A wind turbine, comprising: a turbine blade connected to a rotor hub, said turbine blade comprising a low pressure surface and a high pressure surface; an electrical stall sensor configured on at least one of said low pressure surface or said high pressure surface at a location to detect backflow in a stall condition at said respective pressure surface, said stall sensor further comprising: a power supply; a flap pivotally configured relative to said respective pressure surface so as to be moved from a first position towards a second position by backflow over said respective pressure surface; and a sensor circuit that responds to movement of said flap between said first and second positions and generates a corresponding electrical signal that indicates a stall condition, characterised in that said stall sensor comprises a transducer that produces a varying output parameter as a function of relative movement of said flap such that said electrical signal provides a quantitative indication of magnitude of backflow over the said respective pressure surface (23,24), and wherein said flap is biased towards said first position, said transducer comprising a load cell that measures resistance of said flap to the backflow. a turbine blade connected to a rotor hub, said turbine blade comprising a low pressure surface and a high pressure surface; an electrical stall sensor configured on at least one of said low pressure surface or said high pressure surface at a location to detect backflow in a stall condition at said respective pressure surface, said stall sensor further comprising: a power supply; a flap pivotally configured relative to said respective pressure surface so as to be moved from a first position towards a second position by backflow over said respective pressure surface; and a sensor circuit that responds to movement of said flap between said first and second positions and generates a corresponding electrical signal that indicates a stall condition, characterised in that said stall sensor comprises a transducer that produces a varying output parameter as a function of relative movement of said flap such that said electrical signal provides a quantitative indication of magnitude of backflow over the said respective pressure surface (23,24), and wherein said flap is biased towards said first position, said transducer comprising a load cell that measures resistance of said flap to the backflow. a power supply; a flap pivotally configured relative to said respective pressure surface so as to be moved from a first position towards a second position by backflow over said respective pressure surface; and a sensor circuit that responds to movement of said flap between said first and second positions and generates a corresponding electrical signal that indicates a stall condition, characterised in that said stall sensor comprises a transducer that produces a varying output parameter as a function of relative movement of said flap such that said electrical signal provides a quantitative indication of magnitude of backflow over the said respective pressure surface (23,24), and wherein said flap is biased towards said first position, said transducer comprising a load cell that measures resistance of said flap to the backflow.