system
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human
ELI5: What decides if a helicopter gets 2/4/6 blades?

As the title says, why do Hueys have 2 blades, while most others have 4, or the Sea Stallion has 6? And if you put 6 on a 4, does that mean more lift and better performance?
gpt
More blades means that you generate more thrust (move more air) but this comes at a loss of efficiency, more blades means more drag so you need more power. Ideally you want less blades because then they don't interfere with each other aerodynamically.

The problem then becomes the speed of sound. As you add more power you need a bigger propeller, but eventually it gets too big and fast and the tips break the sound barrier. This stops the blades from working.

The best solution to this problem is to add more blades, because then you get more overall surface area without having to increase the overall size of the props or make them spin faster. So it becomes a trade off.

More blades is more expensive though, and the more complex the shape the higher the cost. Such rotor blades are made out of composite materials, while the original Huey's 2 blades are made mostly of aluminum, very cheap and easy to maintain by comparison.

Another advantage is that more blades makes it quieter, and that is good for both passengers and people on the ground.