Why planes fly

A large airliner weighs around 250 tonnes at take-off, as much as 50 elephants, and yet it lifts off seemingly without effort. How can something so heavy fly? You already know the answer on a small scale: your paper plane sails across the room because forces pull and push on it. On a big aircraft it is exactly the same four forces, just enormously stronger.

Four forces on a plane

Four main forces act on every aircraft in flight. Weight pulls it down, lift pushes it up. The thrust of the engines drives it forward, and air resistance (drag) holds it back. You can picture these four forces as arrows pulling on the plane: whichever wins decides whether the plane climbs, descends, speeds up or slows down.

Where does lift come from?

The secret is in the wing. It is tilted slightly and curved on top, so it deflects the passing air downwards. And just like recoil: if the wing pushes the air down, the air pushes the wing up. At the same time, the air flows faster over the top of the wing, the pressure there drops, and the higher pressure from below gives an extra push upwards. The faster the plane flies, the more air is deflected every second and the bigger the lift becomes. That is why planes first need real speed on the runway before take-off.

The balance of forces in flight

In calm, level flight there is balance: lift is exactly as big as weight, and thrust is exactly as big as drag. The plane then holds its altitude and speed. At take-off the pilots apply full thrust, lift becomes bigger than weight, and the plane climbs. For landing, thrust is reduced, lift shrinks below weight, and the plane descends gently. Your paper plane does the same: with no thrust of its own it slows down, loses lift and glides to the floor in a shallow arc.