Patent Publication Number: US-2019185155-A1

Title: Vertical takeoff and landing aircraft

Description:
RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 15/456,173, filed on Mar. 10, 2017, which claims priority to and is a continuation-in-part application of U.S. patent application Ser. No. 14/738,675, filed on Jun. 12, 2015, now U.S. Pat. No. 9,862,486, which is a continuation of U.S. patent application Ser. No. 13/708,612, filed on Dec. 7, 2012, now U.S. Pat. No. 9,085,355, which issued on Jul. 21, 2015, all of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     The field of aviation encompasses many different versions of personal aircraft. The vast majority of readily accessible, affordable personal aircraft are fixed wing designs that require the use of a hangar or ramp space for storage, and an air strip for take-off and landing. These requirements increase the overall cost of operating the aircraft, and can render it inaccessible to the average citizen. In addition, most personal aircraft rely on traditional controls and require training and expertise to operate them. The degree of difficulty and knowledge required to operate these aircraft is often multiple times that of driving a traditional automobile, for example. 
     In contrast, vertical takeoff and landing (VTOL) aircraft can be operated without use of an air strip. Since their inception at the early part of the twentieth century, VTOL aircraft have been powered by internal combustion, turbo-fans or turbo-shaft engines. These types of engines, though reliable, suffer from the threat of engine loss due to foreign object debris (FOD), largely rely on petroleum based products as their fuel, and are highly complex and extremely expensive. The dangerous and cost prohibitive nature of the turbo-fan and turbo-shaft engines used in today&#39;s modern VTOL aircraft has deterred their use in the personal aircraft market. 
     A large number of VTOL aircraft in use today are in the form of tilt-rotor aircraft, and are employed by various militaries throughout the world. Many of these aircraft provide balanced thrust from rotors located on the main wings, thus making them difficult if not impossible to maneuver after a rotor becomes nonoperational. In addition, the overall complexity of today&#39;s tilt-rotor aircraft has limited their use to well trained, professional pilots. However, the many advantages of the tilt-rotor unit platform, including vertical lifting capabilities combined with the speed and efficiency of a conventional fixed wing aircraft, if available to the general public, could revolutionize the transportation industry. 
    
    
     
       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 same reference numbers in different figures indicate similar or identical items. 
         FIGS. 1A and 1B  are perspective views of an illustrative vertical takeoff and landing (VTOL) aircraft. 
         FIG. 2  is a schematic diagram showing a transition of flight of an illustrative VTOL aircraft from takeoff to a transitional mode to a forward flight mode. 
         FIG. 3A  is a side elevation view of an illustrative VTOL aircraft with extended landing gear. 
         FIG. 3B  is a side elevation view of an illustrative VTOL aircraft in vertical (hover) flight mode. 
         FIG. 3C  is a side elevation view of an illustrative VTOL aircraft shown in  FIG. 2  and shows airflow through rotor units during vertical flight. 
         FIG. 3D  is a side elevation view of an illustrative VTOL aircraft in forward flight mode and shows airflow through rotor units during forward flight. 
         FIG. 4  is a top view of the illustrative VTOL aircraft shown in  FIG. 3B . 
         FIG. 5  is a top view of the illustrative VTOL aircraft shown in  FIG. 1A . 
         FIGS. 6A and 6B  are top views of the illustrative VTOL aircraft shown in  FIG. 3B  and show various components that may be selectively included in the VTOL aircraft. 
         FIG. 7  is a perspective view of a VTOL unmanned aerial vehicle (UAV). 
         FIG. 8  is a top view of the illustrative VTOL UAV shown in  FIG. 7  and shows various components and compartments including locations of batteries and storage compartments. 
         FIG. 9A  is a front elevation view of the illustrative VTOL aircraft shown in  FIG. 3B  and shows the fore rotor unit and aft rotor unit configured to provide thrust in a direction of a vertical axis that bisects the aircraft in symmetrical halves. 
         FIG. 9B  is a front elevation view of the illustrative VTOL aircraft shown in  FIG. 3B  and shows the fore rotor unit and aft rotor unit configured to provide thrust in a direction having an angle α from a vertical axis that bisects the aircraft in symmetrical halves. 
         FIG. 9C  is a schematic diagram showing a transition of flight of an illustrative VTOL aircraft using the rotor units as an air-brake. 
         FIG. 10  is a partial top view of the illustrative VTOL aircraft shown in  FIG. 1A  and shows the fore rotor unit configured to provide directed thrust in a direction between an angle β from a longitudinal axis of the fuselage. 
         FIGS. 11A-11C  are isometric views of an illustrative rotating assembly that connects a rotor to the fuselage of the illustrative VTOL aircraft. 
         FIGS. 12A-12C  are views of a rotor and illustrative apparatus to cause lateral movement of the rotor in accordance the  FIGS. 9B and 10 . 
         FIG. 13A  is a partial cutout perspective view of an illustrative rotor unit. 
         FIGS. 13B and 14  are side elevation views of illustrative rotor units. 
         FIGS. 15A and 15B  are partial cutout isometric views of the illustrative rotor unit shown in  FIG. 13  and show lateral thrust directing apparatuses. 
         FIG. 16  is a top view of an illustrative VTOL aircraft that includes wingtip rotors. 
         FIG. 17  is a top view of an illustrative VTOL aircraft having a shortened fuselage and rotors selected to balance a payload of the VTOL aircraft. 
         FIG. 18  is a top view of an illustrative VTOL aircraft having a traditional wing configuration. 
         FIG. 19  is a top view of an illustrative VTOL aircraft where the fore rotor unit is located between the aft rotor unit and a cockpit. 
         FIG. 20  is a top view of an illustrative multi-occupant VTOL aircraft. 
         FIGS. 21A and 21B  are side elevation views of another illustrative multi-occupant VTOL aircraft. 
         FIGS. 22A and 22B  are top views of an illustrative VTOL aircraft where the operator straddles the fuselage. 
         FIGS. 22C and 22D  are side elevation views of an illustrative VTOL aircraft. 
         FIGS. 23A and 23B  are side elevation views of an illustrative VTOL aircraft and show pivoting arms that connect the rotor units to the fuselage. 
         FIGS. 24A and 24B  are top views of an illustrative VTOL aircraft and show a cluster of rotor units. 
         FIGS. 25A and 25B  are top views of an illustrative VTOL aircraft using non-ducted rotors. 
         FIG. 26  is a side elevation view of another aircraft using rotor units coupled to a rotatable arm. 
         FIG. 27  is a side view of an illustrative VTOL aircraft performing an example degraded rotor landing protocol from a hover mode. 
         FIGS. 28A-C  are perspective views of another illustrative VTOL aircraft configured to perform a degraded rotor landing protocol.  FIG. 28A  illustrates the VTOL aircraft in a forward flight mode.  FIG. 28B  illustrates the VTOL aircraft in a hover mode.  FIG. 28C  illustrates the VTOL aircraft in a vertical configuration while performing the degraded rotor landing protocol. 
         FIGS. 29A-C  are perspective views of yet another illustrative VTOL aircraft configured to perform a degraded rotor landing protocol.  FIG. 29A  illustrates the VTOL aircraft in a forward flight mode.  FIG. 29B  illustrates the VTOL aircraft in a hover mode.  FIG. 29C  illustrates the VTOL aircraft in a vertical configuration while performing the degraded rotor landing protocol. 
         FIG. 30  is a perspective view of an example tilt mechanism configured to adjust a rotor of a VTOL aircraft between a hover configuration to a forward flight configuration. 
         FIGS. 31A and 31B  are side views of an example tilt mechanism configured to adjust a rotor of a VTOL aircraft. 
         FIG. 32  is a flow diagram of an illustrative process of a landing protocol associated with a degraded rotor system. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure is directed, in part, to a vertical takeoff and landing (VTOL) aircraft that includes fixed wing flight capabilities. The VTOL aircraft may include thrust producing rotors located on opposite ends of a fuselage and along a longitudinal axis of the fuselage. The rotors may be electric ducted fans. However, other power sources may be used such as gas powered rotor units. Each rotor unit may be coupled to the fuselage via a rotating or pivoting chassis, which allows each rotor unit to provide directed thrust by movement of the rotor unit about at least one axis. By moving the rotor units, the aircraft can transition from a hover mode to a transition mode and then to a forward flight mode and back. The aircraft may take off without use of an airstrip by liftoff into the hover mode. While in the hover mode, the rotors may generate thrust that is directed primarily in a downward direction relative to the fuselage. Once the aircraft has gained altitude after sustained flight in the hover mode, the rotor units may transition, during the transition mode, to a forward-flight mode where the rotors generate centerline trust directed primarily toward the aft of the fuselage. The rotor units may later transition back to the hover mode to enable the aircraft to land without use of an airstrip. 
     The aircraft may include fixed wings to enable prolonged flight with minimal thrust from the rotors. In some embodiments, the aircraft may include a canard wing at the fore end of the fuselage and a delta-shaped main wing at the aft end of the fuselage. However, other wing configurations may be used as discussed below. The wings may include ailerons, flaps, flaperons, and any other control surfaces necessary to control the aircraft in forward flight. In some embodiments, the rotors may provide directed lateral thrust to assist in yaw and roll control of the aircraft. 
     In accordance with various embodiments, the VTOL aircraft may employ a flight control computer that controls and sustains flight in each of the flight modes (hover mode, transition mode, and forward-flight mode). Operation of the aircraft may be performed by providing simple directional commands from operator controls to the flight control computer, which in turn executes the commands while taking other necessary action to sustain flight and/or avoid objects in the surrounding environment. Thus, control of the aircraft from the operator&#39;s perspective may be akin to control of an aircraft in a video game, and may be made possible with minimal training or aviation expertise. In some embodiments, the VTOL aircraft may be an unmanned aerial vehicle (UAV) or a remote-controlled aircraft. 
     In various embodiments, the flight control computer (and/or control management system) of the VTOL aircraft may include instructions to cause performance of a landing protocol in the event that a rotor is operating in a degraded state (e.g., experiences degraded functionality or ceases operation). The degraded state may include the rotor producing zero thrust (i.e. rotor inoperative) or the rotor producing less thrust than required to maintain substantially level flight in a particular regime (i.e., forward flight mode, transition mode, or hover flight mode). 
     In some embodiments, the flight control computer may recognize that a degraded rotor is operating in a degraded state, and may initiate the landing protocol. The landing protocol may include one or more operable rotors providing vertical thrust while the aircraft assumes a substantially vertical configuration due to the loss of thrust from the degraded rotor. In some embodiments, the flight control computer of the aircraft may be configured to adjust an orientation of the one or more operable rotors to provide substantially vertical thrust. In such embodiments, the orientation adjustment may include an adjustment up to 180 degrees. The one or more operable rotors may increase or maximize a thrust output to slow a descent of the aircraft. In some embodiments, the thrust provided by the one or more operable rotors may be sufficient to stop descent or limit a rate of descent of the VTOL aircraft to less than a rate of freefall (e.g., less than 32 feet-per-second-per-second (fps 2 )). In some embodiments, the flight control computer may be configured to identify a power source for the degraded rotor, and route excess power to one or more operable rotors, such as from a battery associated with the degraded rotor. In some embodiments, the flight control computer may be configured to couple, switch, or isolate power sources to provide the power required to perform the degraded rotor landing protocol. 
     In some embodiments, the one or more operable rotors may slow a rate of acceleration of the aircraft due to gravity (e.g., 20 fps 2 , 15 fps 2 ). In various embodiments, the flight control computer may control a descent of the VTOL aircraft at a pre-determined rate (e.g., 500 fpm, 300 fpm, 100 fpm, etc.). In some embodiments, the flight control computer may control the descent of the VTOL aircraft based on an amount of power available to the operational rotor. In the controlled descent, a first end of the aircraft (i.e., the nose or the tail) may contact a landing surface. In some embodiments, the flight control computer may cause the one or more operable rotors adjust a thrust vector at or close to the landing surface, to change a pitch of the aircraft such that the aircraft impacts the ground with a canopy being generally upright. In some embodiments, the adjustment to the thrust vector may cause the VTOL aircraft to rotate onto the landing gear. Accordingly, the landing protocol may be initiated to safely land the VTOL aircraft canopy side up in the event of a degraded motor and/or rotor system. 
     The apparatuses, systems, and techniques described herein may be implemented in a number of ways. Example implementations are provided below with reference to the following figures. 
       FIGS. 1A and 1B  are perspective views of an illustrative vertical takeoff and landing (VTOL) aircraft. 
       FIG. 1A  is an isometric view of an illustrative VTOL aircraft  100  while the VTOL aircraft is in the hover mode. The aircraft in  FIG. 1  has a fuselage  102  with a fore end  104 , an aft end  106 , a top side  108 , and a bottom side  110 . The VTOL aircraft  100  may include a cockpit  112  located within the top side  108  of the fuselage  102 . The cockpit  112  includes space for at least one operator to sit for manned flight. In some embodiments, the cockpit  112  may include space for one or more passengers and/or other payload. In some embodiments, the VTOL aircraft  100  may be equipped for unmanned flight. The cockpit  112  may include a canopy  114  that provides a weatherproof and relatively quiet environment. The canopy  114  may be formed of acrylic, plastic, glass, or any other transparent material. 
     The VTOL aircraft  100  includes at least two rotor units  116 , including a fore rotor unit  116 ( 1 ) and an aft rotor unit  116 ( 2 ) that produce thrust for propulsion in a push-pull configuration. The rotor units  116  are located along a longitudinal axis  118  of the fuselage  102 . The rotor units may be located along a same plane or along different planes (e.g., one rotor unit located closer to the top side  108  of the fuselage  102  than the other rotor unit, etc.). The rotor units  116  may be powered by electric motors or by gas powered engines. In some embodiments, a hybrid system may be used to convert liquid fuel to electrical energy to drive electric motor powered rotors and/or a fuel powered generator may provide electrical power to electric motors. 
     In accordance with one or more embodiments, the rotors  116  may be counter-rotating, in which the rotor of the fore rotor unit  116 ( 1 ) spins clockwise, while the rotor of the aft rotor unit  116 ( 2 ) spins counter-clockwise, or vice versa. The rotor units  116  may consist of a rotor vane, or both a rotor vane and a straightening vane (stator). In some embodiments that include both the rotor vane and the straightening vane, the rotor vane on both the fore rotor unit and aft rotor unit may rotate in the same direction. The straightening vane may be implemented with a stator that is fixed in place and supports the rotor vane. The straightening vane may be comprised of a plurality of aerodynamic blades, configured to counter the vortex imparted on the thrust from the rotor vane. 
     The push-pull configuration of the rotor units  116  may allow for greater controllability when only one rotor unit provides thrust in the forward-flight mode because the rotor units  116  are both located along the longitudinal axis  118  and in substantial alignment with direction of forward flight. Therefore, there may be minimal imparted yaw or roll on the VTOL aircraft  100  when only one rotor unit provides forward thrust. The VTOL aircraft  100  may operate in a push mode, in which the fore rotor unit  116 ( 1 ) provides little or no thrust or a pull mode, in which the aft rotor unit  116 ( 2 ) provides little or no thrust. The VTOL aircraft may also operate in a glide mode, in which the rotor units  116  provide little or no thrust. 
     In some embodiments, the rotor units  116  are equipped with a plurality of blades surrounded by a duct (ducted rotor units). Ducted rotor units may provide a high thrust-to-diameter ratio compared to an open propeller. Ducted rotor units are also very efficient because the duct reduces losses in thrust due to tip vortices. The reduction of tip vortices increases the efficiency of both rotor units due to reduced drag on the tip of each blade while operating within a duct. Furthermore, the reduction of tip vortices increases the efficiency of the aft rotor unit due to decreased turbulent air flow from the fore rotor unit. This increased efficiency provides an increase in thrust from the rotor unit during the forward flight mode. 
     In various embodiments, the rotors may not include the duct (non-ducted rotors). When the VTOL aircraft is configured with non-ducted rotors, the fuselage  102  and/or the canopy  114  may be equipped with vanes to guide laminar airflow directly to the aft rotor unit  116 ( 2 ). In some embodiments, a dissipation mechanism may be used to divert turbulent air away from the aft rotor unit  116 ( 2 ), thereby increasing laminar air flow to the aft rotor unit  116 ( 2 ) and increasing the efficiency of the aft rotor unit  116 ( 2 ). 
     In some embodiments, the rotor units  116  may include a housing, a stator coupled to an interior of the housing, a plurality of blades, and an electric motor coupled to the stator and coupled to the plurality of blades. The housing (when ducted) guides airflow through the rotor unit, and increases the efficiency of each rotor unit. The rotor units  116  may be approximately the same size. However different sized rotor units may be used to balance a payload of the VTOL aircraft  100  for flight in the hover mode. 
     Each rotor unit ( 107 ) may be mounted in a rotating chassis  120 . The rotating chassis  120  for the fore rotor unit  116 ( 1 ) may be coupled to the fore end  104  of the fuselage  102 , possibly between or within a fore wing  122  (such as a canard wing). The rotating chassis  120  for the aft rotor unit  116 ( 2 ) may be attached to the aft end  106  of the fuselage  102 , possibly between or within an aft wing  124 . In some embodiments, the rotating chassis may be a gimbaled chassis or a pivoting chassis. 
     The rotating chassis  120  may allow for at least partial freedom of movement of the rotor unit about the longitudinal axis  118  and a lateral axis  126  of each of the rotor units  116 . The rotor units  116  may move independently of one another, based on a signal from a control management system. The control management system processes signals from an operator (on board operator, remote operator, or both) and a plurality of stability augmentation sensors, and sends an output signal to each of the rotor units  116 , directing each of the rotor units to move about the longitudinal axis and the lateral axis as necessary. 
       FIG. 1B  shows the illustrative VTOL aircraft  100  in a forward-flight mode. The rotor units  116  may move about the lateral axis  126  to transition from the hover mode (shown in  FIG. 1A ), in which the rotor units  116  direct thrust substantially perpendicular to the longitudinal axis  118  and the lateral axis  126 , through a transition mode, to the forward flight mode shown in  FIG. 1B , in which the rotor units  116  direct thrust substantially in line with the longitudinal axis  118 . The movement about a vertical axis that is perpendicular to the longitudinal axis  118  and the lateral axis  126  may provide directed thrust for yaw control during forward flight. In some embodiments, the rotor units  116  may also include a vane at the exhaust, which may operate to direct thrust from the rotor units  116 . 
     The VTOL aircraft  100  may comprise a monocoque structure, made of carbon fiber, titanium, aluminum, or any other material appropriate for aircraft construction. In another embodiment, the structure may comprise a semi-monocoque design, with a shell and longerons made of carbon fiber, titanium, aluminum, or any other material appropriate for aircraft construction. The aircraft skin is a low friction surface that may include built-in solar cells. 
     In some embodiments, the fore wing  122  may be a canard wing and the aft wing  124  (main wing in this configuration) may be a delta wing. However, other wing configurations may be used and are described below with reference to some illustrative aircraft. The fore wing  122 , coupled to the fore end  104  of the fuselage  102 , may act as a lifting surface and provide pitch stability in the forward-flight mode. The canard wing may have control surfaces  130  such as ailerons, to allow for maneuverability about the longitudinal axis  118  of the VTOL aircraft  100 . The ailerons, elevators, rudder are actuated in normal fashion during forward flight 
     The aft wing  124  is attached to the aft end  106  of the fuselage  102  and may be a lift-generating airfoil. The aft wing  124  may be a conventional shaped airfoil, a delta wing, or an ovular shaped wing. The aft wing  124  may be equipped with control surfaces  132 , comprised of a set of any of the following alone, or in combination: ailerons, flaps, flaperons, and any other control surface which may enable the operator to control the roll, and/or speed of the VTOL aircraft  100  while in the forward-flight mode. The control surfaces  132  on the aft wing  124  may also be configured to move simultaneously in the same direction, thereby providing pitch control of the VTOL aircraft  100  while in forward flight. 
     The VTOL aircraft  100  may include ventral fins  134  that extend outward from the bottom side  110  of the fuselage  102  near the aft end  106 . The ventral fins  134  may provide directional stability (e.g., yaw and pitch stability). In some embodiments, the ventral fins  134  may house and/or support landing gear and/or control surfaces, and also serve as energy absorbing devices during hard landings. The landing gear may be implemented as in a tricycle-style configuration that includes two aft struts and one fore strut which may or may not be retractable after take-off in order to reduce drag. 
     A fixed or movable portion of the fuselage or body surface  136  may enhance inlet airflow entering the rotor units during conversion from hover to forward flight. This may improve the airflow entering the ducts giving better efficiency with less turbulence and/or drag from inlet lip flow separation. 
       FIG. 2  is a schematic diagram showing a transition of flight of an illustrative VTOL aircraft from a hover mode  202  to a transitional mode  204  to a forward-flight mode  206 . 
     The hover mode  202  may be initiated prior to takeoff. In the hover mode  202 , the rotor units  116  may be substantially parallel to the ground. Landing gear  208  may be deployed. The control management system may make rapid micro and/or macro adjustments to the thrust level and direction of each of the rotor units  116  to support flight in the hover mode  202 . The operator may direct the aircraft in the hover mode  202 , which may include providing directional data to the control management system. After take-off into the hover mode  202 , the operator may direct the aircraft to remain in the hover mode, or start a transition to forward flight, via the transition mode  204 . 
     The transition mode  204  may be a temporary mode used to transition between the hover mode  202  and the forward-flight mode  206 . However, the VTOL aircraft  100  may sustain prolonged flight in the transition mode  204 . The transition mode may be controlled largely by the control management system. In the transition mode  204 , the rotor units  116  may rotate substantially simultaneously from a position that directs trust downward toward the ground (used in the hover mode  202 ) to a position that directs thrust rearward to cause forward flight. In the transition mode  204 , the fore wing  122  and the aft wing  124  may begin to generate some lift, and the control surfaces  130 ,  132  may become effective to control the pitch and roll of the VTOL aircraft  100 . The VTOL aircraft  100  initiates the forward-flight mode  206  once airspeed of the VTOL aircraft  100  has reached a threshold airspeed and the wings provide lift. In some embodiments, the thrust from one or more of the rotor units  116  may be significantly reduced once the VTOL aircraft  100  begins operation in the forward-flight mode because the lift provided by the wings may replace the lift generated by the rotor units  116  during the hover mode  202  and during the transition mode  204 . 
     In the forward flight mode  206 , the rotor units  116  may operate in a push-pull capacity to generate thrust and propel the aircraft forward. In the forward flight mode  206 , the rotor units may provide directed thrust under control of the operator and/or the control management system. The directed thrust may increase the stability of the aircraft in forward flight mode. 
       FIGS. 3A-3D  show the illustrative VTOL aircraft  100  in various operational modes.  FIG. 3A  is a side elevation view of an illustrative VTOL aircraft with extended landing gear. The VTOL aircraft  100  may include the landing gear  208  that is fixed or retractable. The aft landing gear may be coupled to the ventral fins  134 . The landing gear may be implemented as in a tricycle-style configuration that includes two aft struts and one fore strut which may or may not be retractable after take-off in order to reduce drag. However, other configurations of the landing gear  208  may be used depending on the type of wings used in the VTOL aircraft  100 . For example, when the VTOL aircraft  100  is configured with the fore wing  122  as a main wing, then the landing gear may include two fore struts and one aft strut (tail dragging configuration). 
     The fore wheel  302  may extend and retract from the fuselage  102 . The fore wheel  302  may include a fore strut  304  that provides some shock absorption for the fore wheel  302 . In some embodiments, the ventral fins  134  may be coupled to an aft strut or may act as the aft strut. 
       FIG. 3A  also shows the rotation of the rotor units  116 , thereby enabling the transition from hover mode  202  to forward flight mode  206 , and vice versa, as described above.  FIG. 3A  also shows a rotor unit configuration whereby conventional launches and landings may be accomplished, as the rotating ducts are clear of the ground plane in forward flight mode. 
       FIG. 3B  is a side elevation view of the illustrative VTOL aircraft  100  in the hover mode  202 .  FIG. 3B  shows the fore rotor unit  116 ( 1 ) being located on a lower plane than the aft rotor unit  116 ( 2 ). However, the fore rotor unit  116 ( 1 ) may be located on a higher plane or a same plane as the aft rotor unit  116 ( 2 ). Turbulent airflow from the fore rotor unit  116 ( 1 ) may be at least partially diverted away from the aft rotor unit based on the placement of the rotor units with respect to the fuselage while the rotor units  116  are still situated along the longitudinal axis  118 . The diversion of airflow allows more undisturbed air to reach the aft rotor unit  116 ( 2 ), possibly increasing an efficiency of the aft rotor unit  116 ( 2 ). 
     As previously discussed, the VTOL aircraft  100  may include a cockpit  112  mounted on the top side  108  of the fuselage  102 . The cockpit  112  may includes space for an operator to sit for manned flight. The cockpit  112  may include hardware, controls, the control management system (or portions thereof), and/or other control, navigational, or flight equipment. In some embodiments, the VTOL aircraft  100  may be equipped for unmanned flight, and therefore may or may not include the cockpit  112 . 
       FIG. 3C  is a side elevation view of an illustrative VTOL aircraft  100  shown in  FIG. 3B  and shows airflow  306  through the rotor units  116  during vertical flight in the hover mode  202 . The airflow  306  may create a low pressure volume  308  above the rotor units  116  and a high pressure volume  310  below the rotor units  116 . The low pressure volume  308  sustains flight by drawing the respective portion of the aircraft toward the low pressure volume  308 . The low pressure volume  308  and the high pressure volume  310  create a pressure differential across the airflow  306 . The pressure differential creates the lift needed to keep the aircraft airborne, while the rotor units  116  move air to create thrust. The higher location of the low pressure volume  308  provides stability to flight of the VTOL aircraft  100  that is akin to attachment of ends of a hammock that create stability to a body supported by the hammock. A low center of mass  312  relative to the pressure centers  314  of the rotor units creates inherent stability via the pendulum effect. 
     The airflow  306  through the rotor units  116  lifts the VTOL aircraft  100  into a hover. The airflow  306  is determined based in part on a speed of rotation of the rotors  116 , a pitch of the blades in the rotor units, and other factors. The rotors  116  may rotate at a same speed during hover, or they may rotate at a different speed during hover. The speed of each of the rotors  116  is determined by the control management system based on operator input and sensor input of stability augmentation sensors. The fore rotor unit  116 ( 1 ) and the aft rotor unit  116 ( 2 ) may be the same size or may be different sizes. The rotor units  116  produce enough lift to maintain a hover in and out of ground effect. 
       FIG. 3D  is a side elevation view of the illustrative VTOL aircraft  100  in the forward flight mode  206  and shows airflow  316  through the rotor units  116  during forward flight. As the operator directs, the VTOL aircraft  100  may transition from the hover mode  200  to the forward-flight mode  206 . The airflow  316  includes a boundary airflow having some laminar and some turbulent airflow. Various considerations may be employed to reduce turbulent airflow into the aft rotor unit  116 ( 2 ). Vanes may be added to wings and/or the fuselage  102  (or other locations) to provide laminar airflow to the aft rotor unit  116 ( 2 ). A dissipation mechanism may divert turbulent air away from the aft rotor unit  116 ( 2 ). 
     In some embodiments, forward flight may be powered by a single rotor unit, such as the fore rotor unit  116 ( 1 ), which may negate a desire to reduce turbulent airflow in some regions around the VTOL aircraft  100  during forward flight. 
     In the forward flight mode  206 , the rotor units  116  provide thrust that is directed substantially parallel to the flight path. The rotor units  116  may operate in a push-pull capacity, with the aft rotor unit  116 ( 2 ) pushing the aircraft forward, and the fore rotor unit  116 ( 1 ) pulling the aircraft forward. The forward-flight airflow generates a pressure differential between the top of the aircraft, fuselage and wings, and the bottom. The pressure differential creates the lift needed to keep the aircraft airborne, while the rotor units push and pull the air through to create thrust for forward movement. In various embodiments, the lift may be generated largely by the wings during flight in forward-flight mode  206  while the lift may be largely or solely generated by thrust of the rotor units  116  during flight in the hover mode  202 . 
       FIG. 4  is a top view of the illustrative VTOL aircraft  100  shown in  FIG. 3B  and shows the rotor units  116  in the hover mode  202 . The VTOL aircraft  100  is shown with the fore wing  122  as a canard wing and the aft wing  124  as a delta wing that is a main wing. The rotor units  116  are aligned with the longitudinal axis  118 . In some embodiments, the fore wing  122  may include the fore control surfaces  130  that rotate an entire portion of the fore wing. However, the fore wing  122  may employ traditional ailerons and/or other control surfaces, vanes, and/or other air directing features. 
       FIG. 5  is a top view of the illustrative VTOL aircraft shown in  FIG. 1A . The VTOL aircraft  100  is shown with the rotor units  116  in the forward-flight mode  206 . The fore control surfaces  130  show a configuration with traditional ailerons. 
       FIG. 6A  is a top view of the illustrative VTOL aircraft  100  shown in  FIG. 3B  and shows various components and compartments including control services and storage compartments. The VTOL aircraft  100  may include some or all of the features described below, in accordance with various embodiments. 
     The aft wing  124  may be a main wing and may provide a lifting surface during flight in the forward-flight mode  206 . The aft wing  124  may include ailerons  602 , which provide roll control during flight in the forward-flight mode  206  and possibly during flight in the transition mode  204 . In some embodiments, the aft wing  124  may include flaps or flaperons for speed and roll control during flight in the forward-flight mode  206  and possibly during flight in the transition mode  204 . 
     In some embodiments, the aft wing  124  may include wing tip thrusters  604  to provide roll stability during flight in the hover mode  202  and possibly during flight in the transition mode  204 . The wing tip thrusters  604  may comprise an exhaust connected to compressed air, which releases high pressure air downward, substantially perpendicular to the aft wing  124 . In some embodiments, the wing tip thrusters may be electrically driven rotors mounted in the aft wing  124  and/or possibly mounted on the fore wing  122 , which is described in more detail with reference to  FIG. 16 . 
     The aft wing  124  may include a storage compartment  606  for storing compressed air, extra batteries, hardware, personal items, and/or other payload. The aft wing  124  may comprise a hinge  608  that enables folding of the aft wing  124  during storage of the VTOL aircraft  100 . The hinge  608  may enable folding an outer section of the wing against or toward an inner section of the wing (or vertically) to reduce a width of the aft wing  124 . A folding mechanism  610  may be used to transition the wing from a folded position to a flight position (via rotation about the hinge  608 ). The folding mechanism  610  may include a motor to aid in lifting the outer section of the aft wing  124 . The folding mechanism  610  may lock and unlock the aft wing  124 . The folding mechanism  610  may transmit a signal to indicate whether the aft wing  124  is locked or unlocked and/or folded or not folded. The hinge  608  and the folding mechanism  610  may be implemented on the fore wing  122 . For example, when the fore wing  122  is the main wing, then the hinge  608  and the folding mechanism may be located on the fore wing  122 . 
     In accordance with one or more embodiments, the VTOL aircraft  100  may include one or more stability augmentation sensors  612  that monitor a position, angle, acceleration, and/or orientation of a portion of the VTOL aircraft. For example, the stability augmentation sensors  612  may comprise gyroscopes and/or accelerometers that monitor the pitch, roll, and yaw of the VTOL aircraft  100 , and changes of each over time. The stability augmentation sensors  612  may provide input via signals to a control management system  614 . 
     The control management system  614  may process the signals from the stability augmentation sensors  612 , as well as input from an operator (pilot) via operator controls, to determine how to direct and power the rotor units  116  to maintain flight in the hover mode  202 , the transition mode  204 , and the forward-flight mode  206 . For example, the stability augmentation sensors  612  may detect a sudden tilt of the aircraft due to a gust of wind or other force exerted on the VTOL aircraft  100 . In response, the stability augmentation sensors  612  may transmit a signal to the control management system  614 . The control management system  614  may cause repositioning of one or more of the rotor units  116  to cause the one or more rotor units to direct a thrust to counter the force of the wind, etc., and thus stabilize flight of the VTOL aircraft  100 . Meanwhile, the control management system  614  may also process commands from the operator. The control management system  614  may prioritize control input from the stability augmentation sensors  612  and the operator to maintain stable flight. For example, when an operator provides a command that may compromise sustained flight, the control management system  614  may ignore the command and/or only execute the command for a limited duration before issuing another command (possibly in response to a signal from the stability augmentation sensors  612 ) to take action to sustain flight. 
     The operator, using flight controls, sends input signals to the control management system  614 . In turn, the control management system receives the signals from the flight controls and/or the stability augmentation sensors  612 . The control management system  614  prioritizes the inputs from the stability augmentation sensors  612  and inputs from the operator and then adjusts a direction/orientation and/or power level of each rotor unit. The control management system  614  may also determine the position of the ailerons  602  and/or other control services for roll control during flight in the forward-flight mode  206  and possibly during flight in the transition mode  204 . However, in some embodiments, the ailerons  602  and/or other control services may be controlled mechanically, such as by use of cables and levers, and thus bypass use of the control management system  614 . During flight in the hover mode  202 , the control management system  614  may send signals to the wing tip thrusters  604  to provide for directed thrust to cause or counter roll control. 
     The rotors  116  may be driven by electric motors  616  located at a hub of each of the rotors  116 . The electric motors  616  may be brushless magnet motors. The electric motors  616  may be powered by one or more battery in a battery set  618 . The battery set  618  may be implemented using one or more cells that are placed in one or more locations in the VTOL aircraft  100 . Placement of the battery or batteries may include consideration of a distribution of a payload such that the center of mass is substantially near a midpoint between the rotor units. Thus, the battery or batteries may be placed in other locations throughout the VTOL aircraft to distribute the weight of the battery or batteries. The battery  118  may be in fluid communication with a cooling duct  620 , which may pass external air and/or fluid past the battery  118  to reduce heat generated by the battery  118 . A wiring grid  622  may connect the battery set  618 , the control management system  614 , and a power management system  624 . 
     The power management system  624  may determine, monitor, and manage battery power of the battery set  618 . The power management system  624  may communicate battery power and/or other information to the control management system  614 , which may then use this information when determining how to control the VTOL aircraft (e.g., determining direction and amount of thrust, transmitting warning messages to an operator, causing and/or preventing modes of flight, etc.). For example, when the power management system  624  indicates that the battery power is depleted or close to being depleted (e.g., past a threshold amount), the control management system  614  may provide a signal to the operator to land the VTOL aircraft using an airstrip while sustaining flight in the forward-flight mode  206  because the battery set  618  may not be capable of powering the rotor units  116  to provide sustained flight in the hover mode  202 . 
     In some embodiments, the power management system  624  may provide for the operator to choose a power conservation mode, in which one rotor provides all of the thrust for the aircraft in a pull mode (e.g., the fore rotor unit  116 ( 1 )) or a push mode (e.g., using the aft rotor unit  116 ( 2 )). 
       FIG. 6B  is a top view of the illustrative VTOL aircraft  100  shown in  FIG. 6A  and shows various components to provide reserve and/or emergency power. The various components may be selectively included and/or used to provide power to the electric motors  616  and/or charge the battery set  618 . 
     In accordance with some embodiments, the VTOL aircraft  100  may be equipped with solar cells  626  in communication with the power management system  624  via the wiring grid  622 . The solar cells  626  may enable charging the battery set  618  while the VTOL aircraft is in flight and/or while the VTOL aircraft  100  is not in flight (e.g., stored in a sunny spot, etc.). The solar cells  626  may be mounted on the main wings (e.g., the aft wing  124  in  FIG. 6B ). However, the solar cells  626  may be mounted on any surface of the VTOL aircraft  100 , or the solar cells ( 709 ) may be integrated in at least a portion of the skin of the VTOL aircraft  100 . 
     In various embodiments, the VTOL aircraft may include a drag alternator  628  that deploys a fan, blade, or wing. The fan, blade or wing may cause motion which may in turn cause the drag alternator to generate a current. The drag alternator  628  may be in communication with the battery  118  and the power management system  624  via the wiring grid  622 . The current generated by the drag alternator  628  may be used to charge the battery set  618 . The fan, blade, or wing of the drag alternator may be stored within the aircraft skin when not in use to negate drag. Upon signal from the operator, the power management system  624 , and/or the control management system  614 , the fan, blade, or wing may be deployed to generate the current that may be converted to DC power to charge the battery set  618 . Use of the drag alternator may be reserved to emergency situations where the VTOL aircraft is able to deploy the drag alternator  628  during glided flight or other situations where the additional drag from the fan, blade, or wing does not hinder flight of the VTOL aircraft  100 . 
       FIG. 7  is a perspective view of a VTOL unmanned aerial vehicle (UAV)  700 . The VTOL UAV  700  may include a recessed fuselage  702  located between one or more wings. In some embodiments, the VTOL UAV  700  may include a single wing  704  (e.g., an ovular-shaped wing surrounding the recessed fuselage  702 ) and one or more stabilizers  706  to reduce aerodynamic side slip and increase directional stability in forward flight. However, the VTOL UAV  700  may only differ from the VTOL aircraft  100  in that the VTOL UAV  700  does not include a cockpit and operator controls located within the aircraft. The VTOL UAV  700  may include a control management system that may or may not receive input from an operator that is located remote from the VTOL UAV  700 . The VTOL UAV  700  may include a monocoque structure, made of carbon fiber, titanium, aluminum, or any other material appropriate for an aircraft frame. In some embodiments, the structure may be a semi-monocoque design, with a shell and longerons made of carbon fiber, titanium, aluminum, or any other material appropriate for an aircraft frame. The skin is a low friction surface that may include built-in solar cells, as discussed above. 
       FIG. 8  is a top view of the illustrative VTOL UAV  700  shown in  FIG. 7  and shows various components and compartments including locations of batteries and storage compartments. The VTOL UAV  700  includes the rotor units  116  that produce thrust for propulsion in a push-pull configuration. The rotor units  116  and/or other components may be configured in a similar manner as discussed above with reference to the VTOL aircraft  100 . For example, the VTOL UAV  700  may include one or more of the wing tip thrusters  604 , the stability augmentation sensors  612 , the control management system  614 , the electric motors  616 , the battery set  618 , the cooling duct  620 , the wiring grid  622 , the power management system  624 , the solar cells  626 , and/or the drag alternator  628 . 
     In some embodiments, the VTOL UAV  700  may include surveillance and/or operational components  802 , such as a plurality of cameras. For example, a camera may be located on the bottom of the fuselage and be capable of viewing objects, landscape, and so forth below the VTOL UAV  700 . Another camera (or the same camera, may be able to view objects, landscape, and so forth in front of and/or behind the aircraft. Other cameras and/or surveillance components (e.g., radios, thermo sensors, etc.) may be employed by the VTOL UAV  700 . The surveillance and/or operational components  802  may transmit signals to the operator or other person or device located remote from the VTOL UAV  700  via transmission of data using a radio, optics, or other transmission techniques. 
       FIG. 9A  is a front elevation view of the illustrative VTOL aircraft  100  shown in  FIG. 3B  and shows the fore rotor  116 ( 1 ) configured to provide thrust in a direction of the vertical axis  128  that bisects the fuselage  102  in symmetrical halves. Thrust from the fore rotor unit  116 ( 1 ) is directed substantially parallel to the vertical axis  128 . 
       FIG. 9B  is a front elevation view of the illustrative VTOL aircraft  100  shown in  FIG. 9A  and shows the fore rotor  116 ( 1 ) configured to provide thrust in a direction having an angle α 1  between a fore rotor unit thrust axis  902  and the vertical axis  128  that bisects the aircraft in symmetrical halves. The rotor unit thrust axis  902  is parallel to a direction of thrust generated by the fore rotor unit  116 ( 1 ). The aft rotor  116 ( 2 ) may be configured to provide thrust in a direction having an angle α 2  between an aft rotor unit thrust axis  904  and the vertical axis  128 . As discussed above, the control management system  614  may cause the rotor units to move via the rotating chassis  120  to provide stabilized flight in response to signals from the stability augmentation sensors  612  and/or inputs from an operator. Thus, the rotor units  116  may move independently of one another and under control of the control management system  614  to maintain flight of the VTOL aircraft  100 . The freedom of movement of the rotor units described herein aids in maintaining stable flight in the hover mode  202 . 
       FIG. 9C  shows the rotor units rotated negatively beyond 90 degrees to rapidly slow the aircraft prior to landing. In this configuration, the thrust generated from the rotor units is directed in the direction of flight, thereby slowing the forward airspeed of the aircraft. 
       FIG. 10  is a partial top view of the illustrative VTOL aircraft  100  shown in  FIG. 1A  and shows the fore rotor  116 ( 1 ) configured to provide directed thrust in a direction between an angle β between the fore rotor unit thrust axis  1002  and the longitudinal axis  118  of the fuselage  102 . As discussed above, the control management system  614  may cause the rotor units to move via the rotating chassis  120  to provide directed thrust in response to signals from the stability augmentation sensors  612  and/or inputs from an operator. Thus, the rotor units  116  may move independently of one another and under control of the control management system  614  to maintain flight of the VTOL aircraft  100 . The freedom of movement of the rotor units described herein allows for directed thrust laterally during flight in the forward-flight mode  206  and possibly during flight in the transition mode  204 . 
       FIGS. 11A-11C  are isometric views of an illustrative rotating assembly  1100  that moveably couples a rotor unit to the fuselage  102  of the illustrative VTOL aircraft  100 . 
       FIG. 11A  shows the illustrative rotating assembly  1100  including a rotor  1102  with an electric motor  1104  located at the center of the rotor unit  1102 . The rotor  1102  is mounted in a rotating chassis  1106 , which allows rotation about its lateral and longitudinal axes. The rotating chassis  1106  may include an inner frame  1110  connected to a beam  1108  (e.g., stator), which supports the rotor  1102  and the electric motor  1104 . The dashed line depicts the rotor unit in the transition mode  204 . 
       FIG. 11B  shows the illustrative rotating assembly  1100  with an outer frame  1112  supporting the inner frame  1110 . The outer frame  1112  and the inner frame  1110  are rotatable with respect to each other about the longitudinal and lateral axes, as shown in  FIG. 11C . The fuselage  102  of the VTOL aircraft  100  may provide the outer frame  1112 , or the outer frame  1112  may be coupled to the fuselage  102 . In some embodiments, the rotating chassis may be a gimbaled chassis and/or a pivoting chassis. 
       FIGS. 12A-12C  are isometric views of an illustrative mechanism  1200  to cause movement of the rotor  1102  via the rotating assembly  1100  in accordance the  FIGS. 9B and 10 . 
       FIG. 12A  shows a first drive assembly  1202  that includes a first motor  1204  and a gear  1206  that causes movement of the rotor unit  1102 . The first drive assembly  1202  may be under control of the control management system  614 , which may selectively power the first motor  1204  in either direction to cause transition of the rotor  1102  about a first axis (e.g., such as the transition shown in  FIG. 9B  and  FIG. 10 ). The first drive assembly  1202  may include the inner frame  1110  described with reference to the rotating assembly  1100 . 
     A second drive assembly  1208  may include a second motor  1210  and pulley  1212  that causes movement of the rotor  1102  for tilt control along an axis. The second drive assembly  1208  may be under control of the control management system  614 , which may selectively power the second motor  1210  in either direction to cause transition of the rotor unit  1102  about a second axis (e.g., such as the transition shown in  FIG. 3A ). The second drive assembly  1208  may include the inner frame  1110  described with reference to the rotating assembly  1100 .  FIG. 12 b    shows a configuration that includes multiple instances of the mechanism  1200 , such as a first mechanism  1214  and a second mechanism  1216 .  FIG. 12C  shows the illustrative mechanism  1200  within the fuselage  102  of the VTOL aircraft  100 . 
       FIG. 13A  is a partial cutout isometric view of an illustrative rotor unit  1300 . The rotor unit  1300  (also referred to herein as a rotor and/or the rotor units  116 ) includes a rotor vane  1302 , a stator  1304 , and an electric motor  1306 . In some embodiments, the rotor unit  1300  may include a duct  1308 , which may be a housing that directs airflow through and from the rotor unit  1300 . The rotor vane  1302  may be rotated by the electric motor  1306 , and pulls air through the rotor unit to produce thrust. As the air flows through the rotor unit vane  1302 , a rotation is imparted on the air in the duct  1308 . The stator  1304  couples the rotor unit vane  1302  and electric motor  1306  to the duct  1308  (which also acts as a housing). 
     guide vanes may be arranged radially within each duct, and actuated to direct thrust 
     In some embodiments, the stator  1304  may include curved blades that provide structural support and act as a straightening vane. The straightening vane includes blades that are pitched in a direction opposite to the blades of the rotor unit vane  1302 . The opposite direction of the pitched blades of the straightening vane reduces the rotation imparted on the air by the rotor unit vane  1302 ; thereby resulting in exhaust air that is substantially less turbulent than without use of the straightening vane. However, a second vane  1302 ( 2 ) may be rotatable and a separate part from the stator  1304 . In some embodiments, the rotor unit  1300  may be a non-ducted rotor unit and may not include the duct  1308 . 
       FIG. 13B  is a side elevation view of the illustrative rotor unit  1300 . The electric motor  1306  may be housed between a fore cone  1310  and an aft cone  1316 , which is located on the side of the rotor unit that exhausts the thrust of air. The fore cone  1310  and the aft cone  1316  are aerodynamic features that reduce drag caused by the rotor unit  1300  during flight of the VTOL aircraft  100 . The fore and aft cones may be ventilated to improve motor cooling. 
       FIG. 14  is a side elevation view of an illustrative multi-vane rotor unit  1400 . The multi-vane rotor unit  1400  may include a fore rotor vane  1302 ( 1 ) and an aft rotor vane  1302 ( 2 ), a fore stator  1304 ( 1 ) and an aft stator  1304 ( 2 ), and a fore motor  1306 ( 1 ) and an aft motor  1306 ( 2 ). In some embodiments, the fore motor  1306 ( 1 ) and the aft motor  1306 ( 2 ) may cause rotation of the respective rotor unit vanes in opposite directions. The fore stator  1304 ( 1 ) may include straight blades while the aft stator  1304 ( 2 ) may be a straightening vane and may include blades curved opposite the curvature of the blades of the rotor unit vane  1302 ( 2 ). When more than one rotor vane is in each duct, rotors may rotate in opposite directions to counter vortex effect, and improve efficiency. 
       FIGS. 15A and 15B  are partial cutout isometric views of the illustrative rotor unit shown in  FIG. 13  and show thrust directing apparatuses.  FIG. 15A  shows the rotor unit  1300  with a rotating inner duct  1502  that is rotatably coupled to the inner duct.  FIG. 15B  shows the rotor unit  1300  with a rotating vane  1504  that is rotatably coupled to the inner duct. The rotating inner ring  1502  and the rotating vane  1504  may provide yaw and roll control during VTOL and horizontal flight in the forward-flight mode  206  and the transition mode  204 , and may provide yaw and roll control during flight in the hover mode  202 . During flight in the forward flight mode  206 , the ailerons (or other control surfaces) may control yaw and roll. 
       FIG. 16  is a top view of an illustrative VTOL aircraft  100  that includes wing tip rotors  1602 . The wing tip rotors  1602  may provide roll stability during flight in the hover mode  202  and possibly during flight in the transition mode  204 . The wing tip rotors  1602  may be located near a tip of the main wing. However, the wing tip rotors  1602  may be located in other locations to direct thrust outward towards the general direction of the wing tips and/or downward from the wingtips. 
     The wing tip rotors  1602  may be smaller sized rotors than the main rotors  116  described above. The wing tip rotors  1602  may also be thinner than the main rotors  116  such that a profile of the wing tip rotors does not project substantially above a top surface of the wing or below a bottom surface of the wing. In some embodiments, the wing tip rotors may not include the rotating chassis  120  and therefore may not be able to move to direct thrust in different directions. However, the wing tip rotors  1602  may be configured to include directional mechanisms, such as the rotating chassis  120 , the rotating inner duct  1502 , and/or the rotating vane  1504 . 
     In the hover mode, the thrust from the main rotor units  116  is directed down, allowing for the VTOL aircraft  100  to hover, both in and out of ground effect. The control management system  614  may vary the rotor speed in the hover mode  202  in order to maintain a level, balanced hover. The control management system  614  may also send signals to the rotor units  116 , directing the rotor units  116  to move slightly about the longitudinal and lateral axes to maintain a stable hover, or to move the VTOL aircraft  100  forward, aft, or laterally. Wing tip rotors  1602  may be controlled by the control management system  614  and may aid the main rotor units  116  in lift and in hover stability. 
     The wing tip rotors  1602  may generate thrust perpendicular to the VTOL aircraft  100  at varying intervals as necessary to maintain a stable hover. The power output of the wing tip rotors  1602  may be determined by the control management system  614 . In some embodiments, the wing tip rotors may include a cover that can be placed over the wing tip rotors  1602  during flight in the forward-flight mode  206  to reduce drag caused by the wing tip rotors  1602 . The wing tip rotors  1602  may have a cover on the top side and the bottom side of the wing which opens below a certain airspeed. The cover may be substantially flush with the aircraft skin in order to decrease drag during flight in the forward-flight mode  206  and the transition mode  204 . 
       FIGS. 17-25B  show various configurations of VTOL aircraft that utilize the rotor units  116  configured substantially as described above. The VTOL aircraft shown and described below may be implemented with an on-board operator (pilot) and/or in a UAV configuration that may or may not include a cockpit. 
       FIG. 17  is a top view of an illustrative VTOL aircraft  1700  having a shortened fuselage and rotor units of differing sizes selected to balance a payload of the VTOL aircraft. When a center of mass of expected payload of the VTOL aircraft  1700  is located closer to the aft end  106 , then the aft rotor unit  116 ( 2 ) may be a larger or more powerful rotor unit than the fore rotor unit  116 ( 1 ) to enable stable flight in the hover mode  202 . The larger rotor unit may produce more thrust at the same revolution-per-minute speed. The VTOL aircraft  1700  may have a stronger push-type thrust from the aft rotor unit  116 ( 2 ) when the aft rotor unit is the larger rotor unit. 
     The VTOL aircraft  1700  may include shortened fuselage  1702  as compared to the fuselage  102  depicted in the VTOL aircraft  100  shown in  FIG. 1 . The shortened fuselage  1702  may allow for increased dissipation of airflow from the fore rotor unit  116 ( 1 ) resulting in increased efficiency of the aft rotor unit  116 ( 2 ). 
       FIG. 18  is a top view of an illustrative VTOL aircraft  1800  having a traditional wing configuration. The VTOL aircraft  1800  includes two lift generating surfaces, a fore wing  1802  and an aft wing  1804 . The fore wing  1802  provides the majority of the lift for the aircraft. The fore wing  1802  has ailerons  1806 , which allow the operator to maneuver the aircraft about the longitudinal axis. The aft wing  1804  acts as a lifting surface and provides pitch stability. The aft wing  1804  has elevators  1808 , which allow the operator to maneuver the aircraft about the lateral axis, to pitch up and down. 
     The VTOL aircraft  1800  has two rotor units mounted in a rotating chassis along a longitudinal axis of a fuselage  1810 . In this embodiment, the fore rotor unit  116 ( 1 ) has a larger diameter than the aft rotor unit  116 ( 1 ). The larger rotor unit may produce more thrust at the same revolution-per-minute speed. The configuration show in  FIG. 18  may result in a stronger pull-type thrust from the fore rotor unit  116 ( 1 ). 
     The VTOL aircraft  1800  may include shortened fuselage  1810  as compared to the fuselage  102  depicted in the VTOL aircraft  100  shown in  FIG. 1 . The shortened fuselage  1810  may allow for increased dissipation of airflow from the fore rotor unit  116 ( 1 ) resulting in increased efficiency of the aft rotor unit  116 ( 2 ). 
       FIG. 19  is a top view of an illustrative VTOL aircraft  1900  where the fore rotor is located between the aft rotor and a cockpit. The cockpit  112 , when included, may located forward of the fore rotor unit  116 ( 1 ). In some embodiments, the cockpit  112  may be located behind the aft rotor  116 ( 2 ), which may be another alternative configuration. 
       FIG. 20  is a top view of an illustrative multi-occupant VTOL aircraft  2000 . The multi-occupant VTOL aircraft  2000  may effectively have two fuselages  2002  with a fore end and an aft end, two main wings, and two rotor units  116 . The fuselages  2002  may be configured substantially parallel to each other, and may be large enough to hold passengers or other payload. The two fuselages  2002  may be coupled by an aerodynamic strut  2004  in substantially the center of mass of the aircraft, and between the fore and aft rotor units. 
       FIGS. 21A and 21B  are side elevation views of another illustrative multi-occupant VTOL aircraft  2100 .  FIG. 21A  shows the multi-occupant VTOL aircraft  2100  in the hover mode  202 .  FIG. 21B  shows the multi-occupant VTOL aircraft  2100  in the forward-flight mode  206 . The multi-occupant VTOL aircraft  2100  may include a single fuselage  2102  that may be used to transport passenger or payload. The VTOL aircraft  2100  may use gas powered engines  2104  to generate thrust, such as gas powered turbo propellers, jet turbines, and/or other non-electric power plants. 
       FIGS. 22A and 22B  are top views of an illustrative VTOL aircraft  2200  where the operator  2202  straddles a fuselage  2204 .  FIG. 22A  shows the VTOL aircraft  2200  in flight in the hover mode  202 . The VTOL aircraft  2200  may include two wings  2206 , the rotor units  116 , and a windshield  2208 . The fuselage  2204  may be shaped to accommodate an operator situated above the fuselage akin to an operator straddling a frame while seated on a motorcycle. The wings  2206  may provide lift while the VTOL aircraft  2200  is in flight in the forward-flight mode  206 .  FIG. 22B  shows the VTOL aircraft  2200  in flight in the forward-flight mode  206 . 
     The windshield  2208  may protect the operator from wind, debris, or any other object in the air. The windshield  2208  may be large enough to protect the operator&#39;s torso and head. In some embodiments, the windshield  2208  may shield the operator&#39;s entire body. The windshield  2208  may be made out of acrylic, plastic, glass, or any other transparent material. The windshield  2208  may be formed from a shatterproof material. 
     With reference to  FIG. 22B , the operator  2202  may control the VTOL aircraft  2200  from a seated position near the center of the fuselage  2204 . The fuselage  2204  may include a seat  2214  and foot rests  2216  for the operator. The operator  2202  may issue control commands to the VTOL aircraft  2200  with handlebars  2218 , similar to motorcycle handlebars. The handlebar  2218  may include a throttle, through which the operator  2202  may change the power output of the rotor units  116 . In some embodiments, the operator  2202  may issue the control commands using a lever (e.g., joystick), a steering wheel, or any other device capable of transmitting the operator&#39;s desired power and direction of the VTOL aircraft  2200 . The mechanical motion of the handlebars  2218  may be transmitted into an electronic signal by the control management system  614 . The control management system  614  may processes signals from an operator and the plurality of stability augmentation sensors  612 , and send an output signal to each rotor unit  116 , directing the rotor units to independently to move about the longitudinal axis and the lateral axis as necessary to transition from the hover mode  202  ( FIG. 22A ) to the forward-flight mode  206  ( FIG. 22B ). The control management system  614  may also receive an input signal from the throttle. The speed of each rotor unit  116  may be determined by operator input through the throttle, and stability augmentation sensor  612  input mixed in the computer management system  614 . The rotor units  116  may produce enough lift to maintain a hover in and out of ground effect. 
       FIGS. 22C and 22D  are side elevation views of an illustrative VTOL aircraft.  FIG. 22C  shows airflow  2210  through the rotor units  116  during flight in the hover mode  202 .  FIG. 22D  shows airflow  2212  through the rotor units  116  and across the VTOL aircraft  2200  during flight in the forward-flight mode  206 . In some embodiments, during forward flight, the backwash from the forward rotor unit(s) may provide enhanced lifting ability of the by directing a majority of the fore rotor unit&#39;s backwash over the upper surfaces, creating a lower pressure area relative to the lower surfaces. 
       FIGS. 23A and 23B  are side elevation views of an illustrative VTOL aircraft  2300  and show mounting arms  2302  that connect the rotor units  116  to the fuselage  2304 . 
     The rotor units  116  may be moveably coupled to the fuselage  2304  via the mounting arms  2302  that include the rotating chassis  120  and mounting arms  2302 . The rotating chassis  120  may allow for freedom of movement about the longitudinal axis of each rotor unit, while the mounting arms  2302  provide the transition from a hover mode  202  as shown in  FIG. 23A , to a forward-flight mode  206  as shown in  FIG. 23B . In some embodiments, the mounting arm  2302  may move the thrust and backwash at least partially away from the fuselage  2304 . For example, the air received by the aft rotor unit  116 ( 2 ) may not flow directly from the fore rotor unit  116 ( 1 ) since the rotor units  116  may be located on different planes due to use of the mounting arms  2302 . In some embodiments, the mounting arms  2302  may be “L” shaped or have other configurations, including structural supports or wings. 
       FIGS. 24A and 24B  are top views of an illustrative VTOL aircraft  2400  and show a cluster of rotor units. The VTOL aircraft may include one or more clusters  2402  of rotor units. In some embodiments, the cluster  2402  may include rotor units aligned along a longitudinal axis of the fuselage. This configuration may result in displacement of the rotor units in the forward flight mode  206 , which causes the rotor units to move above or below the fuselage after rotation and thereby receive cleaner intake air. However, other configurations of the cluster  2402  may be used. The cluster  2402  may be used for the aft rotor units as shown in  FIG. 24A , for the fore rotor units as shown in  FIG. 24B , or in both the fore rotor units and aft rotor units. The rotor units selected for inclusion in the cluster  2402  may be selected to balance power based on a location of a center of mass of the VTOL aircraft  2400 . 
       FIGS. 25A and 25B  are top views of an illustrative VTOL aircraft  2500  using non-ducted rotors  2502 . The non-ducted rotors  2502  may be similar or the same as conventional propeller blades. In some instances, the blades of the non-ducted rotors  2502  may be feathered or otherwise adjusted to change an angle of the blades relative to a drive shaft. 
       FIG. 26  is a side elevation view of another aircraft  2600  using rotor units coupled to a rotatable arm  2602 . The rotor units  116  may be moveably coupled to the fuselage  2604  via the rotatable arm  2602  having a pivot location  2606 . The rotatable arm  2602  may enable the transition from the hover mode  202  to a forward-flight mode  206  by rotation of the rotatable arm  2602 . In some embodiments, the rotatable arm  2602  may move the thrust and backwash at least partially away from the fuselage  2604 . 
       FIG. 27  is a side view of an example degraded rotor landing protocol  2700  performed by a VTOL aircraft  2702 , such as VTOL aircraft  100 ,  700 ,  1700 ,  1800 ,  1900 ,  2000 ,  2100 ,  2200 ,  2300 ,  2400 ,  2500 , or  2600 , from a hover mode. In various embodiments, an operator of the VTOL aircraft  2702  may initiate the degraded rotor landing protocol  2700 . In such embodiments, the operator may initiate the degraded rotor protocol  2700  responsive to a degradation in a rotor system. In various embodiments, a control management system of the VTOL aircraft  2702 , such as control management system  614 , may be configured to initiate the degraded rotor landing protocol  2700 . The control management system may include one or more computing devices, such as a flight control computer, configured with at least one processor and at least one memory coupled to the at least one processor. The at least one memory may include a non-transitory computer-readable media configured to store instructions that, when executed by the one or more processors, cause the VTOL aircraft, or components thereof (e.g., motors, rotors, etc.) to perform operations. Though illustrated as being initiated from a hover mode, the degraded rotor landing protocol  2700  may also be initiated from a transition mode, such as transition mode  204 . 
     The degraded rotor landing protocol  2700  may be initiated based on a determination that a rotor (e.g., rotor unit) of the VTOL aircraft  2702  is operating in a degraded state (e.g., experiencing degraded functionality or ceases to operate). The degraded state may include the rotor unit producing zero thrust (i.e. rotor unit inoperative) or less thrust than required to maintain substantially level flight in the hover mode. In various embodiments, the control management system may recognize the degraded state based on an input received from a motor associated with the degraded rotor unit. In some embodiments, the control management system may recognize the degraded state based on a lack of input from a motor associated with the degraded rotor unit (i.e., motor is inoperative). In some embodiments, the control management system may recognize the degraded state based on a detection of an unanticipated pitch of the VTOL aircraft caused by a reduced thrust from the degraded rotor unit. In such examples, the pitch may be detected by one or more stability augmentation sensors (e.g., gyroscopes and/or accelerometers) of the VTOL aircraft. In some embodiments, the control management system may determine that the unanticipated pitch exceeds a threshold unanticipated pitch amount and/or rate. 
     As illustrated at  2704 , the VTOL aircraft may be established in a hover mode. The control management system may determine that a rotor unit is operating in a degraded state (e.g., degraded rotor unit  2706 ), and may initiate the landing protocol. In various embodiments, the landing protocol may include terminating power to the motor associated with the degraded rotor unit  2706  (i.e., to the first rotor unit), thereby decreasing thrust substantially to zero (e.g., no thrust or a negligible amount of thrust). 
     At  2708 , with little or no thrust produced by the degraded rotor unit  2706 , a fuselage  2710  of the VTOL aircraft may pivot about a rotating chassis, such as rotating chassis  120 ,  1106 , etc.) coupled to the operable rotor unit  2712 , to establish the VTOL aircraft in a substantially vertical configuration, such as that shown at  2714 . Meanwhile, the control management system may continually orient the operable rotor unit  2712  to provide thrust in a downward direction toward the ground, which may cause the operable rotor unit  2712  to pivot up to ninety degrees with respect to the fuselage  2710  during this transition from hover mode to a falling state to maintain the thrust in the downward direction. 
     In various embodiments, the landing protocol may include sending a signal to a motor associated with the operable rotor unit  2712  to increase thrust to an initial thrust setting, which may be a maximum thrust output. The maximum thrust output may cause operation of the operable rotor unit  2712  outside of sustainable operational parameters, which if prolonged, may cause the operational rotor unit  2712  to fail due to overheating, wear, or for other reasons. The initial thrust setting may be determined based on a thrust available, a power available, a desired rate of descent, a payload of the aircraft, a gross weight of the aircraft, and/or other factors that may affect flight conditions in a degraded rotor landing. In various embodiments, the control management system may be configured to identify a power source (i.e., motor, battery, fuel tank, etc.) associated with the degraded rotor unit  2706 . The control management system may then cause power (e.g., electrical current, fuel, etc.) from the power source associated with the degraded rotor unit  2706  to be routed to the operable rotor unit  2712 , to provide additional power (i.e., current) to the operable rotor unit  2712  to increase thrust generated by the operable rotor unit  2712 . 
     In various embodiments, the initial thrust setting may be calculated by the control management system based on the VTOL aircraft altitude and/or power (e.g., battery power, gasoline, etc.) available. Additionally, the initial thrust setting may be based on a gross weight of the VTOL aircraft, a payload, a number of occupants, and/or other aircraft weight considerations. In some embodiments, the initial thrust setting may be based on a desired and/or a maximum initial descent rate and/or rate of acceleration. In such embodiments, the desired and/or maximum initial descent rate and/or rate of acceleration may be based on altitude. For example, a maximum initial descent rate and/or rate of acceleration may include a rate that would minimize a speed of impact (e.g., maximum initial rate of acceleration of 10 fps 2  at 100 feet to preclude velocity increasing over 50 fps). For another example, a maximum initial descent rate may include a rate substantially equivalent to the aircraft altitude (i.e., an aircraft at 500 feet AGL would have no greater than a 500 fpm descent rate). 
     In various embodiments, the control management system may be configured to determine an oscillation of the fuselage  2710  about the rotating chassis of the operable rotor unit  2712  (i.e., oscillation relative to a vertical axis of the VTOL aircraft). The oscillation may be a deviation of an end of the aircraft associated with the degraded rotor unit  2706  from a vertical axis (i.e., oscillation relative to the vertical axis). In some embodiments, the oscillation may be determined based on input from one or more stability augmentation sensors of the VTOL aircraft. In some embodiments, responsive to the oscillation, the control management system may send a signal to the operable rotor unit  2712  to adjust a thrust vector  2716  to dampen the oscillations. In such embodiments, the operable rotor unit  2712  may pivot about the rotating chassis, and adjust the thrust vector  2716  opposite a direction of movement to dampen the oscillation. In some embodiments, responsive to the oscillation, the control management system may send a signal to the motor associated with the operable rotor unit  2712  to surge power and cause a transient increase in vertical thrust (i.e., illustrated as thrust vector  2716 ( 1 )) from the operable rotor unit  2712 . The transient increase in vertical thrust may dampen the oscillation to cause the aircraft to establish in a substantially vertical configuration, illustrated at  2714 . 
     At  2714 , the VTOL aircraft is established in the substantially vertical configuration. In some embodiments, the control management system may establish the operable rotor unit  2712  in a vertical configuration thrust setting. The vertical configuration thrust setting may be the same as or different from the initial thrust setting. In some embodiments, the vertical configuration thrust setting may be determined based on an amount of power available to the operable rotor unit  2712 . In some embodiments, the vertical configuration may include a constant descent rate. In various embodiments, the constant descent rate may be a pre-determined rate (e.g., 500 fpm, 300 fpm, 100 fpm, etc.). In some embodiments, the vertical configuration thrust setting may include a constant decrease in acceleration toward the ground. For example, the vertical configuration thrust setting may include a thrust that decreases an acceleration of the aircraft 1 fps 2  every second to minimize a velocity at impact. 
     In some embodiments, the vertical configuration may include a variable descent rate. In some embodiments, the descent rate may depend on the altitude of the VTOL aircraft. In such embodiments, the control management system of the VTOL aircraft may be configured to determine an altitude of the aircraft, and adjust a descent rate (e.g., thrust produced by the operable motor) based on the altitude. For example, a VTOL aircraft may hover at 2000 feet, and based on a recognition of a degraded functionality of a rotor unit, may initiate a descent rate at 1000 fpm. At 1500 feet, the control management system may adjust the descent rate to 750 fpm, at 1000 feet, the control management system may adjust the descent rate to 500 fpm, and at 500 feet, the control management system may adjust the descent rate to 250 fpm, and so on. For another example, a VTOL aircraft may hover at 50 feet and based on a recognition of a degraded functionality of a rotor unit, may initiate and maintain a constant descent rate of 100 fpm. 
     In some embodiments, the control management system may monitor the altitude of the VTOL aircraft, and may adjust the descent rate and/or rate of acceleration based on a determination that the VTOL aircraft is approaching (e.g., within 10, 15, 20, etc. feet of the altitude, within 3, 5, 10, etc. seconds of reaching the altitude based on descent rate) or has reached a minimum or threshold altitude (e.g., 20 feet, 45 feet, etc.). In such embodiments, the VTOL aircraft may descend at a first descent rate and/or acceleration rate above the threshold altitude, and the control management system may adjust the descent rate and/or acceleration rate to a second (slower) rate to lessen a force of impact with the landing surface. For example, the VTOL aircraft may be established in a 500 fpm descent. The control management system may determine that the aircraft is at 50 feet, and may increase thrust produced by the operable motor  2706  to decrease a descent rate to 200 fpm. 
     In the vertical configuration, a first end of the aircraft (i.e., the nose or the tail, depending on the degraded rotor unit) may contact a landing surface. In various embodiments, the nose and/or tail of the VTOL aircraft may include an impact member  2718 . In such embodiments, the impact member  2718  may include a material and/or structure configured to absorb impact forces, and decrease an effect on occupants of the VTOL aircraft. The material and/or structure may include a foam material, a plastic material, a rubber material, a composite material, a metal material, and/or other material configured to absorb impact forces. 
     In various embodiments, the impact member  2718  may include a deployable member, such as an airbag or balloon, configured to expand or deploy prior to impact to absorb impact forces. In some embodiments, the control management system may identify that a ground contact with the impact member  2718  is imminent. The identification of imminent impact may be based on input from one or more distance sensors (e.g., light detection and ranging sensor, sound navigation and ranging sensor, radar altimeter sensor, a ranging camera, a barometer, etc.) and/or a time and rate(s) of descent. In some embodiments, the control management system may automatically deploy the impact member  2718  based on a degraded functionality of a rotor unit and an initiation of the landing protocol. 
     In some embodiments, a determination that the impact is imminent may be based on a distance to impact being less than a threshold distance. In such embodiments, the control management system may compare the distance with the threshold distance, and determine that the distance is at or less than the threshold distance. Additionally or alternatively, the determination that the impact is imminent may be based on a time to impact is less than a threshold time. In some embodiments, the control management system may calculate the time to impact. In such embodiments, the control management system may compare the time to impact with the threshold time, and determine that the time is within the threshold. For example, the threshold time may be 3 seconds. As the VTOL aircraft descends, the control management system may continually calculate the time to impact, and may determine an impact is imminent when the time to impact is at 3 seconds or less. 
     In various embodiments, responsive to an impact with the ground (e.g., detected by a sensor in the impact member  2718 ) and/or an identification that ground contact with the impact member  2718  is imminent, the control management system may send a signal to the operable rotor unit  2712  to adjust a thrust vector  2716  (i.e., to pivot about the rotating chassis) toward a bottom side of the fuselage (e.g., opposite a direction of the canopy, toward a landing gear, etc.), such as thrust vector  2716 ( 2 ). In some embodiments, the thrust vector  2716 ( 2 ) may include a transient adjustment to the direction of thrust in order to change a pitch of the VTOL aircraft such that the aircraft comes to rest on the landing surface in a generally upright orientation. In some embodiments, the adjustment to the thrust vector  2716 ( 2 ) may cause the VTOL aircraft to rotate onto the landing gear  2720 , as depicted at  2722 . Accordingly, the landing protocol may be initiated to safely land the VTOL aircraft canopy side up in the event of a degraded motor and/or rotor system. 
     In various embodiments, after the impact member  2718  contacts the landing surface and the aircraft is established in a rotation about the impact member  2718 , the control management system may adjust the thrust vector  2716  to provide a substantially vertical thrust, such as thrust vector  2716 ( 1 ). In such embodiments, the operable rotor unit may pivot about the rotating chassis to maintain the thrust vector  2716 ( 1 ), and reduce impact forces of the landing at  2722  (e.g., cushion the landing). 
     Though illustrated as ducted fans, the two or more rotor units of the VTOL aircraft may additionally or alternatively include non-ducted rotors with blades, such as non-ducted rotors  2502 . In various embodiments, the blades may be variable pitch and configured to feather when the respective non-ducted rotor is operating in a degraded capacity. Additionally, in some embodiments with non-ducted rotors, the rotating chassis may be configured to rotate the degraded rotor unit to a position substantially perpendicular to a direction of travel. In such embodiments, both the operable rotor unit and the degraded rotor unit may maintain a substantially similar orientation. 
       FIGS. 28A-C  are perspective views of an example VTOL aircraft  2800  configured to perform a degraded rotor landing protocol.  FIG. 28A  illustrates the VTOL aircraft  2800  in a forward flight mode. As illustrated, the VTOL aircraft  2800  includes rotor units  2802 ( 1 ),  2802 ( 2 ),  2802 ( 3 ), and  2802 ( 4 ), coupled to adjustable wings  2804 . In other embodiments, the rotor units  2802 ( 1 ),  2802 ( 2 ),  2802 ( 3 ), and  2802 ( 4 ), may be coupled to fixed wings via a rotating chassis and/or a tilt mechanism, such as that illustrated in  FIGS. 30A-C . The rotor units  2802  may be configured to adjust approximately 90 degrees from a forward flight configuration shown illustrated in  FIG. 28A  to a vertical flight configuration (e.g., hover configuration) illustrated in  FIG. 28B . 
     The VTOL aircraft shown in  FIG. 28B  is established in a hover mode, the control management system may determine that a rotor unit  2802 ( 1 ) of the VTOL aircraft  2800  is operating in a degraded state, and may initiate the degraded rotor landing protocol, such as degraded rotor landing protocol  2700 . In various embodiments, the landing protocol may include terminating power to a motor associated with the degraded rotor unit  2802 ( 1 ). In some embodiments, the landing protocol may additionally include terminating power to a motor associated with a second rotor unit  2802 ( 2 ), the second rotor unit  2802 ( 2 ) being generally in a same or similar plane with the degraded rotor unit  2802 ( 1 ). In some embodiments, the power to the second rotor unit  2802 ( 2 ) may be gradually decreased, in order to reduce an oscillation of a fuselage while the VTOL aircraft transitions to a vertical configuration illustrated in  FIG. 28C . 
     The VTOL aircraft shown in  FIG. 28C  is established in the vertical configuration, with the operable rotor units  2802 ( 3 ) and  2802 ( 4 ) providing vertical thrust to slow a decent rate and/or acceleration rate toward the landing surface. In various embodiments, the control management system may determine that an impact is imminent, and may adjust a thrust vector produced by the operable rotor units  2802 ( 3 ) and  2802 ( 4 ) to change a pitch of the aircraft such that the aircraft comes to rest on the landing surface with a cockpit being generally upright. In some embodiments, the pitch change may cause the VTOL aircraft to rotate onto landing gear. 
       FIGS. 29A-C  are a perspective views of an example VTOL aircraft  2900  configured to perform a degraded rotor landing protocol.  FIG. 29A  illustrates the VTOL aircraft  2900  in a forward flight mode. The VTOL aircraft  2900  includes rotor units  2902 ( 1 ) and  2902 ( 2 ) coupled to fixed wings  2804 . The rotor units  2902  may be coupled to the wings  2904  via a tilt mechanism, such as the tilt mechanism illustrated in  FIG. 30 . The tilt mechanism may be configured to adjust a position of the rotor units  2902 ( 1 ) and  2902 ( 2 ) approximately 90 degrees from a forward flight configuration shown at  2906  to a vertical flight configuration (e.g., hover configuration) illustrated in  FIG. 29B . 
       FIG. 29B  illustrates the VTOL aircraft  2900  established in a hover mode. In various embodiments, the control management system may determine that a rotor unit  2902 ( 1 ) of the VTOL aircraft  2900  is operating in a degraded state, and may initiate the degraded rotor landing protocol, such as degraded rotor landing protocol  2700 . In various embodiments, the landing protocol may include terminating power to a motor associated with the degraded rotor unit  2902 ( 1 ). 
       FIG. 29C  illustrates the VTOL aircraft  2900  established in a vertical configuration, with the operable rotor unit  2902 ( 2 ) providing vertical thrust to slow a decent rate and/or acceleration rate toward the landing surface. In various embodiments, the control management system may determine that an impact is imminent, and may adjust a thrust vector produced by the operable rotor unit  2802 ( 2 ) to change a pitch of the aircraft such that the aircraft comes to rest on the landing surface with a cockpit being generally upright. In some embodiments, the pitch change may cause the VTOL aircraft to rotate onto landing gear. 
       FIG. 30  is a perspective view an example tilt mechanism  3000  of a rotor unit. The tilt mechanism  3000  may be configured to adjust a position of a rotor unit  3002  approximately 90 degrees from a forward flight configuration shown to a vertical flight configuration (e.g., hover configuration). 
     In various embodiments, the tilt mechanism  3000  may include a joint  3004  (e.g., power transmission joint, universal joint, constant velocity (CV) joint, etc.) configured to adjust the rotor unit between the forward flight configuration, as depicted in  FIG. 29A  and the vertical flight configuration, as depicted in  FIG. 29B . In some embodiments, the tilt mechanism  3000  may also include a strut  3006  configured to rotate the rotor unit outboard, as depicted in  FIG. 29C  (rotor unit  2902 ( 2 )). The strut  3006  may include a gas or liquid strut, a cable and pulley, a gear rack and pinion, and/or other method for adjusting the position of the rotor unit  3002 . 
       FIGS. 31A and 31B  are side views of an example tilt mechanism  3100 , such as tilt rotor mechanism  3000 .  FIG. 31A  illustrates the tilt mechanism  3100  with a rotor unit  3102  in hover configuration. The tilt mechanism  3100  may include a joint  3104 , such as joint  3004 , configured to adjust the rotor unit  3102  from the hover configuration to the outboard degraded landing configuration depicted in  FIG. 29C . 
       FIG. 32  is a flow diagram of an example process  3200 , illustrated as a collection of blocks in a logical flow graph, which represent a sequence of 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 storage 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 order in which the operations are described is not intended to be construed as a limitation, and any number of the described blocks can be combined in any order and/or in parallel to implement the processes. 
       FIG. 32  is a flow diagram of an illustrative process  3200  for a degraded rotor landing protocol. The degraded rotor landing protocol can be performed by a computing system of a VTOL aircraft, such as a flight control computer and/or a control management system, such as control management system  614 , or at the discretion of an operator. 
     At  3202 , a computing system of the VTOL aircraft identifies a degraded condition (e.g., degraded state) of a rotor unit. The degraded operation may include the rotor unit producing zero thrust or less thrust than required to maintain substantially level flight in a hover or transition mode. In various embodiments, the computing system may recognize the degraded operation based on an input (e.g., failure message, degraded condition message, etc.) received from a motor associated with the degraded rotor unit. In some embodiments, the computing system may recognize the degraded operation based on a lack of input from a motor associated with the degraded rotor unit. In some embodiments, the computing system may recognize a failure of a rotor unit to obey an instruction and/or protocol. In some embodiments, the computing system may recognize the degraded operation based on a detection of an unanticipated pitch of the VTOL aircraft caused by a reduced thrust from the degraded rotor unit. In such examples, the pitch may be detected by one or more stability augmentation sensors (e.g., gyroscopes and/or accelerometers) of the VTOL aircraft. In some embodiments, the computing system may determine that the unanticipated pitch exceeds a threshold unanticipated pitch amount and/or rate. 
     At  3204 , the computing system may initiate a degraded rotor unit landing protocol by orienting a second rotor unit (an operable rotor unit) to provide vertical thrust. In some embodiments, the computing system may send a signal to the operable rotor unit and/or a motor associated with the second rotor unit to pivot about a rotating chassis in order to provide the vertical thrust. In some embodiments, the computing system may send a signal to a motor associated with the operable rotor unit to set an initial thrust setting. The initial thrust setting may include an increase in thrust. In some embodiments, the initial thrust setting may be based on atmospheric conditions, a maximum available thrust, an amount of power available, a desired rate of descent, an aircraft altitude, an aircraft gross weight, a number of occupants, a payload, and/or other considerations. 
     In various embodiments, the computing system may send a signal to the motor associated with the degraded rotor unit to cut power to the degraded rotor unit, thereby decreasing thrust substantially to zero (e.g., no thrust or a negligible amount of thrust). In some embodiment, the computing system may send a signal to the motor associated with the degraded rotor unit to produce thrust based on the available power and/or condition of the rotor unit. In such embodiments, the degraded rotor unit may assist in producing at least some lift in the degraded rotor landing protocol. 
     At  3206 , the computing system may establish the aircraft in a rate of descent. In some examples, the rate of descent may be a pre-determined rate of descent. In some examples, the rate of descent may be based on atmospheric conditions, a power available, a thrust available, and/or a payload and/or a gross weight of the aircraft. 
     At  3208 , the computing system may monitor a time or a distance to impact a landing surface. In some examples, the time or the distance may be based on input from one or more distance sensors (e.g., light detection and ranging sensor, radar altimeter sensor, a ranging camera, etc.) and/or a time and rate(s) of descent. In some embodiments, the computing system may determine that a time or a distance to impact the landing surface is within a threshold time or a threshold distance. In such embodiments, the computing system may determine that the impact is imminent. In some embodiments, the computing system may determine that an impact with a ground surface has occurred. In such embodiments, the computing system may determine that the impact has occurred based on an input from an accelerometer, a contact sensor, or the like. 
     In various embodiments, responsive to the identification of imminent impact (e.g., time to impact is within a threshold time or distance to impact is within a threshold distance), the control management system may deploy an impact member. In such embodiments, the impact member may be configured to absorb impact forces resulting from the VTOL aircraft contacting the landing surface. For example, the control management system may deploy an airbag to absorb the initial force of impact with the ground. 
     At  3210 , the computing system may adjust an orientation of the operable rotor unit (e.g., pitch of the aircraft) to land in a generally upright configuration. In some embodiments, the computing system may adjust a thrust vector of the operable motor at a predetermined distance from the ground or at a predetermined time before anticipated impact with the ground to cause the VTOL aircraft to rotate to a generally upright configuration (i.e., onto landing gear, onto a bottom side of the fuselage, etc.). Accordingly, the landing protocol may be initiated to safely land the VTOL aircraft generally upright up in the event of a degraded motor and/or rotor system. 
     CONCLUSION 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as illustrative forms of implementing the claims.