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Helicopters are able to launch, land and fly because their propellers creates thrust, the force of flight that pushes an aircraft in the direction of motion. Propellers create thrust through Bernoulli’s principle, which states that the slower air moves, the greater its pressure, or push. When a propeller spins, the air under it moves slower than the air above it. This causes the air under the propeller to have a greater push than the air above it. This pressure difference causes an upward thrust on the helicopter, allowing it to launch, land and fly.