Why does an airplane fly?

An aircraft in steady flight is subject to four forces: lift, weight, thrust and drag. Weight pulls it toward the Earth. Thrust from propellers or jet engines pushes it forward. Drag, the resistance of the air, pulls backward. Lift, produced mainly by the wings, acts upward and holds the aircraft up. In level, unaccelerated flight lift equals weight and thrust equals drag, and the whole question of why an airplane flies comes down to how a wing makes lift.

A wing is shaped and tilted so that the air flowing past it is deflected downward. Newton's third law provides the simplest account: the wing pushes a large mass of air down, and the air pushes the wing up with equal force. The pressure picture says the same thing in different words. Air passing over the curved, longer upper surface and around the tilted wing speeds up and its pressure falls, while air beneath the wing is slowed and compressed slightly. The difference in pressure between the two surfaces, added up over the area of the wing, is lift. Bernoulli's principle and Newton's laws are not competing explanations but two views of a single flow; the popular idea that air above and below must meet at the trailing edge at the same time is false, and wind-tunnel images show the upper flow arriving well ahead.

Two variables dominate. The first is airspeed: lift grows with the square of speed, which is why an airliner needs about 250 to 300 km/h (135–160 kn) before it can leave the runway and why it can fly on far less power at cruise than at take-off. The second is the angle of attack, the angle between the wing chord and the oncoming air. Raising the nose increases the deflection of air and the lift, up to a point. Beyond a critical angle, typically 15 to 20 degrees for a conventional wing, the airflow over the top separates and lift collapses: this is the stall, and much of a pilot's basic training is about staying away from it.

A flat board at an angle produces lift, as any paper airplane demonstrates, and a symmetrical wing flies inverted perfectly well, which is how aerobatic aircraft sustain inverted flight. Cambered airfoils are chosen because they produce lift with less drag and stall more gently, not because they are the only way to fly. Flaps and slats extend at low speed to increase camber and area, which is why the wing of a landing airliner looks so different from the same wing at cruise. Engines do not lift the aircraft; they supply the forward speed that keeps the air moving over the wing, and a glider with no engine at all flies by trading altitude for that same airflow.