Wind Turbines - Wind Energy Systems for Electric Power Generation - page 20

Fig. 2.11 Power, torque and drag coefficients over tip speed ratio with pitch angle as parameter

Fig. 2.12 Sketch of a blade with laminar and turbulent air flow

illustrates the effect, where α is the angle between blade plane and oncoming wind

direction (as in Fig. 2.4) and the angle between rotor and blade planes.

A variant of the stall method is the so-called active-stall where the blades
are automatically turned into the wind direction when a preset speed is reached.
Figure 2.13 illustrates the specific properties. While in stall the air flow break-
off occurs with rigid blade position, in active-stall the blades perform a self acting
angle variation into the wind to control the air flow breakaway; in pitch eventually
the angle variation to decrease the active rotor area is performed without air flow
breakaway.

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