How Do Airplanes Fly?

Airplanes fly because their wings generate lift, a force that pushes the plane up by creating a pressure difference across the wing and by deflecting a huge mass of air downward. It's not Bernoulli's principle alone, the wing's angle of attack is just as important. The result is staggeringly powerful: a fully loaded Boeing 747 produces enough lift to carry around 400 tons.

The four forces on every plane

Flight is a constant tug-of-war between four forces. Get them balanced and you fly level; tip the balance one way or another and you climb, dive, speed up, or slow down.

• Lift - pushes the plane up, generated by the wings
• Weight - gravity constantly pulling the plane down
• Thrust - the engines pushing the plane forward
• Drag - air resistance pulling the plane back

Where lift really comes from

The popular explanation says air moving faster over the curved top of a wing creates lower pressure, sucking the wing upward. That's part of the story, but it's incomplete and has been overhyped in classrooms for decades.

The bigger piece is Newton's third law. The wing meets the oncoming air at a slight angle, called the angle of attack, and deflects a huge mass of air downward. Push that much air down, and the air pushes the wing right back up.

Both effects are real and they work together. But if you had to pick the one that does the heavy lifting, it's the wing throwing air toward the ground.

You can feel this yourself. Stick your flat hand out of a moving car window and tilt the leading edge up slightly; your hand gets shoved upward as it deflects air down. That's a wing in miniature, and it has nothing to do with one side being more curved than the other.

Angle of attack does the heavy lifting

Tilt the wing's leading edge up a little and lift increases, because more air gets deflected downward. This is exactly why a plane pitches its nose up to climb and to leave the runway on takeoff.

Tilt too far, though, and the smooth airflow breaks away from the wing. That's a stall, and lift collapses suddenly, which is why pilots treat the angle-of-attack limit with so much respect.

A wing doesn't get sucked into the sky. It throws air downward, and the air throws it right back up.

How a 400-ton machine stays up

A fully loaded 747 can weigh around 400 tons, and its wings have to generate that much lift continuously just to hold altitude. The engines don't lift the plane directly; they provide thrust so air keeps streaming fast over those wings.

Cut the thrust and the plane slows, the airflow over the wings weakens, and lift fades away. Flight, in the end, is really just the act of keeping enough air moving over the wings to win the tug-of-war against gravity, second after second after second.

What about gliders, with no engine at all

Gliders prove the engine isn't what creates lift. They generate plenty of it with no thrust, by trading altitude for forward speed and letting gravity do the pushing.

A glider is essentially always descending through the air around it, even when it's climbing relative to the ground. It points its nose slightly down, gains speed, and that speed keeps air streaming over the wings fast enough to stay aloft.

Pilots stretch a flight for hours by riding rising air, like the warm thermals over sun-baked ground that push the whole aircraft upward faster than it sinks. It's the same physics a 747 uses, just without the engines: keep air moving over the wing, and the wing keeps flying.

This is also why a powered plane doesn't simply drop out of the sky if its engines quit. It becomes a heavy glider, trading height for speed, and a skilled pilot can stretch that glide for many miles to reach a runway.

Try It Yourself

Want to mess around with the ideas above? On whatifs.fun, Flight Sim, Paper Airplane and Gravity Playground all let you do exactly that — free, in your browser, no download.

Keep reading: how high birds can fly and how fast sound travels. Both go deeper on the same rabbit hole.