Patent Publication Number: US-11046432-B1

Title: Circumferentially-driven propulsion mechanism

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of and claims priority to U.S. application Ser. No. 14/866,557, filed Sep. 25, 2015, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Many unmanned aerial vehicles (“UAV”) utilize outrunner brushless motors to drive propellers to lift and maneuver the UAV. The typical brushless motor includes a stator in which electromagnets are positioned around the perimeter of the stator. A rotor, or can, is positioned and rotates around the stator. A shaft extends from the rotor to which a propeller is coupled. An electronic speed control provides current to the electromagnets that cause the rotor to rotate about the stator. The shafts of brushless motors wear-out due to stress on the shaft caused by movements (pitch, yaw, and roll) of the UAV, because the movements (and stresses) are not parallel with the shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical components or features. 
         FIG. 1  is a block diagram of a top-down view of a UAV, according to an implementation. 
         FIG. 2  is a block diagram of a side view of a UAV illustrating the forces generated and/or applied to the UAV, according to an implementation. 
         FIG. 3  is a block diagram of a top-down view of a circumferentially-driven propulsion mechanism, according to an implementation. 
         FIG. 4  is a block diagram of another top-down view of a circumferentially-driven propulsion mechanism, according to an implementation. 
         FIG. 5  is a block diagram of a top-down view of another configuration of a circumferentially-driven propulsion mechanism, according to an implementation. 
         FIG. 6  is a block diagram illustrating the determination of forces to be generated by maneuverability propellers in response to a maneuverability command, according to an implementation. 
         FIG. 7  is a flow diagram illustrating an example navigation process, according to an implementation. 
         FIG. 8  is a block diagram illustrating various components of an unmanned aerial vehicle control system, according to an implementation. 
     
    
    
     While implementations are described herein by way of example, those skilled in the art will recognize that the implementations are not limited to the examples or drawings described. It should be understood that the drawings and detailed description thereto are not intended to limit implementations to the particular form disclosed but, on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include,” “including,” and “includes” mean including, but not limited to. Additionally, as used herein, the term “coupled” may refer to two or more components connected together, whether that connection is permanent (e.g., welded) or temporary (e.g., bolted), direct or indirect (e.g., through an intermediary), mechanical, chemical, optical, or electrical. Furthermore, as used herein, “horizontal” flight refers to flight traveling in a direction substantially parallel to the ground (e.g., sea level), and that “vertical” flight refers to flight traveling substantially radially outward from the earth&#39;s center. It should be understood by those having ordinary skill that trajectories may include components of both “horizontal” and “vertical” flight vectors. 
     DETAILED DESCRIPTION 
     This disclosure describes a UAV that includes a circumferentially-driven lifting propulsion mechanism that can be used to lift a UAV such that the perceived gravitational force acting on the UAV is approximately zero. In one implementation, the lifting propulsion mechanism is mounted at or near the center of the UAV and the lifting propulsion mechanism may generate a force with a lifting propeller that is approximately equal and opposite to a force applied to the UAV as a result of gravity (referred to herein as gravitational force). Rather than using a traditional brushless motor that includes a central shaft to which one or more propellers are mounted, the lifting propulsion mechanism may be circumferentially-driven and not include a shaft and the propellers are incorporated into the lifting propulsion mechanism. 
     For example, the lifting propulsion mechanism may include a propeller assembly and a propeller rim enclosure. The propeller assembly may include a plurality of propeller blades that extend radially. An end of each of the propeller blades are coupled to an inner side of a propeller rim that is substantially circular and encompasses or surrounds a perimeter of the plurality of propeller blades. A plurality of magnets is coupled to an outer side of the propeller rim. 
     The propeller rim enclosure is configured to encompass the propeller rim so that the propeller rim, and thus, the connected propellers, can rotate with respect to the propeller rim enclosure. For example, the propeller rim enclosure includes a perimeter wall that is substantially circular in shape and has a diameter that is larger than the diameter of the propeller rim. One or more side walls may extend inward from an upper edge and a lower edge of the perimeter wall to form a cavity into which the propeller rim and magnets coupled to the outer side of the propeller rim are positioned. A plurality of bearings, rails, or other guides, generally referred to herein as propeller rim guides, may also be positioned in the cavity to secure the propeller rim and allow the propeller rim to rotate with respect to the propeller rim enclosure. 
     Coupled to an inner side of the perimeter wall of the propeller rim enclosure is a plurality of electromagnets. Current may be applied to the electromagnets by a lifting propulsion mechanism controller to control the rotational speed of the propeller rim and the connected plurality of propeller blades. As the propeller rim and connected propeller blades rotate, the propeller blades generate a lifting force. The lifting force will cause the UAV to become airborne and remain at an altitude. 
     In some implementations, to stabilize and/or maneuver the UAV, one or more maneuverability propulsion mechanisms may be included and used to generate other forces that cause the UAV to pitch, roll, and/or yaw. The maneuverability propulsion mechanism may be configured in a similar manner compared to the lifting propulsion mechanism and/or may include other forms of propulsion, such as traditional brushless motors with a shaft and propeller, jets, etc. 
     Because the lifting propulsion mechanism can generate a force that is approximately equal to and opposite the gravitational force applied to the UAV, the forces to be applied to the UAV by the maneuverability propulsion mechanisms may be determined without considering the full effect of the gravitational force on the UAV. 
     The implementations described herein increase the efficiency of the UAV and allow the maneuverability propellers to operate in a wider range of rotational speeds, angles, pitches, and/or directions. For example, the lifting propulsion mechanism may be larger in size than the maneuverability propulsion mechanism(s) and selected based on the mass of the UAV and any anticipated payload. In one implementation, the lifting propulsion mechanism may be selected such that it is operating within its most efficient range when causing the propeller blades to generate a force that is approximately equal to and opposite the gravitational force applied to the UAV. 
     During transport, UAVs often need different capabilities (e.g., maneuverability, power efficiency) depending on their position and/or their profile. For example, when a UAV is landing, taking off, or in an area with many objects (e.g., a dense area such as a neighborhood, street, etc.), the UAV needs to be agile so that it can avoid colliding with other objects (stationary and/or moving). In comparison, when a UAV is at a high altitude, traveling at a high velocity (e.g., at a cruising altitude), in an area where there are no or few objects, efficiency and power conservation are desirable. 
     By designing the lifting propulsion mechanism such that the magnets and electromagnets that cause rotation of the propeller blades are at the perimeter of the propeller blades, the leverage is increased compared to traditional brushless motors and thus, the force needed to generate the same amount of torque is less. Specifically, the lever arm is increased. In a traditional shafted brushless motor, the lever arm, which is approximately the radius of the propeller shaft, is small, thereby requiring a large torque to generate the desired force from the propellers. In comparison, with the implementations discussed herein, the lever arm is approximately the radius of the propeller rim. Because the lever arm is larger, the torque required to generate the same force is reduced. By reducing the needed torque, the magnets and/or the electromagnets may be smaller and/or lighter weight. Likewise, there is a greater surface area on which to place the magnets and the electromagnets. 
     The lifting propulsion mechanism provides a primary purpose of providing lift and power efficiency to the UAV. In comparison, the maneuverability propulsion mechanism(s) may be configured with smaller, more agile motors, and the propellers may be smaller propellers designed for providing high agility and maneuverability for the UAV. In one example, the maneuverability propulsion mechanism(s) may be traditional center shafted brushless motors. The maneuverability propulsion mechanism(s) provide a primary purpose of guiding the UAV and providing high agility when needed. For example, when high agility is needed, the lifting propulsion mechanism may be shut down and the UAV may be navigated using the maneuverability propulsion mechanisms. In comparison, when agility is not as important and power efficiency is desirable, the lifting propulsion mechanism may be engaged to provide lift to the UAV. The maneuverability propulsion mechanisms may then be operated at a lower power draw and/or some of the maneuverability motors may be stopped. 
     In addition to, or as an alternative to utilizing a lifting propulsion mechanism in combination with maneuverability propulsion mechanism(s), the position of one or more of the propulsion mechanisms may be adjustable with respect to the body of the UAV. For example, one or more of the maneuverability propulsion mechanisms and/or the lifting propulsion mechanism(s) may be horizontally, vertically, and/or rotationally moved with respect to the body of the UAV while the UAV is in flight. In one implementation, the motor arms (discussed below) may be rotated, thereby rotating the maneuverability propulsion mechanisms with respect to the body of the UAV. 
     While the examples discussed herein primarily focus on UAVs in the form of an aerial vehicle utilizing multiple propellers to achieve flight (e.g., a quad-copter, octo-copter), it will be appreciated that the implementations discussed herein may be used with other forms and/or configurations of aerial vehicles and need not be unmanned. 
     As used herein, a “materials handling facility” may include, but is not limited to, warehouses, distribution centers, cross-docking facilities, order fulfillment facilities, packaging facilities, shipping facilities, rental facilities, libraries, retail stores, wholesale stores, museums, or other facilities or combinations of facilities for performing one or more functions of materials (inventory) handling. A “delivery location,” as used herein, refers to any location at which one or more inventory items (also referred to herein as a payload) may be delivered. For example, the delivery location may be a person&#39;s residence, a place of business, a location within a materials handling facility (e.g., packing station, inventory storage), or any location where a user or inventory is located, etc. Inventory or items may be any physical goods that can be transported using a UAV. 
       FIG. 1  illustrates a block diagram of a top-down view of a UAV  100 , according to an implementation. The UAV  100  includes eight maneuverability propulsion mechanisms  102 - 1 ,  102 - 2 ,  102 - 3 ,  102 - 4 ,  102 - 5 ,  102 - 6 ,  102 - 7 ,  102 - 8  spaced about a body  104  of the UAV  100 . The maneuverability propulsion mechanisms  102 , in this example, are center shafted brushless motors with propellers coupled to the shafts. The propeller may be any form (e.g., graphite, carbon fiber) and of a size sufficient to lift and/or guide the UAV  100  and any payload engaged by the UAV  100  so that the UAV  100  can navigate through the air, for example, to deliver a payload to a delivery location. In addition to the maneuverability propulsion mechanisms  102 , the UAV  100  includes a lifting propulsion mechanism  103 . 
     The lifting propulsion mechanism  103  is of a size and configuration to generate a force that is approximately equal and opposite to the gravitational force applied to the UAV  100 . For example, if the mass of the UAV, without a payload, is 20.00 kilograms (kg), the gravitational force acting on the UAV is 196.20 Newtons (N). If the UAV is designed to carry a payload having a mass between 0.00 kg and 8.00 kg, the lifting propulsion mechanism may be configured such that when generating a force between 196.00 N and 275.00 N, it is operating in its most power efficient range. 
     To counteract the angle of momentum of the lifting propulsion mechanism  103 , the maneuverability propulsion mechanisms  102  may rotate in a direction opposite that of the lifting propulsion mechanism  103  to keep the UAV  100  from rotating with the rotation of the propeller assembly of the lifting propulsion mechanism  103 . 
     While this example includes eight maneuverability propulsion mechanisms and a lifting propulsion mechanism, in other implementations, more or fewer maneuverability propulsion mechanisms, and/or lifting propulsion mechanism may be utilized. Likewise, in some implementations, the propulsion mechanisms may be positioned at different locations on the UAV  100  and/or at different angles with respect to the body of the UAV  100 . For example, rather than or in addition to rotating the propellers of the maneuverability propulsion mechanisms  102  in a direction opposite the rotation of the propellers of the lifting propulsion mechanism  103 , one or more of the maneuverability propulsion mechanisms may be titled at an angle with respect to the lifting propulsion mechanism  103  to generate an angle of momentum at an amount approximately equal and opposite to that generated by the lifting propulsion mechanism  103 . 
     Alternative methods of propulsion may also be utilized for either or both of the lifting propulsion mechanism and/or maneuverability propulsion mechanisms. For example, fans, jets, turbojets, turbo fans, jet engines, and the like may be used to propel the UAV. Additional details for an implementation of a lifting propulsion mechanism are discussed further below with respect to  FIGS. 3-5   
     The body  104  or housing of the UAV  100  may likewise be of any suitable material, such as graphite, carbon fiber, and/or aluminum. In this example, the body  104  of the UAV  100  includes four rigid members  105 - 1 ,  105 - 2 ,  105 - 3 ,  105 - 4 , or beams, also referred to herein as motor arms, arranged in a hash pattern with the rigid members intersecting and joined at approximately perpendicular angles. In this example, rigid members  105 - 1  and  105 - 3  are arranged parallel to one another and are approximately the same length. Rigid members  105 - 2  and  105 - 4  are arranged parallel to one another, yet perpendicular to rigid members  105 - 1  and  105 - 3 . Rigid members  105 - 2  and  105 - 4  are approximately the same length. In some implementations, all of the rigid members  105  may be of approximately the same length while, in other implementations, some or all of the rigid members may be of different lengths. Likewise, the spacing between the two sets of rigid members may be approximately the same or different. 
     While the implementation illustrated in  FIG. 1  includes four rigid members  105  that are joined to form the body  104  and corresponding motor arms, in other implementations, there may be fewer or more components to the body  104 . For example, rather than four rigid members, in other implementations, the body  104  of the UAV  100  may be configured to include six rigid members. In such an example, two of the rigid members  105 - 2 ,  105 - 4  may be positioned parallel to one another. Rigid members  105 - 1 ,  105 - 3  and two additional rigid members on either side of rigid members  105 - 1 ,  105 - 3  may all be positioned parallel to one another and perpendicular to rigid members  105 - 2 ,  105 - 4 . With additional rigid members, additional cavities with rigid members on all four sides may be formed by the body  104 . A cavity within the body  104  may be configured to include a payload engagement mechanism for the engagement, transport, and delivery of item(s) and/or containers that contain item(s) (generally referred to herein as a payload). 
     In some implementations, the UAV may be configured for aerodynamics. For example, an aerodynamic housing may be included on the UAV that encloses the UAV control system  110 , one or more of the rigid members  105 , the body  104 , and/or other components of the UAV  100 . The housing may be made of any suitable material(s) such as graphite, carbon fiber, aluminum, etc. Likewise, in some implementations, the location and/or the shape of the payload (e.g., item or container) may be aerodynamically designed. For example, in some implementations, the payload engagement mechanism may be configured such that, when the payload is engaged, it is enclosed within the body and/or housing of the UAV  100  so that no additional drag is created during transport of the payload by the UAV  100 . In other implementations, the payload may be shaped to reduce drag and provide a more aerodynamic design of the UAV and the payload. For example, if the payload is a container and a portion of the container extends below the UAV when engaged, the exposed portion of the container may have a curved shape. 
     The maneuverability propulsion mechanisms  102  are positioned at both ends of each rigid member  105 . The maneuverability propulsion mechanisms may be any form of motor capable of generating enough speed with the propellers to lift the UAV  100  and any engaged payload thereby enabling aerial transport of the payload. Likewise, the maneuverability propellers may be of any material and size sufficient to provide lift and maneuverability to the UAV. For example, the maneuverability propellers may be 10 inch-12 inch diameter carbon fiber propellers. 
     The lifting propulsion mechanism  103  is positioned toward a center of the body  104  of the UAV. Example configurations of the lifting propulsion mechanism  103  are discussed further below with respect to  FIGS. 3-5 . As noted above, in some implementations, the lifting propulsion mechanism may be selected such that it operates within its most efficient range when generating a force sufficient to maintain the UAV and any attached payload at a commanded altitude. 
     Mounted to the body  104  is the UAV control system  110 . In this example, the UAV control system  110  is mounted to one side and on top of the body  104 . The UAV control system  110 , as discussed in further detail below with respect to  FIG. 8 , controls the operation, routing, navigation, communication, lifting propulsion mechanism control, maneuverability propulsion mechanism control, and the payload engagement mechanism of the UAV  100 . 
     Likewise, the UAV  100  includes one or more power modules  112 . In this example, the UAV  100  includes three power modules  112  that are removably mounted to the body  104 . The power module for the UAV may be in the form of battery power, solar power, gas power, super capacitor, fuel cell, alternative power generation source, or a combination thereof. The power module(s)  112  are coupled to and provide power for the UAV control system  110 , the propulsion mechanisms, and the payload engagement mechanism. 
     In some implementations, one or more of the power modules may be configured such that it can be autonomously removed and/or replaced with another power module while the UAV is landed. For example, when the UAV lands at a delivery location, relay location and/or materials handling facility, the UAV may engage with a charging member at the location that will recharge the power module. 
     As mentioned above, the UAV  100  may also include a payload engagement mechanism (not shown). The payload engagement mechanism may be configured to engage and disengage items and/or containers that hold items. In this example, the payload engagement mechanism is positioned beneath the body of the UAV  100 . The payload engagement mechanism may be of any size sufficient to securely engage and disengage containers that contain items. In other implementations, the payload engagement mechanism may operate as the container, containing the item(s). The payload engagement mechanism communicates with (via wired or wireless communication) and is controlled by the UAV control system  110 . 
     While the implementations of the UAV  100  discussed herein utilize propellers to achieve and maintain flight, in other implementations, the UAV may be configured in other manners. For example, the UAV may include fixed wings and/or a combination of both propellers and fixed wings. For example, the UAV may utilize one or more propulsion mechanisms that include propellers to enable vertical takeoff and landing and a fixed wing configuration or a combination wing and propeller configuration to sustain flight while the UAV is airborne. 
       FIG. 2  is a block diagram of a side view of a UAV  200  illustrating the forces generated and/or applied to the UAV, according to an implementation. The UAV  200  includes a plurality of maneuverability propulsion mechanisms  202  that are configured to stabilize and maneuver the UAV  200  by generating directional forces. Likewise, the UAV  200  includes a lifting propulsion mechanism  203  configured to generate a vertical force sufficient to lift and maintain the UAV at an altitude. 
     As identified above, one or more of the maneuverability propulsion mechanisms  202  may include propellers that rotate in a direction opposite to the rotation of the propellers of the lifting propulsion mechanism  203  to generate an angle of momentum that is opposite to that of the lifting propulsion mechanism  203  to stabilize the UAV  200 . Alternatively, one or more of the maneuverability propulsion mechanisms  202  may be titled or offset such that it rotates on a different plane than the lifting propulsion mechanism  203  and generates an angle of momentum that is offset to the angle of momentum generated by the lifting propulsion mechanism  203 . 
     In another implementation, the UAV  200  may include one or more stability propulsion mechanisms  208 . In one implementation, the UAV  200  may include two stability propulsion mechanism  208 . The stability propulsion mechanism  208  may be positioned to generate forces in a plane that is substantially orthogonal to the rotational plane of the lifting propulsion mechanism  203  and/or the maneuverability propulsion mechanisms  202  so that the stability propulsion mechanism(s) generate an angle of momentum that is approximately equal and opposite to the angle of momentum generated by the rotation of the lifting propulsion mechanism  203  and/or the maneuverability propulsion mechanisms  202 . Likewise, the stability propulsion mechanism(s)  208  may be positioned on opposing ends of the UAV  200 , as illustrated, and oriented so they rotate in opposite directions with respect to one another, thereby balancing the rotational forces in the orthogonal plane in which they are rotating. 
     Continuing with the above example, if the mass of the UAV, without payload, is 20.00 kilograms (kg) the gravitational force  205  acting upon the UAV is 196.133 Newtons (N). If the UAV is designed to carry a payload having a mass between 0.00 kg and 8.00 kg, the lifting propulsion mechanism  203  may be configured such that when generating a force between 196.00 N and 275.00 N, it is operating in its most power efficient range. To keep the UAV at an approximately steady altitude (e.g., hover), the vertical force  206  generated by the lifting propulsion mechanism  203  is approximately equal and opposite the gravitational force  205 . When in hover, the lifting propulsion mechanism  203  may include propellers that rotate at a speed sufficient to generate a vertical force  206  that is approximately equal and opposite to the gravitational force  205  and the propellers of the maneuverability propulsion mechanisms may periodically rotate to generate forces sufficient to stabilize the UAV  200  when other forces (e.g., wind) act upon the UAV. 
     In some implementations, one or more of the maneuverability propulsion mechanisms  202  may be disengaged. Likewise, the direction of rotation of one or more of the maneuverability propulsion mechanisms  202  may be altered to generate either a positive vertical force or a negative vertical force. In another example, the pitch of one of more of the maneuverability propulsion mechanisms  202  and/or the lifting propulsion mechanism  203  may be altered to increase or decrease the generated force without altering the rotational speed of the propellers. In still another example, the angle of one or more of the maneuverability propulsion mechanisms and/or the lifting propulsion mechanism may be altered to generate directional forces other than vertical. The angle of the propulsion mechanisms may be altered by altering the pitch and/or roll of the UAV  200  and/or by altering the angle of the propulsion mechanism with respect to the body of the UAV  200 . 
     In traditional UAV configurations, if a maneuver command is received instructing the UAV  200  to move in a horizontal direction  208 , the forces to be generated by each of the motor and propeller pairs is determined that considers not only the direction of movement but also the gravitational force. For example, in a traditional UAV, if the UAV receives a maneuver command to travel in a horizontal direction  208  with a velocity of 20.0 meters/second (m/s) and without changing altitude, the sum of all the forces acting on the UAV must be considered and a force determined for each of the propellers and motors. 
     To determine the force to be generated by each of the propellers (assuming no other environmental forces), the directional force needed to move the UAV  200  can be separated into horizontal and vertical components. In this example, the gravitational force  205  is 196.133 N in the negative direction. As such, a positive vertical force of 196.133 N is needed to maintain the current altitude of the UAV. The horizontal force needed to move the UAV  200  at a velocity of 20.0 m/s second can be determined from the following equation: 
               F   h     =       C   d     *       ρ   *     v   2       2     *   A           
where C d  is the drag coefficient, ρ is the density of the air, v is velocity, A is the presented area of the UAV, also referred to as the reference area, and F h  is the horizontal force. The drag coefficient, a dimensionless number, includes all the complex dependences such as air conditions, shape of the UAV, inclination of the UAV, form drag, skin friction drag, wage drag, and induced drag. The drag coefficient therefore depends on the configuration of the UAV, among other variables, and may be determined through experimentation and/or during navigation as part of a command feedback. For purposes of this discussion, we will assume that the drag coefficient is 0.10. Density of air depends on the temperature and the particles in the air. Assuming dry air at a temperature of 20 degrees Celsius, the density is 1.2041 kg/m 3 . Finally, the presented area of the UAV depends on the pitch, roll, size, and shape of the UAV. For this example, we will assume the presented area is 0.5 m 2 .
 
     Using the above factors, the horizontal force can be computed as: 
     
       
         
           
             
               F 
               h 
             
             = 
             
               
                 
                   0 
                   . 
                   1 
                 
                 * 
                 
                   
                     
                       1 
                       . 
                       2 
                     
                     ⁢ 
                     0 
                     ⁢ 
                     4 
                     ⁢ 
                     1 
                     * 
                     2 
                     ⁢ 
                     
                       0 
                       2 
                     
                   
                   2 
                 
                 * 
                 
                   0 
                   . 
                   5 
                 
               
               = 
               
                 12.041 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 N 
               
             
           
         
       
     
     Summing the vertical force (196.133 N) and the horizontal force (12.041 N), the UAV must generate a total force (F t ) of 196.502 N (F v   2 +F h   2 =F t   2 ). 
     While such a system provides high agility and maneuverability for the UAV, distributing the total forces needed to control the UAV among all the propellers results in the motors frequently not operating within their most efficient ranges, thereby resulting in an inefficient use of power. 
     With the implementations described herein, the lifting propulsion mechanism  203  can be configured such that when the vertical force generated by the lifting propulsion mechanism  203  is 196.133 N the lifting propulsion mechanism  203  is operating in its most efficient range. When a maneuver command is received instructing the UAV  200  to move in the horizontal direction  208  at a speed of 20.0 m/s, the horizontal force of 12.041 N can be determined and distributed among one or more of the maneuverability propulsion mechanisms  202 . For example, if the maneuverability propulsion mechanisms  202  are at their most efficient range when generating 6.00 N of force, the angle of two of the maneuverability propulsion mechanisms may be altered to be approximately ninety degrees with respect to the UAV  200  and the total horizontal force of 12.041 N distributed between those two maneuverability propulsion mechanisms  202 , thereby propelling the UAV  200  in the commanded direction in a more efficient manner. The remaining maneuverability propulsion mechanisms may be disengaged or periodically engaged to stabilize the UAV  200 . 
     As can be seen from a comparison of the traditional UAV, which requires the total force to be distributed among all the propellers, and the implementations described herein, which allocate the vertical force to a lifting propulsion mechanism and allocate the remaining horizontal force of only 12.041 N to the maneuverability propulsion mechanisms, there is a much larger range of operational values available to the maneuverability propulsion mechanisms. The increased operational range for the maneuverability propulsion mechanisms allows greater operational efficiency in maneuvering the UAV  200 . 
     By separating the vertical force from the horizontal force to be generated by the UAV, the total force to be generated by the maneuverability propulsion mechanisms  203  is greatly reduced. Likewise, the computations to be performed by corresponding propulsion mechanism controllers (e.g., electronic speed controls) are simplified. For example, the lifting propulsion mechanism controller only needs to compute the force necessary for lifting the UAV. Likewise, the maneuverability propulsion mechanism controllers for the maneuverability propulsion mechanisms only need to compute the forces necessary to stabilize and maneuver the UAV  200 . 
     In some implementations, one or more of the propulsion mechanism controllers may include a feedback component that provides information to other propulsion mechanism controllers. For example, the maneuverability propulsion mechanism controllers may provide feedback to the lifting propulsion mechanism controller identifying the forces generated by the maneuverability propulsion mechanisms and the lifting propulsion mechanism controller may utilize this information to increase and/or decrease the force generated by the lifting propulsion mechanism  203 . For example, in some implementations, the control signals sent to the maneuverability propulsion mechanisms may be selected so that the maneuverability propulsion mechanisms operate within the most efficient ranges when maneuvering the UAV. In such an implementation, the forces generated by the maneuverability propulsion mechanisms may be determined and the lifting propulsion mechanism may rotate at a speed necessary to provide any additional lift needed to maintain the UAV at an intended altitude and/or to alter the altitude of the UAV. 
     In comparison, the lifting propulsion mechanism controller may provide feedback information to the maneuverability propulsion mechanism controllers identifying the lifting force generated by the lifting propulsion mechanism. The maneuverability propulsion mechanism controllers may utilize this information to adjust the perceived gravitational force applied to the UAV when computing forces for generating maneuvers or altering the attitude of the UAV  200 . 
     In other implementations, rather than altering the angle of one or more of the maneuverability propulsion mechanisms, the angle of the UAV  200  may be altered to navigate in the desired direction. In such an implementation, the lifting propulsion mechanism and the engaged maneuverability propulsion mechanisms will be generating both horizontal and vertical forces to move the UAV in the desired direction, but the majority of the vertical force will be provided by the lifting propulsion mechanism. In still other implementations, rather than, or in addition to altering the rotational speed of the propulsion mechanisms, the pitch of the propellers included in the propulsion controllers may be altered to either increase or decrease the force generated by the propellers. Likewise, in some implementations, the direction of rotation of one or more of the maneuverability propulsion mechanisms may be altered to alter the direction of generated force. For example, if the angle of the UAV is to be altered, the directional rotation of one or more of the maneuverability propulsion mechanisms on one side of the UAV  200  may rotate in a direction opposite to the maneuverability propulsion mechanisms located on the opposing side of the UAV  200 , thereby altering the angle of the UAV  200 . As will be appreciated, any combination of rotational speed, pitch, angle, and/or direction may be utilized to propel the UAV  200 . 
       FIG. 3  is a block diagram of a top-down view of a circumferentially-driven propulsion mechanism, according to an implementation. In this implementation, illustrated is a lifting propulsion mechanism  303  that may be used to generate sufficient force to lift the UAV discussed above with respect to  FIGS. 1-2 . In other implementations, the propulsion mechanism may be used as a maneuverability propulsion mechanism. The propeller blades  301  of the lifting propulsion mechanism are not mounted to a center shaft. 
     The lifting propulsion mechanism  303  includes a propeller assembly that includes a plurality of propeller blades  301 - 1 ,  301 - 2 ,  301 - 3 , and  301 - 4  that are radially positioned. One end of each of the propeller blades  301  is coupled to an inner side of a propeller rim  309 , as illustrated in the expanded view. The propeller rim is substantially circular, has a first diameter, extends around, and encompasses the propeller blades  301 . 
     Coupled to an exterior or outer side of the propeller rim  309  is a plurality of magnets  311 . The magnets may be any type of permanent magnet, such as neodymium iron boron (NdFeB), praseodymium, samarium cobalt (SmCo), alnico, and/or ceramic or ferrite magnets. Because the magnets are positioned around the perimeter of the propeller blades, the force needed to generate the same amount of torque as a tradition brushless motor is much less because of the increased leverage do the positioning of the magnets. Accordingly, the magnets may be smaller and lighter weight. 
     The opposing ends of the propeller blades may be coupled together, or as illustrated in  FIG. 3 , may be coupled to an inner rim  306 . Because the external portion of the propeller blades generate most of the lifting force, they are rotating faster, the inner portion of the propeller blades may be removed and the blades coupled to an inner rim  306 , thereby reducing weight of the propeller assembly and reducing noise generated by the propeller blades  301 . 
     The propeller rim  309  and magnets  311  are positioned within and encompassed by a propeller rim enclosure  307 . The propeller rim enclosure  307  is substantially circular, has a second diameter that is larger than the first diameter of the propeller rim  309 , and extends around and encompasses the propeller rim  309 . 
     Referring to the expanded view, the propeller rim enclosure  307  includes a perimeter wall  312 , an upper wall  314 , and a lower wall  316 . The upper wall  314  is coupled to an upper edge of the perimeter wall at approximately a perpendicular angle and extends inward toward the propeller rim. The lower wall  316  is coupled to the lower edge of the perimeter wall  312  at approximately a perpendicular angle and extends inward toward the propeller rim. The perimeter wall  312 , upper wall  314 , and lower wall  316  form a cavity into which the propeller rim  309  is positioned. 
     Also positioned within the cavity and coupled to an inner side of the perimeter wall  312  is a plurality of electromagnets  313 . A current is applied to the electromagnets  313  by the lifting propulsion controller, also known as an electronic speed control, so that the electromagnets  313  generate a magnetic field that causes the propeller rim to rotate. When the propeller rim rotates, the propeller blades coupled to the propeller rim also rotate, thereby generating a lifting force as the air passes over the propeller blades. The propulsion mechanism controller, discussed below with respect to  FIG. 8 , can control both the speed and direction of rotation of the propeller rim. As will be appreciated, the propeller assembly and the propeller rim enclosure are positioned such that the electromagnets are within a defined distance of the magnets so that the magnets are within the magnetic field generated by the electromagnets. 
     In some implementations, the propeller rim  309  is supported within the propeller rim enclosure by a plurality of propeller rim guides  308 . The propeller rim guides  308  may be bearings, rails, grooves, etc. that secure the propeller rim  309  within the propeller rim enclosure  307  and allow the propeller rim  307  to rotate with respect to the propeller rim enclosure  307 . In one implementation, a first set of propeller rim guides  308 - 1  may be coupled to an inner side of the upper wall and a second set of propeller rim guides  308 - 2  may be positioned on an inner side of the lower wall  316  such that the propeller rim  309  is positioned between the propeller rim guides  308 . 
     The propeller rim enclosure  307  may be coupled to or otherwise mounted to the frame of a UAV  100 , as illustrated in  FIG. 1 . When mounted, the propeller rim enclosure  307  remains stationary with respect to the propeller assembly and the propeller assembly rotates within the propeller rim enclosure. 
     In addition to the reduced torque required to generate the same amount of force by the propeller blades, the configuration of the lifting propulsion mechanism  303  provides other benefits as well. For example, because the propeller blades  301  are terminated and coupled into the propeller rim  307 , the propeller blades do not have tips. By eliminating the tips of the propeller blades, the noise generated from the rotation of the propeller blades is reduced and tip vortices are reduced or eliminated. Still further, by positioning the propeller assembly such that the propeller rim  309  and magnets are within the cavity of the propeller rim enclosure  307 , the propeller rim enclosure  307  dampens the noise generated by the rotation of the propeller assembly. In some implementations, the propeller rim enclosure may also include a protective shroud, such as a screen, that extends over the top and/or bottom of the propeller assembly so that the propeller assembly, including the propeller blades, is fully encompassed. By encompassing the propeller assembly with a protective shroud, it will prevent objects or debris from coming into contact with the propeller blades. 
       FIG. 4  is a block diagram of another top-down view of a circumferentially-driven propulsion mechanism, according to an implementation. In this example, the upper wall of the propeller rim enclosure  407  has been eliminated to enable illustration of the propeller rim  409 , magnets  411 , and electromagnets  413  positioned within the propeller rim enclosure  407  of the propulsion mechanism  403 . As illustrated, the magnets  411  are coupled to an outer edge or perimeter of the propeller rim  409 . In this example, the magnets are equally spaced and extend around the entire perimeter of the propeller rim  409 . 
     The electromagnets  413  are coupled to an inner edge of the perimeter wall  412  of the propeller rim enclosure  407 . In this example, the electromagnets  413  are equally spaced and extend around the entire inside of the perimeter wall  412 . 
     The propeller blades  401 - 1 ,  401 - 2 ,  401 - 3 , and  401 - 4  extend radially from the inner rim  406  and are coupled to the perimeter rim  409 . While this example illustrates four propeller blades that are equally spaced, in other implementations there may be additional and/or fewer propeller blades. Likewise, the propeller blades need to be equally positioned. 
     In some implementations, there may be more than one propeller assembly positioned within the propeller rim enclosure. For example, two propeller assemblies, each of which includes propeller blades, propeller rims, and magnets, may be positioned within the cavity of the propeller rim enclosure  407 . In such a configuration, the first propeller assembly may rotate in a first direction and the second propeller assembly may rotate in a second, opposite direction. In another implementation, the aerial vehicle may include multiple circumferentially-driven propulsion mechanisms  503  that are positioned adjacent one another. In such a configuration, a portion of the electromagnets may be shared between the circumferentially-driven propulsion mechanisms  503 . In either configuration, the propellers of a first circumferentially-driven propulsion mechanism  503  may rotate in an opposite direction than the propellers of the second circumferentially-driven propulsion mechanism  503 . As discussed above, by rotating the two sets of propeller blades in opposite directions, the angle of momentum generated by the two sets of propeller blades cancel out. 
     While the example illustrated in  FIG. 4  describes the magnets and electromagnets being equally spaced and extending around the entire perimeter of the propeller rim and perimeter wall, other implementations may utilize different configurations. For example,  FIG. 5  is a block diagram of a top-down view of another configuration of a circumferentially-driven propulsion mechanism  503 , according to an implementation. In this configuration, the magnets are positioned along the outside propeller rim  509  such that they are opposite the coupling location of the propeller blades  501 - 1 ,  501 - 2 ,  501 - 3 , and  501 - 4 . In such a configuration, the total number of magnets utilized may be reduced, thereby reducing the weight of the propulsion mechanism  503 . In other configurations, the magnets may be positioned at other locations on the propeller rim. For example, rather than positioning the magnets to correspond with the locations of the propeller blades, in other implementations, the magnets may be positioned along the propeller rim at areas where the propeller blades are not located. 
     In addition to altering the position and/or quantity of the magnets along the propeller rim, the position and/or quantity of the electromagnets  513  may likewise be adjusted, according to an implementation. For example, as illustrated in  FIG. 5 , rather than including electromagnets closely spaced and evenly distributed about the inside of the perimeter wall  512  of the propeller rim enclosure  507 , the electromagnets  513  may be distributed into sectors or groups. For example, the electromagnets  513  may be distributed in pairs around the inside of perimeter wall  512  of the propeller rim enclosure  507 . In other implementations, the magnets  511  and/or electromagnets  513  may be positioned at other locations and/or configurations. Likewise, the magnets  511  and/or electromagnets  513  do not need to be distributed in sectors or pairs. For example, the magnets  511  may be individually positioned around the outside of the propeller rim  509 . In another example, rather than positioning the magnets on the outside of the propeller rim  509 , the propeller blades  501  themselves, or a portion thereof, may be magnetic and operate as the magnets such that they rotate in response to a charge being applied to the electromagnets  513 . For example, the ends of the propeller blades  501  closest to the electromagnets  513  may be formed of a magnet material. 
     In some implementations, the only constraining factor may be that, regardless of the position of the propeller assembly with respect to the propeller rim enclosure, at least one magnet is aligned with at least one electromagnet so that the propeller assembly will be caused to rotate when a current is applied to the electromagnets. In some implementations, the number of magnets  511  may be different from the number of electromagnets  513 . 
     While the examples discussed above describe including the magnets on the propeller rim and the electromagnets on the propeller rim enclosure, in other implementations, the magnets and electromagnets may be reversed such that the magnets are placed on the inside of the perimeter wall of the propeller rim enclosure and the electromagnets are placed on the exterior of the propeller rim. In such a configuration, the electromagnets will rotate with the rotation of the propeller rim. Current may be provided to the electromagnets when coupled to the propeller rim via the propeller rim guides (e.g., bearings) that support and position the propeller rim within the propeller rim enclosure. 
     In still other implementations, rather than placing the magnets on the outer side of the propeller rim and placing the electromagnets on the inside of the perimeter wall of the propeller rim enclosure, in other implementations, the magnets may be positioned on an upper and/or lower edge of the propeller rim and the electromagnets placed on an inside of the upper wall and/or lower wall of the propeller rim enclosure. For example, the magnets may be placed on the upper edge of the propeller rim and the electromagnets may be placed on the inside of the upper wall of the propeller rim enclosure. The propeller rim may be supported by propeller rim guides, such as bearings that are positioned on the inside of the lower wall of the propeller rim enclosure. In such a configuration, the repelling forces generated by the magnets and the electromagnets when the electromagnets are active will not only cause the propeller rim to rotate but will also aid in maintaining the position of the propeller rim within the propeller rim enclosure. 
     In some implementations, rather than including a physical rail (e.g., bearings) that support the propeller rim within the cavity of the propeller rim enclosure, the magnets and electromagnets may be positioned such that the propeller assembly is magnetically levitated within the cavity of the propeller rim enclosure. For example, magnets may be placed on the upper edge and lower edge of the propeller rim and/or the propeller rim may be magnetic and the electromagnets  513  may be placed on the inside of the upper wall and the inside of the lower wall of the propeller rim enclosure. In such a configuration, when current is applied to the electromagnets, the repelling forces between the electromagnets and the magnets cause the propeller rim to be magnetically levitated and rotate within the propeller rim enclosure. 
       FIG. 6  is a block diagram illustrating the determination of forces to be generated by maneuverability propulsion mechanism in response to a maneuverability command and the corresponding state information of the UAV, including the Euler angles, position, and velocity of the UAV, according to an implementation. A maneuverability command, which may be received from a remote source (e.g., controller) or from the UAV control system  110 , etc., may include one or more of a pitch, yaw, roll, pitch rate, yaw rate, and/or roll rate, collectively referred to herein as a set point  602 . The set point may then be provided to a summer  604 . The summer  604  determines the error by computing the difference between the current yaw, pitch, roll, yaw rate, pitch rate, and roll rate of the UAV (collectively the “plant variable”) and the set point  602 . The error is then provided to the controller  606 . 
     The controller  606  utilizes the error from the summer  604  along with the perceived gravity provided by the lifting mechanism controller  610  to produce propulsion mechanism commands. As discussed above, the perceived gravity is determined based on the difference between the actual gravitational force and the vertical force generated by the lifting propulsion mechanism(s) controller  610  of the UAV. In some implementations, the perceived gravity may be approximately zero. The propulsion mechanism commands identify the speed of rotation, pitch, direction of rotation and/or angle for each of the propulsion mechanisms. 
     The propulsion mechanism commands are received by the propulsion maneuverability mechanism(s)  608  and the rotational speed of each is updated based on the propulsion mechanism commands. In some implementations, the propulsion mechanism commands may also be provided back to the lifting propulsion mechanism controller  610 . For example, if the maneuverability propulsion mechanism commands include a vertical component that will result in the UAV gaining altitude if there no perceived gravity, the lifting propulsion mechanism controller  610  may receive those commands and reduce the rotational speed of the lifting propulsion mechanism(s) so that the total vertical forces remain approximately constant and the UAV maintains the commanded altitude. 
     As the UAV is maneuvered according to the propulsion mechanism commands, the UAV state  614  is updated and provided as a feedback to the summer  604 . 
       FIG. 7  is a flow diagram illustrating an example navigation process  700 , according to an implementation. The example process of  FIG. 7  and each of the other processes discussed herein may be implemented in hardware, software, or a combination thereof. In the context of software, the described operations represent computer-executable instructions stored on one or more computer-readable media that, when executed by one or more processors, perform the recited operations. Generally, computer-executable instructions include routines, programs, objects, components, data structures, and the like that perform particular functions or implement particular abstract data types. 
     The computer-readable media may include non-transitory computer-readable storage media, which may include hard drives, floppy diskettes, optical disks, CD-ROMs, DVDs, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, flash memory, magnetic or optical cards, solid-state memory devices, or other types of storage media suitable for storing electronic instructions. Finally, the order in which the operations are described is not intended to be construed as a limitation, and any number of the described operations can be combined in any order and/or in parallel to implement the routine. 
     The example navigation process  700  begins by determining if the UAV is to be operating in an efficiency profile or an agility profile, as in  702 . In the agility profile, the UAV may be configured for high agility. In comparison, in the efficiency profile, the UAV may be configured for increased power efficiency. A profile may be determined and selected based at least in part on the position or location of the UAV. For example, the position of the UAV may include the altitude of the UAV, a velocity of the UAV, whether other objects are within a close proximity of and/or approaching the UAV, etc. Objects may be considered within a close proximity of a UAV if they are within a defined distance (e.g., 50 ft., 100 ft.) of the UAV. If the UAV is traveling at a high altitude and there are no or few objects, it may be determined that the UAV should use an efficiency profile. In comparison, during landing, take off, or when operating in areas with several objects nearby, it may be determined that the UAV should operate using an agility profile. 
     If it is determined that the UAV is to operate according to an efficiency profile, the rotational speed (rpm) of the lifting propulsion mechanism is determined based on the gravitational force applied to the UAV, as in  704 . For example, if the UAV has a mass of 20.00 kg, the gravitational force is 196.133 N. As such, the rpm of the lifting propulsion mechanism is set so that the lifting propulsion mechanism generates a force that is approximately equal and opposite the gravitational force. 
     If it is determined that the UAV is not to operate according to the efficiency profile (e.g., it is to operate according to an agility profile) the lifting propulsion mechanism is disengaged and/or the rpm is reduced, as in  706 . Lifting and navigating the UAV with just the maneuverability propulsion mechanism(s) increases the agility of the UAV, but consumes more power. In some implementations, there may be a transition between an efficiency profile and an agility profile, where the rpm of the lifting propulsion mechanism is reduced so that the lifting forces necessary to maintain altitude of the UAV are generated using both the lifting propulsion mechanism and the maneuverability propulsion mechanism(s). 
     Upon setting the rpm of the lifting propulsion mechanism, the perceived gravitational force, from the perspective of the maneuverability propulsion mechanism(s) is determined, as in  708 . In some implementations, if the lifting propulsion mechanism is generating a force that is approximately equal and opposite to the gravitational force acting on the UAV, the perceived gravitational force from the perspective of the UAV is approximately zero. In such an implementation, the maneuverability propulsion mechanism(s) only need to generate forces sufficient to stabilize the UAV, alter the attitude of the UAV, and/or alter the directional speed of the UAV. 
     As part of the navigation process  700 , a maneuver command is received, which may include one or more of a pitch command, roll command, a yaw command or a thrust command, as in  710 . The maneuver command may be received from a remote source, such as a remote computing component, a remote controller, etc., and/or generated by the UAV control system. For example, if the UAV is to navigate a determined path, the UAV control system may generate maneuver commands to be executed so that the UAV remains and/or follows the determined path. 
     In response to receiving the maneuver command, maneuverability propulsion mechanism control signals are determined for each maneuverability propulsion mechanism based on the perceived gravitational forces and the maneuver command, as in  712 . For example, if the maneuver command is a command to move in a horizontal direction at 20.0 m/s and the UAV is operating according to an efficiency profile such that the perceived gravitational force acting upon the UAV is approximately zero, the example process  700  will determine the horizontal forces needed to navigate the UAV in the horizontal direction at a speed of 20.0 m/s. 
     Based on the determined maneuverability propulsion mechanism control signals, the rpm of one or more of the maneuverability propulsion mechanisms, the pitch of one or more of the maneuverability propulsion mechanisms, the direction of one of more of the maneuverability propulsion mechanisms and/or the angle of one or more of the maneuverability propulsion mechanisms may be altered to generate a force corresponding to the maneuverability propulsion mechanism control signals determined at block  712 , as in  714 . The alterations made to the maneuverability propulsion mechanism may be the same or different for each maneuverability propulsion mechanism. 
       FIG. 8  is a block diagram illustrating an example UAV control system  110  of the UAV  100 . In various examples, the block diagram may be illustrative of one or more aspects of the UAV control system  110  that may be used to implement the various systems and methods discussed herein and/or to control operation of the UAV  100 . In the illustrated implementation, the UAV control system  110  includes one or more processors  802 , coupled to a memory, e.g., a non-transitory computer readable storage medium  820 , via an input/output (I/O) interface  810 . The UAV control system  110  also includes lifting propulsion mechanism controller  803 , maneuverability propulsion mechanism controllers  804 , such as electronic speed controls (ESCs), power supply modules  806  and/or a navigation system  808 . The UAV control system  110  further includes a payload engagement controller  812 , a network interface  816 , and one or more input/output devices  818 . 
     In various implementations, the UAV control system  110  may be a uniprocessor system including one processor  802 , or a multiprocessor system including several processors  802  (e.g., two, four, eight, or another suitable number). The processor(s)  802  may be any suitable processor capable of executing instructions. For example, in various implementations, the processor(s)  802  may be general-purpose or embedded processors implementing any of a variety of instruction set architectures (ISAs), such as the x86, PowerPC, SPARC, or MIPS ISAs, or any other suitable ISA. In multiprocessor systems, each processor(s)  802  may commonly, but not necessarily, implement the same ISA. 
     The non-transitory computer readable storage medium  820  may be configured to store executable instructions, data, flight paths, profiles, flight control parameters, center of gravity information, and/or data items accessible by the processor(s)  802 . In various implementations, the non-transitory computer readable storage medium  820  may be implemented using any suitable memory technology, such as static random access memory (SRAM), synchronous dynamic RAM (SDRAM), nonvolatile/Flash-type memory, or any other type of memory. In the illustrated implementation, program instructions and data implementing desired functions, such as those described herein, are shown stored within the non-transitory computer readable storage medium  820  as program instructions  822 , data storage  824 , and flight controls  826 , respectively. In other implementations, program instructions, data, and/or flight controls may be received, sent, or stored upon different types of computer-accessible media, such as non-transitory media, or on similar media separate from the non-transitory computer readable storage medium  820  or the UAV control system  110 . Generally speaking, a non-transitory, computer readable storage medium may include storage media or memory media such as magnetic or optical media, e.g., disk or CD/DVD-ROM, coupled to the UAV control system  110  via the I/O interface  810 . Program instructions and data stored via a non-transitory computer readable medium may be transmitted by transmission media or signals such as electrical, electromagnetic, or digital signals, which may be conveyed via a communication medium such as a network and/or a wireless link, such as may be implemented via the network interface  816 . 
     In one implementation, the I/O interface  810  may be configured to coordinate I/O traffic between the processor(s)  802 , the non-transitory computer readable storage medium  820 , and any peripheral devices, the network interface or other peripheral interfaces, such as input/output devices  818 . In some implementations, the I/O interface  810  may perform any necessary protocol, timing or other data transformations to convert data signals from one component (e.g., non-transitory computer readable storage medium  820 ) into a format suitable for use by another component (e.g., processor(s)  802 ). In some implementations, the I/O interface  810  may include support for devices attached through various types of peripheral buses, such as a variant of the Peripheral Component Interconnect (PCI) bus standard or the Universal Serial Bus (USB) standard, for example. In some implementations, the function of the I/O interface  810  may be split into two or more separate components, such as a north bridge and a south bridge, for example. Also, in some implementations, some or all of the functionality of the I/O interface  810 , such as an interface to the non-transitory computer readable storage medium  820 , may be incorporated directly into the processor(s)  802 . 
     The maneuverability propulsion mechanism(s) controller  804  communicates with the navigation system  808  and adjusts the rotational speed of each maneuverability propulsion mechanisms, for example by altering the current that is applied to the maneuverability propulsion mechanisms. Likewise, the lifting propulsion mechanism(s) controller  803  communicates with the navigation system  808  and adjusts the rotational speed of the lifting propulsion mechanism(s), for example by altering the current that is applied to the lifting propulsion mechanism(s). 
     The navigation system  808  may include a global positioning system (GPS), indoor positioning system (IPS), or other similar system and/or sensors that can be used to navigate the UAV  100  to and/or from a location. The payload engagement controller  812  communicates with the actuator(s) or motor(s) (e.g., a servo motor) used to engage and/or disengage items. 
     The network interface  816  may be configured to allow data to be exchanged between the UAV control system  110 , other devices attached to a network, such as other computer systems (e.g., remote computing resources), and/or with UAV control systems of other UAVs. For example, the network interface  816  may enable wireless communication between the UAV  100  and a UAV control system that is implemented on one or more remote computing resources. For wireless communication, an antenna of the UAV or other communication components may be utilized. As another example, the network interface  816  may enable wireless communication between numerous UAVs. In various implementations, the network interface  816  may support communication via wireless general data networks, such as a Wi-Fi network. For example, the network interface  816  may support communication via telecommunications networks, such as cellular communication networks, satellite networks, and the like. 
     Input/output devices  818  may, in some implementations, include one or more displays, imaging devices, thermal sensors, infrared sensors, time of flight sensors, accelerometers, pressure sensors, weather sensors, etc. Multiple input/output devices  818  may be present and controlled by the UAV control system  110 . One or more of these sensors may be utilized to assist in landing as well as to avoid obstacles during flight. 
     As shown in  FIG. 8 , the memory may include program instructions  822 , which may be configured to implement the example routines and/or sub-routines described herein. The data storage  824  may include various data stores for maintaining data items that may be provided for determining flight paths, landing, identifying locations for disengaging items, etc. In various implementations, the parameter values and other data illustrated herein as being included in one or more data stores may be combined with other information not described or may be partitioned differently into more, fewer, or different data structures. In some implementations, data stores may be physically located in one memory or may be distributed among two or more memories. 
     Those skilled in the art will appreciate that the UAV control system  110  is merely illustrative and is not intended to limit the scope of the present disclosure. In particular, the computing system and devices may include any combination of hardware or software that can perform the indicated functions. The UAV control system  110  may also be connected to other devices that are not illustrated, or instead may operate as a stand-alone system. In addition, the functionality provided by the illustrated components may, in some implementations, be combined in fewer components or distributed in additional components. Similarly, in some implementations, the functionality of some of the illustrated components may not be provided and/or other additional functionality may be available. 
     Those skilled in the art will also appreciate that, while various items are illustrated as being stored in memory or storage while being used, these items or portions of them may be transferred between memory and other storage devices for purposes of memory management and data integrity. Alternatively, in other implementations, some or all of the software components may execute in memory on another device and communicate with the illustrated UAV control system  110 . Some or all of the system components or data structures may also be stored (e.g., as instructions or structured data) on a non-transitory, computer-accessible medium or a portable article to be read by an appropriate drive, various examples of which are described herein. In some implementations, instructions stored on a computer-accessible medium separate from the UAV control system  110  may be transmitted to the UAV control system  110  via transmission media or signals such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as a wireless link. Various implementations may further include receiving, sending, or storing instructions and/or data implemented in accordance with the foregoing description upon a computer-accessible medium. Accordingly, the techniques described herein may be practiced with other UAV control system configurations. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as exemplary forms of implementing the claims.