Patent Publication Number: US-2012045339-A1

Title: Rotor blades

Description:
The present invention relates to rotor blades, and, in particular, to rotor blades for use in underwater tidal power generation installations. 
     BACKGROUND OF THE INVENTION 
     There is increasing interest in the use of underwater power generating equipment that makes use of the energy of tidal flows. Such equipment is secured to the bed of a body of water, such as a sea, estuary or river, and makes use of a rotary generator to generate electricity. The generator is driven by a number of rotor blades placed in the water flow. An example of such a tidal power generating installation is illustrated in  FIG. 1  of the accompanying drawings. 
     In the example shown in  FIG. 1  of the accompanying drawings, the installation  1  is located on a bed  2  of a body of water  3 . A generating unit  4  is mounted on an underwater support structure  5  which is fixed to the bed  2 . The generating unit  4  includes a rotary generator and associated equipment for generating electricity. The generator is driven by a rotor  6  carried on an input shaft of the generator. The rotor  6  has a plurality of rotor blades  14 . 
       FIGS. 2 and 3  of the accompanying drawings illustrate respective cross sectional views of a rotor blade  14  which comprises a root  16  by which the rotor blade is attached to the rotor of the generator. The blade is attached to the rotor using a root fitting  17 . A spar  20  extends from the root  16  to a tip  18  of the blade. The blade has a leading edge  22  and a trailing edge  24 , and the shape of the blade is defined by first and second skins  26  and  28 . The skins  26  and  28  are generally of a composite fibre/resin material, and are moulded to the correct shape. The skins  26  and  28  are supported by the spar  20  which provides the blade  14  with additional strength. 
       FIG. 4  of the accompanying drawings illustrates a known root fitting  17  for a rotor blade. The root fitting  17  is a single cast metallic part having a flange portion  171 , through which a series of holes  172  extend. The holes  172  are for receiving bolts for attaching the root fitting to a blade mounting. A blade fitting portion  173  extends from the flange portion  171 , and is shaped to engage with a root portion of the blade itself, as illustrated in the previous drawings. 
     Rotor blades for underwater use generate large bending moments about the root fitting  17 , and so this component must be large in order to be able to carry the large loads placed upon it. As shown in  FIG. 4 , the fitting is typically a single metallic casting. Such existing root fitting components have several disadvantages. Firstly, the component is very heavy and cumbersome, which limits the number fabricators that are able to produce such a part, and the process of fitting the blade to the root fitting is complex and requires specialist heavy lifting equipment. Secondly, the root fitting must be of the highest quality in order that it does not fail under the high loads. As such, even the smallest fault in the casting, machining, and finishing processes would result in the scrapping of the entire root fitting. 
     It is, therefore, desirable to provide a root fitting for a rotor blade that can overcome the disadvantages of the known root fittings. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a rotor blade comprising a blade portion, a root portion connected with, and extending from, the blade portion, the root portion having an annular cross section which defines first and second root surfaces, and a plurality of discrete root fittings connected with respective portions of the root portion and adapted for attachment to a blade mounting, wherein each root fitting comprises an elongate body member which extends from a free end of the root portion towards the blade portion, which defines first and second bonding surfaces, adhesively bonded to the first and second root surfaces respectively, and which includes means for attachment to a blade mounting. 
     According to another aspect of the present invention, there is provided a rotor blade comprising a blade portion, a root portion connected with, and extending from, the blade portion, the root portion defining first and second root surfaces, the first root surface being an outer surface of the root portion, and the second root surface being an inner surface of the root portion that bounds an interior volume thereof, and a plurality of discrete root fittings connected with respective parts of the root portion of the blade, each root fitting comprising an attachment portion adapted for attachment to a blade mounting, and an elongate body member defining first and second bonding surfaces, which elongate body member extends from a free end of the root portion towards the blade portion, such that the first bonding surface extends adjacent the first surface of the root portion, and such that the second bonding surface extends adjacent the second root surface of the root portion, the first and second bonding surfaces being adhesively bonded to the first and second root surfaces respectively. 
     The first and second root surfaces may be substantially parallel to one another. 
     Alternatively, portions of the first and second root surfaces may converge towards the end of the root portion. 
     In one example, the first bonding surface is substantially parallel to the first root surface and the second bonding surface is substantially parallel to the second root surface. 
     Portions of the first and second root surfaces may diverge towards an end region of the root portion. 
     Each root fitting may be bolted to the root portion of the blade. In such an example, each root fitting may further comprise a bolt which extends through the body portion of the root fitting and through the root portion of blade. 
     The first and second root surfaces and the first and second bonding surfaces may be substantially smooth. Alternatively, at least one of the first and second root surfaces and the first and second bonding surfaces may be treated to increase bonding strength of an adhesive bonding medium. 
     In one embodiment, the root fittings are arranged as a series around the root portion of the blade, each root fitting being interlocked with the next root fitting in the series. 
     The body member may comprise separate first and second body portions which define the first and second bonding surfaces respectively. 
     In such an example, the root portion may have a third root surface which extends between the first and second root surfaces, and the first body portion may include an end part which has an inner surface extending substantially parallel to, and adjacent, the third surface, and an outer surface, and the second body portion may includes an end part which has an inner end surface extending parallel to, and adjacent, the outer surface of the first body portion. In such an example, the end part of the first body portion may define a threaded hole for reception of a threaded stud therein, the threaded hole having an axis extending away from the root portion, and the end part of the second body portion may define a hole coaxial with the threaded hole, such that a threaded stud received in the threaded hole extends from the end part of the first body portion, through the end part of the second body portion, in a direction away from the root portion of the blade. 
     One embodiment may further comprise a threaded component located between respective end regions of the first and second body portions. 
     The first and second body portions may have respective end regions, each end region defining at least one receiving feature, and the root fittings may be arranged around the root portion of the blade to form a series, such that receiving features of neighbouring root fittings define at least one attachment aperture, the blade may further comprise a plurality of fixing components located in respective attachment apertures, the fixing components having features for enabling attachment of the blade to a rotor. 
     \the first and second body portions may have respective end regions, which define a receiving aperture therebetween, and the blade may comprise a plurality of threaded components, located in respective receiving apertures. 
     The first and second body portions may have respective end regions which interengage with one another. 
     The root fittings may be arranged as a series around the root portion of the blade, each root fitting being interlocked with the next root fitting in the series. 
     In such an example, the second body portion of each root fitting may engage with the first body portion of that fitting, and with the first body portion of the next root fitting in the series. According to another aspect of the present invention, there is provided a root fitting means for attaching a rotor blade to a blade mounting, which rotor blade comprises a blade portion, and a root portion connected with, and extending from, the blade portion, the root portion having an annular cross section which defines first and second root surfaces, the root fitting means comprising a plurality of discrete root fittings adapted for connection with respective portions of the root portion of the rotor blade, and for attachment to a blade mounting, wherein each root fitting comprises an elongate body member which extends from a free end region towards a blade end region, which defines first and second bonding surfaces, adapted to be adhesively bonded to first and second root surfaces respectively of a rotor blade, and which includes means for attachment to a blade mounting. 
     The first and second bonding surfaces may be substantially parallel to one another. Alternatively, portions of the first and second bonding surfaces may converge towards the free end region. Alternatively, portions of the first and second bonding surfaces may diverge towards the free end region. 
     A root fitting embodying the present invention may be adapted to be bolted to the root portion of the blade. 
     The first and second bonding surfaces may be substantially smooth. Alternatively, at least one of the first and second bonding surfaces may be treated to increase bonding strength of an adhesive bonding medium. 
     The body member may comprise separate first and second body portions which define the first and second bonding surfaces respectively. 
     The first body portion may include an end part which has an inner surface, and an outer surface, and wherein the second body portion may include an end part which has an inner end surface extending parallel to, and adjacent, the outer surface of the first body portion. The end part of the first body portion may define a threaded hole for reception of a threaded stud therein, the threaded hole having an axis extending away from the root portion, and wherein the end part of the second body portion may define a hole coaxial with the threaded hole, such that a threaded stud received in the threaded hole extends from the end part of the first body portion, through the end part of the second body portion, in a direction away from the root portion of the blade. 
     The end part of the first body portion may define a hole having an axis extending away from the root portion, and the end part of the second body portion may define a hole coaxial with the hole through the first body portion, thereby allowing a fixing arrangement to extend through the first and second body portions of the elongate body member. 
     A root fitting means embodying an aspect of the present invention may further comprise a threaded component located between respective end regions of the first and second body portions. 
     The first and second body portions may have respective end regions, each end region defining at least one receiving feature, such that when the root fittings are arranged around the root portion of the blade to form a series, receiving features of neighbouring root fittings define at least one attachment aperture in which a fixing component is located, the fixing components having features for enabling attachment of the blade to a rotor. 
     The first and second body portions may have respective end regions, which define a receiving aperture therebetween, in which aperture an attachment component is located. The first and second body portions may have respective end regions which interengage with one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an underwater tidal power generation installation; 
         FIG. 2  illustrates a first cross sectional view of a rotor blade for use in the installation of  FIG. 1 ; 
         FIG. 3  illustrates a second cross sectional view of the blade of  FIG. 2 ; 
         FIG. 4  illustrates a previously considered root fitting for a rotor blade; 
         FIG. 5  illustrates principles of a root fitting embodying the present invention; 
         FIGS. 6 to 22  illustrate respective embodiments of the present invention; 
         FIGS. 22 to 25  illustrate respective features applicable to embodiments of the present invention; and 
         FIG. 26  illustrates a further embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 5  shows a cross sectional side view, and an end view of a rotor blade root and root fitting, and illustrates the principles underlying root fittings embodying the present invention. As described above a rotor blade comprises a blade portion and a root portion which extends from the blade portion. The root portion may be provided by an end region of a spar of the blade, which spar extends along the length of the blade, as is well known in blade design. Embodiments of the present invention are concerned with the design of root fitting which enables the blade to be attached to a blade mounting. In the following examples, the root portion of the blade is provided by a free end of the spar  30  of the rotor blade, but could be provided by any suitable part of the blade. 
     In the example shown in  FIG. 5 , the root portion  30  is annular in cross section. Alternatively, the root portion  30  could be of any tubular cross section, such as ovoid, square or rectangular, depending upon design considerations. A root fitting technique embodying the principles of the present invention comprises a plurality of discrete fittings  31 , which are connected at respective positions with the root portion  30 . For the sake of clarity, only four root fittings are illustrated in  FIG. 5 . It will be readily appreciated that a root fitting technique embodying the present invention can have any number of root fittings, and that, in practice, the number is likely to far exceed four. The precise number of fittings provided, however, does not affect the underlying principles of the present invention. Each root fitting  31  is bonded onto the end of the spar  30 , and is provided with means  32 ,  33  for attachment to a blade mounting (not shown for clarity). One example of such attachment means is a threaded hole  32 , and corresponding threaded stud  33 , the stud extending away from the root portion  30 . 
     In the following, only a single root fitting  31  is shown and described, only for the sake of clarity. It is to be understood that a plurality of such root fittings would be used in practice. A first embodiment of the present invention is shown in  FIG. 6 . In this example, the root portion  30  of the blade defines first and second root surfaces  35  and  36 . In this embodiment, the root surfaces  35  and  36  extend into the root portion, and converge, thereby forming a hollow V-shaped region. The root fitting  31  is provided by a body  100 , which could be a cast metallic part. The body  100  is shaped so as to fit into the V-shaped region of the root portion, and has first and second bonding surfaces  101  and  102 , which engage with the first and second root surfaces  35  and  36  respectively. The root portion  30  and the body  100  are held together by applying an adhesive medium to the root and bonding surfaces, such that those surfaces are bonded together. The root portion  30  and the body  100  may also be attached to one another using a bolt  104  in addition to the adhesive medium. The body  100  may be provided with a threaded hole  103  for receiving a threaded stud (not shown), for attachment to a blade mounting. Any suitable alternative fitting may be provided. The plurality of fittings is arranged around the end of the root portion  30 , so as to provide a plurality of attachment means (the threaded studs) for the rotor blade. 
       FIG. 7  illustrates a second embodiment of the present invention. As before, the spar of the rotor blade provides the root portion  30  to which the root fittings are connected. In the embodiment of  FIG. 7 , the root portion  30  tapers at its end, and has first and second root surfaces  35  and  36  that converge. A body  200 , which may be a cast metallic part, has first and second bonding surfaces  201  and  202  that are arranged to engage with the first and second root surfaces  35  and  36  respectively. The first bonding surface  201  is substantially parallel to the first root surface  35 , and the second bonding surface  202  is substantially parallel to the second root surface  36 . The body  200  is attached to the root portion  30  by the provision of an adhesive medium on the junction between the first and second root and bonding surfaces  35  and  201 ,  36  and  202 . In addition, one or more bolts  205  may be provided that extends through the body  200  and the root portion  30 . The bolt  205  serves to hold the adhesive medium in compression between the surfaces of the root portion and the body, and also serves to prevent splitting of that joint. The body  200  is provided with a threaded hole  203  for receiving a threaded stud (not shown) for attachment to a blade mounting. 
       FIG. 8  shows a third embodiment of the present invention, which is similar to the second embodiment described with reference to  FIG. 7 . In this third example, the root portion has first and second root surfaces  35  and  36  which have portions that taper towards one another. The fitting has a body  300 , which has bonding surfaces  301  and  302  that are substantially parallel to, and engage with, the root surfaces  35  and  36 . In this third example, the body  300  has a portion  30  which extends along the root portion  30  such that the first and second root surfaces and the first and second bonding surfaces extend beyond the tapered region, into a region where those surfaces are substantially parallel to one another. 
     An adhesive medium is located between the root surfaces and the bonding surfaces, in order to bond the body  300  to the root portion  30 . The extension of the root and bonding surfaces to a parallel region of the root portion  30  serves to reduce the risk of the spar delaminating. The body  300  can also be secured to the root portion  30  using one or more optional bolts  305  in addition to the adhesive medium. The body  300  is provided with a threaded hole  303  for receiving a threaded stud (not shown), or other appropriate means, for attachment to a blade mounting. 
     One disadvantage of the structure of the second and third embodiments illustrated in  FIGS. 7 and 8  respectively is that preparing the root and bonding surfaces is difficult, and not always reliable. This is because the V-shaped nature of the body  300 , preparing the bonding surfaces  301  and  302  reliably is complex, particularly at the point at which the surfaces converge. The shape can lead to the adhesive medium being forced out of the body, and therefore out of contact with the bonding and root surfaces, when it is applied to the surfaces during manufacture. 
       FIG. 9  illustrates a fourth embodiment of the present invention that improves on the third embodiment. In this fourth embodiment, the root portion  30  has first and second root surfaces  35  and  36  that converge to for a tapered free end. The root fitting of this embodiment comprises a two piece body, having a first body portion  400   a  and a second body portion  400   b . The first body portion extends along one side of the root portion  30 , and defines a first bonding surface  401  which is bonded to the first root surface  35  using a bonding medium. The first body portion extends  404   a  along the root portion  30  so as to overlap a region of the root portion in which the first and second root surfaces are substantially parallel to one another. 
     The second body portion extends along the other side of the root portion  30  to the first body portion  400   a , and defines a second bonding surface  401  which is bonded to the second root surface  35  using a bonding medium. The second body portion extends  404   b  along the root portion  30  so as to overlap a region of the root portion in which the first and second root surfaces are substantially parallel to one another. 
     The first and second body portions  400   a  and  400   b  are provided with respective threaded holes  403   a  and  403   b  for receiving threaded studs for attachment to a blade mounting. As before, an appropriate alternative attachment means may be provided. The number of threaded holed provided can be varied according to the design of the blade mounting. The body portions  400   a  and  400   b  can be held in place using either or both optional bolts  405  and  406 , which extend through both body portions so as to hold the portion together. The fourth embodiment illustrated in  FIG. 9  enables the first bonding surface  401  to be bonded to the first root surface  35  separately from the bonding of the second bonding surface  402  to the second root surface  36 . The body portions  400   a  and  400   b  can be bonded to the root portion  30  in any order. This separation of the bonding steps means that the surface preparation is a less complex operation and is also more reliable. The adhesive medium can be applied onto an “open” surface, which means that it is simple to coat the surface effectively. The body portions  400   a  and  400   b  can also be placed into contact with the root portion  30  in directions onto the root portion, rather than along the root portion  30 , and so the adhesive medium can remain in contact with the surfaces  35 ,  36 ,  401  and  402 .  FIG. 10  illustrates a fifth embodiment of the present invention in which a root fitting is attached to a free end of the root portion  30 . The free end of the root portion  30  has parallel first and second root surfaces  35  and  36 . A third root surface  37  extends between the first and second root surfaces  35  and  36 , and forms the end of the root portion  30 . 
     The root fitting comprises first and second body portions  500   a  and  500   b . The first body portion  500   a  extends from the end of the root portion  30  along the first root surface  35 , and has a first bonding surface  501  that is bonded to the first root surface  35 . This bond is achieved using an adhesive medium applied to one or both of the first root and bonding surfaces  35  and  501 . The first body portion has an end region  504   a  that tapers in towards the root portion  30 . 
     The first body portion  500   a  also includes an end part  505   a  which extends adjacent, and substantially parallel to, the third end surface of the root portion  30 . This end part  505   a  has an outer surface, and a threaded hole  503   a  for receiving a threaded stud for attachment to a blade mounting. 
     The second body portion  500   b  extends along the second root surface  36  from the end of the root portion  30 , and has a tapered end region  504   b . The second body portion  500   b  defines a second bonding surface  502  which is bonded to the second root surface  36  by way of an adhesive medium. The second body portion  500   b  also includes an end part  505   b  which extends adjacent, and substantially parallel to, the outer surface of the end part  505   a  of the first body portion  500   b . In this way, the end part  505   b  of the second body portion  500   b  can be seen to overlap the end part  505   a  of the first body portion  500   a . The end part  505   b  of the second body portion  500   b  is provided with a through hole  503   b  that is coaxial with the threaded hole  503   a  of the end part  505   a  of the first body portion  500   a , to allow a threaded stud to pass through, thereby enabling attachment with a blade mounting. As before, the threaded hole and stud arrangement may be replaced by any suitable means for attaching the fitting to the blade mounting. 
     It will be readily appreciated that the end parts  505   a  and  505   b  of the first and second body portions  500   a  and  500   b  could be reversed such that the end part  505   b  of the second body portion  500   b  is adjacent the third root surface  37 , and is provided with a threaded hole. The first and second body portions  500   a  and  500   b  can also be secured to the root portion  30  using one or more optional bolts  506 . The choice to use a plurality of bolts is dependent upon design criteria. 
       FIG. 11  illustrates a sixth embodiment of the present invention that comprises a two part fitting having first and second body portions  600 . The structure of the fitting is similar to that shown in  FIG. 10 ; first and second body portions  605   a  and  605   b  extend from the end of the root portion, and overlap at that end. The body portions are bonded to the root portion  30  using an adhesive medium applied between first and second root surfaces  35  and  36  and first and second bonding surfaces  601  and  602 . Threaded and through holes  603   a  and  603   b  are provided in the end parts  605   a  and  605   b  of the first and second body portions  600   a  and  600   b , as before, and could be replaced by any other suitable means for attachment to a blade mounting. In contrast to the embodiment shown in  FIG. 10 , the root portion  30  in this embodiment has root surfaces  35  and  36  that have portions that diverge towards the end of the root portion, so as to define an enlarged root portion end  38 . The body portions  600   a  and  600   b  are shaped accordingly so that the first bonding surface is substantially parallel to the first root surface, and so that the second bonding surface is substantially parallel to the second root surface. 
     It will be readily appreciated that the end region of the root portion  30  can be of any appropriate shape, and that the body or body portions of the fitting can be adjusted to suit the shape of the root portion  30 . 
       FIG. 12  illustrates another embodiment of the present invention, in which the root portion  30  defines root surfaces  35  and  36 , which converge towards the free end of the root portion, to an end region  37 , such that the root portion  30  tapers towards its free end. A two part fitting, comprising a first body portion  700   a  and a second body portion  700   b , is arranged to attach the blade to the rotor. The first body portion  700   a  defines a first bonding surface  701  that is bonded to the first root surface  35  of the root portion  30 , and the second body portion  700   b  defines a second bonding surface that is bonded to the second root surface  36  of the root portion  30 . The first body portion  700   a  has an end region  705   a  from which the body portion extends, and tapers towards the blade. The end region  705   a  defines a threaded hole or holes (not shown) for receiving a threaded fixing arrangement for attachment to the rotor. The second body portion  700   b  extends along the root portion  30  to the opposite side from the first body portion  700   b , and includes an end region  705   b  that extends around and overlaps the end region  705   a  of the first body portion  700   a . The end region  705   b  of the second body portion  700   b  defines a through hole or through holes  703   b , through which the fixing arrangement attached to the first body portion can pass. 
     As before, the threaded hole and stud arrangement may be replaced by any suitable means for attaching the fitting to the blade mounting. The first and second body portions  700   a  and  700   b  can also be secured to the root portion  30  using one or more optional bolts (not shown for the sake of clarity). The choice to use a plurality of bolts is dependent upon design criteria. In addition, or alternatively, one of more bolts can be used to secure together the end regions  705   a  and  705   b.    
     The fitting of  FIG. 12  can be attached to the root portion  30  of the blade as two parts that have “open” surfaces for receiving the bonding agent. This arrangement enables a reliable bond to be achieved, since the bonding agent can be evenly applied, and the part can be held in place relatively straightforwardly. 
       FIG. 13  illustrates another embodiment of the present invention, in which the root portion  30  defines root surfaces  35  and  36  which diverge and then converge in a direction towards the free end of the root portion  30 . This construction is mirrored in the fitting which has a corresponding shape to provide bonding surfaces substantially parallel to the root surfaces  35  and  36 . The fitting is a two part fitting, having body portions  910   a  and  910   b , and can include the features and characteristics of the earlier described embodiments. 
       FIG. 14  illustrates another embodiment of the present invention which makes use of a two part fitting. In the  FIG. 14  example, the root portion  30  of the blade defines first and second root surfaces  35  and  36  that converge towards the free end of the root portion  30 , so that the root portion tapers. The fitting comprises first and second body portion  900   a  and  900   b , which define first and second bonding surfaces  901  and  902  respectively. The first and second bonding surfaces  901  and  902  are bonded with the first and second root surfaces  35  and  36  respectively. The first body portion  900   a  has an end region  905   a , from which the body portion  900   a  extends, tapering along the root portion  30  towards the blade. Similarly, the second body portion  900   b  has and end region  905   b , from which that body portion  900   b  extends, tapering along the root portion  30  towards the blade. 
     The end regions  905   a  and  905   b  engage with one another along an inner surface, which in this example is along the centre line of the fitting, but which can be arranged as appropriate. The end regions are held in contact with one another by way of a number of bolts  906 . The number of bolts shown in  FIG. 14  is exemplary only, and any number of bolts could be used. The fitting of  FIG. 14  can be attached to the root portion  30  of the blade as two parts that have “open” surfaces for receiving the bonding agent, as for the other two part fittings described above. This arrangement enables a reliable bond to be achieved, since the bonding agent can be evenly applied, and the part can be held in place relatively straightforwardly. 
     The embodiment of  FIG. 14  also includes a threaded component  910 , which is shown in more detail in  FIG. 15 .  FIG. 15  highlights details from box A of  FIG. 14 . The threaded component  910  is cylindrical in nature, and is located in correspondingly shaped cavities in the first and second body portions  900   a  and  900   b . The threaded component  910  has a cylindrical body  912 , an inner surface of which is threaded for reception and retention of a threaded fixing arrangement for attaching the fitting to the rotor. A number of raised ridge features  911  are defined on an outer surface of the cylindrical body  912 , and engage with corresponding features in the first and second body portions  900   a  and  900   b . The number of raised ridge features is arbitrary and to be decided by design requirements. In addition to being retained by the ridge feature  911 , the threaded fitting  910  may be bonded to one or both of the end regions  905   a  and  905   b  of the first and second body portions  900   a  and  900   b . The threaded component  910  is preferably a cast metallic part, but can be of any suitable material. 
       FIG. 16  illustrates another embodiment of the present invention which makes use of a two part fitting. In  FIG. 16 , three identical fittings  1000   i ,  1000   ii  and  1000   iii  are illustrated. The fittings are arranged around the root portion of the blade, and only a part of the overall fitting is shown in  FIG. 16 . The description below applies to each of the fittings shown in  FIG. 16 ; the reference numerals have been used sparingly, however, for the sake of clarity of the drawing. 
     In the  FIG. 16  example, the root portion  30  of the blade defines first and second root surfaces  35  and  36  that converge towards the free end of the root portion  30 , so that the root portion tapers. The fitting comprises first and second body portion  1000   a  and  1000   b , which define first and second bonding surfaces  1001  and  1002  respectively. The first and second bonding surfaces  1001  and  1002  are bonded with the first and second root surfaces  35  and  36  respectively. The first body portion  1000   a  has an end region  1005   a , from which the body portion  1000   a  extends, tapering along the root portion  30  towards the blade. Similarly, the second body portion  1000   b  has and end region  1005   b , from which that body portion  1000   b  extends, tapering along the root portion  30  towards the blade. 
     The end regions  1005   a  and  1005   b  engage with one another along an inner surface, which in this example is along the centre line of the fitting, but which can be arranged as appropriate. The fitting of  FIG. 16  can be attached to the root portion  30  of the blade as two parts that have “open” surfaces for receiving the bonding agent, as for the other two part fittings described above. This arrangement enables a reliable bond to be achieved, since the bonding agent can be evenly applied, and the part can be held in place relatively straightforwardly. 
     The end regions  1005   a  and  1005   b  define a receiving feature  1006  on each of two radial surfaces of the regions. The radial surface is the surface of the end region which is positioned adjacent the next fitting in the series around the root portion of the blade. The receiving features  1006  of neighbouring fittings combine to define an aperture which receives a connection part  1007 . The aperture and connection part  1007  have complementary engaging shapes, and increase in width in a direction towards the blade, away from the rotor. In this way the connection part  1007  is held in place between the receiving features of the neighbouring body portions of the fitting. The connection part  1007  may also be bonded in place. 
     The connection part  1007  is preferably a cast metallic part, but could be of any other suitable material. The connection part  1007  defines at least one threaded hole (not shown) for reception of a fixing arrangement for attachment to the rotor. 
       FIG. 17  illustrates another embodiment of the present invention which makes use of a two part fitting. In  FIG. 17 , as in  FIG. 16 , three identical fittings  1100   i ,  1100   ii  and  1100   iii  are illustrated. The fittings are arranged around the root portion of the blade, and only a part of the overall fitting is shown in  FIG. 17 . The description below applies to each of the fittings shown in  FIG. 17 ; the reference numerals have been used sparingly, however, for the sake of clarity of the drawing. 
     In the  FIG. 17  example, the root portion  30  of the blade defines first and second root surfaces  35  and  36  that converge towards the free end of the root portion  30 , so that the root portion tapers. The fitting comprises first and second body portion  1100   a  and  1100   b , which define first and second bonding surfaces  1101  and  1102  respectively. The first and second bonding surfaces  1101  and  1102  are bonded with the first and second root surfaces  35  and  36  respectively. The first body portion  1100   a  has an end region  1105   a , from which the body portion  1100   a  extends, tapering along the root portion  30  towards the blade. Similarly, the second body portion  1100   b  has and end region  1105   b , from which that body portion  1100   b  extends, tapering along the root portion  30  towards the blade. 
     The end regions  1105   a  and  1105   b  engage with one another along an inner surface, which in this example is along the centre line of the fitting, but which can be arranged as appropriate. The fitting of  FIG. 17  can be attached to the root portion  30  of the blade as two parts that have “open” surfaces for receiving the bonding agent, as for the other two part fittings described above. This arrangement enables a reliable bond to be achieved, since the bonding agent can be evenly applied, and the part can be held in place relatively straightforwardly. 
     The end regions  1105   a  and  1105   b  define a receiving feature  1106  on each of two radial surfaces of the regions. The radial surface is the surface of the end region which is positioned adjacent the next fitting in the series around the root portion of the blade. The receiving features  1106  of neighbouring fittings combine to define an aperture which receives a connection part  1107 . In the  FIG. 17  example, the receiving feature  1106  has a stepped profile, and the connection part  1107  has a complementary shape to engage with the stepped profile of the feature  1106 . In this way the connection part  1107  is held in place between the receiving features  1106  of the neighbouring body portions of the fitting. The connection part  1107  may also be bonded in place. 
     The connection part  1107  is preferably a cast metallic part, but could be of any other suitable material. The connection part  1107  defines at least one threaded hole (not shown) for reception of a fixing arrangement for attachment to the rotor. 
       FIG. 18  illustrates another embodiment of the present invention which makes use of a two part fitting. In the  FIG. 18  example, the root portion  30  of the blade defines first and second root surfaces  35  and  36  that converge towards the free end of the root portion  30 , so that the root portion tapers. The fitting comprises first and second body portion  1200   a  and  1200   b , which define first and second bonding surfaces  1201  and  1202  respectively. The first and second bonding surfaces  1201  and  1202  are bonded with the first and second root surfaces  35  and  36  respectively. The first body portion  1200   a  has an end region  1205   a , from which the body portion  1200   a  extends, tapering along the root portion  30  towards the blade. Similarly, the second body portion  1200   b  has and end region  1205   b , from which that body portion  1200   b  extends, tapering along the root portion  30  towards the blade. 
     The end regions  1205   a  and  1205   b  engage with one another along an inner surface, which in this example is along the centre line of the fitting, but which can be arranged as appropriate. The end regions may held in contact with one another by way of a number of bolts (not shown). The fitting of  FIG. 18  can be attached to the root portion  30  of the blade as two parts that have “open” surfaces for receiving the bonding agent, as for the other two part fittings described above. This arrangement enables a reliable bond to be achieved, since the bonding agent can be evenly applied, and the part can be held in place relatively straightforwardly. 
     The embodiment of  FIG. 18  also includes a threaded component  1210 , which is shown in more detail in  FIG. 19 .  FIG. 19  highlights details from box B of  FIG. 18 . The threaded component  1210  defines at least one threaded hole  1211  for reception of a fixing arrangement for attachment to the rotor. The end regions  1205   a  and  1205   b  of the body portions  1200   a  and  1200   b  define apertures therethrough for reception of the threaded component  1210 . In the example shown in  FIGS. 18 and 19 , the threaded component extends radially with respect to the root portion of the blade, and has a constant cross section; in this case a lozenge shaped cross section. The apertures defined in the end regions  1205   a  and  1205   b  correspond to the cross section of the threaded component  1210  such that it is held in place between the first and second body portions  1200   a  and  1200   b . In addition, the apertures in the end regions extend completely through the end regions. The threaded component can have any appropriate cross section, and may be bonded in place in the end regions  1205   a  and  1205   b.    
     The end regions  1205   a  and  1205   b  also define a through hole  1203  through which the fixing arrangement may pass. 
       FIGS. 20 and 21  illustrate a further embodiment of the invention which is similar in construction to that shown in  FIGS. 18 and 19 .  FIG. 21  highlights details from box C of  FIG. 20 . The  FIGS. 20 and 21  embodiment has a pair of body portions  1300   a  and  1300   b , having respective end regions  1305   a  and  1305   b  which define apertures for reception and retention of a threaded component  1310 . As in  FIGS. 18 and 19 , the embodiment shown in  FIGS. 20 and 21  makes use of a threaded component having a constant lozenge shaped cross section. The cross section can be any suitable shape. The threaded component  1310  extends radially with respect to the root portion of the blade. In contrast to the previous embodiment, the apertures defined by the end regions  1305   a  and  1305   b  do not extend completely through the end regions. One other possibility is for one of the end regions to have apertures that extend completely therethrough, and for the other body portion to have an aperture that does not. 
       FIG. 22  illustrates another embodiment of the present invention which makes use of a two part fitting. In the  FIG. 22  example, the root portion  30  of the blade defines first and second root surfaces  35  and  36  that converge towards the free end of the root portion  30 , so that the root portion tapers. The fitting comprises first and second body portion  1400   a  and  1200   b , which define first and second bonding surfaces  1401  and  1402  respectively. The first and second bonding surfaces  1401  and  1402  are bonded with the first and second root surfaces  35  and  36  respectively. The first body portion  1400   a  has an end region  1405   a , from which the body portion  1400   a  extends. Similarly, the second body portion  1400   b  has and end region  1405   b , from which that body portion  1400   b  extends. 
     The fitting of  FIG. 22  can be attached to the root portion  30  of the blade as two parts that have “open” surfaces for receiving the bonding agent, as for the other two part fittings described above. This arrangement enables a reliable bond to be achieved, since the bonding agent can be evenly applied, and the part can be held in place relatively straightforwardly. 
     The end regions  1405   a  and  1405   b  have complementary engaging shapes, in the case of  FIG. 22 , complementary curved shapes: the first end region  1305   a  has pair of protruding curved portions that define a corresponding curved aperture therebetween. The second end region  1405   b  has similar protruding curved portion which engages with the curved aperture of the first end region  1405   a . When in place on the root portion of a blade, a series of the first body portions form a series of such curved apertures for reception of a series of the second body portions. 
     In an alternative example, the end region of the second body portion  1400   b  may also have a pair of protruding curved portions. In that case, a second body portion  1400   b  would engage with two neighbouring first body portions, to provide an interlocked serried of fittings. 
       FIG. 23  shows an end view of a plurality of root fittings  1500  in place on a root portion  30 . The cross section of each of the root fittings shown in  FIG. 23  is sector shaped, so that the fittings fit around the annular root portion  30  of the blade. This sector shaped cross section is applicable to any of the embodiments of the invention described above. As an alternative, the fitting may have a square or rectangular cross section.  FIG. 23  also illustrates another feature that is applicable to any of the embodiments described. This additional feature is a locating surface  1509  provided on one or more of the fittings  1500 ′ to engage with a complementary feature on the blade mounting, in order to ensure correct alignment of the blade. 
       FIG. 24  illustrates an additional feature that is applicable to any of the embodiments of the present invention described above. The body (or one or both of the body portions in the case of the two part fitting) is provided with means for interlocking with adjacent fittings. The fittings  1600  are arranged around the end of the root portion  30  in a series, and each fitting is interlocked with the next fitting in the series.  FIG. 24  illustrates one possible way to achieve interlocking, which is to provide the fitting  1600  with an interlocking portion  1607  with extends out from the fitting  1600  towards the next fitting in the series, and interlocks with a corresponding recess  1608  in the next fitting  1600 . By interlocking the fittings, it is possible to increase the loads that can be carried by a given size of fitting. 
       FIG. 25  is a cross sectional end view illustrating a technique for interlocking the two part embodiments described above. The first body portions  1700   a  are arranged around the outer surface of the annular root portion  30  in a series, as before. The second body portions  1700   b  are arranged in a series around the inner surface of the root portion  30 . The second body portions  1700   b  are offset with respect to the first body portions  1700   a , such that any one of the second body portions  1700   b  overlaps, and is attached to, two of the first body portions  1700   a . The first and second body portions may be attached to one another by the use of an appropriate number of bolts  1706 . Threaded holes for receiving threaded studs can be provided in appropriate positions in the first and/or second body portions  1700   a  and  1700   b . This overlapping interlocking serves to improve the strength of the root fitting as a whole. 
     The first and second root surfaces and/or the first and second bonding surfaces can be smooth, or unfinished. Alternatively, the first and second root surfaces and/or the first and second bonding surfaces can be finished in order to increase the bonding strength of the adhesive medium. For example, the surfaces may be provided with a high roughness surface treatment, or may be grooved or serrated. 
     It will be readily appreciated that the references above to first and second body portions is made for the sake of clarity, and that the body portions can be arranged with the first portion either inside or outside the root portion of the blade, with the second body portion in the corresponding opposite position. 
       FIG. 26  illustrates a cross sectional view of another embodiment of the present invention, in which the root portion  30  defines root surfaces  35  and  36 , which converge towards the free end of the root portion, to an end region  37 , such that the root portion  30  tapers towards its free end. A two part fitting, comprising a first body portion  1800   a  and a second body portion  1800   b , is arranged to attach the blade to the rotor. The first body portion  1800   a  defines a first bonding surface  1801  that is bonded to the first root surface  35  of the root portion  30 , and the second body portion  1800   b  defines a second bonding surface  1802  that is bonded to the second root surface  36  of the root portion  30 . The first body portion  1800   a  has an end region  1805   a  from which the body portion extends, and tapers towards the blade. The end region  1805   a  defines a hole for receiving a fixing arrangement for attachment to the rotor, as will be described in more detail below. The second body portion  1800   b  extends along the root portion  30  to the opposite side from the first body portion  1800   b , and includes an end region  1805   b  that extends around and overlaps the end region  1805   a  of the first body portion  1800   a . The end region  1805   b  of the second body portion  1800   b  defines a through hole  1803   b , through which the fixing arrangement can pass. As described with reference to other embodiments, the root portion  30  may also be bolted to the root fitting using any number of suitable bolts that extend through the first and second body portions  1800   a  and  1800   b  of the root fitting, and through the root portion  30 . 
     The fitting of  FIG. 26 , in similar manner to the embodiment of  FIG. 12 , can be attached to the root portion  30  of the blade as two parts that have “open” surfaces for receiving the bonding agent. This arrangement enables a reliable bond to be achieved, since the bonding agent can be evenly applied, and the part can be held in place relatively straightforwardly. In the  FIG. 26  embodiment, the root fitting is attached to the rotor using a bolt that passes through the holes  1803   a  and  1803  of the root body portions  1800   a  and  1800   b , in a direction indicated by the arrow X in the Figure. Such a bolt has a threaded end that locates in a threaded hole defined in the rotor, and so this arrangement allows the blade to be attached to the rotor from a position external to the rotor housing. The fixing bolt has a head which is accessible from the blade side of the root fitting, thereby enabling a tool to be used externally of the rotor housing to tighten the fixing arrangement. Such a configuration enables more straightforward fitting and removal of the blade from the rotor. 
     In order that the bolt head is accessible externally, the direction X in which the bolt is inserted diverges from the direction in which the root portion extends from the root fitting (as indicated by the arrow Y in  FIG. 26 ). In addition at least one strengthening rib  1807  is provided in order to provide the fitting with the required stiffness and strength. In order to enable the fitting and bolt arrangement to align appropriately, a self aligning washer  1809  can be provided at the interface of the body portions  1800   a  and  1800   b  in line with the holes  1803   a  and  1803   b . The washer  1809  can be spherical washer. 
     A compression tube  1811  can also be provided in the hole  1803   a  of the first body portion  1800   a , so that the bolt passing therethrough can be tensioned to the correct level. 
     As an alternative to the bolts passing through the holes  1803   a  and  1803   b  into the rotor hub, studs that project from the rotor hub can be arranged to extend through the holes. The studs then have threaded end portions that emerge from the root fitting to enable fixing nuts to be attached to the studs. The nuts can be tightened from a position external to the rotor housing. It will be appreciated that any suitable fixing arrangement can be used to attach the root fitting to the rotor hub, such an arrangement utilising the external access feature described above.