How do airplanes fly?

How do airplanes fly?

An airplane flies because its wings deflect air downward, and by Newton's third law the air pushes the wings upward. That upward push is called lift. In steady level flight lift exactly balances the plane's weight, and the engines only have to overcome drag.

You can describe the same lift in two equivalent ways: as a change in the momentum of the air, or as lower pressure above the wing than below it. Both are correct; neither needs the wing to be curved on top.

What are the four forces on an airplane?

Why does a wing make lift?

A wing is tilted slightly upward relative to the oncoming air. The air that meets it is forced to turn downward as it leaves the trailing edge. Pushing air down means giving it downward momentum, and the reaction on the wing is an equal upward force. The curved, rounded top surface helps the air stay attached to the wing while it turns, and that is where the low pressure comes from: the air above is accelerated and its pressure falls, while the pressure below rises.

How much lift does a wing make?

Here $\rho$ is the air density ($1.225$ kg/m³ at sea level), $v$ the speed through the air, $S$ the wing area and $C_L$ the lift coefficient, which grows with angle of attack. To hold up a $1000$ kg plane with $S = 16\ \text{m}^2$ and $C_L = 1.2$, set $L = mg$ and solve: $v = \sqrt{2mg/(\rho S C_L)} \approx 29$ m/s, or about $104$ km/h.

This also answers why planes need a long runway: below that speed the wings cannot make enough lift, and the plane keeps rolling until $v$ is high enough. On a hot day or at a high-altitude airport the air is less dense, $\rho$ is smaller, and the required speed goes up.

What is a stall?

Raising the angle of attack increases lift, but only up to a limit, typically around $15°$. Beyond it the smooth airflow separates from the top of the wing, turbulence takes over and lift drops suddenly while drag rises. A stall is about angle, not about engine power: a plane can stall at any speed if the wing is tilted too much.

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