Based on my personal observations, newer windmills seem to have three blades while older ones tend to have four or even more. This question has excellent discussion on my three is an optimal number. But what changed? For example, did people at some point not understand the relevant tradeoffs? Or does the availability of some new material shift the economics to favor fewer, longer blades?
The blade geometry and structural stability is less meaningfull when you have more blades.
Means; if you have to to build the blades only from direct cutted materials without any airfoil-shape you have to use more blades. Thanks to Betz law, this doesn't even change the efficiency too much, and it's more or less only a investment cost factor.
And, before Ludwig Prandtl had verified the Lifting line theory of Lancaster You simply didn't got any other possibility than just make more plate-blades.
So these two men made the crucial change in knowledge.
