Patent Publication Number: US-2022227487-A1

Title: Rotary-wing aircraft individual rotor blade pitch control system

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to the field of rotary-wing aircraft actuation systems, and more particularly to a rotary-wing aircraft rotor blade pitch control system. 
     BACKGROUND ART 
     Rotary-wing aircraft, such as helicopters, are typically propelled by a main rotor having a hub rotatably supported on a rotor mast and supporting a plurality of rotor blades extending radially outward from the hub. To control vertical lift or altitude, the pitch of the rotating blades is typically adjusted collectively through a rotating swash plate that is coupled to the blades by respective linkages. This adjustment of the pitch of the blades changes a blade&#39;s angle of attack relative to the stream of air moving past it. The higher the angle of attack, the more highly loaded the blade becomes in creating more lift. 
     To control horizontal movement, the pitch of each blade is varied by a given amount once per rotor revolution. Conventionally, this tilting of the rotor is affected by tilting the swash plate, which results in the pitch of each of the blades changing twice per revolution of the hub. For example, to move the aircraft directly forward, the pitch of each blade is increased each time that blade passes over the tail of the aircraft, such that the lift developed by that blade is then temporarily greater than that of the other blades, and thereby results in a forward thrust component being applied to the aircraft by the rotor. 
     The pitch of each blade in a conventional rotor is controlled by a control rod, and the positions of all such rods are typically controlled by a single swashplate. The control rods are mounted circumferentially around the swashplate so that axial movement of the swashplate causes collective changes in pitch. Longitudinal and lateral tilting of the swashplate results in cyclic pitch control. 
     Individual blade control systems have been used to enable the pitch of each blade to be varied independently of the others. Typical approaches to individual blade control utilize either electrical motor actuators and slip rings, or hydraulic actuators, hydraulic swivels and electrical slip ring systems. 
     U.S. Pat. No. 4,519,743, entitled “Helicopter Individual Blade Control System,” is directed to a system in which the pitch of the blades of a helicopter rotor assembly are controlled by individual blade control subsystems that respond to output signals from accelerometers mounted on the blades. 
     U.S. Pat. No. 4,930,988, entitled “Individual Blade Control System for Helicopters,” is directed to a control system for providing individual blade control inputs to a four-bladed helicopter rotor. Motion is transmitted to the rotor blades through a conventional swashplate which drives four blades of the rotor and a translatable differential sleeve and summing linkage which drives only two blades. 
     U.S. Pat. No. 7,674,091, entitled “Rotor Blade Pitch Control,” is directed to a mechanical independent blade control mechanism for controlling the pitch of each of the blades of a rotor blade system independently of the other blades. The system includes a plurality of actuators disposed in the fuselage below the hub of the rotor, each being operable to selectively control the pitch of an associated one of the blades independently of the other blades, and a plurality of mechanical linkages disposed within the annulus of the rotor mast, each coupled between a blade and an actuator and operable to transmit a force output by the actuator to a pitch horn fixed to an inner end of the associated blade. 
     BRIEF SUMMARY 
     With parenthetical reference to the corresponding parts, portions or surfaces of the disclosed embodiment, merely for purposes of illustration and not by way of limitation, an rotor blade pitch control system ( 15 ) for a rotary-wing aircraft ( 16 ) having a plurality of rotor blades mounted to a main rotor and driven about a central axis of rotation at an operational speed and in a rotational direction relative to a non-rotating body of the aircraft is provided comprising: a first rotor blade ( 19   a ) connected to a main rotor ( 17 ) and operatively configured to be driven about a central axis of rotation ( 20 ) relative to a non-rotating body ( 21 ) of the aircraft; the first rotor blade ( 19   a ) rotatable about a first pitch axis ( 24   a ); a first blade pitch control motor ( 30   a ) having a first stator ( 31   a ), a first pitch drive rotor ( 32   a ) and a first pitch follower ( 40   a ); a first linkage ( 50   a ) extending between the first pitch follower ( 40   a ) and the first rotor blade ( 19   a ); a first rotor blade coupling ( 58   a ) between the first linkage ( 50   a ) and the first rotor blade ( 19   a ); the first rotor blade coupling ( 58   a ) having a first rotor blade coupling center ( 59   a ); a first pitch follower coupling ( 51   a ) between the first linkage ( 50   a ) and the first pitch follower ( 40   a ); the first pitch follower coupling ( 51   a ) having a first pitch follower coupling center ( 52   a ); a first hinge coupling ( 53   a ) between the first linkage ( 50   a ) and the main rotor ( 17 ); the first hinge coupling ( 53   a ) having a first hinge axis ( 54   a ); the first pitch drive rotor ( 32   a ) having a first cam surface ( 36   a ) orientated about a first driven axis ( 35   a ) that is eccentric to the central axis of rotation ( 20 ); the first linkage ( 50   a ) extending between the main rotor ( 17 ) and the first pitch follower ( 40   a ) such that the first pitch follower coupling center ( 52   a ) rotates about the central axis of rotation ( 20 ) with rotation of the first hinge coupling ( 53   a ) about the central axis of rotation ( 20 ); the first pitch drive rotor ( 32   a ) operatively configured to be driven about the central axis of rotation ( 20 ) independently of the main rotor ( 17 ) to selectively rotate the first driven axis ( 35   a ) about the central axis of rotation ( 20 ); the first driven axis ( 35   a ) and the first pitch follower coupling center ( 52   a ) having a selectively variable first displacement angle ( 80   a ) defined by an inclusive angle between a line ( 81   a ) extending radially between the central axis of rotation ( 20 ) and the first driven axis ( 35   a ) and a line ( 82   a ) extending radially between the central axis of rotation ( 20 ) and the first pitch follower coupling center ( 52   a ); wherein the first pitch drive rotor ( 32   a ) may be rotated about the central axis of rotation ( 20 ) relative to the main rotor ( 17 ) to control a pitch ( 71   a ) of the first rotor blade ( 19   a ) about the first pitch axis ( 24   a ). 
     The first displacement angle ( 80   a ) may range from a minimum displacement angle (0°) to a maximum displacement angle (180°), the pitch may range from a first pitch angle limit ( 74   a ) to a second pitch angle limit ( 75   a ), and when the first displacement angle is the maximum (180°), the pitch may be the first pitch angle limit ( 74   a ), and when the first displacement angle is the minimum ( 00 ), the pitch may be the second pitch angle limit ( 75   a ). The pitch may comprise a neutral pitch angle ( 76   a ), and when the first displacement angle is about half of the maximum (90°), the pitch may be the neutral pitch angle ( 76   a ). 
     The first linkage may comprise a first transfer link ( 60   a ) and a first pitch link ( 64   a ); the main rotor ( 17 ) may comprise a hinge hub ( 25 ); and the first rotor blade ( 19   a ) may comprise a pitch horn ( 65   a ). The first transfer link ( 60   a ) may be coupled to the first pitch follower ( 40   a ) by the first pitch follower coupling ( 51   a ); the first transfer link ( 60   a ) may be coupled to the hinge hub ( 25 ) of the main rotor ( 17 ) by the first hinge coupling ( 53   a ); the first pitch link ( 60   a ) may be coupled to the first pitch horn ( 65   a ) of the first rotor blade ( 19   a ) by the first rotor blade coupling ( 58   a ); and the first transfer link ( 60   a ) may be coupled to the first pitch link ( 64   a ) by a first intermediate coupling ( 55   a ) having a first intermediate coupling center ( 56   a ). The first rotor blade coupling center ( 59   a ) of the first rotor blade coupling ( 58   a ) may be offset a pitch horn distance ( 66   a ) from the first pitch axis ( 24   a ). The first pitch follower coupling may comprise a ball joint or a universal coupling. 
     The first pitch follower ( 40   a ) may be rotatable relative to the first pitch drive rotor ( 32   a ) about the first driven axis ( 35   a ). The first pitch drive rotor ( 32   a ) may comprise a first annular drive bore ( 34   a ) having a first drive axis coincident with the central axis of rotation ( 20 ); the first cam surface of the first pitch drive rotor ( 32   a ) may comprise a first outer annular rim ( 36   a ) having a first rim axis coincident with the first driven axis ( 35   a ); and the first pitch follower ( 40   a ) may comprise a first annular following bore ( 42   a ) having a first following bore axis coincident with the first rim axis ( 35   a ). The rotor blade pitch control system may comprise an annular bearing ( 43   a ) between the first outer annular rim ( 36   a ) and the first annular following bore ( 42   a ). The first pitch drive rotor ( 32   a ) may radially constrain the first pitch follower ( 40   a ) relative to the central axis of rotation ( 20 ). 
     The rotor blade pitch control system may comprise a controller ( 90 ) that receives input signals and outputs command signals to the first blade pitch control motor ( 30   a ) to control a speed of rotation of the first pitch drive rotor ( 32   a ) about the central axis of rotation ( 20 ) and the first displacement angle ( 80   a ). The controller may vary the first displacement angle ( 80   a ) to vary the pitch ( 71   a ) of the first rotor blade ( 19   a ) about the first pitch axis ( 24   a ). The first displacement angle ( 80   a ) may be variable from 0 degrees to 180 degrees. The controller ( 90 ) may maintain a constant first displacement angle ( 80   a ) to maintain a desired constant pitch ( 71   a ) of the first rotor blade ( 19   a ) about the first pitch axis ( 24   a ). The controller may selectively control the first blade pitch control motor ( 30   a ) such that the first driven axis ( 35   a ) rotates about the central axis of rotation ( 20 ) at a first rotational speed and the main rotor ( 17 ) rotates about the central axis of rotation at a second rotational speed, whereby the controller ( 90 ) controls a speed differential between the first speed of rotation of the first driven axis ( 35   a ) about the central axis of rotation ( 20 ) and the second speed of rotation of the main rotor ( 17 ) about the central axis of rotation ( 20 ). The controller may vary the first displacement angle ( 80   a ) by varying the speed differential from substantially 1 to 1. The controller ( 90 ) may vary the pitch ( 71   a ) of the first rotor blade ( 19   a ) about the first pitch axis ( 24   a ) by varying the speed differential such that the first rotational speed that the first driven axis ( 35   a ) rotates about the central axis of rotation ( 20 ) is different from the second rotational speed that the main rotor ( 17 ) rotates about the central axis of rotation ( 20 ). 
     The rotor blade pitch control system may comprise a unit frame ( 29 ) mounted to a non-rotating body ( 26 ) of the aircraft; the first stator ( 31   a ) of the first blade pitch control motor ( 30   a ) may be mounted to the unit frame ( 29 ); and the first pitch drive rotor ( 32   a ) may have an annular stator-facing portion ( 34   a ) and a plurality of magnets ( 39   a ) supported by the annular stator-facing portion ( 34   a ). 
     The rotor blade pitch control system may comprise: a second rotor blade ( 19   b ) connected to the main rotor ( 17 ) and operatively configured to be driven about the central axis of rotation ( 20 ) relative to the non-rotating body ( 21 ) of the aircraft; the second rotor blade ( 19   b ) rotatable about a second pitch axis ( 24   b ); a second blade pitch control motor ( 30   b ) having a second stator ( 31   b ), a second pitch drive rotor ( 32   b ) and a second pitch follower ( 40   b ); a second linkage ( 50   b ) extending between the second pitch follower ( 40   b ) and the second rotor blade ( 19   b ); a second rotor blade coupling ( 58   b ) between the second linkage ( 50   b ) and the second rotor blade ( 19   b ); the second rotor blade coupling ( 58   b ) having a second rotor blade coupling center ( 59   b ); a second pitch follower coupling ( 51   b ) between the second linkage ( 50   b ) and the second pitch follower ( 40   b ); the second pitch follower coupling ( 51   b ) having a second pitch follower coupling center ( 52   b ); a second hinge coupling ( 53   b ) between the second linkage ( 50   b ) and the main rotor ( 17 ); the second hinge coupling ( 53   b ) having a second hinge axis ( 54   b ); the second pitch drive rotor ( 32   b ) having a second cam surface ( 36   b ) orientated about a second driven axis ( 35   b ) that is eccentric to the central axis of rotation ( 20 ); the second linkage ( 50   b ) extending between the main rotor ( 17 ) and the second pitch follower ( 40   b ) such that the second pitch follower coupling center ( 52   b ) rotates about the central axis of rotation ( 20 ) with rotation of the second hinge coupling ( 53   b ) about the central axis of rotation ( 20 ); the second pitch drive rotor ( 32   b ) operatively configured to be driven about the central axis of rotation ( 20 ) independently of the main rotor ( 17 ) and independently of the first pitch drive rotor ( 32   a ) to selectively rotate the second driven axis ( 35   b ) about the central axis of rotation ( 20 ); and the second driven axis ( 35   b ) and the second pitch follower coupling center ( 52   b ) having a selectively variable second displacement angle ( 80   b ) defined by an inclusive angle between a line ( 81   b ) extending radially between the central axis of rotation ( 20 ) and the second driven axis ( 35   b ) and a line ( 82   b ) extending radially between the central axis of rotation ( 20 ) and the second pitch follower coupling center ( 52   b ); wherein the second pitch drive rotor ( 32   b ) may be rotated about the central axis of rotation ( 20 ) relative to the main rotor ( 17 ) to control a pitch ( 71   b ) of the second rotor blade ( 19   b ) about the second pitch axis ( 24   b ) independently of the control of the pitch of the first rotor blade ( 19   a ) about the first pitch axis ( 24   a ). The rotor blade pitch control system may comprise a controller ( 90 ) that receives input signals and outputs command signals to the second blade pitch control motor ( 30   b ) to control a speed of rotation of the second pitch drive rotor ( 32   b ) about the central axis of rotation ( 20 ) and the second displacement angle ( 80   b ). 
     In another aspect, a rotor blade pitch control system is provided comprising: a first rotor blade ( 19   a ) operatively configured to be driven about a central axis of rotation ( 20 ); the first rotor blade ( 19   a ) rotatable about a first pitch axis ( 24   a ); a first pitch drive rotor ( 32   a ) operatively configured to be driven about the central axis of rotation ( 20 ) independently of rotation of the first rotor blade ( 19   a ) about the central axis of rotation ( 20 ); a first pitch follower ( 40   a ) rotatable relative to the first pitch drive rotor ( 32   a ); the first pitch follower ( 40   a ) and the first rotor blade ( 19   a ) coupled ( 50   a ) such that the first pitch follower ( 40   a ) rotates with rotation of the first rotor blade ( 19   a ) about the central axis of rotation ( 20 ); the first pitch drive rotor ( 32   a ), the first pitch follower ( 40   a ) and the first rotor blade ( 19   a ) coupled ( 50   a ) such that the first pitch drive rotor ( 32   a ) is operatively configured to be driven to control an angular displacement ( 80   a ) of the first pitch drive rotor ( 32   a ) relative to the first pitch follower ( 40   a ) about the central axis of rotation ( 20 ) and thereby control a pitch ( 71   a ) of the first rotor blade ( 19   a ) about the first pitch axis ( 24   a ). 
     The rotor blade pitch control system may comprise: a second rotor blade ( 19   b ) operatively configured to be driven about the central axis of rotation ( 20 ); the second rotor blade ( 19   b ) rotatable about a second pitch axis ( 24   b ); a second pitch drive rotor ( 32   b ) operatively configured to be driven about the central axis of rotation ( 20 ) independently of rotation of the second rotor blade ( 19   b ) about the central axis of rotation ( 20 ) and independently of the first pitch drive rotor ( 32   a ) about the central axis of rotation ( 20 ); a second pitch follower ( 40   b ) rotatable relative to the second pitch drive rotor ( 32   b ); the second pitch follower ( 40   b ) and the second rotor blade ( 19   b ) coupled ( 50   b ) such that the second pitch follower ( 40   b ) rotates with rotation of the second rotor blade ( 19   b ) about the central axis of rotation ( 20 ); the second pitch drive rotor ( 32   b ), the second pitch follower ( 40   b ) and the second rotor blade ( 19   b ) coupled ( 50   b ) such that the second pitch drive rotor ( 32   b ) is operatively configured to be driven to control an angular displacement ( 80   b ) of the second pitch drive rotor ( 32   b ) relative to the second pitch follower ( 40   b ) about the central axis of rotation ( 20 ) and thereby control a pitch ( 71   b ) of the second rotor blade ( 19   b ) about the second pitch axis ( 24   b ) independently of the control of the pitch ( 71   a ) of the first rotor blade ( 19   a ) about the first pitch axis ( 24   a ). 
     The rotor blade pitch control system may comprise a first linkage ( 50   a ) between the first pitch follower ( 40   a ) and the first rotor blade ( 19   a ). The rotor blade pitch control system may comprise: a first pitch follower coupling ( 51   a ) between the first linkage ( 50   a ) and the first pitch follower ( 40   a ); the first pitch follower coupling ( 51   a ) having a first pitch follower coupling center ( 52   a ); the first pitch drive rotor ( 32   a ) having a first cam surface ( 36   a ) orientated about a first driven axis ( 35   a ) that is eccentric to the central axis of rotation ( 20 ); and wherein the angular displacement ( 80   a ) of the first pitch drive rotor ( 32   a ) relative to the first pitch follower ( 40   a ) comprises a selectively variable first displacement angle defined by an inclusive angle between a line ( 81   a ) extending radially between the central axis of rotation ( 20 ) and the first driven axis ( 35   a ) and a line ( 82   a ) extending radially between the central axis of rotation ( 20 ) and the first pitch follower coupling center ( 52   a ). The rotor blade pitch control system may comprise: a main rotor ( 17 ) connected to the first rotor blade ( 19   a ) and operatively configured to be driven about the central axis of rotation ( 20 ); a first hinge coupling ( 53   a ) between the first linkage ( 50   a ) and the main rotor ( 17 ); the first hinge coupling ( 53   a ) having a first hinge axis ( 54   a ); and the first linkage ( 50   a ) extending between the main rotor ( 17 ) and the first pitch follower ( 40   a ) such that the first pitch follower coupling center ( 52   a ) rotates about the central axis of rotation ( 20 ) with rotation of the first hinge coupling ( 53   a ) about the central axis of rotation ( 20 ). 
     The rotor blade pitch control system may comprise a first blade pitch control motor ( 30   a ) operatively configured to drive the first pitch drive rotor ( 32   a ) about the central axis of rotation ( 20 ) and a controller ( 90 ) that receives input signals and outputs command signals to the first blade pitch control motor ( 30   a ) to control a speed of rotation of the first pitch drive rotor ( 32   a ) about the central axis of rotation ( 20 ) and the angular displacement ( 80   a ) of the first pitch drive rotor ( 32   a ) relative to the first pitch follower ( 40   a ) about the central axis of rotation ( 20 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a representative perspective view of a first embodiment of the improved individual rotor blade pitch control system on a main rotor assembly of a helicopter. 
         FIG. 2  is an enlarged representative perspective view of the individual rotor blade pitch control system shown in  FIG. 1 . 
         FIG. 3  is a vertical cross-sectional view of the individual rotor blade pitch control system shown in  FIG. 1 . 
         FIG. 4  is a partial vertical cross-sectional representative view of a pitch rotor and pitch follower assembly of the individual blade pitch control system shown in  FIG. 1 . 
         FIG. 5  is a top plan diagram view of the assembly shown in  FIG. 4 . 
         FIG. 5A  is a top plan diagram view of the assembly shown in  FIG. 5  in an upper pitch angle limit configuration. 
         FIG. 5B  is a top plan diagram view of the assembly shown in  FIG. 5  in lower pitch angle limit configuration. 
         FIG. 5C  is a top plan diagram view of the assembly shown in  FIG. 5  in an intermediate pitch angle limit configuration. 
         FIG. 6A  is a representative perspective view of a pitch control subsystem for the first of the rotor blades shown in  FIG. 1  in an upper pitch angle limit configuration. 
         FIG. 6B  is a representative perspective view of a pitch control subsystem for the first of the rotor blades shown in  FIG. 1  in a lower pitch angle limit configuration. 
         FIG. 6C  is a representative perspective view of a pitch control subsystem for the first of the rotor blades shown in  FIG. 1  in an intermediate pitch angle limit configuration. 
         FIG. 7A  is a representative perspective view of a pitch control subsystem for the second of the rotor blades shown in  FIG. 1  in an upper pitch angle limit configuration. 
         FIG. 7B  is a representative perspective view of a pitch control subsystem for the second of the rotor blades shown in  FIG. 1  in a lower pitch angle limit configuration. 
         FIG. 7C  is a representative perspective view of a pitch control subsystem for the second of the rotor blades shown in  FIG. 1  in an intermediate pitch angle limit configuration. 
         FIG. 8A  is a representative perspective view of a pitch control subsystem for the third of the rotor blades shown in  FIG. 1  in an upper pitch angle limit configuration. 
         FIG. 8B  is a representative perspective view of a pitch control subsystem for the third of the rotor blades shown in  FIG. 1  in a lower pitch angle limit configuration. 
         FIG. 8C  is a representative perspective view of a pitch control subsystem for the third of the rotor blades shown in  FIG. 1  in an intermediate pitch angle limit configuration. 
         FIG. 9A  is a representative perspective view of a pitch control subsystem for the fourth of the rotor blades shown in  FIG. 1  in an upper pitch angle limit configuration. 
         FIG. 9B  is a representative perspective view of a pitch control subsystem for the fourth of the rotor blades shown in  FIG. 1  in a lower pitch angle limit configuration. 
         FIG. 9C  is a representative perspective view of a pitch control subsystem for the fourth of the rotor blades shown in  FIG. 1  in an intermediate pitch angle limit configuration. 
         FIG. 10  is a schematic diagram of the control system for the pitch control subsystems shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed description is an integral part. Unless otherwise indicated, the drawings are intended to be read (e.g., crosshatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description. As used in the following description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate. 
       FIG. 1  is a schematic illustration of helicopter  16  having airframe  21  and main rotor  17  that is driven about center axis of rotation  20 . Main rotor  17  includes rotor blades  19   a ,  19   b ,  19   c  and  19   d  rotationally mounted to main rotor blade hub  22  on main rotor  17  via rotor blade grips  23   a ,  23   b ,  23   c  and  23   d  that allow the rotor blades to be rotated about their pitch axes  24   a ,  24   b ,  24   c  and  24   d , respectively, so that the pitch of the rotor blades may be selectively varied. Rotor blades  19   a ,  19   b ,  19   c  and  19   d  have pitch horns  65   a ,  65   b ,  65   c  and  65   d , respectively, to which a torque may be applied to control the respective pitch angle  71   a ,  71   b ,  71   c  and  71   d  of the rotor blade about its pitch axis  24   a ,  24   b ,  24   c  and  24   d , respectively. 
     Rotor blade hub  22  is driven about central axis of rotation  20  by main rotor shaft  18 , which is driven through a main rotor gear box by one or more aircraft engines. Main rotor shaft  18  and blade hub  22  rotate in a rotational direction and at an operational rotational frequency about center axis of rotation  20 . Although a helicopter is shown and described in this embodiment, rotor blade pitch control system  15  may be used with other types or configurations of rotary-wing aircraft or rotor-craft or in other pitch control applications. 
     As shown in  FIGS. 1-4 , rotor blade pitch control system  15  is mounted between fuselage  21  and main rotor  17  and is generally orientated concentrically with main rotor  17 .  FIG. 1  provides a frame of reference comprising longitudinal axis x-x aligned with the longitudinal axis of helicopter  16 , transverse axis y-y perpendicular to axis x-x, and vertical axis z-z concentric with central axis of rotation  20  of main rotor  17 . 
     As shown in  FIGS. 2-4 , rotor blade pitch control system  15  generally includes first pitch control motor  30   a  mounted to static mast  26  of airframe  21  of helicopter  16 , first linkage  50   a  connecting first pitch control motor  30   a  and first rotor blade  19   a , second pitch control motor  30   b  mounted to static mast  26  of airframe  21  of helicopter  16 , second linkage  50   b  connecting second pitch control motor  30   b  and second rotor blade  19   b , third pitch control motor  30   c  mounted to static mast  26  of airframe  21  of helicopter  16 , third linkage  50   c  connecting third pitch control motor  30   c  and third rotor blade  19   c , fourth pitch control motor  30   d  mounted to static mast  26  of airframe  21  of helicopter  16 , and fourth linkage  50   d  connecting fourth pitch control motor  30   d  and fourth rotor blade  19   d.    
     Rotor blade pitch control system  15  includes cylindrical support frame  29  orientated coaxially with main rotor  17  about central axis  20 . Support frame  29  is fixed to static mast  26  of helicopter  16  and does not rotate relative to fuselage  21  of helicopter  16 . Frame  29  supports each of pitch motors  30   a ,  30   b ,  30   c  and  30   d.    
     Motor  30   a  comprises stator  31   a , fixed to frame  29 , and rotor  32   a  that is driven to rotate about drive axis  33   a  relative to stator  31   a . In this embodiment, drive axis  33   a  is coaxial with central axis  20 . Upper and lower bearings  37   a  act between rotor  32   a  and frame  29  such that rotor  32   a  is driven by motor  30   a  to rotate about axis  33   a  relative to frame  29 . In this embodiment, motor  30   a  is a rotary brushless permanent magnet electric motor with rotor  32   a  having permanent magnets  39   a  spaced around its inwardly-facing annular stator-facing surface  34   a  and stator  31   a  having coils energized to drive rotor  32   a  about axis  33   a  in either rotational direction. 
     Motor  30   b  comprises stator  31   b , fixed to frame  29 , and rotor  32   b  that is driven to rotate about drive axis  33   b  relative to stator  31   b . In this embodiment, drive axis  33   b  is coaxial with central axis  20 . Upper and lower bearings  37   b  act between rotor  32   b  and frame  29  such that rotor  32   b  is driven by motor  30   b  to rotate about axis  33   b  relative to frame  29 . In this embodiment, motor  30   b  is a rotary brushless permanent magnet electric motor with rotor  32   b  having permanent magnets  39   b  spaced around its inwardly-facing annular stator-facing surface  34   b  and stator  31   b  having coils energized to drive rotor  32   b  about axis  33   b  in either rotational direction. 
     Motor  30   c  comprises stator  31   c , fixed to frame  29 , and rotor  32   c  that is driven to rotate about drive axis  33   c  relative to stator  31   c . In this embodiment, drive axis  33   c  is coaxial with central axis  20 . Upper and lower bearings  37   c  act between rotor  32   c  and frame  29  such that rotor  32   c  is driven by motor  30   c  to rotate about axis  33   c  relative to frame  29 . In this embodiment, motor  30   c  is a rotary brushless permanent magnet electric motor with rotor  32   c  having permanent magnets  39   c  spaced around its inwardly-facing annular stator-facing surface  34   c  and stator  31   c  having coils energized to drive rotor  32   c  about axis  33   c  in either rotational direction. 
     Motor  30   d  comprises stator  31   d , fixed to frame  29 , and rotor  32   d  that is driven to rotate about drive axis  33   d  relative to stator  31   d . In this embodiment, drive axis  33   d  is coaxial with central axis  20 . Upper and lower bearings  37   d  act between rotor  32   d  and frame  29  such that rotor  32   d  is driven by motor  30   d  to rotate about axis  33   d  relative to frame  29 . In this embodiment, motor  30   d  is a rotary brushless permanent magnet electric motor with rotor  32   d  having permanent magnets  39   d  spaced around its inwardly-facing annular stator-facing surface  34   d  and stator  31   d  having coils energized to drive rotor  32   d  about axis  33   d  in either rotational direction. 
     In this embodiment, motor axis  33   a , motor axis  33   b , motor axis  33   c , motor axis  33   d  and central axis  20  are coaxial and rotors  32   a ,  32   b ,  32   c  and  32   d  are directly driven by motors  30   a ,  30   b ,  30   c  and  30   d , respectively. However, alternatively such rotors could be indirectly driven through gear trains, belts or other rotational couplings, and could be non-concentric to each other and to central axis  20 . 
     Drive rotor  32   a  of first motor  30   a  is rotationally coupled via follower  40   a  to linkage  50   a  at spherical bearing  51   a , drive rotor  32   b  of second motor  30   b  is rotationally coupled via follower  40   b  to linkage  50   b  at spherical bearing  51   b , drive rotor  32   c  of third motor  30   c  is rotationally coupled via follower  40   c  to linkage  50   c  at spherical bearing  51   c , and drive rotor  32   d  of fourth motor  30   d  is rotationally coupled via follower  40   d  to linkage  50   d  at spherical bearing  51   d.    
     Inner directly driven rotor  32   a  is rotationally coupled to outer follower  40   a  such that follower  40   a  and inner rotor  32   a  are rotatable relative to each other. As further described below, follower  40   a  rotates via linkage  50   a  with main rotor  17 , which in turn is driven by the engine of helicopter  16  about central axis  20 . As further described below, drive rotor  32   a  has driven axis  35   a  that is selectively driven to rotate about central axis  20  by motor  30   a  independently of main rotor  17 . 
     Rotor  32   a  includes an inner bore defined by inner annular surface  34   a  orientated about drive axis  33   a  that is coincident with central axis  20 , and outer annular rim  36   a  orientated about driven axis  35   a . Driven axis  35   a  is parallel to and not coaxial with drive axis  33   a , such that driven axis  35   a  is radially offset eccentric radial distance  38   a  from central axis  20  and drive axis  33   a . Follower  40   a  has an inner annular bore defined by inner annular surface  42   a , which is orientated about driven axis  35   a  and coaxially with outer annular rim  36   a  of rotor  32   a . As shown, spherical coupling  51   a  couples follower  40   a  to one end of linkage  50   a  at coupling center  52   a . Coupling center  52   a  and follower  40   a  will rotate about driven axis  35   a  with rotation of main rotor  17  about central axis  20 . Annular bearing  43   a  acts between rotor  32   a  and follower  40   a  such that follower  40   a  may rotate, via linkage  50   a , with rotation of main rotor  17  relative to drive rotor  32   a . As explained further below, the relative angular positions of drive rotor  32   a  and follower  40   a  about central axis  20  dictate the pitch of rotor blade  19   a  about pitch axis  24   a.    
     As shown, rotor  32   a  is rotationally supported by frame  29 . Upper and lower bearing pairs  37   a  act between the outer cylindrical bearing surfaces of frame  29  and the opposed inner cylindrical bearing surfaces  34   a  of rotor  32   a . Rotor  32   a  is thereby configured to rotate about axis  20  on upper and lower annular bearing pairs  37   a . Thus, rotor  32   a  is mounted on frame  29  by rolling bearings  37   a  such that drive rotor  32   a  is rotatable relative to frame  29  and fuselage  21 . 
     Follower  40   a  is rotationally supported between rotor  32   a  and linkage  50   a . Upper and lower bearing pairs  43   a  act between outer cylindrical bearing surface  36   a  of rotor  32   a  and opposed inner cylindrical bearing surface  42   a  of follower  40   a . Follower  40   a  is configured to rotate about driven axis  35   a  on upper and lower bearing pairs  43   a . Thus, follower  40   a  is mounted on rotor  32   a  by rolling bearings  43   a  such that follower  40   a  is rotatable about central axis  20  relative to rotor  32   a.    
     As shown, the outer end of follower  40   a  is rotationally supported, via spherical bearing  51   a  having coupling center  52   a , by the follower end of linkage  50   a . Linkage  50   a  is rotationally supported, via hinge joint  53   a  having hinge axis  54   a , by hinge hub  25  of main rotor  17 . The pitch horn end of linkage  50   a  is rotationally supported, via spherical bearing  58   a  having coupling center  59   a , by pitch horn  65   a  of rotor blade  19   a . In this embodiment, linkage  50   a  comprises an L-shaped lever transfer link  60   a , having first arm  61   a  and second arm  62   a  that pivot about hinge axis  54   a , and pitch rod  64   a . Pitch rod  64   a  is coupled at one end to the end of pitch horn  65   a  of rotor blade  19   a  by spherical joint  58   a . Pitch rod  64   a  is coupled at the other end to the end of second arm  62   a  by spherical joint  55   a . The end of first arm  61   a  of transfer link  60   a  is coupled to the outer end of follower  40   a  by spherical joint  51   a . Spherical bearing  51   a  couples follower  40   a  to the end of first arm  61   a  of transfer link  60   a  at coupling center  52   a.    
     As shown in  FIG. 3 , spherical bearing  51   a  is a rotary coupling about center  52   a  between first arm  61   a  of linkage  50   a  and follower  40   a . Follower  40   a  has an inner race  95   a  orientated about coupling center  52   a  such that it rotates with rotation of follower  40   a . Race  95   a  has a spherical inner diameter surface orientated about center  52   a . The end portion of arm  61   a , opposite to fulcrum portion  63   a , extends through and is in linear sliding engagement with a through-bore in ball  96   a . Ball  96   a  has an outer spherical diameter surface orientated about center  52   a  and is retained in race  95   a  of follower  40   a , with the outer surface of ball  96   a  in spherical sliding engagement with the inner surface of race  95   a . Thus, race  95   a  rotates with rotation of follower  40   a , and ball  96   a  is rotatable with arm  61   a  in at least two degrees of motion about coupling center  52   a  relative to follower  40   a . The shaft end portion of arm  61   a  may slide in the through-bore of ball  96   a  and is in linear sliding engagement with ball  96   a  such that arm  61   a  may translate linearly through coupling center  52   a  relative to ball  96   a . Spherical bearings  51   b ,  51   c  and  51   d  are configured between linkages  50   b ,  50   c , and  50   d  and followers  40   b ,  40   c ,  40   d , respectively, in substantially the same manner. 
     Spherical bearing  55   a  is a rotary coupling about center  56   a  between arm  62   a  of transfer link  60   a  and pitch rod  64   a . The end portion of pitch rod  64   a  has a race with a spherical inner diameter surface orientated about coupling center  56   a . Arm  62   a  has a clevis pin rotationally supporting a ball with an outer spherical diameter surface orientated about coupling center  56   a . The ball of arm  62   a  is retained in the race of pitch rod  64   a , with the outer surface of the ball in spherical sliding engagement with the inner surface of the race. Thus, the race of pitch rod  64   a  and the ball of arm  62   a  may rotate in at least two degrees of motion about coupling center  56   a  relative to each other. Spherical bearings  55   b ,  55   c  and  55   d  are configured between links  60   b ,  60   c  and  60   d  and rods  64   b ,  64   c  and  64   d , respectively, in substantially the same manner. 
     Similarly, spherical bearing  58   a  is a rotary coupling about center  59   a  between pitch horn  65   a  of rotor blade  19   a  and pitch rod  64   a . The end portion of pitch rod  64   a  has a race with a spherical inner diameter surface orientated about coupling center  59   a . Pitch horn  65   a  has a clevis pin rotationally supporting a ball with an outer spherical diameter surface orientated about coupling center  59   a . The ball of pitch horn  65   a  is retained in the race of pitch rod  64   a , with the outer surface of the ball in spherical sliding engagement with the inner surface of the race. Thus, the race of pitch rod  64   a  and the ball of pitch horn  65   a  may rotate in at least two degrees of motion about coupling center  59   a  relative to each other. Spherical bearings  58   b ,  58   c  and  58   d  are configured between pitch horns  65   b ,  65   c  and  65   d  and rods  64   b ,  64   c  and  64   d , respectively, in substantially the same manner. 
     While in the above embodiment couplings  51   a ,  51   b ,  51   c ,  51   d ,  55   a ,  55   b ,  55   c ,  55   d ,  58   a ,  58   b ,  58   c  and  58   d  comprise spherical bearings, it is contemplated that other various alternative rotational couplings or pivot joints may be employed. For example, and without limitation, gimbal or universal joint type couplings may be used as alternatives. 
     Blade hub  22  and hinge hub  25  are fixed to rotor shaft  18  of rotor  17 . Blade hub  22  with rotor blade  19   a , and hinge hub  25  with hinge joint  53   a , are stacked axially relative to central axis  20 . Coupling center  59   a  of pitch rod  64   a  is offset fixed pitch horn distance  66   a  from pitch axis  24   a . Coupling center  59   a  of pitch rod  64   a  is offset axially from hinge axis  54   a  by a variable axial pitch link distance  70   a  that varies as a function of radial distance  83   a  between coupling center  52   a  and central axis of rotation  20 . Motor  30   a  and follower  40   a  with coupling  51   a  are stacked axially below hinge hub  25  relative to central axis  20 . 
     As show, transfer link  60   a  pivots about hinge axis  54   a . Hinge axis  54   a  is positioned tangent to a circle having radius  68  about central axis  20  such that transfer link  60   a  pivots in a vertical plane B that is radial to central axis  20 . With lever arms  61   a  and  62   a , elbow  63   a  at hinge  53   a  acts as a fulcrum. Radial movement of coupling center  52   a  towards central axis  20  causes coupling center  56   a  to rotate about hinge axis  54   a  down and away from blade hub  22 , which, via pitch rod  64   a  and pitch horn  65   a , rotates blade  19   a  in a first direction  73  about pitch axis  24   a . Radial movement of coupling center  52   a  away from central axis  20  causes coupling center  56   a  to rotate about hinge axis  54   a  up and towards blade hub  22 , which, via pitch rod  64   a  and pitch horn  65   a , rotates blade  19   a  in a second direction  72  about pitch axis  24   a.    
     Based on the angular displacement between main rotor  17  and drive rotor  32   a , coupling center  52   a  has a selectively variable radial displacement distance  83   a  (r) from central axis  20  ranging from a minimum distance (r min), as shown in  FIG. 5B , to a maximum distance (r max), as shown in  FIG. 5A . With the configuration of linkage  50   a , based on the radial displacement distance  83   a  (r) of coupling center  52   a  from central axis  20 , coupling center  59   a  will have a selectively variable pitch angle  71   a  (∠p) about pitch axis  24   a  between upper angular pitch angle limit  74   a  and lower angular pitch angle limit  75   a . As shown, the pitch angle  71   a  (∠p), between upper limit pitch angle  74   a  and lower limit pitch angle  75   a , is selectively varied by selectively varying radial displacement distance  83   a  (r) from central axis  20  from a minimum distance (r min) to a maximum distance (r max). In this embodiment, the maximum radial displacement distance  83   a  from central axis  20  (r max) corresponds to rotor blade  19   a  having first angular pitch angle limit  74   a , and the minimum radial displacement distance  83   a  from central axis  20  (r min) corresponds to rotor blade  19   a  having second angular pitch angle limit  75   a . Thus, system  15  provides rotor blade  19   a  with a pitch angle rotational range  71   a  between upper limit  74   a  and lower limit  75   a  that is based on and a direct function of the radial linear displacement range  84   a  of coupling center  52   a  from central axis  20 . Based on the radial displacement distance  83   a  (r) of coupling center  52   a  from central axis  20 , blade  19   a  has a selectively variable pitch about pitch axis  24   a.    
     With the configuration of linkage  50   a  in this embodiment, based on the radial displacement distance  83   a  (r) of coupling center  52   a  from central axis  20 , coupling center  59   a  will have a selectively variable axial displacement distance  70   a  (y) from hinge axis  54   a  ranging from a minimum distance (y min) to a maximum distance (y max). Based on the axial displacement distance  70   a , blade  19   a  has a selectively variable pitch about pitch axis  24   a.    
     Driven axis  35   a  and coupling center  52   a  have a selectively variable displacement angle  80   a  (∠d) about central axis  20  defined by the inclusive angle between line  81   a , extending between central axis  20  and driven axis  35   a  perpendicular to central axis  20 , and line  82   a  extending between central axis  20  and coupling center  52   a  perpendicular to central axis  20 . As shown, the magnitude of radial displacement distance  83   a  (r) from central axis  20 , between a minimum distance (r min) and a maximum distance (r max), is selectively varied by selectively varying displacement angle  80   a  between zero degrees and 180 degrees. As shown, the relative rotation of main rotor  17  and drive rotor  32   a  may be controlled to vary displacement angle  80   a , and thereby vary radial displacement distance  83   a  (r), and thereby vary axial displacement distance  70   a  (y) and pitch angle  71   a  (∠p) between pitch limits  74   a  and  75   a , to produce a desired pitch of rotor blade  19   a  about pitch axis  24   a  within the operational range  71   a  of rotor blade  19   a.    
     Hinge joint  53   a  and hinge axis  54   a  are driven about central axis  20  via main rotor  17  in a circular path of diameter  68 . Hinge joint  53   a  is selectively driven by main rotor  17  in a primary rotational direction at a primary rotational frequency (ω 2 ). Thus, rotation of main rotor  17  about axis  20  causes rotation of hinge joint  53   a  about axis  20 . Because transfer link  60   a  between hinge hub  25  and follower  40   a  rotationally connects hinge joint  53   a  and main rotor  17  to follower  40   a , follower  40   a  rotates with main rotor  17 . Follower  40   a  will rotate with main rotor  17  about central axis  20  in the primary rotational direction and at the primary rotational frequency (ω 2 ). Thus, rotation of main rotor  17  about axis  20  causes rotation of follower  40  in the same direction and at the same rotational speed. 
     Driven axis  35   a  is moved about central axis  20  via drive rotor  32   a  in a circular path of radius  38   a . Rotor  32   a  is selectively driven by motor  30   a  in the primary rotational direction at a rotational frequency (ω 1 ). Thus, rotation of rotor  32   a  about axis  20  moves driven axis  35   a  about axis  20 . Accordingly, driven axis  35   a  may be selectively driven to rotate about axis  20  at the same time as coupling center  52   a  is driven to rotate about axis  20 . When driven axis  35   a  and coupling center  52   a  are driven to rotate about axis  20  at the same speed (ω 1 =ω 2 ), displacement angle  80   a  (∠d), and thereby radial displacement distance  83   a  (r), axial displacement distance  70   a  (y) and pitch angle  71   a  (∠p), are maintained at a constant. To vary displacement angle  80   a  (∠d), and thereby radial displacement distance  83   a  (r), axial displacement distance  70   a  (y) and pitch angle  71   a  (∠p) to produce a desired pitch angle, driven axis  35   a  is driven my motor  30   a  to rotate about axis  20  at a different speed than the speed of rotation of coupling center  52   a  about central axis  20  (ω 1 ≠ω 2 ) until the desired radial displacement distance  83   a  (r), axial displacement distance  70   a  (y), and pitch angle  71   a  (∠p) is achieved. The relative angular positions of driven axis  35   a  and coupling center  52   a  to each other about central axis  20  is controlled to control radial distance  83   a  of coupling center  52   a  from central axis  20 . Linkage  50   a  then translates any radial displacement  84   a  relative to central axis  20  into rotational displacement  71   a  of rotor blade  19   a  about pitch axis  24   a . In this embodiment, such distance can range from a maximum distance (r max) when displacement angle  80   a  is zero degrees, such that driven axis  35   a  and coupling center  52   a  are angularly aligned about central axis  20  and pitch angle  71   a  is at limit  74   a , as shown in  FIGS. 5A and 6A , to a minimum distance (r min) when displacement angle  80   a  is 180 degrees, such that driven axis  35   a  and coupling center  52   a  are the furthest from each other, and pitch angle  71   a  is at a limit  75   a , as shown in  FIGS. 5B and 6B . 
       FIGS. 5A and 6A  show the alignment between drive rotor  32   a  and driven axis  35   a  about axis  20  and hinge hub  25  and coupling center  52   a  about axis  20  when controlled to provide an upper limit pitch angle  74   a  about pitch axis  24   a . In this embodiment and upper limit pitch configuration, drive rotor  32   a  is controlled such that displacement angle  80   a  is about zero degrees and axial distance  70   a  is at a maximum (y max). As shown, with displacement angle  80   a  at zero degrees, driven axis  35   a  of drive rotor  32   a  and coupling center  52   a  of follower  40   a  are angularly aligned about central axis  20 . 
       FIGS. 5B and 6B  show the alignment between drive rotor  32   a  and driven axis  35   a  about axis  20  and hinge hub  25  and coupling center  52   a  about axis  20  when controlled to provide a lower limit pitch angle  75   a  about pitch axis  24   a . In this embodiment and lower limit pitch configuration, drive rotor  32   a  is controlled such that displacement angle  80   a  is about 180 degrees and axial distance  70   a  is at a minimum (y min). As shown, with displacement angle  80   a  at 180 degrees, driven axis  35   a  of drive rotor  32   a  and coupling center  52   a  of follower  40   a  are angularly separated 180 degrees about central axis  20 . In this embodiment and minimum pitch configuration, driven axis  35   a  of drive rotor  32   a  is controlled such that radial displacement distance  83   a  is at a minimum distance (r min) from central axis  20 . 
     In an intermediate pitch configuration shown in  FIGS. 5C and 6C , the circular motion of driven axis  35   a  of drive rotor  32   a  can be controlled to provide intermediate pitch angle  76   a , which in this embodiment is identified as a neutral pitch angle. To change the pitch angle  71   a  from pitch angle limit  74   a , the speed of rotation (ω 1 ) of drive rotor  32   a  relative to the speed of rotation of main rotor  17  (ω 2 ), and the relative speeds of rotation of driven axis  35   a  and coupling center  52   a , respectively, are controlled such that displacement angle  80   a  is increased above 0 degrees and displacement distance  83   a  is less than the maximum (r&lt;max) and axial distance  70   a  is less than the maximum (y&lt;max). 
     The location of the driven axis  35   a , coupling center  52   a  and coupling center  59   a  relative to each other and central axis  20  and pitch axis  24   a  are selected to provide the desired range of net radial displacement  84   a  and net pitch  71   a.    
     To match the actual pitch angle to the desired pitch angle, the circular motion of drive rotor  32   a  is controlled between the upper limit pitch mode and the lower limit pitch mode to reach the desired pitch angle  71   a . In this embodiment, the circular motion of drive rotor  32   a , and resulting pitch angle  71   a , is maintained at the desired orientation by controller  90  driving motor  30   a  relative to main rotor  17  such that motor  30   a  rotates drive rotor  32   a  and driven axis  35   a  about axis  20  at a first rotation speed (ω 1 ) that is substantially the same as the rotational speed of main rotor  17  (ω 2 ). Thus, the controller maintains the desired pitch by maintaining the speed constant between the speed of rotation of drive rotor  32   a  and driven axis  35   a  and the speed of rotation of main rotor  17  and coupling center  52   a  about axis  20 , respectively. Once a desired relationship between drive rotor  32   a  and main rotor  17  is established, and displacement angle  80   a  is defined, the magnitude of pitch angle  71   a  is constant while drive rotor  32   a  and main rotor  17  spin about axis  20  in the same direction and at the same speed. 
     In this embodiment, the orientation of drive rotor  32   a  and driven axis  35   a  and main rotor  17  and coupling center  52   a  about axis  20  relative to each other, and resulting pitch angle  71   a , is modified or varied by controller  90  driving motor  30   a  relative to main rotor  17  such that motor  30   a  rotates drive rotor  32   a  and driven axis  35   a  about axis  20  at a first rotation speed (ω 1 ) that is not substantially equal to the rotational speed of main rotor  17  (ω 2 ). Thus, controller  90  varies the desired pitch angle by varying the speed differential between the speed of rotation of drive rotor  32   a  and driven axis  35   a  about axis  20  and the speed of rotation of main rotor  17  and coupling center  52   a  about axis  20  from substantially 1 to 1. Once the desired operational pitch angle is reached, controller  90  returns to a speed differential between the speed of rotation of drive rotor  32   a  and driven axis  35   a  about axis  20  and the speed of rotation of main rotor  17  and coupling center  52   a  about axis  20 , of substantially 1 to 1. 
     A representative subassembly  15   a  of motor  30   a , pitch follower  40   a , linkage  50   a  and rotor blade  19   a  is shown in  FIGS. 6A, 6B and 6C . The subassembly  15   a  is configured and operates in substantially the same manner as described above with respect to motor  30   a , pitch follower  40   a , linkage  50   a  and rotor blade  19   a  and as further illustrated in  FIGS. 2, 3 and 10 . A representative subassembly  15   b  of motor  30   b , pitch follower  40   b , linkage  50   b  and rotor blade  19   b  is shown in  FIGS. 7A, 7B and 7C . The subassembly  15   b  of motor  30   b , pitch follower  40   b , linkage  50   b  and rotor blade  19   b  is configured and operates in substantially the same manner as described above with respect to subassembly  15   a  and motor  30   a , pitch follower  40   a , linkage  50   a  and rotor blade  19   a  and as further illustrated in  FIGS. 2, 3, 6A, 6B, 6C and 10 . A representative subassembly  15   c  of motor  30   c , pitch follower  40   c , linkage  50   c  and rotor blade  19   c  is shown in  FIGS. 8A, 8B and 8C . The subassembly  15   c  of motor  30   c , pitch follower  40   c , linkage  50   c  and rotor blade  19   c  is configured and operates in substantially the same manner as described above with respect to subassembly  15   a  and motor  30   a , pitch follower  40   a , linkage  50   a  and rotor blade  19   a  and as further illustrated in  FIGS. 2, 3, 6A, 6B, 6C and 10 . A representative subassembly  15   d  of motor  30   d , pitch follower  40   d , linkage  50   d  and rotor blade  19   d  is shown in  FIGS. 9A, 9B and 9C . The subassembly  15   d  of motor  30   d , pitch follower  40   d , linkage  50   d  and rotor blade  19   d  is configured and operates in substantially the same manner as described above with respect to subassembly  15   a  and motor  30   a , pitch follower  40   a , linkage  50   a  and rotor blade  19   a  and as further illustrated in  FIGS. 2 ,  3 ,  6 A,  6 B,  6 C and  10 . In this manner, the pitch of each of rotor blades  19   a ,  19   b ,  19   c  and  19   d  is individually controlled independently of the other rotor blades. 
     In this embodiment, motors  30   a ,  30   b ,  30   c  and  30   d  are powered by 3 phase AC power source  79 , rectified to DC by power control and an AC to DC rectifier  78 . Because rotor blade pitch control system  15  is on the fuselage side of main rotor  17 , a slip ring is not needed to convey power or control signals across a rotary gap to blade pitch control system  15 . 
     Static mast  26  supports the electronics of rotor blade pitch control system  15 , including microprocessor controller  90  and sensor packages  91   a ,  91   b  and  92 . In this embodiment, controller  90  is configured to automatically control the operation of motors  30   a ,  30   b ,  30   c  and  30   d . Controller  90  receives input signals, including inputs from flight control computer  89  of helicopter  16 , and outputs command signals to motors  30   a ,  30   b ,  30   c  and  30   d  to individually control the speed of rotation of drive rotors  32   a ,  32   b ,  32   c  and  32   d  and displacement angles  80   a ,  80   b ,  80   c , and  80   d , respectively, independently of each other. To provide additional fault tolerance, rather than a common controller, separate controllers may be used to individually control each of motors  30   a ,  30   b ,  30   c  and  30   d  independently. 
     Controller  90  communicates with tachometer  92 , which measures main rotor  17  rotational speed about central axis  20  relative to fuselage  21 . However, alternative and/or additional sensors may be located on main rotor shaft  18 , on hub  22  and/or on fuselage or airframe  21  to provide rotor shaft speed or operational frequency and feedback data, such as, without limitation, feedback drive rotor speed from sensor  91   b  and hinge  54  position and rotor blade pitch from sensor  91   a . Sensors may also be installed in other locations. Additional numbers and types of sensor may be used in the system. 
     Based on sensor data, controller  90  controls the operation of rotor blade pitch control system  15 . Controller  90  may control operation of rotor blade pitch control system  15  based on data such as airspeed, blade pitch angle, amount of rotor thrust, and/or other aircraft parameters and dynamics. 
     As shown in  FIG. 10 , controller  90  receives input signals from a plurality of sensors that measure various operating parameters of helicopter  16  and provides output commands as a function of such measurements. Controller  90  is configured to receive and execute software stored in a memory for executing individual commands to motors  30   a ,  30   b ,  30   c  and  30   d . The software may be implemented via a non-transitory computer readable medium having computer executable instructions that when executed by the processor generate a command. 
     In particular, controller  90  sends commands to motors  30   a ,  30   b ,  30   c  and  30   d  based at least in part on tachometer  92  input to rotate drive rotors  32   a ,  32   b ,  32   c  and  32   d  and driven axes  35   a ,  35   b ,  35   c  and  35   d , respectively, about central axis  20  relative to main rotor shaft  18  and blade  22  in a rotational direction that is the same as the rotational direction of main rotor  17  and hubs  25  and  22  and at a desired operational frequency or speed of rotation relative to the operational frequency or speed of rotation of main rotor  17  about central axis  20  to individually control pitch angles  71   a ,  71   b ,  71   c  and  71   d  of rotor blades  19   a ,  19   b ,  19   c  and  19   d , respectively, independently of each other, as explained above. 
     While the presently preferred form of the rotor blade pitch control system has been shown and described, and several modifications thereof discussed, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the scope of the invention, as defined and differentiated by the claims.