Abstract:
A vertical takeoff and landing aircraft includes rotors that provide vertical and horizontal thrust. During forward motion, the vertical lift system is inactive. A lift fan mechanism positions the fan blades of the aircraft in a collapsed configuration when the vertical lift system is inactive and positions the fan blades of the aircraft in a deployed configuration when the vertical lift system is active.

Description:
BACKGROUND 
     Field 
     The described embodiments relate to a variable geometry lift fan mechanism for a powered-lift aircraft. 
     Aircraft may use fixed wings, such as in a conventional airplane, rotary wings, such as in a helicopter, or a combination of fixed wings and rotary wings. Powered-lift aircraft, which derive lift in some flight regimes from rotary wings and in others from fixed wings, are desirable because they are able to perform very short or vertical takeoffs and landings. A powered-lift aircraft may have rotary wings, or rotors, that provide both vertical and horizontal thrust. Other types of powered-lift aircraft have one or more rotors (lift fans) for vertical thrust, and one or more rotors (propellers) for horizontal thrust. In some powered-lift aircraft the lift fans are inactive during forward flight. 
     In a powered-lift aircraft with lift fans, the fans may have four or more blades to provide the needed lift at rotational speeds while still allowing for quiet operation. The fan blades may have large chords and can be highly twisted. When the fan blades are stationary, these types of fans produce large amounts of aerodynamic drag due to flow separation and large frontal area from the blades. This reduces the performance of the aircraft. If the individual fan blades are aligned with the flow direction of air across the aircraft, both the frontal area and flow separation are reduced resulting in lower drag. While a two-bladed fan can be stopped with the blades aligned in the flow direction, this is not possible with fans of more than two blades. 
     SUMMARY 
     The embodiments herein disclose a lift fan of a powered-lift aircraft. The lift fan of the aircraft is configured to transition from a deployed configuration to a collapsed configuration and vice versa. In one embodiment, the deployed configuration of a lift fan corresponds to the best orientation of the fan blades of the lift fan for producing thrust. For example, 90 degree spacing between the blades of a four-bladed fan may correspond to a deployed configuration of a lift fan. The collapsed configuration of the lift fan describes the orientation of the fan blades of the lift fan when thrust is no longer needed. According to one embodiment, the collapsed configuration reduces the frontal area of the lift fan and the drag produced by the lift fan in forward flight by positioning the fan blades of the lift fan to be in-line with each other. Additionally, the collapsed configuration may reduce the overall width of the aircraft allowing for easier transport and storage. 
     In one embodiment, a mechanism is employed to move the blades of the lift fan between the deployed configuration and the collapsed configuration using the motor that drives the lift fan, obviating the need for additional motors and/or actuators. The torque of the motor can be precisely controlled and used to move the lift fan blades between the deployed and collapsed configurations. Mechanical stops may be incorporated into the blades of a lift fan. The position of the mechanical stops define the angle that the blades of the lift fan may rotate through with respect to one another in order to position the fan in the deployed configuration or the collapsed configuration. 
     A mechanism may be employed to resist the rotation of the blades out of either the deployed configuration or collapsed configuration. In one embodiment, the mechanism is a spring loaded pallet fork and is activated by centrifugal force. When the lift fan is at rest, the pallet fork locks the blades in the collapsed configuration. However, when the rotational velocity of the lift fan exceeds a threshold, centrifugal force causes the pallet fork to disengage, allowing the blades to move into the deployed configuration. The centrifugal force prevents the pallet fork from moving from the deployed configuration. Thus, the blades remain locked in the deployed configuration until the rotational velocity of the lift fan decreases below the threshold, at which point the centrifugal force is low enough for the pallet fork to allow the blades to move into the collapsed configuration. 
     The features and advantages described in this summary and the following detailed description are not intended to be limiting. Many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a lift fan in a collapsed configuration according to one embodiment. 
         FIG. 2  illustrates the lift fan in a deployed configuration according to one embodiment. 
         FIG. 3  illustrates a pallet fork according to one embodiment. 
         FIG. 4  illustrates pallet forks included in a lift fan mechanism according to one embodiment. 
         FIG. 5  illustrates a motor rotor according to one embodiment. 
         FIGS. 6A, 6B, 6C, and 6D  illustrate different stages of the lift fan mechanism transitioning from the collapsed configuration to the deployed configuration according to one embodiment. 
         FIGS. 7A and 7B  respectively illustrate detailed views of the motor rotor according to one embodiment. 
         FIGS. 8A, 8B, and 8C  illustrate a lift fan mechanism according to an alternative embodiment. 
         FIGS. 9A, 9B, and 9C  illustrate a lift fan mechanism according to another alternative embodiment. 
     
    
    
     The figures depict, and the detail description describes, various non-limiting embodiments for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a lift fan of a powered-lift aircraft according to one embodiment. The lift fan may generate vertical thrust for takeoff and landing of the powered-lift aircraft. The lift fan may comprise a stack of two or more sets of one or two fan blades, where the total number of blades is equal to the number needed in a baseline fixed-blade fan. Particularly,  FIG. 1  illustrates a stack of two two-blade sets, for a total of four fan blades. In  FIG. 1 , blade set  101  represents a driven set of blades that is attached to a drive source such as an electric motor. Blade set  103  represents one or more additional following set of blades that are coupled to the driven blade set  101  by a mechanism allowing motion about the axis of rotation  102 . The following blade set  103  may include one additional set of blades or two or more additional sets of blades. 
     In  FIG. 1 , the lift fan is shown in a collapsed configuration. When thrust is no longer needed such as during cruising flight or storage of the aircraft, the following blade set  103  can be rotated to the collapsed configuration shown in  FIG. 1 . The collapsed configuration reduces the frontal area and aerodynamic drag of the lift fan because the position of the fan blades in the collapsed configuration is in line with the travel of the aircraft represented by the arrow  105  in  FIG. 1 . Additionally, the collapsed configuration may reduce the overall width of the aircraft allowing for easier transport and storage. In the collapsed configuration, the driven blade set  101  and the following blade set  103  are positioned such that the fan blades of the driven blade set  101  and the following blade set  103  are in line with one another. That is, in the collapsed configuration a substantially zero-degree spacing (i.e., the stowed angle) is configured between a pair of fan blades of the lift fan. 
       FIG. 2  illustrates the lift fan of  FIG. 1  in a deployed configuration according to one embodiment. When the lift fan is producing thrust, the following blade set  103  is rotated to its optimal thrust producing orientation as shown in  FIG. 2 . For example, in the deployed configuration a 90 degree spacing is configured between the driven blade set  101  and following blade set  103 . However, in alternative embodiments the angle between the driven blade set  101  and following blade set  103  may be any angle greater than a stowed angle associated with a collapsed configuration and may include more than one following blade set. In one embodiment, the driven blade set  101  and following blade set  103  is made from a carbon fiber composite material. Alternatively, the driven blade set  101  and following blade set  103  is made from other materials, such as aluminum alloy. 
     The change from the deployed configuration to the collapsed configuration and vice-versa may be accomplished by several mechanisms. A mechanism may move the following blade set  103  between the deployed configuration and the collapsed configuration and vice-versa using the motor that powers the lift fan, and without the use of additional motors and/or actuators. In one embodiment, the following blade set  103  of a lift fan pivots about the rotational axis of the driven blade set  101  within a range of relative motion defined by mechanical stops that are engaged in both the collapsed and deployed configurations. In one embodiment, the mechanical stops are incorporated into the driven blade set  101  and the following blade set  103 . A retention torque may be applied to hold the following blade set  103  against these mechanical stops so that aerodynamic or other disturbances do not cause the following blade set  103  to bounce on the stops or move out of the desired configuration, such as the deployed configuration or the collapsed configuration. The retention torque may be generated by various means. 
     When the lift fan is not used and is stationary with respect to the powered-lift aircraft, the lift fan is oriented in the collapsed configuration. When the lift fan is required to produce thrust, the lift fan is oriented in the deployed configuration, and spins within some nominal speed range depending on the required thrust. An example of a nominal operating speed range is from 2500 RPM to 3500 RPM. A motor applies torque to the lift fan in order to accelerate it from rest to the desired operating speed. 
     As the lift fan accelerates, the lift fan moves from the collapsed configuration to the deployed configuration before it reaches the lower bound of the operating speed range. The aerodynamic torque and inertia of the driven blade set  101  apply torque to the deployment mechanism in the direction that deploys the following blade set  103 . The torque required to cause the transition from the collapsed configuration to the deployed configuration is called the deployment torque profile. In one embodiment, the deployment torque profile is designed such that the following blade set  103  moves from the collapsed configuration to the deployed configuration before the lift fan reaches the lower limit of the nominal operating speed range. The deployment torque is produced by a motor (e.g., an electric motor) that provides the power to spin the lift fan. The torque of the motor is precisely controlled to create the desired deployment torque profile. 
     As an example of how the deployment torque profile is designed, assume that the operating speed range of the lift fan is 2500 rpm to 4000 rpm. In this case, the desired threshold speed may be 1000 rpm, which is sufficiently far below the lower operating speed limit of 2500 rpm that an unintended collapsing of the mechanism can be prevented. The center of gravity  303  of the pallet fork  301  and the spring constant of the torsion spring  320  can be chosen such that the centrifugal force generated at 1000 rpm is sufficient to overcome the force generated by the torsion spring  320 . To deploy the following fan, a torque is applied to the motor such that the rotor quickly accelerates to 900 rpm, then slowly accelerates from 800 rpm to 1200 rpm. As soon as the speed of the rotor exceeds the threshold speed, the pallet fork rotates from the first position to the second position and the aero torque on the following blades causes the following fan to move from the collapsed angle to the deployed angle as will be further described below. When the following fan is in the deployed angle, the pallet fork moves from the second position to the third position as will be further described below. 
     In this case, the collapsing torque profile may be designed as follows: The lift fan is decelerated quickly from its current speed in the operating speed range to 1200 rpm. The lift fan is then slowly decelerated from 1200 rpm to 800 rpm with a low enough deceleration rate that the aero torque on the following fan is sufficient to prevent it from leaving the mechanical stops. As soon as the speed drops below the threshold, the pallet fork moves from the third position to the second position. A negative torque pulse is then applied. The negative torque pulse decelerates the driven fan such that the relative angle between the driven fan and the following fan transitions from the deployed angle to the collapsed angle. Once the following fan reaches the collapsed angle, the pallet fork moves from the second position to the first position. 
       FIG. 3  illustrates a perspective view of the bottom side of a spring loaded pallet fork  301  according to one embodiment. The spring loaded pallet fork  301  is one example of a mechanism that provides the retention torque required to resist the rotation of the following blade set  103  from the deployed configuration shown in  FIG. 2  to the collapsed configuration shown in  FIG. 1  when the lift fan is providing vertical thrust above a threshold speed. Similarly, the spring loaded pallet fork  301  provides the retention torque required to resist the rotation of the following blade set  103  from the collapsed configuration to the deployed configuration when thrust is no longer needed such as during cruising flight or storage of the aircraft (i.e., below the threshold speed). 
     In one embodiment, the pallet fork  301  comprises a pallet fork body  302 , a center of mass  303 , a first pawl  305  (i.e., a first end), a second pawl  310  (i.e., a second end), a pivot  315 , and a torsion spring  320 . The first pawl  305  and second pawl  310  comprise surfaces that each engage a detent of the lift fan as will be further described below with respect to  FIGS. 4 and 5 . In some embodiments, the pawl surfaces are planar. The first pawl  305  and second pawl  310  each extend in a direction that is approximately 90 degrees from the length of the pallet fork body  302 . Thus, the first pawl  305 , in conjunction with the pallet fork body  302 , makes an L-shape. Similarly, the second pawl  310  and the pallet fork body  302  also make an L-shape. When the following blade set  103  is collapsed and the pallet fork  301  is in a first position, the angle of the surface of the first pawl  305  affects the ease with which the pallet fork  301  moves from the first position to a second position when there is a tangential force between the two surfaces. If the angle is more obtuse, a tangential force will tend to slide the pallet fork  301  from the first position to the second position. If the angle is more acute, a tangential force will tend to lock the pallet fork  301  in the first position. If the angle is equal to the arctan of the coefficient of friction, a tangential force has no affect of the ease with which the pallet fork  301  moves from the first position to the second position. An equivalent situation exists for the surface of the second pawl  310  when the following blade set  103  is in the deployed configuration and the pallet fork  301  is in the third position. 
     The pallet fork  301  rotates about the pivot  315 . The pivot  315  is a cylindrical post that protrudes perpendicularly from the pallet fork body  302 . The pivot  315  is positioned on the pallet fork body  302  such that the pivot  315  is closer to the second pawl  310  than the first pawl  310  and the center of mass  303  is located in between the first pawl  305  and the pivot  315 . 
     In one embodiment, the pallet fork  301  rotates between three distinct positions as mentioned above. The pallet fork  301  is in a first position when the pallet fork  301  is rotated clockwise when viewed from the bottom about the pivot  315  such that the first pawl  305  is engaged in a first detent  510  of a motor rotor  505 , further described below with reference to  FIG. 5 . The first position of the pallet fork  301  is associated with the collapsed configuration of the lift fan, further described below with reference to  FIG. 6A . The pallet fork  301  is in a second position when the pallet fork  301  is rotated such that neither the first pawl  305  nor the second pawl  310  are engaged in the first detent  510  or a second detent  515  of the motor rotor  505 . The second position of the pallet fork  301  is associated with the transitional state of the lift fan between the collapsed and deployed configurations, further described below with reference to  FIGS. 6B and 6C . The pallet fork  301  is in a third position when the pallet fork  301  is rotated such that the second pawl  310  is engaged in the second detent  515  of the motor rotor  505 . The third position of the pallet fork  301  is associated with the deployed configuration of the lift fan, further described below with reference to  FIG. 6D . In some embodiments, the pallet fork  301  may be made from a high strength aluminum or other materials such as titanium or steel. 
     The pallet fork  301  is preloaded to rotate clockwise when viewed from the bottom by the torsion spring  320  such that the first pawl  305  engages the first detent  510  on the motor rotor  505  (i.e. the pallet fork  301  is in its first position). The torsion spring  320  is a wire formed to have a first end  325 , a second end  330 , and a plurality of windings  335 . The torsion spring  320  is mounted to the pallet fork  301  so that the plurality of windings  335  wrap around the pivot  315 . The first end  325  of the torsion spring  320  abuts the pallet fork body  302 , and the second end  325  of the torsion spring  320  abuts a surface of the following blade set  103  to which the pallet fork  301  is mounted. By applying a force to both the pallet fork body  302  and the surface of the following blade set  103 , the torsion spring  320  preloads the pallet fork  301  to rotate clockwise when viewed from the bottom. 
       FIG. 4  illustrates a schematic of the following blade set  103  with two pallet forks  301 A and  301 B diametrically opposed to each other in the lift fan according to one embodiment.  FIG. 4  illustrates a bottom view of the following blade set  103 . As mentioned previously, the center of mass  303  of each pallet fork  301  is offset from the pivot  325  in the direction of the first pawl  305 . When the lift fan is at rest (i.e. not rotating), the torsion spring  320  exerts a force on the pallet fork  301  such that the first pawl  305  extends inwards towards the center of the lift fan as shown in  FIG. 4 . However, when the lift fan starts spinning, centrifugal force is generated. The generated centrifugal force acts on the pallet fork  301  at its center of mass  303  to counter the force of the torsion spring  320 . When the lift fan attains a threshold speed, the centrifugal force is greater than the force exerted by the torsion spring  320 , causing the pallet fork  301  to pivot such that the first pawl  305  moves outward away from the center of the lift fan. 
     The following blade set  103  further comprises a plurality of housings  400 A and  400 B. Each pallet fork  301  is positioned within a corresponding housing  400  of the following blade set. For example, pallet fork  301 A is positioned within housing  400 A and pallet fork  301 B is positioned within housing  400 B. 
     Each housing  400  is defined by a first end and a second end. In one embodiment, the first ends of the housings  400  are represented by a set of first mechanical stops  405 A and  405 B and the second ends of the housings  400  are represented by a second set of mechanical stops  410 A and  410 B. The first mechanical stops  405  and second mechanical stops  410  are protrusions on a surface of the following blade set  103  that physically prevent the following blade set  103  from rotating past a minimum and a maximum angle relative to the driven blade set  101 . The first mechanical stops  405  of the following blade set  103  contact the first mechanical stops  520  on the motor rotor (further described below with reference to  FIG. 5 ) and bear the impact load as the lift fan transitions into the collapsed configuration. The second mechanical stops  410  contact the second mechanical stops  525  on the motor rotor (further described below with reference to  FIG. 5 ) and bear the impact load as the lift fan transitions into the deployed configuration. 
       FIG. 5  illustrates the top side of a motor rotor  505  according to one embodiment. In one embodiment, the motor rotor has a cylindrical feature such as a cylindrical body. The cylindrical body is fixed to the driven blade set  101  and includes various components that limit the rotation of the following blade set  103  to a maximum angle of rotation (e.g., 90 degrees) from the driven blade set  101 . In one embodiment, the motor rotor  505  comprises a first detent  510 , a second detent  515 , a first mechanical stop  520 , a second mechanical stop  525 , and a sliding surface  530 . In this example embodiment, the following blade set  103  rotates clockwise when viewed from the top to transition from the collapsed configuration to the deployed configuration. While the following descriptions of the motor rotor  505  and its components reference only one set of features of the motor rotor  505 , it is understood that embodiments of the motor rotor may comprise any multiple of the described features. The embodiment of the motor rotor  505  depicted in  FIG. 5  comprises two sets of each of the features described below. 
     The first mechanical stop  520  and second mechanical stop  525  of the motor rotor  505  are protrusions on a surface of the motor rotor  505  that physically prevent the following blade set  103  from rotating past a minimum and a maximum angle relative to the driven blade set  101 . The first mechanical stop  520  of the motor rotor  505  contacts the first mechanical stop  405  of the following blade set  103  and bears the impact load as the following blade set  103  finishes transitioning to the collapsed configuration. Similarly, the second mechanical stop  525  contacts the second mechanical stop  410  of the following blade set  103  and bears the impact load as the following blade set  103  finishes transitioning to the deployed configuration. The sliding surface  530  is a smooth surface on the motor rotor  505  that runs concentric to the circumference of the motor rotor  505  along which the first pawl  305  and second pawl  310  slide while the following blade set  103  is transitioning between the collapsed and deployed configurations. 
     The first detent  510  and second detent  515  are notches along the circumference of the cylindrical feature of the motor rotor  505 . When the lift fan is at rest in the collapsed configuration, the first pawl  305  of the pallet fork  301  is engaged with the first detent  510  of the motor rotor  505 , and the second pawl  310  is extended outward away from the center of the lift fan. The first pawl  305  rests inside the first detent  510 , preventing the following blade set  103  from rotating to the deployed configuration. While in the collapsed configuration, the first mechanical stop  520  of the motor rotor  505  contacts the first mechanical stop  405  of the following blade set  103  and prevents the following blade set  103  from rotating further counter-clockwise when viewed from the top. 
     As the following blade set  103  rotates to transition from the collapsed configuration to the deployed configuration, the first pawl  305  of the pallet fork  301  disengages the first detent  510  of the motor rotor  505  and the first pawl  305  and second pawl  310  of the pallet fork  301  slide along the sliding surface  530  of the motor rotor. As described above, the sliding surface  530  is a guide that runs concentric to the circumference of the motor rotor  505  and provides a surface that guides the first pawl  305  and second pawl  310  as the lift fan transitions between the collapsed and deployed configurations. 
     When the lift fan is spinning above a threshold speed, the lift fan is in the deployed configuration. In the deployed configuration, the second pawl  310  of the pallet fork  301  is engaged with the second detent  515  of the motor rotor  505 , and the first pawl  305  is extended outward away from the center of the lift fan. The second pawl  310  rests inside the second detent  515 , preventing the following blade set  103  from rotating to the collapsed configuration. While in the deployed configuration, the second mechanical stop  525  of the rotor  505  contacts the second mechanical stop  410  of the following blade set  103  and prevents the following blade set  103  from rotating further clockwise when viewed from the top. 
     In this example embodiment, the pallet forks  301  are mounted to the following blade set  103 , and the detents  510  and  515  are located on the motor rotor  505 . However, in other embodiments, the pallet forks  301  may be located on the motor rotor, and the detents  510  and  515  may be located on the following blade set  103 . Furthermore, in other various embodiments, the pallet forks  301  and detents  510  and  515  may be located on various other components of the lift fan to achieve the same function as the embodiment described herein. In some embodiments, the motor rotor  505  may be made of steel or may be made of other materials such as aluminum or composites. 
       FIGS. 6A through 6D  illustrate four stages of the lift fan transitioning from the collapsed configuration to the deployed configuration when viewed from the top. The following fan  103  is shown translucent in these figures. In  FIGS. 6A and 6B , the following fan  103  is in the collapsed configuration and is aligned with the driven fan  101 . In  FIGS. 6A through 6D , the following blade set  103  rotates clockwise to transition from the collapsed configuration to the deployed configuration. The following descriptions refer to only one pallet fork  301  and the relevant features. However, it is clear that the following descriptions apply to both pallet forks  301  and relevant features depicted in  FIGS. 6A-6D . Furthermore, other embodiments of the lift fan may comprise more or fewer numbers of pallet forks  301  and relevant features.  FIG. 6A  illustrates the lift fan in the collapsed configuration while the lift fan is at rest. The first pawl  305 A of pallet fork  301 A is engaged with the detent  510 A and the first pawl  305 B of pallet fork  301 B is engaged with the detent  510 B due to the force of the torsion spring  320 . The second pawl  310 A of pallet fork  301 A and the second pawl  310 B of pallet fork  301 B is rotated away from the center of the motor rotor  505 . Thus, the pallet forks  301  are in the first position. The first mechanical stop  405 A of the following blade set  103  is in contact with the mechanical stop  520 A of the rotor  505  and the first mechanical stop  405 B is in contact with the mechanical stop  520 B thereby preventing further rotation of the following blade set  103  in the counter clockwise direction when viewed from the top. 
       FIG. 6B  illustrates the lift fan as it begins to transition from the collapsed configuration to the deployed configuration due to the lift fan accelerating towards the threshold speed. When the threshold speed is exceeded, the pallet forks  301 A,  301 B rotate until the second pawl  310 A of pallet fork  301 A and the second pawl  310 B of pallet fork  301 B contact the sliding surface  530 . At this point, the pallet forks  301 A,  301 B are in its second position. That is, the centrifugal force acting on the pallet forks  301 A,  301 B has rotated the pallet forks  301 A,  301 B such that the first pawl  305 A of pallet fork  301 A and the first pawl  305 B of pallet fork  301 B has moved away from the center of the lift fan, and the second pawl  310 A of pallet fork  301 A and the second pawl  310 B of pallet fork  301 B has moved toward the center of the lift fan. Thus, The first pawl  305 A and the first pawl  305 B are no longer engaged with the first detent  510 A,  510 B, allowing the following blade set  103  to rotate. During rotation, the first pawl  305 A and the first pawl  305 B or the second pawl  310 A and the second pawl  310 B contact the sliding surface  530  and slide along the sliding surface  530  until the second mechanical stop  410 A,  410 B of the following blade set  103  contacts the second mechanical stop  525 A,  525 B of the motor rotor  505 . 
       FIG. 6C  illustrates the lift fan just prior to being in the deployed configuration according to one embodiment. The second pawl  310 A,  310 B are about to engage the second detent  515 A and  515 B respectively. The first pawl  305 A,  305 B is still nearly in contact with the sliding surface  530  and the second mechanical stop  410 A,  410 B of the following blade set  103  is now in contact with the second mechanical stop  525 A,  525 B of the rotor  505 . 
       FIG. 6D  illustrates the lift fan in the fully deployed configuration. Because the lift fan is still spinning above the threshold speed, the centrifugal force acting on the pallet forks  301 A,  301 B is great enough to overcome the force of the tension spring, causing the second pawl  310 A,  310 B to respectively engage with the second detent  515 A,  515 B and the first pawl  305 A,  305 B is rotated away from the center of the motor rotor  505 . The pallet forks  301 A,  301 B are in the third position. The lift fan is fully deployed and the pallet forks  301 A,  301 B prevent the lift fan from transitioning back to the collapsed configuration until the lift fan no longer exceeds the threshold speed. 
       FIG. 7A  illustrates the top side view of the motor rotor  505  according to one embodiment. In one embodiment, the motor rotor  505  comprises features similar to those as described with reference to  FIG. 5  on the top side, but also has an identical set of features on the bottom side as shown in  FIG. 7B . This embodiment has the advantage that a single motor design can be used with the fixed and following fans assembled in either a right side up or an upside down configuration. In both of these configurations, the following blade set  103  is assembled on the top side, hence the need to have stops and detents on both sides of the motor rotor. 
     The bottom side of the motor rotor  505  shown in  FIG. 7B  also comprises a bottom collapsed detent  510 C, a bottom deployed detent  515 C, a bottom collapsed mechanical stop  520 C, a bottom deployed mechanical stop  525 C, and a bottom sliding surface  530 C. The motor rotor  505  depicted in  FIGS. 7A and 7B  allows the following blade set  103  to be mounted on either the top or the bottom of the motor rotor  105  and still be allowed to rotate between and lock in the collapsed and deployed positions. The bottom first detent  510 C and bottom second detent  515 C are notches in a surface of the motor rotor  505 . When the lift fan is in the collapsed configuration, the first pawl  305  of the pallet fork  301  engages in the bottom first detent  510 C if the following blade set  103  is attached to the bottom of the motor rotor  505 . When the lift fan is in the deployed configuration, the second pawl  310  of the pallet fork  301  engages in the bottom second detent  515 C. The bottom first mechanical stop  520 C and bottom second mechanical stop  525 C are protrusions on a surface of the motor rotor  505  that physically prevent the following blade set  103  from rotating past a maximum angle relative to the driven blade set  101 . The bottom first mechanical stop  520 C bears the impact load as the following blade set  103  finishes transitioning to the collapsed configuration. The bottom second mechanical stop  525 C bears the impact load as the following blade set  103  finishes transitioning to the deployed configuration. The bottom sliding surface  530 C is a smooth surface on the motor rotor  505  that runs concentric to the circumference of the motor rotor  505  along which the first pawl  305  and second pawl  310  can slide while the following blade set  103  is transitioning between the collapsed and deployed configurations. 
       FIG. 8A  illustrates an alternative embodiment of a latching mechanism for the folding lift fan depicted in  FIG. 1 . The latching mechanism comprises a guide assembly  905 . The guide assembly  905  is fixed to the motor rotor  505  and comprises a cylindrical feature such as a slot guide  910 , a first detent  915 , a mechanical stop  916 , a second detent  950  (shown in  FIG. 8B ), and a second mechanical stop  951  (shown in  FIG. 8B ). The slot guide  910  is a long, thin opening in the guide assembly  905  through which a tab  930  of the pallet fork  920  can slide as further described below with reference to  FIGS. 8B and 8C . The tab  930  comprises a first face  931  and a second face  932 . In one embodiment, the two faces  931  and  932  act as both mechanical stops and as pawls. The first detent  915  is a notch formed in an inner edge  911  of the slot guide  910 , and the second detent  950  is a notch formed in an outer edge  912  of the slot guide  910 . When the lift fan is in the collapsed configuration, the second face  932  of the tab  930  of the pallet fork  920  engages in the first detent  915  and the first face  931  of the tab  930  engages with the mechanical stop  916 . When the lift fan is in the deployed configuration, the first face  931  of the tab  930  of the pallet fork  920  engages in the second detent  950  and the second face  932  of the tab  930  engages with the second mechanical stop  951 . 
     The latching mechanism also comprises a pallet fork  920 . The pallet fork  920  comprises a pivot hole  921  at a first end of the pallet fork  920 , a friction pad  925 , and a tab  930  at a second end of the pallet fork  920 . An axle can extend through the pivot hole  921 , allowing the pallet fork  920  to rotate about an axis  922 . The pivot hole  921  is positioned on the pallet fork  920  such that the friction pad  925 , the center of gravity, and tab  930  are all on the same side of the pivot hole  921 . 
     In one embodiment, the mechanism comprises a friction pad  925  that is a protrusion from a surface of the pallet fork  920 . In some embodiments, the surface of the friction pad  925  is made of a material with a high coefficient of friction such as brake pad material. The tab  930  protrudes on both the top and bottom of the pallet fork  920  that engage a detent in a guide assembly  905  as will be further described below with reference to  FIGS. 8B and 8C . The pallet fork  920  rotates about the axis  922  between three distinct positions. The pallet fork  920  is in a first position when the pallet fork  920  is rotated counter clockwise about the axis  922  such that the tab  930  is engaged with a first detent  915  of the guide assembly  905 , and the friction pad  925  is engaged with a friction surface  955  as depicted in  FIG. 8B . The first position of the pallet fork  920  is associated with the collapsed configuration of the lift fan. In the collapsed configuration, the motor is not applying torque to the rotor and the friction between the friction pad  925  and the friction surface  955  prevents torque disturbances due to airflow, and other torque disturbances from causing the rotor to rotate. This allows the motor to be de-energized, which saves energy. The pallet fork  920  is in a second position when the pallet fork  920  is rotated such that the tab  930  slides along the slot guide  910 , as further described below. The second position of the pallet fork  920  is associated with the transitional configuration between the collapsed and deployed configurations of the lift fan, as further described below. The pallet fork  920  is in a third position when the pallet fork  920  is rotated such that the tab  930  is engaged in a second detent  950  of the guide assembly  905 , as depicted in  FIG. 8C . The third position of the pallet fork  920  is associated with the deployed configuration of the lift fan. The pallet fork  920  is preloaded to rotate counter clockwise by a spring  960 , shown in  FIGS. 8B and 8C . In one embodiment, the spring  960  comprises a wire formed into a plurality of coils and is mounted such that one end of the spring  960  is connected to the pallet fork  920  and the other end of the spring  960  is connected to a pallet fork mount  935 , further described below. 
     The latching mechanism also comprises a pallet fork mount  935 . The pallet fork mount  935  is fixed to the following blade set  103  and comprises a friction pad hole  940  and a pivot hinge  945 . The friction pad hole  940  is an opening formed in the pallet fork mount  935  through which the friction surface  925  of the pallet fork  920  can extend. The pivot hinge  945  is a protrusion on a surface of the pallet fork mount  935  with a hole through which an axle can extend. The pallet fork  920  is mounted to the pallet fork mount  935  by an axle that extends through both the pivot hole  921  of the pallet fork  920  and the hole in the pivot hinge  945  of the pallet fork mount  935 . This allows the pallet fork  920  to rotate relative to the pallet fork mount  935  about the axis  922 . 
       FIG. 8B  illustrates a top down view of the latching mechanism depicted in  FIG. 8A  in the collapsed position. The folding lift fan is stationary and the spring  960  is applying a force on the pallet fork  920  such that the pallet fork  920  rotates counter clockwise and the tab  930  moves inward toward the pallet fork mount  935 . The friction pad  925  of the pallet fork  920  extends through the friction pad hole  940  and contacts a friction surface  955  of the driven blade set  101 . Additionally, the tab  930  of the pallet fork  920  is engaged in the first detent  915  of the guide assembly  905 . The friction pad  925  and the tab  930  both prevent the following blade set  103  from rotating out of the collapsed configuration. 
       FIG. 8C  illustrates a top down view of the latching mechanism depicted in  FIG. 8A  in the deployed position. As the lift fan starts spinning and exceeds a threshold speed, the centrifugal force acting on the pallet fork  920  causes the pallet fork  920  to overcome the force exerted by the spring  960  and rotate clockwise, moving the tab  930  outward away from the pallet fork mount  935 . This causes the tab  930  to disengage from the first detent  915  and also allows the friction pad  925  to release from the friction surface  955 . This allows the following blade set  103 , pallet fork mount  935 , and pallet fork  920  to rotate counterclockwise relative to the driven blade set  101  and guide assembly  905 . Once the lift fan has rotated fully to the deployed configuration, the second surface  932  of the tab  930  of the pallet fork  920  engages with the second mechanical stop  951 , which prevents further rotation. Because the lift fan is still spinning above the threshold speed, the centrifugal force acting on the pallet fork  920  overcomes the force exerted by the spring  960 , and moves into the third position. The first surface  931  of the tab  930  becomes engaged in the deployed detent  950 , preventing the following blade set  103  from rotating out of the deployed configuration. The pallet fork  920  will prevent the lift fan from transitioning back to the collapsed configuration until the lift fan no longer exceeds the threshold speed. In other embodiments, the pallet fork  920  may have the tab  930  but may not have a friction pad  925 . In other embodiments, the pallet fork  920  may only have the friction pad  925  and not utilize a tab  930 . In some embodiments, the pallet fork  920 , pallet fork mount  935 , and guide assembly  905  may be made from titanium or other metal or composite materials. 
       FIG. 9A  illustrates another alternative embodiment of a latching mechanism for the folding lift fan depicted in  FIG. 1 . The latching mechanism comprises a cylindrical feature such as a slot assembly  1005  that is fixed to the driven blade set  101 . The slot assembly  1005  comprises first slots  1010 A and  1010 B and second slots  1015 A and  1015 B. The first slots  1010  and second slots  1015  are holes formed in the slot assembly. When the lift fan is in the collapsed configuration, the first arm  1025  of the pallet fork  1020  engages in the first slot  1010 , as further described below with reference to  FIG. 9B . When the lift fan is in the deployed configuration, the second arm  1030  of the pallet fork  1020  engages in the second slot  1015 , as further described below with reference to  FIG. 9C . 
     The latching mechanism further comprises pallet forks  1020 A and  1020 B. Referring now to  FIG. 9B , each pallet fork  1020  is mounted to the following blade set  103 , and comprises a first arm  1025 , a second arm  1030 , a pivot  1035 , a torsion spring  1040 , and a center of mass  1045 . The first arm  1025  and second arm  1030  extends from the pivot  1035  at a fixed angle, and the fixed angle is 90 degrees in one embodiment. The first arm  1025  and second arm  1030  engage in slots in a slot assembly  1005 , further described below. The pivot  1035  is a hinge assembly that mounts the pivot  1035  to the following blade set  103  and allows the pallet fork  1020  to rotate clockwise and counter clockwise as shown in  FIGS. 9B and 9C . The torsion spring  1040  is a wire formed into a plurality of coils  1041  with a first end  1042  and a second end  1043 . The first end  1042  abuts a surface on the pallet fork  1020 , the second end abuts a surface on the following blade set  103 , and the plurality of coils  1041  surrounds the pivot  1035 . The torsion spring exerts a force on the pallet fork  1020  such that the pallet fork  1020  is preloaded to rotate counterclockwise, causing the first arm  1025  to engage in a first slot  1010 , further described below. 
     The pallet fork  1020  rotates about the pivot  1035  between three distinct positions. The pallet fork  1020  is in a first position when the pallet fork  1020  is rotated counter clockwise about the pivot  1035  such that the first arm  1025  is engaged with the first detent  1010  of the slot assembly  1005 , as depicted in  FIG. 9B . The first position of the pallet fork  1020  is associated with the collapsed configuration of the lift fan. The pallet fork  1020  is in its second position when the pallet fork  1020  is rotated slightly clockwise from its first position, such that the first arm  1025  is not engaged in the first slot  1010  and the second arm  1030  is not engaged in a second slot  1015 . The second position of the pallet fork  1020  is associated with the transitional configuration between the collapsed and deployed configurations, as further described below. The pallet fork  1020  is in a third position when the pallet fork  1020  is rotated further clockwise such that the second arm  1030  is engaged with the second slot  1015  of the slot assembly  1005 , as depicted in  FIG. 9C . The third position of the pallet fork  1020  is associated with the deployed configuration of the lift fan. 
       FIG. 9B  illustrates the pallet fork  1020 B depicted in  FIG. 9A  in the collapsed configuration. In the collapsed configuration, the lift fan is at rest, and the torsion spring  1040  exerts a force on the pallet fork  1020  such that the first arm  1025  of the pallet fork  1020  is engaged in the first slot  1010  of the slot assembly  1005 , preventing the following blade set  103  from rotating out of the collapsed configuration. 
       FIG. 9C  illustrates the pallet fork  1020 B depicted in  FIG. 9A  in the deployed configuration. As the lift fan spins above a threshold speed, the centrifugal force acting on the center of mass  1045  overpowers the force exerted on the pallet fork  1020  by the torsion spring  1040 , and the pallet fork  1020  rotates clockwise around the pivot  1035 . This causes the first arm  1025  to disengage the first slot  1010 , allowing the following blade set  103  to rotate out of the collapsed configuration. Once the following blade set  103  rotates to the fully deployed configuration, the centrifugal force acting on the pallet fork  1020  causes the second arm  1030  of the pallet fork  1020  to engage in the second slot  1015  of the slot assembly  1005 , preventing the following blade set  103  from rotating out of the deployed configuration. The pallet fork  1020  will prevent the lift fan from transitioning back to the collapsed configuration until the lift fan no longer exceeds the threshold speed. In some embodiments, the slot assembly  1005  and pallet fork  1020  may be made from titanium. In other embodiments, the slot assembly  1005  and the pallet fork  1020  may be made from other metal or composite materials. 
     Although this description has been provided in the context of specific embodiments, those of skill in the art will appreciate that many alternative embodiments may be inferred from the teaching provided. Furthermore, within this written description, the particular naming of the components, capitalization of terms, the attributes, data structures, or any other structural or programming aspect is not mandatory or significant unless otherwise noted, and the mechanisms that implement the described invention or its features may have different names, formats, or protocols. Further, some aspects of the system may be implemented via a combination of hardware and software or entirely in hardware elements. Also, the particular division of functionality between the various system components described here is not mandatory; functions performed by a single module or system component may instead be performed by multiple components, and functions performed by multiple components may instead be performed by a single component. Likewise, the order in which method steps are performed is not mandatory unless otherwise noted or logically required. 
     Unless otherwise indicated, discussions utilizing terms such as “selecting” or “computing” or “determining” or the like refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     Finally, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. Accordingly, the disclosure is intended to be illustrative, but not limiting, of the scope of the invention.