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The number of blades required is proportional to the amount of lift required. There are several factors that affect the amount of lift produced by rotor blades. They are the shape of the air foil, the rotational speed, the angle of attack, the length of the rotor blades, and the strength of the blades. All of these have practical limits. Adding an additional blade helps keep all these parameters within practical limits.

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Q: Why do some helicopters have more than 4 blades?
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Related questions

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Why do some helicopters have different numbers of blades?

If you mean rotor blades, theoretically, only one (provided it is counterbalanced and moves fast enough). But you'll typically see two or more. Helicopters use more than one because it requires less rotor speed to get the same lift, and it also runs a lower risk of failure in the mechanical systems. Many rotocraft use four or five, and some up to six or seven blades. There are a few helicopters that use two main rotors, and they are counter rotating so that their torque cancels and a tail rotor is not necessary. But the mechanical complexity of these main rotor systems impinges on mechanical reliability.

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