DETERMINING WIND SPEED AT A WIND TURBINE

The invention relates to determining wind speed at a wind turbine comprising a rotor and a plurality of rotor blades. The invention involves receiving a rotor speed signal indicative of current speed of the rotor and receiving a pitch angle signal indicative of current pitch angle of the rotor blades. For each of a plurality of wind speeds, a first thrust force on the rotor is obtained based on the received rotor speed and pitch angle signals. The invention involves receiving a blade flap load signal, from a blade flap load sensor of each of the rotor blades, indicative of measured flap loading on the rotor blades, and obtaining a second thrust force on the rotor using a defined blade element model and the received blade flap load signal. The first thrust forces are compared against the second thrust force, and wind speed is determined based on the comparison.