Daniel and Kelly dig into the surprisingly controversial question of why airplanes stay in the air.
They say that any technology sufficiently advanced is indistinguishable from magic. Well, there's one technology that people have lusted after for thousands of years that we now rely on even find routine. I'm talking, of course, about flight. It's been only one hundred years since people were able to leave the ground and fly through the air and return safely. Now it's so routine that people sleep through it. But imagine what that would seem like someone from a thousand or ten thousand years ago. It's essentially like magic. Of course, we know it's actually not. It's science because we can understand it, and we can break it down. We can explain it. We understand why planes fly, right, do we could we actually explain it to visiting ancestors from the deep past? Or is it still some kind of black magic even for modern humans and the best aeronautical engineers. So today on Daniel and Kelly's Extraordinary Universe, we'll be asking an old question, why do planes fly?
Hi?
I'm Kelly Wiener Smith and my favorite way to travel is by train.
Hi. I'm Daniel Whitson. I'm a particle physicist, which means I have to fly around the world to get to the biggest particle accelerator.
And what is your worst flight experience?
My worst flight experience is traveling with my own six month old baby. We had a bascinet and we got her to sleep in the basinet and we thought, oh, this is going to be great. Then the stewardess came by and said, oh, there's some turbulence. You have to take her out and put her in a seat and buckle her in. And he said, no, she's sleeping, and she said sorry, it's the rules. And everyone around us went, oh no, and they were right, because she screamed for the rest of the eight hour flight.
Oh no, that's horrible. I'm so sorry. When you finally get your kid asleep and something wakes them up on a plane, it's the worst.
It's the worst. And everybody knew it was the worst, and we all knew there's nothing we could do, but hey, you know, them's the rules. How about you, what's your worst traveled experience?
It also involves plane with my child. She was probably about six months old around there also, and I brought a change of clothes for her. I was totally prepared for her. She was sitting on my lap and we were about to take off, and right as the plane started moving forward, she had a blowout poop on my leg and so it smelled horrible. But you can't get out of your seat until you rechruising altitude. So everybody was like giving me dirty looks and covering their noses, and it took like twenty minutes before we were allowed to get out of our seats. And then you know, I had to change her in the airplane changing room, which is like so tiny, I thought she was gonna fall off the little tray. And anyway, I brought a change of clothes for her, but I didn't assume i'd have on my pants.
Oops. Classic parenting mistake exactly.
So I smelled horrible the whole flight, and people kept looking at me, and it wasn't a lot of fun.
But them's the breaks you got to leave the house with kids.
Sometimes you just got to look back at those people and say, hey, who's going to pay for your social security? Right? Somebody's got to make the next generation. And to me, Madame poopy Pants, I'll get a.
Little like badge that says that.
Now, whenever I see a parent with a kid, I'm like, what can I carry for you. How can I help like I offer to help all the time.
Yeah, exactly. And the big lesson I learned is never judge, and you never in somebody else's family. You don't know what they're going through or what they're struggling with or how much they slept last night. Just help and smile. Yep.
And kids are always at their worst because they're always the tiredest and they're not used to it, and so like anyway, yes, don't judge. But planes are wonderful. I needed to get from place to place, and I needed to get there quickly. And actually I wouldn't have wanted to take my daughter on a long train ride for the journey that we were going on, because it would have been days. But I was flying to London the other day and we had kind of talked about why do planes fly? And you said it's complicated, And while I was, you know, above the Atlantic, I thought, how complicated?
How well do we understand this?
I hope the answer is really well, So I'm looking forward to today's conversation.
Good Well. This question actually came to me from a listener. Tom Johnson from Ohio wrote to me and asked if we could explain why planes fly because he looked into a little bit and he was kind of confused. So he wanted a clear explanation for why it is that planes stay in the air.
And if you have a question that you want to ask us, we would be happy to answer them. So send us an email at questions at Daniel and Kelly dot org.
When I told Tom that we were going to answer his question on the pod, he said, quote, my wife is absolutely going to roll her eyes tonight when I tell her over sushi, which made me wonder. Tom is in Ohio. Where is he getting a sushi? Probably they have it flown in so even his sushi relies on airplane in wings.
Yeah, and you know, I lived in Ohio for a decade or more, and I can tell you that when sushi has to fly that far, it's not great.
You're too far from the ocean.
At that point, you're saying nobody should fly to Ohio for sushi.
No, I'd say that.
I'm going to go on the record saying you shouldn't fly just to Ohio for sushi.
Ohio has got other things going for it.
You should have Ohio pride. You know, I know you're a transplanted Virginian, but like you come from Ohio. A good thing about Ohio, for example, is it made you You.
Know, I was actually born in New Jersey, which is an even harder state to defend, but I do actually love New Jersey and I will defend New Jersey. But you know, Ohio was a very safe place to grow up where I grew up, and so that's fine. And I think Ohio has made the most astronauts, and so you know, we all would make jokes about you know, what is it about Ohio that makes you want to get as far as possible away from this planet? And I'm not sure if Ohio is still winning an astronaut production, but it was at some point.
All right, well, let's stop flying around the issue and get to the question. I was wondering what people out there thought about how planes stay in the air, how wings work, what stories they had been told, and what they understood about this question. So, as usual, I went to our bank of volunteers to ask them why do planes fly? If you would like to play for a future episodes, please don't be shy right to us to questions at Danielankelly dot org. In the meantime, think about it for a moment. Why do you think planes fly? Here's what a bunch of listeners had to say.
I believe nobody really knows. I would be glad to learn otherwise. I know that I've read in my school book that the air takes a longer way on the top of the wing than on the bottom of the wing, and therefore we have a sort of suction going on that pulls the wing up.
Planes fly because they want to, and because of the differential air.
Pressure forces the air to take a longer path to go over the wing versus under the wing, and this differential in the path length reduces the pressure on the upper surface of the wing, and that provides lift.
There's some confusion surrounding the Bernoulli effect and the airflow speed over the top and bottom of the wings. There's lift, which means that the air going over the curved part of the wing takes longer than the air going underneath the wing, so the lift is created there.
It takes longer for the air to go over the top of the wing.
The aerodynamics of that wing shape causes low pressure to form over the top of the wing and high pressure to form under the wing.
So wasn't it to do with Leonardo da Vinci came up with the original idea to base planes on the optron from wings of beds.
Planes fly because they can't swim.
I think that light blanes such as paper planes can fly just because of hierodynamics, but heavy planes need engines to generate pressure difference. But I am not entirely sure how this works.
So you have to have thrust greater than drug and lift greater than gravity, a higher pressure below and the lower pressure above the top of the wing is curved, so the air has to travel further.
I thought the planes flew because the Russian forts caused air to flow over their wings and created a force that pushes up on the wings.
So I think planes fly because of the way that the wind dends around the wing.
Planes fly because the lift that they generate is greater than the gravitational pull.
The air has.
A longer way over the wing than under the wing.
The top of the wing, because of its shape, it makes a suction, so it's the plane up.
I'm not actually completely sure why plane flies, but I know I've been in them and they tend to do that, and I hope they continue to do so.
I love that the answers were a combination of people who clearly sort of know what the answer is and like gave good answers, and then people who are hilarious.
Because they can't swim. Made me laugh out loud. So bravo.
You know. I love that approach. You don't know the answer, tell us a good joke.
Yeah, absolutely, I think that might be one of the keys to life.
I think I.
Want to see that more of my physics exams. You know, when somebody doesn't know how to solve this quantum mechanical problemly write me a good joke. I'll give you some points anyway.
You will give points for human sure.
Absolutely. I used to regularly have a question on exams that was just a random New Yorker cartoon, and then the question was, write a physics related caption for this cartoon.
Oh nice. I love that you're encouraging creativity.
I had an exam where I could get five extra credit points if I drew a parasite, and you'd think that that would be great, but I was like, I hate this because I don't even do nice stick figures. That's why I married an artist. He takes care of all of it. But the good thing is there's a stage of parasites where they essentially just look like a circle. And so I just I said, oh, there's a medicinariosist done, and I.
Didn't get full credit. All right, So where do we start?
So the beginning of the question of why planes fly goes all the way back to the Right Brothers in nineteen oh three. You know, the Right brothers. They were engineers, they were not physicists. And so while the Right Brothers made a plane fly, they actually didn't understand it at the time. Like, they had no principles, they had no theory, they had no real reason to think their plane was gonna fly. They just kind of made it work. And then they were showered, of course, with awards and fame and all sorts of stuff, which led some people in the physics community to get a little bit resentful.
So if they had no idea, why did they pick the design they picked? Is it because they were building on things that other people had made that maybe they understood what they were doing, and so they just sort of tinkered with that.
Yeah, engineering is a lot of tinkering, right, So let's try this let's try that, and sometimes things work and then later you figure out why that thing floats or why does this thing fly? And that's essentially what happened with the Wright brothers. And they won this fancy award from the Aeronautical Society, and one physicist, Colonel Fullerton, wrote it and said, quote, I think it was a mistake of the Aeronautical Society giving the rights of metal for their contribution to aeronautical science. I agree with they're having the medal, but it should have been for what they have done. In other words, they didn't understand it. They didn't actually advance aeronautical science. They just sort of like made it work, and they left open the question of like why does this work.
I still feel like there should be a medal for the moment when you're about to push your plane off the cliff and you're like, we don't understand what's going on.
But he the Wiley Coyote Award or something.
Maybe it's a subset of the Darwin Award.
Yeah. And this turned out to be really important politically because this is just before World War One, right, and so World War one is beginning and we have nations trying to figure out like how to make planes fly, how to make them stable, and nobody really understood like why some things work, how to make them better. And if you don't have like a model for why planes fly, it's pretty hard to improve their performance. And so just before World War One you had all these folks in Britain and then also on the continent trying to understand the theory of flight so they could take advantage of it militarily.
Okay, So the Right Brothers didn't have a theory, and nobody else had a theory either, so everyone was just tinkering to see how it worked and that it worked for the Right Brothers. Did the fact that it worked for the Right Brothers give insights into why it would work, Like did their design work because it took advantages of some physics principle that became obvious after the facts to the physicists.
It certainly isn't obvious after the fact, because people are still to this day arguing why planes fly. And it's fascinating that the divide in the field we're going to dig into this really goes back to the split that happened in World War One, where you had the sort of like mathematical battle between the scientists. You had the British trying to figure it out and you had the Germans trying to figure it out, and they took different approaches, and those approaches still live on and do battle in science today. In modern aeronautical engineering, the Germans took this approach of thinking about airs a fluid flowing over the wing and analyzing the velocity and thinking about the pressure. But the British didn't like that approach. It made some approximations that made them uncomfortable, and they like using Newton's forces, Newton, of course being British, And so to this day we have two competing theories for why planes fly and how they work. It's fascinating to me that we still haven't figured this out. You're right, we had a working example about one hundred years ago, and that definitely helps, right, It rules some things out, it inspires experiments, So having a working example is useful, but it doesn't always tell you exactly why something happens.
So are these theories, which I'm sure we'll get into the details of in a second, are they different enough that that resulted in the British and the Germans having planes that had different shapes during World War One? Or do both theories sort of predict that the same plane shape is good.
Yeah, they make different predictions about what's important in the shape of the wing, which is fascinating. And I found this quote from a historian David Bloor who said, on the eve of the Great War, none of the British workers in the field of aero dynamics had any workable account of how an airplane could get off the ground, which you know, makes it pretty hard to optimize the performance of your warplanes.
And pretty scary to get into a warplane on top of all the reasons that it's scary to get into a warplane exactly.
And later on we'll hear about Albert Einstein's personal design for an airplane, which really didn't work very well.
So let's dig Should we start with the Germans or should we start with the British.
So the German story is the one that most people have heard about. It's this theory of fluids and flow and Bernoulli's equations and all this kind of stuff. So we should start with there to make contact with what people think about when they think about flight.
All right, let's do that.
So the usual story for why plane flies has to do with the shape of the wing, and so aeronautical engineers call this an airfoil. It's round y in the front and it's point d in the back. It's sort of like a long, thin tear drop. Right. The idea that you're usually told is that the shape of this wing creates a pressure difference, that you have lower pressure above the wing and higher pressure below the wing, and that creates lyft because higher pressure below and lower pressure above basically pushes up on the wing. And it's the shape of the wing that's crucial in creating these pressure differences, which is what creates lyft. So that's the usual story for why planes fly. That comes down to this particular shape of the wing.
So you've got this curved edge moving towards the air quickly. Why does that shape not create the same amount of pressure on the top as it does on the bottom. Because that shape is like symmetrical, right is it? Because when it goes over, it has more space to sort of spread out.
So the shape is not actually symmetrical. Right. Typically it's like flat on the bottom and more curved on the top, and so the shape is not actually symmetrical. Which is why the Bernoulli story tells you that it pushes up. Bernoulli is a guy in the seventeen hundreds who is thinking about fluid flow and mostly about like water through pipes and pressure and volume and like, fluids are still something we're struggling to understand. But Bernoulli had a simplified view of it, which oiler actually came through later and proved all of his equations for him. But Bernoulli's principle tells us that faster moving fluids have lower pressure and slower moving fluids have higher pressure. So if the shape of the wing as it moves through the air makes the air go slower under the wing and faster above the wing on this curved shape, then that'll generate lower pressure above the wing and higher pressure below the wing, and that gives you lift.
And that feels easy to test, right because you just flip the wing over and it should do the opposite, right, like it should send you plummeting downwards.
Yes, already you're identifying a problem with this, But before we take this explanation apart, let's do a little bit more to support it. Because you're right, it kind of is easy to test and you can do a simple test at home. You can just like take a piece of paper and blow air above the piece of paper, and what you see is the paper goes from being sort of droopy to being flat, so it sort of like lifts up. And so it's like a classic symbol at home demonstration. Which you're doing is you're increasing the velocity of the air above the paper, which in theory lowers the air pressure above the paper, which provides lift because now the pressure is higher below the paper and lower above the paper. And you know, the story of like the air moves faster over the wing and slower below the wing is also verified in lots of other experiments, Like you can do these smoke tests where essentially you put a wing in a wind tunnel. Instead of just blowing air over it, you blow smoke over it. Smoke just being like a bunch of particles. You can track those particles and you can measure the velocity. So basically measure the velocity of the air around the wing. See what happened, right, don't just like talk about it, you know, in your salons while you're smoking cigarettes or whatever. Actually figure it out. And this is verified, like wind tunnels and smoke tests tell us that the air does move faster over the top of the wing than below the wing, And so this seems like it all sort of comes together, but as you identify, there are some important limitations to this explanation of why planes fly.
So when you get a window seat near the wing and you look out, it's easier to see like it's bunching up and it gets a little bit like kind of white ish. Is that pressure forming? Like are you sort of bunching up the air molecules to the point where they're visible or something else happening there? Or am I imagining it? And this is like an episode of the Twilight Zone.
Nobody else sees that, Kelly, Yeah, exactly, it's just you.
That was such a great episode.
I love that.
No.
I think what's happening there is you're seeing more water vapor. There are definitely changes in the air pressure below and above the wing, and as you know, water is very sensitive to pressure, the vapor point and all this kind of stuff. What you're seeing there is not the air itself, but water vapor forming, which can show you, just like any smoke tests can show you where the air is flowing. So that is pretty cool.
That sounds obvious now that you say it. But why did I ask that?
But whatever, that's fine, okay, So we decided now we're going to talk about the limitations. Has anybody flipped the wing the other way and then saw what happened?
Did that plane still fly?
So? Planes fly upside down all the time? Right? A big problem with this explanation is that it predicts that the asymmetry of the wing is crucial, right, that the curb bit on the top compared to the flat bottom is really important for making a lift. And so it predicts that if you flip that over, planes should like crash to the ground. Right, that an upside down wing should have anti lift or should have net force downwards. But we don't see that. You know, you can fly planes upside down all the time. Everybody who's been to like an aeronautics show has seen old fashioned planes or new planes. They can fly upside down. So Bernoulli's equation does and answered this question. This can't be the complete story of why wings provide lyft because it gives the wrong explanation or upside down planes.
So you mentioned that Eiler figured out all of these equations to support Bernoulli. Does it turn out that Oiler's equations were missing some important factor because the equation said it should work, but then it doesn't work.
So what was missing in the equations?
Oh yeah, great question. And this really goes to the heart of what's going on here. There's nothing wrong with Bernoulli's equations. And I'm giving Oiler credit only because Oiler came along and like did all the math BERNEULLI had like these leaps of insight, like oh, I think this and that and the other thing, and then Oiler came along and actually like dotted all the i's and crossed all the t's. And I think Oiler would have gotten credit for these equations if he hadn't already gotten credit for like eighty percent of everything in mathematics. It's like so much of stuff in mathematics that like Oiler just let somebody else take credit for because he's already got everything else named after him, which is amazing. So there's no problem with Bernoulli's equations. But there are always a simple description like brenell The's equations describe fluid flow, and they make some assumptions like you're talking about a fluid as if it's incompressible, for example, or you're talking about it as if it's not made of microscopic particles. That's not true, right, Air is made of microscopic particles. It is compressible. That doesn't mean that the equations don't apply. It means that they apply in some limited sense. It also might just be the wrong story. It might not provide the answer the explanation. You know, what we're looking for is an answer to a question which is macroscopic. Like we see the wing go through the air, the wing goes up. We want to know why. That's sort of like a human question, right, it's not a mathematical question. It's not like there's a prediction and number we're trying to calculate. We want like a story that tells us why this is happening. And that's a little bit more slippery than just mathematics. It's something to do with cause and effect and understanding, and you know, it's at the heart of science is coming up with these stories. But it's not always easy, especially when we're zooming out from a microscopic universe, to try to tell a story macroscopically. It's like economics. You know, you can ask like, why do prices go down? Well, I have this theory of inflation. I have a theory that involves prices and supply chains, whatever, and that the theory can be correct, but it doesn't always apply because the conditions it assumes aren't always relevant, and it doesn't always answer the question that you're asking.
I feel like, the next time I get on a plane, I'm going to think to myself, I wish it were simple.
I wish we've really understood this, But plays tend to stay up. So that's good. So we've established that the Germans got everything wrong and World War one? Oh wait, was that World War One or World War two?
This is World War One we're talking about.
Well, they got it wrong both times.
Sorry guys.
Okay, so we know that when a plane over it doesn't crash into the ground.
What was he getting wrong? Bernoulli?
So, Bernoulli's story is very nice, and it's very simple, and there's lots of it that's correct, but as you say, it doesn't explain some things. It doesn't explain why planes can fly upset down, and it also doesn't explain something really crucial at the heart of the story, which is why is the air moving faster over the top of the wing than the bottom of the wing. Like we said, the wing is asymmetric, it's curbing on the top and it's flat on the bottom. But why does that make the air go faster over the top? And you often hear this pop side explanation which is completely wrong, which is that it's a longer trip over the top of the wing, so the air has to go faster in order to meet its air particle partners at the back end of the wing.
Well, why would it have to exactly, why.
Would it have to It doesn't have to. It's nonsense, But you hear this all the time. I was taught this myself. You know that, like when air particles hit the front of the wing, if one goes above and one goes below, then to meet up again at the back of the wing, the one that takes the longer route would have to go faster. And it's an example of like it's a compelling story, but it doesn't actually make any sense because they don't have to take the same time. It's fine, the universe doesn't like crash or implode or hit a seg fault or something. If the air particles don't meet up again.
Yeah right, they're not like buddies. They don't have to get to the same spot at the same time. Yeah all right, so that doesn't have to happen.
So that doesn't have to happen. That's not the explanation for why air moves faster over the top, and so the Bernoulli story is sort of lacking that explanation, like why is there this pressure difference? And it opens this door to this question of like, well, did the speed differences cause the pressure differences or is it the pressure differences that are causing the speed differences. Because Bernoulli's equation doesn't give us causality, just says lower pressure, higher speed. It doesn't say that lower pressure is caused by the higher speed. It could also be the higher speed is caused by the lower pressure. So that tells us that we need more to the story. Bernulli story can't just crowed completely. Uhy planes fly because it doesn't tell us why planes fly upside down, and it doesn't explain why we see air going faster over the top of the wing, which is something we definitely see happening.
Right, So, I know we're about to talk about another explanation. But does looking at birds help at all? Because they have, you know, different kinds of wing shapes and they've kind of figured it out over evolutionary time. This isn't going to give us the pressure answer, but does it give us some insights into like how these wings should be shaped. I'm getting off track, but I just looked at a picture of a bird in my office and.
Now I have to ask it does and bird wings work with similar principles to airplane wings, except of course for the flapping part. But for gliding they do fascinatingly. Insect wings are even more complicated, and insect flight is not something we understand like at all. It's really kind of incredible. We should do a whole another episode about why insects fly.
And if you're ever feeling depressed, there are some slow mode videos of insects trying to take off and it.
Is the most clumsy, amazing thing I've I've ever seen.
It blows my mind that they ever get off the ground, and it makes my daughter left so hard she peeps a little.
So I knew a guy who did these experiments where they put an insect on a pin that like glued it to a pin and then they showed it videos, so it was like thought it was flying and they would like try to take videos of its wings moving to understand it. It's kind of incredible.
All right, this has been an amazing tangent. I've had a lot of fun. Let's go back to World War One, which is less fun. So I think we've talked about the shortfalls in Bernoulli's explanation. Are we ready to move on to what the British folks were thinking?
Yeah, So across the English channel we had folks that are working on a completely different theory, and not a completely different theory just because they disagreed scientifically with the best approach. Remember there's a war here, and so they're not going to collaborate. They're not going to be like going to the same conferences and sharing their ideas. It's sort of like when evolution splits. You know, you get all these weird animals off on an island and they diverge into their own crazy direction and get weird pouches for their babies or whatever. It's what happens when you work in isolation. You get competing nations, and so we have this fascinating sort of social experiment where you like, isolated two groups of scientists and asked them to solve the same problem and they took very different approaches.
So was it like Allopatrick speciation where they weren't encountering each other at all anymore and that's why they come up with different theories because it was no cross talk. Or was it s Patrick expeciation where the ideas could have flowed between each other, but they were still deciding to stay segregated. Was it nationalism or did they just not know?
Is what I'm wondering.
I think it started with nationalism, but then it developed into sort of tribal camps and the Germans criticized the British, and the British criticized the Germans. You know, even after the First War, when these guys could have gotten together and had like a global, unified theory of flight, they continued working in their own direction because they were invested in you know, this guy's advisor told him that the German approach was nonsense, and the German guy's adviser told him the British approach was scheiss, you know or whatever, And so.
Do you speak German?
I do not speak German. I speak Danish, which is sort of Germanic a little bit, but that's a big German anyway. So the British across the English Channel didn't like the German approach. They didn't like thinking about air as a fluid because they were like, this is too idealized. They can't explain what's really happening. The Germans thought, you know, ideal fluids are a fine approximation, but the British didn't like it. And you know, anytime you're making an explanation in science, you're going to be making approximations. It's just a question of what approximations you make. They're always shortcuts. Nobody can completely describe the full complexity of the universe in their equations. It's always a simplification. And there's an art to that, choosing the simplification that captures the essential details of what's happening in reality, so you can provide a useful explanation while dismissing all the irrelevant complexities that you don't need to worry about, which is why, for example, we can talk about how a ball flies through the air and often ignore air resistance and ignore the quantum effects and ignore everything else that's happening, the tug of Jupiter on that ball, because we judiciously choose what approximations to make. So that's what's happening here is two subjective approaches, and the British were focused not on this ideal fluid approach, but instead of using just a simple approach with Newton's law, they were just thinking about the forces.
How do forces act differently on the top and the bottom based on that tear drop shape.
Yeah, so the Newton's approach basically ignores the tear drop shape and it says the shape of the wing isn't actually important. What's crucial is the angle of the wing into the wind. So they're thinking, hey, the wing is tilted a little bit, and so as the wing is moving through the air, the wind gets bounced basically off the wind and goes down. So you have wind flowing onto the wing and you have an angle, and then it bounces down and so the wing goes up. And like anybody who's ridden in a car and put their hand out at an angle knows that you can feel lift right, your hand basically flies and your hand's not a tear drop, it's not an air foil. It's not like carefully germanically engineered to get lift. It's just your hand, you know, But still you can get lift. This is a simple application of Newton's theory. Like action reaction, the wind gets pushed down by the angle of the wing. The aeronautical engineers call it the angle of attack, and so the wing gets pushed up. Very simple.
So if that's true, then you could adjust wing shape to be even better at flying by making it bend even more so that you're stopping even more air as you go. Did the British try that to prove how much better they were?
I wish you'd been around in the aeronautical society, so clear with your questions. So what you're describe me is actually what you see in an airplane when you take off, if you're paying attention to the wing. Modern airplanes actually have a changeable shape, right. They have these flaps and these levers, right, so they can change the shape of the wing, and during takeoff they do just that. They push down the back of the wing so it's more of a curvy shape. It like grabs the air a little bit more, so you get more lift during takeoff. And then when you're flying, they make it flatter, so you're cruising, you get less lyft, but you also get less drag. Drag is the force that pushes the wing backwards, so you want upwards force lift without as much backwards force drag. So what you're seeing is exactly what modern airplanes do. Is they make a curveer wing you take off to get more lift.
Very cool.
I was reading about programmable matter, and I think one of the proposals was to have matter that responds to the environment and makes those changes on its own without you needing to do anything.
But I am glad we have more control over the process right now. I'm not ready to have us let go of that control yet.
And the nice thing about this explanation is it explains why planes can fly upside down. They fly upside down because they have the right angle of attack. It doesn't matter the shape of a wing at all. It just matters that the airplane wing is at the correct angle. So if you fly your airplane upside down, you can do it as long as the wing is angled into the air at the right angle.
So does this completely explain everything? That's never how our podcast episodes end. So what is it missing.
So this is a beautiful story of the limitations of science because both explanations explain something, but neither of them tell the complete story. So, for example, there's a couple of things that the British Newton's theory force explanation doesn't describe. Number one is what's going on on the top of the wing, right, This just focuses on the bottom of the wing, but we see that the pressure is lower on the top of the wing. The Newton's theory approach says, all the lift comes from below, that you're getting this force from the wind that's hitting it from below. But we see this lower pressure above, and it turns out, as we'll talk about later, that it contributes maybe even more to the lift, the low pressure above the wing than the high pressure does below the wing. And the Newton's theory cannot explain this. That's number one, and.
It can't explain why there's low pressure above the wing, right, just like the other one.
Yeah, exactly. Number two is that if you do these smoke studies, we see that the wing affects the flow of the air, not just below the wing. It's like the influence of the wing on the air is larger than just the wing, Like the air begins to flow up above the wing before it hits the wing. There is this upwash in advance of the wing and this downwash after the wing. So it's not just like you have a paddle and it's being hit by molecules and it's getting pushed up. There's a complicated interplay here between the wing and the flow around it and the air itself applying pressure on itself. So you can't ignore the sort of fluid effects of the air if you want to completely describe what's happening.
Yeah, so while you were describing all of that, I thought, oh man, it really feels like fluid dynamics should explained. So maybe the German example should have worked. Why does the fluid example not work then? With all of that complicated stuff happening.
So the fluid example does explain some parts of it. And so where we're going to go in the end is this like weird hybrid of the British German theory in order to have the most complete explanation of why it happens. But what we're doing right now is sort of examining like the failure of the individual ones. So the German explanation. The fluid approach can describe the flow of the fluid around the wing, but the British approach, though it's simple and satisfying, can't describe all of this stuff.
Okay, spoiler alert.
Yeah, so one thing that fails to describe is the sort of holistic flow of the air around the wing. The other is stalling and the Newton's force approach. The only reason you're flying is the angle of attack, and the greater the angle, the greater the force. Right, But anybody who flies knows that if you have too great an angle, you're going to stall, Like if your plane is pointed too far upwards, you're not going to get lift anymore. And what happens technically, what we see in wind tunnels is that if the angle of attack is too great, then as the air flows over the top of the wing, it doesn't merge smoothly with the air flowing below the wing, and you get this like weird turbulence. They call it a separation region because the air doesn't nicely reconnect. And you know, when you're going through a fluid, turbulence is a problem. You want to minimize turbulence. And so if the angle is too large. This is like gap between the air flows as they come off the back of the wing and they don't don't merge nicely. That's called the separation region between these flows, and that creates the stall. You lose your lift. So you can't explain that. Also with the Newton's explanation, you need some fluid theory to explain.
That slightly tangential. We haven't discussed propellers. Do propellers and jet engines play any role here or is that just in like that moves us forward and all of the uppiness comes from the wings.
Yeah, So basically engines just provide forward velocity so that the wings attack the air. All The propellers are a subtle point because propellers have a particular shape, which is related to the shape of a wing, right, because it's converting motion in one direction to air velocity in the other direction. So the shape of a propeller is related to the shape of a wing. But for the question of why planes fly, you can think of its just like something pushy, right, the pushy bit that gets the plane moving through the air. Yeah.
Okay, so you've got the pushy bit, but we don't understand what's happening around the wings yet, So how do you move from figuring out that the German and the British explanation are failing in some way to figuring out the answer to what's going on? Is this like where we are we just don't know or are there more experiments you can do to figure it out?
So what you shouldn't do is ask the world's smartest physicist Einstein to weigh in on this very practical engineering question.
Well, let's find out what he had to say after the break, all right, So for most of the Daniel led episodes, we have to walk through a what did Einstein think? Because he just provided so many good insights into so many things. So let's figure out what did Einstein think about this?
Yeah, so Einstein, he's a smart guy. He developed relativity, he struggled and failed to integrate quantum mechanics into it. So it's not like he solved every problem he attacked. But he was a smart guy. And you know, he was German and this is a relevant question for the Germans at the time, and he was interested in this and he wrote, quote, there's a lot of obscurity surrounding these questions and open puzzles like this, you know, they attract smart people like, oh, maybe I can figure this out. Maybe there's an insight, a moment of clarity that will just untangle this whole mess. And so he thought about flight from the German point of view, and he actually designed his own wing shape, sort of a weird variation on the air foril Is Einstein. Remember, he was not just a clever physicist. He worked in the patent office and so he had seen lots of inventions. He had a practical mind also, So he designed his own wing and he took it to a German aircraft company, and you know, he was an esteemed famous scientist at this point, so they thought, okay, sure, so they worked with him, they collaborated with him, They built the plane, and they sent it on a test flight, and the pilot came back and said he would never fly the thing again because a quote flew like a pregnant duck.
I feel like whenever a question sort of tiptoes into biology and Einstein is involved, you want to back away. So, you know, I think maybe most of us know that ducks can't get pregnant.
They carry eggs, but they don't get pregnant the way we do.
And also, like, you know, he married his cousin, which I think is a bad idea also from a biological perspective.
But anyway, okay, So.
Einstein was great at everything, but not at designing wings.
Yeah, exactly. And so fast forward fifty years or so, what are people talking about today in terms of why planes fly? I went across campus here to a colleague at UC Irvine, Haythian Taha. He's a professor of aeronautical engineering here and he studies lyft. He works on this problem. I asked him what is a simple explanation for lyfts? And he said, quote, this is not settled in the aeronautical engineering community. Please don't laugh. That's a direct quote. So here we are laughing. It's hilarious, you know, like these guys still haven't figured this out more than one hundred years after their right brothers.
But yet still so much money is made us.
So it's nice to know you don't have to have everything worked out before you commercialize something.
But let's not oversell it. It's not that we don't understand why planes fly. It's that we don't have a compelling, simple explanation at the sort of zoomed out level. And again there's always a difference between life the microphysics. Can you model what's happening with individual air molecules and the shape of the wing and come up with a simulation in which planes fly. Absolutely, we can, and we do. This is why, for example, Boeing can design their airplanes on the computer and be confident that they will fly. They don't need to use wing tests anymore and build a bunch of models to experiment because we have accurate simulations for what's going on in the microphysics level. It's just that when you zoom out and now you want to tell a simple story about what's happening, there are competing ways to do that, and all of them ignore some details, and so none of them are completely satisfactory. So when he says this is not settled, he doesn't mean we don't know why planes fly in a sort of microphysics point of view, means we don't have a nice, compelling, one sentence explanation for why this happens the way we can. For example, like a ball flying through the air, why does a baseball fly in a parabola? We know the answer is when you have constant acceleration, you get parabolic motion. Simple, one sentence answer. That's what we're lacking for flight. We have a very complex thing that's happening with microphysics, and it's when we're struggling to come up with a simplified explanation at the macroscopic level.
Do engineers care?
So if you can make a simulation on a computer that tells you everything you need to know, Like, are engineers still interested in this question or has this completely moved to the realm of like physicists who never see the light who are studying it in their offices, Like, yeah, do they care what the answer is anymore?
Or do physicists care?
Only absolutely they care because engineers want simple, working models, you know, when they want to model how a plane is going to fly, they don't want to have to go back to describing every single particle. They want simple sets of equations that let them work quickly and easily the same way. For example, we'd love to be able to predict a hurricane without modeling every single rain drop. Currently we have the technology, and we're building these incredibly powerful computers just to model wind and pressure and predict the weather. We'd love simple equations to be able to do that right rather than having to use incredibly detailed computation. And so anytime we can summarize complexity with simple math, we definitely win. So people are definitely working on this and trying to find a complete holistic approach that captures all this behavior, that lets us do important engineering without simulating all the tiny microphysical details. Absolutely, yeah, people care.
And I want to go on record that I apologize for implying that physicists don't go outside.
What is this outside thing you're talking about? I was going to ask you about that. I have heard about this, but I've been too shy to ask.
Can you told me you've been hiking.
I know that you're maybe even more extreme than I am in terms of outdoor adventures.
No, I do love going outside. That's why I live in California. It's so amazing here.
Oh, the fall right now is absolutely incredible. Okay, So we've talked about a lot of different ideas, a lot of things we know and we don't know. I know the answer at the end is that it's complicated. But is there a simple ish kind of way to summarize where we are right now in our understanding.
Yeah, So I think the simplest way to describe it is that both of Bernoulli and Newton's stories help, but they're both limited, and the limitation is that they're trying to tell a story in terms of like simple causes and simple effects. The airflows and its bounces off the wing, or the pressure is lower above the wing because the velocity is higher, and the real story is that the cause and effect are not so simple. There's lots of things happening here and they are interplaying together, so it's not so easy to disentangle the cause and the effect. There's a lot of things at work here, pressure and velocity and forces all in this fantastic harmony that's making the wing go up, which is why none of these simple stories are satisfactory. We can weave a slightly more complex story to explain why planes fly that involves all of these aspects sort of working together. But you know, humans like simple stories. Ball goes down because of gravity, this kind of thing. We don't have that simple a cause and effect.
I'm always amazed by things that work so well, where when you scratch the surface, we really don't understand, like I was talking to someone who uses deep brain stimulation to treat epilepsy. So, like, when somebody has a seizure, they're having like an electrical storm in their brain, and sometimes they have electrodes implanted into the center of their brain and they essentially just get their brains apped and that helps.
And so I reached out to an expert and I was.
Like, why does zapping someone's brain with electricity stop the epileptic seizure?
And they're like, we don't know.
Just like I'm a little nervous getting onto a plane hearing why does the plane go up?
We don't know. I also would not want to hear why are you shocking my brain?
We don't know.
But in both cases it works, and it seems to work reliably, so we get.
By Also true, I think for bost pharmaceuticals, right, we're like, well, it does have this effect, we don't really understand that's a biochemistry of it, but hey, keep taking it, you know.
That's right.
Yes, we all muddle along.
And I'm not an anti vaxxer at all. I just mean this is sort of the way science is. You always understand science at some level, you don't always need to understand the microphysical explanation in order to make it work. And that's actually a huge gift. Otherwise we couldn't do anything if we needed to understand like the nature of quantum gravity before we made a bowl of chicken soup, we'd never have chicken soup. We'd never understand the world. It's lucky that the world can be understood without knowing all the details, or science would be impossible.
It's lucky you can tinker with the world without understanding it.
Yes, that too. So we do have a story we can tell about why planes fly. And I think the best way to think about why a wing goes up is to focus on pressure, pressure above the wing and pressure below the wing. Fundamentally, wing goes up because pressure grows below the wing and pressure falls above the wing. And both of these are contributed to by the Bernoulli and the Newton story.
Got it, Okay, we managed to summarize something in like three or four sentences.
But we can also dig into it in a little bit more detail, you know, like why does pressure grow below the wing? Well, Newton tells us to focus on the angle of attack, right, and that's correct. The wing goes through the air, and the angle of attack pushes down on the air, and the air pushes back, and this is pressure. Just like if you're scooping up snow with a snowshovel, right, this is going to be increase pressure. The snow is going to get compacted. So why does pressure form below the wing? Not that controversial. It's the angle of attack that's crucial.
And as long as you go like this and you just don't look at the top part of the wing, and so I'm covering the top part of my eyes and you don't think about the top part of the wing, then everything is good. But we don't understand what's happening above?
Is that right well above the wing? We can also think about the pressure. The Newton story can't explain what's happening, but if you think about it in terms of fluid you can. Right, So what's happening above the wing? Initially, if you just like shoot air above the wing, then it wants to flow straight back, which effectively creates like a vacuum underneath. The angle of attack, working together with the shape of the wing, creates this low pressure zone above the wing because the air needs to go down to flow onto that right. So that's exactly what happens, is you create this low pressure zone, which is like a vacuum. It pulls the air into it, and that's pulling the wing up. So you have low pressure above the wing created by the shape of the wing and the angle of attack, which creates low pressure above it. And so these two effects work in harmony. You have a force from below and you have like suction from above. So wings go up not just because the air below them is pushing them up, but the air above them is sucking them up into the sky.
So the Germans got the top right and the British got the bottom right.
Is that right?
Okay?
I feel like there's so many times where I hear that there's a debate and there's people on this side and people on that side, and five years later the answer is almost always, oh, they were both right, and it's some combination of things, but okay, cool.
Yeah, And so mostly this comes from the shape of the front of the wing. Right. Now, the shape of the wing is important because you want to minimize drag. Right, the Newton folks tell you the shape doesn't matter. All that matters is you get the force from the bottom but what you want is to create low pressure on the top without creating a lot of drag and without creating this turbulence zone which gives you a stall. So that's why you have this shape. You have this shape in the front of the wing to give you this uneven pressure distribution, so you get low pressure but not so much that you get a stall. And then you have this smooth ending so the flow comes together nicely without creating turbulence at the end. So the shape of an airfoil and the angle of the attack is optimized to give you smooth flow, low pressure above the wing, and to minimize drag, which is important if you want to take off.
I feel like the next time I get out of flight, I'm going to appreciate it all much more, having a better sense of how this all works.
But you know, in terms of coming to this explanation, the experts still are arguing about, like how to summarize this. I've given you sort of a summary version of it, but I'm sure there are aeronautical engineers out there who have differing opinions and are going to write into us with their theory for why planes fly, or at least for how to describe it. Our original listener Tom Johnson wrote in because he saw this video from the Smithsonian Air and Space Museum that explained it in terms of just Bernoulli right, the Bernoulli explanation of low pressure on top, and he wasn't satisfied with this, and he wrote to me and he said, quote, I contacted the Smithsonian about this video, voicing my concerns, and I was told unequivocally that it's their policy that Berneulli makes planes fly. I wasn't aware that physics cared about policies, but I guess I'm mistaken.
That's amazing. Institutions are fantastic.
Exactly. And so again, you know, while the microphysics is clear about what's happening around the wing and you can track these individual particles, we still do struggle a little bit coming up with a simple explanation. It turns out pressure is important, and velocity is important, and the forces are important. So none of the sort of classic simple explanations can describe everything that's happening around a wing. You need a more complex, a fuller description of what's happening to explain all the phenomena why planes can fly upside down, Why the pressure goes down above the wing. All this kind of stuff. So in the end the British and the Germans have to work together.
Ah, I mean that's better in the end.
So those plane wing shapes, So I'm thinking of other instances where you've got like things moving through the air.
So we've got like.
Wind turbines, and then you've got racing cars. They don't want to go up, they want to go down, so they do the whole shape in reverse. Like, how has this information been used in other contexts?
Yeah, exactly. So Formula one cars, they have a wing in the back, but that wing pushes the car down so that it maintains friction because they need friction in order to go forward. Right, the wheels have to get pushed onto the ground so the wheels can grab and then when they turn the wheels, the car goes forward. If the car lifts up above the ground, then it can't move forward anymore. So they design it in order to generate downward pressure, so they have crucially a different angle, right, But they also think about the foil and they do really complex modeling because you know, millions of dollars mean microseconds and that it's the difference between winning and losing. So yeah, absolutely, they invert all of this theory in order to get a wing that can push down. And the crucial thing there is the angle of attack, right.
And then wind turbines. They don't want the wind turbine to go airborne, but they want it to spin as fast as it can, which feels like maybe some of the principles we talked about today could help that happen.
Yeah, how does this plant to wind turbines?
Yeah, exactly, Well, it's the same principle. Essentially, you have air flow across the propeller and you want to convert that to a sideways force, right, So you want to force up relative to the propeller. You don't want the whole propeller to fly abof the ground, right, but you wanted to spin around the axis, and so again the angle of attack and the shape of it ensures a force that turns it and smooth flow around it because you don't want turbulence in the back of your propeller blade. So each one of those is essentially just a little wingh.
Very cool and guessing that we weren't racing cars before we had planes, so probably the plane stuff came first.
Yeah, the plane stuff came first. Cars were pretty slow until kind of recently, Like you could win big car races by driving it like forty five miles an hour for many years.
That sounds so cute. Now it's much harder to die at that speed. Why would you watch that exactly?
But it's really fun to think about how we explain the world around us, and how challenging it can be to explain some things. It makes me really grateful for the times that we can find a very simple explanation and wonder like why that's possible sometimes and not other times. Is it because the way that we think about the world, or is it just something about the way the world works? You know, alien scientists have a better theory for why wings work because they started off from a completely different point of view mathematically or scientifically. Or is everybody struggling to describe some complex behaviors? Are these things just inherently complex or is it just our language that's making them a challenge?
I wonder if part of that is there some percent of explanations that we have that we feel good about that are actually wrong if you look at them more closely.
Stuff works anyway.
Yeah, and then I wonder if some other stuff just there happens to be a metaphor that we're familiar with that makes it easier to understand.
And there's just not like a ready metaphor for some of the more complicated things.
But yeah, and sometimes we accept an explanation even though it's nonsense, like the air has to go faster over the top of the wing to meet its partners. Like a lot of people go, oh huh, yeah, that makes sense, but it.
Doesn't actually make sense, you know, yep.
And fundamentally, all of science is quote unquote wrong. If you zoom in far enough, you know, everything is just an approximate description, not like scientists are lying to us or we're promoting nonsense, but everything is an approximation, even like describe an electron as a fundamental particle, Yeah, probably it's not. Probably it's made of something smaller. We just haven't seen it yet. But our theory works so far, we've never been able to make an experiment where it breaks. And so all of science is a work in progress in that sense, not just like do we have the best explanation for it? But you know, how far can we push this until we see it break. And seeing it break is an opportunity, right, It's not a disaster. It's a chance to learn something deeper about the universe, to come up with a more accurate description, or to poke through reality to another level and see how it works underneath it all. So it's just part of the journey of science.
I feel like it's a great time to be in science. There's still so many fundamental things left to understand at the same time as we gaining all this new technology that allows us to address questions in different ways.
We have all these new tools. Can't wait to see what we learn in the next couple decades.
Yeah, and it's not just like what's inside an electron, like weird, abstract fundamental stuff that you'll ever see. It's stuff right in front of you, stuff you can see with your eyes. You know, tornadoes and hurricanes and airplanes and fluid flow. All this stuff is complex and unsolved. So smart young people with energy out there go out there figure it out. There's lots left to do, so thank you very much to Tom Johnson for sending us this question. If you have questions about the way the world works or why you can't understand it. Please write to us two Questions at Daniel and Kelly dot org. We love to hear from you.
Thanks Tom.
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