0803: "Airfoil"

This forum is for the individual discussion thread that goes with each new comic.

Moderators: Moderators General, Prelates, Magistrates

squareroot
Posts: 548
Joined: Tue Jan 12, 2010 1:04 am UTC
Contact:

0803: "Airfoil"

Postby squareroot » Fri Oct 08, 2010 4:01 am UTC

Image
Title text: This is a fun explanations to prepare kids for because it's common but totally wrong. Other good comments include, "why does the air have to arrive at the same time?" and ";I saw the Wright brothers plane and those wings were curved the same on the top and bottom!"

I hate this soooo much... my science teacher was insistent upon this being the correct version... :x
...the same is true of many other things he taught us.

OT: I made two xkcd threads in a row!!! WHAT NOW!
<signature content="" style="tag:html;" overused meta />
Good fucking job Will Yu, you found me - __ -

User avatar
LucasBrown
Posts: 299
Joined: Thu Apr 15, 2010 2:57 am UTC
Location: Poway, CA

0803: "Airfoil"

Postby LucasBrown » Fri Oct 08, 2010 4:01 am UTC

Image
Alt text: "This is a fun explanations to prepare kids for because it's common but totally wrong. Other good comments include 'why does the air have to arrive at the same time?' and 'I saw the Wright brothers plane and those wings were curved the same on the top and bottom!'"

"It's complicated" doesn't cut it. Not by a long shot.
And what does Santa have to do with anything?

black_hat_guy
Posts: 111
Joined: Tue Jul 20, 2010 8:34 pm UTC

Re: 0803: "Airfoil"

Postby black_hat_guy » Fri Oct 08, 2010 4:03 am UTC

I think "It's complicated... and we need to move on." is patronizing. Of course, the last one is just mean.
Billy was a chemist.
He isn't any more.
What he thought was H2O
was H2SO4.

richdun
Posts: 10
Joined: Fri Apr 02, 2010 4:39 am UTC

Re: 0803: "Airfoil"

Postby richdun » Fri Oct 08, 2010 4:08 am UTC

Other wrong answers include:

"You should know why that is. Moving on..."

"You're right. I just said that wrong to make sure you were paying attention."

"Oh yeah, smart guy? How about I toss you upside down out of this window and we figure it all out?"

ARandomDude
Posts: 60
Joined: Mon Dec 28, 2009 7:10 pm UTC

Re: 0803: "Airfoil"

Postby ARandomDude » Fri Oct 08, 2010 4:15 am UTC

Can they really fly upside down for long durations of time (and I mean commercial planes, never mind everybody getting pissed because your plane is upside down).

HonoreDB
Posts: 161
Joined: Tue Feb 06, 2007 4:32 pm UTC
Contact:

Re: 0803: "Airfoil"

Postby HonoreDB » Fri Oct 08, 2010 4:16 am UTC

"This is the way it will appear on the test."

or, more soul-crushingly:

"Actually, yes, this is completely wrong. But memorize it anyway."

X9fx1cZ
Posts: 11
Joined: Mon Aug 30, 2010 2:29 am UTC

Re: 0803: "Airfoil"

Postby X9fx1cZ » Fri Oct 08, 2010 4:16 am UTC

I've never understood how the "equal transit time" theory got started. There's really no reason other than a misunderstanding of physics to think that the streamlines would have meet up at the trailing edge of the airfoil. (The theory is flawed due to this assumption as well as the assumption that the upper surface of an airfoil is longer than the lower surface; the pressure differential causing lift due to Bernoulli's principle is more or less accurate.) Another counter to the theory is asking why an aircraft with symmetric airfoils (e.g. a NACA00xx airfoil) could fly.

Of course, you can't really explain the Euler equations/Navier Stokes at the level depicted to explain why the flow behaves as it does.

User avatar
picnic_crossfire
Posts: 193
Joined: Fri Dec 05, 2008 10:42 pm UTC
Location: I do love my ma and pa

Re: 0803: "Airfoil"

Postby picnic_crossfire » Fri Oct 08, 2010 4:21 am UTC

Can anyone here actually answer that student's question?
picnic time!

squareroot
Posts: 548
Joined: Tue Jan 12, 2010 1:04 am UTC
Contact:

Re: 0803: "Airfoil"

Postby squareroot » Fri Oct 08, 2010 4:22 am UTC

Waah! No! My thread was up first! :(

But in case it gets deleted through some terrible fate, I reiterate my comments-

My science teacher was exactly like the first "Wrong" thing. He has an incredible skill - not teaching, sadly, but rather at just making problems disappear.
It was even more painful for me because my dad was a professor of fluid mechanics - when I tried to explain to him that at sufficiently high speeds, the air drag becomes quadratic and therefore firing a bullet sideways could make it fall more slowly... he got a (what seemed to me) smirk and said, "Very interesting. I'll keep that in mind."
:x :x :x
<signature content="" style="tag:html;" overused meta />
Good fucking job Will Yu, you found me - __ -

Taymon
Posts: 30
Joined: Sun Oct 25, 2009 4:52 am UTC

Re: 0803: "Airfoil"

Postby Taymon » Fri Oct 08, 2010 4:24 am UTC

I'm not sure, but I think this might just be the best xkcd of all time.

And I'm somewhat ashamed to admit that despite having heard it explained in some form several times, I still don't really understand how airplanes fly.

DaveMcW
Posts: 35
Joined: Sat May 17, 2008 7:42 pm UTC

Re: 0803: "Airfoil"

Postby DaveMcW » Fri Oct 08, 2010 4:25 am UTC

Last edited by DaveMcW on Fri Oct 08, 2010 4:26 am UTC, edited 1 time in total.

User avatar
cs24
Posts: 29
Joined: Thu Nov 01, 2007 4:58 am UTC

Re: 0803: "Airfoil"

Postby cs24 » Fri Oct 08, 2010 4:26 am UTC

The explanation is wrong because the streamlines don't meet up after the trailing edge of the airfoil: http://www.youtube.com/watch?v=6UlsArvbTeo&t=0m17s

X9fx1cZ
Posts: 11
Joined: Mon Aug 30, 2010 2:29 am UTC

Re: 0803: "Airfoil"

Postby X9fx1cZ » Fri Oct 08, 2010 4:29 am UTC

picnic_crossfire wrote:Can anyone here actually answer that student's question?


One explanation is that while the aircraft's wings will likely produce a negative lift, the elevators can be used to pitch the nose such that level flight can be maintained by changing the angle of attack. This is something you'd see fighters and aerobatic aircraft doing. It would be a bad idea to try to fly inverted in say a 747.


-----
NASA also has a good overview of the theory shown in the comic: http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html
Last edited by X9fx1cZ on Fri Oct 08, 2010 4:31 am UTC, edited 1 time in total.

shotgun.shenanigans
Posts: 3
Joined: Wed Apr 14, 2010 5:15 pm UTC

Re: 0803: "Airfoil"

Postby shotgun.shenanigans » Fri Oct 08, 2010 4:29 am UTC

Gotta say, as a future high school physics teacher, I love this comic. In fact, I remember doing a science fair project on this very concept when I was in high school.

I think this will become a topic in my class at some point. Now to get through the rest of university.

shotgun.shenanigans
Posts: 3
Joined: Wed Apr 14, 2010 5:15 pm UTC

Re: 0803: "Airfoil"

Postby shotgun.shenanigans » Fri Oct 08, 2010 4:31 am UTC

X9fx1cZ wrote:
picnic_crossfire wrote:Can anyone here actually answer that student's question?


One explanation is that while the aircraft's wings will likely produce a negative lift, the elevators can be used to pitch the nose such that level flight can be maintained by changing the angle of attack. This is something you'd see fighters and aerobatic aircraft doing. It would be a bad idea to try to fly inverted in say a 747.


Even then, fighters and aerobatic aircraft would only be able to sustain it for short periods of time. I can't recall if that's due to stresses on the pilot or danger of the plane falling out of the sky though.

X9fx1cZ
Posts: 11
Joined: Mon Aug 30, 2010 2:29 am UTC

Re: 0803: "Airfoil"

Postby X9fx1cZ » Fri Oct 08, 2010 4:32 am UTC

There are definitely going to be stresses on the aircraft, especially along the elevators. As far as how long it can be sustained, that depends on the airfoil, wing loading, conditions, etc.

Inverted flying is mostly airshow stuff.
Last edited by X9fx1cZ on Fri Oct 08, 2010 4:41 am UTC, edited 1 time in total.

User avatar
Steve the Pocket
Posts: 705
Joined: Mon Apr 23, 2007 4:02 am UTC
Location: Going downtuuu in a Luleelurah!

Re: 0803: "Airfoil"

Postby Steve the Pocket » Fri Oct 08, 2010 4:37 am UTC

Someone needs to compile a list of things they actually still teach in schools, even just in the lower grades, that are blatantly wrong. Like that people in Columbus's day thought the world was flat (pretty sure that's what they fed me in elementary school).
Last edited by Steve the Pocket on Fri Oct 08, 2010 4:40 am UTC, edited 1 time in total.
cephalopod9 wrote:Only on Xkcd can you start a topic involving Hitler and people spend the better part of half a dozen pages arguing about the quality of Operating Systems.

Baige.

X9fx1cZ
Posts: 11
Joined: Mon Aug 30, 2010 2:29 am UTC

Re: 0803: "Airfoil"

Postby X9fx1cZ » Fri Oct 08, 2010 4:39 am UTC

It's depressing how widespread this particular theory is, even more so in that it's propagated by people who should know better.

User avatar
radtea
Posts: 137
Joined: Mon Mar 03, 2008 8:57 pm UTC
Contact:

Re: 0803: "Airfoil"

Postby radtea » Fri Oct 08, 2010 4:40 am UTC

This is a nice overview of the "controversy":

http://amasci.com/wing/airfoil.html

The problem is primarily just what is done in the mouse-over text: insisting that a particular handwaving account is "totally wrong", rather than more-or-less useful for a given purpose. As the linked article says:

Newton and Bernoulli do not contradict each other. Explanations which are based on Newton's and on Bernoulli's principles are completely compatible. Air-deflection and Newton's Laws explain 100% of the lifting force. Air velocity and Bernoulli's equation also explains 100% of the lift. For the most part they're just two different ways of simplifying a single complicated subject. Much of the controversy arises because one side or the other insists that only *THEIR* view is correct. They insist that only a *SINGLE* explanation is possible, and the opposing view is therefore wrong.


Premature or simply unjustified reification of explanatory terms has been the bane of the sciences from the word go. Our conceptual accounts of phenomena have always been far more contingent than our mathematical descriptions, and doing science--subjecting our ideas to testing by systematic observation and controlled experiment--requires maintaining the flexibility of mind to interpret the math under the most convenient conceptual framework in a given situation.

The raw Navier-Stokes equation is very tricky to interpret, so we simplify, and then we teach the simplifications (along with the added weight of the historical significance of those simplifications) and then our students reify our simplifications and pretty soon we find ourselves surrounded by engineers.

Besides, everyone knows the lift from a wing is REALLY caused by the residual vorticity created by the initial acceleration of the aerofoil!

Now that that's settled, go amuse yourself with some goofy poetry and pretty pictures: http://www.cindylooyou.com
Coming on Midsummer's Day to a Web Browser Near You: http://www.songsofalbion.com

Azkyroth
Posts: 65
Joined: Mon Jun 29, 2009 6:35 am UTC

Re: 0803: "Airfoil"

Postby Azkyroth » Fri Oct 08, 2010 4:40 am UTC

Where do people live where they discuss physics at even this rudimentary level at an age where kids still believe in Santa?

X9fx1cZ
Posts: 11
Joined: Mon Aug 30, 2010 2:29 am UTC

Re: 0803: "Airfoil"

Postby X9fx1cZ » Fri Oct 08, 2010 4:54 am UTC

Perhaps the best "basic" explanation of lift is that airfoils are "shaped" such that streamlines flow over the airfoil at a higher velocity, creating a pressure difference which due to Bernoulli produces lift.

It's hard to explain lift without delving into aerodynamics and lots of mathematics.

User avatar
xkcdfan
Posts: 140
Joined: Wed Jul 30, 2008 5:10 am UTC

Re: 0803: "Airfoil"

Postby xkcdfan » Fri Oct 08, 2010 4:54 am UTC

Azkyroth wrote:Where do people live where they discuss physics at even this rudimentary level at an age where kids still believe in Santa?

We learned the airplane lift BS when I was in first grade.

X9fx1cZ
Posts: 11
Joined: Mon Aug 30, 2010 2:29 am UTC

Re: 0803: "Airfoil"

Postby X9fx1cZ » Fri Oct 08, 2010 4:56 am UTC

I remember it being taught in grade school.

tuckels
Posts: 29
Joined: Mon Feb 23, 2009 5:43 am UTC

Re: 0803: "Airfoil"

Postby tuckels » Fri Oct 08, 2010 4:59 am UTC

My Chemistry teacher used to say "it's wrong, but it's the answer the markers are looking for" whenever someone pointed out a fallacy to him.

ndu192
Posts: 1
Joined: Fri Oct 08, 2010 4:57 am UTC

Re: 0803: "Airfoil"

Postby ndu192 » Fri Oct 08, 2010 5:01 am UTC

Airplanes can fly upside down because if you change the pitch, the angle of attack at which the wings fly at result in the same pressure difference capable of lifting the plane. If you look at fighter planes and stunt planes, they have nearly symmetrical airfoil shapes, and it's just the wings angle of attack which creates the pressure differential. Likewise, it's very hard to fly upside down in a 747 because the wings aren't designed to fly upside down. It's possible, but you would need to pitch up a lot while upside down to compensate for the inefficient airfoil. There's your answer

dragoneye1589
Posts: 39
Joined: Mon Jun 11, 2007 4:08 am UTC

Re: 0803: "Airfoil"

Postby dragoneye1589 » Fri Oct 08, 2010 5:06 am UTC

picnic_crossfire wrote:Can anyone here actually answer that student's question?


The only answer I've ever gotten that I have been satisfied with is that fluid flow is so complex (see Navier-Stokes) that lift isn't fully understood even by those with PhD's in the subject.

Bernoulli's principle explains it except for the boundary layer and viscosity (both of which are conditions that have to be satisfied for Bernoulli to be applicable at all). Newton explains it if you treat the control volume as a black box. I've even heard explanations that include the Coanda effect.

Planes can fly upside down because once you get to a certain angle of attack lift would then apply on the "top" side of the airfoil. I believe airplanes that are designed to fly upside down usually have symmetrical airfoils in order to get around the fact that cambered airfoils actually generate lift at small negative angles of attack.

Even in my 300 level fluid dynamics class, it was more or less explained as: "When flow changes direction like this, the streamlines are closer together, signifying higher speeds and lower pressures, thus there are unbalanced forces and the airplane rises."

wagner
Posts: 22
Joined: Tue Oct 06, 2009 3:18 pm UTC

Re: 0803: "Airfoil"

Postby wagner » Fri Oct 08, 2010 5:10 am UTC

shotgun.shenanigans wrote:Even then, fighters and aerobatic aircraft would only be able to sustain it for short periods of time. I can't recall if that's due to stresses on the pilot or danger of the plane falling out of the sky though.


Aircraft can fly indefinitely upside down. Depending on the camber of your airfoil, you may have to fly at a fairly large negative angle of attack to maintain sufficient lift, resulting in high drag and inefficient flight. The problem is the engine. Engines need lubrication, and nearly all engines are designed to run at 1G. Long term operation at high or low Gs will result in the oil system not functioning properly and the engine being damaged or destroyed. Airliners and commercial craft will suffer damage in short order. Fighter jets and aerobatic craft are generally capable of extended operation in such flight regimes.

X9fx1cZ
Posts: 11
Joined: Mon Aug 30, 2010 2:29 am UTC

Re: 0803: "Airfoil"

Postby X9fx1cZ » Fri Oct 08, 2010 5:21 am UTC

This is why I'm not planning on specializing in aerodynamics...

markchd
Posts: 2
Joined: Wed Nov 18, 2009 6:20 am UTC

Re: 0803: "Airfoil"

Postby markchd » Fri Oct 08, 2010 5:28 am UTC

Steve the Pocket wrote:Someone needs to compile a list of things they actually still teach in schools, even just in the lower grades, that are blatantly wrong.


http://amasci.com/miscon/miscon4.html has some pretty shocking stuff.

ghjm
Posts: 2
Joined: Fri Oct 08, 2010 4:41 am UTC

Re: 0803: "Airfoil"

Postby ghjm » Fri Oct 08, 2010 5:40 am UTC

Other objections you can make to the various hand-waving attempts at justification of this:

"The air streams have to meet because the air actually stays still, the airplane just moves through it" - "So how do airplanes work in a wind tunnel?"
"Anything involving the engine" - "So how do gliders fly?"
"Anything involving Bernoulli" - "So how do paper airplanes fly, since their wings are completely flat?"


The correct answer is: Wings create lift by moving air downwards, just like holding your hand out the car window. That's it. It's not mysterious at all. Also, just like holding your hand out the car window, you have to tilt it a bit ("positive angle of attack") or you get no lift; and if you tilt it too much ("critical angle of attack") you suddenly stop getting any lift ("stall").

Wings do not have to be any particular shape. They are the shape they are only because it gives higher efficiency. Different airplanes have different wing shapes based on the mission they expect to be flown. Most airplanes aren't expected to be flown upside down, so their wings are designed for very good efficiency right side up but poor efficiency upside down. Aerobatic airplanes are expected to have good flying characteristics while inverted, so their wings are typically symmetrical or close to it.

Flying upside down does not cause any extra stress on the airframe. Most airplanes cannot fly inverted for very long, but only because the fuel system feeds from the bottom of the tank, so the engine will quit. Also, in a non-aerobatic airplane, when you turn it upside down you will suddenly find every previous pilot's lost pencil, coke can, sunglasses etc, since they will now be raining down on your head.

And of course, flying inverted means hanging upside down from the seat belt straps. The world record is something over 4 hours. I wouldn't want to do it for 4 minutes.

User avatar
Stanistani
Posts: 73
Joined: Sat Jan 26, 2008 6:13 pm UTC
Contact:

Re: 0803: "Airfoil"

Postby Stanistani » Fri Oct 08, 2010 5:40 am UTC

squareroot wrote:OT: I made two xkcd threads in a row!!! WHAT NOW!

What now? It goes quietly away. :(

User avatar
Time Kitten
Posts: 28
Joined: Mon Jul 19, 2010 4:38 am UTC
Location: Behind you!
Contact:

Re: 0803: "Airfoil"

Postby Time Kitten » Fri Oct 08, 2010 5:47 am UTC

wagner wrote:
shotgun.shenanigans wrote:Even then, fighters and aerobatic aircraft would only be able to sustain it for short periods of time. I can't recall if that's due to stresses on the pilot or danger of the plane falling out of the sky though.


Aircraft can fly indefinitely upside down. Depending on the camber of your airfoil, you may have to fly at a fairly large negative angle of attack to maintain sufficient lift, resulting in high drag and inefficient flight. The problem is the engine. Engines need lubrication, and nearly all engines are designed to run at 1G. Long term operation at high or low Gs will result in the oil system not functioning properly and the engine being damaged or destroyed. Airliners and commercial craft will suffer damage in short order. Fighter jets and aerobatic craft are generally capable of extended operation in such flight regimes.


Or, even more simple than that; Fuel has to be on the bottom of the tank in order for the go-juice to reach the tubes - usually gravity does this. This is an issue long before any oil or lubrication is taken into account.

I have even more hatred for public schools now. No wonder my egg drop pod got more airtime then any of the class's powered planes.

User avatar
ysth
Posts: 183
Joined: Wed Apr 11, 2007 7:21 pm UTC

Re: 0803: "Airfoil"

Postby ysth » Fri Oct 08, 2010 5:51 am UTC

Stanistani wrote:
squareroot wrote:OT: I made two xkcd threads in a row!!! WHAT NOW!

What now? It goes quietly away. :(

Why is that?

Anyway, silly child, when its upside down, the air hitting the front part of the hump pushes the plane up!
A math joke: r = | |csc(θ)|+|sec(θ)| |-| |csc(θ)|-|sec(θ)| |

User avatar
jmorgan3
Posts: 710
Joined: Sat Jan 26, 2008 12:22 am UTC
Location: Pasadena, CA

Re: 0803: "Airfoil"

Postby jmorgan3 » Fri Oct 08, 2010 5:52 am UTC

dragoneye1589 wrote:The only answer I've ever gotten that I have been satisfied with is that fluid flow is so complex (see Navier-Stokes) that lift isn't fully understood even by those with PhD's in the subject.
...
Even in my 300 level fluid dynamics class, it was more or less explained as: "When flow changes direction like this, the streamlines are closer together, signifying higher speeds and lower pressures, thus there are unbalanced forces and the airplane rises."

While it's true that the exact lift on an airfoil can only be calculated from the full Navier-Stokes equation, I think there are a number of satisfying explanations that give an accurate conceptual picture of what's going on. If you have taken a 300-level fluid dynamics class, you have probably studied the lift produced by a rotating cylinder. The airfoil creates circulation in a different way than the cylinder (the sharp trailing edge), but the situation is otherwise analogous: the circulation means that there is slower, higher pressure air on the bottom and faster, lower pressure air on top.

I posted another explanation here.
This signature is Y2K compliant.
Last updated 6/29/108

Spen
Posts: 70
Joined: Sat Sep 12, 2009 8:28 pm UTC

Re: 0803: "Airfoil"

Postby Spen » Fri Oct 08, 2010 6:01 am UTC

This is the difference between my GCSE and A level physics teachers with the a-level one being the one with the right response, unfortunately school's DT department goes for the wrong approach as the level of knowledge simply isn't there so you end up reading a load of retro textbooks on our own!

User avatar
Icalasari
Posts: 107
Joined: Wed May 26, 2010 5:11 am UTC

Re: 0803: "Airfoil"

Postby Icalasari » Fri Oct 08, 2010 6:16 am UTC

Even though it likely doesn't apply, due to us covering Feyerabend last week I couldn't help but think of that damn paper

Anybody know how long it takes before philosophy papers filter out of the brain and stop affecting enjoyment of webcomics?



Anyways, I was pretty much that kid in elementary. Difference is that if I got the bad answer, I would keep demanding an answer until the teacher told me or finally broke down in tears (sadly, the latter never happened).

User avatar
arbivark
Posts: 531
Joined: Wed May 23, 2007 5:29 am UTC

Re: 0803: "Airfoil"

Postby arbivark » Fri Oct 08, 2010 6:19 am UTC

rats, airfoiled again.

Someone needs to compile a list of things they actually still teach in schools, even just in the lower grades, that are blatantly wrong. Like that people in Columbus's day thought the world was flat (pretty sure that's what they fed me in elementary school).

the one that got me: elementary school: columbus discovered america. jr high: vikings discovered america. me: wait, they're teaching us lies on purpose?
4 years ago i had to quit a job as a grader of the tests kids in my state take to graduate, because we were required to mark it wrong when the kids got a better right answer than the expected right answer.
the exact conflict had to do with a question where the expected answer was 36.4 centimeters, but the question stated "give your answer in centimeters" so the smarter kids answered 36, and we were supposed to mark it wrong.
so i quit, haven't had a regular job since.

K^2
Posts: 68
Joined: Mon Feb 18, 2008 6:33 am UTC

Re: 0803: "Airfoil"

Postby K^2 » Fri Oct 08, 2010 6:22 am UTC

ARandomDude wrote:Can they really fly upside down for long durations of time (and I mean commercial planes, never mind everybody getting pissed because your plane is upside down).

Yes, it can. The lift generated has more to do with circulation around the wing than anything else. You can use two factors to generate circulation. Venturi effect and angle of attack. The asymmetric wing will generate some circulation with zero angle of attack due to the camber of the wing. But it can still use the angle of attack to generate circulation going the other way, that is generate negative lift. Flip the plane upside down, and that negative lift becomes positive lift you need to sustain flight. It's not going to be terribly efficient, however.

Jet fighters and stunt planes will typically have symmetric or at least almost symmetric wings, allowing them to fly right side up and upside down equally well.

Here is a picture of a random lift curve I pulled off the net.

Image

The NACA # tells you the actual shape of the wing. Re is the Reynolds number, telling you relationship between the size of the wing and the airspeed at which it is going to be operating. So if I wasn't lazy, I'd be able to tell you exactly what kind of wing it is. But since I am, I'll just tell you that it is a curved airfoil, which I can tell from the lift curve itself.

On the y axis is the lift generated by the wing, on the x axis is the angle at which wing meets the air stream. Notice that at zero angle, the wing still generates lift. Notice also that at sufficiently negative angle, the wing can generate negative lift. The two blue lines mark two very important angles of attack. First gives optimal glide ratio, second gives you critical angle of attack, at which the aircraft will stall.


The main reason the explanation with air pressures is wrong is because the air stream at the skin actually has zero velocity. It's one of the standard boundary conditions in fluid dynamics. That means that if you are going on Bernoulli equation alone, you would have to assume that pressures are equal. Of course, the pressures are not equal, but computing pressures near the boundary is a complicated problem. Generally, it is far easier to compute circulation, which tells you how much air has been diverted downwards by the wing, and then using Newton's 3rd to do the rest. This is what the Kutta-Joukowski Theorem deals with.

DeathDread
Posts: 5
Joined: Mon Jan 26, 2009 7:06 am UTC

Re: 0803: "Airfoil"

Postby DeathDread » Fri Oct 08, 2010 6:35 am UTC

I refuse to believe this comic. The content contained is absolutely false.



Santa Claus is real, damn it.

ThomasS
Posts: 585
Joined: Wed Dec 12, 2007 7:46 pm UTC

Re: 0803: "Airfoil"

Postby ThomasS » Fri Oct 08, 2010 6:39 am UTC

There is a bit of truth to the explanation, even if the overall argument is false. Specifically, it is true that the air above the wing is moving faster and has a lower pressure than the air below the airfoil, and it is also true that this pressure difference is in some sense why life occurs. Well, that is all true once you get through the boundary layer.

The hard question is "why does the air move faster above the airfoil?". The theory answer does involve vorticity and to apply that theory to practice you do need big computers and still aren't sure afterwards. Or you simply run your airfoils through wind-tunnels, which tend to give reasonably close to accurate numbers. This approach isn't perfect either though because the body of the aircraft, wing length and taper and so on all have hard to predict effects.

Asymmetric airfoils happen to be more efficient, and even more importantly they will stall at lower speeds than symmetric airfoils. (when upside up) This is one reason why aerobatic aircraft are much harder to land than conventional craft. Aerodynamically, any aircraft "can" fly upside down for a sustained period of time, and at least in calm air they should hold together when inverted. However, to keep an engine running happily requires a supply of fuel, oil pressure and so on. Sustained inverted operation requires special systems to keep the fluids flowing. Aerobatic aircraft have them; conventional craft do not.


Return to “Individual XKCD Comic Threads”

Who is online

Users browsing this forum: No registered users and 107 guests