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Abstract The results from a Joule-III research project are presented. The objectives of the project have been improvement of design methods for stall regulated wind turbines related to stall induced vibrations and dynamic stall. The primary concern is the edgewise vibrations in the fundamental blade natural mode shape, which have caused trouble on modern wind turbines of the 500 kW size, corresponding to a rotor diameter of approximately 40 m. A theoretical study, based on quasi-steady aerodynamics, confirms that the basic source driving the vibrations is energy supplied from the aerodynamic forces to the vibration during stalled operation. The phenomenon can be described as negative aerodynamic damping. The theoretical approach identifies the main parameters controlling the phenomenon. They are related to the steady and the dynamic airfoil characteristics, the overall aerodynamic layout of the blade, e.g. chord length and twist, the structural properties of the blade, e.g. structural damping and properties controlling the resulting vibration direction. Furthermore, full aeroelastic calculations and comparison with measurements show that the properties of the supporting structure, i.e. the nacelle and the tower, are important, as this part of the structure may either resist the vibration by supplying damping or sup-port the vibration, when the frequencies of the coupled rotor and tilt-yaw mode shapes in the rotating frame of reference -- usually denoted the backwards and the forwards whirling frequencies -- are close to the blade natural frequency. It is confirmed that qualified aeroelastic calculations can be used for determining the influence of changing the primary design parameters. The design guidelines therefore builds on both the simple quasi-steady models, which can be used for the preliminary choice of the design variables mentioned above, and full aeroelastic calculations. The full aeroelastic calculations refine the design basis and should be used for choosing the final design variables and for final verification of the design. Through this design procedure it is possible to assess the required safety margin against stall induced vibrations.
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