WIND TURBINE BLADE TWIST BASIC INFORMATION AND TUTORIALS

Comparing a propeller turbine blade with an airplane wing, you will see that the airplane wing is flat relative to a turbine blade, which has a twist; that is, the tip of a blade is not parallel to the blade root. This twist is not much, and can be only a few degrees, depending on the blade length.

The reason why a blade must be twisted by an angle and not fl at can be seen from the fact that in order to have a good lift force on a blade the air fl ow must hit the blade at a proper angle.

When a blade rotates, the points at the tip side go faster than the points near the root. Since the wind speed is approximately the same for all the points on a blade, the relative speed of air fl ow with respect to the blade is different for points along the length of a blade.

Figure 4.4 illustrates the relative speed due to the combination of wind speed and the blade motion. Th e angle shown by φ (phi) in the fi gure is the angle of attack. This angle, as shown in Figure 4.5, is not the same for segments of the blade in the tip area, in the middle or at the blade root.

 FIGURE 4.4 Relative speed of air fl ow over different parts of a blade is the reason for blade twist.

FIGURE 4.5 If a blade is not twisted in design, a correct desired angle of attack cannot

be maintained.

In order to have more or less the same angle of attack for all the segments of a blade, these segments must encounter the wind at diff erent angles, as shown in figure 4.4. Otherwise some segments have very inappropriate angles of attack.

In fact, for the three segments shown in figure 4.4, the direction of the relative speed determines the twist angle. In figures 4.4 and 4.5 you are looking at a blade in a direction along the blade and toward the shaft.

Figure 4.5 shows a case where a blade has no twist. In such a case, the chord lines of all the segments of a blade are parallel. As shown, the angle of attack can be correct for certain parts of the blade, but is not correct for the other parts.