{♫Intro♫}.

When the Wright Brothers made their first flight in 1903, people had been trying to fly for centuries. So even though their plane didn't go much faster than 10 kilometers per hour -- basically, running speed -- it was a major achievement.

What's maybe equally amazing, though, is that it didn't take another few centuries for us to achieve supersonic flight. Chuck Yeager became the first person to fly faster than the speed of sound -- more than 1200 kilometers per hour -- in 1947, not even 50 years after the Wright Brothers. But making that flight wasn't just a matter of building stronger engines.

To do it, engineers had to solve hundreds of problems -- including ditching one of the biggest assumptions in aerodynamics. They clearly did it, though. And today, the innovation that helps keep supersonic planes from falling apart is also the main reason why just about all commercial airplanes look the same.

At first, making a faster plane was really just about building better engines and structures. Except, as we got better at flying and started approaching the speed of sound, we noticed that our aircraft… just didn't behave like they were supposed to. We first noticed this with propellers.

Propellers act like sideways wings, and because they're moving with the plane and spinning, they cut through air a lot faster than the rest of the aircraft. But once propellers got moving faster than about half the speed of sound, they suddenly stopped producing as much thrust as expected. And at high speeds, regular wings also didn't get lift like they did at low speeds.

So a plane that flew perfectly at 15% the speed of sound might fall out of the sky at 60%. The problem lingered for about fifteen years, until an aerodynamicist named John Stack found a fundamental flaw in the models and equations everyone used to understand how air moved past objects. In 1933, after looking at experiments in wind tunnels, he realized that air pressure around a quickly-moving object drops, because, ultimately, the air is getting squeezed.

That might not seem like a big deal, but it actually violated one of the biggest assumptions in aerodynamics. For decades, people had assumed -- in one form or another -- that air is incompressible. It doesn't squeeze or stretch as it runs into things, but instead just glides by or bounces off.

In other words, air pressure might change from place to place, but its density doesn't. This assumption means that you don't need to worry about air interacting with itself or how that interaction affects the rest of the world. That makes calculations way easier, and, at slow speeds at least, it's mostly true,.

Which is why people hadn't had to worry about it. But John Stack and other researchers showed that, as you approach the speed of sound, thinking that air is incompressible is just flat-out wrong. And it has to do with what the speed of sound actually means.

Essentially, it's just how fast information -- like that a plane is coming -- can get passed between groups of air molecules. When a plane is going slowly, molecules can push each other out of the way long before the aircraft gets to them. But if the plane is going close to the speed of sound, there's no time for the air to move, so it piles up.

That forms a shockwave that acts almost like a shield. It blocks other air from smoothly moving past the plane, decreases lift, and increases drag forces that slow the aircraft down. Which explained why wings and propellers become less effective at those high speeds.

After this realization, engineers developed all sorts of clever ways of mitigating this problem, from stabilizers that stopped shockwaves from tearing planes apart to giving planes rocket engines that fought the extra drag. These innovations all culminated in Chuck Yeager's famous flight in 1947. But supersonic or not, shockwaves still threatened to tear planes to pieces.

So before high-speed flight became a regular thing, engineers had to make a change to the wings that you can still see on just about any fast plane today. Originally, most planes had wings that went straight out sideways, perpendicular to their bodies. That let air move over as much surface as possible, generating as much lift as possible.

The problem was, if a shockwave formed, air didn't have anywhere to go. It just kind of piled up in front of the wing, which led to all those lift and drag issues. So engineers came up with an alternative: swept wings.

They're wings that come out of the plane's body at an angle, usually with the wings pointed back toward the tail. The angle meant that air could get pushed along the wings instead of piling up around them -- sort of like sliding down a ramp instead of running into a wall. The fact that swept wings kept air moving more than made up for having slightly less air going across the wings and actually creating lift.

And you see this design everywhere today, from military planes to commercial jets, which fly at about 70% the speed of sound. These designs all fundamentally take advantage of the same clever piece of engineering, which is why they all look pretty similar -- and why the basic airplane design hasn't changed much in the last 50 years. Because once you've found a solution, you don't always need to keep switching things up.

Thanks for watching this episode of SciShow, especially to our patrons on Patreon! Your support has meant a lot over the years, and we're thankful to have been given the chance to make hundreds of these videos. If you want to become a patron and support free science education online, you can go to patreon.com/scishow. {♫Outro♫}.