Why do the giant windmills on energy farms seem to always have three blades? Is there a reason three is better than two and better than four?
There seems to be a debate whether three or two is better, with three being
the predominant view.
There’s only like 5 large wind turbine manufacters in the world. US installation is overwhelmingly from two companies, GE and Vesta. I’m not sure if either make
large turbines for two.
Here’s a thread that discusses the number of blades on airplane propellers, and might give some insight. Here’s another. The major difference between propellers and windmill blades may be due to the rotation speed (with windmills being much slower, of course).
Aesthetically, windmills need to have three or five. That should be objectively obvious. 
the proper number of blades depends, of course, on what it’s used for. Just like any other engineering problem.
fast-turning windmills used for electric power generation have few blades, because when they’re turning rapidly the drag of the tips becomes the limiting factor. Fewer blades means there’s less resistance. If the blades are turning faster enough, three or so is enough to extract the energy you’re gonna get.
Slower-turning mills , especially those built long ago, with a lot of friction, aren’t limited by the drag at the edges of the blades – their bigest problem is getting enough torque to overcome friction. so those Dutch windmills often have four or five blades.
Those tiny windmills atop Farmer’s water pumps from the midwest (and newJersey, for that matter) often start from scratch, catching the wind when they can, and often have to overcome a lot of friction from rusty works that get lubricated when the farmer can get around to it. So they have lots of vanes to get plenty of torque.
When in Lubbock, Texas, be sure to visit the Windmill Museum! We got a lot of wind here, so we got a lot of windmills, including some with blades the likes of which you’ve never seen.
My 8th grade science fair experiment (circa 1974) was this exact subject. I made a windmill which turned a bicycle generator*, and found that two blades worked better than any other number up to eight. Of course, this application is fairly high-speed.
- do they still make these? A little generator attached to the bike frame, and could be snapped down to contact the tire sidewall when you needed electricity for a headlight. I recall Bart Simpson had one of these a few years ago, and his bike came to a stop with each 1/2 turn of the crank because of the resistance. That’s sure what it felt like when I was a kid using one.
One-bladed windmills (with a counterweight) are possible, and have certain advantages. But they also have disadvantages.
(couldn’t find a good link)
Brian