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Airplane

Subhash Sharma

We take it for granted that we can fly from one side of the world to the other in a matter of hours, but only a century ago this amazing ability to race through the air was discovered.

Let’s take a closer look at how it works!

How do planes fly?

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: 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: Four forces act on a plane in flight.

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 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.

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.

 

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: Newton’s third law of motion acting on a plane.

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.

 

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.

Photo: An airfoil wing has a curved upper surface and a flat lower surface. Source: explainthatstuff.com
Photo: An airfoil wing has a curved upper surface and a flat lower surface. Source: explainthatstuff.com

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!

Wing vortices

Now a plane doesn’t throw air down behind it in a completely clean way. Each wing actually sends air down by making a spinning vortex (a kind of mini tornado) immediately behind it. It’s a bit like when you’re standing on a platform at a railroad station and a high-speed train rushes past without stopping, leaving what feels like a huge sucking vacuum in its wake. With a plane, the vortex is quite a complex shape and most of it is moving downward, but not all. There’s a huge draft of air moving down in the center, but some air actually swirls upward either side of the wingtips.

What is steering?

Steering anything means you change the direction in which it’s traveling. In scientific terms, changing something’s direction of travel means you change its velocity, which is the speed it has in a particular direction.

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Photo: Steering a plane by banking at a steep angle. Source: explainthatstuff.com

Even if it goes at the same speed, if you change the direction of travel, you change the velocity. Changing something’s velocity (including its direction of travel) means you accelerate it. Again, it doesn’t matter if the speed stays the same: a change in direction always means a change in velocity and acceleration. Newton’s laws of motion tell us that you can only accelerate something (change its speed or direction of travel) by using a force. In other words, by pushing or pulling it somehow. To cut a long story short, if you want to steer something you need to apply a force to it.

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