the rotary wing rotor ro for a helicopter , schematically shown on fig1 , comprises a hub m driven in rotation around its axis z - z by a main gearbox ( not shown ) and blades p transversally connected to said hub m via fastening devices l . thus , the blades p could rotate around the axis z - z , at the rotation frequency ω of said hub m . as shown on fig2 , a blade p according to the present method , system and device comprises an aerodynamic part a and a fastening part b , shorter than said part a . the fastening part b , for example with a cross - shaped section , cooperates with the fastening device l for fastening the blade p to the hub m . the aerodynamic part a comprises a top surface 2 and a bottom surface 3 forming , at the front , a leading edge 4 and , at the rear , a trailing edge 5 . in the vicinity of the leading edge 4 ( see also fig3 ), the bottom surface 3 is cut by a longitudinal slit 6 longitudinally distributing said aerodynamic part a ( according to the span e ) into a front longitudinal part a 1 comprising said leading edge 4 and a rear longitudinal part a 2 comprising said trailing edge 5 . on the other hand , said front a 1 and rear a 2 longitudinal parts are integrally formed through the top surface 2 being continuous . in the embodiment shown on fig3 , said aerodynamic part a comprises : a leading edge spar 7 , forming said leading edge 4 and the parts of the top surface 2 and the bottom surface 3 adjacent to the latter ; such spar 7 can be made in a fiber - resin composite material ( for instance glass - epoxy or carbon - epoxy ) and possibly incorporate a ballast mass 8 extended according to the leading edge 4 ; a bottom surface spar 9 , separated from the leading edge spar 7 by the bottom surface longitudinal slit 6 , the front edge 6 a of the latter being formed by a longitudinal transversal side of the leading edge spar 7 whereas the rear edge 6 r of said longitudinal slit 6 is formed by a longitudinal transversal side of the bottom surface spar 9 ; the latter can also be made in a fiber - resin composite material ; a ridge spar 10 forming the trailing end 5 and , for instance , made in a fiber - resin composite material ; a shell 11 forming the top surface 2 and the bottom surface 3 ( interrupted by the slit 6 ) and enclosing the spars 7 , 9 and 10 being simultaneously integral therewith ; a filling material 12 , for example , a rigid foam with a low resiliency modulus ( for instance , polyurethane ) filling said shell 11 between said spars 7 , 9 and 10 ; and a strip 13 of an elastomer material with a low resiliency modulus , obstructing the slit 6 and being integral ( preferably with a glue ) with the edges 6 a and 6 b of the latter . the shell 11 is made in a fiber - resin material ( for instance , carbon fibers ) and such fibers f 1 are arranged longitudinally with respect to said aerodynamic part of the blade , i . e . according to said span e . possibly , said shell can comprise fibers f 2 being orthogonal to said span , but it does not comprise any fiber being tilted on the latter ( see the cutaway view of the shell 11 shown on fig5 ). moreover , in an area 14 being adjacent to the slit 6 and extending on both parts of the latter , the shell 11 is rigidly integral ( for instance by gluing ) with the leading edge spar 7 and said bottom surface spar 9 . in contrast , outside the area 14 , the shell 11 is connected to the spars 7 , 9 , 10 and to the filling material 12 by a connecting layer of a damping material with a low resiliency modulus . such a connecting layer ( not shown for clarity reasons in the drawing ) can be continuous or discontinuous and be formed with an elastomer material . it should be easily understood that an aerodynamic part a being little rigid in torsion around the span e is consequently obtained , with however a rigid integrity , located around the slit 6 , between the leading edge spar 7 and the bottom surface spar 9 , on the one side , and the shell 11 , on the other side . by selecting a fastening part b being even less rigid in torsion around the spar than the aerodynamic part a ( for instance , 10 to 100 times lower ), the blade p is able to sustain a torsion generating on the blade shank , i . e . on the side 15 of the free end 16 of the latter , a resilient dynamic twist angle v of at least 14 °. furthermore , on the free end 16 of the blade p , an actuator 17 is inserted in the extension of the aerodynamic part a ( see fig4 ). the actuator 17 is piezoelectric and similar to that described in the document ep - 1 , 788 , 646 , as to which it is expressly referred . when the piezoelectric actuator 17 is fastened on the tip of the aerodynamic part a , it is located at least partly in the plane of the chord pc thereof . a removable hood 18 encloses and protects the piezoelectric actuator 17 and the end side 15 of the blade . the piezoelectric actuator 17 exerts a shear action and comprises two surfaces 19 and 20 being adapted to slide one relative to the other when said actuator is electrically supplied . through a coupling part 21 , the surface 19 is formed integrally with the leading edge spar 7 , while the surface 20 is integrally formed with the bottom surface spar 9 . in such a way , as illustrated on fig5 , when said actuator 17 is excited , it generates a slide between said surfaces 19 and 20 , such sliding being directed according to the span and being transmitted to the spars 7 and 9 moving with each other . thus , it results in a relative movement between the front part a 1 and the rear part a 2 ( schematically illustrated by the arrows 22 and 23 on fig5 ) and a buckling of the shell 11 resulting in a torsion distortion of the blade p around the torsion axis t - t arranged in the plane of the chord pc and directed according to the span e . obviously , the strip 13 also suffers a shear distortion ( see fig5 ). on fig6 , an exemplary embodiment is schematically represented for a fastening device l for a blade p so that the latter is able to turn around the axis z - z of the rotor ro . in this exemplary embodiment , the fastening device l comprises : a blade hub 24 made integral of the hub m of the rotor ro by any known means , not shown ; a flange ( or flange portion ) 25 being integral in rotation , on the one side , with said blade hub 24 and , on the other side , with the internal end 26 of the fastening part b of the blade p ; a rigid sleeve ( or sleeve portion ) 27 enclosing , with a big play , said fastening part b , said sleeve 27 , on the one side , comprising a flange ( or flange portion ) 28 arranged opposite the flange 25 and , on the other side , being made integral , through fastening means 29 , with the blade portion 30 making the transition between the aerodynamic part a and the fastening part b ; and at least one device 31 being able to progressively vary the pressure between the flanges 25 and 28 . as shown on fig7 a and 7b , the peripheries 25 a and 25 b of the flanges 25 and 28 being able to suffer a slight resilient distortion , are arranged inside a mobile yoke 32 of the device 31 and being mutually in contact through resilient blocks 33 being interposed between them . the peripheries 25 a and 25 b are submitted , on the one side , to the action of a spring 34 and , on the other side , to the action of controllable cam 35 , said spring 34 and said cam resting on the mobile yoke 32 so as to exert antagonistic actions on said peripheries 25 a and 25 b . the cam 35 is rotationally mounted around an axis 36 mounted on the yoke 31 and can turn around said axis under the control of an actuator represented by arrows f . a return spring 37 is able to bring the cam 35 into the position of fig7 a in the case of a failure of the cam actuator f . in the situation represented on fig7 a , the cam pushes the peripheries 25 a and 28 a by pushing the spring 34 , so that the pressure exerted between the flanges 25 and 28 is big . in such a case , the sleeve 27 is made integral with the blade hub 24 and the twist actuator 17 cannot exert any action on the blade fastening part b , only the aerodynamic part a being able to be twisted . naturally , it results that own torsion frequency of the blade p is then at a maximum and identical to that of said aerodynamic part a . in contrast , in the situation represented on fig7 b , the spring 34 is expanded and pushes the peripheries 25 a and 28 a against the cam 35 , so that the pressure between the flanges 25 and 28 is weak , even nil . the sleeve 27 is thus disengaged from the blade hub 24 and the twist actuator 17 can exert its action on the whole parts a and b of the blade . the own torsion frequency of the blade p is then at a minimum . obviously , through a rotation control of the cam 35 around its axis 36 between the positions illustrated by fig7 a and 7b , it is possible to progressively vary , in both directions , the own torsion frequency on the whole blade comprising the parts a and b thereof and the maximum value corresponding to the own torsion frequency of the sole aerodynamic part a . furthermore , it will be noticed that in the case of a failure of the actuator f or the actuator 17 , for instance , due to an electrical supplying problem , or even in the case of a divergence in the blade twisting appearing , the return spring 37 brings back to the situation of fig7 a , corresponding to the maximum own torsion frequency .