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Remember, a propeller blade is like the wing of a plane. With the thrust from the engine, each blade is able to produce lift, pulling the airplane forward, or, in the cases of helicopters, up. The more blades you have, the more lift you have, because each one is going to help produce more. However, you can't just add more blades indefinitiely. Each one adds weight and surface area to the propeller, creating more drag! This means you need to find a balance, having enough blades to produce lift, but not too much so as to create extra drag.