WIND TURBINE AERODYNAMIC BLADES BASIC INFORMATION AND TUTORIALS

Aerodynamic Blade Wind Turbines

Each blade of a turbine is subject to aerodynamic forces from the wind. The airstream has a relatively fixed direction for the area swept by the blades, at an instant, but since blades are twisted the relative direction of wind with respect to a blade is not the same for various segments of the blade. 

Moreover, whereas the windstream has a constant speed (for a short period under consideration), the speed of air as a result of the blade rotation is smaller for the segments of the blade closer to the hub than the segments closer to the tip of the blade.

Consequently, the relative motion of air with respect to the blade as a result of wind and blade motion varies along the length of a blade. The resulting aerodynamic force, thus, varies in both magnitude and direction along the blade span. The typical force for a segment is shown in figure below (a blade can be assumed to be made of any arbitrary number of segments).

The force on each blade segment consists of two components: one component along the direction of wind (the drag force, almost in the horizontal direction) and one perpendicular to wind (the lift component, in a near vertical plane). These forces are depicted for a two-blade turbine in figure below.

A two blade-turbine is more appropriate to demonstrate the fact that all the components along the wind direction have the same force direction in the two blades, whereas the forces normal to the wind have opposite directions in the two blades, since the blades are symmetric to each other. 

The forces shown correspond to when the blade is feathered and does not catch much energy (the lift components are smaller than the drag components). This is just for the sake of clarity of the figure. This implies, also, the fact that the horizontal force on the blades is greater when a turbine is parked than when it is working.

It is easy to verify that the resultant of the two sets of forces is a push on both blades in the wind direction and a torque about the turbine shaft axis. In other words, all those force components along the wind direction contribute to a backward push on the blades, and do not generate any rotational motion.

However, those force components that are in the opposite direction in a (near) vertical plane are the only ones that generate a torque that makes the turbine rotate.

Note that the just mentioned aerodynamic forces are functions of wind speed, rotational speed, and pitch angle. Therefore, they are not the same for different operating conditions and for when the turbine is parked. 

A blade must be able to withstand these forces in the harshest condition; that is, when these forces are at their highest.

All the forces on a turbine must ultimately be transferred to the ground through the tower.