system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do windmills typically have 4 blades, yet all modern wind turbines have 3?
gpt
Wind turbines have three blades for a balance of stability and efficiency. It's the fewest number of blades you can have while still keeping the structure from shaking itself apart from gyroscopic forces. Since more blades means less efficiency, three is the best we can do.

Windmills were made in a time when precision engineering and machining weren't nearly as advanced as they are now. It follows the same principle - fewer blades is more efficient - but four blades is a *whole* lot easier to manually balance than three is.

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For people asking why two blades isn't stable:

Two blades is only stable if the turbine doesn't rotate laterally. Because an efficient turbine needs to rotate to maximize its angle to the wind, three is much better.

Two-bladed (and even one-bladed) turbines *do* exist. The problem is that because of their instability, they produce a lot more wear on their components and are more prone to failure.

There is still research being done into making two-bladed turbines more viable, and if a good solution to their instability is found, They may well end up becoming the standard. For now, though, three blades is all-around the best option.