How Does An Airplane Stay In The Air?
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You're listening to Shortwave from NPR. Driving around Washington, D.C., I see commercial planes all the time, taking off from Dulles, coming into Reagan. They're kind of a marvel of engineering in the form of a gigantic aluminum bird.

But that's not how Rami Barnwell feels about it. A longtime friend of the show, Rami is not a fan of flying. I always had a stuffed animal with me in childhood when I would fly. I still have it. It's a beanie baby named Bridget. And one time in the Atlanta airport, I left her on the plane train and my family missed our flight because I had to have Bridget to get on the plane. This fear stayed with her into adulthood.

At 25, she flew to her best friend's wedding. And that was the first time as an adult she flew sober. And she was overcome with anxiety. I thought that I was going to throw up the entire time. Like in the airport, I like couldn't get my heart rate down. I was freaking out. Like when I sat down on the flight, I just tried my hardest to fall asleep before it took off. So that's what Rami does now. She's chosen the plain nap strategy.

Falls asleep before it takes off, always in a window seat, always cocooned with an eye mask. But the fear is still there in the back of her mind, and it's entirely to do with the plane. I think the takeoff and in the air are the scariest. For me, the takeoff is like...

This doesn't make sense. It's not going to go in the air. It's not going to work. This is so heavy. How is it going to make it every single time? So you're not disconnected at all from how absolutely absurd it is that we fly? Not at all. It's like very hyper present in my mind. And I actually, when I get on a flight, I'm like,

Having to remind myself that the odds of the plane crashing are smaller than the odds of me getting into a car accident. Every time I fly, I'm like, Raimi, this plane is not going to crash. Probably. Probably. Statistically, a plane crash is super unlikely.

But still, according to a 2016 paper, up to an estimated 40% of Americans feel some fear at the thought of flying. And Raimi has questions. Yeah, I would like to understand how the flight stays in the air once it takes off, despite the weight differential between the plane and the air. Okay. How it gets up there. How it gets up there, yeah. And then I want to know how it stays up there.

Ramey, I'm going to get answers to some of these questions for you. Today on the show, on the wings of Ramey's questions, we take flight with MIT aeronautical engineer Mark Drella. So I'm wondering, do you sympathize with people who are skeptical about how planes manage to stay up in the air? Sympathize? I'm not. I don't.

I don't get it why it's so mysterious. You know, there's so many examples of phenomena which basically demonstrate lift in a very intuitive way. We get into lift, which is the force that holds a plane in the air. All the fluid mechanics of how a plane flies and how this is done stably and safely. You're listening to Shortwave, the science podcast from NPR.

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All right. First, Mark, let's talk about Lyft. How does Lyft work? Well, there's lots and lots of explanations. So, for example, I don't know if you've ever done this when you're a kid. You can kind of fly your hand.

Right, as you hold it out the car window. Yeah. You can let the air support the weight of your hand, right, if you angle it just right. Yeah, it kind of feels like you're resting your hand on a shelf of air. Right, on an air cushion. Yeah. If you've got to angle it up just a little bit, let the air blow underneath, so to speak, it'll support the weight of your arm, right, of your forearm. That's basically what lift is, intuitively. Right.

Air moving past an object pushes on that object. I mean, why is that a mystery, right? And you see it lots of ways. For example...

How does a water skier stay up? Well, the water rushing past the water skier pushes up on the skis and keeps the skier above the water, right? Right. The equations of motion for that situation are the same as for an airplane. Based on basic physics, there is really not that much significant difference between water and air. They're both classified as fluids.

Big difference is simply the density. You know, water is about 800 times more dense than air. But that's the only difference. So if you could visualize air going past an object moving through the air, or if you visualize a water moving past an object moving through the water, they literally look the same. So for example, the Goodyear blimp...

Wow. Ugh.

It occurs to me also, planes have wings. Like, why? Why do you need wings? What are wings? Okay, so just going back to your hand moving out of the car window, right? Yeah. The air needs to an area to push against. Pressure is the major way how fluids impart force to an object. So, for example, water rushing past a water skier,

The water pressure under the skis is larger than the overall surrounding pressure, and that extra pressure on the bottom pushes up on the skier. Just like the air pressure underneath on the bottom surface of a wing pushes up on

on the wing and that holds up the airplane. So you need area. In fact, on very simple high school algebra, force is pressure times area. So does that mean that the bigger the wing, the bigger the area, the greater the force? Absolutely. Okay. Planes also, obviously, they have engines that provide thrust, the force that moves a plane forward and helps it climb into the sky. Right.

But besides the engines, what else is happening such that lift happens? Okay, so again, the simplest explanation is you go back to your hand sticking out the car window. If you hold your hand perfectly horizontally, right, you won't feel a force. To get an upward force, you have to tilt your hand so that you present the bottom of your hand to the oncoming flow, right? That angle is called the angle of attack. ♪

That will make the pressure on the bottom of your hand larger than on top, and that resulting pressure difference imbalance times the area of your hand is the lift acting on your hand.

Exactly the same thing happens with an airplane. This angle of attack part, is that the same thing as when you're trying to surf and you need to catch a wave and you need the surfboard to hit the wave at the right angle? Yeah, yeah. An airplane is far more subtle because the nose is just barely higher up. But it's the same idea, yes. So as that happens...

What's happening to the air around the wing? Yeah, so basically the air pushes on the wing. And in the process, the wing also pushes on the air. This is Newton's third law. When two things push against each other, it's never one way. So for example, when the air pushes up on the airplane, the airplane pushes the air down slightly as a result.

So what happens is, in practice, if you look at the overall picture, if the airplane wasn't there, the air just would sail through straight. But because there's a wing there, the air curves, and that curvature is related to the differences in pressure.

So specifically for lifting wing, the pressure under the wing tends to increase and the pressure above the wing tends to decrease just from the angle of attack and the shape of the wing. Is it that pressure difference that creates the lift? Exactly. So when a plane is taking off, because it has those propellers, you have the speed and the

The wing, so long as it's operational and the angle of attack is right, that lift phenomenon will happen. And that suggests to me that the angle of attack is really important. Mm-hmm.

What mechanisms exist on planes to make sure that angle of attack is right every time? Yeah, so the thing that holds the wing at the correct angle is the tail. And birds have tail feathers for a very similar reason. The tail is very much like the fins on a weather vane or a dart, right? It kind of points the airplane into the wind at the correct angle. So the pilot is never like, okay, the angle of attack will be this. Like, I'm going to calibrate the angle of attack.

Well, okay. First of all, the pilot can control the tail. In fact, that's the primary purpose of the pilot is to control the tail. Right. So the pilot, by moving the control stick, moves the tail and the tail then moves the wing to the right angle. Now, in practice, the airplane kind of passively does the right thing.

Because what happens is when you're rolling down the runway, as the pilot then moves the stick, then the tail tilts the entire airplane, including the wing. And then once the angle of attack is such that the resulting pressure difference equals the weight, the airplane lifts off the ground. And then it kind of keeps going. All right. So once the plane is up there,

We all know it can get a little bumpy. What is turbulence and how are planes able to stay safe in turbulent conditions? Well, it's basically the atmosphere is not perfectly still. So if the airplane flies into air that's rising or descending or just there's a lot of turbulence mixing air, so to speak, then that will modify the pressures on the wings and the fluctuating pressures and kind of fluctuate the airplane. That's what you're feeling.

And airplanes are designed to withstand very, very extreme turbulence. Even the most violent imaginable turbulence that you've ever felt before.

the airplane can easily withstand loads maybe three times bigger. So there's a very big safety margin on how aircraft are built. In fact, I can't remember hearing of an airplane breaking up in the midair because of turbulence. It just doesn't happen.

Okay. Well, that is definitely one fear. And then the other fear, I think, for a lot of people is crashes. What about that? And what about operator error? Well, obviously, you know, if you drive off a cliff, bad things will happen. Or if you drive into a rock face, same thing on an airplane. I mean, that's an operator error. There's nothing wrong with the airplane. It's just it was operated incorrectly. That's why you have two pilots. So we can imagine having...

Two drivers in your car, if one falls asleep, it's okay, the other one's there. The airplane is designed to almost fly hands-off. Another way to say it is, for example, if you're cruising your car down the highway and you let go of the steering wheel, nothing will really happen, right?

Because the car is passively stable. Passive stability is a good thing. It means that if you don't do anything, it'll just keep doing what it's doing. The plane is passively stable? Correct. Wow! Yeah, so the pilot could fall asleep for 10 seconds and it'll be fine.

I brought all of this information to Ramey and she said, That makes so much sense. Like it's so helpful to hear you say the plane wants to stay in the air coupled with the imagery of having my hand out the window. I'm like, oh yeah, it is very hard to make the air not create the waves, not do the thing.

Yeah, you're going to be safe. You're going to be okay when you fly. Yeah, the plane wants to stay in the air. The plane wants to stay in the air.

This episode was produced by Jessica Young. It was edited by our showrunner, Rebecca Ramirez. Tyler Jones checked the facts. Kweisi Lee was the audio engineer. Special thanks to Sasha Rice. Beth Donovan is our senior director. And Colin Campbell is our senior vice president of podcasting strategy. I'm Emily Kwong. Thank you, as always, for listening to Shortwave, the science podcast from NPR.

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