Patent Publication Number: US-2023138684-A1

Title: Ground State Determination Systems for Aircraft

Description:
TECHNICAL FIELD OF THE DISCLOSURE 
     The present disclosure relates, in general, to ground state determination systems for use on aircraft and, in particular, to systems and methods for determining whether an aircraft is on a surface such as the ground, including before takeoff or after landing, based on aircraft sensor data, thrust commands and/or other parameters. 
     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 and generate a lifting force as the aircraft moves forward to support the aircraft 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 and landing vertically. Rotorcraft such as helicopters, tiltrotors, tiltwings, quadcopters and other multicopters are examples of VTOL aircraft. Each of these rotorcraft utilizes one or more rotors to provide lift and thrust to the aircraft. The rotors not only enable vertical takeoff and landing, but may also enable hover, forward flight, backward flight and lateral flight. These attributes make VTOL aircraft highly versatile for use in congested, isolated or remote areas. 
     In an operational scenario, several important functions of VTOL aircraft depend on whether the aircraft is on a surface such as the ground or a landing pad. These functions include activating flight control loops, changing flight control loop gains, syncing positions, activating motors, deploying a payload and changing integrator gains. For example, it may be undesirable to accumulate a large integrated motor command while the aircraft is on the ground. It may also be undesirable to build a positioning error, for example due to global positioning satellite (GPS) drift, which could affect navigation. It is also useful for remote operators of an aircraft having no line of sight to the aircraft to know when the aircraft is on the ground. 
     Previous attempts at determining the ground state of aircraft have relied on mechanical ground proximity switches or weight sensors, which have proven to be unreliable and add weight, cost and complexity to the aircraft. These prior solutions are particularly unreliable and prohibitive when implemented on small or lightweight aircraft. Other prior systems have relied on analyzing the vibrational profile of the aircraft both in flight and on the ground and comparing such vibrational profiles to determine whether the aircraft is grounded, although such systems have also lacked adequate reliability. Accordingly, a need has arisen for ground state determination systems that reliably determine whether an aircraft is on a surface while avoiding the additional weight, cost and complexity associated with previous systems. 
     SUMMARY 
     In a first aspect, the present disclosure is directed to a ground state determination system for an aircraft including sensors configured to detect parameters of the aircraft and a flight control system implementing a ground state module. The ground state module includes a ground state monitoring module configured to monitor the parameters and a ground state determination module configured to compare each of the parameters monitored by the ground state monitoring module to a respective parameter threshold to determine whether the aircraft is on a surface. 
     In some embodiments, the sensors may include a distance sensor to detect an altitude of the aircraft and the parameters monitored by the ground state monitoring module may include the altitude of the aircraft. In such embodiments, the ground state determination module may be configured to compare the altitude of the aircraft to an altitude threshold, thereby determining whether the aircraft is on the surface based at least partially on the altitude of the aircraft. In certain embodiments, the sensors may include an attitude sensor to detect a pitch of the aircraft and the parameters monitored by the ground state monitoring module may include the pitch of the aircraft. In such embodiments, the ground state determination module may be configured to compare the pitch of the aircraft to a pitch threshold, thereby determining whether the aircraft is on the surface based at least partially on the pitch of the aircraft. In some embodiments, the sensors may include an accelerometer to detect a vertical acceleration of the aircraft and the parameters monitored by the ground state monitoring module may include the vertical acceleration of the aircraft. In such embodiments, the ground state determination module may be configured to compare the vertical acceleration of the aircraft to a vertical acceleration threshold, thereby determining whether the aircraft is on the surface based at least partially on the vertical acceleration of the aircraft. In certain embodiments, the sensors may include a velocity sensor to detect a vertical velocity of the aircraft and the parameters monitored by the ground state monitoring module may include the vertical velocity of the aircraft. In such embodiments, the ground state determination module may be configured to compare the vertical velocity of the aircraft to a vertical velocity threshold, thereby determining whether the aircraft is on the surface based at least partially on the vertical velocity of the aircraft. 
     In some embodiments, the flight control system may include a thrust command module configured to provide a thrust command to a propulsion assembly of the aircraft and the parameters monitored by the ground state monitoring module may include the thrust command. In such embodiments, the ground state determination module may be configured to compare the thrust command to a thrust command threshold, thereby determining whether the aircraft is on the surface based at least partially on the thrust command. In certain embodiments, the ground state monitoring module may include a time module configured to monitor time persistence of the parameters. In such embodiments, the ground state determination module may be configured to determine whether the aircraft is on the surface based at least partially on the time persistence of the parameters. 
     In a second aspect, the present disclosure is directed to an aircraft including a fuselage, sensors configured to detect parameters of the aircraft and a flight control system in communication with the sensors. The flight control system implements a ground state module including a ground state monitoring module configured to monitor the parameters and a ground state determination module configured to compare each of the parameters monitored by the ground state monitoring module to a respective parameter threshold to determine whether the aircraft is on a surface. 
     In some embodiments, the aircraft may be a tailsitter aircraft. In certain embodiments, the aircraft may include an airframe having first and second wings with first and second pylons extending therebetween, the fuselage coupled to the pylons. In such embodiments, the aircraft may include a two-dimensional distributed thrust array attached to the airframe, the thrust array including propulsion assemblies coupled to the first wing and propulsion assemblies coupled to the second wing. 
     In a third aspect, the present disclosure is directed to a method for determining a ground state of an aircraft including monitoring parameters of the aircraft; comparing each of the parameters to a respective parameter threshold; and determining whether the aircraft is on a surface in response to comparing each of the parameters to the respective parameter threshold. 
     In some embodiments, monitoring the parameters may include monitoring a thrust command, an altitude of the aircraft, a pitch of the aircraft, a vertical acceleration of the aircraft and/or a vertical velocity of the aircraft. In certain embodiments, monitoring the parameters may include monitoring an altitude of the aircraft and a pitch of the aircraft, comparing each of the parameters to the respective parameter threshold may include comparing the altitude of the aircraft to an altitude threshold and comparing the pitch of the aircraft to a pitch threshold and determining whether the aircraft is on the surface may include determining whether the aircraft is on the surface in response to comparing the altitude of the aircraft to the altitude threshold and comparing the pitch of the aircraft to the pitch threshold. In some embodiments, determining whether the aircraft is on the surface may include determining that the aircraft is on the surface in response to the altitude of the aircraft being less than the altitude threshold and the pitch of the aircraft exceeding the pitch threshold. In certain embodiments, the pitch threshold may be in a range between 30 degrees and 60 degrees. In some embodiments, monitoring the parameters may include monitoring a thrust command to a propulsion assembly of the aircraft, a vertical acceleration of the aircraft and a pitch of the aircraft, comparing each of the parameters to the respective parameter threshold may include comparing the thrust command to a thrust command threshold, comparing the vertical acceleration of the aircraft to a vertical acceleration threshold and comparing the pitch of the aircraft to a pitch threshold and determining whether the aircraft is on the surface may include determining whether the aircraft is on the surface in response to comparing the thrust command to the thrust command threshold, comparing the vertical acceleration of the aircraft to the vertical acceleration threshold and comparing the pitch of the aircraft to the pitch threshold. In some embodiments, determining whether the aircraft is on the surface may include determining that the aircraft is on the surface in response to the thrust command being less than the thrust command threshold, the vertical acceleration of the aircraft exceeding the vertical acceleration threshold and the pitch of the aircraft exceeding the pitch threshold. In certain embodiments, the thrust command threshold may be in a range between 10 percent and 40 percent of hover power thrust. In some embodiments, the vertical acceleration threshold may be equal to or less than zero. 
     In certain embodiments, monitoring the parameters may include monitoring an altitude of the aircraft, a pitch of the aircraft, a thrust command to a propulsion assembly of the aircraft and a vertical acceleration of the aircraft and comparing each of the parameters to the respective parameter threshold may include comparing the altitude of the aircraft to an altitude threshold, comparing the pitch of the aircraft to a pitch threshold, comparing the thrust command to a thrust command threshold and comparing the vertical acceleration of the aircraft to a vertical acceleration threshold. In such embodiments, determining whether the aircraft is on the surface may include a first determination step determining whether the aircraft is on the surface in response to comparing the altitude of the aircraft to the altitude threshold and comparing the pitch of the aircraft to the pitch threshold and a second determination step determining whether the aircraft is on the surface in response to comparing the thrust command to the thrust command threshold, comparing the vertical acceleration of the aircraft to the vertical acceleration threshold and comparing the pitch of the aircraft to the pitch threshold. In such embodiments, determining whether the aircraft is on the surface may include determining that the aircraft is on the surface in response to either or both of the first determination step or the second determination step determining that the aircraft is on the surface. 
     In some embodiments, monitoring the parameters may include monitoring an altitude of the aircraft, a thrust command to a propulsion assembly of the aircraft and a vertical acceleration of the aircraft and comparing each of the parameters to the respective parameter threshold may include comparing the altitude of the aircraft to an altitude threshold, comparing the thrust command to a thrust command threshold and comparing the vertical acceleration of the aircraft to a vertical acceleration threshold. In such embodiments, determining whether the aircraft is on the surface may include a first determination step determining whether the aircraft is on the surface in response to comparing the altitude of the aircraft to the altitude threshold and a second determination step determining whether the aircraft is on the surface in response to comparing the thrust command to the thrust command threshold and comparing the vertical acceleration of the aircraft to the vertical acceleration threshold. In such embodiments, determining whether the aircraft is on the surface may include determining that the aircraft is on the surface in response to either or both of the first determination step or the second determination step determining that the aircraft is on the surface. 
    
    
     
       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.  1 A- 1 B  are schematic illustrations of an aircraft having a ground state determination system that is operable to convert between thrust-borne lift in a VTOL orientation and wing-borne lift in a biplane orientation in accordance with embodiments of the present disclosure; 
         FIG.  2    is a block diagram of one implementation of a plurality of propulsion assemblies and a flight control system for an aircraft having a ground state determination system in accordance with embodiments of the present disclosure; 
         FIG.  3    is a block diagram of autonomous and remote control systems for an aircraft having a ground state determination system in accordance with embodiments of the present disclosure; 
         FIG.  4    is a block diagram of a ground state determination system for an aircraft in accordance with embodiments of the present disclosure; 
         FIGS.  5 A- 5 I  are schematic illustrations of an aircraft having a ground state determination system in a sequential flight operating scenario in accordance with embodiments of the present disclosure; 
         FIGS.  6 A- 6 D  are flowcharts of various methods for determining a ground state of an aircraft in accordance with embodiments of the present disclosure; 
         FIGS.  7 A- 7 B  are schematic illustrations of a helicopter having a ground state determination system in accordance with embodiments of the present disclosure; 
         FIGS.  8 A- 8 F  are schematic illustrations of a helicopter having a ground state determination system in a sequential flight operating scenario in accordance with embodiments of the present disclosure; and 
         FIG.  9    is a flowchart of a method for determining a ground state of a helicopter or other VTOL 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, all features of an actual implementation may not be described in this specification. 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 nevertheless 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 devices 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 by mere contact or by moving and/or non-moving mechanical connections. 
     Referring to  FIGS.  1 A- 1 B  in the drawings, isometric views of a tailsitter aircraft  10  having a ground state determination system that is operable to transition between thrust-borne lift in a VTOL orientation and wing-borne lift in a biplane orientation are depicted.  FIG.  1 A  depicts aircraft  10  in the biplane orientation wherein the propulsion assemblies provide forward thrust with the forward airspeed of aircraft  10  providing wing-borne lift enabling aircraft  10  to have a high speed and/or high endurance forward flight mode.  FIG.  1 B  depicts aircraft  10  in the VTOL orientation wherein the propulsion assemblies provide thrust-borne lift. Aircraft  10  has a longitudinal axis l 0   a  that may also be referred to as the roll axis, a lateral axis  10   b  that may also be referred to as the pitch axis and a vertical axis  10   c  that may also be referred to as the yaw axis. When longitudinal axis  10   a  and lateral axis  10   b  are both in a horizontal plane and normal to the local vertical in the earth&#39;s reference frame, aircraft  10  has a level flight attitude. In the illustrated embodiment, the length of aircraft  10  in the direction of lateral axis  10   b  is greater than the width of aircraft  10  in the direction of longitudinal axis l 0   a  in the VTOL orientation of aircraft  10 , as depicted in  FIG.  1 B . Both the magnitudes of the length and the width of aircraft  10  as well as the difference between the length and the width of aircraft  10  are important relative to the landing stability of aircraft  10  as well as the tip-over stability of aircraft  10  when aircraft  10  is positioned on a surface such as the ground in a tailsitter orientation. 
     In the illustrated embodiment, aircraft  10  has an airframe  12  including wings  14 ,  16  each having an airfoil cross-section that generates lift responsive to the forward airspeed of aircraft  10 . Wings  14 ,  16  may be formed as single members or may be formed from multiple wing sections. The outer skins for wings  14 ,  16  are preferably formed from high strength and lightweight materials such as fiberglass, carbon, plastic, metal or other suitable material or combination of materials. In the biplane orientation of aircraft  10 , wing  14  is an upper wing having a straight wing configuration and wing  16  is a lower wing having a straight wing configuration. In other embodiments, wings  14 ,  16  could have other designs such as anhedral and/or dihedral wing designs, swept wing designs or other suitable wing designs. In the illustrated embodiment, wings  14 ,  16  are substantially parallel with each other. Extending generally perpendicularly between wings  14 ,  16  are two truss structures depicted as pylons  18 ,  20 . In other embodiments, more than two pylons may be present. Pylons  18 ,  20  are preferably formed from high strength and lightweight materials such as fiberglass, carbon, plastic, metal or other suitable material or combination of materials. In the illustrated embodiment, pylons  18 ,  20  are substantially parallel with each other. 
     Aircraft  10  includes a cargo pod  22  that is coupled between pylons  18 ,  20 . Cargo pod  22  may be fixably or removably coupled to pylons  18 ,  20 . In addition, in the coupled position, cargo pod  22  may be fixed, shiftable or rotatable relative to pylons  18 ,  20 . Cargo pod  22  has an aerodynamic shape configured to minimize drag during high speed forward flight. Cargo pod  22  is preferably formed from high strength and lightweight materials such as fiberglass, carbon, plastic, metal or other suitable material or combination of materials. Cargo pod  22  has an interior region that may receive a payload  24  therein such as one or more packages. Aircraft  10  may autonomously transport and deliver payload  24  to a desired location in which case, aircraft  10  may be referred to as an unmanned aerial vehicle (UAV), an unmanned aerial system (UAS) or a drone. In other embodiments, aircraft  10  may not include cargo pod  22 . 
     One or more of cargo pod  22 , wings  14 ,  16  and/or pylons  18 ,  20  may contain flight control systems, energy sources, communication lines and other desired systems. For example, pylon  20  houses flight control system  26  of aircraft  10 . Flight control system  26  is preferably a redundant digital flight control system including multiple independent flight control computers. For example, the use of a triply redundant flight control system  26  improves the overall safety and reliability of aircraft  10  in the event of a failure in flight control system  26 . Flight control system  26  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  26  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  26  may include one or more memory storage modules including, but 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  26  may be a microprocessor-based system operable to execute program code in the form of machine-executable instructions. In addition, flight control system  26  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. 
     One or more of cargo pod  22 , wings  14 ,  16  and/or pylons  18 ,  20  may contain one or more electrical power sources depicted as a plurality of batteries  28  in pylon  20 . Batteries  28  supply electrical power to flight control system  26 , the distributed thrust array of aircraft  10  and/or other power consumers of aircraft  10  such that aircraft  10  may be referred to as an electric vertical takeoff and landing (eVTOL) aircraft. In other embodiments, aircraft  10  may have a hybrid power system that includes one or more internal combustion engines and an electric generator. Preferably, the electric generator is used to charge batteries  28 . In other embodiments, the electric generator may provide power directly to a power management system and/or the power consumers of aircraft  10 . In still other embodiments, aircraft  10  may use fuel cells as the electrical power source. 
     Cargo pod  22 , wings  14 ,  16  and/or pylons  18 ,  20  also contain a wired and/or wireless communication network that enables flight control system  26  to communicate with the distributed thrust array of aircraft  10 . In the illustrated embodiment, aircraft  10  has a two- dimensional distributed thrust array that is coupled to airframe  12 . As used herein, the term “two-dimensional thrust array” refers to a plurality of thrust generating elements that occupy a two-dimensional space in the form of a plane. A minimum of three thrust generating elements is required to form a “two-dimensional thrust array.” A single aircraft may have more than one “two-dimensional thrust array” if multiple groups of at least three thrust generating elements each occupy separate two-dimensional spaces thus forming separate planes. As used herein, the term “distributed thrust array” refers to the use of multiple thrust generating elements each producing a portion of the total thrust output. The use of a “distributed thrust array” provides redundancy to the thrust generation capabilities of the aircraft including fault tolerance in the event of the loss of one of the thrust generating elements. A “distributed thrust array” can be used in conjunction with a “distributed power system” in which power to each of the thrust generating elements is supplied by a local power system instead of a centralized power source. For example, in a “distributed thrust array” having a plurality of propulsion assemblies acting as the thrust generating elements, a “distributed power system” may include individual battery elements housed within the nacelle of each propulsion assembly. 
     The two-dimensional distributed thrust array of aircraft  10  includes a plurality of propulsion assemblies, individually denoted as  30   a ,  30   b ,  30   c ,  30   d  and collectively referred to as propulsion assemblies  30 . In the illustrated embodiment, propulsion assemblies  30   a ,  30   b  are coupled at the wingtips of wing  14  and propulsion assemblies  30   c ,  30   d  are coupled at the wingtips of wing  16 . By positioning propulsion assemblies  30   a ,  30   b ,  30   c ,  30   d  at the wingtips of wings  14 ,  16 , the thrust and torque generating elements are positioned at the maximum outboard distance from the center of gravity of aircraft  10  located, for example, at the intersection of axes  10   a ,  10   b ,  10   c . The outboard locations of propulsion assemblies  30  provide dynamic stability to aircraft  10  in hover and a high dynamic response in the VTOL orientation of aircraft  10  enabling efficient and effective pitch, yaw and roll control by changing the thrust, thrust vector and/or torque output of certain propulsion assemblies  30  relative to other propulsion assemblies  30 . 
     Even though the illustrated embodiment depicts four propulsion assemblies, the distributed thrust array of aircraft  10  could have other numbers of propulsion assemblies both greater than or less than four. Also, even though the illustrated embodiment depicts propulsion assemblies  30  in a wingtip mounted configuration, the distributed thrust array of aircraft  10  could have propulsion assemblies coupled to the wings and/or pylons in other configurations such as mid-span configurations. Further, even though the illustrated embodiment depicts propulsion assemblies  30  in a mid-wing configuration, the distributed thrust array of aircraft  10  could have propulsion assemblies coupled to the wings in a low wing configuration, a high wing configuration or any combination or permutation thereof. In the illustrated embodiment, propulsion assemblies  30  are variable speed propulsion assemblies having fixed pitch rotor blades and thrust vectoring capability. Depending upon the implementation, propulsion assemblies  30  may have longitudinal thrust vectoring capability, lateral thrust vectoring capability or omnidirectional thrust vectoring capability. In other embodiments, propulsion assemblies  30  may operate as single speed propulsion assemblies, may have variable pitch rotor blades and/or may be non-thrust vectoring propulsion assemblies. 
     Propulsion assemblies  30  may be independently attachable to and detachable from airframe  12  and may be standardized and/or interchangeable units and preferably line replaceable units (LRUs) providing easy installation and removal from airframe  12 . The use of line replaceable propulsion units is beneficial in maintenance situations if a fault is discovered with one of the propulsion assemblies. In this case, the faulty propulsion assembly  30  can be decoupled from airframe  12  by simple operations and another propulsion assembly  30  can then be attached to airframe  12 . In other embodiments, propulsion assemblies  30  may be permanently coupled to wings  14 ,  16 . 
     Referring to  FIG.  1 B , component parts of propulsion assembly  30   d  will now be described. It is noted that propulsion assembly  30   d  is representative of each propulsion assembly  30  therefore, for sake of efficiency, certain features have been disclosed only with reference to propulsion assembly  30   d . One having ordinary skill in the art, however, will fully appreciate an understanding of each propulsion assembly  30  based upon the disclosure herein of propulsion assembly  30   d . In the illustrated embodiment, propulsion assembly  30   d  includes a nacelle  32  that houses components including a battery  32   a , an electronic speed controller  32   b , one or more actuators  32   c , an electronics node  32   d , one or more sensors  32   e  and other desired electronic equipment. Nacelle  32  also supports a propulsion system  32   f  including a gimbal  32   g , a variable speed electric motor  32   h  and a rotor assembly  32   i . Extending from a lower end of nacelle  32  is a tail assembly  32   j  that includes one or more aerosurfaces  32   k . In the illustrated embodiment, aerosurfaces  32   k  include stationary horizontal and vertical stabilizers. In other embodiments, aerosurfaces  32   k  may be active aerosurfaces that serve as elevators to control the pitch or angle of attack of wings  14 ,  16  and/or ailerons to control the roll or bank of aircraft  10  in the biplane orientation of aircraft  10 . Aerosurfaces  32   k  also serve to enhance hover stability in the VTOL orientation of aircraft  10 . 
     Flight control system  26  communicates via a wired communications network within airframe  12  with electronics nodes  32   d  of propulsion assemblies  30 . Flight control system  26  receives sensor data from sensors  32   e  and sends flight command information to the electronics nodes  32   d  such that each propulsion assembly  30  may be individually and independently controlled and operated. For example, flight control system  26  is operable to individually and independently control the speed and the thrust vector of each propulsion system  32   f . Flight control system  26  may autonomously control some or all aspects of flight operation for aircraft  10 . Flight control system  26  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  26  to enable remote flight control over some or all aspects of flight operation for aircraft  10 . 
     Aircraft  10  has a landing gear assembly  34  that includes a plurality of landing feet depicted as landing foot  34   a  coupled to a lower or aft end of propulsion assembly  30   a , landing foot  34   b  coupled to a lower or aft end of propulsion assembly  30   b , landing foot  34   c  coupled to a lower or aft end of propulsion assembly  30   c  and landing foot  34   d  coupled to a lower or aft end of propulsion assembly  30   d . By positioning landing feet  34   a ,  34   b ,  34   c ,  34   d  at the lower end of wingtip mounted propulsion assemblies  30 , landing feet  34   a ,  34   b ,  34   c ,  34   d  are positioned at the maximum outboard distance from the center of gravity of aircraft  10  located, for example, at the intersection of axes  10   a ,  10   b ,  10   c , which provides for maximum landing stability and tip-over stability for aircraft  10 . In an operational scenario, several important functions of aircraft  10  depend upon whether aircraft  10  is on a surface such as the ground or a landing pad. These functions include activating flight control loops, changing flight control loop gains, syncing positions, activating motors, deploying a payload and changing integrator gains. For example, it may be undesirable to accumulate a large integrated motor command while aircraft  10  is on the ground. It may also be undesirable to build a positioning error, for example due to global positioning satellite (GPS) drift, which could affect navigation. It may also be useful for remote operators of aircraft  10  having no line of sight or other visual feedback of aircraft  10  to know when aircraft  10  is on the ground. 
     Previous attempts at determining the ground state of aircraft have relied upon mechanical ground proximity switches or weight sensors, which have proven to be unreliable and add weight, cost and complexity to the aircraft. These prior solutions are particularly unreliable and prohibitive when implemented on small or lightweight aircraft such as aircraft  10 . Other prior systems have relied on analyzing the vibrational profile of an aircraft both in flight and on the ground and comparing such vibrational profiles to determine whether the aircraft is grounded, although such systems have also lacked adequate reliability. To address these and other drawbacks of prior systems, aircraft  10  includes a ground state determination system  36 . Ground state determination system  36  includes a ground state module  38  implemented by flight control system  26  that monitors a combination of aircraft parameters such as above ground level (AGL) altitude, vertical acceleration, motor commands and/or attitude from one or more sensors  32   e ,  40   a ,  40   b  or other components and compares such aircraft parameters to respective parameter thresholds to determine whether aircraft  10  is on a surface. Ground state determination system  36  does not rely solely on ground proximity switches or weight sensors, if any at all, which have been shown to be unreliable and may require additional weight, power, cost, complexity or wiring unsuitable for small or lightweight vehicles such as aircraft  10 . 
     It should be appreciated that aircraft  10  is merely illustrative of a variety of aircraft that can implement the embodiments disclosed herein. Indeed, ground state determination system  36  may be implemented on any aircraft. Other aircraft implementations can include helicopters, hybrid aircraft, tiltwing aircraft, tiltrotor aircraft, quad tiltrotor aircraft, gyrocopters, propeller-driven airplanes, compound helicopters, jets, and the like. As such, those skilled in the art will recognize that ground state determination system  36  can be integrated into a variety of aircraft configurations. It should be appreciated that even though aircraft are particularly well- suited to implement the embodiments of the present disclosure, non-aircraft vehicles and devices can also implement the embodiments. 
     Referring next to  FIG.  2   , a block diagram illustrates one implementation of a propulsion and flight control system for an aircraft  100  that is representative of aircraft  10  discussed herein. Specifically, aircraft  100  includes four propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  that form a two-dimensional thrust array of thrust vectoring propulsion assemblies. Propulsion assembly  102   a  includes various electronic components  104   a  including one or more batteries, one or more controllers and one or more sensors including a distance sensor such as an altimeter, which may be located anywhere on propulsion assembly  104   a  or aircraft  100  depending on a number of factors such as the minimum distance requirement for the altimeter. Propulsion assembly  102   a  also includes a propulsion system  106   a  described herein as including an electric motor and a rotor assembly. In the illustrated embodiment, propulsion assembly  102   a  includes a two-axis gimbal  108   a  operated by one or more actuators  110   a . In other embodiments, propulsion assembly  102   a  may include a single-axis gimbal or other mechanism for thrust vectoring. In still other embodiments, propulsion assembly  102   a  may be a non-thrust vectoring propulsion assembly. 
     Propulsion assembly  102   b  includes an electronics node  104   b  depicted as including one or more batteries, one or more controllers and one or more sensors such as a distance sensor. Propulsion assembly  102   b  also includes a propulsion system  106   b  and a two-axis gimbal  108   b  operated by one or more actuators  110   b . Propulsion assembly  102   c  includes an electronics node  104   c  depicted as including one or more batteries, one or more controllers and one or more sensors such as a distance sensor. Propulsion assembly  102   c  also includes a propulsion system  106 c and a two-axis gimbal  108 c operated by one or more actuators  110   c . Propulsion assembly  102   d  includes an electronics node  104   d  depicted as including one or more batteries, one or more controllers and one or more sensors such as a distance sensor. Propulsion assembly  102   d  also includes a propulsion system  106   d  and a two-axis gimbal  108 d operated by one or more actuators  110   d.    
     Flight control system  112  is operably associated with each of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  and is linked to electronic nodes  104   a ,  104   b ,  104   c ,  104   d  by a fly-by-wire communications network depicted as arrows  114   a ,  114   b ,  114   c ,  114   d . Flight control system  112  receives sensor data from and sends commands to propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  to enable flight control system  112  to independently control each of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d , as discussed herein. Aircraft  100  includes a ground state determination system in which flight control system  112  implements a ground state module  116 . Ground state module  116  monitors aircraft parameters detected by the various sensors of aircraft  100  including the distance sensors of electronic nodes  104   a ,  104   b ,  104   c ,  104   d  and/or sensors  118 . Sensors  118  may include any combination of sensors such as an attitude sensor to detect a pitch or tipover point of aircraft  100 , an accelerometer to detect a vertical acceleration of aircraft  100 , a velocity sensor to detect a vertical velocity of aircraft  100  and/or other sensors. Flight control system  112  also includes a thrust command module  120  that issues thrust commands to propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d . Ground state module  116  also monitors the thrust commands that issue from thrust command module  120 . Ground state module  116  compares the aircraft parameters from the various sensors and/or thrust command module  120  to respective parameter thresholds to determine whether aircraft  100  is on a surface. 
     Referring additionally to  FIG.  3    in the drawings, a block diagram depicts a control system  122  operable for use with aircraft  100  or aircraft  10  of the present disclosure. In the illustrated embodiment, system  122  includes two primary computer based subsystems; namely, an aircraft system  124  and a remote system  126 . In some implementations, remote system  126  includes a programming application  128  and a remote control application  130 . Programming application  128  enables a user to provide a flight plan and mission information to aircraft  100  such that flight control system  112  may engage in autonomous control over aircraft  100 . For example, programming application  128  may communicate with flight control system  112  over a wired or wireless communication channel  132  to provide a flight plan including, for example, a starting point, a trail of waypoints and an ending point such that flight control system  112  may use waypoint navigation during the mission. In addition, programming application  128  may provide one or more tasks to flight control system  112  for aircraft  100  to accomplish during the mission such as delivery of a payload to a desired location. Following programming, aircraft  100  may operate autonomously responsive to commands generated by flight control system  112 . 
     In the illustrated embodiment, flight control system  112  includes a command module  134  and a monitoring module  136 . It is to be understood by those skilled in the art that these and other modules executed by flight control system  112  may be implemented in a variety of forms including hardware, software, firmware, special purpose processors and/or combinations thereof. Flight control system  112  receives input from a variety of sources including internal sources such as thrust command module  120 , sensors  118 , controllers/actuators  140 , propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  as well as external sources such as remote system  126 , global positioning system satellites or other location positioning systems and the like. 
     During the various operating modes of aircraft  100  such as the vertical takeoff flight mode, the hover flight mode, the forward flight mode, transition flight modes and the vertical landing flight mode, command module  134  including thrust command module  120  provides commands to controllers/actuators  140 . These commands enable independent operation of propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d  including rotor speed, thrust vector and the like. Flight control system  112  receives feedback from controllers/actuators  140  and propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d . This feedback is processed by monitoring module  136  that can supply correction data and other information to command module  134  and to controllers/actuators  140 . Sensors  118 , such as an attitude and heading reference system (AHRS) with solid-state or microelectromechanical systems (MEMS) gyroscopes, accelerometers and magnetometers as well as other sensors including positioning sensors, speed sensors, environmental sensors, fuel sensors, temperature sensors, location sensors and the like also provide information to flight control system  112  to further enhance autonomous control capabilities. A ground state monitoring module  142  of monitoring module  136  monitors any combination of aircraft parameters detected by or associated with sensors  118 , thrust command module  120  and/or propulsion assemblies  102   a ,  102   b ,  102   c ,  102   d . A ground state determination module  144  compares the aircraft parameters monitored by ground state monitoring module  142  to respective parameter thresholds to determine whether aircraft  100  is on a surface. 
     Some or all of the autonomous control capability of flight control system  112  can be augmented or supplanted by remote flight control from, for example, remote system  126 . Remote system  126  may include one or computing systems that may be implemented on general-purpose computers, special purpose computers or other machines with memory and processing capability. The computing systems may be microprocessor-based systems operable to execute program code in the form of machine-executable instructions. In addition, the computing systems may be connected 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. Remote system  126  communicates with flight control system  112  via communication link  132  that may include both wired and wireless connections. 
     While operating remote control application  130 , remote system  126  is configured to display information relating to one or more aircraft of the present disclosure on one or more flight data display devices  146 . Display devices  146  may be configured in any suitable form, including, for example, liquid crystal displays, light emitting diode displays or any suitable type of display. Remote system  126  may also include audio output and input devices such as a microphone, speakers and/or an audio port allowing an operator to communicate with other operators or a base station. Display device  146  may also serve as a remote input device  148  if a touch screen display implementation is used, however, other remote input devices, such as a keyboard or joystick, may alternatively be used to allow an operator to provide control commands to an aircraft being operated responsive to remote control. 
     Referring to  FIG.  4    in the drawings, a ground state determination system for an aircraft is schematically illustrated and generally designated  200 . Ground state determination system  200  may be implemented on any VTOL aircraft such as aircraft  10  in  FIGS.  1 A- 1 B  or aircraft  100  in  FIG.  2   . Ground state determination system  200  utilizes aircraft sensors  202 , which may be located anywhere on the aircraft. More particularly, ground state determination system  200  utilizes various parameters  204  detected by aircraft sensors  202  to determine whether the aircraft is on a surface. Aircraft sensors  202  include distance sensor  206 , which detects altitude  208  of the aircraft. Distance sensor  206  may be any device capable of measuring distance such as an above ground level (AGL) altitude sensor. Distance sensor  206  may be a radar altimeter, laser altimeter, acoustic altimeter, ultrasonic altimeter, optical sensor or other suitable distance sensor. One or more distance sensors  206  may be located at or near the landing gear of the aircraft such as on aft portions of nacelles  32  of aircraft  10  in  FIGS.  1 A- 1 B . Distance sensor  206 , however, may be located anywhere on the aircraft. Aircraft sensors  202  include attitude sensor  210 , which detects pitch  212  of the aircraft. Attitude sensor  210  may be used, for example, to determine whether the aircraft is in the biplane orientation shown in  FIG.  1 A , the VTOL orientation shown in  FIG.  1 B  or any position therebetween. In some embodiments, pitch  212  may be expressed as an angle by which the pitch of the aircraft deviates from the biplane orientation shown in  FIG.  1 A . Aircraft sensors  202  include accelerometer  214 , which detects vertical acceleration  216  of the aircraft. Accelerometer  214  is capable of detecting both negative, or descending, vertical acceleration and positive, or ascending, vertical acceleration. Aircraft sensors  202  also include velocity sensor  218 , which detects vertical velocity  220  of the aircraft. 
     Flight control system  222  of the aircraft implements thrust command module  224 , which provides one or more thrust commands  226  to one or more aircraft propulsion assemblies  228  such as propulsion assemblies  30   a ,  30   b ,  30   c ,  30   d  in  FIGS.  1 A- 1 B . Thrust command  226  may be expressed as a percentage, fraction or proportion of a maximum thrust amount such as hover power thrust for the aircraft. In some embodiments, thrust command  226  is one of the aircraft parameters used by ground state determination system  200  to determine whether the aircraft is on a surface. 
     Ground state determination system  200  includes ground state module  230  implemented by flight control system  222 . Ground state module  230  includes ground state monitoring module  232 , which monitors the aircraft parameters including altitude  208 , pitch  212 , vertical acceleration  216 , vertical velocity  220 , thrust command  226  or any combination thereof. Ground state module  230  also includes ground state determination module  234 , which compares any combination of aircraft parameters  208 ,  212 ,  216 ,  220 ,  226  monitored by ground state monitoring module  232  to parameter thresholds  236  to determine whether the aircraft is on a surface. For example, in determining whether the aircraft is on a surface, ground state determination module  234  may compare altitude  208  to an altitude threshold, pitch  212  to a pitch threshold, vertical acceleration  216  to a vertical acceleration threshold, vertical velocity  220  to a vertical velocity threshold and/or thrust command  226  to a thrust command threshold. 
     Ground state monitoring module  232  may monitor, and ground state determination module  234  may utilize, any combination of aircraft parameters  208 ,  212 ,  216 ,  220 ,  226  to determine whether the aircraft is on a surface. In some embodiments, ground state monitoring module  232  monitors altitude  208  and pitch  212  of the aircraft and ground state determination module  234  compares altitude  208  and pitch  212  of the aircraft to altitude and pitch thresholds, respectively. Ground state determination module  234  may then determine whether the aircraft is on a surface in response to comparing altitude  208  and pitch  212  of the aircraft to altitude and pitch thresholds. For example, ground state determination module  234  may determine that the aircraft is on a surface in response to altitude  208  being less than the altitude threshold and pitch  212  exceeding the pitch threshold. The altitude threshold may be a predetermined threshold taking into account the location of distance sensor  206  on the aircraft and/or the minimum distance capable of being detected by distance sensor  206 . In one non-limiting example, if distance sensor  206  is located on the landing gear of the aircraft, the altitude threshold may be in a range between one inch to three feet such as one foot. The pitch threshold may be in a range between 30 degrees and 60 degrees such as 45 degrees, or any other pitch threshold that indicates that the tipover point of the aircraft is oriented for vertical takeoff or landing as opposed to forward flight. For example, the pitch threshold may be predetermined such that pitch  212  exceeds the pitch threshold when the aircraft is in the VTOL orientation of aircraft  10  shown in  FIG.  1 B . In alternative embodiments, pitch  212  may be measured, and the pitch threshold may be predetermined, such that pitch  212  is less than the pitch threshold when the aircraft is oriented for vertical takeoff or landing, in which case the criterion for determining that the aircraft is on a surface is that pitch  212  is less than the pitch threshold. 
     In other embodiments, ground state monitoring module  232  monitors thrust command  226  to propulsion assembly  228  as well as vertical acceleration  216  and pitch  212  of the aircraft and ground state determination module  234  compares thrust command  226  to a thrust command threshold, vertical acceleration  216  to a vertical acceleration threshold and pitch  212  to a pitch threshold. Ground state determination module  234  may then determine whether the aircraft is on a surface based on the comparison of thrust command  226 , vertical acceleration  216  and pitch  212  to parameter thresholds  236 . For example, ground state determination module  234  may determine that the aircraft is on a surface in response to thrust command  226  being less than a thrust command threshold, vertical acceleration  216  exceeding a vertical acceleration threshold and pitch  212  exceeding a pitch threshold. The thrust command threshold may be predetermined based on a variety of factors such as the thrust-to-weight ratio of the aircraft. In one non-limiting example, the thrust command threshold may be in a range between 10 percent and 40 percent of hover power thrust such as 25 percent of hover power thrust. In another non-limiting example, the vertical acceleration threshold may be equal to or less than zero such as −1 ft/s 2 . Thus, if vertical acceleration  216  exceeds the vertical acceleration threshold, it may be safely determined that the aircraft is not accelerating downward. Other vertical acceleration thresholds, either greater than or less than zero, may also be selected that ensure that the aircraft is not accelerating downward. In yet other embodiments, ground state determination module  234  may determine whether the aircraft is on a surface based on an error or discrepancy in vertical velocity  220  of the aircraft. More particularly, a difference threshold between the calculated vertical velocity of the aircraft and the actual vertical velocity  220  detected by velocity sensor  218  may be used alone or in conjunction with the other aircraft parameters to determine whether the aircraft is on a surface. 
     In determining the ground state of the aircraft, aircraft parameters  208 ,  212 ,  216 ,  220 ,  226  are subject to fluctuation at or near their respective parameter thresholds  236 . For example, if the thrust command threshold is 25 percent and thrust command  226  is oscillating at or near 25 percent, it is undesirable for ground state determination module  234  to fluctuate between an on-ground and off-ground state determination in response to the fluctuation of thrust command  226 . Thus, ground state monitoring module  232  may include a time module  238  to enforce a confirmation time period that prevents transient fluctuations in the aircraft parameters from affecting the ground state determination of ground state determination system  200 . Time module  238  monitors the time persistence of any combination of aircraft parameters  208 ,  212 ,  216 ,  220 ,  226  and ground state determination module  234  may determine whether the aircraft is on a surface based at least partially on the time persistence of any combination of parameters  208 ,  212 ,  216 ,  220 ,  226  relative to their respective parameter thresholds  236 . The time persistence at which a logic determination is confirmed may depend, for example, on the rate or frequency at which sensor data is being collected or received. In one non-limiting example, ground state determination module  234  may require a three-cycle persistence of any particular aircraft parameter to change a logic state so that one or two ticks or instances of bad or oscillating data will not cause a false logic determination. In one implementation of this example, ground state determination module  234  may require thrust command  226  to be less than the thrust command threshold for three cycles for an on-ground determination to be confirmed. Time persistence may also be measured or required in milliseconds or seconds. 
     In yet other embodiments, ground state monitoring module  232  monitors altitude  208 , pitch  212 , thrust command  226  and vertical acceleration  216  and ground state determination module  234  compares altitude  208  to an altitude threshold, pitch  212  to a pitch threshold, thrust command  226  to a thrust command threshold and vertical acceleration  216  to a vertical acceleration threshold. Ground state determination module  234  may then perform two determination steps related to one another by “OR” logic such that if either or both of the first or second determination steps determine that the aircraft is on a surface, ground state determination module  234  likewise makes a determination that the aircraft is on a surface. In the first determination step, ground state determination module  234  may compare altitude  208  to the altitude threshold and pitch  212  to the pitch threshold. In the second determination step, ground state determination module  234  may compare thrust command  226  to the thrust command threshold, vertical acceleration  216  to the vertical acceleration threshold and pitch  212  to the pitch threshold. A non-limiting example of such embodiments may be expressed as follows:
         IF (altitude  208 &lt;altitude threshold AND pitch  212 &gt;pitch threshold)
           OR   
           IF (thrust command  226 &lt;thrust command threshold AND vertical acceleration  216 &gt;vertical acceleration threshold AND pitch  212 &gt;pitch threshold)
           THEN   Aircraft is on a surface.   
               

     In other embodiments, which may be used for aircraft such as helicopters that maintain a relatively stable pitch throughout its flight regime, ground state determination system  200  may dispense with the comparison between pitch  212  and a pitch threshold when determining whether the aircraft is on a surface. In such embodiments, ground state monitoring module  232  monitors altitude  208 , thrust command  226  and vertical acceleration  216  and ground state determination module  234  compares altitude  208  to an altitude threshold, thrust command  226  to a thrust command threshold and vertical acceleration  216  to a vertical acceleration threshold. Ground state determination module  234  may implement two determination steps. In a first determination step, ground state determination module  234  may determine whether the aircraft is on a surface in response to comparing altitude  208  to the altitude threshold. In a second determination step, ground state determination module  234  may determine whether the aircraft is on a surface in response to comparing thrust command  226  to the thrust command threshold and vertical acceleration  216  to the vertical acceleration threshold. Ground state determination module  234  may determine that the aircraft is on a surface in response to either or both first or second determination steps determining that the aircraft is on a surface. A non- limiting example of such embodiments may be expressed as follows:
         IF (altitude  208 &lt;altitude threshold)
           OR   
           IF (thrust command  226 &lt;thrust command threshold AND vertical acceleration  216 &gt;vertical acceleration threshold)
           THEN   Aircraft is on a surface.   
               

     In yet other embodiments, a third determination step may be added to determine whether the aircraft is on a surface in response to comparing pitch  212  to a pitch threshold and ground state determination module  234  may determine that the aircraft is on a surface in response to any one of the three determination steps determining that the aircraft is on a surface. Such a three-step determination logic may be useful for aircraft that change pitch to convert between forward flight and VTOL flight modes such as aircraft  10  shown in  FIGS.  1 A- 1 B . A non-limiting example of such embodiments may be expressed as follows:
         IF (altitude  208 &lt;altitude threshold)
           OR   
           IF (thrust command  226 &lt;thrust command threshold AND vertical acceleration  216 &gt;vertical acceleration threshold)
           OR   
           IF (pitch  212 &gt;pitch threshold)
           THEN   Aircraft is on a surface.   
               

     In the examples provided herein, the operators used in each expression, including mathematical, relational and logic operators, are reversible and generally interchangeable. For example, any “OR” operator may be replaced with an “AND” operator, or vice versa. Also, any “&lt;” operator may be replaced with a “&gt;” operator, or vice versa. Additionally, any “&lt;” or “&gt;” operator may be replaced with a “≤” or “≥” operator, respectively. Indeed, numerous permutations of the examples provided herein are within the scope of the illustrative embodiments. By dispensing with the need for dedicated proximity or weight switches and instead using inertial, altimeter and/or other aircraft parameters in a self-checking logic scheme, ground state determination system  200  provides a more reliable indicator of whether or not an aircraft is on a surface such as the ground. Also, because ground state determination system  200  may use sensors that are used on an aircraft for other purposes, ground state determination system  200  adds little or no weight or cost to the aircraft. 
     Referring additionally to  FIGS.  5 A- 5 I  in the drawings, a sequential flight-operating scenario of aircraft  240  including ground state determination system  200  is depicted. Ground state determination system  200  is implemented in part by flight control system  222 . As best seen in  FIG.  5 A , aircraft  240  is in a tailsitter position on a surface  242  such as the ground, a helipad or the deck of an aircraft carrier with landing feet  244  in contact with surface  242 . When aircraft  240  is ready for a mission, flight control system  222  commences operations providing flight commands to the various components of aircraft  240 . Flight control system  222  may be operating responsive to 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 transitions between wing-borne flight and thrust-borne flight. While ground state determination system  200  may be used to determine whether aircraft  240  is on surface  242  prior to takeoff, for sake of efficiency the operation of ground state determination system  200  is described herein in relation to determining whether aircraft  240  is on surface  242  during the landing phase of the flight regime. One having ordinary skill in the art, however, will fully appreciate an understanding of the operation of ground state determination system  200  prior to takeoff based upon the disclosure herein of the operation of ground state determination system  200  during landing. 
     As best seen in  FIG.  5 B , aircraft  240  has performed a vertical takeoff and is engaged in thrust-borne lift in the VTOL orientation of aircraft  240 . As illustrated, the rotor assemblies of propulsion assemblies  246  are each rotating in substantially the same horizontal plane. In the illustrated embodiment, pitch angle  212  of aircraft  240  is the angle between reference axis  250  substantially parallel to propulsion assemblies  246  and a horizontal plane H that is normal to the local vertical in the earth&#39;s reference frame, although in other embodiments the pitch angle of aircraft  240  may be measured relative to other axes or planes such as the pitch or longitudinal axes of aircraft  240  described in  FIGS.  1 A- 1 B . In  FIG.  5 B , pitch angle  212  of aircraft  240  is approximately 90 degrees, indicating that aircraft  240  is in the VTOL orientation and has a level flight attitude. In the VTOL orientation, wing  252  is the forward wing and wing  254  is the aft wing. As discussed herein, flight control system  222  independently controls and operates each propulsion assembly  246  including independently controlling speed and thrust vector. During hover, flight control system  222  may utilize differential speed control and/or differential or collective thrust vectoring of propulsion assemblies  246  to provide hover stability for aircraft  240  and to provide pitch, roll, yaw and translation authority for aircraft  240 . 
     After vertical ascent to the desired elevation, aircraft  240  may begin the transition from thrust-borne lift to wing-borne lift. As best seen from the progression of  FIGS.  5 B- 5 D , aircraft  240  is operable to pitch down from the VTOL orientation toward the biplane orientation to enable high speed and/or long range forward flight. As seen in  FIG.  5 C , pitch angle  212  of aircraft  240  is approximately 30 degrees such that aircraft  240  has an inclined flight attitude. Flight control system  222  may achieve this operation through speed control of some or all of propulsion assemblies  246 , thrust vectoring of some or all of propulsion assemblies  246  or any combination thereof 
     As best seen in  FIGS.  5 D and  5 E , aircraft  240  has completed the transition to the biplane orientation with the rotor assemblies of propulsion assemblies  246  each rotating in substantially the same vertical plane. In the biplane orientation, wing  254  is the upper wing positioned above wing  252 , which is the lower wing. Pitch angle  212  is approximately zero degrees as reference axis  250  is in horizontal plane H such that aircraft  240  has a level flight attitude in the biplane orientation. As forward flight with wing-borne lift requires significantly less power than VTOL flight with thrust-borne lift, the operating speed of some or all of the propulsion assemblies  246  may be reduced. In certain embodiments, some of the propulsion assemblies of aircraft  240  could be shut down during forward flight. In the biplane orientation, the independent control provided by flight control system  222  over each propulsion assembly  246  provides pitch, roll and yaw authority for aircraft  240 . 
     As aircraft  240  approaches the desired location, aircraft  240  may begin its transition from wing-borne lift to thrust-borne lift. As best seen from the progression of  FIGS.  5 E- 5 G , aircraft  240  is operable to pitch up from the biplane orientation to the VTOL orientation to enable, for example, a vertical landing operation. As seen in  FIG.  5 F , pitch angle  212  is approximately 30 degrees such that aircraft  240  has an inclined flight attitude of about 30 degrees pitch up. Flight control system  222  may achieve this operation through speed control of some or all of propulsion assemblies  246 , thrust vectoring of some or all of propulsion assemblies  246  or any combination thereof. In  FIG.  5 G , aircraft  240  has completed the transition from the biplane orientation to the VTOL orientation and pitch angle  212  is approximately 90 degrees such that aircraft  240  has a level flight attitude in the VTOL orientation. 
     Once aircraft  240  has completed the transition to the VTOL orientation, aircraft  240  may hover and commence its vertical descent to surface  242 .  FIG.  5 H  shows aircraft  240  several feet above surface  242  prior to landing and  FIG.  51    shows aircraft  240  landed on surface  242 . Ground state determination system  200  monitors various parameters of aircraft  240  and compares such parameters to respective parameter thresholds to determine whether aircraft  240  is on surface  242 . In some embodiments, ground state determination system  200  determines whether aircraft  240  is on surface  242  based on two separate determination processes that each use different combinations of aircraft parameters. If either or both of the two determination processes determine that aircraft  240  is on surface  242 , then ground state determination system  200  likewise makes a determination that aircraft  240  is surface  242 . A first example of one of the two determination processes that may be used by ground state determination system  200  utilizes altitude  208  and pitch  212  of aircraft  240 . In particular, ground state determination system  200  may compare altitude  208  to altitude threshold  258 , which may be on the order of microns, millimeters, feet or meters, and pitch  212  to a pitch threshold such as a pitch threshold in a range between 30 degrees and 60 degrees by determining whether altitude  208  is less than altitude threshold  258  and pitch  212  exceeds the pitch threshold. In this non-limiting example, even though pitch  212  may exceed the pitch threshold in both  FIGS.  5 H and  51   , ground state determination system  200  may determine that aircraft  240  on surface  242  in  FIG.  5 I  but not in  FIG.  5 H  since altitude  208  is less than altitude threshold  258  only in  FIG.  51   . A second example of one of the two determination processes that may be used by ground state determination system  200  utilizes the thrust command(s) to propulsion assemblies  246 , the vertical acceleration of aircraft  240  and pitch  212  of aircraft  240 . In particular, ground state determination system  200  may compare the thrust command to a thrust command threshold such as a thrust command threshold in a range between 10 percent and 40 percent of hover power thrust, the vertical acceleration to a vertical acceleration threshold such as a vertical acceleration threshold equal to or less than zero and pitch  212  to a pitch threshold such as 45 degrees by determining whether the thrust command is less than the thrust command threshold, the vertical acceleration exceeds the vertical acceleration threshold and pitch  212  exceeds the pitch threshold. In this non-limiting example, even though pitch  212  may exceed the pitch threshold in both  FIGS.  5 H and  5 I , the determination of whether or not aircraft  240  is on surface  242  will depend on the thrust command to propulsion assemblies  246  and the vertical acceleration of aircraft  240  in each of these figures as well as the benchmarks set by the thrust command threshold and the vertical acceleration threshold. In other embodiments, one, three or more determination processes may be used to determine whether aircraft  240  is on surface  242  and such processes may be linked by “AND”, “OR” or other logic in making an on-ground determination. Other combinations of aircraft parameters may be used in such processes. Once aircraft  240  has landed as shown in  FIG.  51   , aircraft  242  rests in its tailsitter orientation at the selected landing site. 
     Referring to  FIGS.  6 A- 6 D  in the drawings, various methods for determining whether an aircraft is on a surface are depicted. In  FIG.  6 A , method  300  includes monitoring parameters of the aircraft (step  302 ) and comparing each of the parameters to a respective parameter threshold (step  304 ). Method  300  includes determining whether the aircraft is on a surface in response to comparing each of the parameters to the respective parameter threshold (step  306 ). If, in comparing each of the parameters to their respective parameter thresholds, method  300  determines that the aircraft is on a surface, then the aircraft is on a surface (step  308 ). If, in comparing each of the parameters to their respective parameter threshold, method  300  determines that the aircraft is not on a surface, then the aircraft is not on a surface (step  310 ). After the on-ground determination is executed, method  300  may loop back to any previous step such as step  302  to continue evaluating the on-ground state of the aircraft. In  FIG.  6 B , method  314  includes monitoring the altitude of the aircraft (step  316 ) and monitoring the pitch of the aircraft (step  318 ). Method  314  also includes comparing the altitude of the aircraft to an altitude threshold (step  320 ) and comparing the pitch of the aircraft to a pitch threshold (step  322 ). Method  314  includes determining whether the aircraft is on a surface in response to comparing the altitude of the aircraft to the altitude threshold and comparing the pitch of the aircraft to the pitch threshold (step  324 ). If, in comparing the altitude of the aircraft to the altitude threshold and comparing the pitch of the aircraft to the pitch threshold, method  314  determines that the aircraft is on a surface, then the aircraft is on a surface (step  326 ). If, in comparing the altitude of the aircraft to the altitude threshold and comparing the pitch of the aircraft to the pitch threshold, method  314  determines that the aircraft is not on a surface, then the aircraft is not on a surface (step  328 ). After the on-ground determination is executed, method  314  may loop back to any previous step such as step  316  to continue evaluating the on-ground state of the aircraft. 
     In  FIG.  6 C , method  332  includes monitoring a thrust command to a propulsion assembly of the aircraft (step  334 ), monitoring the vertical acceleration of the aircraft (step  336 ) and monitoring the pitch of the aircraft (step  338 ). Method  332  also includes comparing the thrust command to a thrust command threshold (step  340 ), comparing the vertical acceleration of the aircraft to a vertical acceleration threshold (step  342 ) and comparing the pitch of the aircraft to a pitch threshold (step  344 ). Method  332  includes determining whether the aircraft is on a surface in response to comparing the thrust command to the thrust command threshold, comparing the vertical acceleration of the aircraft to the vertical acceleration threshold and comparing the pitch of the aircraft to the pitch threshold (step  346 ). If, in comparing the thrust command to the thrust command threshold, comparing the vertical acceleration of the aircraft to the vertical acceleration threshold and comparing the pitch of the aircraft to the pitch threshold, method  332  determines that the aircraft is on a surface, then the aircraft is on a surface (step  348 ). If, in comparing the thrust command to the thrust command threshold, comparing the vertical acceleration of the aircraft to the vertical acceleration threshold and comparing the pitch of the aircraft to the pitch threshold, method  332  determines that the aircraft is not on a surface, then the aircraft is not on a surface (step  350 ). After the on-ground determination is executed, method  332  may loop back to any previous step such as step  334  to continue evaluating the on- ground state of the aircraft. 
     In  FIG.  6 D , method  354  includes monitoring the altitude of the aircraft (step  356 ), monitoring the pitch of the aircraft (step  358 ), monitoring a thrust command to a propulsion assembly of the aircraft (step  360 ) and monitoring the vertical acceleration of the aircraft (step  362 ). Method  354  also includes comparing the altitude of the aircraft to an altitude threshold (step  364 ), comparing the pitch of the aircraft to a pitch threshold (step  366 ), comparing the thrust command to a thrust command threshold (step  368 ) and comparing the vertical acceleration of the aircraft to a vertical acceleration threshold (step  370 ). Method  354  includes determining whether the aircraft is on a surface in response to comparing the altitude of the aircraft to the altitude threshold and comparing the pitch of the aircraft to the pitch threshold (step  372 ). If, in comparing the altitude of the aircraft to the altitude threshold and comparing the pitch of the aircraft to the pitch threshold, method  354  determines that the aircraft is on a surface, then the aircraft is on a surface (step  374 ). If, in comparing the altitude of the aircraft to the altitude threshold and comparing the pitch of the aircraft to the pitch threshold, method  354  determines that the aircraft is not on a surface, then method  354  determines whether the aircraft is on a surface in response to comparing the thrust command to the thrust command threshold, comparing the vertical acceleration of the aircraft to the vertical acceleration threshold and comparing the pitch of the aircraft to the pitch threshold (step  376 ). If, in comparing the thrust command to the thrust command threshold, comparing the vertical acceleration of the aircraft to the vertical acceleration threshold and comparing the pitch of the aircraft to the pitch threshold, method  354  determines that the aircraft is on a surface, then the aircraft is on a surface (step  374 ). If, in comparing the thrust command to the thrust command threshold, comparing the vertical acceleration of the aircraft to the vertical acceleration threshold and comparing the pitch of the aircraft to the pitch threshold, method  354  determines that the aircraft is not on a surface, then the aircraft is not on a surface (step  378 ). Determination steps  372  and  376  may be performed in any order or simultaneously. After the on-ground determination is executed, method  354  may loop back to any previous step such as step  356  to continue evaluating the on-ground state of the aircraft. 
     Referring to  FIGS.  7 A- 7 B  in the drawings, a helicopter is schematically illustrated and generally designated  400 . Helicopter  400  includes a rotor hub assembly  402 , which includes a plurality of rotor blade assemblies  404 . Rotor hub assembly  402  is rotatable relative to a fuselage  406  of helicopter  400 . The pitch of rotor blade assemblies  404  can be collectively and/or cyclically manipulated to selectively control direction, thrust and lift of helicopter  400 . A landing gear system  408  including skids provides ground support for helicopter  400 . A tailboom  410  extends from fuselage  406 . A tail rotor  412  includes a tail rotor hub assembly  414  that is rotatably coupled to the aft portion of tailboom  410 . 
     Helicopter  400  includes a ground state determination system  416  configured to determine whether helicopter  400  is on a surface. Ground state determination system  416  includes a ground state module  418  implemented by flight control system  420  that monitors a combination of aircraft parameters such as above ground level (AGL) altitude, vertical acceleration, vertical velocity and/or motor commands from one or more onboard sensors and compares such aircraft parameters to respective parameter thresholds to determine whether helicopter  400  is on a surface such as the ground. Since helicopter  400  is not a tailsitter aircraft and is not required to change pitch between forward flight and VTOL flight modes when landing, the pitch of helicopter  400  is a less relevant factor for ground state determination system  416  when determining whether helicopter  400  is on a surface. Thus, the processes and logic used by ground state determination system  416  to determine the on-ground state of helicopter  400  may dispense with pitch as an operand altogether. Helicopter  400  is illustrative of the wide variety of aircraft types that may utilize the ground state determination systems of the illustrative embodiments. 
     Referring additionally to  FIGS.  8 A- 8 F  in the drawings, a sequential flight operating scenario of helicopter  400  using ground state determination system  416  is depicted. As best seen in  FIG.  8 A , helicopter  400  is positioned on surface  422  prior to takeoff. When helicopter  400  is ready for a mission, flight control system  420  commences operations to provide flight control to helicopter  400  which may be onboard pilot flight control, remote flight control, autonomous flight control or a combination thereof. For example, it may be desirable to utilize onboard pilot flight control during certain maneuvers such as takeoff and landing but rely on autonomous flight control during hover and/or forward flight. In  FIG.  8 B , helicopter  400  has lifted off from landing surface  422 . In  FIGS.  8 C and  8 D , helicopter  400  has gained altitude. In  FIG.  8 E , helicopter  400  enters the landing phase as it begins to vertically descend onto surface  422 . 
     Ground state determination system  416  monitors various parameters of helicopter  400  and compares such parameters to respective parameter thresholds to determine whether helicopter  400  is on surface  422 . In some embodiments, ground state determination system  416  determines whether helicopter  400  is on surface  422  based on two separate determination processes that each use different combinations of aircraft parameters. If either or both of the two determination processes determine that helicopter  400  is on surface  422 , then ground state determination system  416  likewise makes a determination that helicopter  400  is on surface  422 . A first example of one of the two determination processes that may be used by ground state determination system  416  utilizes altitude  424  of helicopter  400 . In particular, ground state determination system  416  may compare altitude  424  to altitude threshold  426 , which may be on the order of microns, millimeters, feet or meters, by determining whether altitude  424  is less than altitude threshold  426 . In this non-limiting example, ground state determination system  416  may determine that helicopter  400  is on surface  422  in  FIG.  8 F  but not in  FIG.  8 E  since altitude  424  is less than altitude threshold  426  only in  FIG.  8 F . A second example of one of the two determination processes that may be used by ground state determination system  416  utilizes the thrust command(s) to rotor hub assembly  402  and the vertical acceleration of helicopter  400 . In particular, ground state determination system  416  may compare the thrust command to a thrust command threshold such as a thrust command threshold in a range between 10 percent and 40 percent of full hover power thrust and the vertical acceleration to a vertical acceleration threshold such as a vertical acceleration threshold equal to or less than zero by determining whether the thrust command is less than the thrust command threshold and the vertical acceleration exceeds the vertical acceleration threshold. In this non-limiting example, the determination of whether or not helicopter  400  is on surface  422  will depend on the thrust command to rotor hub assembly  402  and the vertical acceleration of helicopter  400  as well as the benchmarks set by the thrust command threshold and the vertical acceleration threshold. In other embodiments, one, three or more determination processes may be used to determine whether helicopter  400  is on surface  422  and such processes may be linked by “AND”, “OR” or other logic in making an on-ground determination. Other combinations of aircraft parameters may be used in such process(es). 
     Referring to  FIG.  9    in the drawings, a method for determining whether an aircraft such as helicopter  400  is on a surface is depicted and generally designated  500 . Method  500  includes monitoring the altitude of the aircraft (step  502 ), monitoring a thrust command to a propulsion assembly of the aircraft (step  504 ) and monitoring the vertical acceleration of the aircraft (step  506 ). Method  500  also includes comparing the altitude of the aircraft to an altitude threshold (step  508 ), comparing the thrust command to a thrust command threshold (step  510 ) and comparing the vertical acceleration of the aircraft to a vertical acceleration threshold (step  512 ). Method  500  includes determining whether the aircraft is on a surface in response to comparing the altitude of the aircraft to the altitude threshold (step  514 ). If, in comparing the altitude of the aircraft to the altitude threshold, method  500  determines that the aircraft is on a surface, then the aircraft is on a surface (step  516 ). If, in comparing the altitude of the aircraft to the altitude threshold, method  500  determines that the aircraft is not on a surface, then method  500  determines whether the aircraft is on a surface in response to comparing the thrust command to the thrust command threshold and comparing the vertical acceleration of the aircraft to the vertical acceleration threshold (step  518 ). If, in comparing the thrust command to the thrust command threshold and comparing the vertical acceleration of the aircraft to the vertical acceleration threshold, method  500  determines that the aircraft is on a surface, then the aircraft is on a surface (step  516 ). If, in comparing the thrust command to the thrust command threshold and comparing the vertical acceleration of the aircraft to the vertical acceleration threshold, method  500  determines that the aircraft is not on a surface, then the aircraft is not on a surface (step  520 ). Determination steps  514  and  518  may be performed in any order or simultaneously. After the on-ground determination is executed, method  500  may loop back to any previous step such as step  502  to continue evaluating the on-ground state of the aircraft. 
     The flowcharts and block diagrams in the different depicted embodiments illustrate the architecture, functionality, and operation of some possible implementations of apparatus, methods and computer program products. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified function or functions. In some alternative implementations, the function or functions noted in the block may occur out of the order noted in the figures. For example, in some cases, two blocks shown in succession may be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. 
     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.