Patent Publication Number: US-2023150659-A1

Title: Twin fuselage tiltrotor aircraft

Description:
TECHNICAL FIELD 
     This disclosure relates in general to the field of tiltrotor aircraft and, more particularly, though not exclusively, to twin fuselage arrangements for such aircraft. 
     BACKGROUND 
     An electric vertical takeoff and landing (eVTOL) aircraft is a type of aircraft that uses electric power to supply rotational energy through electric motor(s) to props, rotors, or fans in an aircraft propulsion system for enabling the aircraft to hover, take off, and land vertically. Because of their versatility and lack of a need for a runway, eVTOL aircraft are particularly useful for providing urban air mobility. As used herein, the term eVTOL also includes VTOL aircraft that use hybrid-electric (with an engine running a generator producing electricity and battery stored power) or turbo electric (an engine running a generator providing all power required) propulsion systems. One particular type of eVTOL aircraft is an electric tiltrotor aircraft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To provide a more complete understanding of the present disclosure and features and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying figures, in which like reference numerals represent like elements: 
         FIGS.  1 A- 1 B  illustrate a tiltrotor aircraft having a twin fuselage hexrotor configuration in accordance with embodiments described herein; 
         FIG.  1 C  illustrates a hexrotor arc arrangement of rotors of the tiltrotor aircraft of  FIGS.  1 A and  1 B ; 
         FIGS.  2 A- 2 B  illustrate a tiltrotor aircraft having a twin fuselage quadrotor configuration in accordance with embodiments described herein; 
         FIGS.  3 A- 3 B  illustrate a tiltrotor aircraft having a twin fuselage hexrotor configuration including a cargo pod between the fuselages in accordance with embodiments described herein; and 
         FIGS.  4 A- 4 B  illustrate a tiltrotor aircraft having a twin fuselage hexrotor configuration including a weapons assembly between the fuselages in accordance with embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure describes various illustrative embodiments and examples for implementing the features and functionality of the present disclosure. While particular components, arrangements, and/or features are described below in connection with various example embodiments, these are merely examples used to simplify the present disclosure and are not intended to be limiting. It will of course be appreciated that in the development of any actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, including compliance with system, business, and/or legal constraints, which may vary from one implementation to another. Moreover, it will be appreciated that, while such a development effort might be complex and time-consuming; it 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 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, components, members, apparatuses, etc. described herein may be positioned in any desired orientation. Thus, the use of terms such as “above”, “below”, “upper”, “lower”, “top”, “bottom”, or other similar 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 components described herein may be oriented in any desired direction. When used to describe a range of dimensions or other characteristics (e.g., time, pressure, temperature, length, width, etc.) of an element, operations, and/or conditions, the phrase “between X and Y” represents a range that includes X and Y. 
     Additionally, as referred to herein in this specification, the terms “forward”, “aft”, “inboard”, and “outboard” may be used to describe relative relationship(s) between components and/or spatial orientation of aspect(s) of a component or components. The term “forward” may refer to a spatial direction that is closer to a front of an aircraft relative to another component or component aspect(s). The term “aft” may refer to a spatial direction that is closer to a rear of an aircraft relative to another component or component aspect(s). The term “inboard” may refer to a location of a component that is within the fuselage(s) of an aircraft and/or a spatial direction that is closer to or along a centerline of the aircraft (wherein the centerline runs between the front and the rear of the aircraft) or other point of reference relative to another component or component aspect. The term “outboard” may refer to a location of a component that is outside the fuselage(s) of an aircraft and/or a spatial direction that farther from the centerline of the aircraft or other point of reference relative to another component or component aspect. 
     Further, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Example embodiments that may be used to implement the features and functionality of this disclosure will now be described with more particular reference to the accompanying figures. 
     Described herein are various configurations for twin fuselage tiltrotor aircraft. In particular, both hexrotor and quadrotor configurations are disclosed. Embodiments of tiltrotor aircraft described herein may be suitable for use in a variety of unmanned aircraft applications, including but not limited to unmanned logistics operations, joint surveillance and target attack radar system (JSTAR) applications, and military and other unmanned aerial systems (UAS) applications, to name a few. Embodiments described herein distribute propulsion around the aircraft to achieve large center of gravity (CG) envelopes and employ a long wing to facilitate efficient cruise operation. Twin fuselages may be used for packaging aircraft components and/or to support landing gear, which may include but is not limited to skids, fixed wheels, and retractable wheels, to name a few. Modular payload, such as a cargo pod, a weapons assembly and/or one or more sensors, may be mounted to the underside of a center wing section between the fuselages. In certain embodiments, twin fuselages may be implemented as twin booms. A distinguishing feature of embodiments described herein is the lack of a center boom/fuselage between the twin (outboard) fuselages. 
       FIGS.  1 A and  1 B  illustrate an example tiltrotor aircraft  100  that is convertible between a VTOL or hover (also commonly referred to as helicopter) mode (shown in  FIG.  1 A ), which allows for vertical takeoff and landing, hovering, and low speed directional movement, and a cruise (also commonly referred to as airplane) mode (shown in  FIG.  1 B ), which allows for forward flight. Aircraft  100  includes two fuselages  102   a ,  102   b , and a wing assembly  104  including a center wing section  105  extending between and interconnecting fuselages  102   a ,  102   b , and outboard wing portions  106   a ,  106   b , extending outboard of fuselages  102   a ,  102   b . In accordance with features of embodiments described herein, aircraft  100  further includes three pairs of propulsion systems, including forward propulsion systems  108   a ,  108   b , connected to the forward ends of the fuselages  102   a ,  102   b , aft propulsion systems  110   a ,  110   b , connected proximate the aft ends of the fuselages  102   a ,  102   b , and a pair of wing-mounted propulsion systems  112   a ,  112   b , proximate opposite ends of wing assembly  104 . In particular, in accordance with features of embodiments described herein, and as illustrated in  FIGS.  1 A and  1 B , wing-mounted propulsion systems  112   a ,  112   b , are connected to inboard ends of wing tips  114   a ,  114   b , disposed on outboard ends of outboard wing portions  106   a ,  106   b . As shown in  FIGS.  1 A and  1 B , aircraft  100  further includes a tail assembly  113  connected to aft ends of fuselages  102   a ,  102   b . Although as shown in  FIGS.  1 A and  1 B , tail assembly  113  is illustrated as including a pair of boom-mounted stabilizers, other tail assembly configurations may be implemented without departing from the spirit or scope of embodiments described herein. 
     In the illustrated embodiment, each forward propulsion system  108   a ,  108   b , includes a drive system housing comprising a pylon  120   a ,  120   b , and a rotatable open rotor assembly comprising a plurality of rotor blades  124   a ,  124   b , connected to a rotor shaft and configured to rotate about a rotor axis. As shown in  FIGS.  1 A and  1 B , the rotor assemblies of forward propulsion systems  108   a ,  108   b , include two (2) rotor blades; however, it should be recognized that more blades may be implemented without departing from the spirit and the scope of the embodiments described. Rotation of rotor blades  124   a ,  124   b , generates lift while operating in helicopter mode and thrust while operating in airplane mode. Each pylon  120   a ,  120   b , may house one or more electric motors therein configured to produce rotational energy that drives the rotation of the rotor assembly. Alternatively, each pylon  120   a ,  120   b , may house a gearbox therein that drives the rotation of the rotor assembly, wherein the gearbox receives rotational energy from a driveshaft. 
     In the illustrated embodiment, each wing-mounted propulsion system  112   a ,  112   b , includes a drive system housing comprising a pylon  130   a ,  130   b , and a rotatable open rotor assembly comprising a plurality of rotor blades  134   a ,  134   b , connected to a rotor shaft and configured to rotate about a rotor axis. As shown in  FIGS.  1 A and  1 B , the rotor assemblies of propulsion systems  112   a ,  112   b , include two (2) rotor blades; however, it should be recognized that more blades may be implemented without departing from the spirit and the scope of the embodiments described. It should also be recognized that rotor assemblies of propulsion systems  112   a ,  112   b , may include a different number of rotor blades than rotor assemblies of propulsion systems  108   a ,  108   b . Rotation of rotor blades  134   a ,  134   b , generates lift while operating in helicopter mode and thrust while operating in airplane mode. Each pylon  130   a ,  130   b , may house one or more electric motors therein configured to produce rotational energy that drives the rotation of the rotor assembly. Alternatively, each pylon  130   a ,  130   b , may house a gearbox therein that drives the rotation of the rotor assembly, wherein the gearbox receives rotational energy from a driveshaft. 
     In the illustrated embodiment, each aft propulsion system  110   a ,  110   b , includes a drive system housing comprising a pylon  140   a ,  140   b , and a rotatable open rotor assembly comprising a plurality of rotor blades  144   a ,  144   b , connected to a rotor shaft and configured to rotate about a rotor axis. As shown in  FIGS.  1 A and  1 B , the rotor assemblies of propulsion systems  110   a ,  110   b , include two (2) rotor blades; however, it should be recognized that more blades may be implemented without departing from the spirit and the scope of the embodiments described. It should also be recognized that rotor assemblies of propulsion systems  110   a ,  110   b , may include a different number of rotor blades than rotor assemblies of propulsion systems  108   a ,  108   b ,  112   a ,  112   b.    
     Rotation of rotor blades  144   a ,  144   b  generates lift while operating in helicopter mode. Each pylon  140   a ,  140   b , may house one or more electric motors therein configured to produce rotational energy that drives the rotation of the rotor assembly. Alternatively, each pylon  140   a ,  140   b , may house a gearbox therein that drives the rotation of the rotor assembly, wherein the gearbox receives rotational energy from a driveshaft. 
     In accordance with features of embodiments described herein, and as illustrated in  FIGS.  1 A and  1 B , wing-mounted propulsion systems  112   a ,  112   b , are connected to inboard ends of wing tips  114   a ,  114   b , attached to outboard ends of wing assembly  104 . Wing tips  114   a ,  114   b , together with wing-mounted propulsion systems  112   a ,  112   b , tilt relative to wing assembly  104  between a first position ( FIG.  1 A ), in which propulsion systems  112   a ,  112   b , and wing tips  114   a ,  114   b , are configured in a hover mode, and a second position ( FIG.  1 B ), in which propulsion systems  112   a ,  112   b , and wing tips  114   a ,  114   b , are configured in a cruise mode. It will be recognized that wing tips  114   a ,  114   b , are not necessary and that in alternative embodiments, wing tips are omitted. 
     In accordance with features of embodiments described herein, forward propulsion systems  108   a ,  108   b , (and more specifically, pylons  120   a ,  120   b ) are tiltably connected to forward ends of fuselages  102   a ,  102   b , such that they may be tilted between a first position ( FIG.  1 A ), in which propulsion systems  108   a ,  108   b , are configured in a hover mode, and a second position ( FIG.  1 B ), in which propulsion systems  108   a ,  108   b , are configured in a cruise mode. 
     In accordance with features of embodiments described herein, aft propulsion systems  110   a ,  110   b , are fixedly attached to fuselages  102   a ,  102   b , proximate aft ends thereof (forward of tail assembly  113 ) and do not convert between hover mode ( FIG.  1 A ) and cruise mode ( FIG.  1 B ). 
     The position of the rotor assemblies of the forward propulsion systems  108   a ,  108   b , and wing-mounted propulsion systems  112   a ,  112   b , as well as the pitch of individual rotor blades  124   a ,  124   b ,  134   a ,  134   b ,  144   a ,  144   b , can be selectively controlled in order to selectively control direction, thrust, and lift of aircraft  100 . As previously noted, propulsion systems  108   a ,  108   b ,  112   a ,  112   b , are each convertible, relative to fuselages  102   a ,  102   b , between a vertical position, as shown in  FIG.  1 A , and a horizontal position, as shown in  FIG.  1 B . Propulsion systems  108   a ,  108   b ,  112   a ,  112   b , are in the vertical position during vertical takeoff and landing mode. Vertical takeoff and landing mode may be considered to include hover operations of aircraft  100 . Propulsion systems  108   a ,  108   b ,  112   a ,  112   b , are in the horizontal position during forward flight mode, in which aircraft  100  is in forward flight. In forward flight mode, propulsion systems  108   a ,  108   b ,  112   a ,  112   b , direct their respective thrusts in the aft direction to propel aircraft  100  forward. Aircraft  100  is operable to fly in all directions during the vertical takeoff and landing mode configuration of  FIG.  1 A , although faster forward flight is achievable while in the forward flight mode configuration of  FIG.  1 B . Propulsion systems  108   a ,  108   b ,  112   a ,  112   b , may be tiltable between the vertical and horizontal positions by actuators (not shown) that are tiltable in response to commands originating from a pilot and/or a flight control system. Each of the propulsion systems  108   a ,  108   b ,  110   a ,  110   b ,  112   a ,  112   b , may utilize an electric motor and gearbox unit disposed within a respective pylon  120   a ,  120   b ,  130   a ,  130   b ,  140   a ,  140   b  or a direct drive motor of group of motors on the same shaft as a power source to rotate the respective rotor assembly about the rotor axis via the rotor shaft. 
     It should be noted that, although propulsion systems  108   a ,  108   b , are shown and described as being tiltable between cruise and hover positions, those propulsion systems may be fixed in the hover positions, similarly to propulsion systems  110   a ,  110   b . Additionally and/or alternatively, propulsion systems  112   a ,  112   b , may be connected to opposite ends of the wing assembly  104  such that only pylons  130   a ,  130   b , or a portion thereof, are tiltable relative to the wing assembly  104 . The tiltable pylons  120   a ,  120   b  and  130   a  and  130   b  may also be differentially tiltable and may vary in power to control yaw of the aircraft whereas different thrust for roll and pitch is controlled through differential blade pitch, rpm, and motor power. 
     In accordance with features of embodiments described herein, when aircraft  100  is in cruise mode, the rotor assemblies of aft propulsion systems  110   a ,  110   b , may cease rotation. In embodiments in which forward propulsion systems  108   a ,  108   b , are also fixed (i.e., do not convert between hover and cruise modes), rotor assemblies of propulsion systems  108   a ,  108   b , may also cease rotation when aircraft  100  is in cruise mode. Fewer active rotor assemblies in cruise mode improves blade loading and propulsive efficiency of the props. In addition, stopping or slowing the aft rotors reduces drag. The lift is accomplished with the wings so lift from rotors  110   a ,  110   b , is not necessary. With six rotor assemblies, a rotor assembly can be lost while still allowing aircraft  100  to hover even without motor redundancy per rotor assembly. In the event of a rotor failure, the rotor on the opposite side of the aircraft would be powered down, allowing the aircraft to hover as a quad copter with the four remaining rotors operating at elevated power levels. In accordance with features of embodiments described herein, if the aft left rotor were to fail, the forward right rotor would also be powered down, allowing the thrust on the remaining rotors to balance. Electric power to the motors allows the distributed nature of the aircraft  100  to stay weight efficient without requiring extensive cross-connects. 
     Because the aircraft  100  includes six (6) rotor assemblies, the aircraft may be referred to as a “hexrotor aircraft.”  FIG.  1 C  illustrates a hexrotor arc arrangement  150  when aircraft  100  is in hover mode. In particular, to allow the minimum number of rotors and still allow loss of a rotor as a recoverable failure mechanism in a hover, the rotors are arranged in an arc arrangement both side-to-side and forward-to-aft. As a result, when one rotor is lost and a second rotor is powered down, the aircraft can still be balanced as a quad arrangement on the remaining rotors. If a linear arrangement were used, the power and balance requirements would prevent recovery on just four rotors. Multi-copters with more than six rotors may recover by offsetting opposing rotors, but six in the illustrated arc arrangement is the minimum to allow a recovery after failure and result in significant weight savings due to fewer redundant motors and rotors. 
       FIGS.  2 A and  2 B  illustrate an example tiltrotor aircraft  200  that is convertible between a VTOL or hover (also commonly referred to as helicopter) mode (shown in  FIG.  2 A ), which allows for vertical takeoff and landing, hovering, and low speed directional movement, and a cruise (also commonly referred to as airplane) mode (shown in  FIG.  2 B ), which allows for forward flight. Aircraft  200  includes two fuselages  202   a ,  202   b , and a wing assembly  204  including a center wing section  205  extending between and interconnecting fuselages  202   a ,  202   b , and outboard wing portions  206   a ,  206   b , extending outboard of fuselages  202   a ,  202   b . In accordance with features of embodiments described herein, aircraft  200  further includes two pairs of propulsion systems, including forward propulsion systems  208   a ,  208   b , connected to the forward ends of the fuselages  202   a ,  202   b , and aft propulsion systems  210   a ,  210   b , connected proximate the aft ends of the fuselages  202   a ,  202   b . As shown in  FIGS.  2 A and  2 B , aircraft  200  further includes a tail assembly  213  connected to aft ends of fuselages  202   a ,  202   b . Although as shown in  FIGS.  2 A and  2 B , tail assembly  213  is illustrated as including a pair of boom-mounted stabilizers, other tail assembly configurations may be implemented without departing from the spirit or scope of embodiments described herein. 
     In the illustrated embodiment, each forward propulsion system  208   a ,  208   b , includes a drive system housing comprising a pylon  220   a ,  220   b , and a rotatable open rotor assembly comprising a plurality of rotor blades  224   a ,  224   b , connected to a rotor shaft and configured to rotate about a rotor axis. As shown in  FIGS.  2 A and  2 B , the rotor assemblies of forward propulsion systems  208   a ,  208   b , include two (2) rotor blades; however, it should be recognized that more blades may be implemented without departing from the spirit and the scope of the embodiments described. Rotation of rotor blades  224   a ,  224   b , generates lift while operating in helicopter mode and thrust while operating in airplane mode. Each pylon  220   a ,  220   b , may house one or more electric motors therein configured to produce rotational energy that drives the rotation of the rotor assembly. Alternatively, each pylon  220   a ,  220   b , may house a gearbox therein that drives the rotation of the rotor assembly, wherein the gearbox receives rotational energy from a driveshaft. 
     In the illustrated embodiment, each aft propulsion system  210   a ,  210   b , includes a drive system housing comprising a pylon  240   a ,  240   b , and a rotatable open rotor assembly comprising a plurality of rotor blades  244   a ,  244   b , connected to a rotor shaft and configured to rotate about a rotor axis. As shown in  FIGS.  2 A and  2 B , the rotor assemblies of propulsion systems  210   a ,  210   b , include two (2) rotor blades; however, it should be recognized that more blades may be implemented without departing from the spirit and the scope of the embodiments described. It should also be recognized that rotor assemblies of propulsion systems  210   a ,  210   b , may include a different number of rotor blades than rotor assemblies of propulsion systems  208   a ,  208   b.    
     Rotation of rotor blades  244   a ,  244   b  generates lift while operating in helicopter mode. Each pylon  240   a ,  240   b , may house one or more electric motors therein configured to produce rotational energy that drives the rotation of the rotor assembly. Alternatively, each pylon  240   a ,  240   b , may house a gearbox therein that drives the rotation of the rotor assembly, wherein the gearbox receives rotational energy from a driveshaft. 
     In accordance with features of embodiments described herein, forward propulsion systems  208   a ,  208   b , (and more specifically, pylons  220   a ,  220   b ) are tiltably connected to forward ends of fuselages  202   a ,  202   b , such that they may be tilted between a first position ( FIG.  2 A ), in which propulsion systems  208   a ,  208   b , are configured in a hover mode, and a second position ( FIG.  2 B ), in which propulsion systems  208   a ,  208   b , are configured in a cruise mode. In accordance with features of embodiments described herein, aft propulsion systems  210   a ,  210   b , are fixedly attached to fuselages  202   a ,  202   b , proximate aft ends thereof (forward of tail assembly  213 ) and do not convert between hover mode ( FIG.  2 A ) and cruise mode ( FIG.  2 B ). 
     The position of the rotor assemblies of the forward propulsion systems  208   a ,  208   b , as well as the pitch of individual rotor blades  224   a ,  224   b ,  244   a ,  244   b , can be selectively controlled in order to selectively control direction, thrust, and lift of aircraft  200 . As previously noted, propulsion systems  208   a ,  208   b , are each convertible, relative to fuselages  202   a ,  202   b , between a vertical position, as shown in  FIG.  2 A , and a horizontal position, as shown in  FIG.  2 B . Propulsion systems  208   a ,  208   b , are in the vertical position during vertical takeoff and landing mode. Vertical takeoff and landing mode may be considered to include hover operations of aircraft  200 . Propulsion systems  208   a ,  208   b , are in the horizontal position during forward flight mode, in which aircraft  200  is in forward flight. In forward flight mode, propulsion systems  208   a ,  208   b , direct their respective thrusts in the aft direction to propel aircraft  200  forward. Aircraft  200  is operable to fly in all directions during the vertical takeoff and landing mode configuration of  FIG.  2 A , although faster forward flight is achievable while in the forward flight mode configuration of  FIG.  2 B . Propulsion systems  208   a ,  208   b , may be tiltable between the vertical and horizontal positions by actuators (not shown) that are tiltable in response to commands originating from a pilot and/or a flight control system. Each of the propulsion systems  208   a ,  208   b ,  210   a ,  210   b , may utilize an electric motor and gearbox unit disposed within a respective pylon  220   a ,  220   b ,  240   a ,  240   b  or a direct drive motor of group of motors on the same shaft as a power source to rotate the respective rotor assembly about the rotor axis via the rotor shaft. 
     The tiltable pylons  220   a ,  220   b , may be differentially tiltable and may vary in power to control yaw of the aircraft whereas different thrust for roll and pitch is controlled through differential blade pitch, rpm, and motor power. 
     In accordance with features of embodiments described herein, when aircraft  200  is in cruise mode, the rotor assemblies of aft propulsion systems  210   a ,  210   b , may cease rotation or rotate slower. Fewer active rotor assemblies in cruise mode improves blade loading and propulsive efficiency of the props. In addition, stopping or slowing the aft rotors reduces drag. The lift is accomplished with the wings so lift from rotors  210   a ,  210   b  is not necessary. 
     Because the aircraft  200  includes four (4) rotor assemblies, the aircraft may be referred to as a “quadrotor aircraft.” 
       FIGS.  3 A- 3 B  illustrate an example tiltrotor aircraft  300  that is convertible between a VTOL or hover (also commonly referred to as helicopter) mode (shown in  FIG.  3 A ), which allows for vertical takeoff and landing, hovering, and low speed directional movement, and a cruise (also commonly referred to as airplane) mode (shown in  FIG.  3 B ), which allows for forward flight. The aircraft  300  is substantially identical to the aircraft  100  ( FIGS.  1 A- 1 B ) except that the aircraft  300  includes a cargo pod  302  connected to the underside of wing assembly between twin fuselages  304   a ,  304   b.    
       FIGS.  4 A- 4 B  illustrate an example tiltrotor aircraft  400  that is convertible between a VTOL or hover (also commonly referred to as helicopter) mode (shown in  FIG.  4 A ), which allows for vertical takeoff and landing, hovering, and low speed directional movement, and a cruise (also commonly referred to as airplane) mode (shown in  FIG.  4 B ), which allows for forward flight. The aircraft  400  is substantially identical to the aircraft  100  ( FIGS.  1 A- 1 B ) except that the aircraft  400  includes a weapons assembly  402  disposed between twin fuselages  404   a ,  404   b . It will be recognized that other types of external cargo, such as sensors, may be supported between the fuselages as shown in  FIGS.  3 A,  3 B,  4 A, and  4 B . 
     It should be appreciated that aircraft illustrated herein, such as aircraft  100 , is merely illustrative of a variety of aircraft that can implement the embodiments disclosed herein. Indeed, the various embodiments of the electric drive system line replaceable unit described herein may be used on any aircraft that utilizes motors. Other aircraft implementations can include hybrid aircraft, tiltrotor aircraft, quad tiltrotor aircraft, unmanned aircraft, gyrocopters, airplanes, helicopters, commuter aircraft, electric aircraft, hybrid-electric aircraft, and the like. As such, those skilled in the art will recognize that the embodiments described herein for an electric drive system line replaceable unit 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. 
     The components of rotor assemblies described herein may comprise any materials suitable for use with an aircraft rotor. For example, rotor blades and other components may comprise carbon fiber, fiberglass, or aluminum; and rotor shafts and other components may comprise steel, aluminum, or titanium. 
     Example 1 provides an aircraft comprising first and second fuselages; a wing assembly connecting the first and second fuselages, wherein the first and second fuselages are parallel to one another; first and second forward propulsion systems tiltably attached to forward ends of the first and second fuselages; and first and second aft propulsion systems fixedly attached proximate aft ends of the first and second fuselages. 
     Example 2 provides the aircraft of example 1, further comprising a tail assembly attached to the aft ends of the first and second fuselages. 
     Example 3 provides the aircraft of any of examples 1-2, further comprising first and second wing-mounted propulsion systems tiltably connected to outboard ends of the wing assembly. 
     Example 4 provides the aircraft of example 3, further comprising first and second wing tips fixedly connected to outboard sides of the first and second wing-mounted propulsion systems. 
     Example 5 provides the aircraft of example 4, wherein the first and second wing-mounted propulsion systems and the first and second wing tips are collectively tiltable between a first position when the aircraft is in a hover mode and a second position when the aircraft is in a cruise mode. 
     Example 6 provides the aircraft of any of examples 1-5, wherein each of the propulsion systems includes a rotor assembly comprising a plurality of rotor blades. 
     Example 7 provides the aircraft of example 6, wherein the rotor assemblies of the first and second aft propulsion systems rotate when the aircraft is in the hover mode and cease to rotate when the aircraft is in the cruise mode. 
     Example 8 provides the aircraft of any of examples 1-7, wherein the first and second forward propulsion systems are tiltably connected to the forward ends of the first and second fuselages such that the first and second forward propulsion systems are tiltable between a first position when the aircraft is in the hover mode and a second position when the aircraft is in the cruise mode. 
     Example 9 provides the aircraft of any of examples 1-8, wherein the propulsion systems collectively comprise a hexrotor arc when the aircraft is in the hover mode. 
     Example 10 provides the aircraft of any of examples 1-9, further comprising a cargo pod between the first and second fuselages. 
     Example 11 provides the aircraft of example 10, wherein the cargo pod is connected to an underside of the wing assembly. 
     Example 12 provides the aircraft of any of examples 1-11, further comprising external payload between the first and second fuselages. 
     Example 13 provides the aircraft of example 12, wherein the external payload is connected to an underside of the wing assembly. 
     Example 14 provides the aircraft of example 12, wherein the external payload comprises at least one of a weapons assembly and a sensor. 
     Example 15 provides an eVTOL selectively convertible between a hover mode and a cruise mode, the eVTOL comprising a first fuselage; a second fuselage parallel to the first fuselage; a wing assembly connecting the first and second fuselages; first and second forward propulsion systems attached to forward ends of the first and second fuselages, the first and second forward propulsion systems comprising first and second pylons tiltable relative to the first and second fuselages between a first position when the aircraft is in a hover mode and a second position when the aircraft is in a cruise mode, wherein each of the pylons houses a drive system for providing rotational power to rotor assemblies of the forward propulsion systems; and first and second aft propulsion systems fixedly attached proximate aft ends of the first and second fuselages. 
     Example 16 provides the eVTOL of example 15, further comprising first and second wing-mounted propulsion systems tiltably connected to outboard ends of the wing assembly. 
     Example 17 provides the eVTOL of example 16, further comprising first and second wing tips fixedly connected to outboard sides of the first and second wing-mounted propulsion systems. 
     Example 18 provides the eVTOL of example 17, wherein the first and second wing-mounted propulsion systems and the first and second wing tips are collectively tiltable between a first position when the aircraft is in a hover mode and a second position when the aircraft is in a cruise mode. 
     Example 19 provides the eVTOL of any of examples 16-18, wherein each of the propulsion systems includes a rotor assembly comprising a plurality of rotor blades. 
     Example 20 provides the eVTOL of example 19, wherein the rotor assemblies of the first and second aft propulsion systems rotate when the aircraft is in the hover mode and cease to rotate when the aircraft is in the cruise mode. 
     At least one embodiment is disclosed, and variations, combinations, and/or modifications of the embodiment(s) and/or features of the embodiment(s) made by a person having ordinary skill in the art are within the scope of the disclosure. Alternative embodiments that result from combining, integrating, and/or omitting features of the embodiment(s) are also within the scope of the disclosure. Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a numerical range with a lower limit, RI, and an upper limit, Ru, is disclosed, any number falling within the range is specifically disclosed. In particular, the following numbers within the range are specifically disclosed: R=RI+k*(Ru−RI), wherein k is a variable ranging from 1 percent to 100 percent with a 1 percent increment, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . 50 percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 95 percent, 98 percent, 99 percent, or 100 percent. Moreover, any numerical range defined by two R numbers as defined in the above is also specifically disclosed. Use of the term “optionally” with respect to any element of a claim means that the element is required, or alternatively, the element is not required, both alternatives being within the scope of the claim. Use of broader terms such as comprises, includes, and having should be understood to provide support for narrower terms such as consisting of, consisting essentially of, and comprised substantially of. Accordingly, the scope of protection is not limited by the description set out above but is defined by the claims that follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated as further disclosure into the specification and the claims are embodiment(s) of the present invention. Also, the phrases “at least one of A, B, and C” and “A and/or B and/or C” should each be interpreted to include only A, only B, only C, or any combination of A, B, and C. 
     The diagrams in the FIGURES illustrate the architecture, functionality, and/or operation of possible implementations of various embodiments of the present disclosure. Although several embodiments have been illustrated and described in detail, numerous other changes, substitutions, variations, alterations, and/or modifications are possible without departing from the spirit and scope of the present disclosure, as defined by the appended claims. The particular embodiments described herein are illustrative only and may be modified and practiced in different but equivalent manners, as would be apparent to those of ordinary skill in the art having the benefit of the teachings herein. Those of ordinary skill in the art would appreciate that the present disclosure may be readily used as a basis for designing or modifying other embodiments for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. For example, certain embodiments may be implemented using more, less, and/or other components than those described herein. Moreover, in certain embodiments, some components may be implemented separately, consolidated into one or more integrated components, and/or omitted. Similarly, methods associated with certain embodiments may be implemented using more, less, and/or other steps than those described herein, and their steps may be performed in any suitable order. 
     Numerous other changes, substitutions, variations, alterations, and modifications may be ascertained to one of ordinary skill in the art and it is intended that the present disclosure encompass all such changes, substitutions, variations, alterations, and modifications as falling within the scope of the appended claims. 
     One or more advantages mentioned herein do not in any way suggest that any one of the embodiments described herein necessarily provides all the described advantages or that all the embodiments of the present disclosure necessarily provide any one of the described advantages. Note that in this specification, references to various features included in “one embodiment”, “example embodiment”, “an embodiment”, “another embodiment”, “certain embodiments”, “some embodiments”, “various embodiments”, “other embodiments”, “alternative embodiment”, and the like are intended to mean that any such features are included in one or more embodiments of the present disclosure but may or may not necessarily be combined in the same embodiments. 
     As used herein, unless expressly stated to the contrary, use of the phrase “at least one of”, “one or more of” and “and/or” are open ended expressions that are both conjunctive and disjunctive in operation for any combination of named elements, conditions, or activities. For example, each of the expressions “at least one of X, Y and Z”, “at least one of X, Y or Z”, “one or more of X, Y and Z”, “one or more of X, Y or Z” and “A, B and/or C” can mean any of the following: 1) X, but not Y and not Z; 2) Y, but not X and not Z; 3) Z, but not X and not Y; 4) X and Y, but not Z; 5) X and Z, but not Y; 6) Y and Z, but not X; or 7) X, Y, and Z. Additionally, unless expressly stated to the contrary, the terms “first”, “second”, “third”, etc., are intended to distinguish the particular nouns (e.g., blade, rotor, element, device, condition, module, activity, operation, etc.) they modify. Unless expressly stated to the contrary, the use of these terms is not intended to indicate any type of order, rank, importance, temporal sequence, or hierarchy of the modified noun. For example, “first X” and “second X” are intended to designate two X elements that are not necessarily limited by any order, rank, importance, temporal sequence, or hierarchy of the two elements. As referred to herein, “at least one of”, “one or more of”, and the like can be represented using the “(s)” nomenclature (e.g., one or more element(s)). 
     In order to assist the United States Patent and Trademark Office (USPTO) and, additionally, any readers of any patent issued on this application in interpreting the claims appended hereto, Applicant wishes to note that the Applicant: (a) does not intend any of the appended claims to invoke paragraph (f) of 35 U.S.C. Section 112 as it exists on the date of the filing hereof unless the words “means for” or “step for” are specifically used in the particular claims; and (b) does not intend, by any statement in the specification, to limit this disclosure in any way that is not otherwise reflected in the appended claims.