Speaker 1: They say that any technology sufficiently advanced is indistinguishable from magic. Well,
Speaker 1: there's one technology that people have lusted after for thousands
Speaker 1: of years that we now rely on even find routine.
Speaker 1: I'm talking, of course, about flight. It's been only one
Speaker 1: hundred years since people were able to leave the ground
Speaker 1: and fly through the air and return safely. Now it's
Speaker 1: so routine that people sleep through it. But imagine what
Speaker 1: that would seem like someone from a thousand or ten
Speaker 1: thousand years ago. It's essentially like magic. Of course, we
Speaker 1: know it's actually not. It's science because we can understand it,
Speaker 1: and we can break it down. We can explain it.
Speaker 1: We understand why planes fly, right, do we could we
Speaker 1: actually explain it to visiting ancestors from the deep past?
Speaker 1: Or is it still some kind of black magic even
Speaker 1: for modern humans and the best aeronautical engineers. So today
Speaker 1: on Daniel and Kelly's Extraordinary Universe, we'll be asking an
Speaker 1: old question, why do planes fly?
Speaker 2: Hi?
Speaker 3: I'm Kelly Wiener Smith and my favorite way to travel
Speaker 3: is by train.
Speaker 1: Hi. I'm Daniel Whitson. I'm a particle physicist, which means
Speaker 1: I have to fly around the world to get to
Speaker 1: the biggest particle accelerator.
Speaker 4: And what is your worst flight experience?
Speaker 1: My worst flight experience is traveling with my own six
Speaker 1: month old baby. We had a bascinet and we got
Speaker 1: her to sleep in the basinet and we thought, oh,
Speaker 1: this is going to be great. Then the stewardess came
Speaker 1: by and said, oh, there's some turbulence. You have to
Speaker 1: take her out and put her in a seat and
Speaker 1: buckle her in. And he said, no, she's sleeping, and
Speaker 1: she said sorry, it's the rules. And everyone around us went,
Speaker 1: oh no, and they were right, because she screamed for
Speaker 1: the rest of the eight hour flight.
Speaker 3: Oh no, that's horrible. I'm so sorry. When you finally
Speaker 3: get your kid asleep and something wakes them up on
Speaker 3: a plane, it's the worst.
Speaker 1: It's the worst. And everybody knew it was the worst,
Speaker 1: and we all knew there's nothing we could do, but hey,
Speaker 1: you know, them's the rules. How about you, what's your
Speaker 1: worst traveled experience?
Speaker 3: It also involves plane with my child. She was probably
Speaker 3: about six months old around there also, and I brought
Speaker 3: a change of clothes for her. I was totally prepared
Speaker 3: for her. She was sitting on my lap and we
Speaker 3: were about to take off, and right as the plane
Speaker 3: started moving forward, she had a blowout poop on my
Speaker 3: leg and so it smelled horrible. But you can't get
Speaker 3: out of your seat until you rechruising altitude. So everybody
Speaker 3: was like giving me dirty looks and covering their noses,
Speaker 3: and it took like twenty minutes before we were allowed
Speaker 3: to get out of our seats. And then you know,
Speaker 3: I had to change her in the airplane changing room,
Speaker 3: which is like so tiny, I thought she was gonna
Speaker 3: fall off the little tray. And anyway, I brought a
Speaker 3: change of clothes for her, but I didn't assume i'd
Speaker 3: have on my pants.
Speaker 1: Oops. Classic parenting mistake exactly.
Speaker 3: So I smelled horrible the whole flight, and people kept
Speaker 3: looking at me, and it wasn't a lot of fun.
Speaker 4: But them's the breaks you got to leave the house
Speaker 4: with kids.
Speaker 1: Sometimes you just got to look back at those people
Speaker 1: and say, hey, who's going to pay for your social security? Right?
Speaker 1: Somebody's got to make the next generation. And to me,
Speaker 1: Madame poopy Pants, I'll get a.
Speaker 4: Little like badge that says that.
Speaker 3: Now, whenever I see a parent with a kid, I'm like,
Speaker 3: what can I carry for you. How can I help
Speaker 3: like I offer to help all the time.
Speaker 1: Yeah, exactly. And the big lesson I learned is never judge,
Speaker 1: and you never in somebody else's family. You don't know
Speaker 1: what they're going through or what they're struggling with or
Speaker 1: how much they slept last night. Just help and smile. Yep.
Speaker 3: And kids are always at their worst because they're always
Speaker 3: the tiredest and they're not used to it, and so
Speaker 3: like anyway, yes, don't judge. But planes are wonderful. I
Speaker 3: needed to get from place to place, and I needed
Speaker 3: to get there quickly. And actually I wouldn't have wanted
Speaker 3: to take my daughter on a long train ride for
Speaker 3: the journey that we were going on, because it would
Speaker 3: have been days. But I was flying to London the
Speaker 3: other day and we had kind of talked about why
Speaker 3: do planes fly? And you said it's complicated, And while
Speaker 3: I was, you know, above the Atlantic, I thought, how complicated?
Speaker 4: How well do we understand this?
Speaker 3: I hope the answer is really well, So I'm looking
Speaker 3: forward to today's conversation.
Speaker 1: Good Well. This question actually came to me from a listener.
Speaker 1: Tom Johnson from Ohio wrote to me and asked if
Speaker 1: we could explain why planes fly because he looked into
Speaker 1: a little bit and he was kind of confused. So
Speaker 1: he wanted a clear explanation for why it is that
Speaker 1: planes stay in the air.
Speaker 3: And if you have a question that you want to
Speaker 3: ask us, we would be happy to answer them. So
Speaker 3: send us an email at questions at Daniel and Kelly
Speaker 3: dot org.
Speaker 1: When I told Tom that we were going to answer
Speaker 1: his question on the pod, he said, quote, my wife
Speaker 1: is absolutely going to roll her eyes tonight when I
Speaker 1: tell her over sushi, which made me wonder. Tom is
Speaker 1: in Ohio. Where is he getting a sushi? Probably they
Speaker 1: have it flown in so even his sushi relies on
Speaker 1: airplane in wings.
Speaker 3: Yeah, and you know, I lived in Ohio for a
Speaker 3: decade or more, and I can tell you that when
Speaker 3: sushi has to fly that far, it's not great.
Speaker 4: You're too far from the ocean.
Speaker 1: At that point, you're saying nobody should fly to Ohio
Speaker 1: for sushi.
Speaker 4: No, I'd say that.
Speaker 3: I'm going to go on the record saying you shouldn't
Speaker 3: fly just to Ohio for sushi.
Speaker 4: Ohio has got other things going for it.
Speaker 1: You should have Ohio pride. You know, I know you're
Speaker 1: a transplanted Virginian, but like you come from Ohio. A
Speaker 1: good thing about Ohio, for example, is it made you You.
Speaker 3: Know, I was actually born in New Jersey, which is
Speaker 3: an even harder state to defend, but I do actually
Speaker 3: love New Jersey and I will defend New Jersey. But
Speaker 3: you know, Ohio was a very safe place to grow
Speaker 3: up where I grew up, and so that's fine. And
Speaker 3: I think Ohio has made the most astronauts, and so
Speaker 3: you know, we all would make jokes about you know,
Speaker 3: what is it about Ohio that makes you want to
Speaker 3: get as far as possible away from this planet? And
Speaker 3: I'm not sure if Ohio is still winning an astronaut production,
Speaker 3: but it was at some point.
Speaker 1: All right, well, let's stop flying around the issue and
Speaker 1: get to the question. I was wondering what people out
Speaker 1: there thought about how planes stay in the air, how
Speaker 1: wings work, what stories they had been told, and what
Speaker 1: they understood about this question. So, as usual, I went
Speaker 1: to our bank of volunteers to ask them why do
Speaker 1: planes fly? If you would like to play for a
Speaker 1: future episodes, please don't be shy right to us to
Speaker 1: questions at Danielankelly dot org. In the meantime, think about
Speaker 1: it for a moment. Why do you think planes fly?
Speaker 1: Here's what a bunch of listeners had to say.
Speaker 5: I believe nobody really knows. I would be glad to
Speaker 5: learn otherwise. I know that I've read in my school
Speaker 5: book that the air takes a longer way on the
Speaker 5: top of the wing than on the bottom of the wing,
Speaker 5: and therefore we have a sort of suction going on
Speaker 5: that pulls the wing up.
Speaker 1: Planes fly because they want to, and because of the
Speaker 1: differential air.
Speaker 6: Pressure forces the air to take a longer path to
Speaker 6: go over the wing versus under the wing, and this
Speaker 6: differential in the path length reduces the pressure on the
Speaker 6: upper surface of the wing, and that provides lift.
Speaker 1: There's some confusion surrounding the Bernoulli effect and the airflow
Speaker 1: speed over the top and bottom of the wings. There's lift,
Speaker 1: which means that the air going over the curved part
Speaker 1: of the wing takes longer than the air going underneath
Speaker 1: the wing, so the lift is created there.
Speaker 2: It takes longer for the air to go over the
Speaker 2: top of the wing.
Speaker 6: The aerodynamics of that wing shape causes low pressure to
Speaker 6: form over the top of the wing and high pressure
Speaker 6: to form under the wing.
Speaker 7: So wasn't it to do with Leonardo da Vinci came
Speaker 7: up with the original idea to base planes on the
Speaker 7: optron from wings of beds.
Speaker 1: Planes fly because they can't swim.
Speaker 2: I think that light blanes such as paper planes can
Speaker 2: fly just because of hierodynamics, but heavy planes need engines
Speaker 2: to generate pressure difference. But I am not entirely sure
Speaker 2: how this works.
Speaker 7: So you have to have thrust greater than drug and
Speaker 7: lift greater than gravity, a higher pressure below and the
Speaker 7: lower pressure above the top of the wing is curved,
Speaker 7: so the air has to travel further.
Speaker 8: I thought the planes flew because the Russian forts caused
Speaker 8: air to flow over their wings and created a force
Speaker 8: that pushes up on the wings.
Speaker 2: So I think planes fly because of the way that
Speaker 2: the wind dends around the wing.
Speaker 6: Planes fly because the lift that they generate is greater
Speaker 6: than the gravitational pull.
Speaker 2: The air has.
Speaker 4: A longer way over the wing than under the wing.
Speaker 8: The top of the wing, because of its shape, it
Speaker 8: makes a suction, so it's the plane up.
Speaker 1: I'm not actually completely sure why plane flies, but I
Speaker 1: know I've been in them and they tend to do that,
Speaker 1: and I hope they continue to do so.
Speaker 3: I love that the answers were a combination of people
Speaker 3: who clearly sort of know what the answer is and
Speaker 3: like gave good answers, and then people who are hilarious.
Speaker 4: Because they can't swim. Made me laugh out loud. So bravo.
Speaker 1: You know. I love that approach. You don't know the answer,
Speaker 1: tell us a good joke.
Speaker 3: Yeah, absolutely, I think that might be one of the
Speaker 3: keys to life.
Speaker 4: I think I.
Speaker 1: Want to see that more of my physics exams. You know,
Speaker 1: when somebody doesn't know how to solve this quantum mechanical
Speaker 1: problemly write me a good joke. I'll give you some
Speaker 1: points anyway.
Speaker 4: You will give points for human sure.
Speaker 1: Absolutely. I used to regularly have a question on exams
Speaker 1: that was just a random New Yorker cartoon, and then
Speaker 1: the question was, write a physics related caption for this cartoon.
Speaker 4: Oh nice. I love that you're encouraging creativity.
Speaker 3: I had an exam where I could get five extra
Speaker 3: credit points if I drew a parasite, and you'd think
Speaker 3: that that would be great, but I was like, I
Speaker 3: hate this because I don't even do nice stick figures.
Speaker 3: That's why I married an artist. He takes care of
Speaker 3: all of it. But the good thing is there's a
Speaker 3: stage of parasites where they essentially just look like a circle.
Speaker 3: And so I just I said, oh, there's a medicinariosist done,
Speaker 3: and I.
Speaker 4: Didn't get full credit. All right, So where do we start?
Speaker 1: So the beginning of the question of why planes fly
Speaker 1: goes all the way back to the Right Brothers in
Speaker 1: nineteen oh three. You know, the Right brothers. They were engineers,
Speaker 1: they were not physicists. And so while the Right Brothers
Speaker 1: made a plane fly, they actually didn't understand it at
Speaker 1: the time. Like, they had no principles, they had no theory,
Speaker 1: they had no real reason to think their plane was
Speaker 1: gonna fly. They just kind of made it work. And
Speaker 1: then they were showered, of course, with awards and fame
Speaker 1: and all sorts of stuff, which led some people in
Speaker 1: the physics community to get a little bit resentful.
Speaker 3: So if they had no idea, why did they pick
Speaker 3: the design they picked? Is it because they were building
Speaker 3: on things that other people had made that maybe they
Speaker 3: understood what they were doing, and so they just sort
Speaker 3: of tinkered with that.
Speaker 1: Yeah, engineering is a lot of tinkering, right, So let's
Speaker 1: try this let's try that, and sometimes things work and
Speaker 1: then later you figure out why that thing floats or
Speaker 1: why does this thing fly? And that's essentially what happened
Speaker 1: with the Wright brothers. And they won this fancy award
Speaker 1: from the Aeronautical Society, and one physicist, Colonel Fullerton, wrote
Speaker 1: it and said, quote, I think it was a mistake
Speaker 1: of the Aeronautical Society giving the rights of metal for
Speaker 1: their contribution to aeronautical science. I agree with they're having
Speaker 1: the medal, but it should have been for what they
Speaker 1: have done. In other words, they didn't understand it. They
Speaker 1: didn't actually advance aeronautical science. They just sort of like
Speaker 1: made it work, and they left open the question of
Speaker 1: like why does this work.
Speaker 3: I still feel like there should be a medal for
Speaker 3: the moment when you're about to push your plane off
Speaker 3: the cliff and you're like, we don't understand what's going on.
Speaker 1: But he the Wiley Coyote Award or something.
Speaker 4: Maybe it's a subset of the Darwin Award.
Speaker 1: Yeah. And this turned out to be really important politically
Speaker 1: because this is just before World War One, right, and
Speaker 1: so World War one is beginning and we have nations
Speaker 1: trying to figure out like how to make planes fly,
Speaker 1: how to make them stable, and nobody really understood like
Speaker 1: why some things work, how to make them better. And
Speaker 1: if you don't have like a model for why planes fly,
Speaker 1: it's pretty hard to improve their performance. And so just
Speaker 1: before World War One you had all these folks in
Speaker 1: Britain and then also on the continent trying to understand
Speaker 1: the theory of flight so they could take advantage of
Speaker 1: it militarily.
Speaker 3: Okay, So the Right Brothers didn't have a theory, and
Speaker 3: nobody else had a theory either, so everyone was just
Speaker 3: tinkering to see how it worked and that it worked
Speaker 3: for the Right Brothers. Did the fact that it worked
Speaker 3: for the Right Brothers give insights into why it would work,
Speaker 3: Like did their design work because it took advantages of
Speaker 3: some physics principle that became obvious after the facts to
Speaker 3: the physicists.
Speaker 1: It certainly isn't obvious after the fact, because people are
Speaker 1: still to this day arguing why planes fly. And it's
Speaker 1: fascinating that the divide in the field we're going to
Speaker 1: dig into this really goes back to the split that
Speaker 1: happened in World War One, where you had the sort
Speaker 1: of like mathematical battle between the scientists. You had the
Speaker 1: British trying to figure it out and you had the
Speaker 1: Germans trying to figure it out, and they took different approaches,
Speaker 1: and those approaches still live on and do battle in
Speaker 1: science today. In modern aeronautical engineering, the Germans took this
Speaker 1: approach of thinking about airs a fluid flowing over the
Speaker 1: wing and analyzing the velocity and thinking about the pressure.
Speaker 1: But the British didn't like that approach. It made some
Speaker 1: approximations that made them uncomfortable, and they like using Newton's forces, Newton,
Speaker 1: of course being British, And so to this day we
Speaker 1: have two competing theories for why planes fly and how
Speaker 1: they work. It's fascinating to me that we still haven't
Speaker 1: figured this out. You're right, we had a working example
Speaker 1: about one hundred years ago, and that definitely helps, right,
Speaker 1: It rules some things out, it inspires experiments, So having
Speaker 1: a working example is useful, but it doesn't always tell
Speaker 1: you exactly why something happens.
Speaker 3: So are these theories, which I'm sure we'll get into
Speaker 3: the details of in a second, are they different enough
Speaker 3: that that resulted in the British and the Germans having
Speaker 3: planes that had different shapes during World War One? Or
Speaker 3: do both theories sort of predict that the same plane
Speaker 3: shape is good.
Speaker 1: Yeah, they make different predictions about what's important in the
Speaker 1: shape of the wing, which is fascinating. And I found
Speaker 1: this quote from a historian David Bloor who said, on
Speaker 1: the eve of the Great War, none of the British
Speaker 1: workers in the field of aero dynamics had any workable
Speaker 1: account of how an airplane could get off the ground,
Speaker 1: which you know, makes it pretty hard to optimize the
Speaker 1: performance of your warplanes.
Speaker 3: And pretty scary to get into a warplane on top
Speaker 3: of all the reasons that it's scary to get into
Speaker 3: a warplane exactly.
Speaker 1: And later on we'll hear about Albert Einstein's personal design
Speaker 1: for an airplane, which really didn't work very well.
Speaker 3: So let's dig Should we start with the Germans or
Speaker 3: should we start with the British.
Speaker 1: So the German story is the one that most people
Speaker 1: have heard about. It's this theory of fluids and flow
Speaker 1: and Bernoulli's equations and all this kind of stuff. So
Speaker 1: we should start with there to make contact with what
Speaker 1: people think about when they think about flight.
Speaker 4: All right, let's do that.
Speaker 1: So the usual story for why plane flies has to
Speaker 1: do with the shape of the wing, and so aeronautical
Speaker 1: engineers call this an airfoil. It's round y in the
Speaker 1: front and it's point d in the back. It's sort
Speaker 1: of like a long, thin tear drop. Right. The idea
Speaker 1: that you're usually told is that the shape of this
Speaker 1: wing creates a pressure difference, that you have lower pressure
Speaker 1: above the wing and higher pressure below the wing, and
Speaker 1: that creates lyft because higher pressure below and lower pressure
Speaker 1: above basically pushes up on the wing. And it's the
Speaker 1: shape of the wing that's crucial in creating these pressure differences,
Speaker 1: which is what creates lyft. So that's the usual story
Speaker 1: for why planes fly. That comes down to this particular
Speaker 1: shape of the wing.
Speaker 3: So you've got this curved edge moving towards the air quickly.
Speaker 3: Why does that shape not create the same amount of
Speaker 3: pressure on the top as it does on the bottom.
Speaker 3: Because that shape is like symmetrical, right is it? Because
Speaker 3: when it goes over, it has more space to sort
Speaker 3: of spread out.
Speaker 1: So the shape is not actually symmetrical. Right. Typically it's
Speaker 1: like flat on the bottom and more curved on the top,
Speaker 1: and so the shape is not actually symmetrical. Which is
Speaker 1: why the Bernoulli story tells you that it pushes up.
Speaker 1: Bernoulli is a guy in the seventeen hundreds who is
Speaker 1: thinking about fluid flow and mostly about like water through
Speaker 1: pipes and pressure and volume and like, fluids are still
Speaker 1: something we're struggling to understand. But Bernoulli had a simplified
Speaker 1: view of it, which oiler actually came through later and
Speaker 1: proved all of his equations for him. But Bernoulli's principle
Speaker 1: tells us that faster moving fluids have lower pressure and
Speaker 1: slower moving fluids have higher pressure. So if the shape
Speaker 1: of the wing as it moves through the air makes
Speaker 1: the air go slower under the wing and faster above
Speaker 1: the wing on this curved shape, then that'll generate lower
Speaker 1: pressure above the wing and higher pressure below the wing,
Speaker 1: and that gives you lift.
Speaker 3: And that feels easy to test, right because you just
Speaker 3: flip the wing over and it should do the opposite, right,
Speaker 3: like it should send you plummeting downwards.
Speaker 1: Yes, already you're identifying a problem with this, But before
Speaker 1: we take this explanation apart, let's do a little bit
Speaker 1: more to support it. Because you're right, it kind of
Speaker 1: is easy to test and you can do a simple
Speaker 1: test at home. You can just like take a piece
Speaker 1: of paper and blow air above the piece of paper,
Speaker 1: and what you see is the paper goes from being
Speaker 1: sort of droopy to being flat, so it sort of
Speaker 1: like lifts up. And so it's like a classic symbol
Speaker 1: at home demonstration. Which you're doing is you're increasing the
Speaker 1: velocity of the air above the paper, which in theory
Speaker 1: lowers the air pressure above the paper, which provides lift
Speaker 1: because now the pressure is higher below the paper and
Speaker 1: lower above the paper. And you know, the story of
Speaker 1: like the air moves faster over the wing and slower
Speaker 1: below the wing is also verified in lots of other experiments,
Speaker 1: Like you can do these smoke tests where essentially you
Speaker 1: put a wing in a wind tunnel. Instead of just
Speaker 1: blowing air over it, you blow smoke over it. Smoke
Speaker 1: just being like a bunch of particles. You can track
Speaker 1: those particles and you can measure the velocity. So basically
Speaker 1: measure the velocity of the air around the wing. See
Speaker 1: what happened, right, don't just like talk about it, you know,
Speaker 1: in your salons while you're smoking cigarettes or whatever. Actually
Speaker 1: figure it out. And this is verified, like wind tunnels
Speaker 1: and smoke tests tell us that the air does move
Speaker 1: faster over the top of the wing than below the wing,
Speaker 1: And so this seems like it all sort of comes together,
Speaker 1: but as you identify, there are some important limitations to
Speaker 1: this explanation of why planes fly.
Speaker 3: So when you get a window seat near the wing
Speaker 3: and you look out, it's easier to see like it's
Speaker 3: bunching up and it gets a little bit like kind
Speaker 3: of white ish. Is that pressure forming? Like are you
Speaker 3: sort of bunching up the air molecules to the point
Speaker 3: where they're visible or something else happening there? Or am
Speaker 3: I imagining it? And this is like an episode of
Speaker 3: the Twilight Zone.
Speaker 1: Nobody else sees that, Kelly, Yeah, exactly, it's just you.
Speaker 4: That was such a great episode.
Speaker 1: I love that.
Speaker 7: No.
Speaker 1: I think what's happening there is you're seeing more water vapor.
Speaker 1: There are definitely changes in the air pressure below and
Speaker 1: above the wing, and as you know, water is very
Speaker 1: sensitive to pressure, the vapor point and all this kind
Speaker 1: of stuff. What you're seeing there is not the air itself,
Speaker 1: but water vapor forming, which can show you, just like
Speaker 1: any smoke tests can show you where the air is flowing.
Speaker 1: So that is pretty cool.
Speaker 4: That sounds obvious now that you say it. But why
Speaker 4: did I ask that?
Speaker 3: But whatever, that's fine, okay, So we decided now we're
Speaker 3: going to talk about the limitations. Has anybody flipped the
Speaker 3: wing the other way and then saw what happened?
Speaker 4: Did that plane still fly?
Speaker 1: So? Planes fly upside down all the time? Right? A
Speaker 1: big problem with this explanation is that it predicts that
Speaker 1: the asymmetry of the wing is crucial, right, that the
Speaker 1: curb bit on the top compared to the flat bottom
Speaker 1: is really important for making a lift. And so it
Speaker 1: predicts that if you flip that over, planes should like
Speaker 1: crash to the ground. Right, that an upside down wing
Speaker 1: should have anti lift or should have net force downwards.
Speaker 1: But we don't see that. You know, you can fly
Speaker 1: planes upside down all the time. Everybody who's been to
Speaker 1: like an aeronautics show has seen old fashioned planes or
Speaker 1: new planes. They can fly upside down. So Bernoulli's equation
Speaker 1: does and answered this question. This can't be the complete
Speaker 1: story of why wings provide lyft because it gives the
Speaker 1: wrong explanation or upside down planes.
Speaker 3: So you mentioned that Eiler figured out all of these
Speaker 3: equations to support Bernoulli. Does it turn out that Oiler's
Speaker 3: equations were missing some important factor because the equation said
Speaker 3: it should work, but then it doesn't work.
Speaker 4: So what was missing in the equations?
Speaker 1: Oh yeah, great question. And this really goes to the
Speaker 1: heart of what's going on here. There's nothing wrong with
Speaker 1: Bernoulli's equations. And I'm giving Oiler credit only because Oiler
Speaker 1: came along and like did all the math BERNEULLI had
Speaker 1: like these leaps of insight, like oh, I think this
Speaker 1: and that and the other thing, and then Oiler came
Speaker 1: along and actually like dotted all the i's and crossed
Speaker 1: all the t's. And I think Oiler would have gotten
Speaker 1: credit for these equations if he hadn't already gotten credit
Speaker 1: for like eighty percent of everything in mathematics. It's like
Speaker 1: so much of stuff in mathematics that like Oiler just
Speaker 1: let somebody else take credit for because he's already got
Speaker 1: everything else named after him, which is amazing. So there's
Speaker 1: no problem with Bernoulli's equations. But there are always a
Speaker 1: simple description like brenell The's equations describe fluid flow, and
Speaker 1: they make some assumptions like you're talking about a fluid
Speaker 1: as if it's incompressible, for example, or you're talking about
Speaker 1: it as if it's not made of microscopic particles. That's
Speaker 1: not true, right, Air is made of microscopic particles. It
Speaker 1: is compressible. That doesn't mean that the equations don't apply.
Speaker 1: It means that they apply in some limited sense. It
Speaker 1: also might just be the wrong story. It might not
Speaker 1: provide the answer the explanation. You know, what we're looking
Speaker 1: for is an answer to a question which is macroscopic.
Speaker 1: Like we see the wing go through the air, the
Speaker 1: wing goes up. We want to know why. That's sort
Speaker 1: of like a human question, right, it's not a mathematical question.
Speaker 1: It's not like there's a prediction and number we're trying
Speaker 1: to calculate. We want like a story that tells us
Speaker 1: why this is happening. And that's a little bit more
Speaker 1: slippery than just mathematics. It's something to do with cause
Speaker 1: and effect and understanding, and you know, it's at the
Speaker 1: heart of science is coming up with these stories. But
Speaker 1: it's not always easy, especially when we're zooming out from
Speaker 1: a microscopic universe, to try to tell a story macroscopically.
Speaker 1: It's like economics. You know, you can ask like, why
Speaker 1: do prices go down? Well, I have this theory of inflation.
Speaker 1: I have a theory that involves prices and supply chains, whatever,
Speaker 1: and that the theory can be correct, but it doesn't
Speaker 1: always apply because the conditions it assumes aren't always relevant,
Speaker 1: and it doesn't always answer the question that you're asking.
Speaker 3: I feel like, the next time I get on a plane,
Speaker 3: I'm going to think to myself, I wish it were simple.
Speaker 4: I wish we've really understood this, But plays tend to
Speaker 4: stay up. So that's good. So we've established that the
Speaker 4: Germans got everything wrong and World War one? Oh wait,
Speaker 4: was that World War One or World War two?
Speaker 1: This is World War One we're talking about.
Speaker 4: Well, they got it wrong both times.
Speaker 9: Sorry guys.
Speaker 3: Okay, so we know that when a plane over it
Speaker 3: doesn't crash into the ground.
Speaker 4: What was he getting wrong? Bernoulli?
Speaker 1: So, Bernoulli's story is very nice, and it's very simple,
Speaker 1: and there's lots of it that's correct, but as you say,
Speaker 1: it doesn't explain some things. It doesn't explain why planes
Speaker 1: can fly upset down, and it also doesn't explain something
Speaker 1: really crucial at the heart of the story, which is
Speaker 1: why is the air moving faster over the top of
Speaker 1: the wing than the bottom of the wing. Like we said,
Speaker 1: the wing is asymmetric, it's curbing on the top and
Speaker 1: it's flat on the bottom. But why does that make
Speaker 1: the air go faster over the top? And you often
Speaker 1: hear this pop side explanation which is completely wrong, which
Speaker 1: is that it's a longer trip over the top of
Speaker 1: the wing, so the air has to go faster in
Speaker 1: order to meet its air particle partners at the back
Speaker 1: end of the wing.
Speaker 4: Well, why would it have to exactly, why.
Speaker 1: Would it have to It doesn't have to. It's nonsense,
Speaker 1: But you hear this all the time. I was taught
Speaker 1: this myself. You know that, like when air particles hit
Speaker 1: the front of the wing, if one goes above and
Speaker 1: one goes below, then to meet up again at the
Speaker 1: back of the wing, the one that takes the longer
Speaker 1: route would have to go faster. And it's an example
Speaker 1: of like it's a compelling story, but it doesn't actually
Speaker 1: make any sense because they don't have to take the
Speaker 1: same time. It's fine, the universe doesn't like crash or
Speaker 1: implode or hit a seg fault or something. If the
Speaker 1: air particles don't meet up again.
Speaker 3: Yeah right, they're not like buddies. They don't have to
Speaker 3: get to the same spot at the same time. Yeah
Speaker 3: all right, so that doesn't have to happen.
Speaker 1: So that doesn't have to happen. That's not the explanation
Speaker 1: for why air moves faster over the top, and so
Speaker 1: the Bernoulli story is sort of lacking that explanation, like
Speaker 1: why is there this pressure difference? And it opens this
Speaker 1: door to this question of like, well, did the speed
Speaker 1: differences cause the pressure differences or is it the pressure
Speaker 1: differences that are causing the speed differences. Because Bernoulli's equation
Speaker 1: doesn't give us causality, just says lower pressure, higher speed.
Speaker 1: It doesn't say that lower pressure is caused by the
Speaker 1: higher speed. It could also be the higher speed is
Speaker 1: caused by the lower pressure. So that tells us that
Speaker 1: we need more to the story. Bernulli story can't just
Speaker 1: crowed completely. Uhy planes fly because it doesn't tell us
Speaker 1: why planes fly upside down, and it doesn't explain why
Speaker 1: we see air going faster over the top of the wing,
Speaker 1: which is something we definitely see happening.
Speaker 3: Right, So, I know we're about to talk about another explanation.
Speaker 3: But does looking at birds help at all? Because they have,
Speaker 3: you know, different kinds of wing shapes and they've kind
Speaker 3: of figured it out over evolutionary time. This isn't going
Speaker 3: to give us the pressure answer, but does it give
Speaker 3: us some insights into like how these wings should be shaped.
Speaker 3: I'm getting off track, but I just looked at a
Speaker 3: picture of a bird in my office and.
Speaker 1: Now I have to ask it does and bird wings
Speaker 1: work with similar principles to airplane wings, except of course
Speaker 1: for the flapping part. But for gliding they do fascinatingly.
Speaker 1: Insect wings are even more complicated, and insect flight is
Speaker 1: not something we understand like at all. It's really kind
Speaker 1: of incredible. We should do a whole another episode about
Speaker 1: why insects fly.
Speaker 3: And if you're ever feeling depressed, there are some slow
Speaker 3: mode videos of insects trying to take off and it.
Speaker 4: Is the most clumsy, amazing thing I've I've ever seen.
Speaker 3: It blows my mind that they ever get off the ground,
Speaker 3: and it makes my daughter left so hard she peeps
Speaker 3: a little.
Speaker 1: So I knew a guy who did these experiments where
Speaker 1: they put an insect on a pin that like glued
Speaker 1: it to a pin and then they showed it videos,
Speaker 1: so it was like thought it was flying and they
Speaker 1: would like try to take videos of its wings moving
Speaker 1: to understand it. It's kind of incredible.
Speaker 3: All right, this has been an amazing tangent. I've had
Speaker 3: a lot of fun. Let's go back to World War One,
Speaker 3: which is less fun. So I think we've talked about
Speaker 3: the shortfalls in Bernoulli's explanation. Are we ready to move
Speaker 3: on to what the British folks were thinking?
Speaker 1: Yeah, So across the English channel we had folks that
Speaker 1: are working on a completely different theory, and not a
Speaker 1: completely different theory just because they disagreed scientifically with the
Speaker 1: best approach. Remember there's a war here, and so they're
Speaker 1: not going to collaborate. They're not going to be like
Speaker 1: going to the same conferences and sharing their ideas. It's
Speaker 1: sort of like when evolution splits. You know, you get
Speaker 1: all these weird animals off on an island and they
Speaker 1: diverge into their own crazy direction and get weird pouches
Speaker 1: for their babies or whatever. It's what happens when you
Speaker 1: work in isolation. You get competing nations, and so we
Speaker 1: have this fascinating sort of social experiment where you like,
Speaker 1: isolated two groups of scientists and asked them to solve
Speaker 1: the same problem and they took very different approaches.
Speaker 3: So was it like Allopatrick speciation where they weren't encountering
Speaker 3: each other at all anymore and that's why they come
Speaker 3: up with different theories because it was no cross talk.
Speaker 3: Or was it s Patrick expeciation where the ideas could
Speaker 3: have flowed between each other, but they were still deciding
Speaker 3: to stay segregated. Was it nationalism or did they just
Speaker 3: not know?
Speaker 4: Is what I'm wondering.
Speaker 1: I think it started with nationalism, but then it developed
Speaker 1: into sort of tribal camps and the Germans criticized the British,
Speaker 1: and the British criticized the Germans. You know, even after
Speaker 1: the First War, when these guys could have gotten together
Speaker 1: and had like a global, unified theory of flight, they
Speaker 1: continued working in their own direction because they were invested
Speaker 1: in you know, this guy's advisor told him that the
Speaker 1: German approach was nonsense, and the German guy's adviser told
Speaker 1: him the British approach was scheiss, you know or whatever,
Speaker 1: And so.
Speaker 4: Do you speak German?
Speaker 1: I do not speak German. I speak Danish, which is
Speaker 1: sort of Germanic a little bit, but that's a big
Speaker 1: German anyway. So the British across the English Channel didn't
Speaker 1: like the German approach. They didn't like thinking about air
Speaker 1: as a fluid because they were like, this is too idealized.
Speaker 1: They can't explain what's really happening. The Germans thought, you know,
Speaker 1: ideal fluids are a fine approximation, but the British didn't
Speaker 1: like it. And you know, anytime you're making an explanation
Speaker 1: in science, you're going to be making approximations. It's just
Speaker 1: a question of what approximations you make. They're always shortcuts.
Speaker 1: Nobody can completely describe the full complexity of the universe
Speaker 1: in their equations. It's always a simplification. And there's an
Speaker 1: art to that, choosing the simplification that captures the essential
Speaker 1: details of what's happening in reality, so you can provide
Speaker 1: a useful explanation while dismissing all the irrelevant complexities that
Speaker 1: you don't need to worry about, which is why, for example,
Speaker 1: we can talk about how a ball flies through the
Speaker 1: air and often ignore air resistance and ignore the quantum
Speaker 1: effects and ignore everything else that's happening, the tug of
Speaker 1: Jupiter on that ball, because we judiciously choose what approximations
Speaker 1: to make. So that's what's happening here is two subjective approaches,
Speaker 1: and the British were focused not on this ideal fluid approach,
Speaker 1: but instead of using just a simple approach with Newton's law,
Speaker 1: they were just thinking about the forces.
Speaker 3: How do forces act differently on the top and the
Speaker 3: bottom based on that tear drop shape.
Speaker 1: Yeah, so the Newton's approach basically ignores the tear drop
Speaker 1: shape and it says the shape of the wing isn't
Speaker 1: actually important. What's crucial is the angle of the wing
Speaker 1: into the wind. So they're thinking, hey, the wing is
Speaker 1: tilted a little bit, and so as the wing is
Speaker 1: moving through the air, the wind gets bounced basically off
Speaker 1: the wind and goes down. So you have wind flowing
Speaker 1: onto the wing and you have an angle, and then
Speaker 1: it bounces down and so the wing goes up. And
Speaker 1: like anybody who's ridden in a car and put their
Speaker 1: hand out at an angle knows that you can feel
Speaker 1: lift right, your hand basically flies and your hand's not
Speaker 1: a tear drop, it's not an air foil. It's not
Speaker 1: like carefully germanically engineered to get lift. It's just your hand,
Speaker 1: you know, But still you can get lift. This is
Speaker 1: a simple application of Newton's theory. Like action reaction, the
Speaker 1: wind gets pushed down by the angle of the wing.
Speaker 1: The aeronautical engineers call it the angle of attack, and
Speaker 1: so the wing gets pushed up. Very simple.
Speaker 3: So if that's true, then you could adjust wing shape
Speaker 3: to be even better at flying by making it bend
Speaker 3: even more so that you're stopping even more air as
Speaker 3: you go. Did the British try that to prove how
Speaker 3: much better they were?
Speaker 1: I wish you'd been around in the aeronautical society, so
Speaker 1: clear with your questions. So what you're describe me is
Speaker 1: actually what you see in an airplane when you take off,
Speaker 1: if you're paying attention to the wing. Modern airplanes actually
Speaker 1: have a changeable shape, right. They have these flaps and
Speaker 1: these levers, right, so they can change the shape of
Speaker 1: the wing, and during takeoff they do just that. They
Speaker 1: push down the back of the wing so it's more
Speaker 1: of a curvy shape. It like grabs the air a
Speaker 1: little bit more, so you get more lift during takeoff.
Speaker 1: And then when you're flying, they make it flatter, so
Speaker 1: you're cruising, you get less lyft, but you also get
Speaker 1: less drag. Drag is the force that pushes the wing backwards,
Speaker 1: so you want upwards force lift without as much backwards
Speaker 1: force drag. So what you're seeing is exactly what modern
Speaker 1: airplanes do. Is they make a curveer wing you take
Speaker 1: off to get more lift.
Speaker 4: Very cool.
Speaker 3: I was reading about programmable matter, and I think one
Speaker 3: of the proposals was to have matter that responds to
Speaker 3: the environment and makes those changes on its own without
Speaker 3: you needing to do anything.
Speaker 4: But I am glad we have more control over the
Speaker 4: process right now. I'm not ready to have us let
Speaker 4: go of that control yet.
Speaker 1: And the nice thing about this explanation is it explains
Speaker 1: why planes can fly upside down. They fly upside down
Speaker 1: because they have the right angle of attack. It doesn't
Speaker 1: matter the shape of a wing at all. It just
Speaker 1: matters that the airplane wing is at the correct angle.
Speaker 1: So if you fly your airplane upside down, you can
Speaker 1: do it as long as the wing is angled into
Speaker 1: the air at the right angle.
Speaker 3: So does this completely explain everything? That's never how our
Speaker 3: podcast episodes end. So what is it missing.
Speaker 1: So this is a beautiful story of the limitations of
Speaker 1: science because both explanations explain something, but neither of them
Speaker 1: tell the complete story. So, for example, there's a couple
Speaker 1: of things that the British Newton's theory force explanation doesn't describe.
Speaker 1: Number one is what's going on on the top of
Speaker 1: the wing, right, This just focuses on the bottom of
Speaker 1: the wing, but we see that the pressure is lower
Speaker 1: on the top of the wing. The Newton's theory approach says,
Speaker 1: all the lift comes from below, that you're getting this
Speaker 1: force from the wind that's hitting it from below. But
Speaker 1: we see this lower pressure above, and it turns out,
Speaker 1: as we'll talk about later, that it contributes maybe even
Speaker 1: more to the lift, the low pressure above the wing
Speaker 1: than the high pressure does below the wing. And the
Speaker 1: Newton's theory cannot explain this. That's number one, and.
Speaker 3: It can't explain why there's low pressure above the wing, right,
Speaker 3: just like the other one.
Speaker 1: Yeah, exactly. Number two is that if you do these
Speaker 1: smoke studies, we see that the wing affects the flow
Speaker 1: of the air, not just below the wing. It's like
Speaker 1: the influence of the wing on the air is larger
Speaker 1: than just the wing, Like the air begins to flow
Speaker 1: up above the wing before it hits the wing. There
Speaker 1: is this upwash in advance of the wing and this
Speaker 1: downwash after the wing. So it's not just like you
Speaker 1: have a paddle and it's being hit by molecules and
Speaker 1: it's getting pushed up. There's a complicated interplay here between
Speaker 1: the wing and the flow around it and the air
Speaker 1: itself applying pressure on itself. So you can't ignore the
Speaker 1: sort of fluid effects of the air if you want
Speaker 1: to completely describe what's happening.
Speaker 3: Yeah, so while you were describing all of that, I thought,
Speaker 3: oh man, it really feels like fluid dynamics should explained.
Speaker 3: So maybe the German example should have worked. Why does
Speaker 3: the fluid example not work then? With all of that
Speaker 3: complicated stuff happening.
Speaker 1: So the fluid example does explain some parts of it.
Speaker 1: And so where we're going to go in the end
Speaker 1: is this like weird hybrid of the British German theory
Speaker 1: in order to have the most complete explanation of why
Speaker 1: it happens. But what we're doing right now is sort
Speaker 1: of examining like the failure of the individual ones. So
Speaker 1: the German explanation. The fluid approach can describe the flow
Speaker 1: of the fluid around the wing, but the British approach,
Speaker 1: though it's simple and satisfying, can't describe all of this stuff.
Speaker 4: Okay, spoiler alert.
Speaker 1: Yeah, so one thing that fails to describe is the
Speaker 1: sort of holistic flow of the air around the wing.
Speaker 1: The other is stalling and the Newton's force approach. The
Speaker 1: only reason you're flying is the angle of attack, and
Speaker 1: the greater the angle, the greater the force. Right, But
Speaker 1: anybody who flies knows that if you have too great
Speaker 1: an angle, you're going to stall, Like if your plane
Speaker 1: is pointed too far upwards, you're not going to get
Speaker 1: lift anymore. And what happens technically, what we see in
Speaker 1: wind tunnels is that if the angle of attack is
Speaker 1: too great, then as the air flows over the top
Speaker 1: of the wing, it doesn't merge smoothly with the air
Speaker 1: flowing below the wing, and you get this like weird turbulence.
Speaker 1: They call it a separation region because the air doesn't
Speaker 1: nicely reconnect. And you know, when you're going through a fluid,
Speaker 1: turbulence is a problem. You want to minimize turbulence. And
Speaker 1: so if the angle is too large. This is like
Speaker 1: gap between the air flows as they come off the
Speaker 1: back of the wing and they don't don't merge nicely.
Speaker 1: That's called the separation region between these flows, and that
Speaker 1: creates the stall. You lose your lift. So you can't
Speaker 1: explain that. Also with the Newton's explanation, you need some
Speaker 1: fluid theory to explain.
Speaker 3: That slightly tangential. We haven't discussed propellers. Do propellers and
Speaker 3: jet engines play any role here or is that just
Speaker 3: in like that moves us forward and all of the
Speaker 3: uppiness comes from the wings.
Speaker 1: Yeah, So basically engines just provide forward velocity so that
Speaker 1: the wings attack the air. All The propellers are a
Speaker 1: subtle point because propellers have a particular shape, which is
Speaker 1: related to the shape of a wing, right, because it's
Speaker 1: converting motion in one direction to air velocity in the
Speaker 1: other direction. So the shape of a propeller is related
Speaker 1: to the shape of a wing. But for the question
Speaker 1: of why planes fly, you can think of its just
Speaker 1: like something pushy, right, the pushy bit that gets the
Speaker 1: plane moving through the air. Yeah.
Speaker 3: Okay, so you've got the pushy bit, but we don't
Speaker 3: understand what's happening around the wings yet, So how do
Speaker 3: you move from figuring out that the German and the British
Speaker 3: explanation are failing in some way to figuring out the
Speaker 3: answer to what's going on? Is this like where we
Speaker 3: are we just don't know or are there more experiments
Speaker 3: you can do to figure it out?
Speaker 1: So what you shouldn't do is ask the world's smartest
Speaker 1: physicist Einstein to weigh in on this very practical engineering question.
Speaker 3: Well, let's find out what he had to say after
Speaker 3: the break, all right, So for most of the Daniel
Speaker 3: led episodes, we have to walk through a what did
Speaker 3: Einstein think? Because he just provided so many good insights
Speaker 3: into so many things. So let's figure out what did
Speaker 3: Einstein think about this?
Speaker 1: Yeah, so Einstein, he's a smart guy. He developed relativity,
Speaker 1: he struggled and failed to integrate quantum mechanics into it.
Speaker 1: So it's not like he solved every problem he attacked.
Speaker 1: But he was a smart guy. And you know, he
Speaker 1: was German and this is a relevant question for the
Speaker 1: Germans at the time, and he was interested in this
Speaker 1: and he wrote, quote, there's a lot of obscurity surrounding
Speaker 1: these questions and open puzzles like this, you know, they
Speaker 1: attract smart people like, oh, maybe I can figure this out.
Speaker 1: Maybe there's an insight, a moment of clarity that will
Speaker 1: just untangle this whole mess. And so he thought about
Speaker 1: flight from the German point of view, and he actually
Speaker 1: designed his own wing shape, sort of a weird variation
Speaker 1: on the air foril Is Einstein. Remember, he was not
Speaker 1: just a clever physicist. He worked in the patent office
Speaker 1: and so he had seen lots of inventions. He had
Speaker 1: a practical mind also, So he designed his own wing
Speaker 1: and he took it to a German aircraft company, and
Speaker 1: you know, he was an esteemed famous scientist at this point,
Speaker 1: so they thought, okay, sure, so they worked with him,
Speaker 1: they collaborated with him, They built the plane, and they
Speaker 1: sent it on a test flight, and the pilot came
Speaker 1: back and said he would never fly the thing again
Speaker 1: because a quote flew like a pregnant duck.
Speaker 3: I feel like whenever a question sort of tiptoes into
Speaker 3: biology and Einstein is involved, you want to back away. So,
Speaker 3: you know, I think maybe most of us know that
Speaker 3: ducks can't get pregnant.
Speaker 4: They carry eggs, but they don't get pregnant the way
Speaker 4: we do.
Speaker 3: And also, like, you know, he married his cousin, which
Speaker 3: I think is a bad idea also from a biological perspective.
Speaker 4: But anyway, okay, So.
Speaker 3: Einstein was great at everything, but not at designing wings.
Speaker 1: Yeah, exactly. And so fast forward fifty years or so,
Speaker 1: what are people talking about today in terms of why
Speaker 1: planes fly? I went across campus here to a colleague
Speaker 1: at UC Irvine, Haythian Taha. He's a professor of aeronautical
Speaker 1: engineering here and he studies lyft. He works on this problem.
Speaker 1: I asked him what is a simple explanation for lyfts?
Speaker 1: And he said, quote, this is not settled in the
Speaker 1: aeronautical engineering community. Please don't laugh. That's a direct quote.
Speaker 1: So here we are laughing. It's hilarious, you know, like
Speaker 1: these guys still haven't figured this out more than one
Speaker 1: hundred years after their right brothers.
Speaker 4: But yet still so much money is made us.
Speaker 3: So it's nice to know you don't have to have
Speaker 3: everything worked out before you commercialize something.
Speaker 1: But let's not oversell it. It's not that we don't
Speaker 1: understand why planes fly. It's that we don't have a compelling,
Speaker 1: simple explanation at the sort of zoomed out level. And
Speaker 1: again there's always a difference between life the microphysics. Can
Speaker 1: you model what's happening with individual air molecules and the
Speaker 1: shape of the wing and come up with a simulation
Speaker 1: in which planes fly. Absolutely, we can, and we do.
Speaker 1: This is why, for example, Boeing can design their airplanes
Speaker 1: on the computer and be confident that they will fly.
Speaker 1: They don't need to use wing tests anymore and build
Speaker 1: a bunch of models to experiment because we have accurate
Speaker 1: simulations for what's going on in the microphysics level. It's
Speaker 1: just that when you zoom out and now you want
Speaker 1: to tell a simple story about what's happening, there are
Speaker 1: competing ways to do that, and all of them ignore
Speaker 1: some details, and so none of them are completely satisfactory.
Speaker 1: So when he says this is not settled, he doesn't
Speaker 1: mean we don't know why planes fly in a sort
Speaker 1: of microphysics point of view, means we don't have a nice, compelling,
Speaker 1: one sentence explanation for why this happens the way we can.
Speaker 1: For example, like a ball flying through the air, why
Speaker 1: does a baseball fly in a parabola? We know the
Speaker 1: answer is when you have constant acceleration, you get parabolic motion. Simple,
Speaker 1: one sentence answer. That's what we're lacking for flight. We
Speaker 1: have a very complex thing that's happening with microphysics, and
Speaker 1: it's when we're struggling to come up with a simplified
Speaker 1: explanation at the macroscopic level.
Speaker 4: Do engineers care?
Speaker 3: So if you can make a simulation on a computer
Speaker 3: that tells you everything you need to know, Like, are
Speaker 3: engineers still interested in this question or has this completely
Speaker 3: moved to the realm of like physicists who never see
Speaker 3: the light who are studying it in their offices, Like, yeah,
Speaker 3: do they care what the answer is anymore?
Speaker 4: Or do physicists care?
Speaker 1: Only absolutely they care because engineers want simple, working models,
Speaker 1: you know, when they want to model how a plane
Speaker 1: is going to fly, they don't want to have to
Speaker 1: go back to describing every single particle. They want simple
Speaker 1: sets of equations that let them work quickly and easily
Speaker 1: the same way. For example, we'd love to be able
Speaker 1: to predict a hurricane without modeling every single rain drop.
Speaker 1: Currently we have the technology, and we're building these incredibly
Speaker 1: powerful computers just to model wind and pressure and predict
Speaker 1: the weather. We'd love simple equations to be able to
Speaker 1: do that right rather than having to use incredibly detailed computation.
Speaker 1: And so anytime we can summarize complexity with simple math,
Speaker 1: we definitely win. So people are definitely working on this
Speaker 1: and trying to find a complete holistic approach that captures
Speaker 1: all this behavior, that lets us do important engineering without
Speaker 1: simulating all the tiny microphysical details. Absolutely, yeah, people care.
Speaker 3: And I want to go on record that I apologize
Speaker 3: for implying that physicists don't go outside.
Speaker 1: What is this outside thing you're talking about? I was
Speaker 1: going to ask you about that. I have heard about this,
Speaker 1: but I've been too shy to ask.
Speaker 4: Can you told me you've been hiking.
Speaker 3: I know that you're maybe even more extreme than I
Speaker 3: am in terms of outdoor adventures.
Speaker 1: No, I do love going outside. That's why I live
Speaker 1: in California. It's so amazing here.
Speaker 3: Oh, the fall right now is absolutely incredible. Okay, So
Speaker 3: we've talked about a lot of different ideas, a lot
Speaker 3: of things we know and we don't know. I know
Speaker 3: the answer at the end is that it's complicated. But
Speaker 3: is there a simple ish kind of way to summarize
Speaker 3: where we are right now in our understanding.
Speaker 1: Yeah, So I think the simplest way to describe it
Speaker 1: is that both of Bernoulli and Newton's stories help, but
Speaker 1: they're both limited, and the limitation is that they're trying
Speaker 1: to tell a story in terms of like simple causes
Speaker 1: and simple effects. The airflows and its bounces off the wing,
Speaker 1: or the pressure is lower above the wing because the
Speaker 1: velocity is higher, and the real story is that the
Speaker 1: cause and effect are not so simple. There's lots of
Speaker 1: things happening here and they are interplaying together, so it's
Speaker 1: not so easy to disentangle the cause and the effect.
Speaker 1: There's a lot of things at work here, pressure and
Speaker 1: velocity and forces all in this fantastic harmony that's making
Speaker 1: the wing go up, which is why none of these
Speaker 1: simple stories are satisfactory. We can weave a slightly more
Speaker 1: complex story to explain why planes fly that involves all
Speaker 1: of these aspects sort of working together. But you know,
Speaker 1: humans like simple stories. Ball goes down because of gravity,
Speaker 1: this kind of thing. We don't have that simple a
Speaker 1: cause and effect.
Speaker 3: I'm always amazed by things that work so well, where
Speaker 3: when you scratch the surface, we really don't understand, like
Speaker 3: I was talking to someone who uses deep brain stimulation
Speaker 3: to treat epilepsy. So, like, when somebody has a seizure,
Speaker 3: they're having like an electrical storm in their brain, and
Speaker 3: sometimes they have electrodes implanted into the center of their
Speaker 3: brain and they essentially just get their brains apped and
Speaker 3: that helps.
Speaker 4: And so I reached out to an expert and I was.
Speaker 3: Like, why does zapping someone's brain with electricity stop the
Speaker 3: epileptic seizure?
Speaker 4: And they're like, we don't know.
Speaker 3: Just like I'm a little nervous getting onto a plane
Speaker 3: hearing why does the plane go up?
Speaker 4: We don't know. I also would not want to hear
Speaker 4: why are you shocking my brain?
Speaker 3: We don't know.
Speaker 4: But in both cases it works, and it seems to
Speaker 4: work reliably, so we get.
Speaker 1: By Also true, I think for bost pharmaceuticals, right, we're like, well,
Speaker 1: it does have this effect, we don't really understand that's
Speaker 1: a biochemistry of it, but hey, keep taking it, you know.
Speaker 9: That's right.
Speaker 4: Yes, we all muddle along.
Speaker 1: And I'm not an anti vaxxer at all. I just
Speaker 1: mean this is sort of the way science is. You
Speaker 1: always understand science at some level, you don't always need
Speaker 1: to understand the microphysical explanation in order to make it work.
Speaker 1: And that's actually a huge gift. Otherwise we couldn't do
Speaker 1: anything if we needed to understand like the nature of
Speaker 1: quantum gravity before we made a bowl of chicken soup,
Speaker 1: we'd never have chicken soup. We'd never understand the world.
Speaker 1: It's lucky that the world can be understood without knowing
Speaker 1: all the details, or science would be impossible.
Speaker 4: It's lucky you can tinker with the world without understanding it.
Speaker 1: Yes, that too. So we do have a story we
Speaker 1: can tell about why planes fly. And I think the
Speaker 1: best way to think about why a wing goes up
Speaker 1: is to focus on pressure, pressure above the wing and
Speaker 1: pressure below the wing. Fundamentally, wing goes up because pressure
Speaker 1: grows below the wing and pressure falls above the wing.
Speaker 1: And both of these are contributed to by the Bernoulli
Speaker 1: and the Newton story.
Speaker 3: Got it, Okay, we managed to summarize something in like
Speaker 3: three or four sentences.
Speaker 1: But we can also dig into it in a little
Speaker 1: bit more detail, you know, like why does pressure grow
Speaker 1: below the wing? Well, Newton tells us to focus on
Speaker 1: the angle of attack, right, and that's correct. The wing
Speaker 1: goes through the air, and the angle of attack pushes
Speaker 1: down on the air, and the air pushes back, and
Speaker 1: this is pressure. Just like if you're scooping up snow
Speaker 1: with a snowshovel, right, this is going to be increase pressure.
Speaker 1: The snow is going to get compacted. So why does
Speaker 1: pressure form below the wing? Not that controversial. It's the
Speaker 1: angle of attack that's crucial.
Speaker 3: And as long as you go like this and you
Speaker 3: just don't look at the top part of the wing,
Speaker 3: and so I'm covering the top part of my eyes
Speaker 3: and you don't think about the top part of the wing,
Speaker 3: then everything is good. But we don't understand what's happening above?
Speaker 1: Is that right well above the wing? We can also
Speaker 1: think about the pressure. The Newton story can't explain what's happening,
Speaker 1: but if you think about it in terms of fluid
Speaker 1: you can. Right, So what's happening above the wing? Initially,
Speaker 1: if you just like shoot air above the wing, then
Speaker 1: it wants to flow straight back, which effectively creates like
Speaker 1: a vacuum underneath. The angle of attack, working together with
Speaker 1: the shape of the wing, creates this low pressure zone
Speaker 1: above the wing because the air needs to go down
Speaker 1: to flow onto that right. So that's exactly what happens,
Speaker 1: is you create this low pressure zone, which is like
Speaker 1: a vacuum. It pulls the air into it, and that's
Speaker 1: pulling the wing up. So you have low pressure above
Speaker 1: the wing created by the shape of the wing and
Speaker 1: the angle of attack, which creates low pressure above it.
Speaker 1: And so these two effects work in harmony. You have
Speaker 1: a force from below and you have like suction from above.
Speaker 1: So wings go up not just because the air below
Speaker 1: them is pushing them up, but the air above them
Speaker 1: is sucking them up into the sky.
Speaker 4: So the Germans got the top right and the British
Speaker 4: got the bottom right.
Speaker 9: Is that right?
Speaker 1: Okay?
Speaker 3: I feel like there's so many times where I hear
Speaker 3: that there's a debate and there's people on this side
Speaker 3: and people on that side, and five years later the
Speaker 3: answer is almost always, oh, they were both right, and
Speaker 3: it's some combination of things, but okay, cool.
Speaker 1: Yeah, And so mostly this comes from the shape of
Speaker 1: the front of the wing. Right. Now, the shape of
Speaker 1: the wing is important because you want to minimize drag. Right,
Speaker 1: the Newton folks tell you the shape doesn't matter. All
Speaker 1: that matters is you get the force from the bottom
Speaker 1: but what you want is to create low pressure on
Speaker 1: the top without creating a lot of drag and without
Speaker 1: creating this turbulence zone which gives you a stall. So
Speaker 1: that's why you have this shape. You have this shape
Speaker 1: in the front of the wing to give you this
Speaker 1: uneven pressure distribution, so you get low pressure but not
Speaker 1: so much that you get a stall. And then you
Speaker 1: have this smooth ending so the flow comes together nicely
Speaker 1: without creating turbulence at the end. So the shape of
Speaker 1: an airfoil and the angle of the attack is optimized
Speaker 1: to give you smooth flow, low pressure above the wing,
Speaker 1: and to minimize drag, which is important if you want
Speaker 1: to take off.
Speaker 3: I feel like the next time I get out of flight,
Speaker 3: I'm going to appreciate it all much more, having a
Speaker 3: better sense of how this all works.
Speaker 1: But you know, in terms of coming to this explanation,
Speaker 1: the experts still are arguing about, like how to summarize this.
Speaker 1: I've given you sort of a summary version of it,
Speaker 1: but I'm sure there are aeronautical engineers out there who
Speaker 1: have differing opinions and are going to write into us
Speaker 1: with their theory for why planes fly, or at least
Speaker 1: for how to describe it. Our original listener Tom Johnson
Speaker 1: wrote in because he saw this video from the Smithsonian
Speaker 1: Air and Space Museum that explained it in terms of
Speaker 1: just Bernoulli right, the Bernoulli explanation of low pressure on top,
Speaker 1: and he wasn't satisfied with this, and he wrote to
Speaker 1: me and he said, quote, I contacted the Smithsonian about
Speaker 1: this video, voicing my concerns, and I was told unequivocally
Speaker 1: that it's their policy that Berneulli makes planes fly. I
Speaker 1: wasn't aware that physics cared about policies, but I guess
Speaker 1: I'm mistaken.
Speaker 4: That's amazing. Institutions are fantastic.
Speaker 1: Exactly. And so again, you know, while the microphysics is
Speaker 1: clear about what's happening around the wing and you can
Speaker 1: track these individual particles, we still do struggle a little
Speaker 1: bit coming up with a simple explanation. It turns out
Speaker 1: pressure is important, and velocity is important, and the forces
Speaker 1: are important. So none of the sort of classic simple
Speaker 1: explanations can describe everything that's happening around a wing. You
Speaker 1: need a more complex, a fuller description of what's happening
Speaker 1: to explain all the phenomena why planes can fly upside down,
Speaker 1: Why the pressure goes down above the wing. All this
Speaker 1: kind of stuff. So in the end the British and
Speaker 1: the Germans have to work together.
Speaker 4: Ah, I mean that's better in the end.
Speaker 3: So those plane wing shapes, So I'm thinking of other
Speaker 3: instances where you've got like things moving through the air.
Speaker 4: So we've got like.
Speaker 3: Wind turbines, and then you've got racing cars. They don't
Speaker 3: want to go up, they want to go down, so
Speaker 3: they do the whole shape in reverse. Like, how has
Speaker 3: this information been used in other contexts?
Speaker 1: Yeah, exactly. So Formula one cars, they have a wing
Speaker 1: in the back, but that wing pushes the car down
Speaker 1: so that it maintains friction because they need friction in
Speaker 1: order to go forward. Right, the wheels have to get
Speaker 1: pushed onto the ground so the wheels can grab and
Speaker 1: then when they turn the wheels, the car goes forward.
Speaker 1: If the car lifts up above the ground, then it
Speaker 1: can't move forward anymore. So they design it in order
Speaker 1: to generate downward pressure, so they have crucially a different angle, right,
Speaker 1: But they also think about the foil and they do
Speaker 1: really complex modeling because you know, millions of dollars mean
Speaker 1: microseconds and that it's the difference between winning and losing.
Speaker 1: So yeah, absolutely, they invert all of this theory in
Speaker 1: order to get a wing that can push down. And
Speaker 1: the crucial thing there is the angle of attack, right.
Speaker 3: And then wind turbines. They don't want the wind turbine
Speaker 3: to go airborne, but they want it to spin as
Speaker 3: fast as it can, which feels like maybe some of
Speaker 3: the principles we talked about today could help that happen.
Speaker 4: Yeah, how does this plant to wind turbines?
Speaker 1: Yeah, exactly, Well, it's the same principle. Essentially, you have
Speaker 1: air flow across the propeller and you want to convert
Speaker 1: that to a sideways force, right, So you want to
Speaker 1: force up relative to the propeller. You don't want the
Speaker 1: whole propeller to fly abof the ground, right, but you
Speaker 1: wanted to spin around the axis, and so again the
Speaker 1: angle of attack and the shape of it ensures a
Speaker 1: force that turns it and smooth flow around it because
Speaker 1: you don't want turbulence in the back of your propeller blade.
Speaker 1: So each one of those is essentially just a little wingh.
Speaker 3: Very cool and guessing that we weren't racing cars before
Speaker 3: we had planes, so probably the plane stuff came first.
Speaker 1: Yeah, the plane stuff came first. Cars were pretty slow
Speaker 1: until kind of recently, Like you could win big car
Speaker 1: races by driving it like forty five miles an hour
Speaker 1: for many years.
Speaker 3: That sounds so cute. Now it's much harder to die
Speaker 3: at that speed. Why would you watch that exactly?
Speaker 1: But it's really fun to think about how we explain
Speaker 1: the world around us, and how challenging it can be
Speaker 1: to explain some things. It makes me really grateful for
Speaker 1: the times that we can find a very simple explanation
Speaker 1: and wonder like why that's possible sometimes and not other times.
Speaker 1: Is it because the way that we think about the world,
Speaker 1: or is it just something about the way the world works?
Speaker 1: You know, alien scientists have a better theory for why
Speaker 1: wings work because they started off from a completely different
Speaker 1: point of view mathematically or scientifically. Or is everybody struggling
Speaker 1: to describe some complex behaviors? Are these things just inherently
Speaker 1: complex or is it just our language that's making them
Speaker 1: a challenge?
Speaker 3: I wonder if part of that is there some percent
Speaker 3: of explanations that we have that we feel good about
Speaker 3: that are actually wrong if you look at them more closely.
Speaker 4: Stuff works anyway.
Speaker 3: Yeah, and then I wonder if some other stuff just
Speaker 3: there happens to be a metaphor that we're familiar with
Speaker 3: that makes it easier to understand.
Speaker 4: And there's just not like a ready metaphor for some
Speaker 4: of the more complicated things.
Speaker 1: But yeah, and sometimes we accept an explanation even though
Speaker 1: it's nonsense, like the air has to go faster over
Speaker 1: the top of the wing to meet its partners. Like
Speaker 1: a lot of people go, oh huh, yeah, that makes sense,
Speaker 1: but it.
Speaker 9: Doesn't actually make sense, you know, yep.
Speaker 1: And fundamentally, all of science is quote unquote wrong. If
Speaker 1: you zoom in far enough, you know, everything is just
Speaker 1: an approximate description, not like scientists are lying to us
Speaker 1: or we're promoting nonsense, but everything is an approximation, even
Speaker 1: like describe an electron as a fundamental particle, Yeah, probably
Speaker 1: it's not. Probably it's made of something smaller. We just
Speaker 1: haven't seen it yet. But our theory works so far,
Speaker 1: we've never been able to make an experiment where it breaks.
Speaker 1: And so all of science is a work in progress
Speaker 1: in that sense, not just like do we have the
Speaker 1: best explanation for it? But you know, how far can
Speaker 1: we push this until we see it break. And seeing
Speaker 1: it break is an opportunity, right, It's not a disaster.
Speaker 1: It's a chance to learn something deeper about the universe,
Speaker 1: to come up with a more accurate description, or to
Speaker 1: poke through reality to another level and see how it
Speaker 1: works underneath it all. So it's just part of the
Speaker 1: journey of science.
Speaker 3: I feel like it's a great time to be in science.
Speaker 3: There's still so many fundamental things left to understand at
Speaker 3: the same time as we gaining all this new technology
Speaker 3: that allows us to address questions in different ways.
Speaker 4: We have all these new tools. Can't wait to see
Speaker 4: what we learn in the next couple decades.
Speaker 1: Yeah, and it's not just like what's inside an electron,
Speaker 1: like weird, abstract fundamental stuff that you'll ever see. It's
Speaker 1: stuff right in front of you, stuff you can see
Speaker 1: with your eyes. You know, tornadoes and hurricanes and airplanes
Speaker 1: and fluid flow. All this stuff is complex and unsolved.
Speaker 1: So smart young people with energy out there go out
Speaker 1: there figure it out. There's lots left to do, so
Speaker 1: thank you very much to Tom Johnson for sending us
Speaker 1: this question. If you have questions about the way the
Speaker 1: world works or why you can't understand it. Please write
Speaker 1: to us two Questions at Daniel and Kelly dot org.
Speaker 1: We love to hear from you.
Speaker 4: Thanks Tom.
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