Abstract: The invention relates to a method for designing a flexible blade or an articulated rigid blade with one or more torsion springs, for a wind turbine or a water turbine, the flexible blade being designed to passively control the pitch angle of the wind turbine or of the water turbine during operation, the method comprising the following steps: a) receiving the known geometric profile; b) determining a change in the optimal pitch angle, 0o opt rigid, as a function of the specific speed ?; c) determining the local behaviour of the flexible blade or of the articulated blade and local ratios relating to the aerodynamic loading and to the centrifugal force being exerted on the blade; d) determining local values of the bending modulus B of the flexible blade/the stiffness of the torsion spring and of the mass density p of the blade; and e) providing information relating to the selection of the material.