Patent Publication Number: US-11021234-B1

Title: Variable pitch mechanisms for propeller blades using a compound gearbox

Description:
BACKGROUND 
     Aerial vehicles, including autonomous or automated aerial vehicles, may utilize propellers and corresponding motors to generate lift and/or thrust. It may be desirable to vary pitches of one or more of the propeller blades to alter a thrust profile or otherwise affect lift or maneuverability of the aerial vehicles. However, existing mechanisms for varying pitches of propeller blades suffer from limited range of motion, e.g., less than a 90 degree change of blade pitch. Accordingly, there is a need for propeller blade pitch adjustment mechanisms that can provide a greater range of motion, e.g., greater than a 90 degree change of blade pitch. 
    
    
     
       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 schematic diagram of a first propeller blade pitch adjustment apparatus, according to an implementation. 
         FIG. 2  is a schematic diagram of a propeller hub and a compound gearbox of the first propeller blade pitch adjustment apparatus, according to an implementation. 
         FIG. 3  is a schematic diagram of a first stage gearbox of the compound gearbox of the first propeller blade pitch adjustment apparatus, according to an implementation. 
         FIG. 4  is a schematic diagram of a second stage gearbox of the compound gearbox of the first propeller blade pitch adjustment apparatus, according to an implementation. 
         FIG. 5  is a schematic diagram of the compound gearbox of the first propeller blade pitch adjustment apparatus, according to an implementation. 
         FIG. 6A  is a schematic diagram of a second propeller blade pitch adjustment apparatus, according to an implementation. 
         FIG. 6B  is a schematic, partial cross-section diagram of the second propeller blade pitch adjustment apparatus, according to an implementation. 
         FIG. 6C  is a schematic, partial cross-section diagram of pitch adjustment spools of the second propeller blade pitch adjustment apparatus, according to an implementation. 
         FIG. 7A  is a schematic diagram of a third propeller blade pitch adjustment apparatus, according to an implementation. 
         FIG. 7B  is a schematic, partial cross-section diagram of pitch adjustment spools of the third propeller blade pitch adjustment apparatus, according to an implementation. 
         FIG. 8  is a block diagram illustrating various components of an 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. 
     DETAILED DESCRIPTION 
     Various propeller blade pitch adjustment apparatuses are described herein. Each of the example embodiments of pitch adjustment apparatuses allows a variation in pitch of a propeller blade of at least more than 90 degrees and/or up to and exceeding 360 degrees, and some embodiments allow potentially infinite variation in pitch of a propeller blade. In addition, for various embodiments described herein, the pitch of a propeller blade may be varied by actuation of a pitch adjustment shaft or a control shaft. 
     The pitch adjustment apparatuses as described herein may allow reversal of thrust, or inversion of a thrust profile, of a propeller and corresponding motor without any reduction in propulsive efficiency. For example, the pitches of blades of a propeller may be rotated approximately 180 degrees using any of the pitch adjustment apparatuses described herein, and the rotation of the motor may be reversed, thereby reversing the thrust, or inverting the thrust profile, of the propeller and corresponding motor without any reduction in propulsive efficiency. 
     Such reversal of thrust may be useful for aerial vehicles in order to slow down, stop, or move in a reverse direction, e.g., when the aerial vehicles are on the ground. In addition, reversal of thrust may be useful for aerial vehicles to maintain stable flight in various operating conditions, e.g., in the case of one or more motor failures, or when operating in extreme environments, such as windy environments. Further, reversal of thrust may be useful for aerial vehicles that utilize one or more propellers and corresponding motors for both vertical takeoff and landing (VTOL) and also horizontal flight, e.g., for aerial vehicles having propellers and corresponding motors that tilt between vertical and horizontal flight configurations. Many other applications of thrust reversal, or other desired changes in thrust profiles, using the pitch adjustment apparatuses described herein are possible. Moreover, although the present disclosure describes pitch adjustment apparatuses in the context of aerial vehicles, the pitch adjustment apparatuses described herein may be used in any other vehicles, machines, devices, or other systems that utilize propellers, fans, or other similar structures having blades, and that are rotated by motors or other propulsion sources. 
     In some embodiments, the pitch adjustment apparatus may include a propeller hub and a compound gearbox. The propeller hub may be coupled to and rotated by a first end of a motor shaft that is rotated by a rotor of a motor, and the motor shaft may be a hollow motor shaft. The propeller hub may include one or more propeller blade shafts, and each of the propeller blade shafts may be connected to a propeller blade at a first end that extends out of the propeller hub and may be connected to a blade gear, e.g., a bevel gear, at a second end within the propeller hub. 
     A pitch adjustment shaft may extend within and through the hollow motor shaft, and a pitch adjustment gear, e.g., a bevel gear, may be connected to a first end of the pitch adjustment shaft within the propeller hub. The pitch adjustment gear may be in operative engagement with each of the blade gears associated with respective propeller blade shafts and propeller blades. Accordingly, rotation of the pitch adjustment shaft and pitch adjustment gear may cause corresponding rotations of each of the blade gears, propeller blade shafts, and propeller blades to adjust pitches of the propeller blades. 
     The second end of the motor shaft and the second end of the pitch adjustment shaft may each be coupled to components within a compound gearbox, e.g., gearbox. The gearbox may be configured to allow the motor shaft and the pitch adjustment shaft to rotate in a same direction at a same rotational speed such that during operation of the motor, the propeller blades may rotate to generate thrust based on the rotation of the motor shaft, while also maintaining a desired pitch of the propeller blades during operation. 
     The compound gearbox, e.g., gearbox, may include a first stage gearbox, e.g., a first planetary stage, and a second stage gearbox, e.g., a second planetary stage. The first stage gearbox may include a first sun gear, a plurality of first planet gears carried by a first planetary gear carrier, and a ring gear. The second end of the motor shaft may be connected to and rotate the first sun gear of the first stage gearbox. The plurality of first planet gears may be in operative engagement with the first sun gear and carried by the first planetary gear carrier. The first planetary gear carrier may be rotationally fixed, e.g., relative to a stator of the motor, or another component coupled to the stator. The ring gear may be in operative engagement with the plurality of first planet gears and rotate around the plurality of first planet gears and the first sun gear. 
     The second stage gearbox may include a second sun gear, a plurality of second planet gears carried by a second planetary gear carrier, and the ring gear. The plurality of second planet gears may be in operative engagement with the ring gear, and rotation of the ring gear may cause the plurality of second planet gears to rotate on the second planetary gear carrier. The second planetary gear carrier may be selectively rotatable relative to a fixed position of the first planetary gear carrier. The second sun gear may be in operative engagement with the plurality of second planet gears, and rotation of the plurality of second planet gears may cause the second sun gear to rotate. The second sun gear may be connected to and rotate the second end of the pitch adjustment shaft. 
     When the second planetary gear carrier is held in position relative to the first planetary gear carrier, the motor shaft and the pitch adjustment shaft may rotate in a same direction at a same rotational speed, and the propeller blades may maintain a constant pitch. In order to adjust the pitches of the propeller blades, the second planetary gear carrier may be rotated relative to the first planetary gear carrier. The rotation of the second planetary gear carrier may cause rotation of the second planet gears, the second sun gear, the pitch adjustment shaft, and the pitch adjustment gear, and thereby may cause rotation of the blade gears, propeller blade shafts, and propeller blades to adjust the pitches of the propeller blades. 
     In other embodiments, the pitch adjustment apparatus may include a propeller hub that is coupled to and rotated by a motor shaft, and a propeller blade pitch adjustment assembly within the propeller hub. The motor shaft may be hollow, and the propeller hub may enclose an interior space within which the propeller blade pitch adjustment assembly is at least partially movably situated. 
     The propeller blade pitch adjustment assembly may include one or more pitch adjustment spools that are each coupled to a propeller blade that extends outside the propeller hub. Each pitch adjustment spool may rotate and thereby cause the propeller blade to rotate and adjust its pitch. The one or more pitch adjustment spools may be operatively engaged with a control member that controls the rotation of the pitch adjustment spools. In addition, a control shaft may be rotatably coupled to the control member to adjust a position of the control member. The control shaft may extend through the hollow motor shaft. 
     In some embodiments, two or more pitch adjustment spools may be rotatably connected to each other via a torsion spring, and each pitch adjustment spool may be connected to the control member via a tension cable. The control member may be a plate, block, or other structure that moves within the propeller hub in a direction transverse to axes of rotation of the pitch adjustment spools and propeller blades. The control shaft may be rotatably connected to the control member via a bearing and also move within the propeller hub and the hollow motor shaft in a direction transverse to axes of rotation of the pitch adjustment spools and propeller blades. Movement of the control shaft and control member away from the pitch adjustment spools may pull the tension cables against a biasing force of the torsion spring and rotate the pitch adjustment spools, thereby adjusting pitches of the propeller blades. 
     In further embodiments, two or more pitch adjustment spools may be rotatably connected to each other, and each pitch adjustment spool may include a gear, e.g., a pinion gear, on an outer surface. The control member may include racks, e.g., including gear teeth, on inner surfaces that are operatively engaged with the gears of the pitch adjustment spools, and the control member may move within the propeller hub in a direction transverse to axes of rotation of the pitch adjustment spools and propeller blades. The control shaft may be rotatably connected to the control member via a bearing and also move within the propeller hub and the hollow motor shaft in a direction transverse to axes of rotation of the pitch adjustment spools and propeller blades. Movement of the control shaft and control member may rotate the pitch adjustment spools via the racks and pinion gears, thereby adjusting pitches of the propeller blades. 
       FIG. 1  is a schematic diagram of a first propeller blade pitch adjustment apparatus  100 , according to an implementation. The apparatus  100  may include a propeller hub  110  and a compound gearbox  120  that are arranged on either side of a motor  104 . The motor  104  may be supported on a motor arm  102  or other frame or body portion of an aerial vehicle. For example, the motor  104  may include a stator  106 , and a rotor (not shown) that is coupled to and rotates a motor shaft  108 . The motor shaft  108  may be a hollow motor shaft that extends from opposing sides of the motor  104 . In addition, a pitch adjustment shaft  122  may extend within the hollow motor shaft  108  between components within each of the propeller hub  110  and the compound gearbox  120 , as further described with respect to  FIGS. 2-5 . 
     At a first end, the motor shaft  108  may be coupled to the propeller hub  110 , as further described with respect to  FIG. 2 . Extending out from the propeller hub  110  may be propeller blade shafts  112  and corresponding propeller blades  114 . Alternatively or in addition, the propeller blade shafts  112  and propeller blades  114  may be integrally formed with each other, or only the propeller blades  114  may extend from the propeller hub  110 , with the propeller blade shafts  112  being substantially enclosed within the propeller hub  110 . While  FIG. 1  shows two propeller blade shafts  112 - 1 ,  112 - 2  and two corresponding propeller blades  114 - 1 ,  114 - 2 , any other number and arrangement of propeller blade shafts  112  and propeller blades  114  may extend from the propeller hub  110 . 
     When the motor shaft  108  is rotated by the rotor of the motor  104 , the propeller hub  110  may rotate together with the motor shaft  108 , such that the propeller blade shafts  112  and propeller blades  114  also rotate. Rotation of the propeller blades  114  may generate thrust, dependent on the shape and pitch of the propeller blades. 
     At a second end, the motor shaft  108  may be coupled to a component within the compound gearbox  120 , as further described with respect to  FIGS. 3-5 . As shown in  FIG. 1 , the compound gearbox  120  may include a first planetary gear carrier  123  and a second planetary gear carrier  124 , each of which is further described with respect to  FIGS. 3-5 . The first planetary gear carrier  123  may be coupled or fixed to the motor arm  102  and/or the stator  106  of the motor  104 . Accordingly, the first planetary gear carrier  123  may be rotationally fixed relative to the stator  106  of the motor  104 . The second planetary gear carrier  124  may be rotatable relative to the first planetary gear carrier  123 . For example, the second planetary gear carrier  124  may include gear teeth on an outer surface by which the second planetary gear carrier  124  may be selectively rotated, e.g., by an actuator. 
       FIG. 2  is a schematic diagram of a propeller hub  110  and a compound gearbox  120  of the first propeller blade pitch adjustment apparatus  200 , according to an implementation. The apparatus  200  shown in  FIG. 2  is substantially the same as the apparatus  100  shown in  FIG. 1 , but with some elements removed and/or shown in outline for clarity of description. The apparatus  200  may include a propeller hub  110  and a compound gearbox  120  that are arranged at opposite ends of a motor shaft  108 . The motor shaft  108  may be rotated by a rotor of a motor and may be a hollow motor shaft. 
     As described herein, a second end of the motor shaft  108  may be coupled to a component within the compound gearbox  120 , as further described with respect to  FIGS. 3-5 . The compound gearbox  120  may include a rotationally fixed first planetary gear carrier  123  and a selectively rotatable second planetary gear carrier  124 , each of which is further described with respect to  FIGS. 3-5 . In addition, a pitch adjustment shaft  122  may extend within the hollow motor shaft  108 , and a second end of the pitch adjustment shaft  122  may couple to a component within the compound gearbox  120 , as further described with respect to  FIGS. 3-5 . 
     A first end of the motor shaft  108  may be coupled to the propeller hub  110 . The propeller hub  110  may include a housing  210  that encloses an interior space and one or more openings  212  through which propeller blade shafts  112  or propeller blades  114  may extend out from the propeller hub  110 . Portions of the propeller blade shafts  112 , the propeller blades  114 , and the openings  212  may be circular or cylindrical in order to allow rotation of the propeller blade shafts  112  or propeller blades  114  within the openings  212 . In addition, each of the openings  212  may include one or more bearings or other similar, friction-reducing elements to facilitate rotation of the propeller blade shafts  112  or propeller blades  114 . While  FIG. 2  shows two openings  212 - 1 ,  212 - 2 , any other number and arrangement of openings  212  may be provided in the propeller hub  110  to accommodate the propeller blade shafts  112  or propeller blades  114 . 
     When the motor shaft  108  is rotated by the rotor of the motor, the propeller hub  110 , including the housing  210  and the openings  212 , may rotate together with the motor shaft  108 , such that the propeller blade shafts  112  and propeller blades  114  also rotate with rotation of the motor shaft  108 . Rotation of the propeller blades  114  may generate thrust, dependent on the shape and pitch of the propeller blades. 
     Within the housing  210  of the propeller hub  110 , each of the propeller blade shafts  112  may couple to a blade gear  217 . The blade gears  217  may be bevel gears, e.g., straight bevel gears or spiral bevel gears. In addition, each of the propeller blade shafts  112  and/or blade gears  217  may include one or more bearings or other similar, friction-reducing elements to facilitate rotation of the blade gears  217  relative to the housing  210 . While  FIG. 2  shows two blade gears  217 - 1 ,  217 - 2 , any other number and arrangement of blade gears  217  associated with the propeller blade shafts  112  or propeller blades  114  may be provided within the propeller hub  110 . 
     Further, within the housing  210  of the propeller hub  110 , the pitch adjustment shaft  122  may couple to a pitch adjustment gear  215  that is in operative engagement with each of the blade gears  217 . The pitch adjustment gear  215  may be a bevel gear, e.g., a straight bevel gear or a spiral bevel gear. In addition, the pitch adjustment shaft  122  and/or the pitch adjustment gear  215  may include one or more bearings or other similar, friction-reducing elements to facilitate rotation of the pitch adjustment gear  215  relative to the housing  210 . 
     When the pitch adjustment shaft  122  is rotated via an input to a component of the compound gearbox, as further described herein with respect to  FIGS. 3-5 , the pitch adjustment gear  215  may rotate together with the pitch adjustment shaft  122 . Rotation of the pitch adjustment gear  215  may cause rotation of the blade gears  217  that are in operative engagement with the pitch adjustment gear  215 , and rotation of the blade gears  217  may cause corresponding rotation of the propeller blade shafts  112  and propeller blades  114 . Therefore, rotation of the pitch adjustment shaft  122  may cause rotation, i.e., changes in pitch, of each of the propeller blades  114  that extend from the propeller hub  110 . 
     For example, the operative engagement between the pitch adjustment gear  215  and the blade gears  217  may ensure that each of the blade gears  217  rotates in a same rotational direction in response to rotation of the pitch adjustment gear  215 . Further, if all the blade gears  217  have the same size and the same number of gear teeth, each of the blade gears  217  may rotate a same degree of rotation, i.e., a same change in pitch of the propeller blades  114 , in response to rotation of the pitch adjustment gear  215 . 
     Furthermore, the housing  210  of the propeller hub  110  may be a substantially closed system, such that lubricant may be maintained within the propeller hub  110  to facilitate smooth engagement between the pitch adjustment gear  215  and the blade gears  217  and smooth rotation of the pitch adjustment shaft  122  and the propeller blade shafts  112 , as well as to prevent contamination and deterioration of the components and/or lubricant. 
       FIG. 3  is a schematic diagram of a first stage gearbox of the compound gearbox  120  of the first propeller blade pitch adjustment apparatus  300 , according to an implementation. The apparatus  300  shown in  FIG. 3  is substantially the same as the apparatuses  100 ,  200  shown in  FIGS. 1 and 2 , but with some elements removed and/or shown in outline for clarity of description. The apparatus  300  may include a compound gearbox  120  that is arranged at one end of a motor shaft  108 . The motor shaft  108  may be rotated by a rotor of a motor and may be a hollow motor shaft. 
     As described herein, a first end of the motor shaft  108  may be coupled to a propeller hub  110 , as further described with respect to  FIG. 2 . The propeller hub  110  may rotate with the motor shaft  108 , and the propeller hub  110  may include components that allow adjustment of the pitches of the propeller blades that extend from the propeller hub  110 . 
     The first stage gearbox, or first planetary stage, of the compound gearbox  120  may include a first sun gear  315 , a plurality of first planet gears  325  carried by a first planetary gear carrier  123 , and a ring gear  330 . A second end of the motor shaft  108  may be coupled to the first sun gear  315 , such that the first sun gear  315  may rotate together with rotation of the motor shaft  108 . The plurality of first planet gears  325  may be in operative engagement with the first sun gear  315 . The plurality of first planet gears  325  may be carried on posts  323  or stems associated with the first planetary gear carrier  123 . Although not shown in  FIG. 3 , the first planetary gear carrier  123  may be coupled to each of the posts  323 . In addition, the first planetary gear carrier  123  may be rotationally fixed to the stator  106  of the motor  104 , the motor arm  102 , a frame or body portion of the aerial vehicle, or any other component that does not rotate relative to the stator  106  of the motor  104 . Further, the ring gear  330  may be in operative engagement with each of the plurality of first planet gears  325 . In addition, each of the motor shaft  108 , the first sun gear  315 , the first planet gears  325 , and/or the ring gear  330  may include one or more bearings or other similar, friction-reducing elements to facilitate their rotation relative to the first planetary gear carrier  123  and/or the second planetary gear carrier  124 . While  FIG. 3  shows four first planet gears  325 - 1 ,  325 - 2 ,  325 - 3 ,  325 - 4  on four posts  323 - 1 ,  323 - 2 ,  323 - 3 ,  323 - 4 , any other number and arrangement of first planet gears  325  and posts  323  may be provided within the first stage gearbox. 
     When the motor shaft  108  is rotated by the rotor of the motor, the first sun gear  315  may rotate together with the motor shaft  108 . Rotation of the first sun gear  315  may cause rotation of each of the plurality of first planet gears  325  on the rotationally fixed first planetary gear carrier  123 , and rotation of the plurality of first planet gears  325  may cause rotation of the ring gear  330 . The ring gear  330  may rotate in an opposite rotational direction from the direction of rotation of the first sun gear  315 , and the ring gear  330  may rotate at a different rotational speed from the speed of rotation of the first sun gear  315 . 
       FIG. 4  is a schematic diagram of a second stage gearbox of the compound gearbox  120  of the first propeller blade pitch adjustment apparatus  400 , according to an implementation. The apparatus  400  shown in  FIG. 4  is substantially the same as the apparatuses  100 ,  200 ,  300  shown in  FIGS. 1-3 , but with some elements removed and/or shown in outline for clarity of description. The apparatus  400  may include a compound gearbox  120  that is arranged at one end of a motor shaft  108 . The motor shaft  108  may be rotated by a rotor of a motor and may be a hollow motor shaft. 
     The second stage gearbox, or second planetary stage, of the compound gearbox  120  may include a second sun gear  415 , a plurality of second planet gears  425  carried by a second planetary gear carrier  124 , and the ring gear  330  that is shared between the first stage gearbox and the second stage gearbox. A second end of the pitch adjustment shaft  122  may extend through both the hollow motor shaft  108  and the first sun gear  315  and may be coupled to the second sun gear  415 , such that the pitch adjustment shaft  122  rotates together with rotation of the second sun gear  415 . The plurality of second planet gears  425  may be in operative engagement with the second sun gear  415 . The plurality of second planet gears  425  may be carried on posts  423  or stems associated with the second planetary gear carrier  124 . The second planetary gear carrier  124  may be coupled to each of the posts  423 . In addition, the second planetary gear carrier  124  may be selectively rotatable relative to the stator  106  of the motor  104 , the motor arm  102 , a frame or body portion of the aerial vehicle, or any other component that is fixed relative to the stator  106  of the motor  104 . Further, the ring gear  330  may be in operative engagement with each of the plurality of second planet gears  425 . In addition, each of the pitch adjustment shaft  122 , the second sun gear  415 , the second planet gears  425 , and/or the ring gear  330  may include one or more bearings or other similar, friction-reducing elements to facilitate their rotation relative to the first planetary gear carrier  123  and/or the second planetary gear carrier  124 . While  FIG. 4  shows four second planet gears  425 - 1 ,  425 - 2 ,  425 - 3 ,  425 - 4  on four posts  423 - 1 ,  423 - 2 ,  423 - 3 ,  423 - 4 , any other number and arrangement of second planet gears  425  and posts  423  may be provided within the second stage gearbox. 
     When, as described herein, the ring gear  330  is rotated as a result of rotation of the motor shaft  108  by the rotor of the motor, the second planet gears  425  may rotate on the selectively rotatable second planetary gear carrier  124 . Rotation of the second planet gears  425  may cause rotation of the second sun gear  415 , and rotation of the second sun gear  415  may cause rotation of the pitch adjustment shaft  122 . If the first sun gear  315  and the second sun gear  415  have the same size and the same number of gear teeth, if all the first planet gears  325  and the second planet gears  425  have the same size and the same number of gear teeth, and if the second planetary gear carrier  124  is held in position relative to the first planetary gear carrier  123 , the motor shaft  108  and the pitch adjustment shaft  122  may rotate in the same rotational direction at the same rotational speed. As a result, during operation of the motor, the propeller blades may rotate to generate thrust based on the rotation of the motor shaft, while also maintaining a desired pitch of the propeller blades during operation. 
     In order to adjust pitches of the propeller blades, the second planetary gear carrier  124  may be selectively rotated relative to the first planetary gear carrier  123 . For example, as shown in  FIGS. 1-4 , the second planetary gear carrier  124  may include gear teeth on an outer surface thereof, and an actuator  127  having corresponding gear teeth may be in operative engagement with the gear teeth of the second planetary gear carrier  124  to adjust the rotational position of the second planetary gear carrier  124 . The actuator  127  may be a servo actuator, a geared actuator, a rotary actuator, a rack and pinion actuator, a screw actuator, and/or any other type of actuator. Alternatively or in addition, any other method of actuating the second planetary gear carrier  124  may be used instead of the gear teeth on the outer surface thereof. For example, a portion of the second planetary gear carrier  124  may be directly connected to an actuator  127 , e.g., a servo actuator, a motor, or any other type of actuator, to adjust the rotational position of the second planetary gear carrier  124 . Other types of actuation mechanisms may also be used, such as pulley-type mechanisms to adjust the rotational position of the second planetary gear carrier  124 . 
     As an illustration, if the motor shaft  108  is not rotated by the rotor of the motor, and the first stage gearbox and the second stage gearbox of the compound gearbox  120  are not being actuated by the motor, the motor shaft  108 , the first sun gear  315 , the first planet gears  325 , the ring gear  330 , the second planet gears  425 , the second sun gear  415 , and the pitch adjustment shaft  122  may be stationary. If the second planetary gear carrier  124  is then rotated relative to the first planetary gear carrier  123  to adjust the pitches of the propeller blades  112  via the propeller hub  110 , the rotation of the second planetary gear carrier  124  may cause rotation of the second planet gears  425  such that the second planet gears  425  rotate within the ring gear  330 . The ring gear  330  may be held stationary by the stationary motor shaft  108  and the first stage gearbox. The rotation of the second planet gears  425  within the ring gear  330  may cause the second sun gear  415  to rotate, and thereby may cause rotation of the pitch adjustment shaft  122 . Then, as described herein, the rotation of the pitch adjustment shaft  122  may cause adjustment of the pitches of the propeller blades  112  via the propeller hub  110 . While the adjustment of pitches of the propeller blades  112  is described herein in the context of a stationary motor shaft  108  for ease of illustration, the second planetary gear carrier  124  may also be rotated relative to the first planetary gear carrier  123  during rotation of the motor shaft  108 , e.g., during operation of the motor, to adjust the pitches of the propeller blades  112 . 
     Furthermore, the compound gearbox  120  may be a substantially closed system, such that lubricant may be maintained within the compound gearbox  120  to facilitate smooth engagement between the first sun gear  315 , the first planet gears  325 , the ring gear  330 , the second planet gears  425 , and the second sun gear  415  and smooth rotation of the motor shaft  108  and the pitch adjustment shaft  122 , as well as to prevent contamination and deterioration of the components and/or lubricant. 
       FIG. 5  is a schematic diagram of the compound gearbox  120  of the first propeller blade pitch adjustment apparatus  500 , according to an implementation. The apparatus  500  shown in  FIG. 5  is substantially the same as the apparatuses  100 ,  200 ,  300 ,  400  shown in  FIGS. 1-4 , but with some elements removed and/or shown in outline for clarity of description. The apparatus  500  may include a compound gearbox  120  that is arranged at one end of a motor shaft  108 . The motor shaft  108  may be rotated by a rotor of a motor and may be a hollow motor shaft. 
       FIG. 5  shows an underside view of the compound gearbox  120  with the first planetary gear carrier  123  and the second planetary gear carrier  124  removed for clarity. Thus,  FIG. 5  shows the motor shaft  108 , the first sun gear  315 , the first planet gears  325 , the ring gear  330 , the second planet gears  425 , the second sun gear  415 , and the pitch adjustment shaft  122 , as described herein with respect to  FIGS. 1-4 . Further, while  FIGS. 3-5  show the first stage gearbox and the second stage gearbox having the same number of planet gears, e.g., four first planet gears and four second planet gears, any other number and arrangement of first or second planet gears  325 ,  425  may be provided within the first or second stage gearboxes, e.g., four first planet gears and three second planet gears. 
     Each of the components of the propeller hub  110  and the compound gearbox  120  may be made from any suitable materials, such as metal, plastics, carbon fiber, other materials, or combinations thereof, for example. In addition, the various gears may be coupled to the various shafts using any suitable connection methods, such as keyed connections, frictionally engaged connections, screw connections, set screws, adhesives, other connections, or combinations thereof. Alternatively or in addition, one or more of the various gears may be integrally formed with their respective shafts. Further, while  FIGS. 1 and 2  show the propeller hub  110  having a substantially rectangular prism shape, any other shape or configuration of the propeller hub  110  is possible, e.g., circular prism, elliptical prism, hexagonal prism, octagonal prism, or other polygonal prism. 
     The first propeller blade pitch adjustment apparatuses  100 ,  200 ,  300 ,  400 ,  500 , including the propeller hub  110  and the compound gearbox  120 , as described herein with respect to  FIGS. 1-5 , may allow a variation in pitches of propeller blades of at least more than 90 degrees, and may allow potentially infinite variation in pitches of propeller blades. Accordingly, thrust reversal of a propeller and corresponding motor may be accomplished without any reduction in propulsive efficiency using the first propeller blade pitch adjustment apparatuses to adjust the pitches of propeller blades by approximately 180 degrees and reversing a rotation of the motor. Moreover, various other changes to the thrust profile of a propeller and corresponding motor may be accomplished using the first propeller blade pitch adjustment apparatuses to adjust the pitches of propeller blades as desired. 
     In an alternative embodiment to  FIGS. 1-5 , the compound gearbox  120  may be situated between the motor  104  and the propeller hub  110 , with the first stage gearbox of the compound gearbox  120  adjacent the motor  104  and the second stage gearbox of the compound gearbox  120  adjacent the propeller hub  110 . Accordingly, the first planetary gear carrier may be rotationally fixed relative to and situated adjacent a stator of the motor  104 , and the second planetary gear carrier may be selectively rotatable and situated adjacent the propeller hub  110 . 
     In this alternative embodiment, the pitch adjustment shaft  122  may instead be a hollow pitch adjustment shaft, and the motor shaft  108  may extend within and through the hollow pitch adjustment shaft. The motor shaft may be coupled to and extend through the first sun gear  315  of the first stage gearbox, then extend through the compound gearbox  120  within the hollow pitch adjustment shaft and couple to a portion of the propeller hub  110 , e.g., a portion of the propeller hub  110  distal from the motor  104 . The hollow pitch adjustment shaft may be coupled to the second sun gear  415  at a first end and coupled to a pitch adjustment gear within the propeller hub  110  at a second end. 
     The propeller hub  110  in this alternative embodiment may be substantially similar to that described herein with respect to  FIGS. 1 and 2 , except that the pitch adjustment shaft may be hollow, and the motor shaft may extend through the hollow pitch adjustment shaft and the pitch adjustment gear and couple to a portion of the propeller hub  110 , e.g., a portion of the propeller hub  110  distal from the motor  104 , in order to rotate the propeller hub  110  together with rotation of the motor shaft. 
     The compound gearbox  120  in this alternative embodiment may also be substantially similar to that described herein with respect to  FIGS. 2-5 , except that the pitch adjustment shaft may be hollow, and the motor shaft may couple to and extend through the first sun gear  315  and then extend through the hollow pitch adjustment shaft and the second sun gear  415  en route to the propeller hub  110 , as described herein. 
       FIG. 6A  is a schematic diagram of a second propeller blade pitch adjustment apparatus  600 , and  FIG. 6B  is a schematic, partial cross-section diagram of the second propeller blade pitch adjustment apparatus  600 , according to an implementation. The apparatus  600  may include a propeller hub  110  coupled to a motor shaft  108  that is rotated by a rotor of a motor. The motor shaft  108  may be a hollow motor shaft that extends from the motor. In addition, a control shaft  610  may extend within the hollow motor shaft  108  between components within the propeller hub  110  and a side of the motor opposite the propeller hub  110 . 
     The propeller hub  110  may be coupled to and rotate together with rotation of the motor shaft  108 . In addition, the propeller hub  110  may be formed integrally with the motor shaft  108 . The propeller hub  110  may also include one or more openings  624  through which propeller blade shafts  112  or propeller blades may extend from the propeller hub  110 . The openings  624  may be circular, cylindrical, or otherwise shaped to allow changes in pitches of the propeller blades connected to the propeller blade shafts  112 . In addition, the openings  624  may include bearings or other similar, friction-reducing elements to facilitate rotation of the propeller blade shafts  112  or propeller blades. While  FIGS. 6A and 6B  show two openings  624 - 1 ,  624 - 2  and two propeller blade shafts  112 - 1 ,  112 - 2  or propeller blades, any other number and arrangement of openings  624  and propeller blade shafts  112  and propeller blades may be provided that extend from the propeller hub  110 . 
     A propeller blade pitch adjustment assembly may be situated within the propeller hub  110 . The propeller blade pitch adjustment assembly may include a control shaft  610 , a control member  615 , one or more pitch adjustment spools  620  coupled to the propeller blade shafts  112  or propeller blades, one or more tension cables  622 , and one or more torsion springs  625 . The control shaft  610  may extend within and through the hollow motor shaft  108  and be rotatably coupled to the control member  615 . For example, the control shaft  610  may include bearings or other similar, friction-reducing elements to facilitate rotation of the control shaft  610  relative to the control member  615 . On a side of the motor opposite the propeller hub  110 , the control shaft  610  may extend past the motor and be actuated by an actuator, e.g., a servo actuator, a geared actuator, a motor, a rotary actuator, a rack and pinion actuator, a screw actuator, a linear actuator, and/or any other type of actuator. 
     Alternatively, the control shaft  610  may be coupled directly to the control member  615  and rotate together with the control member  615 , propeller hub  110 , and motor shaft  108 . On a side of the motor opposite the propeller hub  110 , the end of the control shaft  610  may extend past the motor and be rotatably coupled to an actuator. For example, the end of the control shaft  610  may include bearings or other similar, friction-reducing elements to facilitate rotation of the control shaft  610  relative to a connection to the actuator, such that the actuator need not rotate together with the control shaft  610  and motor shaft  108 . 
     The control member  615  may comprise a plate, block, or other structure that may move within the propeller hub  110 . For example, the control member  615  may be pushed or pulled by the control shaft  610  in a direction parallel to an axial length of the control shaft  610 . In addition, the direction of motion of the control member  615  may be substantially transverse to axes of rotation of the pitch adjustment spools  620 , propeller blade shafts  112  and propeller blades. The control member  615  may include protrusions  616  that extend into grooves  617  of the propeller hub  110  to facilitate the movement of the control member  615  upon actuation by the control shaft  610 . Alternatively or in addition, the control member  615  may include grooves into which protrusions of the propeller hub  110  extend to facilitate movement of the control member  615 . Further, any other structures or formations along portions of the mating surfaces of the control member  615  and propeller hub  110  may be used to facilitate movement of the control member  615 , such as guides, tracks, rods, linear bearings, other similar friction-reducing elements, or any other cooperating shapes or structures between the control member  615  and the propeller hub  110 . In some embodiments, the control member  615  may be sized to substantially fill the cross-sectional shape of the propeller hub  110 . While  FIGS. 6A and 6B  show two protrusions  616 - 1 ,  616 - 2  and two grooves  617 - 1 ,  617 - 2  on the control member  615  and propeller hub  110 , respectively, any other number and arrangement of protrusions, grooves, or other structures or formations may be provided to facilitate movement of the control member  615  within the propeller hub  110 . 
     The pitch adjustment spools  620  may be coupled to the propeller blade shafts  112  and propeller blades, and the pitch adjustment spools  620  may be rotatably coupled to each other.  FIG. 6C  is a schematic, partial cross-section diagram of pitch adjustment spools  620  of the second propeller blade pitch adjustment apparatus  600 , according to an implementation. For example, as shown in  FIG. 6C , the pitch adjustment spools  620  may have portions that overlap with each other, such that the pitch adjustment spools  620  may rotate about a same axis of rotation. The interfaces between the pitch adjustment spools  620  may include bearings or other similar, friction-reducing elements to facilitate rotation of the pitch adjustment spools  620  relative to each other. In addition, the pitch adjustment spools  620  may be connected to each other via a torsion spring  625  that may bias the pitch adjustment spools  620  to particular rotational positions relative to each other. The torsion spring  625  may bias the pitch adjustment spools  620  in opposite rotational directions from each other. The torsion spring  625  may be attached to the pitch adjustment spools  620  using screws, rivets, clamps, any other types of fasteners, welds, adhesives, or any other attachment methods. 
     The pitch adjustment spools  620  may be coupled to the control member  615  via tension cables  622 . For example, the tension cables  622  may be steel wires or any other connecting wire or cable. The tension cables  622  may be attached to portions of the pitch adjustment spools  620  at their first ends, and attached to portions of the control member  615  at their second ends. The tension cables  622  may be attached at their first and second ends using screws, rivets, clamps, any other types of fasteners, welds, adhesives, or any other attachment methods. The attachment points of the tension cables  622  to the pitch adjustment spools  620  may be configured such that movement of the control member  615 , e.g., in response to pulling by the control shaft  610 , may cause rotation of the pitch adjustment spools  620  in opposite rotational directions against the biasing force of the torsion spring  625 . The tension cables  622  may also run along grooves or channels provided on surfaces of the pitch adjustment spools  620 . 
     When the motor shaft  108  is rotated by the rotor of the motor, the propeller hub  110  may rotate together with the motor shaft  108 . Rotation of the propeller hub  110  may cause rotation of the control member  615  via the mating surfaces or connections, e.g., protrusions  616  and grooves  617 , between the control member  615  and the propeller hub  110 . Rotation of the propeller hub  110  may also cause rotation of the pitch adjustment spools  620 , propeller blade shafts  112  and propeller blades via the openings  624  through which the propeller blade shafts  112  or propeller blades extend from the propeller hub  110 . As a result, the propeller hub  110 , the control member  615 , the pitch adjustment spools  620 , the tension cables  622 , the torsion spring  625 , the propeller blade shafts  112  and the propeller blades may rotate together with the motor shaft  108 . In contrast, the control shaft  610 , via the rotatable connection to the control member  615  in one embodiment, may not necessarily rotate together with the propeller hub  110  and the remainder of the propeller blade pitch adjustment assembly. In an alternative embodiment in which the control shaft  610  is directly coupled to the control member  615 , the control shaft  610  may rotate together with the propeller hub  110 , the motor shaft  108 , and the remainder of the propeller blade pitch adjustment assembly, and the control shaft  610  may include a rotatable connection to the actuator on an opposite side of the motor, such that the actuator need not rotate together with the control shaft  610 , the motor shaft  108 , the propeller hub  110 , and the remainder of the propeller blade pitch adjustment assembly. 
     When adjustments to pitches of the propeller blades are desired, the control shaft  610  may be moved relative to the propeller hub  110 , e.g., pulled in a direction away from and transverse to axes of rotation of the pitch adjustment spools  620 , propeller blade shafts  112 , and propeller blades. The movement of the control shaft  610  may also move the control member  615  away from and transverse to axes of rotation of the pitch adjustment spools  620 , propeller blade shafts  112 , and propeller blades. The movement of the control member  615  may pull the tension cables  622 , thereby rotating the pitch adjustment spools  620  against the biasing force of the torsion spring  625 . The pitch adjustment spools  620  may rotate in opposite rotational directions in response to pulling by the tension cables  622 , such that the propeller blade shafts  112  and propeller blades may rotate to adjust the pitches of the propeller blades by substantially the same degree of rotation. 
     While  FIGS. 6A and 6B  show two pitch adjustment spools  620 - 1 ,  620 - 2 , two tension cables  622 - 1 ,  622 - 2 , and one torsion spring  625  that interconnects the two pitch adjustment spools  620 - 1 ,  620 - 2 , any other number and arrangement of pitch adjustment spools  620 , tension cables  622 , and torsion springs  625  may be provided in the second propeller blade pitch adjustment apparatus  600 . For example, if the second propeller blade pitch adjustment apparatus  600  includes three or more pitch adjustment spools  620 , the pitch adjustment spools  620  may each be rotatably coupled to a central member, e.g., that is fixed relative to the propeller hub  110 , via a respective torsion spring  625 , and the pitch adjustment spools  620  may each be coupled to the control member  615  by a respective tension cable  622 . In this manner, the pitches of three or more propeller blades may be substantially simultaneously adjusted using the second propeller blade pitch adjustment apparatus  600 . 
     Furthermore, the propeller hub  110  may be a substantially closed system, such that lubricant may be maintained within the propeller hub  110  to facilitate smooth engagement between the control member  615  and the propeller hub  110 , smooth operation of the control shaft  610  and the pitch adjustment spools  620 , and smooth rotation of the propeller blade shafts  112  and propeller blades, as well as to prevent contamination and deterioration of the components and/or lubricant. 
     Each of the components of the second propeller blade pitch adjustment apparatus  600 , including the motor shaft  108 , propeller hub  110 , control shaft  610 , control member  615 , pitch adjustment spools  620 , tension cables  622 , torsion spring  625 , and/or propeller blade shafts  112  and propeller blades, may be made from any suitable materials, such as metal, plastics, carbon fiber, other materials, or combinations thereof, for example. In addition, the pitch adjustment spools may be coupled to the propeller blade shafts or propeller blades using any suitable connection methods, such as keyed connections, frictionally engaged connections, screw connections, set screws, adhesives, other connections, or combinations thereof. Alternatively or in addition, one or more of the pitch adjustment spools may be integrally formed with their respective propeller blade shafts or propeller blades. Further, while  FIGS. 6A and 6B  show the second propeller blade adjustment apparatus  600  having a substantially rectangular prism shape, any other shape or configuration of the apparatus  600  is possible, e.g., circular prism, elliptical prism, hexagonal prism, octagonal prism, or other polygonal prism. 
     The second propeller blade pitch adjustment apparatus  600 , including the propeller hub  110  and propeller blade pitch adjustment assembly enclosed therein, as described herein with respect to  FIGS. 6A-6C , may allow a variation in pitches of propeller blades of at least more than 90 degrees, and may allow a variation in pitches of propeller blades of up to 360 degrees or more, e.g., if the tension cables are wound around the pitch adjustment spools one or more times and with sufficient available travel of the control shaft and control member. Accordingly, thrust reversal of a propeller and corresponding motor may be accomplished without any reduction in propulsive efficiency using the second propeller blade pitch adjustment apparatus to adjust the pitches of propeller blades by approximately 180 degrees and reversing a rotation of the motor. Moreover, various other changes to the thrust profile of a propeller and corresponding motor may be accomplished using the second propeller blade pitch adjustment apparatus to adjust the pitches of propeller blades as desired. 
       FIG. 7A  is a schematic diagram of a third propeller blade pitch adjustment apparatus  700 , according to an implementation. The apparatus  700  may include a propeller hub  110  coupled to a motor shaft  108  that is rotated by a rotor of a motor. The motor shaft  108  may be a hollow motor shaft that extends from the motor. In addition, a control shaft  710  may extend within the hollow motor shaft  108  between components within the propeller hub  110  and a side of the motor opposite the propeller hub  110 . 
     The propeller hub  110  may be coupled to and rotate together with rotation of the motor shaft  108 . In addition, the propeller hub  110  may be formed integrally with the motor shaft  108 . The propeller hub  110  may also include one or more openings  724  through which propeller blade shafts  112  or propeller blades may extend from the propeller hub  110 . The openings  724  may be circular, cylindrical, or otherwise shaped to allow changes in pitches of the propeller blades connected to the propeller blade shafts  112 . In addition, the openings  724  may include bearings or other similar, friction-reducing elements to facilitate rotation of the propeller blade shafts  112  or propeller blades. While  FIG. 7A  shows two openings  724 - 1  (hidden from view),  724 - 2  and two propeller blade shafts  112 - 1 ,  112 - 2  or propeller blades, any other number and arrangement of openings  724  and propeller blade shafts  112  and propeller blades may be provided that extend from the propeller hub  110 . 
     A propeller blade pitch adjustment assembly may be situated within the propeller hub  110 . The propeller blade pitch adjustment assembly may include a control shaft  710 , a control member  715 , and one or more pitch adjustment spools  720  coupled to the propeller blade shafts  112  or propeller blades. The control shaft  710  may extend within and through the hollow motor shaft  108  and be rotatably coupled to the control member  715 . For example, the control shaft  710  may include bearings or other similar, friction-reducing elements to facilitate rotation of the control shaft  710  relative to the control member  715 . On a side of the motor opposite the propeller hub  110 , the control shaft  710  may extend past the motor and be actuated by an actuator, e.g., a servo actuator, a geared actuator, a motor, a rotary actuator, a rack and pinion actuator, a screw actuator, a linear actuator, and/or any other type of actuator. 
     Alternatively, the control shaft  710  may be coupled directly to the control member  715  and rotate together with the control member  715 , propeller hub  110 , and motor shaft  108 . On a side of the motor opposite the propeller hub  110 , the end of the control shaft  710  may extend past the motor and be rotatably coupled to an actuator. For example, the end of the control shaft  710  may include bearings or other similar, friction-reducing elements to facilitate rotation of the control shaft  710  relative to a connection to the actuator, such that the actuator need not rotate together with the control shaft  710  and motor shaft  108 . 
     The control member  715  may comprise a plate, block or other structure that may move within the propeller hub  110 . For example, the control member  715  may be pushed or pulled by the control shaft  710  in a direction parallel to an axial length of the control shaft  710 . In addition, the direction of motion of the control member  715  may be substantially transverse to axes of rotation of the pitch adjustment spools  720 , propeller blade shafts  112  and propeller blades. The control member  715  may include racks  717 , e.g., including gear teeth, that are operatively engaged with gear teeth on the pitch adjustment spools  720 . For example, each rack  717  may be configured to operatively engage with gear teeth of a respective pitch adjustment spool  720 . The control member  715  may also include slots  719  that allow movement of the control member  715  without interfering with the propeller blade shafts  112  and propeller blades that extend from the propeller hub  110 . In alternative embodiments, the slots  719  may comprise openings at portions of the control member  715  that allow movement of the control member  715  without interfering with the propeller blade shafts  112  and propeller blades. Similar to the apparatus  600  of  FIGS. 6A-6C , the control member  715  may include protrusions and/or grooves that cooperate with grooves and/or protrusions of the propeller hub  110  to facilitate the movement of the control member  715  upon actuation by the control shaft  710 . Further, any other structures or formations along portions of the mating surfaces of the control member  715  and propeller hub  110  may be used to facilitate movement of the control member  715 , such as guides, tracks, rods, linear bearings, other similar friction-reducing elements, or any other cooperating shapes or structures between the control member  715  and the propeller hub  110 . In some embodiments, the control member  715  may be sized to substantially fill the cross-sectional shape of the propeller hub  110 . 
     The pitch adjustment spools  720  may be coupled to the propeller blade shafts  112  and propeller blades, and the pitch adjustment spools  720  may be rotatably coupled to each other.  FIG. 7B  is a schematic, partial cross-section diagram of pitch adjustment spools  720  of the third propeller blade pitch adjustment apparatus  700 , according to an implementation. For example, as shown in  FIG. 7B , the pitch adjustment spools  720  may have portions that overlap with each other, such that the pitch adjustment spools  720  rotate about a same axis of rotation. The interfaces between the pitch adjustment spools  720  may include bearings or other similar, friction-reducing elements to facilitate rotation of the pitch adjustment spools  720  relative to each other. 
     The pitch adjustment spools  720  may also include gears, e.g., gear teeth on an outer surface of each of the pitch adjustment spools  720 . The gear teeth of the pitch adjustment spools  720  may operatively engage with the gear teeth of respective racks  717  on the inner surfaces of the control member  715 . As shown in  FIG. 7A , each rack  717  of the control member  715  may be operatively engaged with gear teeth of a respective pitch adjustment spool  720  to cause rotation of the pitch adjustment spools  720  in opposite rotational directions, such that the pitches of propeller blades coupled to the pitch adjustment spools  720  via propeller blade shafts  112  are rotated by a same degree of rotation. 
     When the motor shaft  108  is rotated by the rotor of the motor, the propeller hub  110  may rotate together with the motor shaft  108 . Rotation of the propeller hub  110  may cause rotation of the control member  715  via the mating surfaces or connections, e.g., protrusions and grooves, between the control member  715  and the propeller hub  110 . Rotation of the propeller hub  110  may also cause rotation of the pitch adjustment spools  720 , propeller blade shafts  112  and propeller blades via the openings  724  through which the propeller blade shafts  112  or propeller blades extend from the propeller hub  110 . As a result, the propeller hub  110 , the control member  715 , the pitch adjustment spools  720 , the propeller blade shafts  112  and the propeller blades may rotate together with the motor shaft  108 . In contrast, the control shaft  710 , via the rotatable connection to the control member  715  in one embodiment, may not necessarily rotate together with the propeller hub  110  and the remainder of the propeller blade pitch adjustment assembly. In an alternative embodiment in which the control shaft  710  is directly coupled to the control member  715 , the control shaft  710  may rotate together with the propeller hub  110 , the motor shaft  108 , and the remainder of the propeller blade pitch adjustment assembly, and the control shaft  710  may include a rotatable connection to the actuator on an opposite side of the motor, such that the actuator need not rotate together with the control shaft  710 , the motor shaft  108 , the propeller hub  110 , and the remainder of the propeller blade pitch adjustment assembly. 
     When adjustments to pitches of the propeller blades are desired, the control shaft  710  may be moved relative to the propeller hub  110 , e.g., pushed in a direction toward or pulled in a direction away from and transverse to axes of rotation of the pitch adjustment spools  720  and propeller blade shafts  112  and propeller blades. The movement of the control shaft  710  may also move the control member  715  in a corresponding direction toward or away from and transverse to axes of rotation of the pitch adjustment spools  720  and propeller blade shafts  112  and propeller blades. The movement of the control member  715  may cause the pitch adjustment spools  720  to rotate due to the operative engagement between the gear teeth on the pitch adjustment spools  720  and the gear teeth on the racks  717  of the control member  715 . The pitch adjustment spools  720  may rotate in opposite rotational directions in response to pushing or pulling by the control member  715 , such that the propeller blade shafts  112  and propeller blades may rotate to adjust the pitches of the propeller blades by substantially the same degree of rotation. 
     While  FIG. 7A  shows two pitch adjustment spools  720 - 1 ,  720 - 2 , two racks  717 - 1 ,  717 - 2 , and two slots  719 - 1 ,  719 - 2 , any other number and arrangement of pitch adjustment spools  720 , racks  717 , and slots  719  may be provided in the third propeller blade pitch adjustment apparatus  700 . For example, if the third propeller blade pitch adjustment apparatus  700  includes three or more pitch adjustment spools  720 , the pitch adjustment spools  720  may each be rotatably coupled to a central member, e.g., that is fixed relative to the propeller hub  110 , and gear teeth of each of the pitch adjustment spools  720  may be coupled to a respective rack  717  of the control member  715 . In this manner, the pitches of three or more propeller blades that extend from the propeller hub  110  via respective slots  719  may be substantially simultaneously adjusted using the third propeller blade pitch adjustment apparatus  700 . 
     Furthermore, the propeller hub  110  may be a substantially closed system, such that lubricant may be maintained within the propeller hub  110  to facilitate smooth engagement between the control member  715  and the propeller hub  110 , smooth engagement between the gear teeth of the control member  715  and the gear teeth of the pitch adjustment spools  720 , smooth operation of the control shaft  710  and the pitch adjustment spools  720 , and smooth rotation of the propeller blade shafts  112  and propeller blades, as well as to prevent contamination and deterioration of the components and/or lubricant. 
     Each of the components of the third propeller blade pitch adjustment apparatus  700 , including the motor shaft  108 , propeller hub  110 , control shaft  710 , control member  715 , racks  717 , pitch adjustment spools  720 , and/or propeller blade shafts  112  and propeller blades, may be made from any suitable materials, such as metal, plastics, carbon fiber, other materials, or combinations thereof, for example. In addition, the pitch adjustment spools may be coupled to the propeller blade shafts using any suitable connection methods, such as keyed connections, frictionally engaged connections, screw connections, set screws, adhesives, other connections, or combinations thereof. Alternatively or in addition, one or more of the pitch adjustment spools may be integrally formed with their respective propeller blade shafts. Further, while  FIG. 7A  shows the third propeller blade adjustment apparatus  700  having a substantially rectangular prism shape, any other shape or configuration of the apparatus  700  is possible, e.g., circular prism, elliptical prism, hexagonal prism, octagonal prism, or other polygonal prism. 
     The third propeller blade pitch adjustment apparatus  700 , including the propeller hub  110  and propeller blade pitch adjustment assembly enclosed therein, as described herein with respect to  FIGS. 7A-7B , may allow a variation in pitches of propeller blades of at least more than 90 degrees, and may allow a variation in pitches of propeller blades of up to 360 degrees or more, e.g., with sufficient available travel of the control shaft and control member. Accordingly, thrust reversal of a propeller and corresponding motor may be accomplished without any reduction in propulsive efficiency using the third propeller blade pitch adjustment apparatus to adjust the pitches of propeller blades by approximately 180 degrees and reversing a rotation of the motor. Moreover, various other changes to the thrust profile of a propeller and corresponding motor may be accomplished using the third propeller blade pitch adjustment apparatus to adjust the pitches of propeller blades as desired. 
       FIG. 8  is a block diagram illustrating various components of an example aerial vehicle control system  800  of an example aerial vehicle which may utilize one or more of the propeller blade pitch adjustment apparatuses described herein, according to an implementation. In various examples, the block diagram may be illustrative of one or more aspects of the aerial vehicle control system  800  that may be used to implement the various systems and processes discussed above. In the illustrated implementation, the aerial vehicle control system  800  includes one or more processors  802 , coupled to a non-transitory computer readable storage medium  820  via an input/output (I/O) interface  810 . The aerial vehicle control system  800  may also include an electronic speed control or propulsion controller  804 , a power controller/supply module  806  and/or a navigation system  808 . The aerial vehicle control system  800  further includes a propeller blade pitch controller  812 , a network interface  816 , and one or more input/output devices  818 . 
     In various implementations, the aerial vehicle control system  800  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, propeller blade data or characteristics, blade pitch data or characteristics, propeller blade pitch adjustment apparatus data or characteristics, data or characteristics associated with the aerial vehicle or any other system or machine utilizing the propeller blade pitch adjustment apparatuses, and/or other 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 above, are shown stored within the non-transitory computer readable storage medium  820  as program instructions  822 , data storage  824  and propeller blade, blade pitch, and operational data  826 , respectively. In other implementations, program instructions, data and/or operational data 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 aerial vehicle control system  800 . Propeller blade pitch adjustment apparatus data or characteristics may include data related to motor shafts  108 , pitch adjustment shafts  122 , blade gears  217 , pitch adjustment gears  215 , first sun gears  315 , first planet gears  325 , first planetary gear carriers  123 , ring gears  330 , second planet gears  425 , second planetary gear carriers  124 , second sun gears  415 , control shafts  610 ,  710 , control members  615 ,  715 , pitch adjustment spools  620 ,  720 , tension cables  622 , torsion springs  625 , racks  717 , and/or any other components of the apparatuses  100 - 700  described herein. 
     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 aerial vehicle control system  800  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  816  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 electronic speed control or propulsion controller  804  communicates with the navigation system  808  and adjusts the operational characteristics of each propulsion mechanism to guide the aerial vehicle along a determined flight path and/or to perform other navigational maneuvers. The navigation system  808  may include a GPS or other similar system than can be used to navigate the aerial vehicle to and/or from a location. 
     The aerial vehicle control system  800  may also include a propeller blade pitch controller  812 . The propeller blade pitch controller  812  communicates with components of the aerial vehicle, as discussed above, and controls the actuation of the second planetary gear carrier  124  and pitch adjustment shaft  122 , control shaft  610 , and/or control shaft  710  to adjust pitches of propeller blades. For example, an aerial vehicle control system  800  may operate a motor in a first rotational direction to generate thrust with a corresponding propeller, and if a thrust reversal is desired, the propeller blade pitch controller  812  may actuate the second planetary gear carrier  124  and pitch adjustment shaft  122 , control shaft  610 , and/or control shaft  710  to adjust pitches of the one or more propeller blades, e.g., rotate the blades by approximately 180 degrees, and the aerial vehicle control system  800  may operate the motor in a second rotational direction opposite from the first rotational direction. 
     The network interface  816  may be configured to allow data to be exchanged between the aerial vehicle control system  800 , other devices attached to a network, such as other computer systems, aerial vehicle control systems of other aerial vehicles, and/or an aerial vehicle management system. For example, the network interface  816  may enable wireless communication between numerous aerial vehicles. 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, image capture devices, imaging sensors, 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 aerial vehicle control system  800 . One or more of these sensors may be utilized to determine an aerial vehicle operation, a flight condition, a location, and/or a time at which a change of pitch is desired for one or more propellers of the aerial vehicle. 
     As shown in  FIG. 8 , the memory may include program instructions  820  which may be configured to implement the example processes and/or sub-processes described above. The data storage  824  and propeller blade, blade pitch, and operational data  826  may include various data stores for maintaining data items that may be provided for controlling the actuation of the various propeller blade pitch adjustment apparatuses described herein to adjust pitches of propeller blades. 
     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. 
     Each process described herein may be implemented by the architectures described herein or by other architectures. The processes are illustrated as a collection of blocks in a logical flow. Some of the blocks represent operations that can be implemented in hardware, software, or a combination thereof. In the context of software, the blocks 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. In addition, in some implementations, the computer readable media may include a transitory computer readable signal (in compressed or uncompressed form). Examples of computer readable signals, whether modulated using a carrier or not, include, but are not limited to, signals that a computer system hosting or running a computer program can be configured to access, including signals downloaded through the Internet or other networks. 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 process. Additionally, one or more of the operations may be considered optional and/or not utilized with other operations. 
     Those skilled in the art will appreciate that the aerial vehicle control system  800  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, including computers, network devices, internet appliances, PDAs, wireless phones, pagers, etc. The aerial vehicle control system  800  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 aerial vehicle control system  800 . 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 above. In some implementations, instructions stored on a computer-accessible medium separate from the aerial vehicle control system  800  may be transmitted to the aerial vehicle control system  800  via transmission media or signals, such as electrical, electromagnetic, or digital signals, conveyed via a communication medium, such as a network and/or 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 aerial vehicle control system configurations. 
     Those skilled in the art will appreciate that, in some implementations, the functionality provided by the processes and systems discussed above may be provided in alternative ways, such as being split among more software modules or routines or consolidated into fewer modules or routines. Similarly, in some implementations, illustrated processes and systems may provide more or less functionality than is described, such as when other illustrated processes instead lack or include such functionality respectively, or when the amount of functionality that is provided is altered. In addition, while various operations may be illustrated as being performed in a particular manner (e.g., in serial or in parallel) and/or in a particular order, those skilled in the art will appreciate that, in other implementations, the operations may be performed in other orders and in other manners. Those skilled in the art will also appreciate that the data structures discussed above may be structured in different manners, such as by having a single data structure split into multiple data structures or by having multiple data structures consolidated into a single data structure. Similarly, in some implementations, illustrated data structures may store more or less information than is described, such as when other illustrated data structures instead lack or include such information respectively, or when the amount or types of information that is stored is altered. The various processes and systems as illustrated in the figures and described herein represent example implementations. The processes and systems may be implemented in software, hardware, or a combination thereof in other implementations. Similarly, the order of any process may be changed and various elements may be added, reordered, combined, omitted, modified, etc., in other implementations. 
     From the foregoing, it will be appreciated that, although specific implementations have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the appended claims and the features recited therein. In addition, while certain aspects are presented below in certain claim forms, the inventors contemplate the various aspects in any available claim form. For example, while only some aspects may currently be recited as being embodied in a computer readable storage medium, other aspects may likewise be so embodied. Various modifications and changes may be made as would be obvious to a person skilled in the art having the benefit of this disclosure. It is intended to embrace all such modifications and changes and, accordingly, the above description is to be regarded in an illustrative rather than a restrictive sense.