How does a plane fly?

A loaded Boeing can weigh more than 300 tonnes and still lift off the ground and cross oceans at 900 kilometres per hour. There is no magic behind it: there is physics, and rather elegant physics at that. The whole of flight comes down to the balance of four forces acting on the aircraft at the same time.

The four forces of flight

As a plane moves through the sky, four forces pull on it in different directions. As Europe’s aviation safety agency EASA sums it up, steady flight happens when those forces balance one another:

• Lift: pushes the plane up. It is generated by the wings.
• Weight: gravity pulling the plane toward the ground.
• Thrust: drives the plane forward. It comes from the engines.
• Drag: the friction of the air, which slows the aircraft down.

To take off, lift must overcome weight; to accelerate, thrust must overcome drag. When all four are matched, the plane flies straight and level, gliding stably through the air.

How lift is born

The key to everything is the wing, whose profile is not symmetrical: the upper surface is more curved than the lower one. As the plane advances, the air splits at the leading edge of the wing and rejoins behind it. Here it helps to grasp two complementary ideas, not rival ones:

1. The air speeds up over the wing. Travelling along the curved upper surface, the air moves faster and its pressure drops; underneath, it is slower and at higher pressure. That pressure difference sucks the wing upward (the principle associated with Bernoulli).
2. The wing deflects air downward. The wing “pushes” masses of air toward the ground and, by Newton’s third law (action and reaction), the air pushes the wing upward with equal force.

Both effects are two sides of the same phenomenon. A decisive factor is the angle of attack: the tilt of the wing relative to the incoming air. The greater the angle, the more lift… up to a point, because if it is excessive the airflow separates and the wing “stalls” and stops holding the plane up. That is why, as explained by EAS Barcelona, understanding aerodynamics is not empty theory: it lets the pilot anticipate how the plane will behave in every manoeuvre.

Thrust: no speed, no flight

Wings only generate lift if air is moving over them, and that requires speed. Hence the role of thrust. Whether from jet engines or propellers, thrust launches the plane forward and “manufactures” the relative wind the wings need.

An everyday example makes it clear: if you stick your hand out of a moving car’s window and tilt it slightly upward, you feel the air pushing it up. The faster the car goes, the stronger that push. Your hand is an improvised wing, and the car’s speed acts as the engine.

How it is steered: the three axes

Flying is not just about staying up: you also have to manoeuvre. The plane is controlled on three axes using movable surfaces:

• Ailerons (on the wing edges): make the plane tilt to one side (roll), to turn.
• Elevator (on the horizontal tail): raises or lowers the nose (pitch).
• Rudder (on the vertical tail): points the nose left or right (yaw).

By combining these surfaces with engine power, the pilot governs the aircraft precisely, from take-off to landing.

A common myth

You often hear that “a plane flies only because of the Bernoulli effect” or, conversely, “only because it deflects air.” The reality, as technical explainers like Flightdemy point out, is that both explanations describe the same phenomenon from different angles. Reducing it to a single formula leads to wrong conclusions, such as thinking a plane could not fly upside down (it can: by adjusting the angle of attack, many aerobatic planes fly inverted).

Everyday physics at 10,000 metres

That such a heavy object stays in the air stops being surprising once you see the parts: wings that turn speed into lift, engines that provide that speed, and control surfaces that let you steer the whole thing. Next time you watch a plane take off, you will know it is not defying physics: it is using it with millimetric precision. And that, perhaps, is the most fascinating part of flight.