Patent Publication Number: US-2019185150-A1

Title: Split yoke in a folding rotor blade assembly

Description:
CROSS-REFERENCE To RELATED APPLICATIONS 
     This patent application is related to U.S. Patent Application entitled “Compact Folding Yoke In A Folding Rotor Blade Assembly,” Docket No. 60388-P007US; U.S. Patent Application entitled “Compact Folding Yoke With Flexible Yoke Arms In A Folding Rotor Blade Assembly,” Docket No. 60388-P008US; U.S. Patent Application entitled “Dual Blade Fold Bolts And Inboard Centrifugal Bearing In A Folding Rotor Blade Assembly,” Docket No. 60388-P009US; and U.S. Patent Application entitled “Folding Spindle And Bearing Assembly In A Folding Rotor Blade Assembly,” Docket No. 60388-P010US; and U.S. Patent Application entitled “Outboard Centrifugal Force Bearing With Inboard Blade Fold Axis In A Folding Rotor Blade Assembly,” Docket No. 60388-P011US. Each patent application identified above is filed on the same date as this patent application and is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     This section provides background information to facilitate a better understanding of the various aspects of the disclosure. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. 
     Rotorcraft and tiltrotor aircraft are often transported or stored on vessels or in areas where storage space is limited. In order to reduce the space that each aircraft occupies such that the maximum number of aircraft can be accommodated within the limited storage space, the blade assemblies of some rotor systems can be folded so that each rotor blade is generally parallel with each other in order to reduce the overall profile of the blade assembly. Typically, each rotor blade is folded about a single pivot point positioned outboard of the yoke that attaches the rotor blade to the central drive mast. The single pivot point is also necessarily outboard of an essential set of inboard and outboard bearings that connect the rotor blade to the yoke. The distance between the inboard and outboard bearings is dependent on aircraft configuration where each configuration has an optimal distance for that particular aircraft&#39;s loads and dynamics. As a result, the pivot point of each rotor blade is typically at least that optimal distance out from the rotor blade&#39;s inboard connection to the yoke. 
     In an effort to transport or store larger numbers of rotorcraft and tiltrotor aircraft, current naval vessels have reduced the allotted storage space available for each aircraft. Present rotor blade folding systems cannot accommodate the reduced space parameters. This requirement necessitates a tighter grouping of the rotor blades than is currently available by prior art rotor blade folding systems. 
     SUMMARY 
     An example of a split yoke for a folding rotor blade assembly includes a bilateral hub spring including an upper hub spaced from a lower hub, a yoke arm connected to the bilateral hub spring between the upper hub and the lower hub, a first connection point of the yoke arm to the bilateral hub spring including a removable bolt, and a second connection point of the yoke arm to the bilateral hub spring, wherein the yoke arm pivots relative to the bilateral hub spring about the second connection point when the removable bolt is removed from the first connection point. 
     An example of a system for folding a rotor blade assembly includes a hub spring operatively connected to a central mast, a yoke arm connected to the hub spring at a releasable point and a pivot point, and a plurality of bearings connecting the yoke arm to a rotor blade, the plurality of bearings positioned on the yoke arm outboard of the pivot point. 
     An example of a method for folding a rotor blade assembly comprising a yoke arm connected to a hub spring with a releasable connection and a pivotable connection includes pitching a rotor blade connected to the yoke arm, releasing the releasable connection of the yoke arm, and pivoting the yoke arm about the pivotable connection. 
     This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1A  is a perspective view of a tiltrotor aircraft in a flight ready position according to aspects of the disclosure. 
         FIG. 1B  is a perspective view of a tiltrotor aircraft in a stowed position according to aspects of the disclosure. 
         FIG. 2A  is a partial perspective view of a blade assembly in an unfolded position according to one or more aspects of the disclosure. 
         FIG. 2B  is a partial side view of a blade assembly in an unfolded position according to one or more aspects of the disclosure. 
         FIG. 3  is a partial top view of a rotor blade and yoke arm according to aspects of the disclosure. 
         FIG. 4  is a top view of a rotor blade assembly in a folded position according to aspects of the disclosure. 
         FIG. 5  is a flowchart of the actions performed in converting a tiltrotor aircraft from a flight ready position to a stowed position according to aspects of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. Specific examples of components and arrangements are described below to simplify the disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Referring to  FIGS. 1A and 1B , an illustrative tiltrotor aircraft  100  is shown. Tiltrotor aircraft  100  includes fuselage  102 , landing gear  104 , tail member  106 , wing  108 , wing tip  110 , wing tip  112 , rotor system  114 , and rotor system  116 . Rotor system  114  is housed within nacelle  115  located on an end portion of wing  108  proximate wing tip  110 , while rotor system  116  is housed within nacelle  117  located on an opposite end portion of wing  108  proximate wing tip  112 . Wing tip  110  is pivotable at a location on wing  108  outboard of nacelle  115 . Wing tip  112  is pivotable at a location on wing  108  outboard of nacelle  117 . Nacelles  115  and  117  are pivotable between a helicopter mode where the rotor systems are generally vertical and an airplane mode where the rotor systems are generally horizontal. Nacelle  115  and nacelle  117  are substantially symmetric of each other about fuselage  102 . Rotor system  114  includes a plurality of foldable rotor blades  118 . Rotor system  116  includes a plurality of foldable rotor blades  120 . Rotor blades  118  and  120  may rotate in opposite directions to cancel the torque associated with the operation of each rotor system  114  and  116 . The angle of the pivotable nacelles  115  and  117  relative to the wing, as well as the pitch of rotor blades  118  and  120 , can be adjusted in order to selectively control direction, thrust, and lift of tiltrotor aircraft  100 . Further, rotor systems  114  and  116  are illustrated in the context of tiltrotor aircraft  100 ; however, a singular rotor system with foldable rotor blades can be implemented on other non-tilting rotor and helicopter rotor systems. It should also be appreciated that teachings from tiltrotor aircraft  100  may apply to other aircraft such as airplanes and unmanned aircraft which would benefit from folding rotor blades. 
     Fuselage  102  represents the body of tiltrotor aircraft  100  and may be coupled to rotor systems  114  and  116  such that the rotor systems with rotor blades  118  and  120  may move tiltrotor aircraft  100  through the air. Landing gear  104  supports tiltrotor aircraft  100  when tiltrotor aircraft  100  is landing or when tiltrotor aircraft  100  is at rest on the ground. Vertical axis  122  is generally perpendicular to the longitudinal axis of the wing and is generally positioned at the intersection of the fuselage and the wing.  FIG. 1A  represents tiltrotor aircraft  100  in operational flying position in an airplane mode.  FIG. 1B  represents tiltrotor aircraft  100  in a stowed position where rotor blades  118  have been folded generally parallel with each other and rotor blades  120  have been folded generally parallel with each other in order to reduce the profile of the aircraft to whatever degree is required in response to storage space restrictions. In the stowed position, wing  108  is swivelled approximately 90° to generally align with fuselage  102 . 
     Generally each rotor system includes a mast driven by a power source. A rotor system includes a yoke connected to the mast and rotor blades indirectly connected to the yoke with bearings. There may be inboard bearings connecting a cuff or grip of a rotor blade to the yoke proximate the mast and outboard bearings connecting the rotor blade to an outboard end of a yoke arm. Other combinations of inboard and outboard bearings with or without cuffs or grips are possible as well as the removal of one or the other bearings. The bearings accommodate forces acting on the rotor blades allowing each rotor blade to flex with respect to the yoke/mast and other rotor blades. The weight of the rotor blades and the lift of rotor blades may result in transverse forces on the yoke and other components. Examples of transverse forces may include forces resulting from flapping and coning of the rotor blades. Flapping generally refers to the up-and-down movement of a rotor blade positioned at a right angle to the plane of rotation. Coning generally refers to the upward flexing of a rotor blade due to lift forces acting on the rotor blade. The rotor blades may be subject to other forces, such as axial, lead/lag, and feathering forces. Axial forces generally refer to the centrifugal force on the rotor blades during rotation of the rotor blades. Lead and lag forces generally refer to forces resulting from the horizontal movement of the rotor blades about a vertical pin occurring if, for example, the rotor blades do not rotate at the same rate as the yoke. Feathering forces generally refer to forces resulting from twisting motions that cause a rotor blade to change pitch. The power source, mast, and yoke are components for transmitting torque. The power source may include a variety of components including an engine, a transmission, and differentials. In operation, the mast receives torque from the power source and rotates the yoke. Rotation of the yoke causes the rotor blades to rotate with the mast and yoke. 
     Referring to  FIGS. 2A and 2B , blade assembly  202  is shown in an unfolded position. Each rotor system  114  and  116  comprises a separate blade assembly. In the interest of clarity, a single blade assembly is described herein with the understanding that tiltrotor aircraft  100  comprises a pair of similarly configured blade assemblies. Blade assembly  202  is shown in an unfolded position. In the unfolded position, each rotor blade  204 ,  206 , and  208  is generally equally spaced from each other around mast  209 . For example in the three rotor blade configuration shown, 120° separates each rotor blade. It should also be appreciated that teachings regarding blade assembly  202  can apply to blade assemblies having greater or fewer rotor blades. It should also be appreciated that teachings regarding blade assembly  202  can apply to blade assemblies not intended to fold. 
     Hub spring  210  is connected to mast  209  through a central opening  211  in the hub spring. Hub spring  210  is a bilateral disc comprised of upper hub  212  mounted to lower hub  213 . A split yoke  203  includes a plurality of separate yoke arms where each yoke arm  214 ,  216 , and  218  is individually attached to hub spring  210  between upper hub  212  and lower hub  213  with two bolts  220  at two separate attachment points. Bolts  220  pass through both upper hub  212  and lower hub  213  and the yoke arm. Each yoke arm is in double shear condition between upper hub  212  and lower hub  213 . The double shear condition prevents any rotational moment about the connection of the yoke arm to the hub spring at each bolt  220  created by centrifugal forces acting on the rotor blade during blade assembly rotation. Opposite the connection to hub spring  210 , yoke arms  214 ,  216 , and  218  are connected to rotor blades  204 ,  206 , and  208 , respectively via outboard beams  224 ,  226 , and  228 , respectively. Outboard beams  224 ,  226 , and  228  house outboard bearings  225 ,  227 , and  229  that respond to centrifugal force acting on the rotor blades due to rotation. Rotor blades  204 ,  206 , and  208  include integrally formed split cuffs  230 ,  231 , and  232 , respectively. Yoke arms  214 ,  216 , and  218  are connected to split cuffs  230 ,  231 , and  232 , respectively via inboard beams  234 ,  236 , and  238 , respectively. Each integral split cuff provides a double shear condition that prevents any moment about the connection of the yoke arm to the cuff created by centrifugal forces acting on the rotor blade. Inboard beams  234 ,  236 , and  238  house inboard bearings that allow the rotor blades to flex in response to shear forces on the rotor blades due to rotation. The outboard and inboard bearings are generally elastomeric bearings constructed from a rubber type material that absorb vibration and provide for limited movement of the rotor blades relative to the yoke arm and mast. The centrifugal force (“CF”) load path on each rotor blade is from the rotor blade, to the outboard bearing, and to the yoke arm. Although the location of centrifugal force bearings is disclosed as an outboard configuration within the outboard beams, it should also be appreciated that the location of centrifugal force bearings could alternatively be an inboard configuration within the inboard beams. 
     Swash plate  222  is connected to mast  209 . Pitch links  240  extend from swash plate  222  and connect to pitch horns  242 . A different pitch horn  242  is connected to each split cuff  230 ,  231 , and  232 . The swash plate, pitch links, and pitch horns are operatively connected to an actuator and used to pitch the rotor blades relative to the yoke arm about the central longitudinal axis of each rotor blade. During folding of the rotor blades, the pitch links may extend/telescope or temporarily disengage from their connection to the pitch horns. As an alternative, the pitch horns may extend/telescope, or partially disengage from their connection to the split cuff, to permit folding without positional movement of the pitch horns and pitch links. 
     As illustrated in  FIG. 3 , yoke arm  214  is attached to rotor blade  204 . Rotor blade  204  includes leading edge  322  and trailing edge  324 . Yoke arm  214  is split into a generally “Y” shape including tip  302  opposite ends  304  and  305 . In the interest of clarity, a single yoke arm and rotor blade is described herein with the understanding that a blade assembly comprises a plurality of similarly configured yoke arms and rotor blades. Clamp plate  306  is mounted to tip  302 . Outboard bearing  225  extends between clamp plate  306  and outboard beam  224 . Outboard beam  224  is connected to rotor blade  204 . Inboard beam  234  is mounted to flanges  308  of split cuff  230 . Clamp  312  is mounted to yoke arm  214  at the intersection of ends  304  and  305 . Inboard bearing  310  extends between inboard beam  234  and clamp  312 . Rotor blade  204  is free to rotate about its central longitudinal axis  320  with respect to yoke arm  214 . The central longitudinal axis of a rotor blade may also be referred to as a blade pitch change axis. This rotation allows rotor blade  204  to pitch through an angle in the range of 45° to 90°. Ends  304  and  305  include mounting/pivot holes  314  and  315 , respectively. Mounting/pivot holes  314  and  315  are sized to engage bolts  220 . Distance  316  is the spacing between inboard beam  234  which houses inboard bearing  310  and outboard beam  224  which houses outboard bearing  225 . Distance  316  is an optimal distance between inboard and outboard bearings for a rotor blade assembly of a particular aircraft. The distance is dependent on the particular aircraft&#39;s loads and dynamics. Inboard direction  318  points toward the drive mast of a blade assembly while outboard direction  319  points toward the unconnected end of a rotor blade. 
     Referring to  FIG. 4 , blade assembly  202  is shown in a folded position. Unfolded rotor blade  204  and unfolded rotor blade  208  are depicted in shadow. Rotor blade  204  has central longitudinal axis  424 . Rotor blade  208  has central longitudinal axis  428 . Rotor blade  204  is pivoted about pivot point  402  through angle  406 . Rotor blade  208  is pivoted about pivot point  404  through angle  408 . Actuators operatively connected to the rotor blades facilitate movement of the rotor blades about the pivot points. Angles  406  and  408  may be in the range of 90° to 180°. Physical stops or proximity sensors signal the actuators to cease movement of the rotor blades. 
     Rotor blade  204  cannot pivot about pivot point  402  until the bolt at connection point  412  that connects one end of the yoke arm to the hub spring when in the unfolded position is pulled. Rotor blade  208  cannot pivot about pivot point  404  until the bolt at connection point  414  that connects one end of the yoke arm to the hub spring when in the unfolded position is pulled. The bolts at pivot points  402  and  404  provide pivot axes for the yoke arm and attached rotor blade to pivot with respect to the hub spring. Actuators connected to the bolts at connection points  412  and  414  pull or remove the bolts at connection points  412  and  414  so that the yoke arm is no longer connected to the hub spring at connection points  412  and  414 . The bolts can be completely removed from engagement with the yoke arm and the hub spring or, alternatively, as part of a latch and lock system attached to the hub spring where the removable bolts remain fixed to the yoke arm. Once the bolts at connection points  412  and  414  are removed, the yoke arm and attached rotor blade are free to pivot about the single bolts at pivot points  402  and  404  still connecting the yoke arm to the hub spring. 
     Pivot points  402  and  404  are positioned inboard of the inboard beams of rotor blades  204  and  208 , respectively. Pivot points  402  and  404  are distance  410  from the inboard beams of rotor blades  204  and  208 , respectively. Distance  410  is measured parallel with central longitudinal axes  424  and  428 . Pivot points  402  and  404  are not positioned on central longitudinal axes  424  and  428 . In the folded position, pivot points  402  and  404  are located inboard of central longitudinal axes  424  and  428 , respectively. The pivot point of each rotor blade positioned inboard of the inboard beams and inboard of the folded rotor blade central longitudinal axes allows folded profile  416  to be less than if the pivot point were outboard of the outboard beam. 
     Referring to  FIG. 5 , the actions performed in converting tiltrotor aircraft  100  from a flight ready position to a stowed position are shown. At block  502 , nacelles  115  and  117  which house rotor systems  114  and  116 , respectively, are pivoted to helicopter mode. Each nacelle is rotated nose up to approximately 90° nacelle angle. A 90° nacelle angle is where the longitudinal axis of the nacelle is generally vertical relative to the ground. The blade assemblies of each rotor system are generally horizontal. At block  504 , each rotor blade is pitched about its central longitudinal axis to high collective position. High collective is when the leading edge of each rotor blade is generally facing upward. This is referred to as indexing the rotor blades. Actuators operatively connected to pitch links  240  and pitch horns  242  facilitate the change in pitch of the rotor blades. At block  506 , bolts connecting one end of the to-be-folded rotor blades to the hub spring are pulled. Actuators operatively connected to the bolts facilitate temporary removal of the bolts effectively disengaging one end of the yoke arm from connection to the hub spring. The position and number of identified to-be-folded rotor blades can vary depending on rotor assembly configuration. At block  508 , the rotor blades and attached yoke arms are pivoted. Actuators operatively connected to the rotor blades facilitate pivoting the rotor blades about the still connected pivot points of the yoke arms. The rotor blades are pivoted toward the fuselage until the rotor blades are generally parallel with each other at which point physical stops or proximity sensors signal the actuators to cease movement of the rotor blades. At block  510 , nacelles  115  and  117  are pivoted to airplane mode. Each nacelle is rotated to approximately 0° nacelle angle. 0° nacelle angle is where the longitudinal axis of the nacelle is generally horizontal relative to the ground. The blade assemblies of each rotor system remain generally horizontal. At block  512 , wing tips  110  and  112  are pivoted toward the fuselage. At block  514 , wing  108  is swivelled about vertical axis  122  to lie above and generally align with the fuselage. The entire sequence of converting tiltrotor aircraft  100  from an operational flight ready position to a stowed position can be completed in a range of 1 to 2 minutes in a wind of up to at least 60 knots. It can be interrupted or stopped at any point to facilitate maintenance. Manual operation is possible in the event of a system failure. It is to be understood that several of the previous actions may occur simultaneously or in different order. The order of actions disclosed is not meant to be limiting. 
     The term “substantially” is defined as largely but not necessarily wholly what is specified (and includes what is specified; e.g., substantially 90 degrees includes 90 degrees and substantially parallel includes parallel), as understood by a person of ordinary skill in the art. In any disclosed embodiment, the terms “substantially,” “approximately,” “generally,” and “about” may be substituted with “within [a percentage] of what is specified, where the percentage includes 0.1, 1, 5, and 10 percent. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the disclosure. Those skilled in the art should appreciate that they may readily use the disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the disclosure. The scope of the invention should be determined only by the language of the claims that follow. The term “comprising” within the claims is intended to mean “including at least” such that the recited listing of elements in a claim are an open group. The terms “a,” “an” and other singular terms are intended to include the plural forms thereof unless specifically excluded.