Patent Publication Number: US-11649044-B2

Title: Coaxial rotor systems for VTOL aircraft

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of co-pending application Ser. No. 16/824,647 filed Mar. 19, 2020. 
    
    
     TECHNICAL FIELD OF THE DISCLOSURE 
     The present disclosure relates, in general, to coaxial rotor systems for vertical takeoff and landing aircraft and, in particular, to coaxial rotor systems that include top and bottom rotor assemblies and a motor coupled to one of the rotor assemblies, the coaxial rotor systems utilizing a torque reaction force responsive to the output of the motor to rotate at least one of the rotor assemblies. 
     BACKGROUND 
     In a traditional coaxial rotor system designed for vertical takeoff and landing (VTOL) aircraft, top and bottom coaxial rotors are connected to a transmission in the fuselage by two concentric masts, one inside of the other. Gearing within the transmission rotates the two masts in opposite directions when power is supplied by an engine so that the top and bottom rotors counter rotate. Coaxial rotor systems can be an energy efficient method for generating vertical lift, and therefore have been utilized by some existing VTOL aircraft. For example, because coaxial rotor systems manage aircraft yaw by varying the torque distribution between the top and bottom rotors, VTOL aircraft with coaxial rotor systems do not require a tail rotor, which can save 10-15 percent in total power expenditure. Furthermore, the power efficiency of a coaxial rotor system may be 6-9 percent more efficient than a traditional single rotor system of comparable disc loading and solidity. Conventional, single rotor helicopters also have a limited top speed due to the problem of retreating blade stall, in which the rotor blade on the retreating side of the rotor disc in forward flight experiences loss of lift due to the rotorcraft&#39;s linear forward flight velocity exceeding the rotor blade&#39;s minimum angular velocity for lift production. Aircraft having coaxial rotor systems overcome the phenomena of retreating blade stall since one or more rotor blades advance on both sides of the rotorcraft during flight, allowing for a faster forward airspeed. 
     Nonetheless, the adoption of coaxial rotor systems by existing aircraft has been limited due to their complexity and costliness. For example, existing coaxial rotor systems require a transmission, which adds weight and negatively impacts the power efficiency of the aircraft. Furthermore, existing coaxial rotor systems may require adverse yaw compensation during autorotation due to changes in aerodynamics between the top and bottom rotors, which is counterintuitive and hazardous in emergency scenarios. Accordingly, a need has arisen for lighter, more power efficient coaxial rotor systems that do not require a transmission and address the deficiencies of existing coaxial rotor systems that have led to their limited adoption in modern aircraft. 
     SUMMARY 
     In a first aspect, the present disclosure is directed to a coaxial rotor system for a rotorcraft including a mast, a top rotor assembly and a bottom rotor assembly. The top rotor assembly is coupled to the distal end of the mast. The bottom rotor assembly includes a motor configured to provide rotational energy to the mast, thereby rotating the top rotor assembly. The bottom rotor assembly experiences a torque reaction force responsive to the motor rotating the mast such that the top and bottom rotor assemblies counter rotate. 
     In some embodiments, the mast may form a mast plate disposed in the bottom rotor assembly and the bottom rotor assembly may be rotatably coupled to the mast plate via a thrust bearing. In certain embodiments, the top and bottom rotor assemblies may each include fixed pitch rotor blades. In other embodiments, the top and bottom rotor assemblies may each include spring-loaded rotor blades having a manually adjustable pitch. In some embodiments, the bottom rotor assembly may include a bottom rotor hub and the motor may be coupled to the underside of the bottom rotor hub. In certain embodiments, the bottom rotor assembly may include a planetary gear system and the motor may be rotatably coupled to the mast via the planetary gear system. 
     In some embodiments, the motor may be rotatably coupled to the mast via a bearing. In certain embodiments, the motor may be a yokeless motor. In some embodiments, the mast may include an upper mast hingeably coupled to a mast base and the coaxial rotor system may include a directional control assembly configured to tilt the upper mast relative to the mast base. In such embodiments, the directional control assembly may be configured to longitudinally and laterally tilt the upper mast relative to the mast base. In certain embodiments, the upper mast may be hingeably coupled to the mast base via a cardan joint. In some embodiments, the directional control assembly may be disposed below the bottom rotor assembly. In certain embodiments, the directional control assembly may be rotatably coupled to the mast via a bearing. In some embodiments, the directional control assembly may include a rotor tilting subassembly including a horizontal arm coupled to the upper mast and a vertical arm including a tilt actuator coupled to the horizontal arm. In such embodiments, the tilt actuator may be configured to tilt the upper mast via the horizontal arm. In some embodiments, the rotor tilting subassembly may include a longitudinal rotor tilting subassembly and a lateral rotor tilting subassembly spaced approximately 90 degrees from the longitudinal rotor tilting subassembly about the mast. In certain embodiments, the coaxial rotor system may include a slip ring below the bottom rotor assembly configured to transmit power to the motor. 
     In some embodiments, the top and bottom rotor assemblies may each include variable pitch rotor blades and the coaxial rotor system may include a collective control pitch assembly disposed between the top and bottom rotor assemblies configured to vary collective pitch of the rotor blades responsive to rotation of the mast. In certain embodiments, the collective control pitch assembly may include a translational sleeve configured to rotate with the mast and pitch arms interposed between the translational sleeve and the rotor blades. In some embodiments, the collective control pitch assembly may include a limiter base rotatably coupled to the underside of the translational sleeve and a spring interposed between the limiter base and the bottom rotor assembly configured to bias the translational sleeve toward the top rotor assembly. 
     In some embodiments, the limiter base may be rotatably coupled to the translational sleeve via a thrust bearing. In certain embodiments, the collective control pitch assembly may include an adjustable collective limiter interposed between the limiter base and the bottom rotor assembly configured to limit downward translation of the translational sleeve to control maximum collective pitch of the rotor blades. In some embodiments, the distal ends of the pitch arms may be coupled to trailing ends of the rotor blades and the proximal ends of the pitch arms may be rotatably coupled to the translational sleeve. In certain embodiments, the pitch arms may include upper pitch arms interposed between the translational sleeve and the rotor blades of the top rotor assembly and lower pitch arms interposed between the translational sleeve and the rotor blades of the bottom rotor assembly. In some embodiments, the mast may include one or more splines and the translational sleeve may form one or more spline grooves to receive the splines. In such embodiments, the translational sleeve may translate along the splines and the splines may constrain rotation of the translational sleeve relative to the mast. In certain embodiments, the mast may be rotatably coupled to the top rotor assembly up to a predetermined number of revolutions via a thrust bearing. In such embodiments, the translational sleeve may form a ball screw nut cavity having internal threads and the collective control pitch assembly may include a ball screw having external threads complementary to the internal threads of the ball screw nut cavity, the ball screw coupled to the top rotor assembly. Also in such embodiments, the translational sleeve may translate downward and the ball screw may translate out of the ball screw nut cavity responsive to relative rotation between the mast and the top rotor assembly. 
     In a second aspect, the present disclosure is directed to an aircraft including a fuselage, a mast, a top rotor assembly and a bottom rotor assembly. The mast is rotatably coupled to the fuselage. The top rotor assembly is coupled to the distal end of the mast. The bottom rotor assembly includes a motor configured to provide rotational energy to the mast, thereby rotating the top rotor assembly. The bottom rotor assembly experiences a torque reaction force responsive to the motor rotating the mast such that the top and bottom rotor assemblies counter rotate. 
     In some embodiments, the base of the mast may be rotatably coupled to the fuselage via a thrust bearing. In certain embodiments, the mast may be rotatably coupled to the fuselage via a gimbal attachment such that the mast has a gimballing degree of freedom relative to the fuselage. In some embodiments, the aircraft may include a yaw control system including a tailboom control surface rotatably coupled to the aft end of the fuselage. In such embodiments, the yaw control system may include a vertical fin rotatably coupled to the aft end of the tailboom control surface. In some embodiments, the yaw control system may include a yaw control actuator configured to synchronously rotate the tailboom control surface and the vertical fin. In certain embodiments, the aircraft may include one or more batteries in a subfloor of the fuselage configured to power the motor. 
    
    
     
       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 a rotorcraft having a coaxial rotor system in accordance with embodiments of the present disclosure; 
         FIG.  2    is a side view of a coaxial rotor system utilized on previous aircraft; 
         FIGS.  3 A- 3 C  are various views of a coaxial rotor system in accordance with embodiments of the present disclosure; 
         FIGS.  4 A- 4 F  are side and front views of a rotorcraft in longitudinal and lateral motion utilizing a coaxial rotor system in accordance with embodiments of the present disclosure; 
         FIGS.  5 A- 5 E  are various views of a yaw control system for a rotorcraft utilizing a coaxial rotor system in accordance with embodiments of the present disclosure; 
         FIGS.  6 A- 6 C  are various views of a coaxial rotor system having a collective control pitch assembly in accordance with embodiments of the present disclosure; 
         FIG.  7    is a cross-sectional view of a coaxial rotor system in accordance with embodiments of the present disclosure; 
         FIG.  8    is a block diagram of a propulsion and control system for a rotorcraft having a coaxial rotor system in accordance with embodiments of the present disclosure; and 
         FIG.  9    is a block diagram of a control system for a rotorcraft having a coaxial rotor system 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, a rotorcraft in the form of a helicopter is schematically illustrated and generally designated  10 . Rotorcraft  10  includes a fuselage  12  and a tailboom  14  extending from fuselage  12  in the aft direction. Fuselage  12  houses a power system  16  to provide power to the various systems of rotorcraft  10 . Power system  16  includes batteries  18  disposed in a subfloor compartment  20  and an aft cabin area  22  of fuselage  12 . Locating batteries  18  in subfloor compartment  20  of fuselage  12  lowers the center of gravity of rotorcraft  10  for improved maneuvering. Batteries  18  may be any type of battery such as lithium ion or fluoride ion batteries. In other embodiments, power system  16  may include an internal combustion engine, a generator, a renewable energy source such as solar panels or any other type of power source. The forward section of fuselage  12  includes an occupant cabin  24  for one or more pilots and/or occupants. In other embodiments, rotorcraft  10  may be an unmanned aerial system and include no occupants. 
     Fuselage  12  forms a payload bay  26  in which a payload  28  is received. In some embodiments, payload  28  may be a releasable payload that is secured to payload bay  26  while rotorcraft  10  is grounded, and which is released from payload bay  26  while rotorcraft  10  is either on the ground or in flight. The different types of payloads that are receivable by payload bay  26  are numerous. For example, payload  28  may include a weapon, video camera, infrared imaging device, high definition camera, chemical sensor, cargo, passenger belongings, additional batteries or any other suitable payload. In a more specific example, a weapon such as a bomb or cargo such as a deliverable package may be released from payload bay  26  either on the ground or during flight. Payload bay  26  may also store passenger luggage or belongings in embodiments in which rotorcraft  10  serves as an air taxi. Payload  28  may be configured to provide data to a flight control system  30  of rotorcraft  10  and flight control system  30  may be configured to control, manipulate or release payload  28 . In other embodiments, fuselage  12  may lack payload bay  26  and instead aft cabin area  22  of fuselage  12  may be used as an occupant cabin, thereby increasing the occupant cabin space of rotorcraft  10 . 
     The primary propulsion system for rotorcraft  10  is a coaxial rotor system  32 . Coaxial rotor system  32  includes a top rotor assembly  34  coupled to the distal end of a mast  36 . A bottom rotor assembly  38  includes a motor  40  that provides rotational energy to mast  36 , thereby rotating top rotor assembly  34 . Thus, motor  40  has an output that drives top rotor assembly  34 . Motor  40  may be powered by batteries  18 . Bottom rotor assembly  38  experiences a torque reaction force in response to motor  40  rotating mast  36  such that top and bottom rotor assemblies  34 ,  38  counter rotate as indicated by directional arrows  42 ,  44 . Previously, the rotors of coaxial rotor systems have been counter rotated using opposite gearing in a transmission. Because coaxial rotor system  32  uses the torque reaction between top and bottom rotor assemblies  34 ,  38  to counter rotate top and bottom rotor assemblies  34 ,  38 , rotorcraft  10  does not require a transmission, which reduces weight and increases the available volume in fuselage  12  to provide design flexibility with respect to cabin space. In the illustrated embodiment, top and bottom rotor assemblies  34 ,  38  include fixed pitch rotor blades  46 , although in other embodiments rotor blades  46  may be variable pitch rotor blades. Although top and bottom rotor assemblies  34 ,  38  are each illustrated as including two rotor blades, top and bottom rotor assemblies  34 ,  38  may have any number of rotor blades. 
     Coaxial rotor system  32  includes a directional control assembly  48 . Directional control assembly  48  tilts mast  36  in the longitudinal and lateral directions for longitudinal and lateral directional control of rotorcraft  10 . Longitudinal and lateral directional control is achieved by displacing the center of gravity of rotorcraft  10  relative to the rotor axis. Rotorcraft  10  thus does not require rotor blades  46  to have conventional cyclic control, although in other embodiments either or both of top or bottom rotor assemblies  34 ,  38  may include a conventional swashplate-based cyclic blade pitch mechanism. Because top and bottom rotor assemblies  34 ,  38  are rotationally isolated from fuselage  12 , rotorcraft  10  uses control surfaces in a yaw control system  50  to manage the yaw of rotorcraft  10 . More particularly, yaw control of rotorcraft  10  is achieved by deflecting rotor airflow through an articulated tailboom control surface  52  during hover and an articulated vertical fin  54  during forward flight. Tailboom control surface  52  is rotatable about axis  56  and vertical fin  54  is rotatable about canted axis  58 . Tailboom control surface  52  and vertical fin  54  may be synchronously or independently rotatable and may be actuated using mechanical links or one or more electric servomotors. Rotorcraft  10  also includes landing skids  60 . 
     It should be appreciated that rotorcraft  10  is merely illustrative of a variety of aircraft that can implement the embodiments disclosed herein. Indeed, coaxial rotor system  32  may be implemented on any aircraft. Other aircraft implementations can include hybrid aircraft, tiltwing aircraft, tiltrotor aircraft, quad tiltrotor aircraft, unmanned aircraft, gyrocopters, propeller-driven airplanes, compound helicopters, drones, electric recreational VTOL aircraft, air taxis, payload transport drones and the like. As such, those skilled in the art will recognize that coaxial rotor system  32  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 to  FIG.  2    in the drawings, rotorcraft  100  includes a previous type of coaxial rotor system  102 . Coaxial rotor system  102  includes top and bottom rotor assemblies  104 ,  106  coupled to transmission  108  via two concentric masts  110 ,  112 . Top rotor assembly  104  is coupled to transmission  108  via inner mast  110  and bottom rotor assembly  106  is coupled to transmission  108  via outer mast  112 . Masts  110 ,  112  are counter rotated relative to one another by opposite gearing in transmission  108 , which receives rotational energy from an engine  114 . Because rotorcraft  100  manages yaw using differential torque between top and bottom rotor assemblies  104 ,  106 , adverse yaw compensation may be required during autorotation due to the changes in aerodynamics between top and bottom rotor assemblies  104 ,  106 , which is counterintuitive and hazardous in emergency scenarios. Transmission  108 , which is required by coaxial rotor system  102 , also reduces the amount of fuselage cabin space that may be used for other purposes such as battery, payload or occupant space. Coaxial rotor system  102  also relies upon cyclic control of the rotor blades of top and bottom rotor assemblies  104 ,  106  for longitudinal and lateral movement of rotorcraft  100 . Cyclic control mechanisms for coaxial rotor systems, however, can be complex and difficult to maintain. 
     Referring to  FIGS.  3 A- 3 C  in the drawings, a coaxial rotor system for a rotorcraft is schematically illustrated and generally designated  200 . Coaxial rotor system  200  is supported by mast  202 , which includes an upper mast  204  hingeably coupled to a mast base  206 . Upper mast  204  forms a mast plate  208  that is rotatably coupled to fuselage  210  and, more specifically, to a lower portion  204   a  of upper mast  204  via thrust bearing  212 . Mast plate  208  and thrust bearing  212  are at least partially enclosed by casing  204   b , which is located above fuselage  210 . In other embodiments, mast plate  208 , thrust bearing  212  and casing  204   b  may be inside fuselage  210 . 
     Top rotor assembly  214  includes a top rotor hub  216  and fixed pitch rotor blades  218  extending therefrom. Top rotor hub  216  is fixedly coupled to the distal end of mast  202 . Bottom rotor assembly  220  is rotatably coupled to mast  202 . Bottom rotor assembly  220  includes a bottom rotor hub  222  and fixed pitch rotor blades  224  extending therefrom. Mast  202  forms mast plate  226  disposed in bottom rotor hub  222 . Bottom rotor assembly  220  is rotatably coupled to mast plate  226  via thrust bearing  228 . Mast plate  226  and thrust bearing  228  form the interface at which lift generated by bottom rotor assembly  220  is transferred to fuselage  210  and the remainder of the rotorcraft. Bottom rotor assembly  220  is rotationally isolated, and therefore free to rotate, in relation to fuselage  210  due to the inclusion of thrust bearing  228 . Also, because the upper portion of upper mast  204  is rotatably coupled to fuselage  210  via thrust bearing  212 , top and bottom rotor assemblies  214 ,  220  are rotationally isolated from fuselage  210  such that net torque experienced by top and bottom rotor assemblies  214 ,  220  is not transferred to fuselage  210 . 
     Motor  230 , which may include a casing, is fixedly coupled to the underside of bottom rotor hub  222 . Power is transmitted to motor  230  via a slip ring  232 , which is located on mast  202  below bottom rotor assembly  220 . Slip ring  232  may be a high current rotational electrical connector such as a brush slip ring or a liquid metal-based slip ring. Motor  230  is rotatably coupled to mast  202  via ball bearing  234  to permit motor  230  to freely rotate about mast  202 . Motor  230  is radially symmetric to minimize imbalances during the operation of coaxial rotor system  200 . Motor  230  is a yokeless motor that lacks a central shaft and allows for mast  202  to be inserted therethrough, adding rigidity to coaxial rotor system  200 . Bottom rotor assembly  220  may include any number of stacked yokeless motors depending on the power requirements of coaxial rotor system  200 . In other embodiments, motor  230  may alternatively be coupled to top rotor assembly  214 . 
     Motor  230  provides rotational energy to mast  202  to rotate top rotor assembly  214 . More particularly, motor  230  rotates ring gear  236  disposed therein, which acts as an input to planetary gear system  238 . Motor  230  is rotatably coupled to mast  202  via planetary gear system  238  to provide a suitable gear ratio between the output of motor  230  and the rotational speed of top rotor assembly  214 . In the illustrated embodiment, planetary gear system  238  is encased by bottom rotor hub  222 . Although planetary gear system  238  is not required to rotate mast  202 , planetary gear system  238  may be useful if the baseline output torque of motor  230  is insufficient to rotate top rotor assembly  214  at a desirable rotational speed. In other embodiments, planetary gear system  238  may be a magnetic planetary gear system. When motor  230  rotates mast  202  via planetary gear system  238 , bottom rotor assembly  220  experiences a torque reaction force such that top and bottom rotor assemblies  214 ,  220  counter rotate. This counter rotating motion between top and bottom rotor assemblies  214 ,  220  is generated based on Newton&#39;s Third Law wherein every action has an equal and opposite reaction. Because motor  230  is free to rotate in relation to fuselage  210 , the motor output torque is reacted by top rotor assembly  214 , causing top and bottom rotor assemblies  214 ,  220  to rotate in opposite directions with substantially equal torque sharing. The rotational speeds of top and bottom rotor assemblies  214 ,  220  may therefore inherently compensate for variations in flight condition and thrust setting. 
     Coaxial rotor system  200  includes directional control assembly  240 , which longitudinally and laterally tilts upper mast  204  relative to mast base  206 . Cardan joint  242 , which hingeably couples upper mast  204  to mast base  206 , allows for both longitudinal and lateral tilting of upper mast  204  relative to mast base  206 . Directional control assembly  240  is located below bottom rotor assembly  220  and casing  204   b  and therefore interposed between bottom rotor assembly  220  and fuselage  210 . Directional control assembly  240  includes a rotor tilting subassembly  244 . Rotor tilting subassembly  244  includes a longitudinal rotor tilting subassembly  246  and a lateral rotor tilting subassembly  248 . Longitudinal rotor tilting subassembly  246  is spaced approximately 90 degrees from lateral rotor tilting subassembly  248  about mast  202 . Longitudinal and lateral rotor tilting subassemblies  246 ,  248  include horizontal arms  250 ,  252 , respectively, having proximal ends coupled to upper mast  204  via ball bearing  254 . Longitudinal and lateral rotor tilting subassemblies  246 ,  248  includes diagonal support arms  250   a ,  252   a  having proximal ends coupled to casing  204   b  and distal ends coupled to horizontal arms  250 ,  252 , respectively. Longitudinal rotor tilting subassembly  246  includes vertical arm  256  interposed between horizontal arm  250  and fuselage  210 . Vertical arm  256  includes a tilt actuator  258 . Lateral rotor tilting subassembly  248  includes vertical arm  260  interposed between horizontal arm  252  and fuselage  210 . Vertical arm  260  includes tilt actuator  262 . Tilt actuators  258 ,  262  may include servomotors, stepper motors or other types of actuators. The bottom ends of vertical arms  256 ,  260  may be coupled to fuselage  210  via ball joint fittings  264  and the top ends of vertical arms  256 ,  260  may be coupled to horizontal arms  250 ,  252  via ball joints to permit movement in multiple planes. Tilt actuator  258  tilts upper mast  204  in the longitudinal direction and tilt actuator  262  tilts upper mast  204  in the lateral direction, thereby achieving longitudinal and lateral directional control for the rotorcraft. Longitudinal and lateral tilting of upper mast  204  may alternatively be achieved mechanically or manually via mechanical linkages to horizontal arms  250 ,  252 . 
     By integrating motor  230  into bottom rotor assembly  220  and rotationally isolating coaxial rotor system  200  from fuselage  210 , coaxial rotor system  200  does not require a transmission, thereby increasing power efficiency and reducing the weight of the rotorcraft. In some examples, removal of the transmission may increase the power efficiency of the rotorcraft by approximately 5 percent compared to coaxial rotor aircraft that require a transmission. The lack of a transmission also results in less transmission losses, better mechanical efficiency and reduced maintenance due to simplified design. Additional space is also available in fuselage  210  where the transmission was located in previous aircraft, resulting in increased design flexibility with respect to the interior of fuselage  210 . The use of motor  230  also reduces emissions and noise as compared to previous coaxial rotor systems. The inclusion of directional control assembly  240  eliminates the need for a cyclic rotor pitch mechanism, which has a high part count and can be difficult to maintain. Coaxial rotor system  200  also lowers the pilot&#39;s workload due to automatic torque balancing between top and bottom rotor assemblies  214 ,  220  when changing flight regimes. Unlike aircraft having multiple small rotors such as quadcopter drones, rotorcraft having coaxial rotor system  200  benefit from higher power efficiency due to a larger rotor diameter, lower disc loading and autorotation capability. Coaxial rotor system  200  does not require a tail rotor, further reducing weight and complexity. In some examples, rotorcraft having coaxial rotor system  200  may be 21 to 29 percent more power efficient in hover than a conventional helicopter of comparable size due to the removal of the tail rotor and transmission and power efficiency advantages of coaxial rotors as compared to traditional single rotor systems. Because autorotation capability increases with helicopter rotor inertia, coaxial rotor system  200  offers another benefit when compared to a traditional coaxial rotor since motor  230  adds to the inertia of bottom rotor assembly  220 . 
     Referring to  FIGS.  4 A- 4 F  in the drawings, longitudinal and lateral directional control of rotorcraft  266  having coaxial rotor system  200  is schematically illustrated. In FIGS.  4 A- 4 C, longitudinal rotor tilting subassembly  246  of directional control assembly  240  tilts coaxial rotor system  200  in the longitudinal direction. More specifically, longitudinal rotor tilting subassembly  246  forwardly tilts coaxial rotor system  200  to move rotorcraft  266  in a forward direction of travel  268 . Longitudinal rotor tilting subassembly  246  aftwardly tilts coaxial rotor system  200  to move rotorcraft  266  in an aftward direction of travel  270 . In  FIGS.  4 D- 4 F , lateral rotor tilting subassembly  248  of directional control assembly  240  laterally tilts coaxial rotor system  200 . More specifically, lateral rotor tilting subassembly  248  rightwardly tilts coaxial rotor system  200  to move rotorcraft  266  in a rightward direction of travel  272 . Lateral rotor tilting subassembly  248  leftwardly tilts coaxial rotor system  200  to move rotorcraft  266  in a leftward direction of travel  274 . 
     Referring to  FIGS.  5 A- 5 E  in the drawings, a rotorcraft  300  having yaw control system  302  is schematically illustrated. Tailboom control surface  304  is rotatably mounted to the aft end of fuselage  306 . Tailboom control surface  304  is supported by, and rotates about, tailboom support shaft  308  including one or more bearings  310 . The forward end of tailboom support shaft  308  is coupled to airframe  312  of fuselage  306 . Yaw control system  302  also includes vertical fin  314 , which is rotatably coupled to the aft and of tailboom control surface  304 . Vertical fin  314  is supported by, and rotates about, vertical fin support shaft  316  including one or more bearings  318 . Since some airflow is directed inward due to the rotor streamtube effect, vertical fin support shaft  316  is canted to minimize adverse yaw from vertical fin  314  while rotorcraft  300  hovers. Yaw control system  302  includes yaw control actuator  320 , which synchronously rotates tailboom control surface  304  and vertical fin  314 . Yaw control actuator  320  may be a servomotor, stepper motor or any other type of actuator. Yaw control actuator  320  rotates tailboom support shaft  308 , which in turn rotates tailboom control surface  304 , via gears  322 . The rotation of tailboom support shaft  308  rotates vertical fin support shaft  316  via bevel gears  324 , thus rotating vertical fin  314 . Although in the illustrated embodiment tailboom control surface  304  and vertical fin  314  rotate synchronously using a geartrain including gears  322 ,  324 , in other embodiments tailboom control surface  304  and vertical fin  314  may be independently actuated by separate and respective actuators. Yaw control system  302  may alternatively utilize a tie rod arrangement to rotate tailboom control surface  304  and vertical fan  314 . In hover, rotorcraft  300  generates downward airflow  326  with reduced swirl compared to conventional single rotor helicopters. As best seen in  FIGS.  5 C- 5 E , tailboom control surface  304  is rotatable in either direction to deflect downward airflow  326 , resulting in a moment on fuselage  306  in either direction as desired. In forward flight, vertical fin  314  is rotatable in either direction to deflect horizontal airflow  328 , resulting in a desired moment on fuselage  306 . 
     Referring to  FIGS.  6 A- 6 C  in the drawings, a coaxial rotor system is schematically illustrated and generally designated  400 . Mast  402  is rotatably coupled to fuselage  404  and supports top and bottom rotor assemblies  406 ,  408 . Bottom rotor assembly  408  includes motor  410 , which provides rotational energy to mast  402  to rotate top rotor assembly  406 . Bottom rotor assembly  408  experiences a torque reaction force in the opposite direction when motor  410  rotates mast  402  such that top and bottom rotor assemblies  406 ,  408  counter rotate. Coaxial rotor system  400  includes a directional control assembly  412  to control the longitudinal and lateral movement of the rotorcraft. Top rotor assembly  406  includes top rotor hub  414  from which variable pitch rotor blades  416  extend. Rotor blades  416  are rotatably coupled to top rotor hub  414  via rotatable yoke members  418 . Bottom rotor assembly  408  includes bottom rotor hub  420  from which variable pitch rotor blades  422  extend. Rotor blades  422  are rotatably coupled to bottom rotor hub  420  via rotatable yoke members  424 . 
     Coaxial rotor system  400  includes a collective control pitch assembly  426  interposed between top and bottom rotor assemblies  406 ,  408  to vary the collective pitch of rotor blades  416 ,  422  in response to the rotation of mast  402 . Collective control pitch assembly  426  includes a translational sleeve, or cam,  428  that rotates with mast  402 . Translational sleeve  428  has a generally hollow cylindrical shape that surrounds mast  402 . Mast  402  includes splines  430  and translational sleeve  428  forms spline grooves  432  to receive splines  430 . Splines  430  constrain translational sleeve  428  to rotate with mast  402  but permit a translational degree of freedom that allows translational sleeve  428  to move up and down along mast  402 . Collective control pitch assembly  426  includes upper pitch arms  434  interposed between translational sleeve  428  and rotor blades  416  of top rotor assembly  406 . Collective control pitch assembly  426  also includes lower pitch arms  436  interposed between translational sleeve  428  and rotor blades  422  of bottom rotor assembly  408 . The distal ends of upper pitch arms  434  are coupled to the trailing ends of rotor blades  416  via rotatable yoke members  418  and the proximal ends of upper pitch arms  434  are rotatably coupled to translational sleeve  428  via ball bearing  438 . The distal ends of lower pitch arms  436  are coupled to the trailing ends of rotor blades  422  via rotatable yoke members  424  and the proximal ends of lower pitch arms  436  are rotatably coupled to translational sleeve  428  via ball bearing  440 . Due to the rotatable connection between pitch arms  434 ,  436  and translational sleeve  428  via ball bearings  438 ,  440 , pitch arms  434 ,  436  rotate in opposite directions independently of translational sleeve  428 . 
     The upper end of translational sleeve  428  forms a ball screw nut cavity  442  having internal threads. Collective control pitch assembly  426  includes a ball screw  444  having external threads complementary to the internal threads of ball screw nut cavity  442 . Ball screw  444  is translatable into and out of ball screw nut cavity  442  by a twisting motion. The top side of ball screw  444  is coupled to the underside of top rotor hub  414 . Collective control pitch assembly  426  includes a limiter base  446  rotatably coupled to the underside of translational sleeve  428  via thrust bearing  448 . Spring  450  is interposed between limiter base  446  and bottom rotor hub  420  and biases translational sleeve  428  upward toward top rotor assembly  406 . An adjustable collective limiter  452  is coupled to the top side of bottom rotor hub  420  such that adjustable collective limiter  452  is interposed between limiter base  446  and bottom rotor assembly  408 . Adjustable collective limiter  452  may include a servomotor, stepper motor or other actuator. Thrust bearing  448  between limiter base  446  and translational sleeve  428  allows limiter base  446  to rotate relative to translational sleeve  428  to prevent or reduce friction when adjustable collective limiter  452  contacts limiter base  446 . 
       FIG.  6 B  shows collective control pitch assembly  426  positioned to provide little or no collective pitch to rotor blades  416 ,  422 .  FIG.  6 C  shows collective control pitch assembly  426  positioned to increase the collective pitch of rotor blades  416 ,  422  to generate lift for the rotorcraft. Collective control pitch assembly  426  increases the collective pitch angle of rotor blades  416 ,  422  when top and bottom rotor assemblies  406 ,  408  counter rotate. Unlike coaxial rotor system  200  in  FIGS.  3 A- 3 C , mast  402  is rotatably coupled to top rotor hub  414  via thrust bearing  454 . Mast  402  is permitted to rotate for a predetermined number of revolutions relative to top rotor hub  414 . For example, mast  402  may be permitted to rotate up to a half revolution, a full revolution or two full revolutions relative to top rotor hub  414 . Translational sleeve  428  converts differential rotation between mast  402  and top rotor assembly  406  into translational motion along mast  402 . More specifically, translational sleeve  428  translates downward and ball screw  444  rotates out of ball screw nut cavity  442  in response to relative rotation between mast  402  and top rotor assembly  406 . Pitch arms  434 ,  436  transfer the vertical motion of translational sleeve  428  to the trailing edges of rotor blades  416 ,  422 , thus causing collective blade pitch to increase when translational sleeve  428  translates downward. Adjustable collective limiter  452  limits the downward translation of translational sleeve  428  to control the maximum collective pitch of rotor blades  416 ,  422 . Adjustable collective limiter  452  has an adjustable height that limits the collective pitch to a desired level as set by the pilot or flight control system. For example, adjustable collective limiter  452  may be lowered during takeoff if a high level of collective pitch is desired and raised during forward flight if a lower level of collective pitch is desired, or vice versa. 
     Collective control pitch assembly  426  adds operational flexibility and safety to coaxial rotor system  400 . For example, the length of pitch arms  434 ,  436  may be selected such that the collective pitch of bottom rotor blades  422  is higher than the collective pitch of top rotor blades  416  to obtain optimum performance. In one non-limiting example, the length of pitch arms  434 ,  436  is such that the collective pitch of bottom rotor blades  422  is 1 to 5 degrees, such as 2.5 degrees, higher than the collective pitch of top rotor blades  416 . Collective control pitch assembly  426  may also automatically change the collective pitch of rotor blades  416 ,  422  when transitioning to autorotation. For example, if motor  410  fails during flight, output torque will be reduced, translational sleeve  428  will spring upward and the collective pitch of rotor blades  416 ,  422  will revert to a low pitch setting such as 1 to 4 degrees to support autorotation. No pilot action is required since the transition to the autorotation collective pitch setting is automatic and no adjustment to the yaw of the rotorcraft is required since coaxial rotor system  400  is rotationally isolated from fuselage  404 . In yet other embodiments, either or both of top or bottom rotor assemblies  406 ,  408  may include a conventional swashplate-based collective blade pitch mechanism in lieu of, or in addition to, collective control pitch assembly  426 . 
     Referring to  FIG.  7    in the drawings, a coaxial rotor system for a rotorcraft is schematically illustrated and generally designated  500 . Mast  502  is rotatably coupled to fuselage  504  and supports top and bottom rotor assemblies  506 ,  508 . Bottom rotor assembly  508  includes motor  510  to provide rotational energy to mast  502  to rotate top rotor assembly  506 . Bottom rotor assembly  508  experiences a torque reaction force when motor  510  rotates mast  502  such that top and bottom rotor assemblies  506 ,  508  counter rotate. Top and bottom rotor assemblies  506 ,  508  include spring-loaded rotor blades  512 ,  514 , which have manually adjustable pitches. Rotor blades  512 ,  514  are spring loaded onto yoke members  516 ,  518  via springs  520 ,  522 . A push-turn-release mechanism allows an operator to optimize flight performance by manually adjusting the pitch of each rotor blade  512 ,  514  at selected points between hover and high speed forward flight regimes. For example, between flights or when grounded, the pilot may push, turn and release each rotor blade  512 ,  514  into a desired pitch position. 
     Coaxial rotor system  500  includes directional control assembly  524 . The base of mast  502  is rotatably coupled to fuselage  504  via thrust bearing  526 . The base of mast  502  and thrust bearing  526  are encased in casing  528 , which is attached to fuselage  504  via gimbal attachment  530  including lugs that may protrude in orthogonal directions. Gimbal attachment  530  provides mast  502  with a gimballing degree of freedom  532  relative to fuselage  504  for both longitudinal and lateral directional control of the rotorcraft. Coaxial rotor system  500  is tiltable in gimballing degree of freedom  532  in both the longitudinal and lateral directions using one or more tilt actuators  534 . Tilt actuators  534  may be spaced 90 degrees apart from one another about mast  502  to provide both longitudinal and lateral directional control. 
     Referring to  FIG.  8    in the drawings, a propulsion and control system for a rotorcraft such as rotorcraft  10  in  FIGS.  1 A- 1 B  is schematically illustrated and generally designated  600 . Coaxial rotor system  602  includes top and bottom rotor assemblies  604 ,  606 . Bottom rotor assembly  606  includes electric motor  608 . Electronics node  610  includes, for example, controllers  612 , sensors  614  and communications elements  616  as well as other components suitable for use in the operation of coaxial rotor system  602 . Each rotor assembly  604 ,  606  includes a plurality of rotor blades radiating therefrom. In some embodiments, coaxial rotor system  602  includes a collective control pitch assembly (not shown) to adjust the collective pitch of the rotor blades. Coaxial rotor system  602  includes a directional control assembly  618  for directional flight control of rotorcraft  600 . 
     Fuselage  620  includes power system  622  that may serve as the electrical energy source for coaxial rotor system  602 , including rotor assemblies  604 ,  606 , motor  608  and electronics node  610 . Power system  622  may include one or more batteries  624 . Battery  624  may be charged by an electrical energy generation system (not shown), such as an internal combustion engine, or may be charged at a ground station. Battery  624  may also be interchangeably removed and installed to enable efficient refueling which may be particularly beneficial in embodiments of rotorcraft  600  wherein the sole electrical energy source is battery  624 . In embodiments that include an electrical energy generation system such as an internal combustion engine housed within fuselage  620 , the electrical energy generation system may include one or more fuel tanks such as liquid fuel tanks. In one non-limiting example, an internal combustion engine may be used to drive an electric generator that produces electrical energy that is fed to coaxial rotor system  602  to power rotor assemblies  604 ,  606 , motor  608  and electronics node  610 . In other embodiments, rotorcraft  600  may implement a hybrid power system including both an internal combustion engine and batteries. This type of hybrid power system may be beneficial in that the energy density of liquid fuel exceeds that of batteries enabling greater endurance for rotorcraft  600 . In the hybrid power system, battery  624  may provide a backup electrical power source to enable rotorcraft  600  to safely land in the event of a failure of the internal combustion engine. In yet other embodiments, coaxial rotor system  602  may include a battery to provide backup battery power in the event of a failure of power system  622 . As another alternative, coaxial rotor system  602  may be hydraulically driven within a hydraulic fluid system wherein one or more high pressure hydraulic sources or generators are housed within fuselage  620  to provide hydraulic power to coaxial rotor system  602 . 
     In the illustrated embodiment, rotorcraft  600  includes a flight control system  626  housed within fuselage  620 . Flight control system  626 , such as a digital flight control system, may preferably be a redundant flight control system and more preferably a triply redundant flight control system including three independent flight control computers. Use of triply redundant flight control system  626  improves the overall safety and reliability of rotorcraft  600  in the event of a failure of flight control system  626 . Flight control system  626  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 coaxial rotor system  602 . Flight control system  626  may be implemented on one or more general purpose computers, special purpose computers or other machines with memory or processing capability. For example, flight control system  626  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. Flight control system  626  may be a microprocessor-based system operable to execute program code in the form of machine executable instructions. In addition, flight control system  626  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. 
     Flight control system  626  communicates via a wired and/or wireless communications network with electronics node  610  of coaxial rotor system  602 . In some embodiments, electronics node  610  may instead be centralized into fuselage  620 . Flight control system  626  receives sensor data from and sends flight command information to electronics node  610  such that coaxial rotor system  602  may be controlled and operated. Flight control system  626  is configured to receive inputs from flight sensors  628  such as, but not limited to, gyroscopes, accelerometers or any other suitable sensing equipment configured to provide flight control system  626  with spatial, positional or force dynamics information, feedback or other data that may be utilized to manage the operation of rotorcraft  600 . For example, flight control system  626  may use sensor data from flight sensors  628  to generate and send flight command information to electronics node  610  to control coaxial rotor system  602 . Rotorcraft  600  may include global positioning system  630  configured to determine, receive and/or provide data related to the location of rotorcraft  600  including flight destinations, targets, no-fly zones, preplanned routes, flight paths or any other geospatial location-related information. Global positioning system  630  may be configured for bidirectional communication with flight control system  626 , unidirectional communication from global positioning system  630  to flight control system  626  or unidirectional communication from flight control system  626  to global positioning system  630 . 
     Rotorcraft  600  may include wireless communication components  632  such as radio communication equipment configured to send and receive signals related to flight commands or other operational information. Wireless communication components  632  may be configured to transmit video, audio or other data gathered, observed or otherwise generated, carried by or obtained by rotorcraft  600 . In some embodiments, flight control system  626  may also be operable to communicate with one or more remote systems via wireless communication components  632  using a wireless communications protocol. The remote systems may be operable to receive flight data from and provide commands to flight control system  626  to enable flight control over some or all aspects of flight operation. In other embodiments, rotorcraft  600  may instead be a manned or piloted vehicle. In both manned and unmanned missions, flight control system  626  may autonomously control some or all aspects of flight operation. 
     Payload  634  is receivable by payload bay  636  and may include a video camera, thermal camera, infrared imaging device, high definition camera, weapon, chemical sensor, cargo, personal belongings such as luggage or any other suitable payload. Payload  634  may be configured to provide data to flight control system  626  and flight control system  626  may be configured to control, manipulate or release payload  634 . In piloted implementations, one or more pilots may operate rotorcraft  600  from within cockpit  638 . Yaw control system  640  extends aft of fuselage  620  and includes tailboom control surface  642  and vertical fin  644 . Tailboom control surface  642  and vertical fin  644  may be synchronously or independently actuated by yaw control actuator  646 . Power system  622  provides power to yaw control actuator  646 . 
     Referring to  FIG.  9    in the drawings, a block diagram depicts an aircraft control system  700  operable for use with rotorcraft  10  of  FIGS.  1 A- 1 B  or any other aircraft of the illustrative embodiments. In the illustrated embodiment, system  700  includes three primary computer based subsystems; namely, an autonomous system  702 , a pilot system  704  and a remote system  706 . As discussed herein, the rotorcraft of the present disclosure may be operated autonomously responsive to commands generated by flight control system  708  that preferably includes a non-transitory computer readable storage medium including a set of computer instructions executable by a processor. Flight control system  708  may be a triply redundant system implemented on one or more general purpose computers, special purpose computers or other machines with memory and processing capability. For example, flight control system  708  may include one or more memory storage modules including, but is not limited to, internal storage memory such as random access memory, non-volatile memory such as read only memory, removable memory such as magnetic storage memory, optical storage, solid-state storage memory or other suitable memory storage. Flight control system  708  may be a microprocessor-based system operable to execute program code in the form of machine-executable instructions. In addition, flight control system  708  may be selectively connectable to other computer systems via a proprietary encrypted network, a public encrypted network, the Internet or other suitable communication network that may include both wired and wireless connections. 
     In the illustrated embodiment, flight control system  708  includes a command module  710  and a monitoring module  712 . It is to be understood by those skilled in the art that these and other modules executed by flight control system  708  may be implemented in a variety of forms including hardware, software, firmware, special purpose processors and combinations thereof. Flight control system  708  receives input from a variety of sources including internal sources such as sensors  714 , controllers  716 , coaxial rotor system  718  and pilot system  704  as well as external sources such as remote system  706 , global positioning system satellites or other location positioning systems and the like. For example, flight control system  708  may receive a flight plan including starting and ending locations for a mission from pilot system  704  and/or remote system  706 . Thereafter, flight control system  708  is operable to autonomously control all aspects of flight of an aircraft of the present disclosure. 
     During the various operating modes of rotorcraft  700 , command module  710  provides commands to controllers  716 . These commands enable operation of coaxial rotor system  718  including, for example, controlling the rotational speed of the rotors, adjusting directional control, adjusting the thrust vectors and the like. Flight control system  708  receives feedback from controllers  716  and coaxial rotor system  718 . This feedback is processed by monitoring module  712  that can supply correction data and other information to command module  710  and/or controllers  716 . Sensors  714 , such as positioning sensors, attitude sensors, speed sensors, environmental sensors, fuel sensors, temperature sensors, location sensors and the like also provide information to flight control system  708  to further enhance autonomous control capabilities. 
     Some or all of the autonomous control capability of flight control system  708  can be augmented or supplanted by a remote flight control system  706 . Remote system  706  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. For example, the computing systems may include one or more memory storage modules including, but is not limited to, internal storage memory such as random access memory, non-volatile memory such as read only memory, removable memory such as magnetic storage memory, optical storage memory, solid-state storage memory or other suitable memory storage. 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. The communication network may be a local area network, a wide area network, the Internet or any other type of network that couples a plurality of computers to enable various modes of communication via network messages using suitable communication techniques such as transmission control protocol/internet protocol, file transfer protocol, hypertext transfer protocol, internet protocol security protocol, point-to-point tunneling protocol, secure sockets layer protocol or other suitable protocol. Remote system  706  communicates with flight control system  708  via a communication link  720  that may include both wired and wireless connections. 
     Remote system  706  preferably includes one or more flight data display devices  722  configured to display information relating to one or more aircraft of the present disclosure. Display devices  722  may be configured in any suitable form, including, for example, liquid crystal displays, light emitting diode displays, cathode ray tube displays or any suitable type of display. Remote system  706  may also include audio output and input devices such as a microphone, speakers and/or an audio port allowing an operator to verbally communicate with, for example, a pilot on board rotorcraft  700 . Display devices  722  may also serve as a remote input device  724  if a touch screen display implementation is used, however, other remote input devices, such as a keyboard or joysticks, may alternatively be used to allow an operator to provide control commands to an aircraft being operated responsive to remote control. 
     Some or all of the autonomous and/or remote flight control of an aircraft of the present disclosure can be augmented or supplanted by onboard pilot flight control from pilot system  704 . Pilot system  704  may be integrated with autonomous system  702  or may be a standalone system preferably including a non-transitory computer readable storage medium including a set of computer instructions executable by a processor and may be implemented by a general purpose computer, a special purpose computer or other machine with memory and processing capability. Pilot system  704  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 memory, solid-state storage memory or other suitable memory storage. Pilot system  704  may be a microprocessor-based system operable to execute program code in the form of machine-executable instructions. In addition, pilot system  704  may be 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. Pilot system  704  may communicate with flight control system  708  via a communication channel  726  that preferably includes a wired connection. 
     Pilot system  704  preferably includes a cockpit display device  728  configured to display information to an onboard pilot. Cockpit display device  728  may be configured in any suitable form, including, for example, as one or more display screens such as liquid crystal displays, light emitting diode displays and the like or any other suitable display type including, for example, a display panel, a dashboard display, an augmented reality display or the like. Pilot system  704  may also include audio output and input devices such as a microphone, speakers and/or an audio port allowing an onboard pilot to verbally communicate with, for example, air traffic control or an operator of a remote system. Cockpit display device  728  may also serve as a pilot input device  730  if a touch screen display implementation is used, however, other user interface devices may alternatively be used to allow an onboard pilot to provide control commands to an aircraft being operated responsive to onboard pilot control including, for example, a control panel, mechanical control devices or other control devices. As should be apparent to those having ordinary skill in the art, through the use of system  700 , an aircraft of the present disclosure can be operated responsive to a flight control protocol including autonomous flight control, remote flight control or onboard pilot flight control and combinations thereof. 
     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.