Abstract:
Disclosed is a turbine blade for a steam turbine, comprising an aerofoil section and a root section and is characterized according to the invention in particular in that the aerofoil section is designed for use in a low-pressure stage of the steam turbine and contains fiber composite materials at least in regions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2006/067923, filed Oct. 30, 2006 and claims the benefit thereof. The International Application claims the benefits of European application No. 05025359.0 filed Nov. 21, 2005, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a turbine blade for a steam turbine, with an aerofoil section and with a root section, which aerofoil section contains, at least in regions, a composite fiber material, the aerofoil section having a filling body which is arranged in the aerofoil center which is surrounded completely by the composite fiber material. The invention relates, furthermore, to a steam turbine having a turbine blade of this type. 
       BACKGROUND OF THE INVENTION 
       [0003]    Turbine blades of this type, in particular turbine blades of this type designed as moving blades, are manufactured in the prior art from steel or titanium. Turbine blades in general and particularly end-stage blades are exposed as a consequence of their function to high centrifugal force stresses, since, to achieve high efficiency, they should constitute as high a flow-off surface as possible and consequently must possess a large blade length. High-strength steels are therefore used for conventional applications. Where these can no longer be employed for reasons of centrifugal force stresses, titanium blades are used, which, because of the lower density, also experience lower centrifugal force stresses. However, these blades are substantially more cost-intensive than steel blades. However, even where titanium blades are concerned, the flow-off surfaces are limited to approximately 16 m 2  for full-revolution machines (50 Hz), thus entailing corresponding consequences for the blade lengths which can be achieved. 
         [0004]    On account of the practical limitation with regard to the blade length, in the prior art the number of low-pressure streams is increased in low-pressure stages of steam turbines. This may take place, for example, by a change from single-stream to two-stream turbine stages or by using a plurality of low-pressure subturbines. The rotational speed of the turbo set may also be reduced. In this case, larger flow-off surfaces can then be utilized. However, all these measures entail sometimes considerable costs. 
       SUMMARY OF INVENTION 
       [0005]    An object on which the invention is based is to provide a steam turbine with a turbine blade of the type initially mentioned, which makes it possible to have a particularly high efficiency of the steam turbine and at the same time can be operated in the steam turbine in an operationally reliable way. 
         [0006]    EP A1 462 606, U.S. Pat. No. 3,883,267, U.S. Pat. No. 5,240,377, FR A 1 178 140 and EP A1 593 811 disclose in each case a turbine blade of the type initially mentioned. In these blades, the composite fiber material used or the corresponding layer consisting of it around the filling body is partially protected against erosion, but not against penetrating moisture, and this may cause damage particularly in the wet steam region. Measures for protection against erosion are also known from EP A1 577 422 and DE 24 50 253 A1. A complete blade consisting of composite fiber material and having a protective layer applied by electroplating is already known from DE 22 43 132 A1. 
         [0007]    This object is achieved, according to the invention, by means of a generic turbine blade which has the features of the claims. Furthermore, the object is achieved, according to the invention, by means of a steam turbine according to the claims. The subclaims which in each case refer back contain advantageous developments of the invention. 
         [0008]    According to the invention, therefore, composite fiber blades are used as low-pressure stage or end stage blades. In a comparison of various materials in terms of strengths, the advantage of composite fiber materials for use as end stage blade material is clearly shown. Thus, the strength in relation to the density (R p0,2 /ρ) is 115 m 2 /s 2  for high-strength tempered steel, 221 m 2 /s 2  for titanium, but 563 m 2 /s 2  for the fiber-reinforced material CFK-HM. On account of the substantially higher strength of the composite fiber material, either turbine blades manufactured with conventional dimensions can be exposed to higher load or the turbine blades can be produced with a greater length. The centrifugal force stresses which in this case arise can then be readily absorbed by the turbine blade, without any loss of operational reliability, on account of the substantially increased strength/density ratio. 
         [0009]    Owing to the high strength/density ratio of a turbine blade containing composite fiber material according to the invention, a greatly enlarged flow-off surface can be provided by the aerofoil section being designed for use in a low-pressure stage of the steam turbine in spite of the high centrifugal force stresses. This may take place, particularly, by the provision of a particularly great blade length. 
         [0010]    The efficiency of the steam turbine can consequently be increased considerably. 
         [0011]    In the sector of industrial turbines, for example, the use according to the invention of the composite fiber material makes it possible for turbine blade of predetermined dimension to be exposed to high load by allowing a higher counter pressure of the end stages (air condensation), by a higher permissible rotational speed of the drive turbines or by enlarging the end stage blades for drives of variable rotational speed. This likewise results in a higher efficiency of the steam turbine. 
         [0012]    As already mentioned, for the sector of power station turbines, there is the potential for a very considerable lengthening of existing end stage blades, together with a substantial enlargement of the flow-off surfaces which can be achieved. For example, it is been possible hitherto for turbo sets of half-revolution design and having flow-off surfaces of 20 m 2  per stream to be replaced with the aid of the turbine blades according to the invention by full-revolution turbo sets of identical flow-off surface. On account of the smaller overall size of full-revolution turbo sets, considerable cost saving becomes possible. Also, by the turbine blades according to the invention being used, the number of low-pressure streams can be reduced. The multi-stream power station applications, for example, one of three lower-pressure parts can be saved. Also, two-stream lower-pressure turbines can be replaced by single-stream machines, with the result that considerable cost savings can likewise be achieved. In addition, by means of the solution according to the invention, in any event a reduction in the plant overall size, with the flow-off cross section remaining the same, can be achieved. 
         [0013]    The turbine blade according to the invention is particularly suitable for the last moving blade row of a steam turbine, but may likewise be used, according to the invention, for the last but two and, if appropriate, last but three blade row. It may likewise be combined with preliminary stage blades consisting of steel or titanium. 
         [0014]    The aerofoil section containing, according to the invention, at least in regions, a composite fiber material, of the turbine blade according to the invention has the composite fiber material preferably at least in the outer wall region. Advantageously, the entire aerofoil section may also consist of composite fiber material. Furthermore, advantageously, in an aerofoil section which becomes more slender toward the aerofoil tip, the number of fibers in the longitudinal direction of the aerofoil section decreases. 
         [0015]    The abovementioned object is achieved according to the invention, furthermore, by means of a generic turbine blade in which the aerofoil section contains, at least in regions, a composite fiber material, at least the region which contains the composite fiber material being surrounded by a deformable moisture-impermeable protective layer which prevents the penetration of moisture into the composite fiber material during the operation of the turbine blade. Furthermore, the object is achieved by means of a steam turbine which is provided with a turbine blade of this type. 
         [0016]    Consequently, moisture absorption by the aerofoil section can be effectively prevented during operation in the steam turbine. Moisture absorption is an undesirable time-dependent process which may cause an increase in weight of the component and consequently a potential unbalance of the rotor. Moreover, moisture absorption of this type may give rise to a deformation of the composite fiber material and, under continuous action, to damage to the matrix and consequently to a failure of the component containing the composite fiber material. By a moisture-impermeable protective layer being provided according to the invention, the consequences listed above and putting the operational reliability of the steam turbine at risk are avoided. So that the protective layer undergoes the expected deformations of the basic material of the aerofoil section without damage or loss of its sealing function, the protective layer according to the invention is designed to be deformable. In this case, according to the invention, the protective layer is designed so as to be deformable. The protective layer does not lose its moisture impermeability over its useful life in spite of deformations, occurring during the operation of the blade, of that region of the aerofoil section which contains the composite fiber material. This may be achieved, in particular, in that the protection layer has an elastic insert region which overshoots the utilized expansion region of the basic material. In addition to the higher steam turbine efficiency made possible by the use according to the invention of the composite fiber material in the aerofoil section, the embodiment according to the invention of the turbine blade can be employed in a particularly operationally reliable way owing to the protective layer according to the invention which is moisture-impermeable. 
         [0017]    Advantageously, the moisture-repelling protective layer surrounds the aerofoil section completely. Furthermore, it may also be expedient if the protective layer surrounds the entire turbine blade, that is to say even the blade root. In an embodiment which is advantageous according to the invention, the protective layer should be configured in such a way that a reliable adhesion of the protective layer is afforded even under the impact of drops. Furthermore, the design of the basic material of the aerofoil section should be such that continuing drop impacts cause no fatigue or spoiling of the basic material. 
         [0018]    Furthermore, the abovementioned object is achieved, according to the invention, by means of a generic turbine blade in which both the aerofoil section and the root section in each case contain, at least in regions, a composite fiber material. Moreover, the object is achieved by means of a steam turbine which is provided with a turbine blade of this type. 
         [0019]    As already mentioned above, by composite fiber material being used in the aerofoil section, the turbine blade can be configured with a large flow-off surface on account of the low density of the composite fiber material. 
         [0020]    This increases the efficiency of the steam turbine. Furthermore, owing to the simultaneous use of composite fiber material in the root section of the turbine blade, a correspondingly secure and reliable anchoring of the turbine blade in the rotor shaft of the steam turbine can be ensured. Thus, in particular, fibers of the composite fiber material can be led continuously through the aerofoil section and the root section, so that the aerofoil section and the root section make a stable connection and a break-off of the aerofoil section during the operation of the turbine blade can be effectively avoided even when higher forces occur. The operation reliability of the turbine blade during operation is consequently ensured. 
         [0021]    In order to ensure the fracture safety of the components containing the composite fiber material, the composite fiber material advantageously contains glass fibers, synthetic fibers, such as, for example, aramide fibers, and/or synthetic fibers. In particular, fiber-reinforced material CFK-HM may be used as composite fiber material. 
         [0022]    In a further advantageous embodiment, the composite fiber material has fibers which are routed in the region of the aerofoil section at an angle deviating from a main axis of the turbine blade, in particular at the angle ±15°, ±−30° and/or ±45° with respect to the main axis. A high torsion resistance of the aerofoil section is consequently achieved. The composite fiber plies may be arranged mirror-symmetrically with respect to the aerofoil center surface, with the result that distortion is avoided. 
         [0023]    By contrast, an asymmetric arrangement leads to distortion. This may be utilized in an advantageous alternative embodiment, if appropriate, for self-setting purposes. Owing to the nature of the arrangement of such fibers or plies, anisotropy may also be utilized, within a limited range, to achieve a directed change of the blade geometry as a function of the operational stresses. 
         [0024]    In this respect, distortion of this type may be provided, in which the blade cascade opens in the case of rotational overspeeds, so that less energy is extracted from the flow and therefore does not contribute to further run-up. The distortion may likewise be utilized for setting an optimized flow profile as a function of the flow and of the load. Thus, for example, the blade cascade can be closed in the case of a lower throughflow and be opened correspondingly in the case of a higher throughflow. 
         [0025]    In order to achieve an optimization of the blade in terms of cost and of rigidity, it is expedient if the aerofoil section has a filling body which is arranged in the aerofoil center and which is surrounded completely by the composite fiber material. 
         [0026]    So that the functioning of the deformable moisture-impermeable protective layer surrounding the region having the composite fiber material can be monitored and in order to rule out a failure of the aerofoil section, it is expedient if an electrically conductive layer is arranged below the protective layer. This electrically conductive layer serves as a warning mechanism, by means of which damage to the protective layer can be detected, whereupon counter measures, such as, for example, a replacement or exchange of the affected component, or a repair of the protective layer can be carried out in due time. An electrically conductive layer of this type may be provided either individually or in pairs with an insulation layer lying between them. 
         [0027]    In the latter instance, for the layer build-up of the aerofoil section, there is in the surface region of the latter a successive arrangement of the composite fiber material, of an electrically conductive, in particular metallic layer, of an insulation layer, of a further electrically conductive, in particular metallic layer and of the protective layer. To monitor the functioning of the protective layer, the insulation resistance with the respect to the surroundings or between the two electrically conductive layers can then be measured. Also, the electrical capacitance of the arrangement comprising the electrically conductive layer, the insulation layer and the further electrically conductive layer can be measured in order to monitor the functioning of the protective layer. If only one electrically conductive layer is provided, it is accordingly appropriate to measure the insulation resistance with respect to the surroundings or the electrical resistance of the electrically conductive layer in order to monitor the functioning of the protective layer. 
         [0028]    In a further advantageous embodiment, below the protective layer, water-soluble chemical substances are arranged, which are detectable in dissolved form, in particular chemically, optically and/or radiologically. This measure constitutes an alternative possibility for monitoring the functioning of the protective layer. Thus, for example, the condensate of the water/steam circuit of the steam power station can be checked continuously. If the chemical substances arranged below the protective layer can be detected in it, this indicates damage to the protective layer. 
         [0029]    In a further expedient embodiment, an onflow edge of the turbine blade is provided with edge reinforcement for protection against drop impacts. Such edge reinforcement may be provided by gluing onto the turbine blade or by laminating into the turbine blade. Also, such edge reinforcement may be produced by means of a thickened protective or intermediate layer. Furthermore, it is possible to thicken the protective layer correspondingly or to glue on or embed an additional protective component. Also, the basic component of the turbine blade itself may be configured with a turbine-like edge reinforcement. Alternatively, protection against drop impacts may be achieved by means of a laminate build-up of the turbine blade in which the fibers run in the transverse direction. 
         [0030]    Furthermore, it is expedient if the root section of the turbine blade has a contact element for making contact with a blade root mounting in a rotor shaft of a steam turbine, the contact element containing composite fiber material and/or a metallic material. The contact element may consist selectively of composite fiber material or of metallic materials. The corresponding metallic materials should be selected such that they allow a load-bearing and dimensionally stable connection with the rotor shaft and prevent an overstressing of that composite fiber material of the blade root which surrounds the contact element. In particular, the contact element may be formed by a metallic sleeve. If the above-described deformable moisture-permeable protective layer is provided, this should advantageously be specially reinforced in the root region, particularly in the contact region, or be protected against damage by means of protective elements. 
         [0031]    In a particularly advantageous embodiment, the root section has a deflection element, by means of which a substantial number of fibers of the aerofoil are deflected, and/or a guide element, by means of which an advantageous fiber routing in the blade root is diverted into a fiber routing adapted to the geometry of the aerofoil section. The deflection element and/or the guide element may also in each case consist of composite fiber material or of a metallic material. In particular, the contact element and the guide element or the contact element and the deflection element may in each case be formed by the same element. 
         [0032]    Advantageously, furthermore, the root section is designed as a plug root which can be plugged into a blade root mounting of a rotor shaft of the turbine in a direction which is radial with respect to the rotor shaft. Expediently, in this case, the fibers of the composite fiber material are led around sleeves serving as contact elements. Moreover, advantageously, in a plug root of this type the aerofoil curvature can be copied in the root region by assignment to different pin positions of the plug root, so as advantageously to give rise to low deflections from the root region to the aerofoil region. 
         [0033]    The outlay in terms of guide elements consequently remains restricted. 
         [0034]    In an advantageous embodiment, the deformable moisture-impermeable protective layer also surrounds the root section. A penetration of moisture even into the composite fiber material contained in the root section is consequently effectively prevented. The useful life of the turbine blade can thereby be further increased. 
         [0035]    In a further advantageous embodiment, the root section of the turbine blade is designed as a sliding root which can be pushed into a blade root mounting of a rotor shaft of the turbine in a direction which is essentially axial with respect to the rotor shaft. An essentially axial direction is to be understood as meaning that the push-in direction may deviate from the axial direction by up to ±40°. In particular, the root section has a curved configuration, the root curvature following essentially that curvature of the aerofoil section which prevails in the vicinity of the root. The transmission of force to blade slots is achieved by means of deflection and contact elements. Contact elements may also perform the function of guide elements. The outlay in terms of guide elements is consequently minimized. 
         [0036]    In an advantageous embodiment of the steam turbine according to the invention, the latter has a device for observing the oscillation behavior of the turbine blade. A change in the characteristic frequency of the turbine blade can consequently be recognized, and this may be attributable to moisture absorption by the composite fiber material in the aerofoil section during the operation of the steam turbine. Such a change in the characteristic frequency of the turbine blade should then give cause for checking the functionality of the abovementioned deformable moisture-impermeable protective layer and, if appropriate, for repairing the protective layer, so that a failure of the component can be prevented. 
         [0037]    In a further advantageous embodiment, the steam turbine has at least one heatable guide vane. Owing to heating, moisture on the guide vane can be evaporated, and corresponding damage to other turbine blades by drop impacts can be prevented. Alternatively, a device for sucking away wetness may also be provided on at least one guide vane. 
         [0038]    The production of the composite fiber blades preferably takes place by means of the conventional methods, in which fibers are wound and impregnated with the matrix material or applied in the form of what are known as prepregs. They are then brought into their final form in what is known as a die, a curing of the matrix also taking place. For this purpose, contact, deflection or guide elements are optionally also introduced or applied at this early stage. It may then be necessary to machine the blades at specific locations, for example by grinding, for example in order to achieve the required dimensional stability, tolerance adherence and surface quality. Also, already mounted contact, deflection or guide elements may be machined or these elements may be applied after the shaping operation. As already mentioned above, furthermore, edge protection may be mounted, which is integrated into the blade profile by means of subsequent adapting work, such as, for example, by grinding. Afterwards, coating with the layers required for the protective layer and for the warning system is carried out. In this case, individual layers may be designed to be reinforced at specific locations in order to improve protective or reinforcing functions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    Exemplary embodiments of a turbine blade according to the invention are explained in more detail below with reference to the accompanying diagrammatic drawings in which: 
           [0040]      FIG. 1  shows a view of a first exemplary embodiment of a turbine blade according to the invention, 
           [0041]      FIG. 2  shows the section II-II according to  FIG. 1 , 
           [0042]      FIG. 2   a  shows a first embodiment of the detail X according to  FIG. 2 , 
           [0043]      FIG. 2   b  shows a second embodiment of the detail X according to  FIG. 2 , 
           [0044]      FIG. 2   c  shows the detail Y according to  FIG. 2 , 
           [0045]      FIG. 3   a  shows a part view of a second exemplary embodiment of the turbine blade according to the invention, 
           [0046]      FIG. 3   b  shows the section III-III according to  FIG. 3   a,    
           [0047]      FIG. 4   a  shows a sectional view of a third exemplary embodiment of a turbine blade according to the invention with a view in the direction of the root section of the blade, 
           [0048]      FIG. 4   b  shows a sectional view of a rotor shaft of a steam turbine in the region of a shaft slot with a root section, fastened in it, of a turbine blade according to  FIG. 4   a,  and 
           [0049]      FIG. 4   c  shows the detail Z according to  FIG. 4   b.    
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0050]      FIG. 1  shows the first exemplary embodiment of a turbine blade  10  according to the invention which is configured particularly for use in a low-pressure stage of a steam turbine. The turbine blade  10  comprises an aerofoil section  12  and a root section  14  in the form of a plug root. The root section  14  has plug-in tabs  16  for a pin connection. The aerofoil section  12  is manufactured from composite fiber material  18  which contains glass fibers and/or carbon fibers. 
         [0051]    The main fiber direction  20  runs along a main axis  21  of the turbine blade  10 . 
         [0052]    In a region near the root section  14 , the aerofoil section  12  has an additional composite fiber ply  22 . The additional composite fiber ply  22  contains additional fibers which run at a deviating angle with respect to the main axis  21  of the turbine blade  10 , for example at an angle of ±15°, ±30° or ±45°, and are provided for stiffening the aerofoil section  12 . A plurality of additional composite fiber plies  22  of this type may also be provided. In this case, these plies may be arranged mirror-symmetrically with respect to the aerofoil center surface, with the result that distortion is avoided. An asymmetric arrangement of the additional composite fiber plies leads to distortion. This may be utilized, if appropriate, for self-setting purposes. 
         [0053]      FIG. 2  shows the section II-II in the aerofoil section  12  according to  FIG. 1 . This shows a filling body  24  arranged in the region of great aerofoil thickness for optimization in terms of weight and of rigidity. This filling body is surrounded by the composite fiber material  18 . The turbine steam  26  flows onto the turbine blade  10  from the left according to  FIG. 2 . For protection against drop impacts, the onflow edge, facing the inflowing turbine steam  26 , of the turbine blade  10  is provided with edge reinforcement  28 . The edge reinforcement  28  is illustrated in more detail in  FIG. 2   c.  It consists of metal and is fastened to the onflow edge  27  of the turbine blade  10  by means of an adhesive bond  40  having a run-out  42  appropriate in terms of adhesive bonding and of the composite fibers. 
         [0054]      FIG. 2   a  illustrates the first embodiment of the build-up of the turbine blade  10  according to  FIG. 2  in a surface region of the latter. The inner composite fiber material  18  is in this case surrounded by a first electrically conductive layer  36  in the form of a metallic layer, by an insulation layer  34 , by a second electrically conductive layer  32  in the form of a metallic layer and finally by a protective layer  30 . The protective layer  30  is designed to be moisture-repelling for sealing off the aerofoil section  12  with respect to liquid. 
         [0055]    The protective layer  30  consequently prevents a penetration of moisture into the composite fiber material  18 . Furthermore, the protective layer  30  is designed to be deformable such that, without any loss of its sealing function, it compensates the deformations to be expected during the operation of the turbine blade  10 . The successive arrangement of the electrically conductive layer  32 , of the insulation layer  34 , and of the electrically conductive layer  36  serves for monitoring the functioning of the protective layer  30 . For this purpose, the insulation resistance of the electrically conductive layers  30 ,  32  with respect to the surroundings or between the layers or the capacitance of the layer arrangement is measured, in order to ascertain whether moisture has penetrated through the protection layer  30  into the interior of the aerofoil section  12 . 
         [0056]      FIG. 2   b  shows a second embodiment of the build-up of the turbine blade  10  according to  FIG. 2  in the surface region of the latter. Here, the composite fiber material  18  is surrounded by a layer of indication material  30  which is surrounded, in turn, by the protective layer  30 . The indication material  38  is in the form of water-soluble substances which are detectable in dissolved form chemically, optically and/or radiologically. The indication material  38  consequently serves for detecting a leak in the protective layer  30 . To be precise, if moisture penetrates into the interior of the aerofoil section  12 , the water-soluble chemical substances of the indication material  38  are dissolved and can be detected in the condensate which has come from the steam leaving the turbine. 
         [0057]      FIG. 3   a  shows a second exemplary embodiment of a turbine blade  110  according to the invention. A root section  43  adjoins an only partially shown aerofoil section  12  with composite fiber material  18 . In this case, the fibers of the composite fiber material  18  are routed from the aerofoil section  12  into the root section  43  and in the latter are led around a contact and deflection element  46  in the form of a metallic sleeve, whereupon the fiber then runs back again into the aerofoil section  12 . The element  46  consequently fulfills a deflection function. At the same time, it also performs a contact function in that it makes contact with a shaft slot  48  of a rotor shaft  47  of a steam turbine. Furthermore, according to  FIG. 3   a,  the turbine blade  110  comprises what is known as a guide element  44 , by means of which an advantageous fiber routing in the blade root is diverted into a fiber rooting, adapted to the geometry of the aerofoil section  12 , of the composite fiber material  18 . 
         [0058]      FIG. 3b  shows the section III-III according to  FIG. 3   a.  The root section  43  is designed in the form of a plug root with plug-in tabs  45  for plugging into corresponding shaft slots  48  running transversely with respect to a longitudinal axis  50  of a rotor shaft  47 . The plug-in tabs  45  are then fastened by means of plug-in pins arranged transversely to them in the shaft slots  48 . Each of these plug roots  45  has one of the contact elements and deflection elements  46 . 
         [0059]      FIG. 4   a  illustrates a third exemplary embodiment of a turbine blade  210  according to the invention with a root section  52  in the form of a sliding root. The root section  52 , which is illustrated more precisely in a sectional view in  FIG. 4   b,  is pushed into a shaft slot  60  running in the axial direction of the rotor shaft. The root section  52  is in this case provided with a curvature, as illustrated in  FIG. 4   a,  and has a deflection element  56 , around which a substantial number of fibers of the composite fiber material  18  are led. These fibers are surrounded by a guide or contact element  54 . This element initially fulfills the function of diverting an advantageous fiber routing in the root section  52  into a fiber rooting adapted to the geometry of the aerofoil section  12 . Furthermore, the element  54  fulfills the function of making contact with a shaft slot  60  of the rotor shaft  58 . The guide and contact element  54  surrounds the composite fiber material  18  of the root section  14  completely and is also contiguous to the composite fiber material  18  in the lower region of the fiber aerofoil section  12 . 
         [0060]    This region is illustrated more precisely in  FIG. 4   c.  In order to cause no damage to the guide and contact element  54  or to the composite fiber material  18  in the event of deformations of the aerofoil section  12 , a gap  62  between the composite fiber material  18  and the element  54  is provided.