Why do wind turbines only have three blades?

22 June 2008

Question

Why do wind turbines only have three blades?

Answer

Not all wind turbines do have three blades. I've seen some in Spain which have four and some older ones only have two. Some old-fashioned windmills have up to six or eight. Three seems to be the optimum for wind turbines. There's a few reasons behind that. One of them is that if you have too many blades on a wind turbine each blade as it moves through the air leaves a vortex behind it. It's very like if you look at a plane taking off you can see swirling air behind to two wigs of the plane. Wind turbine blades are very much like a plane's wing. You get a swirl left behind a moving blade. If those interfere with one another that can cause big problems. That's similar to a boat propeller only having three blades. If you had more blades you'd think you'd move more water but actually it would become self-demolishing in terms of the benefit. That's the optimum choice between weight and materials and efficiency.If you have only two blades because the wind's moving faster higher up than lower down then it tends to, if the blade's pointing upwards, it puts a big twisting force on the bearings. It can damage the bearings with the horizontal force.You can actually steer a boat that way. With big boat propellers, really big propellers the water is denser at the bottom of the propeller than it is at the top. The propeller as it turns round is creating more of a push to the side at the bottom of its run than at the top. If you know which way your propeller rotates by giving the engine a big burst of power you can enable it to do what's called prop walking. It gives you an extra kick in one direction which can make it really easy. That's how these really clever people get their boats into really small spaces just because they've learned they can use that trick just to get some extra movement in one particular direction.

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