wind turbine stalling
Wind turbine stalling works by increasing the angle at which the relative wind strikes the blades (angle of attack), and it reduces the induced drag (drag associated with lift). Stalling is simple because it can be made to happen passively (it increases automatically when the winds speed up), but it increases the cross-section of the blade face-on to the wind, and thus the ordinary drag. A fully stalled turbine blade, when stopped, has the flat side of the blade facing directly into the wind. Compare with furling.
A fixed-speed horizontal-axis wind turbine (HAWT) inherently increases its angle of attack at higher wind speed as the blades speed up. A natural strategy, then, is to allow the blade to stall when the wind speed increases. This technique was used on many early HAWTs, until it was realised that stalled blades generate a large amount of vibration (noise). Standard modern turbines all furl the blades in high winds. Since furling requires acting against the torque on the blade, it requires active pitch angle control which is only cost-effective on very large turbines. Many turbines use hydraulic systems. These systems are usually spring loaded, so that if hydraulic power fails, the blades automatically furl. Other turbines use an electric servomotor for every rotor blade. They have a small battery-reserve in case of an electric-grid breakdown.