Patent Publication Number: US-11396374-B2

Title: Aircraft having radially extendable tailboom assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of co-pending application Ser. No. 15/241,436 filed Aug. 19, 2016. 
    
    
     TECHNICAL FIELD OF THE DISCLOSURE 
     The present disclosure relates, in general, to aircraft that take off and land on their tail and tilt horizontally for forward flight and, in particular, to tail sitter aircraft having a main lifting surface forward of a tailboom assembly that radially extends to form landing gear having a stable ground contact base. 
     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 a 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 oriented 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. 
     Another example of a VTOL aircraft is a tail sitter aircraft that lands and takes off on its tail section. The longitudinal fuselage axis is generally horizontal during forward flight and is generally vertical for hover, takeoff and landing. A fixed propulsion system is typically used to generate vertical thrust during hover, takeoff and landing and horizontal thrust during forward flight, wherein the wings provide lift. It has been found, however, the tail sitter aircraft having forward wings are unstable on the ground due to a high center of gravity and a large wing surface exposed to wind. Attempts have been made to design tail sitter aircraft with aft wings to lower the center of gravity while the aircraft is on the ground. It has been found, however, that having an aft main lifting surface reduces the aerodynamic stability of the aircraft during forward flight. 
     SUMMARY 
     In a first aspect, the present disclosure is directed to a tail sitter aircraft including a fuselage having a forward portion, an aft portion and a longitudinally extending fuselage axis. A main lifting surface is supported by the forward portion of the fuselage. A propulsion system is operably associated with the main lifting surface and is operable to provide thrust during forward flight, vertical takeoff, hover and vertical landing. A tailboom assembly extends from the aft portion of the fuselage. The tailboom assembly includes a plurality of rotatably mounted tail arms having control surfaces and landing members wherein, in a forward flight configuration, the tail arms are radially retracted to reduce tail surface geometry and provide yaw and pitch control with the control surfaces and, wherein, in a landing configuration, the tail arms are radially extended relative to one another about the fuselage axis to form a stable ground contact base with the landing members. 
     In some embodiments, the main lifting surface may include a pair of generally oppositely disposed wings. In other embodiments, the main lifting surface may include at least three generally circumferentially distributed wings. In certain embodiments, the propulsion system may be a distributed propulsion system such as a propulsion system including a plurality of independently controllable cross-flow fans including independently controllable variable thrust cross-flow fans. In other embodiments, the propulsion system may include at least a pair of rotor assemblies including independently controllable rotor assemblies. 
     In some embodiments, the tailboom assembly may include at least three rotatably mounted tail arms. In other embodiments, the tailboom assembly may include at least four rotatably mounted tail arms. In certain embodiments, the tailboom assembly may have an actuator assembly operable to transition the tail arms between the forward flight configuration and the landing configuration. The actuator assembly may include a cross arm assembly coupled to each of the tail arms. The actuator assembly may also include one or more linear actuators, one or more rotary actuators or combinations thereof. In addition, the actuator assembly may include a locking system operable to secure the tail arms in one or both of the forward flight configuration and the landing configuration. 
     In a second aspect, the present disclosure is directed to a tail sitter aircraft including a fuselage having a forward portion, an aft portion and a longitudinally extending fuselage axis. First and second wings are supported by the forward portion of the fuselage and provide a main lifting surface for the aircraft. A propulsion system includes first and second rotor assemblies that are respectively attached to the first and second wings. The propulsion system is operable to provide thrust during forward flight, vertical takeoff, hover and vertical landing. A tailboom assembly extends from the aft portion of the fuselage. The tailboom assembly includes a plurality of rotatably mounted tail arms having control surfaces and landing members wherein, in a forward flight configuration, the tail arms are radially retracted to reduce tail surface geometry and provide yaw and pitch control with the control surfaces and, wherein, in a landing configuration, the tail arms are radially extended relative to one another about the fuselage axis to form a stable ground contact base with the landing members. 
     In a third aspect, the present disclosure is directed to a tail sitter aircraft including a fuselage having a forward portion, an aft portion and a longitudinally extending fuselage axis. At least three generally circumferentially distributed wings are supported by the forward portion of the fuselage and provide a main lifting surface for the aircraft. A distributed propulsion system is operably associated with the wings and is operable to provide thrust during forward flight, vertical takeoff, hover and vertical landing. A tailboom assembly extends from the aft portion of the fuselage. The tailboom assembly includes a plurality of rotatably mounted tail arms having control surfaces and landing members wherein, in a forward flight configuration, the tail arms are radially retracted to reduce tail surface geometry and provide yaw and pitch control with the control surfaces and, wherein, in a landing configuration, the tail arms are radially extended relative to one another about the fuselage axis to form a stable ground contact base with the landing members. 
    
    
     
       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-1B  are schematic illustrations of a tail sitter aircraft in accordance with embodiments of the present disclosure; 
         FIGS. 1C-1D  are schematic illustrations of a variable thrust cross-flow fan system for a tail sitter aircraft in accordance with embodiments of the present disclosure; 
         FIGS. 2A-2F  are schematic illustrations of a tail sitter aircraft in a sequential flight operating scenario in accordance with embodiments of the present disclosure; 
         FIGS. 3A-3B  are schematic illustrations of a tail sitter aircraft in accordance with embodiments of the present disclosure; and 
         FIGS. 4A-4D  are various views of a tailboom assembly for a tail sitter 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. 
     Referring to  FIGS. 1A-1B  in the drawings, a tail sitter aircraft is schematically illustrated and generally designated  10 . Tail sitter aircraft  10  includes a fuselage  12 , a plurality of wings  14 ,  16 ,  18  and a tailboom assembly  20 . As illustrated, wings  14 ,  16 ,  18  are forward of tailboom assembly  20  during forward flight and are considered to be supported by a forward portion of fuselage  12  while tailboom assembly  20  extends from an aft portion of fuselage  12 . Preferably, wings  14 ,  16 ,  18  each have an airfoil cross-section operable to generate lift responsive to the forward airspeed of tail sitter aircraft  10  and form the main lifting surface of tail sitter aircraft  10 . Tail sitter aircraft  10  has a propulsion system depicted as a distributed propulsion system  22  including a plurality of propulsion assemblies in the form of cross-flow fans located in chordwise channels of wings  14 ,  16 ,  18 . For example, cross-flow fans  22 A- 22 E are located in chordwise channels  24 A- 24 E depicted in phantom in wing  16  in  FIG. 1B . 
     Preferably, as discussed herein, the cross-flow fans have variable thrust capacities and may be operated independent of one another. Cross-flow fans of the present disclosure may be operated responsive to one or more electrical motors, hydraulic motors and/or liquid fuel powered engines. One or more cross-flow fans may be operated on a common drive shaft or each cross-flow fan may be operated by a unique drive system. Preferably, variable thrust control, as discussed herein, for each cross-flow fan is independent. Alternatively, more than one cross-flow fan could share a common variable thrust control actuator or system. As illustrated, air enters cross-flow fans  22 A- 22 E from a forward intake portion of a respective chordwise channel  24 A- 24 E and exits cross-flow fans  22 A- 22 E into an aft discharge portion of a respective chordwise channel  24 A- 24 E, thereby generating thrust generally parallel to a longitudinal fuselage axis  26 . The aft portions of chordwise channels may include flaperons or other flow directing members to enable thrust vectoring. 
     Tailboom assembly  20  includes a plurality of control surfaces used during forward flight depicted as rudder  28  for yaw control and elevators  30 ,  32  for pitch control, in the illustrated configuration. It is noted that tail sitter aircraft  10  may fly in other orientations wherein the control surfaces may serve alternate functions. For example, if wings  16 ,  18  are above fuselage  12  and wing  14  is below fuselage  12 , then control surface  32  would operate as the rudder and control surfaces  28 ,  30  would operate as the elevators. In addition, even though tail sitter aircraft  10  is depicted and described as having three wings that are circumferentially distributed uniformly about fuselage  12 , it should be understood by those skilled in the art that a tail sitter aircraft of the present disclosure could have other numbers of wings both greater than and less than three and/or have wings that are oriented in a nonuniform manner. In the present example, instead of wings  14 ,  16 ,  18  being oriented at 120-degree circumferential intervals (120/120/120), the wings could be oriented as 105/150/105, 90/180/90 or other desired wing orientation permutation. Also, even though control surface  28 ,  30 ,  32  are depicted as being circumferentially offset from wings  14 ,  16 ,  18  by 60 degrees, it should be understood by those skilled in the art that control surfaces for a tail sitter aircraft of the present disclosure could have other orientations relative to the wings including being circumferentially inline with the wings. Further, even though the same number of wings and control surfaces has been depicted, it should be understood by those skilled in the art that the number of control surfaces and the number of wings are independent of each other. 
     Tailboom assembly  20  includes a plurality of tail arms  34 ,  36 ,  38  that are operable to be radially retracted in forward flight, as best seen in  FIG. 1A , forming a small tail surface geometry wherein control surfaces  28 ,  30 ,  32  provide yaw and pitch control. In addition, tail arms  34 ,  36 ,  38  are operable to be radially extended for landing, as best seen in  FIG. 1B , forming a stable ground contact base. As illustrated, each tail arm  34 ,  36 ,  38  includes one of the control surface  28 ,  30 ,  32 . As illustrated, each tail arm  34 ,  36 ,  38  includes a landing member  40 ,  42 ,  44  such as a fixed or retractable skid member or shock absorbing member such as a pneumatic shock strut or mechanical spring assembly. Landing members  40 ,  42 ,  44  may also include wheels to assist in ground maneuvers. The length of tail arms  34 ,  36 ,  38  as well as the angle tail arms  34 ,  36 ,  38  make with longitudinal fuselage axis  26  in the landing configuration may be determined based upon the location of the center of gravity  46  of tail sitter aircraft  10 . Preferably, center of gravity  46  should be located within the tip over angle from the ground contact base  48  of landing member  40 ,  42 ,  44 , wherein the tip over angle may be about 55 degrees. As discussed herein, tailboom assembly  20  includes an actuator assembly  50  operable to transition tail arms  34 ,  36 ,  38  between the forward flight configuration and the landing configuration. Tail arms  34 ,  36 ,  38  may also include one more sensors that indicate the position of tail arms  34 ,  36 ,  38  such as fully retracted and fully deployed as well as fault positions if tail arms  34 ,  36 ,  38  fail to reach the fully retracted and/or the fully deployed positions. Tail arms  34 ,  36 ,  38  may also include a fail safe mechanism to bias tail arms  34 ,  36 ,  38  toward the landing configuration in the event of a tail arm fault. 
     Referring to  FIG. 1C , therein is depicted an embodiment of a variable thrust cross-flow fan, such as variable thrust cross-flow fan  22 A of  FIG. 1B . Variable thrust cross-flow fan  22 A includes a cross-flow fan assembly  52  including driver plates  54 ,  56  which are coupled to and are rotatable about a longitudinal axis by a drive shaft  58 . Cross-flow fan assembly  52  includes a plurality of blades  58  that are each rotatably coupled between driver plates  54 ,  56 . As illustrated, blades  58  are disposed radially outwardly from the longitudinal axis such that blades  58  follow a generally circular path of travel when cross-flow fan assembly  52  rotates about the longitudinal axis. Variable thrust cross-flow fan  22 A includes a control assembly  60  that is coupled to each of blades  58 . In the illustrated embodiment, control assembly  60  includes a control cam  62  that is rotatable with and translatable relative to cross-flow fan assembly  52 . Control assembly  60  also includes a plurality of linkages  64  that are slidably coupled to control cam  62  via follower pins  66  in follower slots  68  of driver plate  54  and fixably coupled to blades  58  via driver pins  70  that extend through linkage holes (not visible) of driver plate  54 . 
     When cross-flow fan assembly  52  is rotated by drive shaft  58  and control cam  62  is positioned concentrically with cross-flow fan assembly  52 , follower pins  66  do not move relative to follower slots  68  and blades  58  do not rotate relative to driver plates  54 ,  56 . In this state, blades  58  are in a neutral configuration wherein each of the blades  58  has a substantially zero pitch during an entire revolution of cross-flow fan assembly  52 , as illustrated in  FIG. 1C . In this neutral configuration, all of blades  58  have a substantially zero angle of attack and therefore produce little or no thrust. To produce thrust, blades  58  are rotated relative to driver plates  54 ,  56  in response to shifting control cam  62  forward, in the chordwise direction of wing  16 , relative to cross-flow fan assembly  52 . When control cam  62  is position eccentrically relative to cross-flow fan assembly  52  and cross-flow fan assembly  52  is rotated by drive shaft  58 , follower pins  66  cyclically slide within follower slots  68  which cyclically pivots linkages  64  and cyclically rotates blades  58  relative to driver plates  54 ,  56 . 
     Referring additionally to  FIG. 1D , variable thrust cross-flow fan  22 A is disposed within chordwise channel  24 A of wing  16 . In the illustrated configuration, the center of rotation of control cam  62  has been shifted forward from a concentric location  72  to an eccentric location  74 . In this configuration, as each blade  58  follows the generally circular path of travel, the blades transition between positive pitch, zero pitch, negative pitch, zero pitch and back to positive pitch during each revolution of cross-flow fan assembly  52 . As illustrated, blades  58  have an airfoil cross section and travel in a counterclockwise direction. As blades  58  approach forward intake  76  of chordwise channel  24 A, the blades have progressively greater positive pitch reaching a maximum positive pitch proximate axis  78 . Thereafter, as blades  58  retreat from forward intake  76 , the blades have progressively lesser positive pitch reaching zero pitch proximate axis  80 . As blades  58  approach aft discharge  82  of chordwise channel  24 A, the blades have progressively greater negative pitch reaching a maximum negative pitch proximate axis  78 . Thereafter, as blades  58  retreat from aft discharge  82 , the blades have progressively lesser negative pitch, reaching zero pitch proximate axis  80 . Each blade  58  repeats this cycle on each revolution of cross-flow fan assembly  52 . 
     As blades  58  follow the generally circular path of travel with the cyclically varying angle of attack described herein, air passes through cross-flow fan assembly  52  as indicated by low-density intake airflow arrows  84  and high-density discharge airflow arrow  86  with the resultant thrust indicated by arrow  88 . The magnitude of thrust  88  generated by variable thrust cross-flow fan system  22 A is determined by factors including the magnitude of the eccentricity applied to control cam  62 , the rotational speed of cross-flow fan assembly  52 , the cross sectional shape of blades  58 , the pitch cycle of blades  58 , the number of blades  58  and other factors known to those having skill in the art. 
     Referring next to  FIGS. 2A-2F  in the drawings, a sequential flight-operating scenario of tail sitter aircraft  10  is depicted. Tail sitter aircraft  10  may be a manned or unmanned aircraft and may be operated responsive to onboard pilot flight control, remote flight control or autonomous flight control. Tail sitter aircraft  10  is preferably a fly-by-wire aircraft including an onboard flight control computing system that is operable to receive sensor data from and send flight commands to propulsion system controllers, flaperon controllers, control surface controllers, tailboom actuator assembly controllers, landing member actuator controllers, other systems controllers and the like. Preferably, onboard flight control computing system is operable to individually and independently control and operate each of the propulsion assemblies. As best seen in  FIG. 2A , tail sitter aircraft  10  is in its vertical takeoff and landing configuration. Preferably, all propulsion assemblies are operating to provide maximum thrust and control during vertical takeoff and hover operations. As tail sitter aircraft  10  continues its vertical assent to a desired elevation, it may begin the transition from the vertical takeoff and landing configuration toward the forward flight configuration. 
     As best seen in  FIG. 2B , as tail sitter aircraft  10  transitions from vertical takeoff and landing to forward flight, tailboom actuator assembly  50  radially retracts tail arms  34 ,  36 ,  38  such that tailboom  20  has a reduced tail surface geometry. Tail sitter aircraft  10  also begins to transition its longitudinal fuselage axis  26  from the vertical attitude toward the horizontal attitude. As best seen in  FIG. 2C , tail sitter aircraft  10  has completed the transition to forward flight mode. During forward flight, it may be desirable to maximize flight efficiency, which in turn increases the endurance of tail sitter aircraft  10 . One way to increase efficiency is to fly tail sitter aircraft  10  with a single wing, in this case wing  18 , in the down position and with wings  14 ,  16  extending upwardly at approximately 30 degrees relative to a horizontal axis. In the illustrated embodiment, this position results in desirably oriented control surfaces depicted as rudder  28  for yaw control and elevators  30 ,  32  for pitch control. In addition, once tail sitter aircraft  10  is in the forward flight mode, the thrust requirements are reduced compared to the thrust requirements of vertical takeoff and hovering. Accordingly, in forward flight mode, the thrust output of one or more of the propulsion assemblies may be reduced by, for example, reducing the eccentricity of control cam  62  relative to cross-flow fan assembly  52 . Alternatively or additionally, one or more of the propulsion assemblies may be shut down during forward flight. It is noted that during forward flight, aerodynamic forces tend to bias tail arms  34 ,  36 ,  38  toward the radially retracted, forward flight configuration. 
     Continuing with the current example, as tail sitter aircraft  10  approaches the destination, all propulsion assemblies are preferably reengaged to provide full propulsion capabilities while remaining in forward flight mode, as best seen in  FIG. 2D . Thereafter, tail sitter aircraft  10  may begin its transition from forward flight mode to vertical takeoff and landing mode, as best seen in  FIG. 2E , wherein longitudinal fuselage axis  26  shifts from the horizontal attitude toward the vertical attitude. As tail sitter aircraft  10  continues its vertical descent, as best seen in  FIG. 2F , tailboom actuator assembly  50  radially extends tail arms  34 ,  36 ,  38  such that tailboom  20  forms a stable ground contact base. In addition, tailboom actuator assembly  50  may deploy landing members  40 ,  42 ,  44 , if they were retracted during forward flight mode. 
     Referring to  FIGS. 3A-3B  in the drawings, a tail sitter aircraft is schematically illustrated and generally designated  100 . Tail sitter aircraft  100  includes a fuselage  102 , a pair of wings  104 ,  106  and a tailboom assembly  108 . As illustrated, wings  104 ,  106  are forward of tailboom assembly  108  during forward flight and are considered to be supported by a forward portion of fuselage  102  while tailboom assembly  108  extends from an aft portion of fuselage  102 . Preferably, wings  104 ,  106  each have an airfoil cross-section operable to generate lift responsive to the forward airspeed of tail sitter aircraft  100  and form the main lifting surface of tail sitter aircraft  100 . Tail sitter aircraft  100  includes a pair of propulsion assemblies depicted as rotor assemblies  110 ,  112 . Rotor assembly  110  includes a fixed nacelle  114  that houses an engine and transmission that provide torque and rotational energy to a drive shaft that rotates a rotor hub assembly  116  and a plurality of rotor blade assemblies  118 . Likewise, rotor assembly  112  includes a fixed nacelle  120  that houses an engine and transmission that provide torque and rotational energy to a drive shaft that rotates a rotor hub assembly  122  and a plurality of rotor blade assemblies  124 . 
     Even though tail sitter aircraft  100  is depicted as having a particular number of rotor assemblies, it should be understood by those skilled in the art that a tail sitter aircraft of the present disclosure could have any desired number of rotor assemblies, wherein the operation of the rotor assemblies may be independent. Likewise, even though rotor assemblies  110 ,  112  are depicted as having a particular number of rotor blade assemblies, it should be understood by those skilled in the art that rotor assemblies of the present disclosure could have any desired number of rotor blade assemblies. Also, even though rotor blade assemblies  118 ,  124  are depicted as having a particular length and twist, it should be understood by those skilled in the art that rotor blade assemblies of the present disclosure could have any desired configuration suitable for providing vertical thrust in landing configuration and forward thrust in forward flight configuration. 
     Tailboom assembly  108  includes a plurality of control surfaces used during forward flight depicted as rudders  126   a ,  126   b  for yaw control and elevators  128 ,  130  for pitch control. Tailboom assembly  108  also includes a plurality of tail arms  132 ,  134 ,  136 ,  138  that are operable to be radially retracted in forward flight, as best seen in  FIG. 3A , forming a small tail surface geometry wherein control surfaces  126   a ,  126   b ,  128 ,  130  provide yaw and pitch control. In addition, tail arms  132 ,  134 ,  136 ,  138  are operable to be radially extended for landing, as best seen in  FIG. 3B , forming a stable ground contact base. As illustrated, tail arm  132  includes rudder  126   a , tail arm  134  includes elevator  128 , tail arm  136  includes rudder  126   b  and tail arm  138  includes elevator  130 . Also, each tail arm  132 ,  134 ,  136 ,  138  includes a landing member  140 ,  142 ,  144 ,  146 , illustrated as pneumatic shock struts with wheels. The length of tail arms  132 ,  134 ,  136 ,  138  as well as the angle tail arms  132 ,  134 ,  136 ,  138  make with a longitudinal fuselage axis  148  in the landing configuration may be determined based upon the location of the center of gravity  150  of tail sitter aircraft  100 . Preferably, center of gravity  150  should be located within the tip over angle from the ground contact base  152  of landing members  140 ,  142 ,  144 ,  146 . As discussed herein, tailboom assembly  108  includes an actuator assembly  154  operable to transition tail arms  132 ,  134 ,  136 ,  138  between the forward flight configuration and the landing configuration. 
     Referring next to  FIGS. 4A-4D  of the drawings, various views of a tailboom assembly for a tail sitter aircraft are illustrated and generally designated  200 . Tailboom assembly  200  includes a mounting assembly  202  that is operable to be connected to an aft portion of the fuselage of a tail sitter aircraft by bolting, pining, threading, welding or other suitable coupling technique. In the illustrated embodiment, tailboom assembly  200  includes three tail arms  204 ,  206 ,  208 . As discussed herein, tail arms  204 ,  206 ,  208  are operable to be radially retracted in a forward flight mode to form a small tail surface geometry wherein control surfaces  210 ,  212 ,  214  provide yaw and pitch control. In addition, tail arms  204 ,  206 ,  208  are operable to be radially extended in a landing mode to form a stable ground contact base with landing members  216 ,  218 ,  220 . Mounting assembly  202  includes a frame member  222  that may be a component of tailboom assembly  200  or a component of the aft portion of the fuselage of a tail sitter aircraft, depending upon the implementation. Mount assembly  202  also includes a tail arm attachment fitting  224  that is securably coupled to frame member  222  by three brackets  226 ,  228 ,  230 . In the illustrated embedment, brackets  226 ,  228 ,  230  are bolted to frame member  222  and tail arm attachment fitting  224 , however, those skilled in the art will recognize that brackets of the present disclosure may be secured to frame members and tail arm attachment fittings of the present disclosure using other coupling techniques including pins, rivets, welding, adhesion and the like. Alternatively, brackets of the present disclosure could be integral with frame members and/or tail arm attachment fittings of the present disclosure. Tail arm attachment fitting  224  has a plurality of flanges  232 ,  234 ,  236 ,  238 ,  240 ,  242 , each having an eyehole (not visible) for receiving a fastener therethrough. 
     Tail arms  204 ,  206 ,  208  each include a dual clevis bracket assembly  244 ,  246 ,  248  attached to a forward end thereof. In the illustrated embodiment, dual clevis bracket assemblies  244 ,  246 ,  248  are bolted to tail arms  204 ,  206 ,  208 , however, those skilled in the art will recognize that dual clevis bracket assemblies of the present disclosure may be secured to tail arms of the present disclosure using other coupling techniques including pins, rivets, welding, adhesion and the like. As illustrated, dual clevis bracket assembly  244  mates with flanges  232 ,  234  of tail arm attachment fitting  224  and is rotatably coupled therewith using bolt connections. Likewise, dual clevis bracket assembly  246  mates with flanges  236 ,  238  of tail arm attachment fitting  224  and is rotatably coupled therewith using bolt connections and dual clevis bracket assembly  248  mates with flanges  240 ,  242  of tail arm attachment fitting  224  and is rotatably coupled therewith using bolt connections. Even though dual clevis bracket assemblies  244 ,  246 ,  248  are depicted as being rotatably coupled to tail arm attachment fitting  224  using bolt connections, it should be understood by those skilled in the art that dual clevis bracket assemblies of the present disclosure may be rotatably coupled to tail arm attachment fitting of the present disclosure using other coupling techniques including pins, bearings and the like. 
     The illustrated mounting assembly  202  enables tail arms  204 ,  206 ,  208  to rotate relative to the fuselage of a tail sitter aircraft such that tail arms  204 ,  206 ,  208  are operable to radially retract in a forward flight configuration and radially extend in a landing configuration, as discussed herein. Even though mounting assembly  202  has been depicted and described as having a particular array of components in a particular configuration, it should be understood by those skilled in the art that a mounting assembly of the present disclosure may have fewer components, more components and/or different components. 
     Tailboom assembly  200  includes an actuator assembly  250  operable to transition tail arms  204 ,  206 ,  208  between the forward flight configuration and the landing configuration. In the illustrated embodiment, actuator assembly  250  includes three brace arms  252 ,  254 ,  256  that are rotatably mounted to tail arms  204 ,  206 ,  208  at outer support members, only outer support member  258  of tail arm  208  being visible in the drawings. In addition, brace arms  252 ,  254 ,  256  are rotatably coupled to a central support member  260  that guilds movement of brace arms  252 ,  254 ,  256  during transitions between the forward flight configuration and the landing configuration. Preferably, brace arms  252 ,  254 ,  256  form a truss structure between tail arms  204 ,  206 ,  208  when the tail sitter aircraft is on the ground to reduce or prevent bending moments in tail arms  204 ,  206 ,  208 . In the illustrated embodiment, brace arms  252 ,  254 ,  256  are moved together as a unit by actuator  262 , thereby rotating tail arms  204 ,  206 ,  208  together during transitions between the forward flight configuration and the landing configuration. Preferably, actuator  262  is an electrically operated actuator that may be a linear actuator, a rotary actuator or a combination thereof. As illustrated, actuator  262  is supported by a tail arm, namely tail arm  204 , but could alternatively be supported by mounting assembly  202  or another component of the tail sitter aircraft. 
     Actuator  262  includes a brake system operable to secure actuator  262  in desired positions, thereby enabling actuator  262  to serve as a lock against unwanted rotational movement of tail arms  204 ,  206 ,  208 . For example, it may be desirable to lock tail arms  204 ,  206 ,  208  in the forward flight configuration and/or the landing configuration for security and safety. Alternatively or additionally, a locking system separate from actuator  262  may be used to secure tail arms  204 ,  206 ,  208  in the forward flight configuration and/or the landing configuration. Even though a single actuator  262  has been depicted and described for transitioning tail arms  204 ,  206 ,  208  between the forward flight configuration and the landing configuration, it should be understood by those skilled in the art that tail arm transitioning for a tail sitter aircraft of the present disclosure could utilize other actuation protocols including having individual actuators for each tail arm. In addition, actuator  262  or another actuation system may be used to deploy landing members  216 ,  218 ,  220  in embodiments having retractable landing members. 
     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.