How Does An Airplane Stay In The Air?

There are many statistics out there that prove that flying on a commercial airplane is safe, that plane crashes are overall pretty unlikely. Still, up to an estimated 40% of Americans feel some fear at the thought of flying. So, amid the travel rush of the holiday season, we ask MIT aeronautical engineer Mark Drela https://aeroastro.mit.edu/people/mark-drela/: How does a plane lift off and stay up in the air? Interested in more stories on physics? Email us at [email protected] mailto:[email protected]. We'd love to hear from you! Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Emily Kwong?

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• 0:23Emily Kwong?

  Driving around Washington DC, 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 Ramey Barnwell feels about it. A longtime friend of the show, Ramey is not a fan of flying.

• 0:42Speaker 01

  I always had a stuffed animal with me in childhood. When I would fly, I still have it. It's a beanie baby named Bridgette. 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 Bridgette to get on the plane.

• 0:58Emily Kwong?

  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.

• 1:08Speaker 01

  I thought that I was going to throw up the entire time. Like, in the airport, I 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.

• 1:22Emily Kwong?

  So that's what Ramey does now. She's chosen the plane 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.

• 1:38Speaker 01

  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?

• 1:51Emily Kwong?

  So you're not disconnected at all from how absolutely absurd it is that we fly?

• 1:57Speaker 01

  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, Ramey, this plane is not going to crash, probably. Probably.

• 2:21Emily Kwong?

  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.

• 2:35Speaker 01

  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.

• 2:45Emily Kwong?

  OK. How it gets up there?

• 2:47Speaker 01

  How it gets up there, yeah. And then I want to know how it stays up there.

• 2:51Emily Kwong?

  Raimi, I'm going to get answers to some of these questions for you. Today on the show, on the wings of Raimi's questions, we take flight with MIT aeronautical engineer, Mark Drell. So I'm wondering, do you sympathize with people who are skeptical about how planes manage to stay up in the air?

• 3:10Speaker 02

  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.

• 3:23Emily Kwong?

  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 Short Wave, the science podcast from NPR.

• 3:44Emily Kwong?

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• 5:05Emily Kwong?

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• 6:44Emily Kwong?

  All right, first, Mark, let's talk about lift. How does lift work?

• 6:48Speaker 02

  Well, there's lots and lots of explanations. So, for example, I don't know if you've ever done this with your kid. You can kind of fly your hand, right, as you hold it out the car window.

• 6:58Emily Kwong?

  Yeah.

• 6:58Speaker 02

  You can let the air support the weight of your hand, right? If you angle it just right.

• 7:03Emily Kwong?

  Yeah, it kind of feels like you're resting your hand on a shelf of air.

• 7:07Speaker 02

  Right, on an air cushion.

• 7:08Emily Kwong?

  Yeah.

• 7:09Speaker 02

  If you 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. 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?

• 7:38Emily Kwong?

  Right.

• 7:38Speaker 02

  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.

• 8:19Speaker 02

  So, for example, the Goodyear blimp is almost mathematically identical to a submarine.

• 8:28Emily Kwong?

  Wow.

• 8:29Speaker 02

  Mathematically, you cannot tell the difference.

• 8:35Emily Kwong?

  It occurs to me also, planes have wings. Like, why? Why do you need wings? What are wings?

• 8:43Speaker 02

  Okay, so just going back to your hand moving out of the car window, right?

• 8:49Emily Kwong?

  Yeah.

• 8:50Speaker 02

  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 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.

• 9:33Emily Kwong?

  So does that mean that the bigger the wing, the bigger the area, the greater the force?

• 9:40Speaker 02

  Absolutely.

• 9:41Emily Kwong?

  Okay. Planes also, obviously, they have engines that provide thrust, the force that moves a plane forward and helps it climb into the sky. But besides the engines, what else is happening such that lift happens?

• 9:55Speaker 02

  Okay, so again, the simplest explanation is if 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.

• 10:21Speaker 02

  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.

• 10:38Emily Kwong?

  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 like hit the wave at the right angle?

• 10:49Speaker 02

  Yeah, yeah, an airplane is far more subtle because the nose just barely higher up.

• 10:55Emily Kwong?

  Yeah.

• 10:56Speaker 02

  But it's the same idea, yes.

• 10:59Emily Kwong?

  So as that happens, what's happening to the air around the wing?

• 11:04Speaker 02

  Yeah, so basically the air pushes on the wing.

• 11:08Emily Kwong?

  Uh huh.

• 11:09Speaker 02

  And in the process, the wing also pushes on the air. Right, 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. Okay, 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.

• 11:54Emily Kwong?

  Is it that pressure difference that creates the lift?

• 11:57Speaker 02

  Exactly.

• 11:58Emily Kwong?

  So when a plane is taking off, because it has those propellers, you have the speed. And 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.

• 12:17Speaker 02

  Mm hmm.

• 12:18Emily Kwong?

  What mechanisms exist on planes to make sure that angle of attack is right every time?

• 12:23Speaker 02

  Yeah, so the thing that holds the wing at the correct angle is the tail. And birds have tail feathers for very similar reasons.

• 12:31Emily Kwong?

  Whoa.

• 12:32Speaker 02

  Right, 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.

• 12:40Emily Kwong?

  So the pilot is never like, okay, the angle of attack will be this, like I'm going to calibrate the angle of attack.

• 12:46Speaker 02

  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.

• 12:53Emily Kwong?

  That's what a pilot is doing!

• 12:54Speaker 02

  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. Right? And then it kind of keeps going.

• 13:26Emily Kwong?

  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?

• 13:36Speaker 02

  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 then 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, the airplane can easily withstand loads maybe three times bigger. Okay?

• 14:18Speaker 02

  So, there's a very big safety margin on how aircraft are built. So, you almost, in fact, I can't remember hearing of an airplane breaking up in the midair because of turbulence. It just doesn't happen.

• 14:32Emily Kwong?

  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?

• 14:42Speaker 02

  Well, obviously, you know, if you drive off a cliff, think bad things will happen, or if you drive into a rock face. Same thing on 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, you can imagine having two drivers in your car. If one falls asleep, it's okay. The other one's there.

• 15:08Speaker 02

  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 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.

• 15:31Emily Kwong?

  The plane is passively stable?

• 15:33Speaker 02

  Correct.

• 15:34Emily Kwong?

  Wow.

• 15:35Speaker 02

  Yeah, so the pilot could fall asleep for 10 seconds and it'll be fine.

• 15:42Emily Kwong?

  I brought all of this information to Rani, and she said-

• 15:45Speaker 01

  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 with my hand. Yeah, you're gonna be safe. You're gonna be OK when you fly.

• 16:10Emily Kwong?

  Yeah, the plane wants to stay in the air.

• 16:12Speaker 01

  The plane wants to stay in the air.

• 16:19Emily Kwong?

  This episode was produced by Jessica Young. It was edited by our showrunner, Rebecca Ramirez. Tyler Jones checked the facts. Kwasi 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 Short Wave, the science podcast from NPR.

• 16:47Speaker 03

  Stay tuned for the following announcements and messages from our sponsors.