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Helicopter propellers have rotor blades that are similar to an airplane wing; the only difference is they contain a twist in the middle.

Once air comes in contact with the rotor blade the Bernoulli principle is applied. The air pressure will decrease once it passes over the curve because the limited amount of space increases the speed of the air. Below the rotor blade the air pressure stays the same, thus is it higher than the air on top. This creates low pressure above the rotor and high pressure below the rotor.