There’s No One Way to Explain How Flying Works

Let’s be clear—airplanes are complicated. Sure, it’s entirely possible to get a piece of paper and fold it in a particular way so that it flies. But the physics of flight isn’t trivial. It’s even harder to give an explanation of the forces on a flying aircraft in a short video—which is what I did with my recent WIRED video on the physics of flying.

One of the most common comments to this video was something like this:

What the heck is wrong with this video? Some dude is trying to explain how planes fly and he didn’t even say the word “Bernoulli.” Everyone already knows that planes fly because of Bernoulli’s principle. This guy needs to go back to school.

OK, the part about going back to school is probably true (I can always learn more). But it turns out you don’t really need Bernoulli’s principle to explain how airplanes can fly.

Let’s suppose I wanted to explain the flight of a plane. I could use this common explanation:

A wing is curved on the top. When it runs into the air, some air goes over the top and some goes underneath. Since the top air path is longer, it has to go faster. According to Bernoulli’s principle, faster air reduces pressure. With less pressure on the top, the plane gets pushed up. It’s physics.

Alas, there are problems with an explanation like this. The first problem is that air doesn’t travel over the top at a faster speed because it’s a longer path—that’s just not true (check out this great video explaining the common problems with the flying explanation). The second problem is attacking a complicated idea (flying) with another difficult concept (Bernoulli’s principle). By using Bernoulli’s principle, the physics of flight becomes like a word association game. I say lift force, you say Bernoulli. Don’t worry about what that means, just remember that Bernoulli is the right answer.

You can see these kinds of explanations in other areas. My favorite is the question of what causes the seasons on Earth. Or like I phrase it: Why is it hotter in the summer? If you ask people on the street, I bet a majority of them will say something about the tilt of the Earth’s axis. Indeed, it is warmer on Earth (in the Northern hemisphere) because of Earth’s tilted axis. But if you press further and ask why this makes it warmer, there is a good chance they’ll respond that the tilt of the Earth brings us closer to the sun, thereby heating things up. HINT: This is wrong. The real reason that it’s warmer in the summer is that the sun travels higher in the sky for a longer time. Both of these factors means more solar heating of the ground and hotter weather. In fact, because the Earth’s orbit isn’t completely circular, we are actually farther from the sun during the summer—but it’s still hotter than the winter.

So, in the end people get this false sense of understanding (whether it’s about flying planes or the reason for the seasons).

In the case of wing lift, I like to use more fundamental ideas—like the momentum principle. This is the same idea that you would use to explain the forces on a wall when a ball hits it. It’s more likely a human will be able to relate to the idea of a bouncing ball than something like Bernoulli’s principle. Here’s the short version of the way a wing works. The wing crashes into air in such a way that it pushes it down. Since forces come in pairs, pushing the air down means the air pushes up on the wing. Boom, that force is what we call lift.

This goes along with my Number 1 Rule in Science Communication:

You can rarely be 100 percent correct in your explanation, but you can be 100 percent wrong. The goal isn’t to be correct in your writing, it’s to not be wrong.

The flying video doesn’t tell the whole story, but it isn’t wrong. Also, explaining the lift force using the momentum principle isn’t new (for instance see this paper) and it doesn’t tell the full story. The interaction with the wing and the air not only depends on the bottom of the wing but what happens with the air on the top of the wing. In physics, it’s always complicated.

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