Why does an airplane stay in the air?

A wing is shaped and angled so that the air passing over it is deflected downward. Turning a stream of air requires a force on that air, and by Newton's third law the air pushes back on the wing with an equal and opposite force: that reaction is lift. Seen from the pressure side, the same process appears as air flowing faster over the curved upper surface and more slowly beneath, leaving a lower pressure on top than below. Both descriptions are the same physics viewed from different angles; neither is complete without the other. What matters is that the wing sends a large mass of air downward every second, and the resulting pressure imbalance, added up across the whole span, is enough to carry the aircraft.

The amount of lift depends on three things a pilot can feel directly: airspeed, angle of attack (the angle between the wing and the oncoming air) and the density of the air. Doubling the speed quadruples the lift, which is why an airliner needs a long runway to reach the speed at which its wings can carry 300 tonnes. Raising the angle of attack adds lift up to a point; beyond a critical angle the flow separates from the upper surface and lift collapses, the stall. Thin air at altitude or on hot days reduces lift, so aircraft must fly faster in those conditions to compensate, and flaps are extended at low speed to enlarge and curve the wing when there is not enough speed to spare.

Lift alone does not explain sustained flight. Pushing air past the wing costs energy, because the wing produces drag along with lift, and because the swirling vortices left behind the wingtips represent energy spent on the air. Engines supply the thrust that overcomes that drag. In steady level flight, lift equals weight and thrust equals drag: nothing is accelerating, and the airplane simply continues on its way. A glider makes the same trade using gravity instead of engines, descending gently through the air to keep its speed up. Buoyancy plays no role in a heavier-than-air machine; it stays up only for as long as it keeps moving fast enough to make the air do the work.