Abstract:
A method of making a rotor yoke includes preparing a cured composite rotor yoke preform on a single-sided curing tool and machining at least one portion of the cured rotor yoke preform to form a rotor yoke. In one embodiment, preparing the cured composite rotor yoke is accomplished by applying successive layers of uncured polymeric composite material to a single-sided tool in a configuration to produce an uncured rotor yoke preform; disposing a semi-rigid caul to an untooled side of the uncured rotor yoke preform; enclosing the uncured rotor yoke preform and the semi-rigid caul by substantially hermetically sealing a vacuum bag to the tool; substantially evacuating a volume defined by the vacuum bag and the tool of air; and heating the uncured rotor yoke preform, sometimes under pressure, to form the cured composite rotor yoke preform.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to rotorcraft and, in particular, to yokes for coupling helicopter blades to a mast. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    Each blade of the main rotor assembly of a rotorcraft must be connected to a main support mast, usually by means of a rotor yoke, in a manner allowing several degrees of freedom. Such an interconnection is subjected to high and repeated stresses of both torsional and centrifugal natures, and is therefore an extremely important component of the aircraft. Each blade must be able to rotate about its longitudinal axis to provide pitch control. Each blade must be able to flap in a direction perpendicular to the rotor plane to accommodate vertical loads. In some instances, each blade must be able to pivot within the rotor plane to provide for lead-lag control. The manner in which the blades are secured to the main support mast enables a rotorcraft to be controlled and maneuvered in flight. 
         [0003]    Various types of rotor yokes have been utilized to interconnect the rotorcraft blades and the support mast. Metal rotor yokes have suffered from the disadvantages of weight, cost, high maintenance requirements, and low useful life. There have been several attempts to eliminate one or more of the articulations in such couplings in order to simplify construction and reduce costs. Some rotor yokes are pivotally secured to the support mast, and are characterized by a flat plate construction resilient enough to act as a virtual hinge and thereby accommodate flapping of the blades. 
         [0004]    More recently, glass fibers and other composite materials have been employed in the fabrication of rotorcraft rotor system components. In comparison to a machined metal forging, glass fibers and other composite materials have more favorable fatigue characteristics resulting in longer useful life. In addition, the use of such materials simplifies construction and reduces costs. Referring to  FIGS. 1  and  2 , such composite rotor yokes, such as a rotor yoke  101  are conventionally cured in a rigid, closed mold, such as mold  103 , to form the overall shape of the rotor yoke. One of the problems encountered concerning such rotorcraft rotor yokes, however, has been distortion or “marcelling” of the fibers in the rotor yoke during the curing process. Because the uncured rotor yoke is forced to conform to the cavity, such as cavity  105 , formed by the closed mold, mechanical stresses are induced in the uncured rotor yoke. The fibers are substantially unconstrained during certain portions of the curing cycle when the resin matrix in which the fibers are disposed is in a semi-liquid or liquid state. The induced stress in the uncured rotor yoke is relieved via movement or distortion of the fibers within the resin matrix. The fibers are captured in their distorted or marcelled state when the resin crosslinks in thermosetting composite materials or when the resin is cooled in thermoplastic composite materials. 
         [0005]    There are many designs of rotorcraft yokes well known in the art; however, considerable shortcomings remain. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The novel features believed characteristic of the invention are set forth in the appended claims. However, the invention itself, as well as a preferred mode of use, and further objectives and advantages thereof, will best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, in which the leftmost significant digit(s) in the reference numerals denote(s) the first figure in which the respective reference numerals appear, wherein: 
           [0007]      FIGS. 1 and 2  are stylized, cross-sectional views illustrating a conventional method for manufacturing a composite rotor yoke for a rotorcraft; 
           [0008]      FIG. 3  is a top, plan view of an illustrative embodiment of a fiber-reinforced, polymeric-matrix, composite rotor yoke; 
           [0009]      FIG. 4  is a stylized side, elevational view depicting a portion of an illustrative method for making the composite rotor yoke of  FIG. 3 ; 
           [0010]      FIG. 5  is a stylized, cross-sectional view depicting an illustrative curing assembly including a preform of the composite rotor yoke of  FIG. 3 ; 
           [0011]      FIG. 6  is a top, plan view of an illustrative embodiment of a composite rotor yoke preform corresponding to the composite rotor yoke of  FIG. 3 ; 
           [0012]      FIG. 7  is a top, plan view of an illustrative embodiment of a rotor hub incorporating a pair of composite rotor yokes of  FIG. 3 ; and 
           [0013]      FIG. 8  is a stylized, cross-sectional view depicting an illustrative curing assembly, alternative to the curing assembly of  FIG. 5 . 
       
    
    
       [0014]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0016]    In the specification, reference may be made to the spatial relationships between various components and to the spatial orientation of various aspects of components as the devices are depicted in the attached drawings. However, as will be recognized by those skilled in the art after a complete reading of the present application, the devices, members, apparatuses, etc. described herein may be positioned in any desired orientation. Thus, the use of terms such as “above,” “below,” “upper,” “lower,” or other like terms to describe a spatial relationship between various components or to describe the spatial orientation of aspects of such components should be understood to describe a relative relationship between the components or a spatial orientation of aspects of such components, respectively, as the device described herein may be oriented in any desired direction. 
         [0017]    The present invention represents a composite rotor yoke for a rotorcraft produced using a single-sided, rigid, curing tool. Preferably, the composite rotor yoke is laid-up using an automated fiber placement process. After curing, the cured composite rotor yoke is machined to final shape including, in some embodiments, faying surfaces of the composite rotor yoke. 
         [0018]      FIG. 3  depicts an illustrative embodiment of a rotorcraft rotor yoke  301 . Rotor yoke  301  comprises a polymeric-matrix composite body  302 , made from materials such as glass fibers disposed in an epoxy matrix, although the present invention contemplates other materials for rotor yoke  301 . Outer periphery  303  of rotor yoke  301  and inner peripheries  305   a - 305   c  of rotor yoke  301  are machined to final dimensions. Moreover, openings  307   a  and  307   b  that are used in attaching rotor blades, such as rotor blades  701   a - 701   d  of  FIG. 7 , to rotor yoke  301  are machined openings, as are mast attachment openings  309   a - 309   h . Faying surfaces, such as faying surfaces  311   a - 311   c , which are shown bordered in phantom in  FIG. 3 , are also machined surfaces in certain embodiments of rotor yoke  301 . Thus, rotor yoke  301  defines a machined outer periphery, such as outer periphery  303 ; machined inner peripheries, such as inner peripheries  305   a - c ; machined openings, such as openings  307   a ,  307   b  and  309   a - 309   h ; and/or machined faying surfaces, such as faying surfaces  311   a - 311   c.    
         [0019]    For the purposes of this disclosure, the term “machined” means generated using a rotating cutting tool having cutting features, such as teeth, abrasive particles, and the like, that incrementally remove material from rotor yoke  301  as the cutting tool is traversed along rotor yoke  301 . Moreover, the term “faying surface,” for the purposes of this disclosure, means a surface of an element that will be in contact with another component when the element is attached or joined to the other component. 
         [0020]    It should be noted that the particular geometric configuration of rotor yoke  301  is merely exemplary of the geometric configurations contemplated by the present invention, as the particular geometric configuration of a rotor yoke is implementation specific. 
         [0021]    Referring to  FIG. 4  in the drawings, a composite rotor yoke preform  415  of rotor yoke  301  (shown in  FIG. 3 ) is fabricated by applying successive layers of uncured polymeric composite material  401  onto a single-sided tool  403 . Note that the geometric configuration of tool  403  is merely exemplary of the widely diverse geometric configurations of single-sided tools contemplated by the present invention. In one embodiment, fibers of composite material  401  extend substantially continuously around loops  405  and through blade-attachment areas or lugs  407  of preform  415 , and thus rotor yoke  301 , as represented by a line  409 . Alternatively, in another embodiment, fibers of composite material  401  extend substantially linearly across portions of preform  415 , and thus rotor yoke  301 , but along different angular paths in successive layers to form faceted, straight fiber plies in preform  415 , and thus rotor yoke  301 , as represented by lines  411   a - 411   c . Preferably, composite material  401  is applied to tool  403  using an automated fiber placement machine, such as an automated fiber placement machine  413 . 
         [0022]    Uncured, composite rotor yoke preform  415  is formed when the desired number of plies or layers of composite material  401  have been applied to tool  403  in the desired geometry. It should be noted that, in one embodiment, rotor yoke preform  415  is oversized, i.e., is larger in physical dimensions than rotor yoke  301 . For example, the outside dimensions of rotor yoke preform  415  is larger than the desired outside dimensions of rotor yoke  301  to provide material that is removed during the machining process to form machined outer periphery  303  (shown in  FIG. 3 ). Moreover, dimensions of openings  417 ,  419 , and  421  are smaller than corresponding openings defined by inner peripheries  305   a - 305   c  of rotor yoke  301  to provide material that is removed during the machining process to form inner peripheries  305   a - 305   c . In other embodiments, however, a preform such as preform  415  is formed to produce a substantially net shape rotor yoke  301 , as is discussed herein with respect to  FIG. 8 . 
         [0023]    Referring now to  FIG. 5 , to prepare rotor yoke preform  415  for curing, a semi-rigid caul  501  is placed on an untooled side  503  of rotor yoke preform  415 . Caul  501  preferably exhibits non-isotropic mechanical properties that, along with the shape of caul  501 , is tailored to fit an outside shape of rotor yoke preform  415 . A substantially airtight sheet, referred to herein as a “vacuum bag”  505 , is disposed over caul  501  and rotor yoke preform  415 , such that vacuum bag  505  is substantially hermetically sealed to tool  403 , forming a curing assembly  507 . Other materials, such as breather materials, release films, and the like, may be used as is known to the art. Air between caul  501 , rotor yoke preform  415 , tool  403  and vacuum bag  505  is substantially evacuated and a negative pressure is maintained within vacuum bag  505  with respect to atmosphere. Curing assembly  507  is subjected to heat and, in some embodiments, pressure, to consolidate and cure rotor yoke preform  415 . 
         [0024]    Referring to  FIGS. 3 and 6 , when the cured, rotor yoke preform  415  is removed from vacuum bag  505 , caul  501 , and tool  403 , the cured rotor yoke preform  415  is ready to be machined. Material defining openings  417 ,  419 , and  421  is machined away from cured rotor yoke preform  415  to form inner peripheries  305   a - 305   c . Material is also machined away cured rotor yoke preform  415  to form outer periphery  303  and faying surfaces  311   a - 311   c . Openings  307   a ,  307   b , and  309   a - 309   h  are machined, preferably using a five-axis machine tool, into cured rotor yoke preform  415 . Rotor yoke  301  is formed when all machining operations are complete. 
         [0025]      FIG. 7  depicts a rotor hub  703  comprising, in the illustrated embodiment, a pair of rotor yokes  301 , a coupling  705  that mechanically couples rotor yokes  301  to a mast  707 , and fittings  709   a - 709   d  that mechanically couple rotor yokes  301  to rotor blades  701   a - 701   d , respectively. The configuration of rotor hub  703  depicted in  FIG. 7  is merely exemplary of the widely various embodiments of a rotor hub contemplated by the present invention. 
         [0026]    As discussed herein, a preform for rotor yoke  301  is prepared in substantially a net shape form, rather than in the oversized form described in relation to preform  415  of  FIG. 4 . In such an embodiment, as shown in  FIG. 8 , one or more dams  801  are disposed under caul  501  and vacuum bag  505  adjacent to edges  803  of an uncured preform  805 . Other materials, such as breather materials, release films, and the like, may be used as is known to the art. During the curing process, such as described in relation to  FIG. 5 , the one or more dams  801  confine preform  805  to define the edges of rotor yoke  301 . 
         [0027]    The present invention provides significant advantages, including: (1) providing a composite rotor yoke that exhibits less marcelling of the reinforcing fibers thereof; (2) providing a method of producing a composite rotor yoke that inhibits voids in the composite rotor yoke; (3) enabling the use of toughened, low-flow resins in composite rotor yokes; and (4) providing a composite rotor yoke that requires less composite fabrication time, cost, and effort with less part-to-part variation than conventional rotor yokes. 
         [0028]    The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below. It is apparent that an invention with significant advantages has been described and illustrated. Although the present invention is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof.