Abstract:
A rotor blade ( 1 ), in particular of a main rotor of a rotary-wing aircraft, made of fiber-reinforced plastic includes a blade section ( 10 ) and a connecting section ( 12 ) for fastening the rotor blade ( 1 ) to a drive device ( 18 ) which includes a sleeve-shaped connecting device ( 14 ), is further developed in that the connecting device ( 14 ) includes flat fiber layers (S 1  to S 18 ) running substantially in the plane of extension of the connecting section ( 12 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European patent application 09 400025.4 filed May 27, 2009, the contents of which is incorporated in its entirety by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a rotor blade, in particular of a main rotor of a rotary-wing aircraft or helicopter, made of fibre-reinforced plastic. This comprises a blade section and a connecting section for fastening the rotor blade to a drive device. The connecting section comprises a sleeve-shaped connecting device and lies at the end of the rotor blade opposite to the blade tip and therefore facing a drive axis. It connects the rotor blade at least indirectly to the drive device. For this it comprises at least one sleeve-shaped connecting device, for example, for a bolt connection. The rotor blade more favourably has two sleeve-shaped connecting devices which are disposed adjacent to one another in the plane of rotation of the blade since tilting moments of the blade from its forward travel or as a result of its inertia can be absorbed. 
     2. Description of Related Art 
     Nowadays rotor blades are usually manufactured using the wet or prepreg method of construction. This offers a low degree of automation and is associated with a large amount of manual work and is as a result very cost-intensive and liable to error. Even small improvements of the rotor blade or its method of manufacture can therefore have a cost-reducing or quality-enhancing effect. The development of new rotor blade systems is primarily directed towards reducing the power requirement, the weight and the maintenance expenditure as well as towards increasing the lifetime and reducing the manufacturing costs. The lifetime is substantially determined by the introduction of forces and the transmission of forces between the rotor blade and the drive device. The introduction of forces into the rotor blade is usually effected via loop and bolt connections which are dynamically highly loaded. Loop connections are regarded as solutions appropriate to fibres. It is found, however, that their operating strength is determined by the resin properties. As a result, additional structural-2-elements can be required to increase the dynamic strength. Bolt connections have provided useful as detachable connections. They make it possible, inter alia, to form a folding hinge which is used primarily in the military area. 
     GB-A 2 131 373 discloses a rotor blade for a main rotor of a rotary-wing aircraft, made of fibre-reinforced plastic comprising a blade section and a connecting section for fastening the rotor blade to a drive device which comprises a sleeve-shaped connecting device with flat fibre layers running substantially in the plane of extension of the connection section. An axial-radial elastomeric bearing is incorporated in the blade root for connection of the rotor blade. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore the object of the invention to further simplify the manufacture of a rotor blade of the type specified initially. 
     This object is achieved in a rotor blade of the type specified initially in that the connection device comprises flat fibre layers running substantially in the plane of extension of the connecting section or consists thereof. The connecting section of the rotor blade extends substantially in its plane of rotation. According to the invention, said section is formed from fibre layers likewise extending in the plane of rotation or from a corresponding fibre package. The invention therefore goes away from forming a loop-shaped connecting device by a so-called standing loop in which the fibre layers forming the loop stand perpendicularly to the plane of rotation of the rotor blade. Rather, when forming the loop- or sleeve-shaped connecting device, it follows the principle of the bearing stress connection of fibre layers located in the plane of rotation. This therefore makes it possible to achieve a very flat connecting section having only a small overall height. Thus, the connecting section has a lower aerodynamic resistance. 
     According to an advantageous embodiment of the invention, the fibre layers forming the connecting section comprise both unidirectional layers and also additional fibre layers running pivoted at an angle with respect to these. The alignment of the fibres in the unidirectional fibre layers corresponds to the longitudinal direction of the rotor blade. The unidirectional fibre layers can therefore in particular transmit the centrifugal forces of the rotor blade optimally and in a material-saving manner. The fibres of the additional layers can run as additional reinforcing layers between the unidirectional fibre layers at an almost arbitrary angle, for example of +/−30, +/−45, +/−60 or 0/90 degrees. They can comprise approximately 50-60% unidirectional fibre layers, 35-45% of +/−45 degree fibre layers and about 5-10% of 0/90 fibre layers. These additional layers can also be formed with biaxial or triaxial scrims or fabrics. The additional fibre layers can be provided in the connecting section in the same or different fractions to the unidirectional fibres between them. Thus, the entire cross-section in the connection zone can be filled with fibre material. 
     According to a further advantageous embodiment of the invention, the unidirectional layers run into the blade section of the rotor blade. They thus form a component both of the connecting section and of the blade section. There they can run continuously to the blade tip. In particular, they can form spars of the blade section which run in the longitudinal direction of the blade and form the leading edge of the blade. They thereby ensure a good connection of the blade section to the connecting section. The additional layers can also run at least partly into the blade section and contribute to its formation. Layers of the blade skin can also be tied into the connecting section. However, the continuous course of the unidirectional layers has the advantage compared to this that they run in accordance with the force flow and the direction of the centrifugal force and consequently transmit this with minimal usage of material into the connecting section in the best possible manner. 
     According to a further advantageous embodiment of the invention, the unidirectional fibre layers which tie into the blade section of the rotor blade, are predominantly disposed in a near-surface region of the connecting device. They therefore surround the additional or reinforcing layers, with the result that the connecting section experiences an increase in its bending stiffness and an improvement in its strength properties. 
     According to a further advantageous embodiment of the invention, the connecting section comprises additional reinforcing layers running at an angle of substantially 90 degrees with respect to the longitudinal axis of the rotor blade. The strength of the connecting sections can also be further improved by this means. 
     According to a further advantageous embodiment of the invention, all the fibre layers comprise fabric or scrim having a glass fibre and a carbon fibre component. The rotor blade and in particular its connecting section are therefore constructed in a mixed design of glass and carbon fibres which combined the advantages of both materials. 
     The sleeve-shaped connecting device is therefore formed according to the invention according to the principle of a bearing-stress connection. This means that the fibre layers in the area of bolt connection are interrupted by a sleeve-shaped gap. The gap can, as in the prior art, be omitted by incorporating the fibre layers of the rotor blade in the connecting section. According to an advantageous embodiment of the invention, the sleeve-shaped connecting device is formed by holes substantially perpendicular to the plane of extension of the connecting section. The hole leads to a bearing-stress connection which loads the fibre layers of the connecting section during operation more or less exclusively in their plane of extension and therefore optimally appropriate to the fibres. When a force is introduced via a bolt, as a result of the bearing stress connection, instead of a standing loop according to the prior art, there is no longer any deflection of forces into the fibre layers which could lead to delaminations and cracks in the loop. Rather, the loaded fibre layers run almost free from deflection between the connecting section and the rotor blade. As a result, a maximum load-bearing capacity is in turn possible with minimal material usage. This favours cost-effective production, long lifetime, small thickness dimension of the connecting section and a low weight of the rotor blade. 
     According to a further advantageous embodiment of the invention, the connecting device has a liner-shaped metal reinforcement. As a result, on the one hand, the introduction of force into the connecting section in fibre composite design can be improved and made more uniform and on the other hand, the wear of the connecting device can be reduced. 
     According to a further advantageous embodiment of the invention, the edge distances of the sleeve-shaped connecting device in and at right angles to the longitudinal axis of the rotor blade are different. Thus, the different cases of failure of the bearing stress connection can be countered. In the longitudinal direction of the blade, by dimensioning the edge distance of the connecting device at the front side, a sufficient shear strength can be achieved to prevent tearing out of the connecting device due to shear failure. The edge distance of the connecting device to the side edge running in the longitudinal direction of the blade possibly together with a distance from one another, determines its tensile strength in order to eliminate cheek rupture. The lateral edge distance can be kept somewhat lower by a somewhat higher fraction of unidirectional fibre layers. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The principle of the invention will be described in further detail hereinafter with reference to a drawing as an example. In the drawings: 
         FIG. 1  shows a load connection according to the prior art, 
         FIG. 2  shows a connecting section according to the invention and 
         FIG. 3  shows fibre layers for forming a rotor blade together with connecting section, and 
         FIG. 4  shows a sectional view of a further embodiment of a rotor blade. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows the prior art. For this purpose,  FIG. 1   a  shows a section of a rotor system at the position of the rotor blade connection. A blade section a, which can be immediately identified, goes over into a connecting section b in which a loop connection is formed. This forms the interface to a drive device c which embraces the connecting section b in a fork shape. A bolt d connects the device c to the connecting section b. 
       FIG. 1   b  shows a sectional view according to  FIG. 1   a  in the area of the bolt d. The blade section consists of endless glass fibres, so-called rovings e. These are wound in loops f around wound fibre liners k. In the sectional view in  FIG. 1   b , the planes of the rovings e therefore run parallel to the axial direction of the bolt d and the liners k or perpendicular to the plane of extension of the connecting section b. They form the “standing” loops f which give the connecting section b a height H. Between these is an intermediate space g which is filled with horizontally coated fibre material or a chopped fibre mass. Thus, an unfavourable dividing surface i is formed between the loops f and the fibre material of the intermediate space g. 
     Comparable views are shown in  FIG. 2 . In  FIG. 2   a  a short part of the blade section  10  can be identified from the rotor blade  1 , which goes over into the connecting section  12 . This is embraced by a fork-shaped interface of a drive device  18  to which it is detachably fastened by means of a detachable bolt  20 . 
       FIG. 2   b  shows a sectional view according to  FIG. 2   a  through the connecting section  12 . This is exclusively formed by fibre layers S 1  to S 18  coated horizontally one above the other. The connecting devices  14  form two perpendicularly running holes  16  which pass through the fibre layers S 1  to S 18 . The superposed fibre layers S 1  to S 18 , forming the connecting section  12 , together have a thickness h. 
     The holes  16  are only drilled subsequently in the connecting section  12 . This results in a very uniform and homogeneous formation of the regular cylindrical bearing stress of the holes  16  whereon their subsequent production can be identified without any doubts. Due to the undisturbed reveal formation, the remaining cross-section of the connecting section  12  is fully loadable as far as the edges of the bores  16 , resulting in an optimal utilisation of the cross-section and therefore minimal dimensions of the connecting section  12 . The holes  16  pass through the fibre layers S 1  to S 18  of the connecting section  12  perpendicularly to their plane of extension and therefore load these optimally in a manner appropriate to the fibres. During introduction of force via the bolts  20 , due to the bearing stress connection of the holes  16 , there is no deflection of force into the fibre layers S 1  to S 18  which could lead to delaminations in the connecting section  12 . As a result, a maximum load-bearing capacity can be achieved with minimal material usage. This favours cost-effective production, long lifetime, small external dimension and a low weight of the rotor blade  1 . 
     In the plan view of the connecting section  12  according to  FIG. 2   c , it can be identified that the holes  16  have a shorter edge distance R 1  from the side edge  22  of the connecting section  12  compared to its front side  24  (edge distance R 2 ). The method of production according to the invention makes it possible to achieve a material-saving adaptation of the connecting section  12  to the ensuing loads. Shear failure of the bearing stress connection due to tearing out of at least one of the holes  16  in the longitudinal direction of the blade is countered by a sufficiently dimensioned edge distance R 2 . Together with the height h of the connecting section  12 , this defines the two transmission surfaces for shear stresses per hole  16 . The layers with cross-running fibres in particular absorb this loading. 
     A failure of the connecting section  12  in the direction transverse to the longitudinal direction of the blade through the two holes  16 , i.e. a “cheek rupture” would correspond to a tensile failure. The cross-section thereby loaded is calculated from the width of the connecting section  12  multiplied by its height h minus the loaded bearing stress surfaces of the holes  16 . Since sufficiently loadable cross section is available between the holes  16 , the edge distance R 1  can be smaller. The tensile loading is substantially absorbed by the unidirectional layers S 2 , S 3 , S 5 , S 7  and S 8  (cf.  FIG. 3 ). 
     The side views or sectional views according to  FIG. 1   a  or  1   b  and  2   a  or  2   b  illustrate another advantage of the method of construction according to the invention: the connecting section  12  having a height h of about 36 mm is significantly smaller than that of the prior art having a height H of about 58 mm. Since the thickness H or h determines the region of the connecting section b or  12  exposed aerodynamically to the incoming flow, the connecting section  12  according to the invention offers significantly lower aerodynamic resistance. 
     In the plan views according to  FIG. 1   c  or  2   c , on the other hand, the larger dimensions in the plane of extension of the connecting section  12  are clear. With comparable load-bearing capacity, the edge distance R 2  according to the invention is approximately one and a half times as large as in the prior art. The edge distance R 1  is also larger. However, since these dimensions extend in the aerodynamically non-effective plane of rotation and the rotor blade connection in this plane is scarcely subjected to any constructive restrictions, these enlarged dimensions can be accepted. 
       FIG. 3  shows an example for a sectional and laying plan of the fibre layers S 1  to S 18 . In the direction of the arrow E, these are inserted in a production mould for a resin injection method in order to form the lower shell of a rotor blade  1 . The layer S 1  to be laid first in the mould consists of a multi-axial fabric having an angular alignment of +/−45 degrees with respect to the longitudinal axis of the rotor blade  1  and forms the lower blade skin. The following layers S 2  and S 3  are unidirectional fibre layers forming parts of a spar of the rotor blade  1 . These run flat and rectangularly in the blade section  10  and expand abruptly in the connecting section  12  on its base surface. 
     The layer S 4  is a reinforcing layer consisting of a triaxial fabric. This no longer fills the entire connecting section  12  as shown by a comparison with the layer S 3 . 
     The following layer S 5  again consists of unidirectional fibre material. This extends through the entire blade section  10  and expands in the connecting section  12  on its width. This likewise forms a part of the spar in the rotor blade  1 . This also no longer fills the entire length of the connecting section  12  in the longitudinal direction of the rotor blade  1 . This is followed by the layers S 6  to S 8  correspondingly. 
     Unlike the previous principle, the layer S 10  forms a reinforcing layer consisting of unidirectional fibre material. In contrast to the previous layers S 2 , S 3 , S 5 , S 7  and S 8  of unidirectional fibre material, this is not involved in forming the spar in the rotor blade  1 . The following layers S 11  to S 18  are also reinforcing layers which, with the exception of the last layer S 18 , no longer fill the complete connecting section  12 . 
     The layers S 1  to S 3  as well as S 5 , S 7  and S 8  form a lower shell of the blade section  10  and are guided further in the connecting region  12  of the rotor blade  1 . There they are, as it were, fanned out by providing the reinforcing layers S 4 , S 6 , S 9  to S 18  in between and to this end. 
       FIG. 3  shows fibre layers S 2  to S 18 , whose transition from the connecting section  12  into the blade section  10  runs almost at right angles to the longitudinal axis of the blade. At the transition, they each have an edge K which runs largely at right angles to the longitudinal axis A of the blade and jumps back from layer to layer. The staggered arrangement of the edges K results in a soft transition from the connecting section  12  into the blade section  10  without a stiffness jump. 
       FIG. 4  shows in sectional view a further embodiment with the fibre layers T 1  to T 16  following the fundamental structure according to  FIG. 3  but the layers T 2  to T 16  have a modified form with regard to the transition between the connecting section  12 ′ into the blade section  10 ′: their edges L, of which only those of layers T 2  to T 4  are designated as an example, do not run at right angles in layers T 2  to T 8  but at least partially at an inclination to the longitudinal axis A of the blade. The edges L of the layers T 9  to T 16  on the other hand have a type of recess which is almost symmetrical to the longitudinal axis A. Its form is therefore similar to that of a tooth root. The tips of the layers T 15  and T 16  are cut off so that they additionally acquire an edge M which is inclined to the axis A. 
     The edges L are also like the edges K (cf.  FIG. 3 ). This prevents abrupt stiffness transitions and thus improves the strength behaviour of the connection. Due to the staggered recesses of the layers T 9  to T 16 , a cavity is formed which is filled with a foam core or the like. The layer T 16  is followed by at least one other layer which was not shown because this would cover the arrangement identifiable in  FIG. 4 . 
     Since the preceding rotor wing which has been described in detail comprises an exemplary embodiment, it can be broadly modified in the usual manner by the person skilled in the art without departing from the scope of the invention. In particular, the specific cut of the fibre layers and the sequence of their arrangement can be effected in a different form to that described here. Likewise, the mechanical coupling to the connecting section can be configured in a different form if this is necessary for reasons of space or design reasons. Furthermore, the use of the indefinite article “a” or “an” does not exclude the fact that the relevant features can also be multiply present. 
     REFERENCE LIST 
     
         
         a Blade section 
         b Connecting section 
         c Drive device 
         d Bolt 
         e Roving 
         f Loops 
         g Intermediate space 
         i Dividing surface 
         k Liners 
           1  Rotor blade 
           10 ,  10 ′ Blade section 
           12 ,  12 ′ Connecting section 
           14  Connecting device 
           16  Hole 
           18  Drive device 
           20  Bolt 
           22  Side edge 
           24  Front side 
         A Longitudinal axis of rotor blade 
         H, h Height 
         K Edge of layers S 2  to S 18   
         L Edge of layers T 2  to T 16   
         M Edge of layers T 15 , T 16   
         R 1 , R 2  Edge distance 
         S 1  to S 18  Fibre layers