Abstract:
An aircraft ( 40   a ) is provided that includes a plurality of arms ( 41, 42, 43, 44 ) with selected arms having the ability to either adjust their length, have arm segments operative to move about an articulated joint in two or three dimensions, or have one arm operative to adjust an angle between the one arm and another arm, or any combination of the foregoing. Thrust generators are repositionably mounted on selected arms, and a control system enables automated, on-board, or remote control of the thrust generators, repositioning of the thrust generators on the arms, adjustment in the length of the selected arms, the movement of selected arms about the articulated joints, and adjustment of the angle between two or more arms, all while maintaining directional control of the aircraft in flight or on the ground. The aircraft has operational capabilities that exceed existing designs and facilitates manned and unmanned delivery of cargo and transportation of passengers.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure is directed to dynamically reconfigurable aerial vehicles and, more particularly, to aircraft having multiple thrust generators that can be repositioned relative to one another while in flight or on the ground and a related control system that maintains in-flight stability during repositioning of the thrust generators. 
         [0003]    2. Description of the Related Art 
         [0004]    One form of aircraft that utilizes multiple thrust generators is the multicopter. This type of craft utilizes multiple rotors oriented to have a downward thrust. Typically, these rotors are mounted to a fixed frame, usually multiple arms extending outward from a central hub. Because these craft have inherent dynamic instability, sophisticated control systems are used to vary the amount of thrust produced by each rotor to enable controlled ascent, descent, hovering and directional control. 
         [0005]    Referring to  FIG. 1 , shown therein is a multicopter  20  having four rotors  22  mounted on respective arms  24  that extend from a central hub  26 , usually in a symmetric manner. Each arm  24  holds a respective rotor  22  at a terminal end  28 . A multicopter can have any number of arms, but the most common type of multicopter is a quadcopter, which has the four arms  24  shown in  FIG. 1 . A common variation is the tricopter, which has three arms, hexacopter, which has six arms, and the octocopter  30 , which has eight arms  32 , as shown in  FIG. 2 . 
         [0006]    A quadcopter can be of two types depending how the direction of motion is defined by its controller electronics. There is the “Plus” (+) type where the direction of forward motion is along one of the arms, as shown by the denoted F 1  in  FIG. 1 , and there is the “x” type, where the direction of forward motion is along the middle of two adjacent arms, denoted by F 2  in  FIG. 1 . 
         [0007]    Flight control is maintained by an on-board receiver coupled to a control system that combines data from small on-board MEMs gyroscopes, accelerometers (the same as those found in smart phones) and signals received at the on-board receiver from a remote transmitter. 
         [0008]    Generally, the rotor spins in an opposite direction from the two rotors on either side of it (i.e. in a quadcopter the rotors on opposite corners of the frame spin in the same direction). A multicopter controls roll and pitch by speeding up the rotors on one side of the craft while slowing down the opposing other rotors. If the user wanted the craft to roll left, the user would input signals that would cause the rotors on the right side of the frame to speed up and the rotors on the left slide to slow down. Similarly if the user intended to move the craft forward, the user would cause the back rotors to speed up and the front rotors to slow down. 
         [0009]    The craft turns clockwise or counterclockwise (“yaws” as viewed from the top) by speeding up the set of rotors that spin counterclockwise (or clockwise), and slowing down the other set of rotors that spin clockwise (or counterclockwise). Horizontal motion is accomplished by temporarily speeding up or slowing down some rotors so that the vehicle rolls into the desired direction of travel and increasing the overall thrust of all rotors so the overall thrust vector causes the craft to move in the intended direction. Altitude is controlled by speeding up or slowing down all rotors at the same time. 
         [0010]    One drawback to current designs is the difficulty or near impossibility of accommodating for an engine-out situation or for a change in the amount or shift in the location of a load carried by the aircraft. In addition, when the craft is flying within a narrow airspace, whether intentionally or inadvertently, there is the high risk of rotor strike with a ground-based object, such as trees, buildings, and other structures. Hence, there is a need for a multicopter design that facilitates continued stable flight in response to changes in the moment of inertia, center of gravity, and center of mass, such as engine failure or when there is a change in the load. There is also a need for an aircraft that can reconfigure itself while in flight to accommodate clearance with ground-based features. 
       BRIEF SUMMARY 
       [0011]    An aircraft is provided that includes a plurality of arms with selected arms having the ability to either adjust their length, have arm segments operative to move about an articulated joint in two or three dimensions, or have one arm operative to adjust an angle between the one arm and another arm or any combination of the foregoing. In addition, thrust generators are repositionably mounted on selected arms, and a control system enables one or more of automated, on-board, and remote control of the operation of the thrust generators as well as repositioning of the thrust generators on the arms, adjustment in the length of the selected arms, the movement of selected arms about the articulated joints, and adjustment of the angle between two or more arms, all while maintaining directional control of the aircraft in flight or on the ground. The aircraft has operational capabilities that exceed existing designs and facilitates manned and unmanned delivery of cargo and transportation of passengers. 
         [0012]    In accordance with one aspect of the present disclosure, an aircraft is provided that includes a frame having a plurality of arms, each arm having a longitudinal axis, a first arm of the plurality of arms operative to adjust a length of the first arm along a longitudinal axis, and the first arm or a second arm of the plurality of arms operative to move about an articulated joint, and at least one arm of the plurality of arms operative to adjust an angle between the at least one arm and at least one other arm of the plurality of arms. The aircraft further includes a plurality of thrust generators, and a control system coupled to the first or second and the at least one of the plurality of arms and to the plurality of thrust generators and operative to control the operation of the plurality of thrust generators, and to control adjustment of the length of the first arm in flight and on the ground while maintaining directional control of the aircraft in flight and on the ground, the control system further operative to control the movement of the first arm or a second arm of the plurality of arms about the articulated joint in flight and on the ground while maintaining stability of the aircraft in flight and on the ground, and to control adjustment of the angle between the at least one arm and the at least one other arm in flight and on the ground while maintaining directional control of the aircraft in flight and on the ground. 
         [0013]    In accordance with another aspect of the present disclosure, an aircraft is provided that includes a frame having a plurality of arms, at least one of the plurality of arms operative to adjust a length of the arm along a longitudinal axis to accommodate an asymmetric shift in thrust or load, a plurality of thrust generators, at least one thrust generator mounted on a respective arm of the plurality of arms, and a control system coupled to the at least one of the plurality of arms and to the plurality of thrust generators and operative to control the operation of the plurality of thrust generators and to control adjustment of the length of the at least one of the plurality of arms in flight and on the ground while maintaining directional control of the aircraft in flight or on the ground or both. 
         [0014]    In accordance with still yet another aspect of the present disclosure, an aircraft is provided that includes a frame having a plurality of arms, each arm having a longitudinal axis, and at least one arm operative to move about an articulated joint, a plurality of thrust generators, at least one thrust generator mounted on a respective arm of the plurality of arms, and a control system coupled to the at least one arm and to the plurality of thrust generators and operative to control the movement of the at least one arm about the articulated joint in flight and on the ground while maintaining stability and controllability of the aircraft in flight and on the ground. 
         [0015]    In accordance with a further aspect of the present disclosure, an aircraft is provided that includes a frame having a plurality of arms, each arm having a longitudinal axis, at least one arm of the plurality of arms operative to adjust an angle between the at least one arm and at least one other arm of the plurality of arms, a plurality of thrust generators, at least one thrust generator mounted on a respective arm of the plurality of arms, and a control system coupled to the at least one arm of the plurality of arms and to the plurality of thrust generators and operative to control the operation of the plurality of thrust generators and to control adjustment of the angle between the at least one arm and the at least one other arm in flight and on the ground while maintaining directional control of the aircraft in flight and on the ground. 
         [0016]    In accordance with a further aspect of the present disclosure, an aircraft is provided that includes a frame having a plurality of arms, each arm having a longitudinal axis, a first arm of the plurality of arms operative to adjust a length of the first arm along a longitudinal axis, and the first arm or a second arm of the plurality of arms operative to move about an articulated joint, and at least one arm of the plurality of arms operative to adjust an angle between the at least one arm and at least one other arm of the plurality of arms, a plurality of thrust generators, at least one thrust generator mounted on a respective arm of the plurality of arms, and a control system coupled to the first or second and the at least one of the plurality of arms and to the plurality of thrust generators and operative to control the operation of the plurality of thrust generators and to control adjustment of the length of the first arm in flight and on the ground while maintaining directional control of the aircraft in flight and on the ground, the control system further operative to control the movement of the first arm or a second arm of the plurality of arms about the articulated joint in flight and on the ground while maintaining stability of the aircraft in flight and on the ground, and to control adjustment of the angle between the at least one arm and the at least one other arm in flight and on the ground while maintaining directional control of the aircraft in flight and on the ground. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0017]    The foregoing and other features and advantages of the present disclosure will be more readily appreciated as the same become better understood from the following detailed description when taken in conjunction with the accompanying drawings, wherein: 
           [0018]      FIG. 1  is an illustration of a known multicopter; 
           [0019]      FIG. 2  is an illustration of a known octocopter; 
           [0020]      FIG. 3  is an axonometric view of a known quadcopter showing its roll, pitch, and yaw motion and the roll, pitch and yaw axes; 
           [0021]      FIG. 4A  illustrates a quadcopter with arms contracted, and  FIG. 4B  illustrates the arms extended in accordance with the present disclosure; 
           [0022]      FIGS. 5A and 5B  are enlarged bottom views the quadcopter arm of  FIGS. 4A and 4B , respectively, illustrating the telescopic extension design formed in accordance with the present disclosure. 
           [0023]      FIG. 6  is a bottom plan view of a spiral mechanism configured to change the effective length of the quadcopter arms in accordance with the present disclosure; 
           [0024]      FIGS. 7A and 7B  are axonometric views of a quadcopter arm illustrating multiple positions for mounting a rotor on an arm in accordance with the present disclosure; 
           [0025]      FIGS. 8A and 8B  are axonometric views of a quadcopter arm illustrating slidable mounting of the rotor mount on a multicopter arm in accordance with the present disclosure; 
           [0026]      FIGS. 9A, 9B, and 9C  are a top plan view of a “Plus” (+) type quadcopter with three different configurations in which arm extension and contraction is decoupled, all arm lengths equal ( FIG. 9A ), side arms extended ( FIG. 9B ), and front and rear arms extended ( FIG. 9C ); 
           [0027]      FIGS. 10A-10C  are top plan views of an X type quadcopter with arms in a normal position ( FIG. 10A ), the forward and rearward arms in a narrow position ( FIG. 10B ), and forward and rearward arms in a wide position ( FIG. 10C ); 
           [0028]      FIG. 11  is a simplified diagram of an X type quadcopter showing four rotors, four arms, and the disc at the center; 
           [0029]      FIGS. 12A and 12  B are top plan views of a Plus (+) type quadcopter with arms in a normal position, and then the two side arms in a narrow position in the same forward direction, respectively; 
           [0030]      FIGS. 13A and 13B  are a view of a hexacopter with arms in a normal position, and the arms in a narrow position, respectively; 
           [0031]      FIG. 14  is an illustration of a control system formed in accordance with the present disclosure; 
           [0032]      FIGS. 15A-15C  illustrate various configurations of articulated arms formed in accordance with the present disclosure; and 
           [0033]      FIG. 16  illustrates a multicopter without a central hub and with articulated arm segments formed in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various disclosed implementations. However, one skilled in the relevant art will recognize that implementations may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures or components or both associated with rotors, thrust generators, remote control transmitters and receivers, landing gear, and frame construction for multicopters and in general aircraft and airplanes have not been shown or described in order to avoid unnecessarily obscuring descriptions of the implementations. 
         [0035]    Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open inclusive sense, that is, as “including, but not limited to.” The foregoing applies equally to the words “including” and “having.” 
         [0036]    Reference throughout this description to “one implementation” or “an implementation” means that a particular feature, structure, or characteristic described in connection with the implementation is included in at least one implementation. Thus, the appearance of the phrases “in one implementation” or “in an implementation” in various places throughout the specification are not necessarily all referring to the same implementation. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more implementations. 
         [0037]    The present disclosure is intended to be applicable, in principle, to any multicopter with an arbitrary number of arms in general. However, the representative embodiments described and illustrated herein will be directed to a multicopter with four arms, also known as a quadcopter. It is to be further understood that the present disclosed implementations are intended to be used with all forms of known thrust generators for aircraft including, without limitation, propeller and rotary wings as well as all forms of jet engines and other air breathing engines, such as piston, gas generator, turbo jet, turbo fan, turboprop and turbo shaft, and ramjet. In the following detailed description the term “rotor” will be used to designate a preferred form of thrust generator for the representative implementations. The rotor is defined as a propeller coupled to a motor or engine. In the typical case of a quadcopter, the motor is an electric motor typically powered by an external source of electricity, such as a battery. However, solar power, transmitted electromagnetic energy, ambient electromagnetic energy, gasoline or nitro fuel, and other forms of harvested or generated electricity can be used as a power source, which are all known in the art. 
         [0038]    The agility (ability to change its state of motion quickly) or its opposite, stability (ability of holding the current state of motion) of a quadcopter depends on the position of the rotor relative to the center of gravity of the vehicle and its moments of inertia (rotational mass) of the vehicle for the three orthogonal axes, which are (1) Roll axis (2) Pitch axis, and (3) Yaw axis, all shown in  FIG. 3 , along with the defined direction of motion. In the present disclosure, the agility and stability characteristics are altered by extending and retracting the arms and the attached thrust generators and by moving the arms of the quadcopter in a horizontal plane. 
         [0039]    Referring now to the invention in more detail, in  FIGS. 4A and 4B  there are shown two configurations of a quadcopter  40  having four arms  42  extending from a central hub  44  with thrust generators in the form of rotors  46  mounted on a distal section  48  extending from a proximal section  50  of each arm  42 . In  FIG. 4A , the arms  42  are shown in a contracted configuration in which the distal sections  48  are contracted within the proximal ends  50 . In  FIG. 4B , the arms  42  are in an extended configuration in which the distal sections  48  extend out of the proximal ends  50 . In the extended configuration of  FIG. 4B , there is a greater moment of inertia (I) than in the contracted configuration of  FIG. 4A . Hence, in the contracted configuration of  FIG. 4A , the vehicle  40  has more agility but may be slightly less stable, while in the extended configuration shown in  FIG. 4B , the vehicle  40  has more stability during the flight. 
         [0040]    In more detail, let the moment of inertia (rotational mass) about the roll axis be I RA , moment of inertia about the pitch axis be I PA , and moment of inertia about the yaw axis be I YA . The values of these three components I RA , I PA  and I YA  are different in the configuration shown in  FIG. 4A  than in  FIG. 4B . 
         [0041]    Now referring to  FIG. 11 , and using a simple model of a quadcopter  40  where the center hub  44  is a disc of radius R and mass M disc , each of the four identical arms  42  (extending from the center hub  44 ) has a length of L and mass M arm  and each of the four identical rotors  46  has a mass M rotor . Uniform mass distribution in the arms is assumed. The arms can extend in length. 
         [0042]    The moment of inertia about the Roll axis is 
         [0000]        I   RA   =M   disc   R   2 /4+2 M   arm ( L ) 2 /3+2 M   rotor ( L ) 2    
         [0043]    The moment of inertia about the Pitch axis is 
         [0000]        I   PA   =M   disc   R   2 /4+2 M   arm ( L ) 2 /3+2 M   rotor ( L ) 2    
         [0044]    The moment of inertia about the Yaw axis is 
         [0000]        I   YA   =M   disc   R   2 /2+4 M   arm ( L ) 2 /3+4 M   rotor ( L ) 2    
         [0045]    Now in this simple model, if we expand the arms so that the length L becomes mL (where m is a multiplicative factor) without any change in the mass, then those three components of I become modified as, 
         [0000]        I   RA   =M   disc   R   2 /4+2 M   arm (mL) 2 /3+2 M   rotor (mL) 2    
         [0000]        I   PA   =M   disc   R   2 /4+2 M   arm (mL) 2 /3+2 M   rotor (mL) 2    
         [0000]        I   YA   =M   disc   R   2 /2+4 M   arm (mL) 2 /3+4 M   rotor (mL) 2    
         [0046]    which shows how the change in length affects all the components of the moment of inertia I. 
         [0047]    Let us consider a mid-size quadcopter with some realistic values, L=20 centimeters, M disc =400 grams, M arm =100 grams, M rotor =100 grams. Using the three equations above, this set of values yields 
         [0000]        I   RA   =I   PA =97867 gm·cm 2 , and  I   YA =195734 gm·cm 2  
 
         [0048]    Now if we consider a 25% expansion of arm length (m=1.25), then the new values are, 
         [0000]        I   RA   =I   PA =141367 gm·cm 2 , and  I   YA =282734 gm·cm 2  
 
         [0049]    This shows that the increase of the arm length decreases the agility and increases the stability and decreases the agility for roll, pitch and yaw motions. 
         [0050]    It will be appreciated that various designs can be used to increase or decrease the effective length of the arms, including telescoping referenced above, as well as the use of lead screws, a scissoring mechanism, and the like. 
         [0051]    In further details, still referring to the  FIGS. 4A and 4B , the extension and contraction of the arms  42  can be done when the quadcopter  40  is on the ground. It can also be devised such that the extension and retraction can be performed remotely or in an automated way while quadcopter  40  is in flight. 
         [0052]    There are a number of possible ways of changing the length of the arms  42  remotely during the flight or on the ground. For example, the arm  42  can be made of a set of telescoping sections  48 ,  50  as shown in  FIGS. 5A and 5B . The distal section  48  can be structured to be slidably received within the proximal section  50  and slid in and out to change the effective length of the arm  42 .  FIG. 5A  shows the arm  42  in the contracted configuration, and  FIG. 5B  shows the arm  42  in the extended configuration. The distal section  48  may be manually moved or mechanically moved into and out of (or along) the proximal section  50 . Manually, the arms  42  can be individually adjusted in their length when on the ground. Bolts, screws, detents, and other known fasteners can be used to retain the distal section  48  in a selected position relative to the proximal section  50 . The in-and-out slidable movement can be achieved in various ways, for example, using a lead screw mechanism, a rack-and-pinion arrangement, a spring mechanism (either pushed or pulled), pneumatic, hydraulic, electromagnetic, and other known means. This movement is preferably controlled by the control system, which is coupled to the mechanism that changes the length of the arm. 
         [0053]    The control of the mechanical movement is preferably done remotely using radio frequency controls signals, which will be described in more detail below. In addition, the onboard control system will compensate for the changing weight moments as needed, which is also described in more detail below. It is to be understood that other forms of communication can be used including, without limitation, Bluetooth and Wi-Fi. Movement can also be done autonomously through a processor coupled to sensors that responds to sense signals from the sensors or with pre-programmed instructions. 
         [0054]    Referring to  FIG. 6 , shown therein is another mechanism of contracting and extending all the arms  42  simultaneously. A chuck-spiral mechanism  60  is utilized in which a spiral grooved wheel  62  will contract and extend the arms  42  in response to the wheel turning clockwise/counterclockwise. Because the chuck-spiral mechanism is known in the art, it will not be described in greater detail. 
         [0055]    Referring now to  FIGS. 7A and 7B , another way of changing the agility and stability of a quadcopter  70  is illustrated. Here, the quadcopter  70  has an arm  72  extending from a hub  74  (only one arm is shown for simplicity). The rotor  76  is mounted on the arm  72  in a manner that enables changing the location of the rotor  76  on the arm  72 . If the rotor  76  is mounted closer to the center hub  74  of the quadcopter  70  ( FIG. 7A ), it will have a lower agility than when mounted further along the length of the arm  72  ( FIG. 7B ). 
         [0056]    The rotor  76  can be mounted in a few predefined fixed locations along the arm  72  as shown in  FIG. 7A  using suitable and well-known fastening means. This requires manual relocation of the rotor  76  when it is on the ground. Alternatively, the rotor  76  or can be put on a movable mount  78  that can slide along the arm  72  as shown in  FIGS. 8A and 8B . The rotor  76  can be fixed at any location along the arm, as discussed above with respect to  FIGS. 7A and 7B . Again, the change of the rotor location can be performed on the ground or remotely during the flight. The movement of the thrust generator or movable mount  78  can be achieved in various ways, for example, using a lead screw mechanism, a rack-and-pinion arrangement, a spring mechanism (either pushed or pulled), pneumatic, hydraulic, electromagnetic, and other known means, all of which are preferably controlled by the control system. The control system that maintains stability while changing the rotor position on the arm during flight is described in more detail below. 
         [0057]    Referring now to  FIGS. 9A-9C , more control over the flight characteristics can be achieved when the extension of the four arms of the quadcopter can be decoupled. For example, here for a plus (+) type quadcopter  40   a , shown in  FIG. 9A , the arms  41 ,  42 ,  43 , and  44  are at equal length from the hub  45 . The rotors  46  are thus positioned at the same distance from the hub  45  and from each other. In  FIG. 9B , the two side arms  42 ,  44  are extended and the front and rear arms  41 ,  43  remain contracted so that the respective rotors are no longer equidistantly spaced. There will thus be increased stability and less agility for roll operations (stability against sideways movement) if the direction of movement is forward (F). On the other hand, if the front and rear arms  41 ,  43  only are extended and the right and left arms  42 ,  44  remain contracted, as shown in  FIG. 9C , then the quadcopter  40   a  will have more stability and less agility for pitch operation (stability against forward and backward movement). 
         [0058]    In the same manner, instead of changing the length of the arms, the location of the rotors on the arms can be changed in order to achieve the same effects. By moving the rotor locations on the side arms a user can change the stability and agility of the roll operation. By moving the rotor locations on the front and back arms a user can change the stability and agility of the pitch operation. 
         [0059]    This implementation of the present disclosure has a number of advantages, first and foremost of which is the ability of changing the agility and stability of the quadcopter, since there are situations where either is desired. Examples of situations where a greater stability is desired: (1) A novice learning how to fly a quadcopter, (2) Aerial photography or videography, (3) Navigating through crowded space, for example, an area with trees and other tall structures. Examples of situations where a greater agility is desired: (1) Aerobatics, (2) Fun flying with fast movements, (3) For having a better control against changing wind, (4) Quick turning (yaw). 
         [0060]    Besides the above advantages, the present implementation, with its ability of extending and contracting the arms and moving the location of the rotors on the arms, will also allow the user (1) to make the quadcopter more compact for carrying; (2) to have a better chance of freeing the quadcopter remotely if it gets caught in tree branches or cables during a flight; and (3) to have the ability of moving quadcopter parts out of the field of view for wide-angle photography and videography. 
         [0061]    It will be understood that the foregoing implementation in a quadcopter, which has adjustable length aims or adjustable rotor position, thus allowing the change of its stability/agility, can be performed either remotely during the flight or in between flights and can be applied to all multicopter configurations. Moreover, these implementations may be combined or used individually. 
         [0062]    In another implementation of the present disclosure, the angle of the arms relative to one another can be changed by mounting the arms to the central hub to enable pivotal movement of the arms relative to the hub. Referring to  FIGS. 10A-10C , there are shown three configurations of a quadcopter  100  having four arms  102 ,  104 ,  106 ,  108  extending from a central hub  110  and having rotors  112  mounted on the distal section of each arm  102 ,  104 ,  106 ,  108 . As shown in  FIGS. 10A-10B , the forward motion is illustrated as F, in which the two arms  102  and  104  located forward of the pitch axis are the left and right forward arms, and in which the two arms  106 ,  108  rearward of the pitch axis are the left and right rearward arms respectively. In  FIG. 10A  the arms  102 ,  104 ,  106 ,  108  are in a normal position or configuration. In  FIG. 10B  a modified position is shown with the forward arms  102 ,  106  moved towards each other and coming closer to the roll axis, and the rearward arms  106 ,  108  likewise are moved toward each other and closer to the roll axis. In  FIG. 10C , the opposite movement has occurred, and in this modified position the left forward and rearward arms  102 ,  106  are moved towards each other, coming closer to the pitch axis, and similarly the right forward and rearward arms  104 ,  108  are moved towards each other, coming closer to the roll axis. Movement can be achieved in various ways, for example, using a lead screw mechanism, a rack-and-pinion arrangement, a spring mechanism (either pushed or pulled), pneumatic, hydraulic, electromagnetic, and other known means. Preferably the movement is controlled by the controller coupled to the mechanism for moving the arms. 
         [0063]    Let the moment of inertia be I (rotational mass) about the roll axis as I RA , moment of inertia about the pitch axis as I PA , and moment of inertia about the yaw axis as I YA . The value of I RA  and I PA  are different in configurations shown in  FIGS. 10B and 10C  than in the configuration of  FIG. 10A . Hence they have different flight characteristics than the configuration of  FIG. 10A . For the configuration of  FIG. 10B , pitch agility is less than the configuration of  FIG. 10A , and roll agility is greater than the configuration of  FIG. 10A . For the configuration in  FIG. 10C , the pitch agility is greater than the configuration in  FIG. 10A , and roll agility is less than the configuration in  FIG. 10A . Yaw agility is not affected by this change of configuration, and is the same for all the three configurations. 
         [0064]    Now referring to  FIG. 11 , and using a simple model of a quadcopter  40  where the center hub  44  is a disc of radius R and mass M disc , each of the four identical arms  42  (extending from the center hub  44 ) has a length of L and mass M arm  and each of the four identical rotors  46  has a mass M rotor . Uniform mass distribution in the arms is assumed. The arms can move a rotational motion in a 2-dimensional plane with the axis of rotation coincident with the yaw axis. In this simple model the configuration is symmetric about the roll axis, and the arms form an angle of φ with the roll axis. 
         [0065]    The moment of inertia about the Roll axis is 
         [0000]        I   RA   =M   disc   R   2 /4+4 M   arm ( L  sin φ) 2 /3+4 M   rotor ( L  sin φ) 2  
 
         [0066]    The moment of inertia about the Pitch axis is 
         [0000]        I   PA   =M   disc   R   2 /4+4 M=   arm ( L  cos φ) 2 /3+4 M   rotor ( L  cos φ) 2  
 
         [0067]    Referring back to  FIGS. 10A-10C , considering a realistic situation where M disc =400 gram, M arm =100 gram, and M rotor =100 gram, R=6 cm, and L=20 cm; we get 
         [0000]        I   RA =3600+213333(sin φ) 2  
 
         [0000]        I   PA =3600+213333(cos φ) 2  
 
         [0068]    For φ=45 deg (normal quadcopter configuration,  FIG. 4A ), this yields: 
         [0000]        I   RA =110266 gm·cm 2  
 
         [0000]        I   PA =110266 gm·cm 2  
 
         [0069]    For φ=30 deg (narrow configuration,  FIG. 4B ), this yields: 
         [0000]        I   RA =56933 gm·cm 2  
 
         [0000]        I   PA =163600 gm·cm 2  
 
         [0070]    This shows that by changing φ from 45 to 30 degrees, we get a ˜48% decrease of I RA  and a ˜48% increase of I PA . In other words, it increases the roll agility (reduces the roll stability) and reduces the pitch agility (increases the pitch stability). 
         [0071]    Similarly, for φ=60 deg (wide configuration,  FIG. 4C ), this yields: 
         [0000]        I   RA =163600 gm·cm 2  
 
         [0000]        I   PA =56933 gm·cm 2  
 
         [0072]    This shows that by changing φ from 45 to 60 degrees, we get a ˜48% increase of I RA  and a ˜48% decrease of I PA . In other words, it reduces the roll agility (increases the roll stability) and increases the pitch agility (reduces the pitch stability). 
         [0073]    In further details of the mechanism, still referring to the implementation of  FIGS. 10A-10C , the change of configuration of the arms can be done when the quadcopter is on the ground. It can also be devised such that the change of configuration can be performed remotely or in an automated way while quadcopter is in flight. 
         [0074]    The arms can be independent of each other and configured to rotate in response to actuation of servo motors. There are a number of possible ways of changing the angle of the arms, including but not limited to: (1) Servo motors attached to individual arms that can be controlled remotely; (2) one motor is placed between the two forward arms  102 ,  104 , which will pull together the otherwise spring-loaded arms. There will be another identical motor for the rearward arms  106 ,  108 ; (3) a hand cranked version that can be adjusted on ground; and (4) a ball-and-indent or ball-and-detent type mechanical arrangement for moving the arms by hand to predetermined locations. 
         [0075]    Variations of this technique of changing the quadcopter configuration are also possible. For example, the rotation of the arms can be about an axis that is parallel to (but not coincident with) the yaw axis. Also, the configuration need not be perfectly symmetric, and the angle φ with the roll axis can be different for individual arms. Another possible variation is illustrated in  FIGS. 12A-12B  using the plus (+) type quadcopter  120  having a forward arm  122 , a rearward arm  124 , and left and right side arms  126 ,  128 , extending from a central hub  130 . Forward motion is depicted with F as shown. For a standard quadcopter configuration shown in  FIG. 12A , both the side arms  126 ,  128  are moved forward into a new configuration in  FIG. 12B , thus increasing the roll agility, and moving the center of gravity forward at the same time. This technique may be useful if the original center of gravity is somewhat off-centered that needs to be corrected. 
         [0076]    Referring next to  FIGS. 13A and 13B , the same principle and techniques described before can also be used for other type of multicopters. A hexacopter  140  is shown here in two configurations having six arms  142 ,  144 ,  146 ,  148 ,  150 ,  152  extending from and central hub  154  and having a rotor  156  at the distal section of each arm  142 ,  144 ,  146 ,  148 ,  150 ,  152 . In  FIG. 13A , the normal configuration with equal angles between the adjacent arms (with the direction and motion is along the forward arm  142 ), and in  FIG. 13B , a modified configuration where the two angled forward arms  152  and  144  have moved closer to the forward arm  142 , and corresponding rearward angled arms  150  and  146  have moved closer to rear arm  148 . This results to a higher value of I PA  (moment of inertia about the pitch axis) and a lower value of I (moment of inertia about the roll axis). 
         [0077]    The present disclosed implementation has significant advantages over the conventional design of multicopters. First and foremost of which is the ability of changing the agility and stability in the pitch and roll directions. 
         [0078]    Examples of situations where a greater overall stability is desired and where the normal configuration ( FIG. 10A ) could be used: (1) A novice learning how to fly a quadcopter, (2) Aerial video or photography, and (3) an asymmetrical shift in the thrust or the load. 
         [0079]    Examples of situations where narrow configuration ( FIG. 10B ) could be used (1) Navigating through crowded space, (for example, an area with trees and other tall structures) since this configuration has a narrow width. (2) Where more pitch agility (sideways agility) is desired (aerobatic sideways flip, for instance). 
         [0080]    Examples of situations where wide configuration ( FIG. 10C ) could be used: (1) Where more roll agility (front/back agility) is desired (aerobatic head flip, for instance). (2) For having a better control against changing head-wind or tail-wind. 
         [0081]    Secondly, the present disclosure is fairly easy to implement using mechanical or electromechanical techniques and tools. 
         [0082]    Besides the above main advantages, the present invention, with its ability of moving the arms sideways will also allow the user (1) to make the quadcopter more compact for carrying or fitting it a narrow space; (2) a better chance of freeing the quadcopter remotely if it gets caught in tree branches or cables during a flight; (3) having a bigger field of view of a mounted camera by moving the arms out of the way appropriately; and (4) the ability of moving the center of gravity back and forth along any direction by adjusting the individual arm angles differently, a feature that can be very useful when a payload mounted on the quadcopter is somewhat off-centered or shifts during flight. 
         [0083]      FIG. 14  illustrates a control system  200  for the implementations described above. The control system  200  includes a remote transmitter  202  and a multicopter  204  with an on-board receiver  206  and Flight Control Board (FCB)  208 . 
         [0084]    The quadcopter designs described above can be flown using the foregoing Remote Control (R/C) Transmitter  200 , receiver  206 , and FCB  208 . The transmitter  200  is operated by the user on ground. The receiver  206  and FCB  208  are located on the vehicle, typically near or in the center hub  210 . These are readily available components and will not be described in greater detail herein. 
         [0085]    In use, the operator uses the transmitter  200  to send out basic four commands: (1) Pitch, to go forward/backward, (2) Roll, to go sideways, (3) Yaw, to turn the body clockwise or counterclockwise, and (4) Throttle, to move up/down. There four commands are sent via four radio channels. Usually, joysticks are used for operator input as shown that. 
         [0086]    The receiver  206  receives those commands via radio transmission and sends them directly to inputs on the FCB  208 . The FCB  208  is loaded with the flight control software, which processes the input signals (pitch, roll, yaw, and throttle), and accordingly sends control signals to the four (or six for hexacopter or eight for octocopter) rotors individually. Below is a block diagram: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0087]    For example, if a Pitch command is sent to move forward, then the FCB  208  spins up rotors  3  and  4 , and spins down rotors  1  and  2 . The net effect is the craft will tilt forward and move along that direction. 
         [0088]    Similarly, if a Yaw command is sent, then the FCB  208  spins down the rotors  1  and  3 , and spins up rotors  2  and  4 . The net effect is to turn the craft clockwise. 
         [0089]    This type of FCB is considered a standard accessory for multirotors and drones. There are several companies that make them. Some of them contain only basic sensor components like gyros, while other also contain 3-axis accelerometers and more, like barometer, magnetometer. More advanced ones have options for connecting GPS and similar positioning devices. Some also offers the option of customizing the flight control software. 
         [0090]    The preferred control system for the aircrafts mentioned here is based upon physical models using Lagrangian dynamics. Control parameters for generation of these models would be the thrust of the motor, the length of each arm, the position of the rotor on the arms, and the angle between the arms. The software on-board, which is run by a micro-processor, computer, ASIC, or microchip adjusts these control parameters in mid-flight to fly the craft in optimum possible way. Sensors on the FCB, such as accelerometers or other known sensing devices, such as gyroscopes, altimeters, airspeed indicators, etc., are mounted on the FCB or on the exterior of the hub or the arms, as needed, in order to generate sensing signals to the controller on the FCB, which are then processed as described above to generate control signals. 
         [0091]      FIGS. 15A-15C  illustrate the concept of a quadcopter  150   a ,  150   b ,  150   c , respectively, having articulated arms  152 . The arms  152  can have an arbitrary number of identical or non-identical segments  154  of any shape, connected with rotary joints  156 , as shown in  FIG. 15A . One or more arms  153  can have multiple rotors  158 , depicted in  FIG. 15B . In addition, one or more arms  157  can also be branched to have segments  154  or entire articulated arms  159  extending therefrom as shown in  FIG. 15C . While a central hub  160  is shown in this implementation, it is to be understood that the central hub  160  can be eliminated and the receiver and FCB mounted within or on one of the arms  152 . 
         [0092]      FIG. 16  illustrates yet another implementation of a multicopter  170  in which a central hub is missing. In this design, the multicopter  170  has a plurality of arm segments  172 ,  174 ,  176 ,  178 , and  180  coupled together with articulated joints  184 . Ideally the articulated joints  184  maintain the arm segments  172 ,  174 ,  176 ,  178 , and  180  in a single plane by providing for rotation about a single axis. However, it is within the scope of this disclosure to utilize articulated joints that enable movement within three degrees of freedom, such as a ball joint, in which the arm segments  172 ,  174 ,  176 ,  178 , and  180  can move in three dimensions instead of two. A plurality of rotors  182  are provided along selected arm segments  172 ,  176 ,  178 , and in the case of arm segment  180  there are two rotors  182 . The receiver and FCB (not shown) would be mounted or integrated into one or more of the arm segments  172 ,  174 ,  176 ,  178 , and  180 . 
         [0093]    The implementation in  FIG. 16  provides a vehicle that can change its shape dynamically (in the air) or while on the ground to accommodate different terrain or terrain features, as well as buildings, bridges, antenna, and other natural and man-made structures. It also enables response to changes in thrust, load, and external conditions, such as wind. 
         [0094]    The foregoing implementations can have a variety of uses in both manned and unmanned (drone) applications including, without limitation, scientific research, law enforcement and military deployment, cargo delivery and passenger transport services, and as toys for amusement and education. Although no landing gear, fuselage, cargo container, or passenger compartment has been illustrated or described in conjunction with the instant implementations, one of ordinary skill in this technology can utilize known designs and commercially available components to design and add these features on the described aircraft. 
         [0095]    The various embodiments described above can be combined to provide additional implementations. Aspects of the implementations can be modified, if necessary, to employ concepts of the various patents, applications and publications to provide yet further implementations. Thus, it will be understood that the implementations described herein can be combined in multiple ways or used individually. For example one version could have extendable arms used in combination with adjustable angle arms or with movable thrust generators, or with both of these features, or with the articulated arms or a combination of all of these features. This applies equally well to the adjustable angle feature, the movable thrust generators, and the articulated arms. 
         [0096]    U.S. provisional patent application Ser. No. 61/929,249 filed Jan. 20, 2014 and U.S. provisional patent application Ser. No. 62/027,246 filed Jul. 22, 2014, are incorporated herein by reference, in their entirety. 
         [0097]    These and other changes can be made to the described implementations in light of the foregoing detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific implementations disclosed in the specification and the claims, but should be construed to include all possible implementations along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the foregoing disclosure.