Abstract:
An aircraft includes a fuselage and a wing extending from each lateral side of the fuselage. A nacelle is pivotably se cured to each wing. The nacelle has a rotor located thereat, with the rotor having a rotor tip path plane defined by rotation of the rotor about a rotor axis of rotation. When the rotor tip path plane is changed relative to the wing, the nacelle pivots relative to the wing about a nacelle hinge axis to reduce flapping required by the rotor. A method of operating an aircraft includes changing a rotor tip path plane orientation relative to a wing of the aircraft. The rotor disposed at a nacelle, with the nacelle pivotably secured to the wing. The nacelle is pivoted relative to the wing to reduce an overall tip path plane change requirement of the rotor.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates to vertical takeoff or landing (VTOL) aircraft, such as tilt wing aircraft or Rotor Blown Wing (RBW) aircraft. More specifically, the present disclosure relates to VTOL aircraft having cyclic rotor control. 
         [0002]    VTOL aircraft are rotor-driven aircraft capable of transitioning between conventional wing-borne flight, also referred to as airplane mode, and rotor borne flight, also referred to as helicopter mode. In some configurations, the VTOL aircraft is a tilt wing aircraft, in which the wings and rotors mounted at the wings are rotatable relative to the fuselage. In other configurations, the VTOL aircraft is a tail-sitter aircraft, in which the fuselage, wings and rotors all rotate together to transition between airplane mode and helicopter mode. Such aircraft have increased flexibility over many other aircraft in that they are capable of vertical takeoff and/or landing and have increased maneuverability due to their ability to operate in both airplane mode and helicopter mode. 
       BRIEF SUMMARY 
       [0003]    In one embodiment, an aircraft includes a fuselage and a wing extending from each lateral side of the fuselage. A nacelle is pivotably secured to each wing. The nacelle has a rotor located thereat, with the rotor having a rotor tip path plane defined by rotation of the rotor about a rotor axis of rotation. When the rotor tip path plane is changed relative to the rotor axis of rotation, the nacelle pivots relative to the wing about a nacelle hinge axis to reduce flapping required by the rotor. 
         [0004]    Additionally or alternatively, in this or other embodiments the nacelle pivots in the desired direction to rotor tip path plane change. 
         [0005]    Additionally or alternatively, in this or other embodiments a first rotor tip path plane of a first rotor is changed in a first direction and a second rotor tip path plane of a second rotor is changed in a second direction. 
         [0006]    Additionally or alternatively, in this or other embodiments a first nacelle pivots in an opposite direction to a second nacelle. 
         [0007]    Additionally or alternatively, in this or other embodiments the pivot angle is limited to between about 3-7 degrees. 
         [0008]    Additionally or alternatively, in this or other embodiments a first rotor tip path plane of a first rotor is changed in a first direction and a second rotor tip path plane of a second rotor is changed in a similar direction. 
         [0009]    Additionally or alternatively, in this or other embodiments a first nacelle pivots in a similar direction to a second nacelle. 
         [0010]    Additionally or alternatively, in this or other embodiments a damper is operably connected to the wing and to the nacelle to limit a pivot angle of the nacelle relative to the wing. 
         [0011]    Additionally or alternatively, in this or other embodiments each wing is rotatable relative to the fuselage. 
         [0012]    Additionally or alternatively, in this or other embodiments each wing is rotatably fixed relative to the fuselage. 
         [0013]    In another embodiment, a method of operating an aircraft includes changing a rotor tip path plane orientation relative to an axis of rotation of the rotor. The rotor disposed at a nacelle, with the nacelle pivotably secured to a wing of the aircraft. The nacelle is pivoted relative to the wing to reduce an overall tip path plane change requirement of the rotor. 
         [0014]    Additionally or alternatively, in this or other embodiments a first rotor tip path plane of a first engine is changed in a first direction, and a second rotor tip path plane of a second engine is changed in a second direction. 
         [0015]    Additionally or alternatively, in this or other embodiments a first nacelle is pivoted in an opposite direction to a second nacelle. 
         [0016]    Additionally or alternatively, in this or other embodiments a first nacelle is pivoted in a similar direction to a second nacelle. 
         [0017]    Additionally or alternatively, in this or other embodiments a pivot angle of the nacelle relative to the wing is limited. 
         [0018]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0020]      FIG. 1  is a side view of an embodiment of a tilt wing aircraft; 
           [0021]      FIG. 2  is a view looking aft of an embodiment of a tilt wing aircraft in airplane mode; 
           [0022]      FIG. 3  is a plan view of an embodiment of a tilt wing aircraft in helicopter mode; 
           [0023]      FIG. 4  is a first side view of an embodiment of a tilt wing aircraft in helicopter mode; 
           [0024]      FIG. 5  is a second side view of an embodiment of a tilt wing aircraft in helicopter mode; 
           [0025]      FIG. 6  is a plan view of an embodiment of a nacelle attachment to a wing; and 
           [0026]      FIG. 7  is a side view of an embodiment of a nacelle attachment to a wing. 
           [0027]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    Shown in  FIGS. 1 and 2  is an embodiment of a tilt wing aircraft  10 . The aircraft  10  includes a fuselage  12  with a wing  14  extending from each lateral side of the fuselage  12 . Each wing  14  includes and engine  16  affixed thereto, contained in a nacelle  18 . The engine  16  drives rotation of a rotor  20  to provide thrust and, in hover mode, lift for the aircraft  10 . While the figures and description herein refer to an aircraft  10  having two engines  16 , one skilled in the art will appreciate that the invention may also be applied to aircraft having other numbers of engines, for example, four engines with two at each wing. Further, while the disclosure is provided herein in the context of a tilt-wing aircraft, it is to be appreciated that aspects can readily be applied to other aircraft configurations such as rotor blown wing (RBW) aircraft or tail-sitter aircraft. As will be referenced throughout this disclosure, the aircraft  10  has a roll axis  22  extending longitudinally along the aircraft  10 , a pitch axis  24  extending laterally across the aircraft  10  through the wings  14  and perpendicular to the roll axis  22 , and a yaw axis  26  extending through an intersection of the pitch axis  24  and the roll axis  22 , and perpendicular to both the pitch axis  24  and the roll axis  22 . 
         [0029]    The wings  14  are configured to rotate relative to the fuselage  12 . In some embodiments the rotation is about the pitch axis  24 . The wings  14  rotate to transition the aircraft from conventional airplane mode, shown in  FIGS. 1 and 2 , to hover mode, shown in  FIG. 3  and from hover mode to airplane mode. In airplane mode, a rotor axis of rotation  28  is substantially parallel to the roll axis  22  during normal forward flight while in hover mode the rotor axis of rotation  28  is substantially parallel to the yaw axis  26 . While a tilt wing configuration, in which the wings  14  rotate relative to the fuselage  12 , is described herein, in other embodiments the aircraft  10  is a tail-sitter configuration, in which the wings  14  and the fuselage  12  rotate together about the pitch axis  24  between to transition between airplane mode and hover mode. When executing certain operational maneuvers, such as rotating the aircraft about a yaw axis in helicopter mode, or rotating the aircraft about a roll axis in airplane mode, rotor cyclic pitch control is utilized to execute the maneuver. Rotor cyclic pitch control tilts a rotor plane of rotation, or tip path plane (TPP), changing the angle of attack of the rotor. This change of rotor TPP can often result in rotor blade pitch change or “flapping”, due to a dissymmetry of lift on the rotor. 
         [0030]    A typical tilt wing aircraft has two rotors, one located at each wing. To execute a yaw maneuver in hover mode, cyclic pitch of a first rotor is changed in a first direction, while cyclic pitch of a second rotor is changed in a second direction opposite the first direction. Similarly opposite cyclic pitch changes are made in airplane mode to execute a roll maneuver. The wing of the tilt wing aircraft is typically configured to be torsionally stiff, to resist rotor forces acting on it. During maneuvers such as those described above, the cyclic pitch change of the rotor results in a force which tilts the nacelle in a direction creating a component of the rotor thrust force in the desired direction. The motion of the nacelle offers reduced flapping required by the rotor (i.e., pitch change of rotor blades) thereby reducing dissymmetry of lift. 
         [0031]    Referring again to  FIG. 3 , in normal hover flight a rotor tip path plane (TPP)  30 , defined by rotation of the rotor  20  about the rotor axis of rotation  28  is substantially horizontal. To perform some maneuvers during operation of the aircraft  10 , for example, rotation of the aircraft  10  about the yaw axis  26 , cyclic pitch change is applied to each of the rotors  20  by a flight control system (not shown) based on pilot input. The cyclic pitch change has the effect of tilting the rotor TPP  30  in a selected direction to a selected angle relative to the wing. To yaw the aircraft  10  (view looking down on the aircraft  10  as in  FIG. 3 ), a left side rotor TPP  30   a  is tilted to a first angle  32   a,  for example pitched downwardly (shown in  FIG. 4 ), while a right side rotor TPP  30   b  is tilted to a second angle  32   b  (shown in  FIG. 5 ) opposite to the first angle, for example, pitched upwardly. The result of this change in rotor TPPs  30   a,    30   b  is that the aircraft  10  will yaw in a clockwise direction. To execute a yaw maneuver in the counterclockwise direction, the rotor TPP  30   a  and  30   b  changes are reversed. 
         [0032]    Referring again to  FIG. 4 , changing the rotor TPP  30   a  results in a change to the direction of a rotor wake  34 , which is perpendicular to the rotor TPP  30   a.  To provide the TPP change, the nacelle  18  attachment to the wing  14  is complaint, allowing the nacelle  18  to rotate relative to the wing  14 . Rotation of the nacelle  18  relative to the wing  14  provides a thrust component in the desired direction. This reduces the flapping requirement of the rotor system. 
         [0033]    Similarly, referring now to  FIG. 1 , in normal airplane flight mode, the rotor tip path plane (TPP)  30  is substantially vertical. To roll the aircraft  10  while in airplane mode, cyclic pitch change is applied to each of the rotors  20  by the flight control system  42  based on pilot input. The cyclic pitch change has the effect of tilting the rotor TPP  30  in a selected direction to a selected angle relative to the wing. To roll the aircraft  10  (view looking aft as in  FIG. 2 ), the left side rotor TPP  30   a  is pitched downwardly to the first angle  32   a,  while the right side rotor TPP  30   b  is pitched upwardly to the second angle  32   b  opposite to the first angle, for example, pitched upwardly. 
         [0034]    Similar to when in hover mode, changing the rotor TPP  30   a  results in a change to the direction of the rotor wake  34 , which is perpendicular to the rotor TPP  30   a.  The rotor wake  34  impacts the wing  14 , which generates a roll opposing force acting in a direction opposite a selected roll direction, slowing the roll rate and increasing an amount of change in the rotor TPP  30   a  to effect the selected roll. The nacelle  18  rotates relative to the wing  14  to create the amount of change to the rotor TPP  30   a,  or rotor flapping, required to effect the maneuver. 
         [0035]    Referring to  FIGS. 6 and 7 , details of an exemplary embodiment of the nacelle  18  attachment to the wing  14  will be described below. Referring to  FIG. 6 , the structure includes a fixed wing mount  48  secured to the wing  14 . In some embodiments, as shown, two fixed wing mounts  48  are utilized, one at each lateral side of the nacelle  18 . The fixed wing mounts  48  connect to nacelle mount  50  of the nacelle  18 , resulting in a hinged connection between the nacelle  18  and the wing  14  rotating about a nacelle hinge axis  52 . Referring to  FIG. 7 , when the orientation of the TPP  30  is changed, the nacelle  18  rotates about the hinge axis  52  to create the TPP change. In some conditions, it is desired to limit the rotation of the nacelle  18  relative to the wing  14 . For example, at engine startup it may be desired to lock the nacelle  18  in position. To lock the nacelle  18  position, the nacelle  18  includes a retractable snubber bearing  54 , which is then retracted after startup to allow for rotation of the nacelle  18 . Further, in some embodiments, the arrangement includes a damper  56  connecting the fixed wing mount  48  to the nacelle mount  50  to limit the amount of rotation of the nacelle  18 . Limiting rotation of the nacelle  18  via the damper  56  allows for rotation of the nacelle  18 , while preventing overrotation of the nacelle  18 , which could lead to vibration of the nacelle  18  and thus the aircraft  10 . In some embodiments, the damper  56  limits a rotation angle  60  of the nacelle  18  to between about 3-7 degrees. 
         [0036]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. By way of example, while described in the context of aircraft, it is understood that aspects could be used in other fluid media in addition to or instead of air, such as in underwater craft using propellers. Further, while shown as being manned, it is understood that aspects could be used in unmanned aircraft. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.