Conventional, or fixed-wing, aircraft generally use a propeller or jet engine to provide thrust?, which moves the craft forward through the air. The movement of air over the wings produces lift, which allows the aircraft to fly. An exception is gliders which have no engines and gain their thrust from gravity. That is, they must descend in relation to the air in order to maintain their forward speed. Helicopters use a spinning rotor (a rotary wing) to provide both lift and thrust.
Lighter than air craft float in air in the same way that a ship floats in water, by displacing the air around the craft with a lighter gas such as helium, hydrogen, or hot air. They may have propellers to provide forward thrust.
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