The traditional Dutch windmill had four blades. Modern power-generating windmills invariably have only three. Why the difference?
The bigger the windmill, the fewer blades they tend to have. Over a certain size the materials dictate a 2 blade design. There are smaller ones (think farm windmills to draw water) that have 10+ blades. It’s a combination of size, materials, and expected wind speeds.
If you’ve ever seen the traditional Dutch windmills turning you’ll notice they turn very slowly. They were designed for slow speed high-torque applications, like pumping water. The same as the traditional farmers windmills seen on elevated derricks. Multiple blades with a high twist ratio gives a high solidity ratio, the ratio of the total area of the blades to the total area swept by the blades. Again, high solidity, low speed, high torque.
To generate electricity you need high speed, thus low solidity, less blade area. You could do it with one blade, but the balance problems are hard to engineer around, and a bitch to start.
Two blades are better, but still hard to balance. Three makes it much easier to balance and a good torque to speed ratio for generating electricity.
My guess is they stopped at three blades because it gives the optimal ratio between solidity and balance.
Thanks to both of you. I learned something today!
Does it make a difference that old-fashioned windmills relied on simple drag on flat panels, while modern ones are airfoils?
Old fashioned windmills, such as those in Holland, did indeed use an airfoil, provided by a sail attached to a lattice:
When filled with air, the sail forms an airfoil, which drives the windmill.
You will notice that almost all current wind turbines use three blades, even very large ones. One reason is “judder”, which occurs when the turbine changes direction into the wind. On a two-blade turbine, this can cause very serious vibration, and shorten the life of the unit. Here’s a photo I took last week, south of Lubbock:
