The Unsteady Aerodynamics of Turbine Yaw and Over-Speed Protection - Small Wind Turbines: Analysis, Design, and Application - page 162

Fig. 8.14 Close up of 500 W
turbine furling mechanism.
Photo from Wright [ 21 ]
Figure 8.16 shows that a yaw axis offset causes a non-zero yaw during normal
operation of the 500 W turbine. Each data point represents 15+ seconds of rela-
tively constant wind speed, and a corresponding average yaw angle. When the
turbine generates power, the thrust offset causes an average yaw angle of about
208, which changes little over the range of measured wind speed. This reduces the
power output by around 10% from the unyawed case. Furthermore, there can be
significant hysteresis in furling, whereby the wind speed required for unfurling is
considerably lower than that for the original furling. In field measurements of a
10 kW turbine, Bowen et al. [ 25 ] found significant reductions in the power output
attributable to this effect. They described the measured power curve as having
''two separate concentrations of data points''—see their Fig. 9—due to hysteresis.
Despite the drawbacks of furling, and the complexity it adds to analysing yaw
behaviour, its widespread use indicates it is one of the simplest over-speed pro-
tection methods for small wind turbines.
8.7.2 Pitching
The main part of a pitching turbine (rotor, generator, and tail) is hinged below and
behind the centre of mass such that an increasing wind speed will eventually cause
the moment due to turbine thrust to exceed the counteracting moment due to