Something that was never clear to me at this level of detail is how a tailwind enables an airplane to move faster. In other words, if the airflow is coming from behind, the lift equation should fall apart and the airplane should fall out of the sky.
This looks incredible as usual. What puzzles me, though, is why some people find flying puzzling. At least the kind that we do, ie. helicopters and fixed-wing aircraft. It's easy to accept a fan works: just put your hand there and feel the draft. A wing is just like a linear fan pushing air down. It's completely intuitive to understand for me. The difficulties are just in making it practical and controllable. Conversely, many people don't seem concerned at all with bird or insect flight, which I find a lot harder to understand.
I think many of us were taught in school that airfoil shape was somehow magical -- that the fact that it was bowed more on the top was responsible for the fact that it worked.
This is only partially true, though; a totally flat wing can also support flight. The shaped nature of the wing contributes to its efficiency (and other factors) but do not make other wing shapes incapable of supporting flight.
The reality is that the Wright brothers' innovation was not the airfoil shape or even the lightweight motor. It was the control surfaces, to allow the operator to adjust the plane's attitude on the three axes of rotation, allowing actively stabilized flight.
Paper airplanes and kites demonstrate all the same principles of heavier-than-air flight (the Wright brothers even had a kite version of their airframe they used for testing), despite the fact that they generally do not exhibit shaped airfoils.
The Wrights did use a rudder and "horizontal rudder" on the 1903 Flyer, but they were for some time determined to achieve roll control by warping the wings rather than using control surfaces, and were only forced to adopt ailerons as other pioneers began demonstrating how superior a paradigm that was. So they don't deserve too much credit on that score!
"Control surfaces" was more specific than I intended; what I meant was that their plane allowed them to control all three axes of rotation, and that was the innovation - that they could control pitch, yaw, and roll independently and that allowed them to have active stable flight.
Without those controls, flight is basically impossible, and with them, you could use nearly any airfoil shape (modulo engine power, drag, and stall speed considerations) and achieve heavier-than-air flight.
This is an incredible simplification of flying.
We still don't have a very good understanding of turbulence.
Navier Stokes equations still make Aerospace engineers drink.
Yes its stupid simple if you care about the simplest of analogies, but if you try to understand it, there are reasons why 80% of people drop out.
I think you mean incredible oversimplification
Because an airplane doesn't move its wings like a bug or helicopter, and it's wings aren't shaped like fan blades. One might look at a plane and conclude that since the wings and engines are parallel to the ground, it must only move laterally.
Of course it moves its wings. That's what the runway is for.
I guess this is somewhat counter intuitive:
https://www.youtube.com/watch?v=NBsvzMi9-f8
So yeah, fans are puzzling too.
Surely it's a less impressive result that something powered by mains electricity can move the air in a draft than that a multi-hundred-ton aircraft can fly over the highest mountains.
It's the size of the aerodynamic forces and the complexity of the physical mechanisms that create them that many people have trouble with. In particular: intuitions can be pretty wrong, most simplified explanations are wrong under simple experiments, and the problems exhibit scale variance that is unfamiliar (e.g. Reynolds number).
One time I was working on air data computer for a transonic aircraft that could fly up to about M0.95 - during flight test, an air data probe mounted on a nose boom was used to supply impact and static pressures, angle of attack and sideslip etc. for various air data calculations like airspeed and altitude.
I was fascinated that there was a term in the calculation that related to the aircraft flap position - what's happening way out on the trailing edge of the wing actually has a meaningful effect of pressures measured on a boom out the front of the nose during certain regimes of flight.
Blows my mind that my computer isn't even turning on its fans considering how many shaders are running in this thing.
They’re simple shaders and it’s amazing what a computer can do billions of times per second.
Yeah, I mean come on, we're not rendering something hard and expensive to compute, like a React website that has to list items in a shopping cart.
Personally all the fluid simulation shaders I've written usually makes my fan go off, and I'm counting a few of those here so that's impressive in my eyes.
Yeah. It's impressive to my eyes as well. I was just trying to make a joke about how normal websites need 100% of your CPU to render some text and images, and here's this guy doing multiple fluid simulations on a web page written in custom WebGL and it runs on a potato.
I think they pause when they scroll out of view?
This framing has always irked me:
In this article we’ll investigate what makes airplanes fly by looking at the forces generated by the flow of air around the aircraft’s wings. More specifically, we’ll focus on the cross section of those wings to reveal the shape of an airfoil – you can see it presented in yellow below:
It's invoking some kind of magic to this mysterious shape that "makes airplanes fly". But of course that's bunk. You can make a wing out of rectilinear boxes and get an aircraft that will fly just fine, and for the same reason, as the one with the "airfoil".
Fancy asymmetric airfoils are an engineering solution to the problem of how to get wings to work efficiently and reliably. And they're interesting and worth studying, but they are absolutely not "how airplanes fly".
It'd be like clicking on an article that purported to explain "How computer programs work" .... and then proceeded to describe a debugger.
> It'd be like clicking on an article that purported to explain "How computer programs work" .... and then proceeded to describe a debugger.
To be nitpicky: I wasn't irked by the article's title, but by the framing in the lede.
To be constructively nitpicky: a box used as a wing is absolutely an "airfoil" inasmuch as the term has any meaning. It's not the shape being "special" that makes a wing work, it's the shape that the airflow around it takes, which is to first approximation just a function of its "tilt" along its major axis relative to the flow direction. The business about shape is all just optimization, not what you want to describe if you want to know how an airplane flies.
Very amusing, enjoyed the constructive nitpick!
I suggest reading the article.
It goes into great lengths to show how lift is generated with symmetric airfoils or even flat planes and that the asymmetric airfoil is for efficiency and conditions.
It does not invoke any kind of magic. Even the part you quoted doesn't. Indeed they even animate lift of flat plane at different angles of attack showing lift.
Why does the first slider with the cube not say what the “one property” the slider controls is? Viscosity? Airspeed?
Also, it says "this substance", which I initially thought referred to the cube as it was just mentioned in the previous sentence. But I guess it's the "fluid".
You're kind of correct on both guesses. You can get that change by changing the viscosity OR the airspeed.
He elaborates later on, but you're changing the Reynolds Number - a calculated value from the velocity, fluid density, viscosity and length. The cool thing about a Reynolds Number is that you get identical (in theory) airflow characteristics for two setups with the same Reynold's Number, even if e.g. the airspeed is different.
From the HTML:
<div class="slider_viscosity" id="fdm_hero_sl0">(...)Dare I ask for the code?
It's right there. unminified and unobfuscated. just click save
Wow, I never realized this detail. What a wonderful thing.
IIRC you can view the source and it's all custom WebGL available for viewing.
If you like his work, here's his patreon: https://www.patreon.com/ciechanowski/
If you'd like to see more of their work, ranked by what HN thought was most interesting: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
You can also see all-time top posts: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
The mechanical watch post is 6th on the list
It's pretty interesting that many airfoils used in aircraft design were derived by NACA in the 1920s and 1930s [1]. You'd think that with modern computer software it would be possible to design better airfoils, but apparently, those shapes have already been mathematically perfected by hand and by experiment. So nowadays if you want to design a plane you can just look up the desired NACA airfoil from a table based on the speed, air pressure, etc that you require.
Eh, it's more like you can get to 90% of where you want to be with the 100 year old airfoils (though several of the other series are quite a bit newer).
https://aviation.stackexchange.com/questions/20798/are-naca-...
You'd think that with modern computer software it would be possible to design better airfoils, but apparently, those shapes have already been mathematically perfected by hand and by experiment.
No, modern computer software indeed does better, but there's not a whole lot of room to do better, small changes to bump performance a percentage point or two. These are optimizations which can be (and are sometimes) skipped for many commercial projects.
His mechanical watch internals page is also equally amazing.
It is currently the 6th most popular post on HN:
Wake up babe, new Bartosz Ciechanowski just dropped
LOL! Yeah, so the next should be at least in 2025-Q1.
This man to me is a modern day Da Vinci of the Web
Absolutely beautiful article and presentation
His work is always a good reminder the open web is still an amazing place!
What? A ciechanow.ski airfoil explainer? But I have things to do today!
I thought this was going to be about pipeline workflows... but then I saw it was Bartosz!
I know what I'll be spending a stupid amount of time reading today :)
The plane is just up in the air moving relative to the air around it, it doesn't care how the air it's in is moving relative to the ground.
A tail wind is just saying that the air is moving in a certain direction with respect to the ground (the same direction the plane is flying). The plane doesn't give a shit about that.
Yes that makes sense at a high level, but there must be a point of transition between calm air and a jet stream that makes the wings useless to the airplane for at least a few seconds.
It's relevant in practice when landing against head wind. You need to have extra speed to not stall when you enter the slower air near ground.
Do you mean landing with a tailwind? A headwind should allow the plane to create the same amount of lift it needs to avoid stalling at lower ground speeds.
Yes, that’s correct, but the headwind stops being so headwind-y near the ground, so your plane needs to go a bit faster to compensate for the loss of headwind-ness in the seconds before touchdown.
Yes, the wings are useless once the air is moving close to the speed of the plane. Thankfully, we have jet engines that help planes move a lot faster than the 100-200 knots that jet streams can reach. They'll still affect the flight but only temporarily.
These sudden changes do indeed happen in stormy weather, as adjacent layers of air can move with different velocities relative to the ground (the technical term is “wind shear”). If an airplane climbs or descends through those it will look like your speed (relative to air) is suddenly increased or decreased by some amount and you would have to compensate. It’s also a bigger problem for large, heavy airplanes as you have more work to do to accelerate for a given amount of speed loss.
Jet stream boundary is usually not this sharp, and the airplane would fly much faster than the difference anyway.
Presumably the plane would accelerate as it climbed through the velocity gradient, never falling to a point of negative air-relative airspeed.
Indeed it does, that's effectively what turbulence is.
If you took a plane flying in still air and magically, instantaneously replaced all the air around it with a tail wind equal to its velocity then yes the plane would stall and fall out of the sky.
Fortunately that kind of instantaneous change doesn't happen in real life.
Airplanes are always traveling forward relative to the wind, at some angle of attack. Tailwinds don't work by blowing against the airplane's surface and pushing it forward. Since the airplanes are themselves traveling at, say, N kt forward relative to the wind, then if they are inside a 10 kt tailwind, they'll be doing N+10 kt over the ground, if they are inside a 50 kt tailwind, they'll be doing N+50 kt over the ground. If they are inside a 25 kt headwind, the'll be doing N-25 kt over the ground.
The bottleneck when it comes to a plane going faster is drag which increases with the square of velocity relative to the air. More drag means the plane has to consume more fuel to stay at its current velocity. So if a plane normally goes 600 mph with no wind then a 100 mph tailwind will allow that plane to go 700 mph relative to the ground to experience the same amount of drag as if it were flying at 600 mph on a day with no wind.
Our intuitive experience with wind on the ground is wrong. Next time it’s windy outside imagine the entire volume of air stretching out for miles and miles moving across with the wind speed, we’re just standing at the bottom of this vast air ocean. It will blow your mind and you’ll think about wind differently from then on. So with that in mind, once the airplane is in the air, it doesn’t “know” if there’s a headwind or a tailwind at all, unless you have a way to reference the ground somehow (for example, with a GPS) - just like a boat doesn’t “know” it’s carried by a current downstream. If you are still on the ground, it is very possible that the tailwind is strong enough for you to not be able to takeoff in the available runway - but then you would go in the opposite direction or more likely sit the storm out :)
It works by reducing the amount of CSS the plane needs to carry.
Planes move through the air, and relative to the air mass.
Yes airplanes have to travel faster (in terms of ground speed) to not stall. This is why head winds are preferred for landings and take offs as it allows ground speed to be lower. But during cruise you want a tailwind to reduce the amount of drag for a given ground speed.