What happens when an airplane takes off?

Answer 1

There are four basic forces acting on the airplane. First is GRAVITY, which pulls the airplane down. The second is THRUST, the power of the plane's engines; this can be the force of the propeller, or the jet engine. The third force is DRAG; as the airplane moves through the air, the air DRAGS the airplane back to slow it down. THRUST counteracts DRAG.

The fourth force is LIFT. The shape of the airplane's wing forces the air to travel a little faster over the top of the wing than over the bottom of the wing. The air moving sideways along the surface of the wing causes lower pressure, so the faster air generates lower air pressure on the top of the wing, and higher pressure on the bottom. The higher pressure on the bottom provides LIFT, which pushes the airplane up.

So the higher the THRUST, the greater the LIFT, so as the pilot spins up the engines, the plane goes faster and faster (opposing the DRAG) until the LIFT exceeds the force of GRAVITY. And the airplane flies.

In landing, the pilot throttles back the engine, decreasing the THRUST, which causes lower LIFT, and GRAVITY pulls the airplane down. It's a delicate balance; we want to keep all these forces in balance until the wheels are on the ground, at which time the pilot cuts the engines and the THRUST, and allows DRAG and GRAVITY to take over.