## 5 HAWT Blade Design
### Off-Design Conditions and Power Regulation  
Early wind turbine generator and gearbox technology required that blades rotate at a fixed rotational velocity therefore running at non design tip speed ratios incurring efficiency penalties in all but the rated wind conditions [1]. For larger modern turbines this is no longer applicable and it is suggested that the gearbox maybe obsolete in future turbines [27]. Today the use of fixed speed turbines is limited to smaller designs therefore the associated off-design difficulties are omitted.  
The design wind speed is used for optimum dimensioning of the wind turbine blade which is dependent upon site wind measurements. However, the wind conditions are variable for any site and the turbine must operate at off-design conditions, which include wind velocities higher than rated. Hence a method of limiting the rotational speed must be implemented to prevent excessive loading of the blade, hub, gearbox and generator. The turbine is also required to maintain a reasonably high efficiency at below rated wind speeds.  
As the oncoming wind velocity directly affects the angle of incidence of the resultant airflow onto the blade, the blade pitch angle must be altered accordingly. This is known as pitching, which maintains the lift force of the aerofoil section. Generally the full length of the blade is twisted mechanically through the hub to alter the blade angle. This method is effective at increasing lift in lower than rated conditions and is also used to prevent over speed of the rotor which may lead to generator overload or catastrophic failure of the blade under excessive load [1].  
Two methods of blade pitching are used to reduce the lift force and therefore the rotational velocity of the rotor during excessive wind speeds. Firstly decreasing the pitch angle reduces the angle of attack which therefore reduces the lift generated. This method is known as feathering. The alternative method is to increase the pitch angle which increases the angle of attack to a critical limit inducing the stall condition and reducing lift. The feathering requires the maximum amount of mechanical movement in pitching the blade. However, it is still favoured as stalling can result in excessive dynamic loads. These loads are a result of the unpredictable transition from attached to detached airflow around the blade which may lead to undesirable fluttering [1].  
Utilising the stall condition a limiting speed can be designed into the rotor blade known as passive stall control [1]. Increased wind velocity and rotor speed produce an angle at which stall is initiated therefore automatically limiting the rotor speed. In practice accurately ensuring stall occurs is difficult and usually leads to a safety margin. The use of a safety margin indicates that normal operation occurs at below optimum performance, consequently this method is utilised only by smaller turbines [28].  
Full blade feathered pitching at the hub is used by the majority of today's wind turbine market leaders (Table 4). Feathered pitching offers increased performance, flexibility and the capability of fully pitching the blades to a parked configuration. Manufacturers are reported as using collective pitch [29], in that all the blades are pitched at identical angles. However, further load reductions can be found by pitching blades individually [30]. This requires no extra mechanism in most designs and it is expected to be widely adopted [29,30].