Abstract:
A method and apparatus provide for automatically controlling the flight of a tiltrotor aircraft while the aircraft is in flight that is at least partially rotor-borne. The method and apparatus provide for automatically tilting nacelles in response to a longitudinal-velocity control signal so as to produce a longitudinal thrust-vector component for controlling longitudinal velocity of the aircraft. Simultaneously, cyclic swashplate controls are automatically actuated so as to maintain the fuselage in a desired pitch attitude. The method and apparatus also provide for automatically actuating the cyclic swashplate controls for each rotor in response to a lateral-velocity control signal so as to produce a lateral thrust-vector component for controlling lateral velocity of the aircraft. Simultaneously, collective swashplate controls for each rotor are automatically actuated so as to maintain the fuselage in a desired roll attitude. The method and apparatus provide for yaw control through differential longitudinal thrust produced by tilting the nacelles.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates in general to the field of flight control of aircraft. In particular, the present invention relates to apparatus and methods for controlling the flight of a tiltrotor aircraft.  
       DESCRIPTION OF THE PRIOR ART  
       [0002]     A rotary wing aircraft, such as a helicopter or the tiltrotor aircraft  11  shown in  FIG. 1 , produces lift with at least one main rotor  13 , which comprises multiple wings, or blades  15 , attached to a rotating hub  17 . Each blade  15  has an airfoil cross-section, and lift is produced by moving blades  15  in a circular path as hub  17  rotates. As shown in the figures, the left and right sides of aircraft  11  are generally mirror images of each other, having corresponding components on each side of aircraft  11 . As described herein, a single reference number may be used to refer to both left and right (as viewed if seated in the aircraft) components when the description applies to both components. Specific reference numbers are used for clarity to refer to specific left or right components when the description is specific to either the left or right component. For example, “rotor  13 ” may be used in descriptions of both the left rotor and the right rotor, and “rotor  13 A” and “rotor  13 B” may be used in descriptions that are specific to the left and right rotors, respectively.  
         [0003]     The amount of lift produced can be varied by changing the angle of attack, or pitch, of blades  15  or the speed of blades  15 , though the speed of rotor  13  is usually controlled by use of a RPM governor to within a narrow range for optimizing performance. Varying the pitch for each blade  15  requires a complex mechanical system, which is typically accomplished using a swashplate assembly (not shown) located on each hub  17 .  
         [0004]     Each swashplate assembly has two primary roles: (1) under the direction of the collective control, each swashplate assembly changes the pitch of blades  15  on the corresponding rotor  13  simultaneously, which increases or decreases the lift that each rotor  13  supplies to aircraft  11 , increasing or decreasing each thrust vector  19  for causing aircraft  11  to gain or lose altitude; and (2) under the direction of the cyclic control, each swashplate assembly changes the angle of blades  15  on the corresponding rotor  13  individually as they move with hub  17 , creating a moment in a generally horizontal direction, as indicated by arrows  21 , for causing aircraft  11  to move in any direction around a horizontal 360-degree circle, including forward, backward, left and right.  
         [0005]     Typically, the collective blade pitch is controlled by a lever that the pilot can move up or down, whereas the cyclic blade pitch is controlled by a control stick that the pilot moves in the direction of desired movement of the aircraft. The collective control raises the entire swashplate assembly as a unit, changing the pitch of blades  15  by the same amount throughout the rotation of hub  17 . The cyclic control tilts the swashplate assembly, causing the angle of attack of blades  15  to vary as hub  17  rotates. This has the effect of changing the pitch of blades  15  unevenly depending on where they are in the rotation, causing blades  15  to have a greater angle of attack, and therefore more lift, on one side of the rotation, and a lesser angle of attack, and therefore less lift, on the opposite side of the rotation. The unbalanced lift creates a moment that causes the pitch or roll attitude of aircraft  11  to change, which rotates the thrust vectors and causes aircraft  11  to move longitudinally or laterally.  
         [0006]     A tiltrotor aircraft, such as aircraft  11 , also has movable nacelles  23  that are mounted to the outer ends of each fixed wing  25 . Nacelles  23  can be selectively rotated, as indicated by arrows  27 , to any point between a generally vertical orientation, as is shown in  FIG. 1 , corresponding to a “helicopter mode” for rotor-borne flight using blades  15  to provide lift, and a horizontal orientation, corresponding to an “airplane mode” for forward flight using fixed wings  25  to produce lift. Aircraft  11  may also operate in partial helicopter mode at low speeds, in which rotors  13  and fixed wings  25  both provide part of the required lift for flight. The operation of aircraft  11  typically includes a vertical or short takeoff, a transition from helicopter mode to airplane mode for forward flight, and then a transition back to helicopter mode for a vertical or short landing.  
         [0007]     Due to the many variables involved in the control of flight of a tiltrotor aircraft, a computer-controlled flight control system (FCS)  28  automates many of the functions required for safe, efficient operation. FCS  28  actuates flight-control components of aircraft  11  in response to control inputs generated by one or more of the following: (1) an on-board pilot; (2) a pilot located remote from the aircraft, as with an unmanned aerial vehicle (UAV); (3) a partially autonomous system, such as an auto-pilot; and (4) a fully autonomous system, such as in an UAV operating in a fully autonomous manner. FCS  28  is provided with software-implemented flight control methods for generating responses to these control inputs that are appropriate to a particular flight regime.  
         [0008]     In the automatic control methods of current tiltrotor aircraft, when a command for a change in longitudinal velocity is received by FCS  28  while aircraft  11  is in full or partial helicopter mode, FCS  28  induces longitudinal acceleration of aircraft  11  by changing the pitch attitude of aircraft  11  to direct thrust vectors  19  forward or rearward. The change of pitch attitude is accomplished by FCS  28  commanding the swashplates to tilt forward or rearward using cyclic control, which causes aircraft  11  to pitch downward in the direction that the aircraft is commanded to fly. For example, when aircraft  11  is commanded by a pilot to fly in the forward direction by moving the cyclic control forward, FCS  28  commands the swashplate for each rotor  13  to tilt forward, and rotors  13  create a forward pitch moment. As shown in  FIG. 2 , the moment causes the plane of blades  15  to tilt forward and also pitches aircraft  11  in the nose-down direction, which is visible in comparison to ground  29 . Thrust vectors  19  are thus rotated toward the forward direction, and the result is movement in the direction shown by arrow  30 .  
         [0009]     There are several undesirable influences on aircraft  11  using this flight control method, especially in a gusty or windy environment. When the pitch attitude of aircraft  11  is changed due to a command to move in the forward/rearward direction, there is a change in the angle of attack of wings  25  and a corresponding reduction in lift produced by wings  25 , and this may produce an undesirable change in the vertical velocity and/or altitude of aircraft  11 , which must be countered by changing the vertical climb command. This pitch-attitude-to-vertical-velocity coupling is especially true when hovering or in a low-speed flight condition, and is more pronounced in the presence of a headwind. Using the current automatic flight control method in this situation, aircraft  11  cannot accelerate in the forward direction without a nose-down pitch attitude, and the resulting uncommanded and unwanted vertical motion interferes with the precise vertical control of aircraft  11 .  
         [0010]     In the automatic control methods of current tiltrotor aircraft, when a command for a change in lateral velocity is received by FCS  28  while the aircraft is in full or partial helicopter mode, FCS  28  induces lateral acceleration of aircraft  11  by changing the roll attitude of aircraft  11  to direct thrust vectors  19  to the left or right. This is accomplished using differential collective blade pitch control, which causes fuselage  23  to tilt right or left in the direction that aircraft  11  is commanded to fly. For example, when aircraft  11  is commanded to fly to the right, FCS  28  commands the collective controls on rotors  13  such that right rotor  13  produces less lift than that being produced by left rotor  13 . The resulting thrust imbalance causes aircraft  11  to roll to the right, as shown in  FIG. 3 , directing thrust vectors  19  to the right and causing aircraft  11  to move in the direction of arrow  31 .  
         [0011]     This automatic flight control method of tilting aircraft  11  during lateral maneuvering also causes several problems. When aircraft  11  is operating in the area of ground effects, which it must do each time it is in close proximity to a large surface, such as ground  29  during takeoff and landing, the rolling of aircraft  11  will cause one rotor  13  to be closer to ground  29  than the other rotor  13 . This difference in relation to ground  29  will cause the ground effects to be greater on one side of aircraft  11  than on the other, which will cause the lift of each rotor  13  to change differently. This difference will cause an additional roll moment on aircraft  11 , and this interferes with the precise control of aircraft  11 . The rolling of aircraft  11  also tends to blow the air cushion out from under one side of aircraft  11 , further degrading the controllability.  
         [0012]     When aircraft  11  is moving laterally, or is hovering in a sideward wind, and wings  25  are tilted to the left or right, there is more drag or wind resistance. There is also an increase in down loading, which is the loading of the top of wings  25  by the dynamic pressure caused by rotors  13  and the lateral aircraft velocity. Both of these conditions degrade the controllability in the lateral and vertical axes and require more power than flying level in the same wind conditions.  
         [0013]     Aircraft  11  is also subject to upsets from wind gusts, with wind from any direction causing large position displacements when using the current control methods. For example, if aircraft  11  experiences a wind gust from the left side, aircraft  11  will roll to the right. When aircraft  11  rolls to the right, thrust vectors  19  are also rotated to the right, which makes the lateral velocity of aircraft  11  increase to the right. In current tiltrotor aircraft, if FCS  28  is programmed to hold the aircraft over a specified point on the ground, FCS  28  will command aircraft  11  to roll back to the left, causing thrust vectors  19  to oppose the gust and to move aircraft  11  back to the position it occupied before the gust. This method of control has the disadvantage of allowing the gust to displace aircraft  11  a significant distance from its original position before FCS  28  can drive aircraft  11  back to the original position.  
         [0014]     Other problems with the current methods of control include high response time to FCS commands and reduced passenger comfort. Response time to forward and lateral velocity commands is high due to the requirement that the attitude of aircraft  11  change for these commands to be executed, and the high inertia of a large, manned tiltrotor, such as aircraft  11 , translates into low response frequencies of the system. A significant disadvantage for tiltrotors used to carry passengers is that passenger comfort is compromised by tilting fuselage  23  of aircraft  11  while maneuvering while hovering or in low-speed flight, such as while approaching for a landing and when moving aircraft  11  into position to accelerate to forward flight.  
         [0015]     In the automatic control methods of current tiltrotor aircraft, when a command to change the yaw velocity (i.e., the velocity of change of heading) of aircraft  11  is received by FCS  28  while the aircraft is in full or partial helicopter mode, FCS  28  induces a yawing moment using differential longitudinal cyclic control. For example, when aircraft  11  is commanded to yaw to the left, such as when a pilot depresses the left rudder pedal, FCS  28  commands the swashplate for right rotor  13 B to tilt forward and commands the swashplate of left rotor  13 A to tilt rearward. As shown in  FIG. 4 , the planes of blades  15 A and  15 B and the direction of thrust vectors  19 A,  19 B are tilted in opposite directions, with vector  19 A having a rearward thrust component and vector  19 B having forward thrust component. Thrust vectors  19 A,  19 B create a yaw moment, resulting in rotation of aircraft  11  generally about a vertical yaw axis  32  in the direction shown by arrow  33 .  
       SUMMARY OF THE INVENTION  
       [0016]     There is a need for an improved apparatus and improved methods for controlling tiltrotor aircraft with minimized tilting of the fuselage of the aircraft and enhanced accuracy of control.  
         [0017]     Therefore, it is an object of the present invention to provide an improved apparatus and improved methods for controlling tiltrotor aircraft.  
         [0018]     The present invention provides a flight control system (FCS) implementing the control methods of the invention for automatic flight control of a tiltrotor aircraft while operating at low airspeeds or in a hover, especially during operation in gusty and turbulent wind conditions. In response to a control input for a change in longitudinal velocity, such as a pilot pushing forward on the cyclic control, the FCS commands the nacelles to rotate in the same direction for directing thrust vectors of the rotors in a longitudinal direction. Simultaneously, the FCS automatically holds the fuselage at a desired pitch attitude by use of the longitudinal cyclic swashplate controls.  
         [0019]     In response to a control input for a change in lateral velocity, such as a pilot pushing sideways on the cyclic control, the FCS commands the lateral cyclic swashplate controls for directing thrust vectors of the rotors in a lateral direction. Simultaneously, the FCS automatically holds the fuselage to a desired roll attitude by differential use of rotor collective controls.  
         [0020]     In response to a control input for a change of yaw velocity, such as a pilot depressing a rudder pedal, the FCS commands the nacelles to rotate for directing thrust vectors of the rotors in different directions, creating a moment that causes the aircraft to yaw.  
         [0021]     The present invention provides significant advantages over the prior art, including: (1) providing longitudinal and lateral velocity control while maintaining the fuselage in a desired attitude; (2) reducing response time to forward and lateral velocity commands; (3) increasing accuracy of aircraft control; (4) reducing position displacements caused by wind gusts; (5) reducing the pitch-attitude to vertical-velocity coupling; (6) reducing the responses to ground effects; and (7) reducing the power required for lateral flight. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention taken in conjunction with the accompanying drawings in which like numerals identify like parts, and in which:  
         [0023]      FIG. 1  is a perspective view of a prior-art tiltrotor aircraft;  
         [0024]      FIG. 2  is a side view of the tiltrotor aircraft of  FIG. 1  executing a command to fly forward using a prior-art control method;  
         [0025]      FIG. 3  is a front view of the tiltrotor aircraft of  FIG. 1  executing a command to fly to the right using a prior-art control method;  
         [0026]      FIG. 4  is a side view of the tiltrotor aircraft of  FIG. 1  executing a command to yaw to the left using a prior-art control method;  
         [0027]      FIG. 5  is a side view of a tiltrotor aircraft using apparatus and control methods according to the present invention to maintain position in a hover;  
         [0028]      FIG. 6  is a side view of the tiltrotor aircraft of  FIG. 4  executing a command to fly forward using a control method according to the present invention;  
         [0029]      FIG. 7  is a front view of the tiltrotor aircraft of  FIG. 4  using a control method according to the present invention to maintain position in a hover;  
         [0030]      FIG. 8  is a front view of the tiltrotor aircraft of  FIG. 4  executing a command to fly to the right using a control method according to the present invention;  
         [0031]      FIG. 9  is a side view of the tiltrotor aircraft of  FIG. 4  executing a command to yaw to the left using a control method according to the present invention;  
         [0032]      FIG. 10  is a perspective view of an unmanned tiltrotor aircraft according to the present invention; and  
         [0033]      FIG. 11  is a perspective view of a civilian passenger version of a tiltrotor aircraft according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]     Referring now to  FIG. 5 , a tiltrotor aircraft  34  is depicted in a hover above ground  35 . Aircraft  34  is constructed in the same manner as aircraft  11 , described above, but the flight control system (FCS)  36  in aircraft  34  uses the control methods of the present invention to automatically control the flight of aircraft  34  in response to control inputs by a pilot or electronic system. Rotors  37 , comprising hub  39  and multiple blades  41 , are powered by engines carried within nacelles  43 . Nacelles  43  are rotatably mounted to the outer ends of wings  45 , and wings  45  are affixed to fuselage  47 . As described above, the pitch of each blade  41  is controlled by collective and cyclic swashplate controls (not shown) located within hub  39 . As described herein, a single reference number may be used to refer to both left and right components (as viewed when seated in the aircraft) when the description applies to both components. Specific reference numbers are used for clarity to refer to specific left or right components when the description is specific to either the left or right component.  
         [0035]     In the method of the present invention, a control input for a change in longitudinal velocity, such as a pilot pushing forward or pulling rearward on the cyclic control, causes FCS  36  to command nacelles  43  to rotate in the same direction for directing thrust vectors  49  of rotors  37  in a longitudinal direction. Simultaneously, FCS  36  automatically holds the pitch attitude of fuselage  47  to a desired pitch attitude, which may be a generally level pitch attitude, by use of the longitudinal cyclic swashplate controls. For example,  FIG. 6  shows aircraft  34  configured for forward motion, with nacelles  43  tilted forward to give each thrust vector  49  a forward vector component. These components tends to drive aircraft  34  forward in the direction shown by arrow  51 , while the swashplate controls in each rotor  37  are used to control the pitch attitude of fuselage  47 . In addition to a response to a control input, FCS  36  can generate commands in response to a longitudinal position error, in which nacelles  43  are commanded so as to return aircraft  34  to a previous position or to fly to a selected position.  
         [0036]     This longitudinal velocity control method differs from the prior-art control method in that change of the pitch attitude of fuselage  47  is not required to change the longitudinal velocity of aircraft  34 . Maintaining a generally level pitch attitude prevents the angle of attack for wings  45  from changing and prevents the undesirable change in vertical forces that cause problems in controlling the vertical aircraft position using the prior-art control methods. Specifically, when hovering or in a low-speed flight condition, especially in the presence of a headwind, the longitudinal velocity control method of the present invention will reduce the pitch-attitude to vertical-velocity coupling by allowing aircraft  34  to accelerate in the forward direction without a nose-down pitch attitude. In addition, the method of the present invention allows the attitude of aircraft  34  to be controlled to the most favorable condition during the conversion from helicopter mode to airplane mode.  
         [0037]     The control methods of the present invention also include an improved method of lateral velocity control of aircraft  34 , the method being implemented in FCS  36 . Aircraft  34  is shown in a hover above ground  35  in  FIG. 7 , with the left rotor labeled as  37 A and the right rotor labeled as  37 B. Each rotor  37 A,  37 B produces a vertical thrust vector  49 A,  49 B, respectively, for lifting aircraft  34 . In response to a control input for a change in lateral velocity, such as a pilot pushing sideways on the cyclic control, FCS  36  commands the lateral cyclic swashplate controls for directing thrust vectors  49 A,  49 B of rotors  37 A,  37 B in a lateral direction. Simultaneously, FCS  36  automatically holds the roll attitude of fuselage  47  in a desired roll attitude, which may be a generally level roll attitude, by differential use of rotor collective controls. In addition to a response to a control input, FCS  36  can generate commands in response to a lateral position error, in which the lateral cyclic swashplate controls are commanded so as to return aircraft  34  to a previous position or to fly to a selected position.  
         [0038]     For example,  FIG. 8  shows aircraft configured for movement to the right (as viewed if seated in the aircraft). When command to move to the right, swashplate controls tilt the plane of rotors  37 A,  37 B to the right, causing thrust vectors  49 A,  49 B to have a horizontal component to the right, and this vector component causes aircraft  34  to move in the direction shown by arrow  53 . While the cyclic swashplate controls induce sideward movement, the differential collective blade control is used to hold the aircraft level, meaning that the collective controls for rotors  37 A,  37 B are actuated independently from each other to maintain the desired fuselage attitude. This combination of controls allows aircraft  34  to move laterally in a stable and precise manner while holding aircraft  34  in a level roll attitude. A key advantage to the control method of the present invention is that holding fuselage  47  in a level attitude during lateral flight minimizes ground-effect problems and wing down-loading problems encountered when rolling aircraft  34  using the prior-art method.  
         [0039]     Additionally, the lateral velocity control method of the invention provides for improved lateral gust response, which may be reduced by as much as around 80%. When a lateral gust hits aircraft  34 , FCS  36  will immediately command the lateral cyclic swashplate control in the direction opposing the gust while the differential collective blade control is commanded to hold aircraft  34  level. Aircraft  34  will still have a tendency to roll with the gust, but thrust vectors  49 A,  49 B can quickly be redirected to oppose the gust without the need to roll aircraft  34  beyond the amount required to bring aircraft  34  back to a generally level roll attitude or other desired roll attitude. As described above, FCS  36  may also generate commands to the cyclic swashplate controls in response to a lateral position error for returning aircraft  34  to the position aircraft  34  occupied prior to the displacement caused by the gust.  
         [0040]     The swashplate cyclic controls are limited by physical constraints and the geometry of the system, such that there is a limited amount of total cyclic allowed for all cyclic command inputs. The total cyclic used at any one time is the square root of the sum of the squares of the longitudinal cyclic and the lateral cyclic. As described above, the methods of the invention include using longitudinal cyclic controls for controlling the aircraft pitch attitude and using lateral cyclic controls for controlling the lateral velocity of the aircraft. Longitudinal cyclic is also required to control the aircraft pitch moment as the location of the center of gravity of aircraft  34  changes. To reduce the total cyclic swashplate commands, the present invention also includes a control method for controlling yaw in aircraft  34  without the requirement of using longitudinal cyclic controls.  
         [0041]     The yaw control method provides for differential nacelle control, in which nacelles  43  of aircraft  34  are rotated independently to direct their thrust vectors  49  in different directions, creating a yaw moment. For example,  FIG. 9  shows aircraft  34  configured for yawing in a direction with the nose of aircraft  34  moving to the left (as viewed if seated in the aircraft). Left nacelle  43 A has been rotated rearward, and right nacelle  43 B has been rotated forward, directing thrust vectors  49 A and  49 B in different directions. Thrust vector  49 A has a longitudinal thrust component pointing toward the rear of aircraft  34 , and thrust vector  49 B has a longitudinal thrust component pointing toward the front of aircraft  34 . This longitudinal thrust differential creates a yaw moment, causing aircraft  34  to rotate in the direction of arrow  55  about a yaw axis  57 . An advantage of this yaw control method is that removing the yaw control commands from the total cyclic commands provides for more cyclic control range to be available for control of pitch attitude, center-of-gravity changes, and lateral aircraft velocity control. This allows for increased longitudinal center-of=gravity range, increased capability to hover in a crosswind, increased maneuver envelope for the pitch, roll, and yaw axes, reduced rotor flapping, and simplified prioritization of cyclic commands. Also, the yaw control is not limited by cyclic authority limits.  
         [0042]     While shown in  FIGS. 5-9  as used with a manned, military-style aircraft  34 , the improved FCS and control methods of the present invention may also be applied to control any type of tiltrotor aircraft.  FIG. 10  shows an unmanned aerial vehicle  59  (UAV) constructed as a tiltrotor aircraft. The enhanced accuracy of control permitted by the methods of the present invention is especially beneficial with the remote and often automated operation of UAVs. Specific functions that are enabled or enhanced include automatic launch and automatic recovery from a secondary vehicle, such as from the deck of a ship at sea, and maneuvering around a particular location or target in windy conditions with the required accuracy. Also, the reduced response time to forward and lateral velocity commands provides for a greater maneuver bandwidth, which is a great advantage for automatically controlled aircraft.  
         [0043]     A civilian passenger version of a tiltrotor aircraft  61  is depicted in  FIG. 11 . As discussed above, the advantages realized from using the control methods of the invention include improved passenger comfort. By holding aircraft  61  in generally level pitch and roll attitudes while maneuvering in hover or low-speed flight, the passengers aboard aircraft  57  are not subjected to the tilting and associated change of relative direction of acceleration due to gravity, or g-forces, felt when using the prior-art methods of control.  
         [0044]     The present-invention provides significant advantages over the prior art, including: (1) providing longitudinal and lateral velocity control while maintaining the fuselage in a desired attitude; (2) reducing response time to forward and lateral velocity commands; (3) increasing accuracy of aircraft control; (4) reducing position displacements caused by wind gusts; (5) reducing the pitch-attitude to vertical-velocity coupling; (6) reducing the responses to ground effects; and (7) reducing the power required for lateral flight.  
         [0045]     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. For example, it should be appreciated that these control methods may also be applicable to other tiltrotor aircraft, such as a Quad tiltrotor aircraft having four nacelles.