller  41 . 
     More specifically, detector  80  may monitor the state of battery  42 , and, for example, may detect that battery  42  has no charge or is low on charge as an abnormal state of aircraft  10 . Note that battery  42  being low on charge is a state in which the charge capacity is 10% or less where a charge capacity of 100% represents a state in which battery  42  is fully charged. 
     Detector  80  may detect, as an abnormal state of aircraft  10 , that aircraft  10  is not descending even though receiver  41   a  has received an instruction signal instructing aircraft  10  to descend. More specifically, detector  80  may monitor the operational state of rotor units  30 , and detector  80  may detect, as an abnormal state of aircraft  10 , that rotor units  30  are not being rotated so as to generate thrust that causes aircraft  10  to descend, even though receiver  41   a  has received an instruction signal instructing aircraft  10  to descend. Detector  80  may detect the elevation of aircraft  10 , and detect, as an abnormal state of aircraft  10 , that the elevation of aircraft  10  is not decreasing even though receiver  41   a  has received an instruction signal instructing aircraft  10  to descend. 
     Controller  41  transmits a release command to release unit  26  in accordance with a release instruction signal transmitted from the radio control device. 
     Although not illustrated in  FIG. 7 , note that aircraft  10  includes balloon  20  that functions as a shock absorber, as described above. 
     In aircraft  10  having the configuration described above, the gas release control may be implemented via, for example, the information processing and operations illustrated in  FIG. 8A . In other words, detector  80  detects an abnormal state of aircraft  10  (S 11 ). Controller  41  releases the first gas contained in first gas chamber  21   a  to the outside space by switching release unit  26  to an open state (S 12 ). In other words, in the gas release control illustrated in  FIG. 8A , release unit  26  releases the first gas contained in first gas chamber  21   a , when, as the predetermined timing, detector  80  detects an abnormal state. 
     In aircraft  10  having the configuration described above, the gas release control may be implemented via, for example, the information processing and operations illustrated in  FIG. 8B . In other words, receiver  41   a  receives a release instruction signal transmitted from the radio control device (S 11   a ). Controller  41  releases the first gas contained in first gas chamber  21   a  to the outside space by switching release unit  26  to an open state (S 12 ). Stated differently, in the gas release control illustrated in  FIG. 8B , release unit  26  releases the first gas contained in first gas chamber  21   a , when, as the predetermined timing, receiver  41   a  receives a release instruction signal indicating release of the first gas contained in first gas chamber  21   a.    
     By performing one of the gas release controls described with reference to  FIG. 8A  and  FIG. 8B , the valve in release unit  26  places first gas chamber  21   a  in fluid communication with the outside space, as illustrated in  FIG. 9 . This releases the first gas contained in the first gas chamber  21   a  to the space outside first gas chamber  21   a . Since the released first gas is less dense than air, the specific gravity of aircraft  10  gradually becomes heavier as the first gas is released, and the magnitude of the buoyant force imparted by the gas that filled first gas chamber  21   a  decreases to less than the gross weight of aircraft  10 . Aircraft  10  then begins to descend. 
     (Advantageous Effects, Etc., of Embodiment 1) 
     Aircraft  10  according to this embodiment includes: a plurality of rotor units  30 , each of which includes propeller  32  and motor  33  that drives propeller  32 ; balloon  20  including first shock absorber  20   a  that defines first gas chamber  21   a  containing a first gas less dense than air and second shock absorber  20   b  that is different than first shock absorber  20   a ; and release unit  26  that is disposed in first shock absorber  20   a  and releases the first gas contained in first gas chamber  21   a , at a predetermined timing. 
     With this configuration, since the first gas that is less dense than air can be released via release unit  26  at a predetermined timing, for example, when aircraft  10  becomes uncontrollable mid-flight, aircraft  10  can be made to swiftly descend. Moreover, even if the first gas is released, since the state of second shock absorber  20   b  can be maintained, when aircraft  10  descends, the plurality of rotor units  30  and on-board devices in aircraft  10  such as camera  44  can be inhibited from directly contacting an object on the ground or a floating object before landing. Accordingly, even in cases in which aircraft  10  is brought down to the land by releasing the first gas, the plurality of rotor units  30 , the on-board devices of aircraft  10 , and/or the object can be prevented from being damaged. 
     Moreover, in this embodiment, second shock absorber  20   b  is disposed at least in the lower portion of aircraft  10 . Accordingly, it is possible to maintain the state of second shock absorber  20   b  disposed on the lower portion of aircraft  10 , even when aircraft  10  is caused to descend by releasing the first gas. As such, even while aircraft  10  is descending, the plurality of rotor units and other on-board devices of the aircraft can be effectively inhibited from directly contacting an object. 
     Moreover, in this embodiment, second shock absorber  20   b  laterally covers the plurality of rotor units  30 , across a height of the plurality of rotor units  30  in the up-and-down direction. Accordingly, it is possible to maintain the state of second shock absorber  20   b  that laterally covers the plurality of rotor units  30 , across a height of the plurality of rotor units  30  in the up-and-down direction, even when aircraft  10  is caused to descend by releasing the first gas. As such, even while aircraft  10  is descending, the plurality of rotor units  30  can be effectively inhibited from directly contacting an object. 
     Moreover, in this embodiment, second shock absorber  20   b  defines second gas chamber  21   b  containing a second gas that is more dense than the first gas. This makes it possible to implement second shock absorber  20   b  with a simple configuration. 
     Moreover, in this embodiment, first shock absorber  20   a  is disposed in the upper portion of aircraft  10 . This makes it possible to efficiently release the first gas, which is less dense than air, contained in first gas chamber  21   a  defined by first shock absorber  20   a . It also makes it possible to position the center of gravity of aircraft  10  in the lower portion of aircraft  10  and thus inhibit aircraft  10  from flipping top over bottom. 
     Moreover, in this embodiment, release unit  26  includes a valve that selectively places the first gas chamber in fluid communication with the outside space. In other words, release unit  26  releases the first gas contained in the first gas chamber to the outside space by opening the valve at a predetermined timing. This makes it possible to implement a simple configuration for releasing the first gas via release unit  26  at a predetermined timing. 
     Moreover, in this embodiment, release unit  26  is configured as an electromagnetic valve capable of freely switching between open and closed states. Accordingly, even after the gas release control has been performed and the first gas has been released, aircraft  10  can be reused if first shock absorber  20   a  of aircraft  10  is reinflated with the first gas. 
     Moreover, in this embodiment, aircraft  10  further includes receiver  41   a  that receives a signal. Furthermore, the predetermined timing is when receiver  41   a  receives a release instruction signal indicating release of the first gas contained in the first gas chamber  21   a . Release unit  26  releases the first gas contained in the first gas chamber when receiver  41   a  receives the release instruction signal. This makes it possible to release the first gas via release unit  26  by, for example, an operator using a control terminal to transmit a release instruction signal. With this, the operator can cause aircraft  10  to swiftly descend by releasing the first gas via release unit  26  at a predetermined timing. 
     Moreover, in this embodiment, aircraft  10  further includes detector  80  that detects an abnormal state of the aircraft. Furthermore, the predetermined timing is when the abnormal state is detected by detector  80 . Release unit  26  releases the first gas contained in first gas chamber  21   a  when detector  80  detects an abnormal state. This makes it possible to release the first gas via release unit  26  when aircraft  10  is in an abnormal state. This in turn makes it possible for aircraft  10  to automatically descend in the case of an abnormal state. 
     Moreover, in this embodiment, aircraft  10  further includes receiver  41   a  that receives an instruction signal transmitted from a control terminal operated by an operator. Detector  80  detects, as the abnormal state, that aircraft  10  is not descending even though receiver  41   a  has received an instruction signal instructing aircraft  10  to descend. This makes it possible to release the first gas via release unit  26  when aircraft  10  is in an abnormal state, namely, when aircraft  10  cannot descend. This in turn makes it possible for aircraft  10  to automatically descend in the case of an abnormal state in which aircraft  10  cannot descend even when instructed to do so by the operator. 
     Moreover, in this embodiment, balloon  20  has a flattened shape in the up-and-down direction. 
     This makes it less likely that aircraft  10  will tilt relative to the axis of symmetry (central axis P) of balloon  20  mid-flight, resulting in a more stable flight of aircraft  10 . 
     Moreover, in this embodiment, rotor units  30  and ventilation holes  22  in which rotor units  30  are disposed are located closer to the peripheral edge of balloon  20  than central axis P of balloon  20 . 
     Accordingly, in aircraft  10 , sufficient space between the plurality of rotor units  30  can be secured. Thus, according to this embodiment, since sufficient space between the plurality of rotor units  30  can be secured, flight of aircraft  10  can be stabilized. 
     Moreover, in this embodiment, balloon  20  gradually decreases in height from its central region toward its peripheral edge. 
     With this, when viewed from a lateral side, balloon  20  has a streamline shape. Thus, according to this embodiment, it is possible to reduce the resistance of aircraft  10  to air mid-flight. Furthermore, when ventilation holes  22  are arranged at intervals of a predetermined angle around the central axis of balloon  20  extending in the up-and-down direction, ventilation holes  22  are located at relatively slim portions of balloon  20 , making it possible to keep the lengths of ventilation holes  22  relatively short. The shorter the lengths of ventilation holes  22 , the less the loss in air pressure is as air passes through ventilation holes  22 . Thus, in this case, it is possible to secure a sufficient amount of air flow through ventilation holes  22 , which makes it possible to secure sufficient propulsion by rotor units  30 . 
     Moreover, in this embodiment, coupler  25  is provided in the central region of balloon  20  with one end connected to the upper portion of balloon  20  and the other end connected to the lower portion of balloon  20 . 
     In other words, in the central region of balloon  20 , the upper portion and lower portion of balloon  20  are connected via coupler  25 . Accordingly, balloon  20  can easily assume a desired shape, such as a flattened shape. When the shape of balloon  20  is stable, ventilation holes  22  formed in balloon  20  are also stable, making it possible to achieve actual ventilation holes  22  similar to their design shape. Thus, it is possible to secure a sufficient amount of air flow through ventilation holes  22 , which makes it possible to secure sufficient propulsion by rotor units  30 . Moreover, stabilizing the shape of ventilation holes  22  formed in balloon  20  makes it easier to approximately match the shapes of all ventilation holes  22 . This further equalizes the amount of air flowing through ventilation holes  22 , which stabilizes the flight of aircraft  10 . 
     Moreover, in this embodiment, coupler  25  has a tubular shape. 
     As such, the central regions of the upper and lower portions of balloon  20  (i.e., the regions surrounding central axis P of balloon  20 ) are connected to one another via tubular coupler  25 , across the entire perimeter of the central regions. Thus, according to this embodiment, it is further easier for balloon  20  to maintain a desirable shape. 
     Moreover, in this embodiment, the space inside coupler  25  is in fluid communication with the space outside balloon  20 . 
     As such, air fills the space inside coupler  25  rather than gas for exerting buoyant force, such as helium. 
     Moreover, in this embodiment, each ventilation hole  22  has a shape that gradually expands in cross sectional area from the central region in the up-and-down direction toward the top end portion and from the central region in the up-and-down direction toward the bottom end portion. 
     Giving ventilation holes  22  such a shape reduces a loss in air pressure as air flows into ventilation holes  22  and a loss in air pressure as air flows out of ventilation holes  22 . As such, even when rotor units  30  generate little thrust, it is possible to secure a sufficient amount of air flow through ventilation holes  22 , which makes it possible to secure sufficient propulsion by rotor units  30 . Thus, since the same thrust is achieved, it is possible to reduce the amount of energy used by rotor unit  30 . 
     Moreover, in this embodiment, balloon  20  gradually decreases in height from its central region toward its peripheral edge, and with respect to each ventilation hole  22 , the height h measured near the peripheral edge of balloon  20  is less than the height H measured near the central region of balloon  20 . 
     With this, in each ventilation hole  22  in balloon  20 , air flows into ventilation hole  22  from a direction originating from the peripheral edge of balloon  20  and exits ventilation hole  22  in a direction heading toward the peripheral edge of balloon  20 . As a result, air flowing into one ventilation hole  22  can be inhibited from interfering with air flowing into another ventilation hole  22 , and air flowing out of one ventilation hole  22  can be inhibited from interfering with air flowing out of another ventilation hole  22 . Thus, according to this embodiment, disruption of airflow due to interference of air flowing into and out of ventilation holes  22  can be inhibited, which stabilizes flight of aircraft  10 . 
     Moreover, in this embodiment, rotor units  30  are disposed in the central regions in the up-and-down direction of ventilation holes  22 . In other words, rotor units  30  are disposed so as to overlap a central plane in the up-and-down direction of balloon  20 . 
     As such, air flowing from the top of ventilation hole  22  toward rotor unit  30  and air flowing from rotor unit  30  toward the bottom of ventilation hole  22  can be stabilized, which stabilizes flight of aircraft  10 . 
     Moreover, in this embodiment, ventilation holes  22  are arranged at intervals of a predetermined angle around central axis P of balloon  20  extending in the up-and-down direction. 
     Since the plurality of rotor units  30  are therefore disposed at intervals of a predetermined angle around central axis P of balloon  20  and blow air downward, flight of aircraft  10  can be stabilized. 
     Moreover, in this embodiment, balloon  20  has rotational symmetry about a line extending in the up-and-down direction. 
     As such, the working point of the buoyant force imparted by the gas filling balloon  20  can be located on the axis of symmetry (i.e., central axis P) of balloon  20 . As such, aircraft  10  can be inhibited from tilting mid-flight (i.e., tilt relative to the up-and-down (vertical) direction of central axis P of balloon  20 ), which stabilizes flight of aircraft  10 . 
     Moreover, in this embodiment, in a top view, the peripheral edge of balloon  20  includes reference curve sections  23  and small curvature radius sections  24  having a smaller radius of curvature than reference curve sections  23 . Reference curve sections  23  and small curvature radius sections  24  are alternately arranged around the peripheral edge. There are the same number of reference curve sections  23  as there are ventilation holes  22  and the same number of small curvature radius sections  24  as there are ventilation holes  22 . Each small curvature radius section  24  is disposed adjacent to a different one of ventilation holes  22 , in a more peripheral position than the ventilation hole  22  it is disposed adjacent to. 
     Here, tension working in sections of the peripheral edge of balloon  20  in a top view near ventilation holes  22  is lower than tension working in sections of the peripheral edge of balloon  20  in a top view further away from ventilation holes  22 . This is because tension is working on portions of balloon  20  that form the walls of ventilation holes  22 . When tension working on balloon  20  is regionally low, wrinkles easily form where working tension is low. 
     In light of this, in this embodiment, the radius of curvature of sections of the peripheral edge of balloon  20  in a top view near ventilation holes  22  is less than the radius of curvature of sections of the peripheral edge of balloon  20  in a top view further from ventilation holes  22 . As such, the difference between tension working in sections of the peripheral edge of balloon  20  in a top view near ventilation holes  22  and tension working in sections of the peripheral edge of balloon  20  in a top view further away from ventilation holes  22  can be reduced. Thus, according to this embodiment, wrinkles can be kept from forming in balloon  20 , and the aesthetics of balloon  20  can be maintained. 
     Moreover, in this embodiment, on-board devices are housed in the space inside coupler  25 . The housed on-board devices include at least controller  41  that controls rotor units  30  and battery  42  that supplies power to rotor units  30 . 
     The space inside coupler  25  is in fluid communication with the space outside balloon  20 . As such, maintenance such as changing battery  42  disposed in the space inside coupler  25  can be done without releasing the gas for providing buoyancy, such as helium, from balloon  20 . 
     Moreover, in this embodiment, on-board devices are disposed at the bottom end portion of the space inside coupler  25 . 
     As such, the center of gravity of aircraft  10  can be lowered, thereby stabilizing flight of aircraft  10 . 
     Moreover, in this embodiment, coupler  25  is transparent, and light emitter  46  is housed in the space inside coupler  25 . 
     In this embodiment, light emitted by light emitter  46  passes through the transparent coupler  25 . As such, if the outer layer of balloon  20  is made of a semi-transparent material, for example, light emitted by light emitter  46  will strike the inner surface of balloon  20 , whereby the color of the entire balloon  20  can be changed to the color of light emitted by light emitter  46 . Thus, according to this embodiment, the color of balloon  20  can be changed mid-flight to easily achieve a dramatic effect, for example. 
     Variations of Embodiment 1 
     Variation 1 
     In aircraft  10  according to Embodiment 1, release unit  26  is implemented as a valve, but release unit  26  is not limited to this example. For example, as illustrated in  FIG. 10 , aircraft  10 A includes balloon  20 A having, instead of release unit  26 , hole opener  26 Aa that opens a hole in a predetermined region of first shock absorber  20 Aa.  FIG. 10  illustrates aircraft  10 A according to Variation 1 of Embodiment 1 when gas release control is performed. In  FIG. 10 , (a) illustrates the whole aircraft  10 A after gas release control has been performed, and (b) illustrates enlarged views of release unit  26 A of aircraft  10  before and after gas release control is performed. Note that  FIG. 10  is the same cross section as illustrated in  FIG. 4 . 
     As illustrated in (b-1) in  FIG. 10 , in a state before gas release control has been performed, release unit  26 A is disposed on the upper portion of first shock absorber  20 Aa in aircraft  10 A. Release unit  26 A includes, for example, hole opener  26 Aa including gunpowder capable of creating a small explosion. Hole opener  26 Aa creates a small explosion upon receiving a release command from controller  41  at a predetermined timing ((b-2) in  FIG. 10 ) that opens hole  26 Ac in first shock absorber  20 Aa, which places first gas chamber  21 Aa in fluid communication with the outside space ((b-3) in  FIG. 10 ). This releases the first gas contained in the first gas chamber  21 Aa to the space outside first gas chamber  21 Aa. Since the released first gas is less dense than air, the specific gravity of aircraft  10 A gradually becomes heavier as the first gas is released, and the magnitude of the buoyant force imparted by the gas that filled first gas chamber  21 Aa decreases to less than the gross weight of aircraft  10 A. Aircraft  10 A then begins to descend. 
     In this way, with aircraft  10 A according to Variation 1 of Embodiment 1, release unit  26 A is configured to open hole  26 Ac in first shock absorber  20 Aa, which places first gas chamber  21 Aa in fluid communication with the outside space. Release unit  26 A releases the first gas contained in first gas chamber  21 Aa to the outside space at a predetermined timing by opening hole  26 Ac. This makes it possible to implement a simple configuration for releasing the first gas via release unit  26 A at a predetermined timing. 
     Note that, as illustrated in (b-1) in  FIG. 10 , region  26 Ab in which hole  26 Ac is opened in first shock absorber  20 Aa is preferably more fragile than other regions of first shock absorber  20 Aa. More specifically, region  26 Ab in which hole  26 Ac is opened may be made to be fragile by being thinner than other regions of first shock absorber  20 Aa or by being made of a more fragile material than the vinyl chloride material used to make first shock absorber  20 Aa (e.g., latex). This makes it easy to open hole  26 Ac in the region of first shock absorber  20 Aa where hole  26 Ac is to be formed. 
     Moreover, the hole opener is not limited to opening the hole via a small explosion. The hole opener may open a hole in region  26 Ab in which hole  26 Ac is to be formed using a needle or sharp blade. In other words, the hole opener is not limited to the above example, and may have any configuration that can open a hole in first shock absorber  20 Aa. 
     Variation 2 
     Moreover, aircraft  10 B may be implemented by adding, to aircraft  10  according to Embodiment 1, compression components  71  for speeding up the releasing of the first gas when the gas release control is performed.  FIG. 11  illustrates aircraft  10 B according to Variation 2 of Embodiment 1 when gas release control is performed. Note that (a) in  FIG. 11  illustrates aircraft  10 B before the gas release control is performed, and corresponds to the cross section as illustrated in  FIG. 4 . Note that (b) in  FIG. 11  illustrates the overall state of aircraft  10 B after the gas release control has been performed. 
     As illustrated in  FIG. 11 , inside first shock absorber  20   a  of aircraft  10 B, compression components  71  are implemented as compression springs, which are disposed in a state in which they are expanded beyond their resting state. The respective ends of each compression component  71  are connected to the upper portion and lower portion of first shock absorber  20   a . Stated differently, compression components  71  are connected to the upper portion and lower portion of first shock absorber  20   a  in a state in which a compressive force is exerted that pulls the upper portion and lower portion of first shock absorber  20   a  toward one another. In this way, even though compression components  71  exert a compressive force that pulls the upper portion and the lower portion of first shock absorber  20   a  toward one another, since first gas chamber  21   a  of first shock absorber  20   a  is filled with the first gas, the compressive force and the pressure of the first gas are in equilibrium. Thus, first shock absorber  20   a  can maintain its cross-sectional elliptical shape without being deformed by compression components  71 . 
     Here, as illustrated in (b) in  FIG. 11 , since the first gas is released via release unit  26  when the gas release control is performed, the pressure of first gas and the compressive force from compression components  71  fall out of equilibrium. Accordingly, compression components  71  exert a compressive force that pulls the upper portion and lower portion of first shock absorber  20   a  toward one another. As a result, compression components  71  causes first gas chamber  21   a  to contract. This releases the first gas contained in the first gas chamber  21   a  more quickly. Thus, by performing the gas release control, aircraft  10 B can be caused to descend more quickly. 
     Variation 3 
     In aircraft  10  according to Embodiment 1, first shock absorber  20   a ,  20 Aa is made of the same vinyl chloride as second shock absorber  20   b , but may be made of a material that is more fragile than vinyl chloride (for example, latex). 
     Variation 4 
     In aircraft  10  according to Embodiment 1, second shock absorber  20   b  is the portion of balloon  20  that defines second gas chamber  21   b , but second shock absorber  20   b  is not limited to this example. For example, second shock absorber  20   b  may be made of a solid material, such as a sponge material or rubber material. In other words, second shock absorber  20   b  may be made of any material so long as the material can absorb the impact when colliding with an object. 
     Variation 5 
     In aircraft  10  according to Embodiment 1, ventilation holes  22  in which the plurality of rotor units  30  are disposed each extend across first shock absorber  20   a  and second shock absorber  20   b , but ventilation holes  22  are not limited to this example. For example, the ventilation holes may be formed exclusively in the second shock absorber. Note that when the ventilation holes are formed exclusively in the second shock absorber, the first shock absorber may be small in size and disposed more centrally than the ventilation holes. Moreover, a plurality of the first shock absorbers may be disposed in regions so as to avoid the upper and lower areas of the ventilation holes. 
     Variation 6 
     In Embodiment 1, the release instruction signal is received by receiver  41   a  included in controller  41 , but this is merely one example. The release instruction signal may be received by a receiver included in a different controller independent from controller  41 . In such cases, the different controller is preferably supplied with power from a different battery than battery  42 . In other words, the different receiver may be included in a controller in an auxiliary control system different from the control system including controller  41 . Accordingly, even when controller  41 , battery  42 , etc., in the main control system malfunction, the operator can still operate a control terminal to control release unit  26 . With this, even when the main control system malfunctions and can no longer perform control, since the auxiliary control system can be used to control release unit  26 , the first gas contained in first shock absorber  20   a  can be released to cause aircraft  10  to quickly descend. 
     Embodiment 2 
     Next, Embodiment 2 will be described. 
     Embodiment 2 is implemented to solve problems that arise when aircraft  10  falls from the air. One example of aircraft  10  falling from the air is illustrated in  FIG. 12 , in which aircraft  10  falls to ground  100  when the gas release control described in Embodiment 1 is performed. Another example of a cause of the aircraft falling from the air is when the gross weight of the aircraft is slightly greater than its buoyant force and, for example, the aircraft encounters an unexpected flight situation, such as one or more of rotor units  30  becoming uncontrollable. Note that  FIG. 12  illustrates aircraft  10  according to Embodiment 1 having fallen to ground  100  as a result of the gas release control being performed. 
     (Configuration of Holding Components) 
     Since aircraft  10  according to Embodiment 2 is the same as aircraft  10  according to Embodiment 1, detailed description thereof will be omitted. Here, the two holding components  54  included in fixing component  50  whose functions were not described in detail in Embodiment 1 will be described with reference to  FIG. 13 . 
       FIG. 13  is an enlarged view of holding components  54  and camera  44  in aircraft  10 . In  FIG. 13 , (a) illustrates an enlarged view of holding components  54  and camera  44  when aircraft  10  is flying. In  FIG. 13 , (b) illustrates an enlarged view of holding components  54  and camera  44  when aircraft  10  has fallen to ground  100 . 
     As illustrated in (a) in  FIG. 13 , holding components  54  hold disc  40 , which supports camera  44 , in a state in which camera  44  is protruding downward. For example, when aircraft  10  is flying, holding components  54  hold disc  40 , which supports camera  44 , at the bottom end of recess  29 , and each have an overall length of first length L 1 . In other words, when aircraft  10  is flying, holding components  54  hold camera  44  so as to protrude downward, below balloon  20 , thereby inhibiting balloon  20  from entering the frame of the video being captured by camera  44  and making it possible to capture video having a wide field of view. 
     As illustrated in (b) in  FIG. 13 , the overall length of each holding component  54  can be shortened along a predetermined axis extending up and down, i.e., the axis along which camera  44  protrudes from balloon  20 . More specifically, the overall length of each holding component  54  can be shortened to a position at which camera  44  is housed in recess  29 . In other words, when disc  40  is in a position at which camera  44  is housed in recess  29 , the overall length of each holding component  54  is second length L 2 . Holding components  54  are flexible, which allows them to contract and expand up and down. When camera  44  is pushed up from below, along the predetermined axis, holding components  54  contract. 
     For example, as illustrated in  FIG. 13 , holding components  54  may be implemented as sliding rails each having three sections and capable of sliding up and down. The three sections in each holding component  54  are capable sliding along one another, and may have ball bearings to help them slide smoothly. Note that each holding component  54  may be a sliding rail having two or four or more sections. 
     Since holding components  54  are capable of contracting, even when camera  44  contacts ground  100 , camera  44  can be housed in recess  29  of balloon  20 , reducing the impact imparted on camera  44 . 
     Advantageous Effects, Etc., of Embodiment 2 
     Aircraft  10  according to this embodiment includes: a plurality of rotor units  30  each of which includes propeller  32  and motor  33  that drives propeller  32 ; balloon  20  functioning as a shock absorber that laterally covers the plurality of rotor units  30 , across a height of the plurality of rotor units  30  in an up-and-down direction; camera  44  disposed protruding downward, along a predetermined axis, beyond balloon  20 ; and holding components  54  that hold camera  44  and whose overall lengths can be shortened in the up-and-down direction. 
     Accordingly, even when aircraft  10  accidentally contacts an object, camera  44 , which is an on-board device, can recede into balloon  20 . This reduces the impact imparted on camera  44  of aircraft  10  and/or the object, and reduces damage to camera  44  and/or the object. 
     Moreover, in this embodiment, balloon  20  includes recess  29  of a size capable of housing camera  44 , and the overall length of each holding component  54  can be shortened to a position at which camera  44  is housed in recess  29 . Accordingly, even when aircraft  10  accidentally contacts an object, camera  44 , which is an on-board device, can recede into recess  29  of balloon  20 . In other words, even if aircraft  10  contacts an object, if holding components  54  contract to the position at which camera  44  is housed in recess  29 , balloon  20  will contact the object in areas around recess  29 , so the impact imparted to camera  44  and/or the object can be effectively reduced. 
     Moreover, in this embodiment, holding components  54  are flexible, and when the camera is pushed up from below, contract. Accordingly, even when camera  44  of aircraft  10  accidentally contacts an object, camera  44  is pushed thereby causing holding components  54  to contract and camera  44  to recede so as to be housed in recess  29  of balloon  20 . In other words, even if camera  44  of aircraft  10  contacts an object, camera  44  will resultantly be housed in recess  29  of balloon  20 , and the next thing that will contact the object is the bottom end of recess  29  of balloon  20 . This effectively reduces the impact imparted to camera  44  and/or the object. 
     Variations of Embodiment 2 
     Variation 1 
     In Embodiment 2, holding components  54  are implemented as sliding rails, but holding components  54  are not limited to this example. For example, as illustrated in  FIG. 14 , holding components  54 A implemented as bellows may be used.  FIG. 14  is an enlarged view of holding components  54 A and camera  44  in the aircraft according to Variation 1 of Embodiment 2. 
     Moreover, the holding components are not limited to sliding rails or bellows; a configuration in which one of two sleeves having different diameters is inserted into the other may be used, and a configuration using three or more metal wires, strings, or cords may be used. 
     Variation 2 
     In Embodiment 2, holding components  54  passively contract when camera  44  is pushed up from below, but holding components  54  are not limited to this example. For example, holding components  54  may be configured to detect when camera  44  is about to contact an object and actively contract. 
       FIG. 15  is a block diagram illustrating a configuration of aircraft  10 C according to Variation 2 of Embodiment 2.  FIG. 16A  is a flow chart of one example of contraction control for holding components  54 B in aircraft  10 C according to Variation 2 of Embodiment 2.  FIG. 16B  is a flow chart of one example of contraction control for holding components  54 B in aircraft  10 C according to Variation 2 of Embodiment 2.  FIG. 17  is an enlarged view of holding components  54 B and camera  44  in aircraft  10 C. 
     Aircraft  10 C according to Variation 2 of Embodiment 2 differs from aircraft  10  according to Embodiment 1 in that release unit  26  is omitted and replaced with holding components  54 B driven by controller  41 , as illustrated in  FIG. 15 . Moreover, what is detected by detector  81  in aircraft  10 C according to Variation 2 of Embodiment 2 differs from what is detected by the detector in aircraft  10  according to Embodiment 1. The remaining components in aircraft  10 C according to Variation 2 of Embodiment 2 are the same as in aircraft  10  according to Embodiment 1. Accordingly, the following description will focus on the points of difference with aircraft  10  according to Embodiment 1; description of other components is omitted. 
     Holding components  54 B are implemented as, for example, electric cylinders whose overall lengths are adjusted via a motor. Note that holding components  54 B are not limited to electric cylinders, and may be implemented as, for example, hydraulic or pneumatic cylinders. The overall length of each holding component  54 B is shortened by holding components  54 B being driven by controller  41  at a predetermined timing. 
     Detector  81  detects the state of aircraft  10 C, and transmits a predetermined signal indicating a result of the detection to controller  41 . More specifically, detector  81  is implemented as a distance measuring unit configured to measure a distance to an object below aircraft  10 C. Detector  81  may be implemented as, for example, a ranging device capable of detecting the presence of an object within a predetermined distance by reflecting laser light or sound waves off an object. For example, detector  81  may analyze image data captured by camera  44  included in aircraft  10 C to detect an object present in the surrounding area of aircraft  10 C. 
     In this way, by using detector  81  which uses laser light, sound waves, or image data, aircraft  10 C can recognize an object relatively far away (for example, tens of meters) from aircraft  10 C. Detector  81  is triggered to transmit the predetermined signal indicating the result of the detection to controller  41  when the object comes within a predetermined distance (for example, a few meters) from aircraft  10 C. 
     Note that when detector  81  uses image data captured by camera  44 , camera  44  may function as detector  81 . 
     In aircraft  10 C having the configuration described above, the contraction control may be implemented via, for example, the information processing and operations illustrated in  FIG. 16A . In other words, detector  81  detects the state of aircraft  10 C (S 21 ). Controller  41  shortens the overall length of each holding component  54 B in accordance with the detection result from detector  81  (S 22 ). In other words, with the contraction control illustrated in  FIG. 16A , controller  41  shortens the overall length of each holding component  54 B when, as the predetermined timing, the distance to the object detected by detector  81  is less than a predetermined distance. 
     In aircraft  10 C having the configuration described above, the contraction control may be implemented via, for example, the information processing and operations illustrated in  FIG. 16B . In other words, receiver  41   a  receives a contraction instruction signal transmitted from the radio control device (S 21   a ). Controller  41  shortens the overall length of each holding component  54 B when, as the predetermined timing, receiver  41   a  receives the contraction instruction signal (S 22 ). In other words, with the contraction control illustrated in  FIG. 16B , controller  41  shortens the overall length of each holding component  54 B when, as the predetermined timing, receiver  41   a  receives a contraction instruction signal instructing that the overall length of each holding component  54 B be shortened. 
     As a result of the contraction control described in  FIG. 16A  and  FIG. 16B , the overall length of each holding component  54 B shortens from first length L 1  to second length L 2 , as illustrated in  FIG. 17 . This makes it possible to house camera  44  in recess  29  of balloon  20  at a predetermined timing. 
     In this way, for example, controller  41  can automatically shorten the overall length of each holding component  54 B when, as the predetermined timing, the distance to an object below aircraft  10 C is less than a predetermined distance. With this, when aircraft  10 C contacts an object, an on-board device, such as camera  44 , can be housed within balloon  20  so as not to protrude beyond balloon  20 . This makes it possible to inhibit an on-board device from contacting an object and prevent damage to the on-board device and/or the object. 
     Moreover, for example, controller  41  can shorten the overall length of holding components  54 B as a result of the operator using the radio control device to transmit a contraction instruction signal. With this, as a result of the operator shortening the overall length of holding components  54 B at a predetermined timing, an on-board device can be housed within balloon  20  so as not to protrude beyond balloon  20 , even when aircraft  10 C is flying. This makes it possible to inhibit an on-board device from contacting an object and prevent damage to the on-board device and/or the object. 
     In  FIG. 17 , each holding components  54 B is implemented as a cylinder device such as an electric cylinder, but holding components  54 B are not limited to cylinder devices. For example, as illustrated in  FIG. 18 , the holding components may be implemented as holding component  54 C having a structure including a plurality of links each supportably rotatable at three axes of rotation. Even with the structure of holding component  54 C, the overall length of holding component  54 C can be shortened. 
     Moreover, the overall lengths of holding components  54 B,  54 C are shortened at a predetermined timing. Thereafter, holding components  54 B,  54 C may be extended at a different timing. In other words, holding components  54 B,  54 C are variable-length components capable of contracting and extending. For example, receiver  41   a  receives a length-change instruction signal (contraction instruction signal or extension instruction signal) transmitted from the radio control device. Controller  41  changes (extends or shortens) the overall length of holding components  54 B,  54 C when, as the predetermined timing, receiver  41   a  receives the length-change instruction signal. 
     Variation 3 
     In Embodiment 2 described above, aircraft  10  has, but it not limited to, the same configuration as described in Embodiment 1; for example, release unit  26  may be omitted from aircraft  10 . Moreover, balloon  20  is exemplified as gas chamber  21  being divided into first shock absorber  20   a  and second shock absorber  20   b , but balloon  20  may have a single gas chamber. In such cases, balloon  20  preferably contains the same gas as the first gas. Moreover, in Embodiment 1, at least the first shock absorber is required to be a balloon, but this example is not limiting; so long as the shock absorber laterally covers the plurality of rotor units  30 , across the height of the plurality of rotor units  30  in the up-and-down direction, the shock absorber may be made of any material. In other words, in Embodiment 2, the shock absorber may be made of a solid material such as a sponge material or rubber material. 
     Embodiment 3 
     Next, Embodiment 3 will be described. 
     Embodiment 3 is implemented mainly to solve problems that arise when the aircraft falls from the air, similar to Embodiment 2. As cases in which the aircraft falls from the air are described in Embodiment 2, repeated description thereof will be omitted. 
       FIG. 19  illustrates aircraft  10 D according to Embodiment 3 when shape-change control is performed. Note that (a) in  FIG. 19  illustrates aircraft  10 D before the shape-change control is performed, and corresponds to the cross section as illustrated in  FIG. 3 . Note that (b) in  FIG. 19  illustrates the overall state of aircraft  10 D after the shape-change control has been performed. 
     Aircraft  10 D according to Embodiment 3 differs from the aircraft according to Embodiment 1 in regard to the configuration of fixing component  50 C, as illustrated in  FIG. 19 . As such, fixing component  50 C will be described. 
     Fixing component  50 C differs from fixing component  50  in aircraft  10  according to Embodiment 1 in that arms  52 C are capable of contracting. 
     The plurality of arms  52 C each include movable part  52   b  including distal end section  52   a , and fixed part  52   c . The plurality of arms  52 C each contract by housing movable part  52   b  inside fixed part  52   c . The plurality of arms  52 C are configured as, for example electric cylinders. In each arm  52 C, the portion of movable part  52   b  that is exposed in ventilation hole  22  is fixed to ventilation hole  22 . Accordingly, as a result of each of the plurality of arms  52 C contracting, the external shape of balloon  20  changes, as illustrated in (b) in  FIG. 19 . More specifically, the shape of balloon  20  having a height H 1  and a width W 1  before the plurality of arms  52 C are contracted changes to a shape in which the width of balloon  20  narrows, to a width W 2  smaller than width W 1 , when the plurality of arms  52 C contract. The pressure inside balloon  20  causes the height of balloon  20  to increase by the amount by which the width of balloon  20  narrows, to a height of H 2  greater than height H 1 . 
     This causes the bottom end of recess  29  of balloon  20  to lower in position, whereby an on-board device, such as camera  44 , is housed inside recess  29 . 
     In other words, the plurality of arms  52 C of fixing component  50 C are driven by drive unit  90  (to be referenced later) and change the external shape of balloon  20  functioning as the shock absorber at a predetermined timing. More specifically, the plurality of arms  52 C change the external shape of balloon  20  so that part or all of an on-board device, such as camera  44 , does not protrude beyond balloon  20 . The plurality of arms  52 C are connected to ventilation holes  22  in four locations inside balloon  20 , and change the external shape of balloon  20  by pulling ventilation holes  22  closer together. 
     With this, the plurality of arms  52 C cause recess  29 , which defines at least part of the external shape of balloon  20 , to protrude downward. Even more specifically, the plurality of arms  52 C cause at least part of balloon  20  in the surrounding area of camera  44  to protrude beyond camera  44 . 
     Note that all four of the plurality of arms  52 C contract, but by causing only the two arms  52 C that contract in opposite directions to contract, two locations inside balloon  20  can be brought closer to each other. Accordingly, the configuration is not limited to four arms being capable of contracting; two arms that contract in opposite directions may be capable of contracting. 
     (Aircraft Operation Control Example) 
     With aircraft  10 D according to this embodiment, shape-change control is performed at a predetermined timing to change the external shape of balloon  20 . 
     Next, this shape-change control according to aircraft  10 D will be described with reference to  FIG. 19  through  FIG. 21B . 
       FIG. 20  is a block diagram illustrating a configuration of aircraft  10 D according to Embodiment 3.  FIG. 21A  is a flow chart of one example of the shape-change control performed by drive unit  90  in aircraft  10 D according to Embodiment 3.  FIG. 21B  is a flow chart of another example of the shape-change control performed by drive unit  90  in aircraft  10 D according to Embodiment 3. 
     Aircraft  10 D according to Embodiment 3 differs from aircraft  10 C according to Variation 2 of Embodiment 2 in that it includes arms  52 C instead of holding components  54 B. The remaining components in aircraft  10 D according to Embodiment 3 are the same as in aircraft  10  according to Embodiment 1. 
     Controller  41  contracts the plurality of arms  52 C at a predetermined timing. Drive unit  90  changes the external shape of balloon  20  as a result of the plurality of arms  52 C contracting. 
     In aircraft  10 D having the configuration described above, the shape-change control may be implemented via, for example, the information processing and operations illustrated in  FIG. 21A . In other words, detector  81  detects the state of aircraft  10 D (S 31 ). Controller  41  changes the external shape of balloon  20  in accordance with the result of the detection by detector  81  (S 32 ). In other words, with the shape-change control illustrated in  FIG. 21A , controller  41  changes the external shape of balloon  20  when, as the predetermined timing, the distance detected by detector  81  is less than a predetermined distance. 
     In aircraft  10 D having the configuration described above, the shape-change control may be implemented via, for example, the information processing and operations illustrated in  FIG. 21B . In other words, receiver  41   a  receives a shape-change instruction signal transmitted from the radio control device (S 31   a ). Controller  41  changes the external shape of balloon  20  when, as the predetermined timing, receiver  41   a  receives the shape-change instruction signal (S 32 ). In other words, in the shape-change control illustrated in  FIG. 21B , controller  41  changes the external shape of balloon  20  when, as the predetermined timing, receiver  41   a  receives the shape-change instruction signal, which instructs the changing of the external shape of balloon  20 . 
     As a result of the shape-change control described with reference to  FIG. 21A  and  FIG. 21B  being performed, the plurality of arms  52 C contract, which causes the external shape of balloon  20  to change, as illustrated in  FIG. 19 . This makes it possible to house camera  44  in recess  29  of balloon  20  at a predetermined timing. 
     (Advantageous Effects, Etc., of Embodiment 3) 
     Aircraft  10 D according to this embodiment includes: a plurality of rotor units  30  each including propeller  32  and motor  33  that drives propeller  32 ; balloon  20  that laterally covers the plurality of rotor units  30 , across a height of the plurality of rotor units  30  in an up-and-down direction; and a plurality of arms  52 C that are driven by drive unit  90  to change the external shape of balloon  20  at a predetermined timing. 
     With this configuration, since the external shape of balloon  20  can be changed at a predetermined timing, when, for example, aircraft  10 D encounters an unexpected flight situation, the external shape of balloon  20  can be changed such that an on-board device, such as camera  44 , does not protrude beyond balloon  20 , making it possible to inhibit the on-board device(s) from impacting an object. Moreover, for example, by changing the external shape of external shape balloon  20  of aircraft  10 D, aircraft  10 D can be used for entertainment purposes. 
     In this embodiment, aircraft  10 D further includes camera  44  that protrudes beyond balloon  20 , and drive unit  90  changes the external shape of balloon  20  such that a part or all of camera  44  does not protrude beyond balloon  20 . In other words, when, for example, aircraft  10 D encounters an unexpected flight situation, the external shape of balloon  20  can be changed such that camera  44 , which is protruding beyond balloon  20 , no longer protrudes beyond balloon  20 . As such, even if aircraft  10 D contacts an object, camera  44  can be effectively inhibited from directly contacting the object. Accordingly, even if aircraft  10 D, for example, encounters an unexpected flight situation and contacts an object, camera  44  and/or the object can be prevented from being damaged. 
     In this embodiment, drive unit  90  changes the external shape of balloon  20  by causing at least part of the external shape of balloon  20  to protrude beyond camera  44 . In other words, when, for example, aircraft  10 D encounters an unexpected flight situation, the external shape of balloon  20  can be changed such that camera  44 , which is protruding beyond balloon  20 , no longer protrudes beyond a part of balloon  20 . As such, even if aircraft  10 D contacts an object, balloon  20  will contact the object before camera  44  does. Accordingly, even if aircraft  10 D, for example, encounters an unexpected flight situation and contacts an object, camera  44  can be prevented from being damaged. 
     In this embodiment, drive unit  90  changes the external shape of balloon  20  by causing at least part of balloon  20 , in a surrounding area of camera  44 , to protrude beyond camera  44 . In other words, when, for example, aircraft  10 D encounters an unexpected flight situation, the external shape of balloon  20 , in an area surrounding an on-board device, such as camera  44 , can be changed such that the on-board device does not protrude beyond a part of balloon  20 . As such, even if aircraft  10 D contacts an object in any sort of orientation, balloon  20  will contact the object before the on-board device does. Accordingly, even if aircraft  10 D, for example, encounters an unexpected flight situation and contacts an object, the on-board device and/or object can be prevented from being damaged. 
     In this embodiment, aircraft  10 D further includes detector  81  functioning as the distance measuring unit that measures the distance to an object below aircraft  10 D. Moreover, the predetermined timing is when the distance measured by detector  81  is less than a predetermined distance. Furthermore, drive unit  90  changes the external shape of balloon  20  when the distance measured by detector  81  is less than the predetermined distance. 
     In this way, controller  41  can automatically contract the plurality of arms  52 C when, as the predetermined timing, the distance to an object below aircraft  10 D is less than the predetermined distance. With this, when aircraft  10 D contacts an object, an on-board device, such as camera  44 , can be housed within balloon  20  so as not to protrude beyond balloon  20 , since it is possible to change the external shape of balloon  20  to its post-change form. This makes it possible to inhibit an on-board device from contacting an object and prevent damage to the on-board device and/or the object. 
     In this embodiment, aircraft  10 D further includes receiver  41   a  that receives a signal. Moreover, the predetermined timing is when receiver  41   a  receives a shape-change instruction signal, which instructs the changing of the external shape of balloon  20 . Furthermore, the plurality of arms  52 C change the external shape of balloon  20  when receiver  41   a  receives the shape-change instruction signal. 
     As such, for example, controller  41  can contract the plurality of arms  52 C as a result of the operator using the radio control device to transmit the shape-change instruction signal. With this, as a result of the operator shortening the overall length of the plurality of arms  52 C at a predetermined timing, an on-board device can be housed within balloon  20  so as not to protrude beyond balloon  20 , even when aircraft  10 D is flying. This makes it possible to inhibit an on-board device from contacting an object and prevent damage to the on-board device and/or the object. 
     The overall lengths of the plurality of arms  52 C are shortened at a predetermined timing. Thereafter, the plurality of arms  52 C may be extended at a different timing. In other words, the plurality of arms  52 C are variable-length components capable of contracting and extending. The plurality of arms  52 C are connected to ventilation holes  22  in four locations inside balloon  20 , and change the external shape of balloon  20  by pulling ventilation holes  22  closer together or pushing them farther apart. 
     Variations of Embodiment 3 
     Variation 1 
     In aircraft  10 D according to Embodiment 3 described above, drive unit  90  is exemplified as, but not limited to, changing the external shape of balloon  20  by contracting the plurality of arms  52 C included in fixing component  50 C; for example, the configuration illustrated in  FIG. 22  may be implemented. 
       FIG. 22  illustrates aircraft  10 E according to Variation 1 of Embodiment 3 when shape-change control is performed. Note that (a) in  FIG. 22  illustrates aircraft  10 E before the shape-change control is performed, and corresponds to the cross section as illustrated in  FIG. 3 . Note that (b) in  FIG. 22  illustrates the overall state of aircraft  10 E after the shape-change control has been performed. 
     Aircraft  10 E according to Variation 1 of Embodiment 3 differs from aircraft  10 D according to Embodiment 3 in that it includes external shape changing unit  72 , as illustrated in  FIG. 22 . 
     External shape changing unit  72  includes connectors  72   a , string-like components  72   b , and recessed region  72   c  in which part of the external shape of balloon  20 E is recessed. 
     Connectors  72   a  are located in the central region of the inside of balloon  20 E. Each connector  72   a  is connected to one end of a string-like component  72   b . Connectors  72   a  severs one end of each string-like component  72   b  by disconnecting string-like components  72   b  upon being driven by drive unit  90  having received a shape-change command from controller  41 . 
     String-like components  72   b  each have one end connected to a connector  72   a  and the other end connected to the part of balloon  20 E corresponding to the base of recessed region  72   c . In other words, the shape of recessed region  72   c  is maintained by string-like components  72   b  connecting the base of recessed region  72   c  to another part of balloon  20 E, inside balloon  20 E, via connectors  72   a.    
     In aircraft  10 E configured in this manner, drive unit  90  disconnects connectors  72   a  at a predetermined timing to cause recessed region  72   c  to protrude beyond camera  44  via the internal pressure of balloon  20 E, as illustrated in (b) in  FIG. 22 . This causes recess  29  of balloon  20 E to protrude from an area surrounding camera  44  to form recess  29 E that surrounds the entire perimeter of camera  44 . Accordingly, with a simple configuration, part of balloon  20 E can be caused to protrude beyond an on-board device, such as camera  44 , at a predetermined timing. 
     Variation 2 
     For example, instead of the configuration described in Variation 1, the configuration illustrated in  FIG. 23  may be implemented. 
       FIG. 23  illustrates aircraft  10 F according to Variation 2 of Embodiment 3 when shape-change control is performed. Note that (a) in  FIG. 23  illustrates aircraft  10 F before the shape-change control is performed, and corresponds to the cross section as illustrated in  FIG. 3 . Note that (b) in  FIG. 23  illustrates the overall state of aircraft  10 F after the shape-change control has been performed. 
     Aircraft  10 F according to Variation 2 of Embodiment 3 differs in that it includes gas supply unit  73 , as illustrated in  FIG. 23 . Aircraft  10 F also differs in that balloon  20 F includes first region  75  that is inflated and second region  74  that is deflated. 
     Gas supply unit  73  is capable of supplying gas to second region  74 . More specifically, gas supply unit  73  is a valve that is disposed between first region  75  and second region  74  and selectively places first space  75   a  defined by first region  75  in fluid communication with second space  74   a  that second region  74  is capable of defining. 
     Drive unit  90  supplies gas contained in first space  75   a  to second region  74  by opening gas supply unit  73  at a predetermined timing. In other words, at a predetermined timing, drive unit  90  causes gas supply unit  73  to inflate second region  74  by supplying gas to second region  74 , which causes second region  74  to protrude from balloon  20 F. 
     In aircraft  10 F configured in this manner, at a predetermined timing, drive unit  90  opens gas supply unit  73  to cause second region  74  to protrude beyond camera  44 , using the gas filling balloon  20 F, as illustrated in (b) in  FIG. 23 . This causes recess  29  of balloon  20 F to protrude from an area surrounding camera  44  to form recess  29 F that surrounds the entire perimeter of camera  44 . Accordingly, with a simple configuration, part of balloon  20 F can be caused to protrude beyond an on-board device at a predetermined timing. 
     Variation 3 
     In Variation 2, gas supply unit  73  is exemplified as a valve, but as illustrated in  FIG. 24 , may be implemented as gas supply unit  73 A configured as a canister filled with gas. 
       FIG. 24  illustrates aircraft  10 G according to Variation 3 of Embodiment 3 when shape-change control is performed. Note that (a) in  FIG. 24  illustrates aircraft  10 G before the shape-change control is performed, and corresponds to the cross section as illustrated in  FIG. 3 . Note that (b) in  FIG. 24  illustrates the overall state of aircraft  10 G after the shape-change control has been performed. 
     Aircraft  10 G according to Variation 3 of Embodiment 3 differs in that it includes gas supply unit  73 A instead of gas supply unit  73 , as illustrated in  FIG. 24 . 
     Drive unit  90  supplies gas from gas supply unit  73 A to second region  74  at a predetermined timing. In other words, at a predetermined timing, drive unit  90  causes gas supply unit  73 A to inflate second region  74  by supplying gas to second region  74 , which causes second region  74  to protrude from balloon  20 F. 
     In aircraft  10 G configured in this manner, at a predetermined timing, drive unit  90  supplies gas filled in gas supply unit  73 A to cause second region  74  to protrude beyond camera  44 , as illustrated in (b) in  FIG. 24 . This causes recess  29  of balloon  20 F to protrude from an area surrounding camera  44  to form recess  29 F that surrounds the entire perimeter of camera  44 . Accordingly, with a simple configuration, part of balloon  20 F can be caused to protrude beyond an on-board device at a predetermined timing. 
     Variation 4 
     For example, instead of the configurations described in Variations 1 through 4, the configuration illustrated in  FIG. 25  may be implemented. 
       FIG. 25  illustrates aircraft  10 H according to Variation 4 of Embodiment 3 when shape-change control is performed. Note that (a) in  FIG. 25  illustrates aircraft  10 H before the shape-change control is performed, and corresponds to the cross section as illustrated in  FIG. 4 . Note that (b) in  FIG. 25  illustrates the overall state of aircraft  10 H after the shape-change control has been performed. 
     Aircraft  10 H according to Variation 4 of Embodiment 3 differs in that it includes external shape changing unit  76 , as illustrated in  FIG. 25 . 
     External shape changing unit  76  includes connectors  76   a , string-like components  76   b , and protruding region  77  in which part of the external shape of balloon  20 H is protruding. 
     Connectors  76   a  are located on the outer surface of balloon  20 H. Each connector  76   a  is connected to one end of a string-like component  76   b . Connectors  76   a  each sever one end of a string-like component  76   b  by disconnecting string-like component  72   b  upon being driven by drive unit  90  having received a shape-change command from controller  41 . 
     String-like components  76   b  each have one end connected to a connector  76   a  and the other end connected to a distal end of protruding region  77  of balloon  20 H. With this, protruding region  77  has a distal end that connects to part of the outer surface of balloon  20 H via connector  76   a  to give protruding region  77  a shape that follows the contour of the outer surface of balloon  20 H. 
     In aircraft  10 H configured in this manner, drive unit  90  disconnects connectors  76   a  at a predetermined timing to cause protruding region  77  to protrude beyond camera  44  via the internal pressure of balloon  20 H, as illustrated in (b) in  FIG. 25 . Accordingly, with a simple configuration, part of balloon  20 H can be caused to protrude beyond an on-board device, such as camera  44 , at a predetermined timing. 
     Variation 5 
     In Embodiment 3 described above, the external shape of balloon  20  is changed by contracting plurality of arms  52 C, but this example is not limiting. For example, cord-like components may be attached in two different locations, and the cord-like components may be reeled in with a reel to bring the two different locations closer together. This makes it possible to change the external shape of balloon  20  with a simple configuration. 
     Other Embodiments 
     The embodiments described above include fixing component  50 ,  50 C, but fixing component  50 ,  50 C may be omitted. In such cases, the plurality of rotor units  30  are fixed directly to ventilation holes  22 . Moreover, the balloon need not include the plurality of ducts  28 . 
     The above embodiments have been presented as examples of techniques according to the present disclosure. The accompanying drawings and the detailed description are provided for this purpose. 
     Therefore, the components described in the accompanying drawings and the detailed description include, in addition to components essential to overcoming problems, components that are not essential to overcoming problems but are included in order to exemplify the techniques described above. Thus, those non-essential components should not be deemed essential due to the mere fact that they are illustrated in the accompanying drawings and described in the detailed description. 
     The above embodiments are for providing examples of the techniques according to the present disclosure, and thus various modifications, substitutions, additions, and omissions are possible in the scope of the claims and equivalent scopes thereof. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to an aircraft including a plurality of rotor units and a shock absorber.