Abstract:
In a rotary wing aircraft a bar ( 1 ) of composite matrix material has a longitudinal axis ( 8 ) perpendicular to a cross section with upper and lower surfaces ( 2, 3 ). At least one torsion box ( 17 ) comprises an upper and a lower layers ( 10, 21, 11, 22 ) respectively offset of a longitudinal axis ( 8 ) and of the upper and the lower surfaces ( 2, 3 ), said upper and lower layers ( 10, 21, 11, 22 ) being laterally joint, while cross sections are filled with the first group of fibers extending essentially parallel to the longitudinal axis ( 8 ).

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to European patent application No. EP 13400005.8, filed on Mar. 28, 2013, the disclosure of which is incorporated in its entirety by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    (1) Field of the Invention 
         [0003]    The invention is related to the rotary wing aircrafts technical domain. The invention concerns so-called rotors in such rotary wing aircrafts, e.g. main rotors capable of providing hovering/forward advance thrust or tail rotors for anti-torque. In some of these rotors, a bar of composite matrix material is provided for joining of the rotor blades to a rotary hub assembly of the rotor. 
         [0004]    (2) Description of Related Art 
         [0005]    Especially in hingeless main rotors, such a bar has a flat cross section close to the fixation at the rotary hub assembly, for the flat cross section to serve as a flapping hinge while being mainly loaded by flapping moments as well as by lead lag moments. Both moments and their deflections together with the flat cross section for a low moment of inertia in flapping direction and a large cross section for a high moment of inertia in lead lag direction result in a torsion moment at the bar. The high moment of inertia for the bar in lead lag direction is necessary to get enough cross section area for the centrifugal forces acting on the bar and for a minimum stiffness due to dynamic reasons. 
         [0006]    In the prior art, the document U.S. Pat. No. 4,648,800 describes a fiber reinforced composite flexbeam. The flexbeam is a structural member in a rotor connector for a rotary wing aircraft to connect the rotor blade to the rotor hub. In order to achieve torsional stiffness and structural integrity, reinforcing doublers are inserted into end portions. Such doublers are contoured to conform to the desired shape of the connector to which the flexbeam will be attached. Such doublers are formed of a torsionally stiff material such as steel, composite material, graphite reinforced epoxy. 
         [0007]    The document EP 2246257 describes a flexbeam having a connecting region to a rotor head, with a first connection point and a second connection point that lie adjacent to one the other along a line perpendicular to a radial extension of the rotor head at the end of the flexbeam facing the rotor head. The first connection point comprises a plurality of fastening attachments located one above the other and spaced upwardly and with a circular recess. The second connection point is provided with more fastening attachments. 
         [0008]    The document U.S. Pat. No. 5,690,474 describes a composite flexbeam having a pitch region which includes a core laminate of unidirectional fiberglass material and face laminates of unidirectional graphite material bonded to mating surfaces defined by the core laminate. For addressing the problems associated with delamination or splintering in the inboard transition region, this document departs from teaching from older flexbeams that are incorporating an external composite overwrap or an edge cap. 
         [0009]    The document JP 2010143484 describes a method for determining the cross section of a flexible beam while reducing a layer-layer shearing stress. The flexible beam has a hub side connection portion, a blade side connection portion to which a rotor blade is fixed via a member allowing feathering motion and lead and lug motion and a flexible portion ranging between the hub portion and the blade portion. The cross section perpendicular to a feathering axis forms inclined faces at both cross ends. 
         [0010]    The document US 2006/0204364 discloses a disbond resistant multi-laminate composite product having a first member; a second member of composite material bonded to the first member; a third member of composite material bonded to at least the second member and being so positioned and arranged such that when the second and third members are stressed at least a portion of the third member will have higher stress than the adjacent second member. The third member preferably has a terminal portion adjacent the second member in which terminal portion the strain energy release rate is higher than in the adjacent second member when the second and third members are stressed. The second and third members are preferably so positioned and arranged such that the third member initiates delamination from the second member before the second member initiates delamination from the first member when the first, second and third members are under stress. A stress level indicator is preferably associated with the third member to measure the stress levels in the third member. 
         [0011]    The document U.S. Pat. No. 6,375,426 discloses an edge member for use on a composite flexure having an exposed edge surface. The edge member includes an innermost composite ply attached to the edge surface, an outermost composite ply attached to the innermost composite ply, and at least one interior composite ply disposed between the innermost composite ply and the outermost composite ply. 
         [0012]    The document U.S. Pat. No. 5,820,344 discloses a helicopter rotor system including a contoured flexure strap having a simplified geometrical configuration. The flexure strap serves to join the rotary hub assembly with the rotor blades and is formed of fiberglass. The primary load carrying fibers of the flexure strap preferably extend in the same direction as the longitudinal axis of the flexure strap in order to withstand the centrifugal forces acting on the strap. A cloth wrap of composite matrix material surrounds portions of the flexure strap to provide redundancy which prevents the fibers in the flexure strap from delamination when subjected to forces arising during operation of the helicopter. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    It is an object of the invention to provide a bar of composite matrix material, particularly a bar for joining a rotary wing aircraft rotor blade to a hingeless rotary hub assembly, said bar having a cross section suitable to react to torsion moments while allowing flexibility. 
         [0014]    The solution is provided with the features of claim  1 . Subclaims define embodiments. 
         [0015]    According to embodiments of the invention a bar, particularly a bar for joining a rotary wing aircraft rotor blade to a hingeless rotary hub assembly, is made of composite matrix material. The bar has a longitudinal axis perpendicular and essentially central with regard to its cross section with upper and lower surfaces. The cross section of the bar corresponds essentially to any of the cross sections of the bar for joining a rotor blade to a hingeless rotary hub assembly, e.g. a flexbeam, at regions that are flexible, to allow the rotor blade to undergo rather small movements in a flapping direction, but relatively more important movements in a lead-lag direction and in a pitch angle direction. Any interlaminar shear stresses τ ils  in said flexible regions of the bar of the invention are τ more than or equal to (≧) 20 N/mm 2 . 
         [0016]    Said composite matrix material of the bar of the invention comprises a first group of fibers which extend parallel to the longitudinal axis and a second group of fibers which extend at an angle to the longitudinal axis. A torsion box is provided along the longitudinal axis inside the cross sections of the bar. Said torsion box comprises an upper and a lower layer made of the second group of fibers. Said upper layer is offset of the longitudinal axis of the cross section and of the upper and the lower surface. The lower layer is offset of the longitudinal axis of the cross sections and of the lower and the upper surface. The cross sections of the bar between the upper and the lower layer, between the upper layer and the upper surface and between the lower layer and the lower surface are filled with the first group of fibers extending parallel to the longitudinal axis. 
         [0017]    The bar provides for a relatively high amount of unidirectional fibers especially at the outer area of the cross section to react to the high moments and centrifugal forces occurring e.g. at the root area of a rotor blade. Consequently the bar provides as well for a flat design able to withstand the resulting torsion moment without a thickened up flexible area allowing the desired position of the virtual flapping hinge. The torsion box of the bar is stiff in torsion with the effect that this high torsion stiffness takes most of the incident torsion moments. As a consequence the already high loaded unidirectional fibers next to the upper and lower surfaces of the cross section of the bar are then less shear loaded. The torsion box of the bar helps to avoid inappropriate shear loads for the unidirectional fibers. 
         [0018]    According to an embodiment the second group of fibers extends at an angle of more or less+/−45° relative to the longitudinal axis for high torsion stiffness. 
         [0019]    According to a further embodiment the torsion box is closed by centrally located unidirectional layers of the first group of fibers or by additional layers of the second group of fibers with preferably more or less+/−45° lay-up as an U-section to connect the upper and lower layers to a closed torsion box, which is particularly suitable to transfer torsion loads. As a result the by bending moments and centrifugal loads already high loaded unidirectional fibers at the upper and lower surfaces of the bar have to carry significantly less torsion loads, absorbed by the torsion box inside. 
         [0020]    According to a further embodiment the two upper and lower layers are positioned respectively distant from the upper and lower surfaces of the cross sections of the bar where the highest longitudinal forces occur, said respective distances of the two upper and lower layers from the upper and lower surfaces being determined by the respective shares of unidirectional layers of the first group of fibers or additional layers of the second group of fibers across the cross section. For high longitudinal forces the angled layers of the upper or lower shear layers have not the appropriate fiber direction. These longitudinal forces have maxima at the upper or lower surface of the cross section of the inventive bar and decrease linearly from said upper or lower surface to the center of the cross section of the inventive bar. 
         [0021]    According to a further embodiment the respective distances of the upper and lower layers from each other increase from the lateral ends towards the longitudinal axis of the cross sections. 
         [0022]    According to a further embodiment lateral parts of the upper unidirectional layers and the lower unidirectional layers each have respectively a share of up to 50% in direction of the longer sides of the cross section. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0023]    Embodiments of the invention are described with reference to the following description and drawings. 
           [0024]      FIG. 1  shows an aircraft, a rotor and a blade with a bar seen in a lateral view, according to the invention, 
           [0025]      FIG. 2  shows a top view of the bar according to the invention, 
           [0026]      FIG. 3  shows a schematic cross sectional view of a bar according to the invention, 
           [0027]      FIG. 4  shows shares of fiber groups in a cross sectional view representative for any of the bars according to the invention, 
           [0028]      FIGS. 5-10  show partial cross sectional views of different bars according to the invention, and 
           [0029]      FIG. 11  shows a partial cross sectional view along a bar according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    On  FIG. 1 , a rotary wing aircraft C e.g. a helicopter, has at least one rotor D with a plurality of rotor blades E. The rotor D is e.g. a main rotor and/or an anti-torque rotor and/or a propulsion rotor. The rotor D has a bar  1  that is provided with an upper surface  2  and a lower surface  3 . The bar  1  is a flexbeam that joins a rotor blade E to a hingeless rotary hub assembly F. The bar  1  is joint at a blade end  4  to the rotor blade E and at a root end  5  to the hingeless rotary hub assembly. 
         [0031]    According to  FIG. 2  corresponding features are referred to with the references of  FIG. 1 . At regions between cross sections A-A to B-B the bar  1  is flexible with interlaminar shear stresses τ ils  that are exceeding twenty N/mm 2  (i.e. τ ils ≧20 N/mm 2 ) to allow the rotor blade E to undergo movements in a flapping direction, a lead-lag direction and in a pitch angle direction. An opening  6  and a recess  7  perpendicular through the upper surface  2  and the lower surface  3  are provided in the bar  1  towards the root end  5  for better adapted torsion resistance and/or flexibility of the bar  1 . Bar  1  has a longitudinal axis  8 . 
         [0032]    According to  FIG. 3  corresponding features are referred to with the references of  FIG. 1, 2 . The essentially rectangular cross section B-B of the bar  1  has dimensions of approximately 180-400 mm by 20-120 mm, particularly  180  by 30 mm. The essentially rectangular cross section B-B of the bar  1  comprises an upper layer  10  and a lower layer  11  of a second group of fibers with a lay-up of more or less forty five degrees (i.e. +/−45° lay-up) relative to the longitudinal axis  8 . Said upper and lower layers  10 ,  11  are essentially parallel to the upper surface  2  and the lower surface  3  of the longer sides of the essentially rectangular cross section B-B and said upper and lower layers  10 ,  11  are approximately equally offset of the longitudinal axis  8  and offset of the upper surface  2  and the lower surface  3 . 
         [0033]    Central unidirectional layers  12  of the first group of fibers are arranged essentially parallel to the longitudinal axis  8  between the upper layer  10  and the lower layer  11  of the cross section B-B of the bar  1 . Upper unidirectional layers  13  of the first group of fibers are arranged essentially parallel to the longitudinal axis  8  between the upper layer  10  and the upper surface  2  and lower unidirectional layers  14  are arranged between the lower layer  11  and the lower surface  3 . 
         [0034]    The central unidirectional layers  12  of the first group of fibers or additional layers  15  of the second group of fibers with preferable of more or less forty-five degrees (i.e. +/−45°) lay-up connect the upper and lower layers  10 ,  11  at lateral left and right sides  16 ,  18  of the cross section B-B to a closed torsion box  17 . 
         [0035]    According to  FIG. 4  corresponding features are referred to with the references of  FIG. 1-3 . The central unidirectional layers  12  of the first group of fibers arranged essentially parallel to and around the longitudinal axis  8  between the upper layer  10  and the lower layer  11  have a ten to eighty-five percent share (10%-85% share) in direction of the shorter sides of the cross sections. 
         [0036]    The upper unidirectional layers  13  of the first group of fibers arranged essentially parallel to the longitudinal axis between the upper layer  10  and the upper surface  2  and the lower unidirectional layers  14  arranged between the lower layer  11  and the lower surface  3  each have respectively a five to forty-two point five percent share (5%-42.5% share) in direction of the shorter sides of the cross sections of the bar  1 . 
         [0037]    The upper layer  10  and the lower layer  11  with the second group of fibers with +/−45° fabric layers providing the torsion box  17  each have a two point five to forty percent share (2.5%-40% share) in direction of the shorter sides of the cross sections. 
         [0038]    The central unidirectional layers  12 , the upper unidirectional layers  13  and the lower unidirectional layers  14  are each provided as rovings made of the first group of fibers. The upper layer  10  and the lower layer  11  are fabric layers made of the second group of fibers. The central unidirectional layers  12  may be separated by a core element  9 , said core element  9  comprising fabric layers and/or foam and/or rovings and/or thermoplastic fillers and/or scrap respectively processed, e.g. cemented, with resin. 
         [0039]    The central unidirectional layers  12  separated by the core element  9  each have respectively a share of up to fifty percent (50%) in direction of the longer sides of the cross sections of the bar  1 . The upper unidirectional layers  13  and the lower unidirectional layers  14  are each separated by a central gap in direction of the longer sides of the cross sections of the bar  1 . Lateral parts of the upper unidirectional layers  13  and the lower unidirectional layers  14  each have respectively a share of up to 50% in direction of the longer sides of the cross sections of the bar  1 . 
         [0040]    According to  FIG. 5  corresponding features are referred to with the references of  FIGS. 1-4 . A wedge shaped cross section  20  of bar  1  has a decreasing height towards a left side  16 . A curved upper fabric layer  21  and a curved lower fabric layer  22  each made of the second group of fibers have respectively increasing distances relative to each other towards the longitudinal axis  8 . A closed torsion box  17  is provided by the central unidirectional layers  12  connecting laterally the upper and lower layers  21 ,  22  of bar  1 . 
         [0041]    According to  FIG. 6  corresponding features are referred to with the references of  FIGS. 1-5 . A wedge shaped cross section  20   a  of bar  1  with the decreasing height towards the left side  16 , the curved upper fabric layer  21  and the curved lower fabric layer  22  each made of the second group of fibers and having respectively increasing distances relative to each other towards the longitudinal axis  8  is provided with the closed torsion box  17  by a laterally mounted U shaped cap  23  with brackets turned towards the longitudinal axis  8  and connecting the upper and lower layers  21 ,  22 . 
         [0042]    According to  FIG. 7  corresponding features are referred to with the references of  FIGS. 1-6 . A wedge shaped cross section  20   b  of bar  1  with the decreasing height towards the left side  16 , the curved upper fabric layer  21  and the curved lower fabric layer  22  each made of the second group of fibers and having respectively increasing distances relative to each other towards the longitudinal axis  8  is provided with the closed torsion box  17  by the laterally connecting upper and lower fabric layers  21 ,  22 . 
         [0043]    According to  FIG. 8  corresponding features are referred to with the references of  FIGS. 1-7 . A wedge shaped cross section  20   c  of bar  1  with the decreasing height towards the left side  16 , the curved upper fabric layer  21  and the curved lower fabric layer  22  each made of the second group of fibers and having respectively increasing distances relative to each other towards the longitudinal axis  8  is provided with the closed torsion box  17  by a laterally mounted connecting block  24  for connection of the upper and lower layers  21 ,  22 . 
         [0044]    According to  FIG. 9  corresponding features are referred to with the references of  FIGS. 1-8 . A wedge shaped cross section  20   d  of bar  1  with the decreasing height towards the left side  16 , the curved upper fabric layer  21  and the curved lower fabric layer  22  each made of the second group of fibers and having respectively increasing distances relative to each other towards the longitudinal axis  8  is provided with the closed torsion box  17  by a U shaped cap  25  mounted inside the cross section with brackets turned away from the longitudinal axis  8  and connecting the upper and lower layers  21 ,  22 . 
         [0045]    According to  FIG. 10  corresponding features are referred to with the references of  FIGS. 1-9 . A wedge shaped cross section  20   e  of bar  1  with the decreasing height towards the left side  16 , the curved upper fabric layer  21  and the curved lower fabric layer  22  each made of the second group of fibers and having respectively increasing distances relative to each other towards the longitudinal axis  8  is provided with the closed torsion box  17  made by upper and lower fabric layers  21 ,  22  of an endless tissue. 
         [0046]    According to  FIG. 11  corresponding features are referred to with the references of  FIGS. 1-10 . A curved cross section  20   f  of bar  1  with decreasing height towards an extended longitudinal end  26  is provided with the curved upper fabric layer  21  and the curved lower fabric layer  22  each made of the second group of fibers and having respectively increasing distances relative to each other along the longitudinal axis  8  towards inside the torsion box  17 . The connected upper and lower fabric layers  21 ,  22  are laterally integrated to the extended right end  26 . 
         [0047]    The central unidirectional layers  12 , the upper unidirectional layers  13  and the lower unidirectional layers  14  of the cross sections  20 - 20   f  are respectively provided as rovings made of the first group of fibers. 
         [0000]    
       
         
               
             
               
               
             
           
               
                   
               
               
                 Reference List 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 bar 
               
               
                 2 
                 upper surface 
               
               
                 3 
                 lower surface 
               
               
                 4 
                 blade end 
               
               
                 5 
                 root end 
               
               
                 6 
                 opening 
               
               
                 7 
                 recess 
               
               
                 8 
                 longitudinal axis 
               
               
                 9 
                 core element 
               
               
                 10 
                 upper layer 
               
               
                 11 
                 lower layer 
               
               
                 12 
                 central unidirectional layer 
               
               
                 13 
                 upper unidirectional layer 
               
               
                 14 
                 lower unidirectional layer 
               
               
                 15 
                 additional layer 
               
               
                 16 
                 left side 
               
               
                 17 
                 torsion box 
               
               
                 18 
                 right side 
               
               
                 19 
                 n/a 
               
               
                 20 
                 cross sections 
               
               
                 21 
                 curved upper layer 
               
               
                 22 
                 curved lower layer 
               
               
                 23 
                 laterally mounted u-shaped cap 
               
               
                 24 
                 connecting block 
               
               
                 25 
                 u-shaped cap 
               
               
                 26 
                 longitudinal end 
               
               
                 C 
                 Rotary wing aircraft e.g. a helicopter 
               
               
                 D 
                 Rotor: e.g. helicopter main rotor 
               
               
                 E 
                 Rotor blade 
               
               
                 F 
                 Hingeless rotary hub assembly