         !! 1               "#$% &     ’   ()*+ , ’!  -./01+//2  % 2 + 0  3+ 2  2 4 2 ++  0  2 - / 50 0  2  -0  4 4/ %4   6/ ) 0 34  4 0    4  2  04  -00 + 0  4 2 2/ 50 +  ++  0 2    6   0  2/ 5 7    2 0  2++    +0   4  2  4 3  0 0 0  +  0  +/ 50  0  3  0  2 #8"& 9: -   4-  0  2 +  0  2++  24/   ; 0  +   3++ +    2  ++   + ( /  0     ( 3     4  / &  4  6/ & 3  +     < ( (  04  +  + +  2 +/ &   + ++   2   < (   6   +2   +/ , +  + (  =      +     +2  3 < ( >   4        +2  + < 0 +  +/ & 0 ++  3     ( #8"& 9:  + ?    +  +    2@   +  2/  !  !   "  Wind turbines operate in a severe environment where wind changes its velocity and direction momentarily. Consequently, the aerodynamic forces are variable during the blade rotation and thus failures due to fatigue are frequent. To evaluate these forces there is a need for numerical codes able to compute the aerodynamic properties of the elastic blade structure. To obtain numerical results for blade aerodynamic loads with sufficient quality, a RANS simulation with an appropriate fine grid mesh in the computational domain is needed. However it is very difficult to obtain these results in the case of extreme winds and gusts modelling. Here, a 30& second period of flow time is needed, but the time step in the case of CFD usually corresponds to an angular blade step les than one fourth of degree in the case of dynamic stall. Thus to simulate an extreme wind gust, a minimum of 10 000 time steps is needed. This is not very practical because of high computational cost, therefore simplified aerodynamic models based on blade element method (BEM) or vortex methods are used. To account for fluid&structure interactions, these methods are generally coupled with structural dynamics methods based on beam theory. Recently, Rasmussen (2003) showed that from eight European wind turbine