Abstract:
A blade pitch control mechanism for a propeller assembly ( 10 ) includes an actuation shaft ( 24 ) having a spherical member ( 28 ) located at a propeller axis ( 22 ). A yoke ( 40 ) is located around the spherical member ( 28 ) and is rotatable thereon about a yoke tilt axis ( 62 ) substantially perpendicular to the propeller axis ( 22 ). The yoke ( 40 ) includes a plurality of propeller blade attachments ( 44 ), each receptive of a propeller blade ( 14 ). The plurality of blade attachments ( 44 ) are configured such that rotation of the yoke ( 40 ) about the yoke tilt axis ( 62 ) results in rotation of each propeller blade ( 14 ) about a blade pitch change axis ( 48 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein generally relates to propellers and propeller-driven aircraft. More specifically, the subject disclosure relates to propeller blade pitch control. 
         [0002]    Propellers producing thrust or pressure in a non-uniform flow field are subject to sinusoidal variation of the thrust load on the propeller blades, known in the propeller science as a 1 P (once per revolution) load. This phenomenon occurs, for example, when the propeller is subjected to an angular inflow, an inflow that is non-parallel to the propeller shaft. Such angular inflow is common when an aircraft is in a yaw or G-producing maneuver such as a turn or a climb at takeoff 
         [0003]    The 1 P load manifests itself as a moment on the propeller shaft that lags the inflow angle by 90 degrees as a result of the advancing blade seeing a higher angle of attack and the receding blade seeing a lower angle of attack. This load must be reacted by the aircraft structure and is countered by various control surfaces on the aircraft, resulting in additional structural weight to react the load and larger control surfaces, and associated aircraft drag, to counter the moment. The art would well receive a way to reduce the 1 P load which would, in turn, result in an aircraft weight savings and reduction in reduction in control surface size necessary to counter the load. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    According to one aspect of the invention, a blade pitch control mechanism for a propeller assembly includes an actuation shaft having a spherical member located at a propeller axis. A yoke is located around the spherical member and is rotatable thereon about a yoke tilt axis substantially perpendicular to the propeller axis. The yoke includes a plurality of propeller blade attachments, each receptive of a propeller blade. The plurality of blade attachments are configured such that rotation of the yoke about the yoke tilt axis results in rotation of each propeller blade about a blade pitch change axis. 
         [0005]    According to another aspect of the invention, a propeller assembly includes a propeller shaft extending along a propeller axis and a plurality of propeller blades in operable communication with the propeller shaft. The propeller assembly further includes a blade pitch control mechanism having an actuation shaft including a spherical member located at the propeller axis. A yoke is located around the spherical member and is rotatable thereon about a yoke tilt axis substantially perpendicular to the propeller axis. The yoke includes a plurality of blade attachments, each receptive of a propeller blade of the plurality of propeller blades. The plurality of blade attachments are configured such that rotation of the yoke on the spherical member about the yoke tilt axis results in rotation of each propeller blade about a blade pitch change axis. 
         [0006]    According to yet another aspect of the invention, a method of cyclic pitch change of a plurality of propeller blades of a propeller assembly includes rotating a first propeller blade of the plurality of propeller blades about a blade pitch change axis in response to a flow force encountered by the first propeller blade. The rotation of the first propeller blade is translated into rotation of a yoke about a yoke tilt axis substantially perpendicular to a propeller axis of the propeller assembly. The second propeller blade, located substantially opposite the first propeller blade, is rotated about the blade pitch change axis in response to rotation of the yoke about the yoke tilt axis. 
         [0007]    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 
         [0008]    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: 
           [0009]      FIG. 1  is a partially-exploded view of an embodiment of a propeller assembly; 
           [0010]      FIG. 2  is cross-sectional view of an embodiment of a propeller assembly; 
           [0011]      FIG. 3  is an exploded view of an embodiment of an actuation shaft of the propeller assembly of  FIG. 2 ; 
           [0012]      FIG. 4  is a partial cross-sectional view of an embodiment of a propeller assembly; 
           [0013]      FIG. 5  is a view of an embodiment of a propeller blade; and 
           [0014]      FIG. 6  is a cross-sectional view of another embodiment of a propeller assembly. 
       
    
    
       [0015]    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 
       [0016]    Referring to  FIG. 1 , shown is a partially exploded view of a propeller assembly  10 . The propeller assembly  10  includes a hub  12  which is receptive of a plurality of propeller blades  14  through a plurality of hub openings  16 . The propeller assembly  10  is driven by a propeller shaft  18 . A pitch change actuator  20  is operably connected to the plurality of propeller blades  14  to change the pitch of the propeller blades  14  during operation of the propeller assembly  10 . 
         [0017]    Referring now to  FIG. 2 , a cross-sectional view of an embodiment of the propeller assembly  10  is shown. The propeller shaft  18  is located at a propeller axis  22  which and drives rotation of the propeller assembly  10  about the propeller axis  22 . An actuation shaft  24  is located in the hub  12  along the propeller axis  22  and is fixedly attached to an actuation piston  26  at one end of the actuation shaft  24 . The actuation shaft  24  includes a spherical section  28  located between a first portion  30  and second portion  32  of the actuation shaft  24  between adjacent stop surfaces  34 . As shown in  FIG. 3 , in some embodiments, the spherical portion  28  includes a through hole  36  extending along the propeller axis  22 . A pin  38  extends from the first portion  30  through the through hole  36  into the second portion  32  to secure the first portion  30 , second portion  32  and spherical section  28  together forming the actuation shaft  24 . Referring again to  FIG. 2 , the spherical section  28  may be formed from any suitable material, for example a hardened metal and/or wear resistant coating, or wear resistant composite material. At least one yoke  40  extends around the spherical section  28  and is supported thereon so as to permit articulation as indicated by arrows  42  within the confines of stop surfaces  34 . The pitch change actuator piston  26  is slidably located in the pitch change actuator  20  such that linear motion of the piston  26  along the propeller axis  22  results in linear motion of the spherical section  28  and corresponding motion of the at least one yoke  40 . 
         [0018]    The plurality of propeller blades  14  are rotably located in the propeller hub  12  and confined in the yoke  40  at a yoke channel  44  formed in the yoke  40 , via suitable pin and roller assemblies  46  connected to the plurality of propeller blades  14 . As shown in  FIG. 4 , the pin and roller assemblies  46  are positioned offset in the yoke  40  such that translation of the yoke  40  along propeller axis  22  will result in rotation of blades  14  about a blade pitch change axis  48 , and causes the plurality of rotor blades  14  to increase or decrease pitch in unison, this being well known in the art and commonly referred to as collective pitch change. 
         [0019]    As a result of this invention, motion of the yoke  40  about the spherical section  28  will also result in rotation of the propeller blade  14  about its pitch change axis  48 . Referring to  FIG. 2 , it can be seen that motion of yoke  40  about spherical section  28  causes pitch change of opposite blades  14  in opposite directions. In the embodiment shown in  FIG. 2 , two propeller blades  14  are shown, but propeller assemblies  10  having other quantities of propeller blades  14  are contemplated within the present scope. In the embodiment shown in  FIG. 2 , the yoke channel  44  is defined by two yoke bars  50  arranged in parallel, but other configurations which provide equivalent motion, for example a series of pins and links, may be utilized. Each propeller blade  14  extends outwardly from the hub  12  at the hub openings  16 . Each propeller blade  14  is retained by one or more bearings  52  which allow for rotation of the propeller blade  14  about the pitch change axis  48 , changing a pitch of the propeller blade  14 . 
         [0020]    Referring to  FIG. 5 , in some embodiments, each pin and roller assembly  46  is located off-center at the propeller blade  14  relative to the pitch change axis  48 . Because of this off-center position, a force applied to the pin and roller assembly  46  via the yoke  40  results in rotation of the propeller blade  14  about the pitch change axis  48 . 
         [0021]    Referring again to  FIG. 2 , in baseline operation of the propeller assembly  10 , pitch of the propeller blades  14  is changed via axial movement of the yoke  40  along the propeller axis  22 . This axial movement is achieved by changing a pressure in a portion of a pressure chamber  54 . For example, to move the yoke  40  axially toward a first end  56  of the hub  12 , a pressure P f , is increased in a shaft side  58  of the pressure chamber  54  to be greater than a pressure P c  in an opposing side  60  of the pressure chamber  54 . This results in an axial translation of the piston  26 , and therefore the yoke  40 , toward the first end  56 . Since the pin and roller assembly  46 /yoke  40  interface is offset, the axial movement of the yoke  40  along the propeller axis  22  results in a rotation of the propeller blades  14  about the pitch change axis  48 , or a change in blade pitch. Alternatively, the pitch change can be reversed by increasing the pressure P c  to be greater than P f  to move the piston  26  away from the first end  56  thereby translating the yoke  40  away from the first end  56 . In this embodiment, the movement of the yoke  40  and pressurization change in portions of the pressure chamber  54  are achieved via addition and removal of hydraulic fluid from the pressure chamber  54 , by activation of a flight control by the pilot. It is to be appreciated, however, that other means of moving the yoke  40 , mechanical and/or electrical, may be used. 
         [0022]    Utilization of the spherical portion  28  increases the ability of the pitch of the propeller blades  14  to be adjusted as described earlier. Forces acting on the propeller blades  14  often vary as the propeller blades  14  rotate about the propeller axis  22 , for example, when flow incident to the propeller blade  14  is non-parallel to the propeller axis  22 . To mitigate the effects of these forces, the yoke  40  is permitted to rotate about the yoke center  62  as shown by arrows  42 . This freedom of rotation allows forces acting on the propeller blades  14  to change the pitch of the propeller blades  14  cyclically as the propeller blades  14  move around the propeller axis  22  being influenced cyclically by the variation in in-flow to the propeller  10 . For example, as a first propeller blade  14  is subjected to greater forces than a second propeller blade  14 , the forces drive a change in pitch in the first rotor blade  14 . This, in turn, causes rotation of the yoke  40  about the yoke center  62  which changes the pitch of the second propeller blade  14  by an equal and opposite amount. As the propeller blades  14  rotate about the propeller axis  22 , the forces acting on the first propeller blade  14  change, and through the use of the spherical portion  28 , so does the pitch of the first and second propeller blades  14 . The spherical portion  28  effectively reduces the cyclical forces acting on the propeller blades  14  and reduces the moments on the propeller shaft  18 , airframe, and other components due to the cyclical loading of the propeller blades  14 . 
         [0023]    In some embodiments, as shown in  FIG. 6 , the propeller assembly  10  includes a yoke stop  64 . The yoke stop  64  is a tubular piece located around the actuation shaft  24  at a shaft opening  66  in the hub  12  and is slidable therein. The yoke stop  64 , when engaged, disables articulation of the yoke  40  about the spherical portion  28  of actuation shaft  24  thus stopping the cyclical pitch change in conditions when it is not desired, for example, low power approach and/or reversing conditions. The yoke stop  64  is engaged by increasing P f  which forces the yoke stop  64  along the propeller axis  22  toward the yoke  40 . The yoke stop  64  includes an engagement feature, in this embodiment, a stop ramp  68 , which is engageable with a complimentary feature of the yoke  40 , for example a yoke ramp  70 . When the yoke stop  64  is moved toward the yoke  40  by the increase in P f , the stop ramp  68  engages with the yoke ramp  70  to prevent rotation of the yoke  40  about the yoke center  62 . In some embodiments the yoke stop  64  is engaged only at low blade angles typically associated with low power and reverse because it is travel is limited by stop surface  72 . To disengage the yoke stop  64 , P f  is reduced to allowed movement of the yoke stop  64  away from the yoke  40 . In some embodiments a return spring (not shown) may be used to assist in disengaging the yoke stop. 
         [0024]    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. 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.