Whenever the flowing medium velocity is changing with respect to a propeller, that propeller needs a pitch control mechanism to operate as desired. Commercial wind turbines are designed to produce optimum power, 15 m/s of wind speed. However, the wind speed always fluctuates up and down around this optimum wind speed. To generate the optimum power, the turbine blades should adjust, up and down, according to the wind speed. This adjustment comes from turning the blades around their longitudinal axis (to pitch). When the wind speed is decreased the blade pitch is such that it exposes more surface area to the wind. Conversely, when wind speed increases the blade pitch is such that it exposes less surface area to the wind. If a blade is not designed for stall, increased wind speed will force the rotor to turn faster without a pitch control mechanism. The pitch mechanism allows the wind to flow around the blade as smoothly as possible. To do this, air particles cannot hit the blade head on, rather they must flow almost tangent to the blade just as an airplaneís wing operate in the air. Once you realize that air should flow around the blade, rather then hitting the blade, to create the lift, the concept becomes easier to grasp.
There are two important conditions which will cause the turbine to shutdown. The first, when the wind speed exceeds the allowable maximum wind speed and the second, during an emergency. In these cases, feathering the blades, which is pitching it such that it exposes minimum surface area to the wind, will reduce torque generated by the wind to a minimum and will reduce the stress on the breaking system.
Now we know that pitch is an essential part of a large wind turbine and we also know that flow should gently move around the blade rather than hitting it in any way. Combining this knowledge, in light of velocity triangles, allows us to visually see how pitch works. Without understanding velocity polygons it would be very difficult to comprehend it. As discussed velocity polygon article, the velocity triangle consists of three velocity vectors. These are, the velocity of the wind represented in blue, the rotational velocity of the blade represented in green and relative wind velocity seen by the observer on the blade represented in red.
There are two kinds of pitch control mechanism. The first is called "Active Pitch Control" where the rotor blades turn around their longitudinal axis (to pitch) by a computer controlled mechanism. This type of pitch control requires expensive equipment however provides good pitch control. Active pitch controls are used in one third of the large turbines currently installed. The second pitch control mechanism is called "Passive" or "Stall Pitch Control". In this type of design the blade does not rotate around its longitudinal axis, but is designed such that it naturally creates a stall and lower rotation speed. This type blade requires precise blade design and structurally strong towers.
Letís design a wind turbine where the length of one blade is almost 47 meter (~ 150 feet) and operates at an optimum power production which is wind speeds of 15 meter per second. The flash animation above shows a turbine with velocity triangles on the blade marked by the red dots. There are two sliders in this animation; the first one on the top left corner adjusts the location of the red dots on the blade which affect the shape of the velocity triangle. The second slider on the bottom left corner, controls the wind speed which also affects the shape of the velocity triangle. Notice that at the center of rotation axis you see is an elongated blue ellipse representing the cross section of the blade at the locations of the red dots. This cross section should always be parallel to the red relative velocity.
Letís slide the slider named "Distance from Center" up and down. First we notice that the cross section ellipse changes its angle of attack. This is because when we go away from the center, the rotation speed increases, and decreases when we come closer to the center. Since the wind speed is fixed, this movement changes the magnitude of relative velocity which affects the bladeís cross section angle. Notice also that when the red dots are going away from rotation center, the blade width decreases as it should, and the angle of attack of the cross section almost becomes perpendicular to the rotation axis. On the other hand, when the red dots come closer to rotation center, the cross section get larger and the angle of attack becomes almost parallel to the rotation axis. This is why the turbine blades are built in a twisted shape. This is the perfect design for, if the wind was blowing at 15 meter per second. As we all know the wind speed is always changing so letís see what would happen if we change the wind speed. Please first set the distance from center to 25 meters to better visualize what will happen.
When you change the wind speed you will notice that the blue cross section of the blade is not changing, but the red colored cross section emerges under the blue one and it responds to the changes in the wind speed. The angle of this red cross section is parallel to the red relative velocity and the internal computer system that controls the pitch of the blade by motors. Therefore the red cross section represents the active pitch control mechanism. On the other hand, the blue cross section does not change the angle of attack, and the relative velocity is no longer parallel to the blue cross section (unless of course wind speed is 15 m/s). This creates a stall in the blade and slows down the rotation, therefore the blue cross section corresponds to the passive or stall pitch control mechanism. When the distance from the center slider is moved up and down, the angle between red and blue cross section will change only slightly. By turning the blade from the blue cross section position to the red, the pitch mechanism effectively maintains the most efficient position for the blades.