the optimal number of blades on a windmill is 3. Airplane props, helicopter blades, and ceiling fans can all have any number of blades. Why?
In: 7
How do you define optimal? That’s kind of key here as the needs and wants of wind turbine designers are not those of aircraft manufactures.
There are a number of criteria that going into an “optimal” blade configuration, in no special order –
1) Cost – more blades = more money
2) Vibrations – more blades = a more even spin without vibration
3) Drag – more blades = more drag = more fuel consumption
4) Power – more blades = more surface area for force exchange = more power
5) Size & Speed – The power produced/obtained from spinning blades is complexly related to the number of blades, the size of each blade, and the speed each blade spins.
As you can see, items 1-4 are pretty complex, but let’s boil them down to – we want as few blades as possible, right? But # 5 says that the number of blades, along with their speed and size, directly impacts the power. Putting all these rules together, you basically get an ELI5 of – the smaller and faster the blades are, the more blades you’ll need to use. The larger and slower, the fewer you can use and 3 is pretty much the fewest blades you’d use to create a well balanced, long term function machine.
Short Version – ceiling fans, airplane props, and helicopter blades are all short and spin really fast, so you want a bunch. Wind power turbines can be any size but are typically priced per-blade as opposed to per-blade-size. so they go fewer big blades because it’s cheap and effective.
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