Patent Publication Number: US-11383823-B2

Title: Single-axis gimbal mounted propulsion systems for aircraft

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of co-pending application Ser. No. 15/972,461 filed May 7, 2018, which claims the benefit of U.S. Provisional Application No. 62/594,433, filed Dec. 4, 2017 and is a continuation-in-part of application Ser. No. 15/606,242 filed May 26, 2017, which is a continuation-in-part of application Ser. No. 15/200,163 filed Jul. 1, 2016, the entire contents of each is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD OF THE DISCLOSURE 
     The present disclosure relates, in general, to aircraft operable to transition between thrust-borne flight and wing-borne flight and, in particular, to aircraft having a distributed thrust array including a plurality of propulsion assemblies each having a gimbal mounted propulsion system operable for thrust vectoring. 
     BACKGROUND 
     Fixed-wing aircraft, such as airplanes, are capable of flight using wings that generate lift responsive to the forward airspeed of the aircraft, which is generated by thrust from one or more jet engines or propellers. The wings generally have an airfoil cross section that deflects air downward as the aircraft moves forward, generating the lift force to support the airplane in flight. Fixed-wing aircraft, however, typically require a runway that is hundreds or thousands of feet long for takeoff and landing. 
     Unlike fixed-wing aircraft, vertical takeoff and landing (VTOL) aircraft do not require runways. Instead, VTOL aircraft are capable of taking off, hovering and landing vertically. One example of VTOL aircraft is a helicopter which is a rotorcraft having one or more rotors that provide lift and thrust to the aircraft. The rotors not only enable hovering and vertical takeoff and landing, but also enable, forward, backward and lateral flight. These attributes make helicopters highly versatile for use in congested, isolated or remote areas where fixed-wing aircraft may be unable to takeoff and land. Helicopters, however, typically lack the forward airspeed of fixed-wing aircraft. 
     A tiltrotor aircraft is another example of a VTOL aircraft. Tiltrotor aircraft generate lift and propulsion using proprotors that are typically coupled to nacelles mounted near the ends of a fixed wing. The nacelles rotate relative to the fixed wing such that the proprotors have a generally horizontal plane of rotation for vertical takeoff, hovering and landing and a generally vertical plane of rotation for forward flight, wherein the fixed wing provides lift and the proprotors provide forward thrust. In this manner, tiltrotor aircraft combine the vertical lift capability of a helicopter with the speed and range of fixed-wing aircraft. Tiltrotor aircraft, however, typically suffer from downwash inefficiencies during vertical takeoff and landing due to interference caused by the fixed wing. 
     A further example of a VTOL aircraft is a tiltwing aircraft that features a rotatable wing that is generally horizontal for forward flight and rotates to a generally vertical orientation for vertical takeoff and landing. Propellers are coupled to the rotating wing to provide the required vertical thrust for takeoff and landing and the required forward thrust to generate lift from the wing during forward flight. The tiltwing design enables the slipstream from the propellers to strike the wing on its smallest dimension, thus improving vertical thrust efficiency as compared to tiltrotor aircraft. Tiltwing aircraft, however, are more difficult to control during hover as the vertically tilted wing provides a large surface area for crosswinds typically requiring tiltwing aircraft to have either cyclic rotor control or an additional thrust station to generate a moment. 
     SUMMARY 
     In a first aspect, the present disclosure is directed to an aircraft that includes an airframe having a longitudinal direction. A distributed thrust array is attached to the airframe. The distributed thrust array includes a plurality of propulsion assemblies. Each of the propulsion assemblies is independently controlled by a flight control system. Each propulsion assembly includes a housing and a gimbal coupled to the housing that is operable to tilt about a single axis. A propulsion system is coupled to and is operable to tilt with the gimbal. The propulsion system includes an electric motor having an output drive and a rotor assembly having a plurality of rotor blades. The rotor assembly is rotatable with the output drive of the electric motor in a rotational plane to generate thrust having a thrust vector with a magnitude in the longitudinal direction. Actuation of each gimbal is operable to tilt the respective propulsion system relative to the airframe in the longitudinal direction to change the rotational plane of the respective rotor assembly relative to the airframe, thereby controlling the magnitude of the respective thrust vector in the longitudinal direction. 
     In some embodiments, the aircraft may have a thrust-borne flight mode and a wing-borne flight mode. In certain embodiments, the distributed thrust array may be a two dimensional thrust array and/or may include at least four propulsion assemblies. In some embodiments, each propulsion assembly may be a line replaceable propulsion unit. In certain embodiments, the distributed thrust array may be operable to provide yaw authority in hover responsive to differential longitudinal thrust vectoring and/or longitudinal movement authority in hover responsive to thrust vectoring. In some embodiments, each propulsion assembly may include an actuator coupled between the housing and the gimbal to tilt the gimbal about the axis. 
     In certain embodiments, for each propulsion assembly, the axis of the gimbal may pass through the propulsion system. For example, the axis may pass through the center of mass of the propulsion system or through a location within a predetermined distance from the center of mass of the propulsion system. As another example, the axis of the gimbal may pass through the center of mass in hover of the propulsion system or through a location within a predetermined distance from the center of mass in hover of the propulsion system. As a further example, the axis of the gimbal may pass through a location between the center of mass and the center of mass in hover of the propulsion system. In some embodiments, a maximum longitudinal angle of the thrust vector of each propulsion system may be between about ten degrees and about thirty degrees, may be between about fifteen degrees and about twenty-five degrees or may be about twenty degrees. 
     In certain embodiments, the airframe may include first and second wings having at least first and second pylons extending therebetween and having a plurality of tail members extending therefrom with each tail member having a control surface. In such embodiments, the control surfaces may be operable to provide yaw authority in hover responsive to differential control surface maneuvers. Also, in such embodiments, each of the control surfaces may be positioned within a distance of two rotor diameters of a respective propulsion system such that each of the control surfaces is disposed within the prop wash of the respective propulsion system during hover. In some embodiments, a pod assembly may be coupled to the airframe between the first and second pylons. 
     In a second aspect, the present disclosure is directed to an aircraft having a thrust-borne flight mode and a wing-borne flight mode. The aircraft includes an airframe having first and second wings with at least first and second pylons extending therebetween and with a plurality of tail members extending therefrom. The airframe has a longitudinal direction. A pod assembly is coupled to the airframe between the first and second pylons. A two dimensional distributed thrust array is attached to the airframe. The thrust array includes at least four line replaceable propulsion units. Each of the propulsion units is independently controlled by a flight control system. Each propulsion unit includes a housing and a gimbal coupled to the housing that is operable to tilt about a single axis. A propulsion system is coupled to and is operable to tilt with the gimbal. The propulsion system includes an electric motor having an output drive and a rotor assembly having a plurality of rotor blades. The rotor assembly is rotatable with the output drive of the electric motor in a rotational plane to generate thrust having a thrust vector with a magnitude in the longitudinal direction. Actuation of each gimbal is operable to tilt the respective propulsion system relative to the airframe in the longitudinal direction to change the rotational plane of the respective rotor assembly relative to the airframe, thereby controlling the magnitude of the respective thrust vector in the longitudinal direction to provide yaw authority in hover responsive to differential longitudinal thrust vectoring and longitudinal movement authority in hover responsive to longitudinal thrust vectoring. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the features and advantages of the present disclosure, reference is now made to the detailed description along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
         FIGS. 1A-1F  are schematic illustrations of an aircraft in accordance with embodiments of the present disclosure; 
         FIGS. 2A-2I  are schematic illustrations of an aircraft in a sequential flight operating scenario in accordance with embodiments of the present disclosure; 
         FIG. 3  is a block diagram of a two-dimensional distributed thrust array having two-axis gimbal mounted propulsion systems for an aircraft in accordance with embodiments of the present disclosure; 
         FIGS. 4A-4D  are schematic illustrations of an aircraft performing various flight maneuvers in accordance with embodiments of the present disclosure; 
         FIGS. 5A-5I  are schematic illustrations of a line replaceable propulsion unit operating a two-axis gimbal for an aircraft in accordance with embodiments of the present disclosure; 
         FIGS. 6A-6D  are schematic illustrations of an aircraft performing measures to counteract an actuator fault in a propulsion assembly in accordance with embodiments of the present disclosure; 
         FIG. 7  is a block diagram of a two-dimensional distributed thrust array having single-axis gimbal mounted propulsion systems for an aircraft in accordance with embodiments of the present disclosure; 
         FIGS. 8A-8B  are schematic illustrations of an aircraft performing various flight maneuvers in accordance with embodiments of the present disclosure; 
         FIGS. 9A-9C  are schematic illustrations of a line replaceable propulsion unit operating a single-axis gimbal for an aircraft in accordance with embodiments of the present disclosure; and 
         FIGS. 10A-10D  are schematic illustrations of a tail member having a control surface and a line replaceable propulsion unit coupled thereto for an aircraft in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     While the making and using of various embodiments of the present disclosure are discussed in detail below, it should be appreciated that the present disclosure provides many applicable inventive concepts, which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative and do not delimit the scope of the present disclosure. In the interest of clarity, not all features of an actual implementation may be described in the present disclosure. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming but would be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     In the specification, reference may be made to the spatial relationships between various components and to the spatial orientation of various aspects of components as the devices are depicted in the attached drawings. However, as will be recognized by those skilled in the art after a complete reading of the present disclosure, the devices, members, apparatuses, and the like described herein may be positioned in any desired orientation. Thus, the use of terms such as “above,” “below,” “upper,” “lower” or other like terms to describe a spatial relationship between various components or to describe the spatial orientation of aspects of such components should be understood to describe a relative relationship between the components or a spatial orientation of aspects of such components, respectively, as the device described herein may be oriented in any desired direction. As used herein, the term “coupled” may include direct or indirect coupling by any means, including moving and/or non-moving mechanical connections. 
     Referring to  FIGS. 1A-1F  in the drawings, various views of an aircraft  10  having a distributed thrust array including gimbal mounted propulsion systems operable for thrust vectoring are depicted.  FIGS. 1A, 1C, 1E  depict aircraft  10  in thrust-borne flight which may also be referred to as the vertical takeoff and landing or VTOL flight mode of aircraft  10 .  FIGS. 1B, 1D, 1F  depict aircraft  10  in wing-borne flight which may also be referred to as the forward or high speed forward flight mode of aircraft  10 . In the illustrated embodiment, the airframe  12  of aircraft  10  includes wings  14   a ,  14   b  each having an airfoil cross-section that generates lift responsive to the forward airspeed of aircraft  10 . Wings  14   a ,  14   b  may be formed as single members or may be formed from multiple wing sections. The outer skins for wings  14   a ,  14   b  are preferably formed from high strength and lightweight materials such as fiberglass, carbon, plastic, metal or other suitable material or combination of materials. As illustrated, wings  14   a ,  14   b  are straight wings. In other embodiments, wings  14   a ,  14   b  could have other designs such as polyhedral wing designs, swept wing designs or other suitable wing design. 
     Extending generally perpendicularly between wings  14   a ,  14   b  are two truss structures depicted as pylons  16   a ,  16   b . In other embodiments, more than two pylons may be present. Pylons  16   a ,  16   b  are preferably formed from high strength and lightweight materials such as fiberglass, carbon, plastic, metal or other suitable material or combination of materials. Wings  14   a ,  14   b  and pylons  16   a ,  16   b  may be coupled together at the respective intersections using mechanical connections such as bolts, screws, rivets, adhesives and/or other suitable joining technique. Extending generally perpendicularly from wings  14   a ,  14   b  are landing gear depicted as tail members  18   a ,  18   b ,  18   c ,  18   d  that enable aircraft  10  to operate as a tailsitting aircraft. In the illustrated embodiment, tail members  18   a ,  18   b ,  18   c ,  18   d  are fixed landing struts. In other embodiments, tail members  18   a ,  18   b ,  18   c ,  18   d  may include passively operated pneumatic landing struts or actively operated telescoping landing struts with or without wheels for ground maneuvers. Tail members  18   a ,  18   b ,  18   c ,  18   d  each include a control surface  20   a ,  20   b ,  20   c ,  20   d , respectively, that may be passive or active aerosurfaces that serve as vertical stabilizers and/or elevators during wing-borne flight and serve to enhance hover stability during thrust-borne flight. 
     Wings  14   a ,  14   b  and pylons  16   a ,  16   b  preferably include central passageways operable to contain flight control systems, energy sources, communication lines and other desired systems. For example, as best seen in  FIG. 1A , wing  14   a  houses the flight control system  32  of aircraft  10 . Flight control system  32  is preferably a redundant digital flight control system. In the illustrated embodiment, flight control system  32  is a triply redundant digital flight control system including three independent flight control computers. Use of triply redundant flight control system  32  having redundant components improves the overall safety and reliability of aircraft  10  in the event of a failure in flight control system  32 . Flight control system  32  preferably includes non-transitory computer readable storage media including a set of computer instructions executable by one or more processors for controlling the operation of aircraft  10 . Flight control system  32  may be implemented on one or more general-purpose computers, special purpose computers or other machines with memory and processing capability. For example, flight control system  32  may include one or more memory storage modules including, but is not limited to, internal storage memory such as random access memory, non-volatile memory such as read only memory, removable memory such as magnetic storage memory, optical storage, solid-state storage memory or other suitable memory storage entity. Flight control system  32  may be a microprocessor-based system operable to execute program code in the form of machine-executable instructions. In addition, flight control system  32  may be selectively connectable to other computer systems via a proprietary encrypted network, a public encrypted network, the Internet or other suitable communication network that may include both wired and wireless connections. 
     In the illustrated embodiment, wings  14   a ,  14   b  and/or pylons  16   a ,  16   b  may contain one or more of electrical power sources depicted as batteries  22  in wing  14   a , as best seen in  FIG. 1A . Batteries  22  supply electrical power to flight control system  32 . In some embodiments, batteries  22  may be used to supply electrical power for the distributed thrust array of aircraft  10 . Wings  14   a ,  14   b  and/or pylons  16   a ,  16   b  also contain a communication network  24  that enables flight control system  32  to communicate with the distributed thrust array of aircraft  10 . 
     In the illustrated embodiment, the distributed thrust array includes four propulsion assemblies  26   a ,  26   b ,  26   c ,  26   d  that are independently operated and controlled by flight control system  32 . It should be noted, however, that the distributed thrust array of the present disclosure could have any number of independent propulsion assemblies including six, eight, twelve, sixteen or other number of independent propulsion assemblies. Propulsion assemblies  26   a ,  26   b ,  26   c ,  26   d  are independently attachable to and detachable from airframe  12 . For example, propulsion assemblies  26   a ,  26   b ,  26   c ,  26   d  are preferably standardized and interchangeable units that are most preferably line replaceable propulsion units enabling easy installation and removal from airframe  12 . Propulsion assemblies  26   a ,  26   b ,  26   c ,  26   d  may be coupled to wings  14   a ,  14   b  using quick connect and disconnect couplings techniques including bolts, pins, cables or other suitable coupling techniques. In addition, the use of line replaceable propulsion units is beneficial in maintenance situations if a fault is discovered with one of the propulsion units. In this case, the faulty propulsion unit can be decoupled from airframe  12  by simple operations and another propulsion unit can then be attached to airframe  12 . In other embodiments, propulsion assemblies  26   a ,  26   b ,  26   c ,  26   d  may be permanently coupled to wings  14   a ,  14   b  by riveting, bonding and/or other suitable technique. 
     As illustrated, propulsion assemblies  26   a ,  26   b ,  26   c ,  26   d  are coupled to the outboard ends of wings  14   a ,  14   b . In other embodiments, propulsion assemblies  26   a ,  26   b ,  26   c ,  26   d  could have other configurations including close coupled configurations, high wing configurations, low wing configurations or other suitable configuration. In the illustrated embodiment, the four independently operating propulsion assemblies  26   a ,  26   b ,  26   c ,  26   d  form a two-dimensional thrust array with each of the propulsion assemblies having a symmetrically disposed propulsion assembly. For example, propulsion assemblies  26   a ,  26   c  are symmetrically disposed propulsion assemblies and propulsion assemblies  26   b ,  26   d  are symmetrically disposed propulsion assemblies. It should be noted, however, that a two-dimensional thrust array of the present disclosure could have any number of independent propulsion assemblies including six, eight, twelve, sixteen or other number of independent propulsion assemblies that form the two-dimensional thrust array with each of the propulsion assemblies having a symmetrically disposed propulsion assembly. 
     In the illustrated embodiment, each propulsion assembly  26   a ,  26   b ,  26   c ,  26   d  includes a housing  28   a ,  28   b ,  28   c ,  28   d , that contains components such as an electric motor, a gimbal, one or more actuators and an electronics node including, for example, batteries, controllers, sensors and other desired electronic equipment. Only electric motors  30   a ,  30   b  and electronics nodes  32   a ,  32   b  are visible in  FIG. 1A . The electric motors of each propulsion assembly  26   a ,  26   b ,  26   c ,  26   d  are preferably operated responsive to electrical energy from the battery or batteries disposed with that housings, thereby forming a distributed electrically powered thrust array. Alternatively or additionally, electrical power may be supplied to the electric motors and/or the batteries disposed with the housing from batteries  22  carried by airframe  12  via communications network  24 . In other embodiments, the propulsion assemblies may include internal combustion engines or hydraulic motors. 
     Flight control system  32  communicates via communications network  24  with the electronics nodes of each propulsion assembly  26   a ,  26   b ,  26   c ,  26   d , such as electronics node  32   a  of propulsion assembly  26   a  and electronics node  32   b  of propulsion assembly  26   b . Flight control system  32  receives sensor data from and sends flight command information to the electronics nodes of each propulsion assembly  26   a ,  26   b ,  26   c ,  26   d  such that each propulsion assembly  26   a ,  26   b ,  26   c ,  26   d  may be individually and independently controlled and operated. For example, flight control system  32  is operable to individually and independently control the operating speed and thrust vector of each propulsion assembly  26   a ,  26   b ,  26   c ,  26   d . Flight control system  32  may autonomously control some or all aspects of flight operation for aircraft  10 . Flight control system  32  is also operable to communicate with remote systems, such as a ground station via a wireless communications protocol. The remote system may be operable to receive flight data from and provide commands to flight control system  32  to enable remote flight control over some or all aspects of flight operation for aircraft  10 . The autonomous and/or remote operation of aircraft  10  enables aircraft  10  to perform unmanned logistic operations for both military and commercial applications. 
     Each propulsion assembly  26   a ,  26   b ,  26   c ,  26   d  includes a rotor assembly  34   a ,  34   b ,  34   c ,  34   d . Each rotor assembly  34   a ,  34   b ,  34   c ,  34   d  is directly or indirectly coupled to an output drive of a respective electrical motor  30   a ,  30   b ,  30   c ,  30   d  that rotates the rotor assembly  34   a ,  34   b ,  34   c ,  34   d  in a rotational plane to generate thrust for aircraft  10 . In the illustrated embodiment, rotor assemblies  34   a ,  34   b ,  34   c ,  34   d  each include three rotor blades having a fixed pitch. In other embodiments, the rotor assemblies could have other numbers of rotor blades both less than and greater than three. Alternatively or additionally, the rotor assemblies could have variable pitch rotor blades with collective and/or cyclic pitch control. Each electrical motor  30   a ,  30   b ,  30   c ,  30   d  is paired with a rotor assembly  34   a ,  34   b ,  34   c ,  34   d , for example electrical motor  30   a  and rotor assembly  34   a , to form a propulsion system  36   a ,  36   b ,  36   c ,  36   d . As described herein, each propulsion system  36   a ,  36   b ,  36   c ,  36   d  may have a single-axis or a two-axis tilting degree of freedom relative to housings  28   a ,  28   b ,  28   c ,  28   d  and thus airframe  12  such that propulsion systems  36   a ,  36   b ,  36   c ,  36   d  are operable for thrust vectoring. In the illustrated embodiment, the maximum angle of the thrust vector may preferably be between about 10 degrees and about 30 degrees, may more preferably be between about 15 degrees and about 25 degrees and may most preferably be about 20 degrees. Notably, using a 20-degree thrust vector yields a lateral component of thrust that is about 34 percent of total thrust. In other embodiments, the propulsion systems may not have a tilting degree of freedom in which case, propulsion systems  36   a ,  36   b ,  36   c ,  36   d  may not be capable of thrust vectoring. As such, aircraft  10  may have no thrust vectoring capabilities, single-axis thrust vectoring capabilities or two-axis thrust vectoring capabilities associated with each propulsion assembly  26   a ,  26   b ,  26   c ,  26   d.    
     Aircraft  10  may operate as a transport aircraft for a pod assembly  50  that is fixed to or selectively attachable to and detachable from airframe  12 . In the illustrated embodiment, pylons  16   a ,  16   b  include receiving assemblies for coupling with pod assembly  50 . The connection between pylons  16   a ,  16   b  and pod assembly  50  may be a fixed connection that secures pod assembly  50  in a single location relative to airframe  12 . Alternatively, pod assembly  50  may be allowed to rotate and/or translate relative to airframe  12  during ground and/or flight operations. For example, it may be desirable to have pod assembly  50  low to the ground for loading and unloading cargo but more distant from the ground for takeoff and landing. As another example, it may be desirable to change the center of mass of pod assembly  50  relative to airframe  12  during certain flight conditions such as moving the center of mass of pod assembly  50  forward relative to airframe  12  during high speed wing-borne flight. Similarly, it may be desirable to lowering the center of mass of pod assembly  50  relative to airframe  12  during hover in the event of a partial or total failure of one of the propulsion assemblies. As illustrated, pod assembly  50  may be selectively coupled to and decoupled from airframe  12  to enable sequential pickup, transportation and delivery of multiple pod assemblies  50  to and from multiple locations. 
     Airframe  12  preferably has remote release capabilities of pod assembly  50 . For example, this feature allows airframe  12  to drop pod assembly  50  at a desire location following transportation. In addition, this feature allows airframe  12  to jettison pod assembly  50  during flight, for example, in the event of an emergency situation such as a propulsion assembly or other system of aircraft  10  becoming compromised. One or more communication channels may be established between pod assembly  50  and airframe  12  when pod assembly  50  is attached therewith. A quick disconnect harness may be coupled between pod assembly  50  and airframe  12  such that flight control system  32  may send commands to pod assembly  50  to perform functions. For example, flight control system  32  may operate doors of pod assembly  50  between open and closed positions to enable loading and unloading of a payload to be transported within pod assembly  50 . 
     Referring additionally to  FIGS. 2A-2I  in the drawings, a sequential flight-operating scenario of aircraft  10  is depicted. In the illustrated embodiment, pod assembly  50  is attached to airframe  12  and may contain a desired payload. It is noted, however, that pod assembly  50  may be selectively disconnected from airframe  12  such that a single airframe can be operably coupled to and decoupled from numerous pod assemblies for numerous missions over time. In addition, aircraft  10  may perform missions without having a pod assembly attached to airframe  12 . As best seen in  FIG. 2A , aircraft  10  is in a tailsitting position on the ground. When aircraft  10  is ready for a mission, flight control system  32  commences operations to provide flight control to aircraft  10  which may be autonomous flight control, remote flight control or a combination thereof. For example, it may be desirable to utilize remote flight control during certain maneuvers such as takeoff and landing but rely on autonomous flight control during hover, high speed forward flight and/or transitions between wing-borne flight and thrust-borne flight. 
     As best seen in  FIG. 2B , aircraft  10  has performed a vertical takeoff and is engaged in thrust-borne flight with pod assembly  50  lifted into the air. As illustrated, the rotor assemblies  34   a ,  34   b ,  34   c ,  34   d  are each rotating in the same horizontal plane forming of a two-dimensional distributed thrust array. As noted, flight control system  32  independently controls and operates each propulsion assembly  26   a ,  26   b ,  26   c ,  26   d  including independently controlling operating speeds and thrust vectors. During hover, flight control system  32  may utilize differential speed control of rotor assemblies  34   a ,  34   b ,  34   c ,  34   d  for stabilizing aircraft  10  and for providing yaw authority. This may be achieved by increasing the speed of the rotor assemblies rotating clockwise, such as rotor assemblies  34   a ,  34   c  and/or decreasing the speed of the rotor assemblies rotating counter clockwise, such as rotor assemblies  34   b ,  34   d.    
     Alternatively or additional, flight control system  32  may utilize differential thrust vectoring of propulsion systems  36   a ,  36   b ,  36   c ,  36   d  for stabilizing aircraft  10  and for providing yaw authority. This may be achieved by differential longitudinal thrust vectoring of two symmetrically disposed propulsion systems such as propulsion systems  36   a ,  36   c . This may also be achieved by differential thrust vectoring of all propulsion systems  36   a ,  36   b ,  36   c ,  36   d  by suitably clocking the thrust vectors at approximately 90 degrees from one another. Alternatively or additional, flight control system  32  may utilize differential control surface maneuvers of control surfaces  20   a ,  20   b ,  20   c ,  20   d  for stabilizing aircraft  10  and for providing yaw authority. This may be achieved by differential longitudinal control surface maneuvers of two symmetrically disposed control surfaces such as control surfaces  20   a ,  20   c.    
     In embodiments of aircraft  10  having two-axis thrust vectoring capabilities associated with each propulsion assembly  26   a ,  26   b ,  26   c ,  26   d , aircraft  10  has redundant direction control during hover which serves as a safety feature in the event of a partial or complete failure in one propulsion assembly. As discussed herein, flight control system  32  is operable to send commands to a symmetrically disposed propulsion assembly to counteract a thrust vector error in the compromised propulsion assembly. Alternatively or additional, flight control system  32  is operable to send commands to any one or all of the other propulsion assemblies to counteract a thrust vector error in the compromised propulsion assembly. This feature improves the overall safety of aircraft  10  and provides redundant direction control to aircraft  10 . 
     After vertical assent to the desired elevation, aircraft  10  may begin the transition from thrust-borne flight to wing-borne flight. As best seen from the progression of  FIGS. 2B-2E , aircraft  10  is operable to pitch forward from thrust-borne flight to wing-borne flight to enable high speed and/or long range forward flight. Flight control system  32  may achieve this operation by increasing the speed of rotor assemblies  34   c ,  34   d  and/or decreasing the speed of rotor assemblies  34   a ,  34   b , collective thrust vectoring of propulsion systems  36   a ,  36   b ,  36   c ,  36   d , collective control surface maneuvers of control surfaces  20   a ,  20   b ,  20   c ,  20   d  or any combination thereof. 
     As best seen in  FIG. 2E , rotor assemblies  34   a ,  34   b ,  34   c ,  34   d  are each rotating in the same vertical plane forming of a two-dimensional distributed thrust array. As wing-borne forward flight requires significantly less power then thrust-borne vertical flight, the operating speed of some or all of propulsion assembly  26   a ,  26   b ,  26   c ,  26   d  may be reduced. In certain embodiments, some of the propulsion assemblies of an aircraft of the present disclosure could be shut down during wing-borne forward flight. In forward flight mode, the independent control of flight control system  32  over each propulsion assembly  26   a ,  26   b ,  26   c ,  26   d  provides pitch, roll and yaw authority using, for example, collective or differential thrust vectoring, differential speed control, collective or differential control surface maneuvers or any combination thereof. In addition, as in thrust-borne vertical flight, when aircraft  10  is engaged in wing-borne forward flight, flight control system  32  is operable to send commands to a symmetrically disposed propulsion assembly or multiple other propulsion assemblies to counteract an error in one of the propulsion assemblies. 
     As aircraft  10  approaches its destination, aircraft  10  may begin its transition from wing-borne flight to thrust-borne flight. As best seen from the progression of  FIGS. 2E-2H , aircraft  10  is operable to pitch aft from wing-borne flight to thrust-borne flight to enable, for example, a vertical landing operation. Flight control system  32  may achieve this operation by increasing the speed of rotor assemblies  34   a ,  34   b  and/or decreasing the speed of rotor assemblies  34   c ,  34   d , collective thrust vectoring of propulsion systems  36   a ,  36   b ,  36   c ,  36   d , collective control surface maneuvers of control surfaces  20   a ,  20   b ,  20   c ,  20   d  or any combination thereof. Once aircraft  10  has completed the transition to thrust-borne vertical flight, aircraft  10  may commence its vertical descent to a surface. As best seen in  FIG. 2I , aircraft  10  has landing in a tailsitting orientation at the destination location and may, for example, remotely drop a payload carried within pod assembly  50 . 
     Referring next to  FIG. 3 , the redundant directional control feature of an aircraft  100  having a distributed thrust array including two-axis gimbal mounted propulsion systems will now be described. Aircraft  100  includes a distributed thrust array depicted as four propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  forming a two-dimensional thrust array. Propulsion assembly  102   a  includes electronics node  104   a , two-axis gimbal  106   a  operated by actuators  108   a ,  110   a  and propulsion system  112   a . Propulsion assembly  102   b  includes electronics node  104   b , two-axis gimbal  106   b  operated by actuators  108   b ,  110   b  and propulsion system  112   b . Propulsion assembly  102   c  includes electronics node  104   c , two-axis gimbal  106   c  operated by actuators  108   c ,  110   c  and propulsion system  112   c . Propulsion assembly  102   d  includes electronics node  104   d , two-axis gimbal  106   d  operated by actuators  108   d ,  110   d  and propulsion system  112   d . Each of electronics nodes  104   a ,  104   b ,  104   c ,  104   d  includes one or more batteries, one or more controllers such as an electronic speed controller and one or more sensors for monitoring parameters associate with the components of the respective propulsion assembly. As discussed herein, each of propulsion systems  112   a ,  112   b ,  112   c ,  112   d  includes an electric motor having an output drive and a rotor assembly having a plurality of rotor blades. Each rotor assembly is rotatable with the respective output drive of the electric motor in a rotational plane to generate thrust. A flight control system  114  is operably associated with propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  and is communicably linked to electronic nodes  104   a ,  104   b ,  104   c ,  104   d  by communications network  116 . Flight control system  114  receives sensor data from and send commands to electronic nodes  104   a ,  104   b ,  104   c ,  104   d  to enable flight control system  114  to independently control each of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d.    
     For example, as best seen in  FIG. 4A , aircraft  100  has longitudinal control authority responsive to collective thrust vectoring of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d . As illustrated, aircraft  100  has a longitudinal axis  120  and is operable for movement in the longitudinal direction as indicated by arrow  122 . In the illustrated embodiment, flight control system  114  has sent commands to operate each of actuators  108   a ,  108   b ,  108   c ,  108   d  to tilt each of propulsion systems  112   a ,  112   b ,  112   c ,  112   d  in the forward direction. Actuators  110   a ,  110   b ,  110   c ,  110   d  are in an unactuated state. In this configuration, propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  generate thrust vectors having aftward directed longitudinal components  124   a ,  124   b ,  124   c ,  124   d . In hover, such collective thrust vectoring of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  provides longitudinal control authority to aircraft  100 . 
     The longitudinal thrust vectoring operation will now be described with reference to an exemplary propulsion assembly  102 , depicted as a line replaceable propulsion unit, in  FIGS. 5A-5C . Propulsion assembly  102  includes a housing  126  and a gimbal  106  that is coupled to housing  126 . Gimbal  106  includes an outer gimbal member  128  and an inner gimbal member  130 . Outer gimbal member  128  is pivotally coupled to housing  126  and is operable to tilt about a first axis. Inner gimbal member  130  is pivotally coupled to outer gimbal member  128  and is operable to tilt about a second axis that is orthogonal to the first axis. In the illustrated embodiment, actuator  108  is coupled between housing  126  and outer gimbal member  128  such that operation of actuator  108  shift linkage  132  to tilt outer gimbal member  128  about the first axis relative to housing  126 . Actuator  110  is coupled between housing  126  and inner gimbal member  130  such that operation of actuator  110  shifts linkage  134  to tilt inner gimbal member  130  about the second axis relative to outer gimbal member  128  and housing  126 . A propulsion system  112  is coupled to and is operable to tilt with gimbal  106  about both axes relative to housing  126 . In the illustrated embodiment, the rotor assembly has been removed from propulsion system  112  such that only electric motor  136  and output drive  138  are visible in the figures. 
     As best seen in the comparison of  FIGS. 5A-5C , actuator  108  is operated to tilt propulsion system  112  longitudinally between a fully forward configuration shown in  FIG. 5A  and a fully aft configuration shown in  FIG. 5C  as well as in an infinite number of positions therebetween including the fully vertical configuration shown in  FIG. 5B . This operation longitudinally shifts the thrust vector of propulsion assembly  102  to enable the longitudinal control authority of aircraft  100  depicted in  FIG. 4A . The maximum longitudinal tilt angle of gimbal  106  may preferably be between about 10 degrees and about 30 degrees, may more preferably be between about 15 degrees and about 25 degrees and may most preferably be about 20 degrees. As should be understood by those having ordinary skill in the art, the magnitude of the longitudinal component  124  of the thrust vector is related to the direction of the thrust vector, which is determined by the longitudinal tilt angle of gimbal  106 . 
     As best seen in  FIG. 4B , aircraft  100  has lateral control authority responsive to collective thrust vectoring of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d . As illustrated, aircraft  100  has a longitudinal axis  120  and is operable for movement in the lateral direction as indicated by arrow  142 . In the illustrated embodiment, flight control system  114  has sent commands to operate each of actuators  110   a ,  110   b ,  110   c ,  110   d  to tilt each of propulsion systems  112   a ,  112   b ,  112   c ,  112   d  to the right (from a forward looking perceptive from longitudinal axis  120 ). Actuators  108   a ,  108   b ,  108   c ,  108   d  are in an unactuated state. In this configuration, propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  generate thrust vectors having leftwardly directed lateral components  144   a ,  144   b ,  144   c ,  144   d . In hover, such collective thrust vectoring of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  provides lateral control authority to aircraft  100 . 
     The lateral thrust vectoring operation will now be described with reference to propulsion assembly  102  in  FIGS. 5D-5F . As best seen in the comparison of  FIGS. 5D-5F , actuator  110  is operated to tilt propulsion system  112  lateral between a fully right configuration shown in  FIG. 5D  and a fully left configuration shown in  FIG. 5F  as well as in an infinite number of positions therebetween including the fully vertical configuration shown in  FIG. 5E . This operation laterally shifts the thrust vector of propulsion assembly  102  to enable the lateral control authority of aircraft  100  depicted in  FIG. 4B . The maximum lateral tilt angle of gimbal  106  may preferably be between about 10 degrees and about 30 degrees, may more preferably be between about 15 degrees and about 25 degrees and may most preferably be about 20 degrees. As should be understood by those having ordinary skill in the art, the magnitude of the lateral component  144  of the thrust vector is related to the direction of the thrust vector, which is determined by the lateral tilt angle of gimbal  106 . 
     Using both the longitudinal and lateral control authority provided by collective thrust vectoring of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  provides omnidirectional horizontal control authority for aircraft  100 . For example, as best seen in  FIG. 4C , aircraft  100  has diagonal control authority responsive to collective thrust vectoring of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d . As illustrated, aircraft  100  has a longitudinal axis  120  and is operable for movement in the diagonal direction as indicated by arrow  152 . In the illustrated embodiment, flight control system  114  has sent commands to operate each of actuators  108   a ,  108   b ,  108   c ,  108   d  and actuators  110   a ,  110   b ,  110   c ,  110   d  to tilt each of propulsion systems  112   a ,  112   b ,  112   c ,  112   d  forward/right. In this configuration, propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  generate thrust vectors having aft/leftward directed components  154   a ,  154   b ,  154   c ,  154   d . In hover, such collective thrust vectoring of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  provides diagonal control authority to aircraft  100 . 
     The diagonal thrust vectoring operation will now be described with reference to propulsion assembly  102  in  FIGS. 5G-5I . As best seen in the comparison of  FIGS. 5G-5I , actuators  108 ,  110  are operated to tilt propulsion system  112  diagonally between a fully aft/right configuration shown in  FIG. 5G  and a fully forward/left configuration shown in  FIG. 5I  as well as in an infinite number of positions therebetween including the fully vertical configuration shown in  FIG. 5H . This operation diagonally shifts the thrust vector of propulsion assembly  102  to enable the diagonal control authority of aircraft  100  depicted in  FIG. 4C . The maximum diagonal tilt angle of gimbal  106  may preferably be between about 10 degrees and about 30 degrees, may more preferably be between about 15 degrees and about 25 degrees and may most preferably be about 20 degrees. As should be understood by those having ordinary skill in the art, the magnitude of the diagonal component  154  of the thrust vector is related to the direction of the thrust vector, which is determined by the diagonal tilt angle of gimbal  106 . 
     In addition to collective thrust vectoring of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d , aircraft  100  is also operable to engage in differential thrust vectoring of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d . For example, as best seen in  FIG. 4D , aircraft  100  has yaw authority responsive to differential thrust vectoring of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d . As illustrated, aircraft  100  has a longitudinal axis  120  and is operable for rotation thereabout as indicated by arrow  162 . In the illustrated embodiment, flight control system  114  has sent commands to operate each of actuators  108   a ,  108   b ,  108   c ,  108   d  and actuators  110   a ,  110   b ,  110   c ,  110   d  to tilt propulsion system  112   a  forward/right, to tilt propulsion system  112   b  aft/right, to tilt propulsion system  112   c  aft/left and to tilt propulsion system  112   d  forward/left. In this configuration, propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  generate thrust vectors having horizontal components  164   a ,  164   b ,  164   c ,  164   d . In hover, such differential thrust vectoring of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  provides yaw authority to aircraft  100 . 
     As discussed herein, outer gimbal member  128  is pivotally coupled to housing  126  and is operable to tilt about the first axis and inner gimbal member  130  is pivotally coupled to outer gimbal member  128  and is operable to tilt about the second axis that is orthogonal to the first axis. In the illustrated embodiment, in order to minimize the energy required to tilt propulsion system  112  relative to housing  126  to change the thrust vector direction of propulsion assembly  102 , the first and second axes pass through propulsion system  112 . The precise location of the intersection of the axes through propulsion system  112  may be determined based on factors including the mass of propulsion system  112 , the size and shape of propulsion system  112 , the desired rotational velocity of propulsion system  112  during thrust vectoring and other factors that should be understood by those having ordinary skill in the art. In one implementation, the first and second axes may pass through the center of mass of propulsion system  112 . Alternatively, it may be desirable to have the first and second axes pass through a location near the center of mass of propulsion system  112  such as within a predetermined distance from the center of mass of propulsion system  112 . The predetermined distance may be selected based upon criteria such as a defined volume surrounding the center of mass that contains a predetermined portion of the total mass of propulsion system  112 . For example, the first and second axes may pass through a location within a volume centered at the center of mass of propulsion system  112  that contains no more than ten percent of the mass of propulsion system  112 . Such a volume may be expressed, for example, as being within one centimeter, one inch or other predetermined distance from the center of mass of propulsion system  112 . 
     Due to dynamic effects caused by the rotation of the rotor assembly and the lift generated by the rotor assembly during flight operations, such as during thrust-borne flight operations, the center of mass in hover of propulsion system  112  may not coincide with the actual center of mass of propulsion system  112 . To compensate for the dynamic effects, the first and second axes may pass through the center of mass in hover of propulsion system  112 . Alternatively, it may be desirable to have the first and second axes pass through a location near the center of mass in hover of propulsion system  112  such as within a predetermined distance from the center of mass in hover of propulsion system  112 . In one example, it may be desirable to have the first and second axes pass through a location between the center of mass of propulsion system  112  and the center of mass in hover of propulsion system  112 . 
     Referring now to  FIGS. 6A-6D , the redundant directional control feature of aircraft  100  will now be described. In the illustrated embodiment, aircraft  100  includes a distributed thrust array depicted as four of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  that form a two-dimensional thrust array. As discussed herein and as best seen in  FIG. 3 , each propulsion assembly  102   a ,  102   b ,  102   c ,  102   d  includes an electronics node, a two-axis gimbal operated by two independent actuators and a propulsion system  112   a ,  112   b ,  112   c ,  112   d  that is operable to tilt with the gimbal relative to the propulsion assembly housing and the airframe of aircraft  100 . Flight control system  114  is operable to independently control the operating speeds of each electric motor and is operable to independently control the positions of each actuator such that for each propulsion assembly  102   a ,  102   b ,  102   c ,  102   d , a thrust vector can be resolved within a thrust vector cone. Importantly, in the event of an actuator fault or other fault in one of the propulsion assemblies, flight control system  114  sends commands to at least the symmetrically disposed propulsion assembly to counteract the fault. For example, to overcome a thrust vector error in one of the propulsion assemblies, flight control system  114  autonomously engages in corrective operations such as adjusting the thrust vector of the symmetrically disposed propulsion assembly to counteract the thrust vector error. Adjusting the thrust vector of the symmetrically disposed propulsion system may include tilting the propulsion system about the first axis, tilting the propulsion system about the second axis, changing the operating speed of the electric motor and combinations thereof. This autonomous corrective operation capability serves as redundancy in the directional control of aircraft  100  allowing aircraft  100  to have flight control in hover even during fault conditions. 
     Referring specifically to  FIG. 6A , a thrust vector error in propulsion assembly  102   b  has occurred due to, for example, a static actuator fault causing propulsion system  112   b  of propulsion assembly  102   b  to cease tilting in the longitudinal direction. The thrust vector error is depicted as dashed arrow  170 . Flight control system  114  recognizes the thrust vector error of propulsion assembly  102   b  and sends commands to at least propulsion assembly  102   d  to counteract the single-axis static actuator fault in propulsion assembly  102   b . In this case, the commands may include shifting actuator  108   d  to adjust the thrust vector of propulsion assembly  102   d  to include a corrective component depicted as solid arrow  172  that maintains the stability of aircraft  100 . In addition, flight control system  114  may command propulsion assemblies  102   a ,  102   c  to perform addition corrective actions to assist in counteracting the thrust vector error of propulsion assembly  102   b . Even in the fault condition, as propulsion assembly  102   b  continues to provide significant thrust in the vertical direction and thrust vector capability in the lateral direction, it may be desirable to maintain the operation of propulsion assembly  102   b  until aircraft  100  makes a safe landing allowing the autonomous corrective actions of flight control system  114  to counteract the thrust vector error. 
     Referring specifically to  FIG. 6B , a thrust vector error in propulsion assembly  102   c  has occurred due to, for example, a static actuator fault causing propulsion system  112   c  of propulsion assembly  102   c  to cease tilting in both the longitudinal and lateral directions. The thrust vector error is depicted as dashed arrow  174 . Flight control system  114  recognizes the thrust vector error of propulsion assembly  102   c  and sends commands to at least propulsion assembly  102   a  to counteract the two-axis static actuator fault in propulsion assembly  102   c . In this case, the commands may include shifting actuators  108   a ,  110   a  to adjust the thrust vector of propulsion assembly  102   a  to include a corrective component depicted as solid arrow  176  that maintains the stability of aircraft  100 . In addition, flight control system  114  may command propulsion assemblies  102   b ,  102   d  to perform addition corrective actions to assist in counteracting the thrust vector error of propulsion assembly  102   c . Even in the fault condition, as propulsion assembly  102   c  continues to provide significant thrust in the vertical direction, it may be desirable to maintain the operation of propulsion assembly  102   c  until aircraft  100  makes a safe landing allowing the autonomous corrective actions of flight control system  114  to counteract the thrust vector error. 
     Referring specifically to  FIG. 6C , a thrust vector error in propulsion assembly  102   c  has occurred due to, for example, a single-axis dynamic actuator fault causing propulsion system  112   c  of propulsion assembly  102   b  to tilt uncontrolled in the lateral direction. The thrust vector error is depicted as dashed arrows  178   a ,  178   b  that represent a continuum between maximum error positions. Flight control system  114  recognizes the thrust vector error of propulsion assembly  102   c  and sends commands to at least propulsion assembly  102   a  to counteract the single-axis dynamic actuator fault in propulsion assembly  102   c . In this case, the commands may include continually shifting actuator  110   a  to dynamically adjust the thrust vector of propulsion assembly  102   a  to include the time dependent corrective component depicted as solid arrows  180   a ,  180   b  representing the continuum between maximum corrective positions. The corrective action maintains the stability of aircraft  100 . In addition, flight control system  114  may command propulsion assemblies  102   b ,  102   d  to perform addition corrective actions to assist in counteracting the thrust vector error of propulsion assembly  102   c . Even in the fault condition, as propulsion assembly  102   c  continues to provide significant thrust in the vertical direction, it may be desirable to maintain the operation of propulsion assembly  102   c  until aircraft  100  makes a safe landing allowing the autonomous corrective actions of flight control system  114  to counteract the thrust vector error. 
     Referring specifically to  FIG. 6D , a thrust vector error in propulsion assembly  102   d  has occurred due to, for example, a two-axis dynamic actuator fault causing propulsion system  112   d  of propulsion assembly  102   d  to tilt uncontrolled in the longitudinal and lateral directions. The thrust vector error is depicted as dashed arrows  182   a ,  182   b ,  182   c ,  182   d  within dashed circle  184  representing the universe of error positions. Flight control system  114  recognizes the thrust vector error of propulsion assembly  102   d  and sends commands to at least propulsion assembly  102   b  to counteract the two-axis dynamic actuator fault in propulsion assembly  102   d . In this case, the commands may include continually shifting actuators  108   b ,  110   b  to dynamically adjust the thrust vector of propulsion assembly  102   b  to include the time dependent corrective component depicted as solid arrows  186   a ,  186   b ,  186   c ,  186   d  within solid circle  188  representing the universe of corrective positions. The corrective action maintains the stability of aircraft  100 . In addition, flight control system  114  may command propulsion assemblies  102   a ,  102   c  to perform addition corrective actions to assist in counteracting the thrust vector error of propulsion assembly  102   b . Even in the fault condition, as propulsion assembly  102   d  continues to provide significant thrust in the vertical direction, it may be desirable to maintain the operation of propulsion assembly  102   d  until aircraft  100  makes a safe landing allowing the autonomous corrective actions of flight control system  114  to counteract the thrust vector error. 
     In addition to performing autonomous corrective actions to counteract a thrust vector error, flight control system  114  may autonomously command aircraft  100  to perform other flight maneuvers. Depending upon the type of fault and the magnitude of the thrust vector error caused by the fault, flight control system  114  may command aircraft  100  to return to a maintenance center or other predetermined location. Under other fault situations, flight control system  114  may command aircraft  100  to initiate a jettison sequence of the pod assembly or other payload and/or perform an emergency landing. If the fault is not critical and/or is suitably overcome by the corrective actions described herein, flight control system  114  may command aircraft  100  to continue the current mission. In this case, flight control system  114  may command aircraft  100  to adjust the center of mass of the pod assembly or other payload relative to the airframe such as lowering the elevation of the pod assembly relative to the airframe as this may improve hover stability. 
     Referring next to  FIG. 7 , the directional control of an aircraft  200  having a distributed thrust array including single-axis gimbal mounted propulsion systems will now be described. Aircraft  200  includes a distributed thrust array depicted as four of propulsion assemblies  202   a ,  202   b ,  202   c ,  202   d  that form a two-dimensional thrust array. Propulsion assembly  202   a  includes electronics node  204   a , single-axis gimbal  206   a  operated by actuator  208   a  and propulsion system  112   a . Propulsion assembly  202   b  includes electronics node  204   b , single-axis gimbal  206   b  operated by actuator  208   b  and propulsion system  112   b . Propulsion assembly  202   c  includes electronics node  204   c , single-axis gimbal  206   c  operated by actuator  208   c  and propulsion system  112   c . Propulsion assembly  202   d  includes electronics node  204   d , single-axis gimbal  206   d  operated by actuator  208   d  and propulsion system  112   d . Each of electronics nodes  204   a ,  204   b ,  204   c ,  204   d  includes one or more batteries and one or more controllers such as an electronic speed controller. As discussed herein, each of propulsion systems  202   a ,  202   b ,  202   c ,  202   d  includes an electric motor having an output drive and a rotor assembly having a plurality of rotor blades. Each rotor assembly is rotatable with the respective output drive of the electric motor in a rotational plane to generate thrust. A flight control system  214  is operably associated with propulsion assemblies  202   a ,  202   b ,  202   c ,  202   d  and is communicably linked to electronic nodes  204   a ,  204   b ,  204   c ,  204   d  by communications network  216 . Flight control system  214  send commands to electronic nodes  204   a ,  204   b ,  204   c ,  204   d  to enable flight control system  214  to independently control each of propulsion assemblies  202   a ,  202   b ,  202   c ,  202   d.    
     For example, as best seen in  FIG. 8A , aircraft  200  has longitudinal control authority responsive to collective thrust vectoring of propulsion assemblies  202   a ,  202   b ,  202   c ,  202   d . As illustrated, aircraft  200  has a longitudinal axis  220  and is operable for movement in the longitudinal direction as indicated by arrow  222 . Flight control system  214  has sent commands to operate each of actuators  208   a ,  208   b ,  208   c ,  208   d  to tilt each of propulsion systems  212   a ,  212   b ,  212   c ,  212   d  in the forward direction. In this configuration, propulsion assemblies  202   a ,  202   b ,  202   c ,  202   d  generate thrust vectors having aftward directed longitudinal components  224   a ,  224   b ,  224   c ,  224   d . In hover, such collective thrust vectoring of propulsion assemblies  202   a ,  202   b ,  202   c ,  202   d  provides longitudinal control authority to aircraft  200 . 
     The longitudinal thrust vectoring operation will now be described with reference to an exemplary propulsion assembly  202 , depicted as a line replaceable propulsion unit, in  FIGS. 9A-9C . Propulsion assembly  202  includes a housing  226  and a gimbal  206  that is pivotally coupled to housing  126  and is operable to tilt about a single axis. In the illustrated embodiment, actuator  208  is coupled between housing  226  and gimbal  206  such that operation of actuator  208  shifts linkage  232  to tilt gimbal  206  about the axis relative to housing  226 . A propulsion system  212  is coupled to and is operable to tilt with gimbal  206  about the axis relative to housing  226 . In the illustrated embodiment, the rotor assembly has been removed from propulsion system  212  such that only electric motor  236  and output drive  238  are visible in the figures. 
     As best seen in the comparison of  FIGS. 9A-9C , actuator  208  is operated to tilt propulsion system  212  longitudinally between a fully forward configuration shown in  FIG. 9A  and a fully aft configuration shown in  FIG. 9C  as well as in an infinite number of positions therebetween including the fully vertical configuration shown in  FIG. 9B . This operation longitudinally shifts the thrust vector of propulsion assembly  202  to enable the longitudinal control authority of aircraft  200  depicted in  FIG. 8A . The maximum longitudinal tilt angle of gimbal  206  may preferably be between about 10 degrees and about 30 degrees, may more preferably be between about 15 degrees and about 25 degrees and may most preferably be about 20 degrees. As should be understood by those having ordinary skill in the art, the magnitude of the longitudinal component  224  of the thrust vector is related to the direction of the thrust vector, which is determined by the longitudinal tilt angle of gimbal  206 . 
     In the illustrated embodiment, the single gimbal axis is located below propulsion system  212 . In other single gimbal axis embodiments and similar to propulsion assembly  102  of  FIGS. 5A-5I , the single gimbal axis could alternately pass through propulsion system  212 . For example, the single gimbal axis could pass through the center of mass of propulsion system  212  or through a location near the center of mass of propulsion system  212 , such as within a predetermined distance from the center of mass of propulsion system  212 . As another example, the single gimbal axis could pass through the center of mass in hover of propulsion system  212 , through a location near the center of mass in hover of propulsion system  112 , such as within a predetermined distance from the center of mass in hover of propulsion system  112 , or through a location between the center of mass of propulsion system  212  and the center of mass in hover of propulsion system  212 . 
     In addition to collective thrust vectoring of propulsion assemblies  202   a ,  202   b ,  202   c ,  202   d , aircraft  200  is also operable to engage in differential longitudinal thrust vectoring of propulsion assemblies  202   a ,  202   b ,  202   c ,  202   d . For example, as best seen in  FIG. 8B , aircraft  200  has yaw authority responsive to differential longitudinal thrust vectoring of propulsion assemblies  202   b ,  202   d . As illustrated, aircraft  200  has a longitudinal axis  220  and is operable for rotation thereabout as indicated by arrow  226 . Flight control system  214  has sent commands to operate actuator  208   b  to tilt propulsion system  212   b  forward and to operate actuator  208   d  to tilt propulsion system  212   d  aftward. In this configuration, propulsion assembly  212   b  generates a thrust vector having an aftward directed longitudinal component  228   b  and propulsion assembly  212   d  generates a thrust vector having a forward directed longitudinal component  228   d . In hover, such differential longitudinal thrust vectoring of symmetrically disposed propulsion assemblies, such as propulsion assemblies  202   b ,  202   d , provides yaw authority to aircraft  200 . 
     Referring to  FIGS. 10A-10D , various independent mechanisms for providing yaw authority in hover to an aircraft of the present disclosure will now be described. Aircraft  10  described above will be used as the example aircraft for the present discussion wherein aircraft  10  would include four of the illustrated tail sections. Each tail section includes a tail member  300  depicted with a propulsion assembly  302  and a control surface  304  coupled thereto. Propulsion assembly  302  includes a rotor assembly  306  and may represent any propulsion assembly discussed herein including propulsion assemblies operable for single-axis thrust vectoring, two-axis thrust vectoring or no thrust vectoring. Control surface  304  is an active control surface operable for tilting in the longitudinal direction of aircraft  10  by actuator  308  via linkage  310  responsive to commands from flight control system  32 . In the illustrated embodiment, rotor assembly  306  has a rotor diameter D and control surface  304  is less than two rotor diameters (2D) and preferably between one rotor diameter and two rotor diameters from rotor assembly  306 . Locating control surface  304  within the specified distance from rotor assembly  306  enables control surface  304  to operate in the propwash of rotor assembly  306  in both thrust-borne flight and wing-borne flight. 
     If aircraft  10  utilizes embodiments of propulsion assembly  302  with no thrust vectoring, aircraft  10  has two independent yaw authority mechanisms in hover. In one approach, differential speed control is used to change the relative rotor speeds of the rotor assemblies rotating clockwise compared to the rotor assemblies rotating counterclockwise causing a torque imbalance in aircraft  10 , which provides yaw authority. This operation may be represented by the tail section configuration in  FIG. 10A . In the other approach, differential longitudinal control surface maneuvers of control surfaces  304  of two symmetrically disposed tail sections are used to create a yaw moment responsive to propwash blowing over the tilted control surfaces  304 . This operation may be represented by the tail section configuration in  FIG. 10C . Depending upon the yaw authority requirement, it may be desirable to use one yaw authority mechanism instead of another due to factors such as the response rate and yaw moment of a particular yaw authority mechanism. For example, a yaw authority mechanism with a faster response rate may be preferred for small and/or continuous corrections while a yaw authority mechanism with a larger yaw moment may be preferred for large correction and/or certain aircraft maneuvers such as large rotations about the longitudinal axis. In addition, an aircraft  10  having non thrust vectoring propulsion assemblies  302  may use a combination of differential speed control and differential longitudinal control surface maneuvers to provide yaw authority. This operation may be represented by the tail section configuration in  FIG. 10C . 
     If aircraft  10  utilizes embodiments of propulsion assembly  302  having single-axis or two-axis thrust vectoring, aircraft  10  has three independent yaw authority mechanisms in hover. In one approach, differential speed control is used to change the relative rotor speeds of the rotor assemblies rotating clockwise compared to the rotor assemblies rotating counterclockwise causing a torque imbalance in aircraft  10 , which provides yaw authority. This operation may be represented by the tail section configuration in  FIG. 10A . In another approach, differential longitudinal control surface maneuvers of control surfaces  304  of two symmetrically disposed tail sections are used to create a yaw moment responsive to propwash blowing over the tilted control surfaces  304 . This operation may be represented by the tail section configuration in  FIG. 10C . In the next approach, differential thrust vectoring is used to generate a yaw moment. In either single or two-axis thrust vectoring embodiments, this may be achieved by differential longitudinal thrust vectoring of two symmetrically disposed propulsion systems (see  FIG. 8B ). In addition, in two-axis thrust vectoring embodiments, this may be achieved by differential thrust vectoring of all propulsion systems by suitably clocking the thrust vectors at approximately 90 degrees from one another (see  FIG. 4D ). This operation may be represented by the tail section configuration in  FIG. 10B . Depending upon the yaw authority requirement, it may be desirable to use a faster response rate yaw authority mechanism for small and/or continuous corrections and a yaw authority mechanism with a larger yaw moment for large correction and/or certain aircraft maneuvers. 
     In addition, an aircraft  10  having thrust vectoring propulsion assemblies  302  may use a combination of differential speed control, differential longitudinal control surface maneuvers and differential thrust vectoring to provide yaw authority. For example, aircraft  10  could utilize differential speed control in combination with differential longitudinal control surface maneuvers, which may be represented by the tail section configuration in  FIG. 10C . As another example, aircraft  10  could utilize differential speed control in combination with differential thrust vectoring, which may be represented by the tail section configuration in  FIG. 10B . As a further example, aircraft  10  could utilize differential longitudinal control surface maneuvers in combination with differential thrust vectoring, which may be represented by the tail section configuration in  FIG. 10D . In a final example, aircraft  10  could utilize differential speed control in combination with differential longitudinal control surface maneuvers and differential thrust vectoring, which may be represented by the tail section configuration in  FIG. 10D . 
     The foregoing description of embodiments of the disclosure has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosure. The embodiments were chosen and described in order to explain the principals of the disclosure and its practical application to enable one skilled in the art to utilize the disclosure in various embodiments and with various modifications as are suited to the particular use contemplated. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the embodiments without departing from the scope of the present disclosure. Such modifications and combinations of the illustrative embodiments as well as other embodiments will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.