A method of determining a value of a stress related quantity of a rotor blade of a wind turbine is provided, the method including: emitting a primary radar signal towards a portion of the blade; receiving a secondary radar signal emanating from the blade due to interaction with the primary radar signal; analyzing at least the received secondary radar signal; and deriving, based on the analysis, the value of the stress related quantity as related to or indicating a blade acceleration and/or a first temporal derivative of the blade acceleration and/or a higher temporal derivative of the blade acceleration.

The method is for operating a wind turbine having a rotor with at least one rotor blade, a tower and a pitch setting system. The method includes providing first information which is representative for the tilt bending moment acting on the rotor. Second information which is representative for the thrust force acting on the rotor is provided. Third information which is representative for a critical area of thrust forces and tilt bending moments is provided. Fourth information is determined depending on the first, the second and the third information. The fourth information is representative for whether the tilt bending moment and the thrust force lie within the critical area. If this is the case, an output signal is generated which is configured to cause the pitch setting system to change the pitch angle of the at least one rotor blade in order to leave the critical area.

A method of estimating a position of at least a part of a rotor blade of a rotor of a wind turbine during operation of the wind turbine is provided. The rotor blade is deflected due to a deflection motion of the rotor blade towards a tower of the wind turbine and the position is indicative of the deflection. The method includes measuring a first parameter indicative of an absolute and/or a relative position of at least a second part of the rotor blade or of a further rotor blade of the rotor. The method further includes measuring second parameters indicative of an absolute and/or a relative position of at least a third part of the rotor blade or of the further rotor blade of the rotor. The method further includes estimating a state of the rotor blade and deriving the position from the estimated state of the rotor blade.