Why do wind turbines have three blades?

I think Stuart C is on the right track. The odd numbers mean you don't get the torque when one blade gets into the tower shadow.

The second reason is leverage. You want to put your blade surface as far from the axis of rotation as possible, because you get more leverage that way. More force at a lower rpm. The same blade area with 6 blades (half as long) would only produce 1/4 the power.

To frame your question differently, why not have "infinity" blades on the mill in order to get "infinity" times the power output. Of course this can't be the case, or else one windmill would power the earth. So what's the answer to your question? The answer is that there is a compromise to be reached and the best number of blades lies somewhere between 1 and infinity. But one of the reasons that the optimal number is so low is because at a certain point, too many blades can start to make the wind divert around the mill instead of blowing through it. If there is too much surface area in the circle defined by the blades, wind will re-route around the circle and energy will not be captured. There is also the fact that turbulence is a power-lower. If there were too many blades, the small amout of turbulence caused by a leading blade would diminish the power generated by a following blade. By having good spacing, the turbulence is blown behind the following blade, and doesn't affect energy capture. There are several other reasons that have to do with: initial installation costs, maintenance, angular momentum, total mass of structure, and as you mentioned windforce on the tower.

An odd number of blades is usually chosen, for the following reason. If the machine had two blades, when one passes in front of the tower it passes into a dead area with little wind pressure, at the same time as the other blade is in the full force of the wind. This can damage the axle or bearings, when it continues thousands of times.

Therefore uneven numbers of blades are preferable. Three blades are chosen because one single blade would require a counter weight, and more is less economical to make.

I believe the modern way of thinking is that the power you can get out of the wind only depends on the circular area the blades cover, and adding more blades only adds more friction to the system. That's probably an oversimplification, though.

If they only had two, the blades would have to be really *long* in order to have enough surface area to catch enough moving air to generate power....which would lead to materials weight and/or durability issues.

If they had four or more....there it gets interesting, but I'd be willing to bet that again, total turbine weight and aerodynamic drag *in rotation* would be issues too (a smaller number of blades would have less mass, less inertia, and create less of that turbulent *chop* during blade rotation that would cut into efficiency).

It might well be for the same reason that helicopters generally come with only two blades per rotor: for the application, that is what gets the job done best with the least hassle.

Or....people may just be taking the *cheap* way out, seeing as how Big Oil busts their humps to make sure there's as little money as possible in wind-farming and other alternative-energy technologies. [*]

I hope this helps....I'm not a trained engineer myself, just a bit of a Green hobbyist myself, I am sure there are others here who can provide all of the details and gnarly equations needed to back me up or refute me....and I don't mind this.

Thanks for your time! ^_^

They have three because...

If you only had two, there would be a possibility of no movement--the blades are aligned with the wind. If there were four, this is a problem too. If the top blade and the bottom blade were both perpandicular to the wind, the windmill wouldn't move, they would be countering each other. Maybe more blades would work, but simplicity is golden.

They have Moore surface area. Just a few ounces of pressure over a large area is how it works.