[HN Gopher] Airfoil
___________________________________________________________________
Airfoil
Author : todsacerdoti
Score : 1338 points
Date : 2024-02-27 16:32 UTC (6 hours ago)
(HTM) web link (ciechanow.ski)
(TXT) w3m dump (ciechanow.ski)
| Ancapistani wrote:
| 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 :)
| Krastan wrote:
| Wake up babe, new Bartosz Ciechanowski just dropped
| Brajeshwar wrote:
| LOL! Yeah, so the next should be at least in 2025-Q1.
| ipqk wrote:
| If you like his work, here's his patreon:
| https://www.patreon.com/ciechanowski/
| diggan wrote:
| 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...
| frfl wrote:
| 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
| flightster wrote:
| Dare I ask for the code?
| yeknoda wrote:
| It's right there. unminified and unobfuscated. just click save
| maxmcd wrote:
| Wow, I never realized this detail. What a wonderful thing.
| Tmpod wrote:
| It's a bit sad this isn't the norn for education articles
| (and mostly everything else too).
|
| Bartosz's dedication and craftsmanship is really inspiring.
| pitzips wrote:
| IIRC you can view the source and it's all custom WebGL
| available for viewing.
| nuz wrote:
| Absolutely beautiful article and presentation
| mint2 wrote:
| Why does the first slider with the cube not say what the "one
| property" the slider controls is? Viscosity? Airspeed?
| H8crilA wrote:
| From the HTML: <div class="slider_viscosity"
| id="fdm_hero_sl0">(...)
| philote wrote:
| 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".
| cryptopian wrote:
| 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.
| Etheryte wrote:
| I think this is entirely intentional. All articles by Bartosz
| build up from simple basic principles and avoiding specific
| technical terms is a good way to onboard viewers of mixed
| backgrounds without scaring anyone off. Viscosity is actually
| mentioned for the first time only roughly three quarters into
| the whole thing.
| mint2 wrote:
| I think it would benefit by being broken up into modules and
| a little less simplified, at least naming the things.
| bhasi wrote:
| His mechanical watch internals page is also equally amazing.
| RaoulP wrote:
| It is currently the 6th most popular post on HN:
|
| https://news.ycombinator.com/item?id=31261533
| globular-toast wrote:
| 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.
| pants2 wrote:
| 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.
| globular-toast wrote:
| Of course it moves its wings. That's what the runway is for.
| jahewson wrote:
| Rectilinear fan blades are shaped very similarly to aircraft
| wings. And it does only move laterally until the ailerons are
| moved away from being parallel with the ground.
| d--b wrote:
| I guess this is somewhat counter intuitive:
|
| https://www.youtube.com/watch?v=NBsvzMi9-f8
|
| So yeah, fans are puzzling too.
| convivialdingo wrote:
| If you watch it very slowly, the paper initially folds under
| the mouth and then it blows out straight.
|
| I'm guessing the initial puff creates a high pressure area on
| top of the paper, rolling it downward and back. Them after
| the puff has pushed the air away, there is now a low pressure
| zone on top of the paper which lifts it up as the air below
| is rushing upwards around the sides of the paper.
| andrewla wrote:
| 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.
| amenhotep wrote:
| 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!
| andrewla wrote:
| "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.
| BWStearns wrote:
| Ailerons were really only invented when they were (and
| named in French) because the Wrights were extremely
| litigious, they sued Curtiss for using ailerons and
| basically destroyed American aviation for a decade allowing
| the French a temporary lead. This had an interesting
| cultural effect of lots of things becoming named in French
| across aviation (including things like the weather code for
| mist being "br" for brume to this day).
| p_l wrote:
| Because the explanation in school misses something like 90%
| of the detail replacing it with zero-explanation magical
| thinking.
|
| For example, yes, the air above the wing moves _faster_ than
| the air below the wing, and it 's related to shape of the
| airfoil.
|
| However, it has nothing to do with magical "air has longer to
| travel".
|
| It starts with how combining flows at the trailing edge of
| the airflow create a vortex which induces an opposite vortex
| around the wing, which is a bit counter-intuitive (but it has
| nothing on why swept wings work, which can be summarised for
| practical aircraft design purposes of "because if we
| calculate at an angle we get better values and reality is
| crying in the corner")
| djsavvy wrote:
| Growing up I got the "air has longer to travel on the top
| of the wing than the bottom" explanation, and it always
| smelled like BS. This is the first explanation of flight
| aerodynamics that really made sense to me -- incredible
| article as always from this author.
| plopz wrote:
| The whole air has longer to travel thing is obviously hand
| waving a lot of different properties that are all combining
| to get better efficiencies. For example, don't forget the
| coanda effect and its contributions to the shape of a wing.
| Luckily we can always just return to the navier-stokes
| equations to help us out.
| tekla wrote:
| This is an incredible (over)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.
| jebarker wrote:
| I think you mean incredible oversimplification
| NovemberWhiskey wrote:
| 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.
| globular-toast wrote:
| It's just a matter of scale. What's impressive to me with the
| big aircraft is that we can organise thousands(?) of people
| to build something that big. But when it comes to the
| _principle_ of flying it 's just a bigger version of the fan.
| If you were to say they used the same amount of energy as a
| fan then _that_ would be impressive. But they don 't, they
| burn tons of fossil fuels. Geese can fly over the highest
| mountains too and all they eat is grass.
| NovemberWhiskey wrote:
| > _it 's just a bigger version of the fan_
|
| I mean, actually, it isn't - that's the whole point about
| scale variance and Reynolds number and why wings that work
| for insects are not the wings that work for jumbo jets.
| risenshinetech wrote:
| Who are these mysterious people you're interacting with who are
| concerned with (but don't understand) the physics of flight,
| but who are also not concerned with bird or insect flight?
| nuz wrote:
| Blows my mind that my computer isn't even turning on its fans
| considering how many shaders are running in this thing.
| naikrovek wrote:
| They're simple shaders and it's amazing what a computer can do
| billions of times per second.
| baobabKoodaa wrote:
| 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.
| nuz wrote:
| 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.
| baobabKoodaa wrote:
| 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.
| efnx wrote:
| I think they pause when they scroll out of view?
| fisherjeff wrote:
| What? A ciechanow.ski airfoil explainer? But I have things to do
| today!
| tandr wrote:
| It is too late for us to be saved, my friend, way too late...
| pcurve wrote:
| This man to me is a modern day Da Vinci of the Web
| anibalin wrote:
| It's truly impressive. the amount of time and dedication its
| uncanny.
| flokie wrote:
| His work is always a good reminder the open web is still an
| amazing place!
| 1970-01-01 wrote:
| 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.
|
| https://www.skytough.com/post/tailwinds-make-plane-faster
| CrazyStat wrote:
| 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.
| 1970-01-01 wrote:
| 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.
| bityard wrote:
| Indeed it does, that's effectively what turbulence is.
| CrazyStat wrote:
| 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.
| fh973 wrote:
| 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.
| rockostrich wrote:
| 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.
| Toutouxc wrote:
| 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.
| The5thElephant wrote:
| On the flip side you also get ground-effect when you are
| low to the ground where the high-pressure underneath the
| wing gets trapped against the ground creating a cushion
| of pressure increasing lift.
| bouchard wrote:
| Which can be a bit of a challenge when trying to land,
| especially for aerodynamically efficient aircraft.
| lanternfish wrote:
| Presumably the plane would accelerate as it climbed through
| the velocity gradient, never falling to a point of negative
| air-relative airspeed.
| rockostrich wrote:
| 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.
| lovecg wrote:
| 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.
| mechhacker wrote:
| It also changes a lot for sailboats, and even more for faster
| sailing craft (windsurfers, etc.).
|
| You can feel a much stronger pressure in the sail when moving
| towards the wind on a fast windsurfer/windfoil as you can do
| 15-20kts 45deg towards the wind, giving you an apparent wind
| that is 10-14kts stronger than the true wind.
|
| On the same craft, going away/downwind, you will feel the
| apparent wind at a similar angle 10-14kts less. In fact,
| because of the change in drag and forces, you'll probably be
| going faster and feel even less wind on the downwind leg.
|
| When you turn, this can be a big benefit for going downwind
| (jibing) as at some point the sail feels zero apparent wind
| (your motion cancelling out the true wind), feels very light
| in your hand, and easy to rotate to face the other way. Even
| knowing the physics of it, the timing and execution is still
| something that takes a lot of practice...especially on big
| race gear with a 9.0m2 sail.
| barbegal wrote:
| 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.
| fouronnes3 wrote:
| Planes move through the air, and _relative_ to the air mass.
| ryandrake wrote:
| 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 _head_ wind, the'll be doing N-25 kt
| over the ground.
| rockostrich wrote:
| 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.
| jahewson wrote:
| It works by reducing the amount of CSS the plane needs to
| carry.
| lovecg wrote:
| 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 :)
| klabb3 wrote:
| I've noticed this effect while diving. When you're in a
| current, you're basically the same density so you're moving
| with the water. In mid-water with poor visibility, this is
| really freaky, because you have no way of telling in what
| direction you're moving, and how fast. If you "forget" your
| orientation, you can't really recover it. Thankfully, you
| always have highly accurate depth gauge, but as for lateral
| movement, it's an eerie feeling. You could just pop up
| anywhere.
| colechristensen wrote:
| This is a kind of relativity thing.
|
| The only connection a plane has to the universe is the air
| around it. It simply does not know or care what the ground is
| doing until the ground is quite close.
|
| A small plane in a very high wind is perfectly happy having a
| "backward" ground track.
|
| Same thing as if you were trying to swim upstream in a fast
| river. How fast you move through the water doesn't have
| anything to do with how fast the water is moving across the
| land.
| BWStearns wrote:
| It's like swimming in a stream. Even if you did nothing you
| would move basically at the rate that the water is moving.
|
| Lift only comes from the interaction of the air and the wing,
| so if there's zero relative motion then you will fall out of
| the sky, regardless of if you have a 200 knot groundspeed.
|
| This also means that, if the wind at altitude is above your
| plane's stall speed, you can hover in place by flying straight
| into the wind! (example here:
| https://www.youtube.com/watch?v=n_e6ijREScE)
|
| Similarly, if you are in a packet of air that is moving at 200
| knots, the fact that you are moving at 500 knots indicated
| airspeed does not mean you are flying supersonic from an
| aerodynamic perspective, despite having a groundspeed of 700
| knots.
| danmaz74 wrote:
| Both drag and lift depend on the speed of the airplane relative
| to the wind, not relative to the ground. So, if the maximum
| efficiency dictates that the airplane should travel at speed X
| _relative to the wind_ , and the wind is flowing at speed Y in
| the same direction as the airplane needs to travel, then the
| airplane, flying at maximum efficiency, will be travelling at
| speed X+Y relative to the ground.
| porphyra wrote:
| 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.
|
| [1] https://en.wikipedia.org/wiki/NACA_airfoil
| colechristensen wrote:
| 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.
| g129774 wrote:
| "better" airfoils are used in experimental craft design. for
| example mark drela wrote and used xfoil to design wings for
| mit's project daedalus, a human powered long distance flight
| aircraft. this is the case where, like sibling commenter
| stated, you need that extra % to get better performance
| characteristics. you can still run xfoil, it's a delightfully
| oldschol fortran program.
| petsfed wrote:
| I have a friend whose PhD is in computational flow dynamics,
| as applied to airfoil design. He works almost exclusively in
| fortran (which is wild to me, for someone under 35, but I
| guess its the "industry" standard). I just asked him about
| xfoil and he observed that there are more modern programs for
| (as he put it) more "realistic/complex" designs, but said it
| was a good starting place.
| g129774 wrote:
| oh i'm sure state of the art has advanced since then. i
| have the necessary physics background, but it's not
| otherwise my domain: i once used xfoil to design an airfoil
| for an autonomous model glider as a hackerspace project
| back when i had free time for things like that many years
| ago. the glider was also loaded into x-plane to develop and
| test the autonomous part. so whenever various experimental
| aircraft projects popup, i'm likely to look into them, and
| then also notice the peculiar foils they use.
| bouchard wrote:
| Not really, they're a nice first step though, or if you require
| something "good enough".
| jayyhu wrote:
| NACA and the other published airfoils[1] are generally a good
| starting point for hobbyist/RC folks. However if you want to
| eke out that last 5% bit of performance (ie. you are a
| company/institution), you would start with one of the above
| airfoils and optimize them to fit your flight envelope &
| mission profile. Here's a neat video of optimizing a round
| profile into an airfoil optimized for supersonic speeds [2].
|
| [1] http://airfoiltools.com [2]
| https://www.youtube.com/watch?v=FHYTBguMfWc
| namirez wrote:
| Not quite true! Modern airplanes are way more complex. First of
| all, all modern airplanes have supercritical airfoils which go
| back to the 60s and 70s. Secondly, the airfoil of the wing root
| is typically different than the wing tip. Finally, new
| composite wings are adaptive during flight. They change their
| shape slightly to maximize efficiency.
| H8crilA wrote:
| Case in point would be modern gliders (sailplanes). One
| simple parameter that describes their aerodynamic performance
| is the maximum achievable Lift/Drag ratio, and that
| dimension-less ratio has climbed from ~30 in the 1960s to as
| high as 75 today. That means modern gliders can, using the
| same altitude/energy, go over 2 times further horizontally.
| The L/D is not the ultimate decider of performance but it is
| quite representative of the aerodynamic performance
| improvements.
|
| BTW, all lift based flying objects have an L/D ratio (which
| depends mainly on the airspeed), this includes birds, fighter
| jets, commercial airliners; and the discrepancies can be
| pretty interesting. For example if one looks at the L/D of
| the Concorde vs a subsonic jet it becomes clear why it was so
| damn expensive to operate. Or why the U-2 looks like a glider
| :). I cannot find any aerodynamic performance data on any
| famous long endurance (>24h) unmanned drone, but I bet it's
| rather high as well.
| dmoy wrote:
| > Concorde
|
| Another good example is the space shuttle. It does actually
| glide back down. But it glides like a brick at first (1:1
| during its initial braking into the atmosphere), and then
| like a less dense brick (2:1 while it's still supersonic),
| and then like a brick with shitty wings (a whopping 4:1 or
| whatever on final approach). Which is about what the
| Concorde is during landing, 4:1, yea.
|
| Pretty crazy stuff
|
| (Obviously the space shuttle was a tradeoff for, you know,
| getting it into orbit via rocket)
| CPLX wrote:
| The big variation now though is that the airfoil shape varies
| quite a bit from one end of the wing to the other.
| jameshart wrote:
| NACA airfoils aren't so much a numbered set of standard, tested
| designs as a useful set of mathematical curve formulae for
| making airfoil-like shapes, and describing them using
| parameters.
|
| NACA published empirically determined wind tunnel performance
| numbers for selected parameters, which was useful research but
| not a declaration of 'these are the good values, you should
| only use these'.
|
| It's a bit like saying all satellites follow TLE orbits derived
| by NASA/NORAD in the 1950s - they do, but only because that's
| just a standard way of writing down the orbital elements that
| describe a particular ellipse, not a catalog of 'known good'
| orbits.
| atlas_hugged wrote:
| I swear Bartosz' posts deserve to be pinned to the top of hacker
| news on release day at this point.
| civil_engineer wrote:
| The wings of an airplane in level flight direct air downward with
| a force equal to the airplane's weight. If one were to build a
| large scale on the ground, as an airplane flies over it, the
| scale would register the weight of the airplane. The wings act
| like a scoop forcing air downward behind the wing. At least
| that's the way I think about it when I'm out flying around in my
| Cessna.
| WanderPanda wrote:
| That's my mental model as well. The incompressible fluid-based
| explanations never made much sense to me
| ivanjermakov wrote:
| Although it is a nice mental model, that's not quite true.
|
| > The wings act like a scoop forcing air downward behind the
| wing
|
| Only bottom side of the wing acts as a scoop, creating positive
| pressure. Upper side, in opposite, creates negative pressure
| which "sucks" the plane into it, creating additional lift.
|
| It surprised me how much lift is coming from the negative
| pressure - about a half:
| https://aviation.stackexchange.com/a/16202
| danmaz74 wrote:
| Actually, it _is_ quite true. Gravity is exercising on the
| airplane a force F equal to the weight of the plane, towards
| the ground. For the airplane to stay at the same height, air
| needs to exercise a force that is equal and opposite to that
| of gravity. For an airplane buoyancy is negligible, so the
| force comes from accelerating enough air towards the ground
| so that F = M*A when M is the mass of air being accelerated,
| and A the (average) acceleration.
|
| Notice that this isn't a separate effect from the effect of
| pressure - it's just a different way of seeing the same
| effect. The wing is accelerating the air both upwards and
| downwards, but because the pressure is higher below the wing
| than it is above it, more air is accelerated down than it is
| accelerated up - which lifts the airplane, but makes the air
| go down.
| jameshart wrote:
| Except that negative pressure is not a thing. Air molecules
| are not grabbing the wings and pulling them up - they are
| just not pushing down on the top as much as the ones
| underneath are pushing upwards.
| bloppe wrote:
| Ya, I was hoping for more nuance related to this. I'm sure the
| air foils generate lift, but atmospheric pressure at cruising
| altitude is ~4psi, and the pressure differential across the
| foil must be only a tiny fraction of that. According to my
| understanding of Bernoulli's principle, you'd have to quadruple
| the speed to cut the pressure in half, and I can't imagine the
| top air traveling _that_ much faster than the bottom air.
|
| Yet a 747 can produce 850000 pounds of lift with only 729000
| square inches of wing? Feels like a very incomplete description
| at best
| p_l wrote:
| The airfoil shape causes formation of vortex around the wing,
| which ridiculously changes the relative speeds and pressures
| involved. At low pressure you compensate with speed, which is
| squared in lift equation.
| p_l wrote:
| ... I'm honestly surprised it's possible to get PPL(A) without
| learning about wing vortices responsible for lift generation.
|
| In order to use "scoop" approach for lift, you need to have
| either _very low_ wing loading (think paper airplanes) or very
| high speeds (above transsonic range).
| Rapzid wrote:
| > If one were to build a large scale on the ground, as an
| airplane flies over it, the scale would register the weight of
| the airplane
|
| No, it wouldn't.
|
| I think the article does a pretty good job building a more
| complete understanding than the simplistic "deflection" mental
| model.
| diimdeep wrote:
| This is the future of education. Very approachable and seems to
| target the most common denominator of knowledgeable people that
| out there.
|
| I wonder will there be articles in the future with more math and
| code snippets?
| elwell wrote:
| The future of education is as an entertainment. There will be
| no need to educate oneself in the AI future (except for merely
| egotistical reasons?).
| cloogshicer wrote:
| Imagine a world in which _all_ education was this level of
| quality.
|
| Imagine how much more you'd know, be able to do and understand.
|
| I really wish good education was valued more highly in society.
| pomian wrote:
| Yes, that. Call it enhanced learning? For instance, add Dan
| Carlin - Hardcore History podcasts for your history lectures.
| If everyone listened to those podcasts, then all you would need
| is a good teacher/professor to discuss what you learned - and
| there is 'so much' learned from any one of his episodes.
| justinzollars wrote:
| I'm taking a sailing class, and learned sails in addition to
| keels utilize this concept.
| colechristensen wrote:
| For anyone really interested, this is the authoritative reference
| for NACA, etc. airfoils.
|
| _Theory of Wing Sections_ by Abbott and von Doenhoff (1959)
|
| https://www.amazon.com/Theory-Wing-Sections-Aeronautical-Eng...
| simple10 wrote:
| The javascript code is not minified and is easy to follow.
| Beautifully done.
|
| https://ciechanow.ski/js/base.js
|
| https://ciechanow.ski/js/airfoil.js
| 0xbadcafebee wrote:
| As part of building my own truck-top camper, I got into
| researching aerodynamics of vehicles in order to try to reduce
| loss of fuel efficiency. The most interesting ideas I found were
| that aerodynamics don't matter much on most vehicles until they
| pick up significant speed.
|
| Most automobiles are pretty heavy, so the engine has to do
| significant work just to get it to move. At a certain point, the
| vehicle can change gears to get the engine to do less work and
| use less fuel. But around the same time, the force of the air is
| increasing. By the time an automobile goes over about 50mph, the
| air forces are getting increasingly strong, and the engine has to
| work harder to keep the vehicle moving. At this point, beginning
| to lower the air's coefficient of drag on the vehicle will lessen
| the work the engine needs to do to keep the car moving at speed.
| So you can optimize the design of the vehicle's exterior to
| reduce the drag coefficient, which will reduce things like flow
| separation and turbulence, creating fewer rear pressure zones and
| causing less drag.
|
| So you might wonder, why aren't more cars teardrop-shaped like
| the airfoil? The answer is, it depends. Most people want
| something that looks good more than they want efficient operation
| at speed. But sometimes having more drag actually helps. For
| example, the Lotus Elise: while it is smaller and looks more
| sleek than a Tesla Model 3, it actually has a much worse drag
| coefficient than a Tesla Model 3. The Lotus has way more force
| acting against it at speed than the Tesla. But the Lotus is a
| sports car, and sports cars benefit greatly from increased
| traction, and you can get more of that traction by increasing the
| downforce on the car. So the Lotus's design sacrifices top-speed
| drag coefficiency in order to gain some downforce which helps
| traction when cornering at speed.
|
| What about pickup trucks? Even though modern pickups actually
| have lots of subtle design changes to improve drag coefficient,
| they all tend to have open beds, which is _terrible_ for drag. It
| creates this giant messy turbulent pressure area in the bed which
| drags on the tailgate and the rest of the car. By adding a truck
| topper, the drag is significantly reduced, but you don 't see
| most trucks driving around with a topper on. But trucks naturally
| have worse gas mileage, so nobody really thinks twice about the
| aerodynamics.
|
| (To be fair, the air's impact on gas mileage is minimal unless
| you're going quite fast. But for trucks with extremely bad gas
| mileage, like 18-wheelers, it makes much more difference. That's
| why they often have airfoils on the front of the truck, gaps
| between cab and container closed, and skirts to reduce drag from
| the undercarriage. Strangely though, the biggest improvement to
| reduce drag coefficient actually comes from modern European big-
| rigs whose containers are actually tapered like a teardrop. The
| rear of the vehicle's shape makes the most difference to how
| severe flow separation is, and thus how big of a pressure area
| develops, pulling on the rear of the vehicle. If we wanted to
| make trucking more fuel efficient globally we'd change the shape
| of the containers to be more like teardrops, but that would make
| handling and shipping them much more awkward)
|
| You'll usually only see these effects on automobiles at higher
| speeds, due to the vehicle needing to overcome gravity before the
| air forces build up. Lighter vehicles (say, bicycles) with less
| impact on them from gravity will be impacted earlier (at lower
| speeds) by the force of the air, so optimizing drag coefficient
| is much more important, which is why bicycle racers have to put
| so much into aerodynamics at significantly lower speeds than an
| automobile. Interestingly, the drag coefficient on a bicycle and
| rider is actually equivalent to that of a small car.
| onetimeuse92304 wrote:
| I wish every presentation on how planes fly started with an
| actual flat plane. A wing that has a flat crossection. I think
| the shape of the airfoil of the wing is absolutely distracting
| and prevents people from understanding what is really happening.
|
| Every person who ever stuck a flat object outside the window of a
| moving car knows that you do not need a fancy shape to have lift.
|
| And so many people are stuck thinking that the shape of the
| airfoil is responsible for the plane to be able to fly,
| supposedly because the air needs to run a longer way around the
| foil above the wing than below the wing. And this somehow causes
| pressure difference due to Bernoulli law and this is what keeps
| the plane up. Which is almost total BS because planes can
| obviously fly inverted.
|
| Now I admit I only skimmed the article, and although the
| animations are beautiful, I am missing what really is key to
| understanding of what is happening.
|
| I am looking for a bigger, far away view of the wing and showing
| what happens to the air BEHIND the wing.
|
| Because how the plane really works is as it flies forward, it
| diverts large masses of air downwards. It pushes off of air.
|
| Part of the air is diverted by the lower portion of the wing, but
| the much larger portion of lift is generated by larger masses of
| air above and behind the wing. Those can be thought as being
| sucked down behind the wing (if you look at it from the point of
| view of a stationary air mass, not from the point of view of the
| wing).
|
| And the main role of the airfoil is to keep that mass of air
| behind the wing stuck to the airfoil at wide range of angles and
| speeds as possible, because a flat sheet is very poor at doing
| this.
| p_l wrote:
| Unfortunately, your explanation is entirely wrong... and you're
| attacking a "lies to children" simplification with your mention
| of "needs to run longer way around" bit.
| avn2109 wrote:
| Well in defense of the GP, the "planes can observably fly
| upside down" point (and its close cousin the "flat wing cross
| sections can fly too" point) is a good one, this pokes holes
| in the usual two-dimensional "the air goes faster on top"
| themed explanation that omits any discussion of vortex
| shedding/third-dimensional effects.
| p_l wrote:
| Oh, to be quite honest, I loved trolling my high school
| teachers with "your explanation fails, here is a real world
| airfoil, please explain it" and I would draw a symmetrical
| airfoil or - for extra trolling - a trapezoid one. (At that
| point I had already flown solo)
|
| But the same I found myself unable to pass by someone
| pushing "flat plane at an angle".
| dameyawn wrote:
| Yea, I agree and try to explain it this way to friends.
| Airfoils help, but it's ultimately just the wing pushing air
| down and why planes can fly upside down.
|
| FWIW, aerospace engineering degree, used xFoil, did tons of
| fluid sims, etc.
| p_l wrote:
| And it's an "even more wrong" explanation than the "lies for
| children" diagram used in school physics class.
|
| For reference, actual "proper" discussion of lift in
| textbooks on aerodynamics have tendency to start with a
| sphere/cylinder.
| Xirgil wrote:
| Do you have any recommended reading on this topic? I'd like
| to brush up.
| p_l wrote:
| There used to be a good one from NASA, written for K-12
| but 100% adhering to actual science not "lies for
| children".
|
| EDIT: This is a good starting point for the frankly
| awesome material from NASA Glenn Research Centre: https:/
| /www.grc.nasa.gov/WWW/K-12/VirtualAero/BottleRocket/a...
|
| Unfortunately it partly bitrotted due to using java
| applets for interactive demos, but I think most of it is
| still reachable - I'll try to find it later when I'm at
| the desk.
|
| Personally I learnt from a 1980 book that was still part
| of mandatory reading for glider pilot course in Poland in
| 2005.
| danmaz74 wrote:
| I once found an explanation that finally made it clear to me
| why the shape of the airfoil can create lift. Yes, the air
| above the wing needs to travel a longer distance with the
| typical section used in wings, which means that it goes faster
| than the air below the wing. It also leaves the wing moving
| downwards - and when this downward-moving, faster flux of air
| meets the slower one from below, the result is that a mass of
| air is pushed downwards - exactly as needed to lift the plane,
| as you correctly said.
|
| As the article says, you can have lift by just changing the
| inclination of a symmetrical airfoil, but an asymmetrical one
| can generate lift even without inclination (and with lower
| drag). The article also explains that acrobatic airplanes have
| symmetrical wing sections exactly because they need to be able
| to fly just as easily inverted.
| lovecg wrote:
| To me the most intuitive and practical mental image is
| imagining two large bubbles of lower pressure above the wing
| that hold the wing up by suction (you can see those literally
| as condensation under certain conditions). As you increase the
| angle of attack the bubbles get larger and stronger, until the
| angle is so large that they "break off" and the wing stalls.
| ubj wrote:
| Obligatory XKCD comic (also with the name "Airfoil"):
|
| https://xkcd.com/803/
| CrimsonCape wrote:
| I grew up duck hunting and learned intimately how ducks use their
| wings and the variations of shapes at different velocities as
| they slow down to land on the water. I also grew up boating and
| swimming and have a likewise similar understanding of paddling,
| tracking a canoe straight, and using boat motor trim to "get on
| the plane".
|
| I guess I struggle with articles like this because it's already
| so intuitive as a mix of air and fluid dynamics. In fact, fixed
| airfoils are so boring when you see what a duck can do.
|
| https://www.youtube.com/watch?v=-3CVZYY8xS4
|
| So for all the fancy physics talk, this duck is literally just
| paddling air with his wings. The same physiology I use to stay
| afloat when treading water while swimming.
| mondrian wrote:
| Fixed airfoil physics become really important at very high
| speeds.
| jameshart wrote:
| Or if your Cessna isn't equipped with a flapping system.
| solardev wrote:
| But when that fancy duck wants to get to Paris in a hurry, it
| still has to hop on a fixed-wing Concorde like everyone else.
| kjkjadksj wrote:
| Give it a strong selective pressure towards speed and a few
| million years and you will have your supersonic duck.
| LargoLasskhyfv wrote:
| Reverse dragooning by expelling fiery farts made of
| hypergolics fueled by fantastic fermentation?
| solardev wrote:
| Intercontinental ballistic mallards? Just another trillion-
| dollar boondoggle from the military-ornithological complex.
| aredox wrote:
| For an example of a flat-ish airfoil that performs well enough
| for model airplanes (and is easier to build than a NACA & co
| airfoil), see the KFm airfoil family:
|
| https://en.wikipedia.org/wiki/Kline%E2%80%93Fogleman_airfoil
|
| Very useful when making model airplanes out of foamboard.
| kqr wrote:
| Huh, odd. I was under the impression that for swept/delta
| winged paper airplanes one wants a smooth top surface to
| encourage attachment and any steps on the bottom to provide
| decalage. (I.e. the area ahead of the step acts as a canard-
| like surface.)
|
| Is this an airfoil that works for tailed aircraft but not
| tailless ones, perhaps?
|
| Edit: I just skimmed the book on paper planes by KF and indeed
| they are using the variation with the step on the bottom for
| their paper planes.
|
| I'm actually even more surprised now. How on Earth did they
| manage to patent the idea of reflex on a delta wing to give a
| tailless plane stability? This seems like the thing that (a)
| was known since early human-carrying gliders, and (b)
| implicitly discovered by anyone that folds a lot of paper
| airplanes. I will definitely read their book in more detail.
| kqr wrote:
| I agree with the other commenter that the specific shape of the
| cross section of the wing is overemphasised in almost all
| material including this one. Any shape longer than it's thick
| will, at a reasonable angle of attack, provide lift.
|
| This article did provide a barn door model also, but it was quite
| far down.
|
| The shape is mainly about efficiency and increasing the range of
| reasonable angles of attack, and then further nuances.
| tjkrusinski wrote:
| It's amazing how the article did such an incredible job
| building a deep understanding of how the airfoil works, yet you
| managed to completely miss that and find something to so small
| to critique.
| MattRix wrote:
| I think many of these kinds of comments are driven by a form
| of insecurity. They subconsciously wish they had written the
| article and are envious of the attention the author is
| receiving... so they find whatever small nitpick they can in
| order to tear it down.
| milliams wrote:
| I thought they made it very clear and talked at length that the
| shape isn't the key important factor. They do also then go on
| to talk about the benefits of different shapes and why they are
| chosen.
| ajkjk wrote:
| Pretty impressive. I was curious how they made the whole thing so
| I went to look at the source for the images. It's mostly in one
| 10000 line JS file which draws all the graphics onto <canvas> in
| JS, plus a bunch of WebGL that goes over my head. The code looks
| like function draw_car(ctx, rot) {
| ctx.save(); let sc = 0.04; ctx.scale(sc, -sc);
| ctx.lineWidth \*= 1 / sc; ctx.translate(-286, -51);
| ctx.beginPath(); ctx.moveTo(463.93652, 9.89137);
| ctx.bezierCurveTo(462.12793, 6.72347, 461.22363, 3.5344,
| 461.22363, 0.32417); ctx.bezierCurveTo(447.58911,
| -1.16177, 434.20691, 2.81333, 434.85754, 5.10777); ...
|
| I wonder what their workflow was. Surely all those curves weren't
| programmed by hand?
| plopz wrote:
| Just a small note, that code looks like Context2d rather than
| WebGL unless you were looking at something else in the code
| dubcanada wrote:
| Not sure what you are looking at but
| https://ciechanow.ski/js/airfoil.js is the JS that contains the
| code for the graphics/visuals. And it is completely readable.
|
| The 2d part though is probably generated.
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