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The diagrams below explain a little bit about the apparent wind (think of this as the direction of the wind as the blade 'feels' it). They explain how the apparent wind speed actually gets faster as it travels through the wing 'tip' (compared with the wing 'root' - the part where it is joined to the hub). This means that it can travel dangerously fast at the tips (it can reach supersonic speeds - faster than the speed of sound!).

If the wind travels too fast across the foil or at the wrong 'angle of attack', the airflow 'stalls'. In an aircraft a stall results in a total loss of lift and the aeroplane will fall out of the sky and crash! In an wind turbine the result is a loss of rotation and then electricity generation. In severe cases, it can cause the blades to fail and break off. These could damage houses or even injure people.

When the environmental conditions of a wind turbine are poor they must be 'feathered' to prevent them from working. This allows the wind to pass over them without the turbine spinning.