Why constant torque along the whole wind turbine blade?

[Alexander Tangen]

Hi, I have investigated the theory behind different types of wind turbines, and found out that the chord length is derived in a way that gives a constant torque along the whole blade.

Are there any reasons for that? I heard that someone mentioned something about the wake, but is it possible to get a good explanation on why we prefer the same torque along the whole blade?

[Bill Glynn]

Hi Alexander,

I think you are referring to the reason why turbine blades are cut with a pitch that changes throughout the length of the blade.
Unlike the old style Dutch wind mills that are known as drag type the newer style are said to be flying. The newer style blades are shaped more like airplane wings than a propeller. The lift of the "wing" is in a direction that helps encourage turbine spin. That's why they spin so much faster than the Dutch mills.
On an airplane the entire wing is flying at the same airspeed but a turbine blade is not. Although the entire blade is spinning at the same rotational speed, the tip of the blade is flying much faster than portions of the blade closer to the hub due to the longer circumference traveled by the tip.
If the entire blade length was cut to the same pitch then much of the length would be at the wrong angle to fly properly and efficiency would suffer due to turbulence. When the pitch is varied throughout the length of the blade then maximum efficiency can be obtained. Every portion of the blade is at the correct angle for the blade airspeed at any given radius from the hub.
Hope this helps.

Bill