How flights fly

How do planes fly?

Forces acting on a flying plane: thrust, weight, drag, and lift

If you’ve ever watched a jet plane taking off or coming in to land, the first thing you’ll have noticed is the noise of the engines. Jet engines, which are long metal tubes burning a continuous rush of fuel and air, are far noisier (and far more powerful) than traditional propeller engines. You might think engines are the key to making a plane fly, but you’d be wrong. Things can fly quite happily without engines, as gliders (planes with no engines), paper planes, and indeed gliding birds readily show us.

Photo: Four forces act on a plane in flight. When the plane flies horizontally, lift from the wings exactly balances the plane’s weight. But the other two forces do not balance: the thrust from the engines pushing forward always exceeds the drag (air resistance) pulling the plane back. That’s why the plane moves through the air. Photo by Kemberly Dawn Groue courtesy of US Air Force.

If you’re trying to understand how planes fly, you need to be clear about the difference between the engines and the wings and the different jobs they do. A plane’s engines are designed to move it forward at high speed. That makes air flow rapidly over the wings, which throw the air down toward the ground, generating an upward force called lift that overcomes the plane’s weight and holds it in the sky. So it’s the engines that move a plane forward, while the wings move it upward.

Diagram showing Newton's third law of motion applied to the wings and engines of a plane.

Photo: Newton’s third law of motion explains how the engines and wings work together to make a plane move through the sky. The force of the hot exhaust gas shooting backward from the jet engine pushes the plane forward. That creates a moving current of air over the wings. The wings force the air downward and that pushes the plane upward. Photo by Samuel Rogers (with added annotations by explainthatstuff.com) courtesy of US Air Force. Read more about how engines work in our detailed article on jet engines.

How do wings make lift?

Airfoils

Okay, so the wings are the key to making something fly—but how do they work? In most science books, you’ll read that airplane wings have a curved upper surface and a flatter lower surface, making a cross-sectional shape called an airfoil (or aerofoil, if you’re British):

Photo showing airfoil wing on the NASA Centurion solar-powered plane.
Photo: An airfoil wing has a curved upper surface and a flat lower surface. This is the wing on NASA’s solar-powered Centurion plane. Photo by Tom Tschida courtesy of NASA Armstrong Flight Research Center.

When air rushes over the curved upper wing surface, it has to travel further and go slightly faster than the air that passes underneath. According to a basic theory of physics called Bernoulli’s law, fast-moving air is at lower pressure than slow-moving air, so the pressure above the wing is lower than the pressure below, creating the lift that holds the plane up. Although this explanation of how wings work is widely repeated, it’s not the whole story. If it were the only factor involved, planes couldn’t fly upside down. Flipping a plane over would produce “downlift” and send it crashing to the ground!

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