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Helicopters are affected by the same four forces that planes are: lift, weight, drag, and thrust. However, lift and thrust come from somewhere else - instead of being produced by the wings, they are produced by the main rotor, which is a giant propeller. The propeller is connected to an engine, which provides thrust. As it spins, the airfoil shaped propeller blades (just like airplane wings) force the air over the top to speed up, which decreases pressure, while the air underneath keeps its higher pressure. The high pressure air pushes up on the blades, lifting the helecopter into the air.