Changing the Wind Speed Changes Wind Direction Relative to the Rotor Blade
In this next picture we have taken one rotor blade from the previous page off its hub, and we look from the hub towards the tip, at the back side (the lee side) of the rotor blade. The wind in the landscape blows between, say 8 m/s and 16 m/s (from the bottom of the picture), while the tip of the blade rotates towards the left side of the picture.
In the picture you can see how the angle of attack of the wind changes much more dramatically at the root of the blade (yellow line) than at the tip of the blade (red line), as the wind changes. If the wind becomes powerful enough to make the blade stall , it will start stalling at the root of the blade.
Now, let us cut the rotor blade at the point with the yellow line. In the next picture the grey arrow shows the direction of the lift at this point. The lift is perpendicular to the direction of the wind. As you can see, the lift pulls the blade partly in the direction we want, i.e. to the left. It also bends the rotor blade somewhat, however.
Rotor Blade Profiles (Cross Sections)
As you can see, wind turbine rotor blades look a lot like the wings of an aircraft. In fact, rotor blade designers often use classical aircraft wing profiles as cross sections in the outermost part of the blade.
The thick profiles in the innermost part of the blade, however, are usually designed specifically for wind turbines. Choosing profiles for rotor blades involves a number of compromises including reliable lift and stall characteristics, and the profile's ability to perform well even if there is some dirt on the surface (which may be a problem in areas where there is little rain).
Rotor Blade Materials
Most modern rotor blades on large wind turbines are made of glass fibre reinforced plastics, (GRP), i.e. glass fibre reinforced polyester or epoxy.
Using carbon fibre or aramid (Kevlar) as reinforcing material is another possibility, but usually such blades are uneconomic for large turbines.
Wood, wood-epoxy, or wood-fibre-epoxy composites have not penetrated the market for rotor blades, although there is still development going on in this area. Steel and aluminium alloys have problems of weight and metal fatigue respectively. They are currently only used for very small wind turbines.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 19 September 2003