0803: "Airfoil"

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Re: 0803: "Airfoil"

Postby Eternal Density » Fri Oct 08, 2010 6:51 am UTC

markchd wrote:
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.
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Re: 0803: "Airfoil"

Postby K^2 » Fri Oct 08, 2010 6:55 am UTC

ThomasS wrote: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.

Pressure difference is 100% of the reason why lift occurs. That's what pushes directly on the wing. It's why the pressure is lower above that's the tricky part, and the explanation "because air moves faster" doesn't hold water, even though the two are ultimately related.

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The simple explanation is almost correct

Postby LaelOdhner » Fri Oct 08, 2010 6:57 am UTC

To play the devil's advocate, I'd like to point out that the simple/wrong explanation isn't too far off. The assumption that the streamlines on a wing "want to meet at the back" is pretty much how lift was calculated on early airfoils. This is known as the Kutta condition. Wikipedia has a good explanation; I'll provide a bad one here.

The Kutta condition is the assumption that the streamlines moving over a body with a sharp corner at the back will always meet at that trailing corner. This brazen assertion has the virtue of being correct often enough to be useful. It is certainly valid for thin plates or wing sections that are tilted to only a few degrees. If you compute the potential flow over the wing (that is, the velocity field of an inviscid, incompressible fluid) while assuming that the Kutta condition is valid, then the pressure profile of the wing calculated using Bernoulli's principle will predict the net lift with surprising accuracy. This idea is commonly used when applying the Kutta-Joukowski theorem that I see some commenters mentioning (the relationship between circulation and lift).

People commonly mistake the Kutta condition to mean that the flow over the top of the wing has to "hurry up" in order to meet the flow over the bottom. That's not how it works; if an air molecule and his little buddy are split apart by the leading edge of the wing, the will not meet each other on the trailing edge. One will generally come out ahead of the other, though the paths they trace will intersect downstream. Nevertheless, it is easy to see how the myth was started.

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Re: 0803: "Airfoil"

Postby wbeaty » Fri Oct 08, 2010 6:58 am UTC

X9fx1cZ wrote:It's hard to explain lift without delving into aerodynamics and lots of mathematics.


Nah, math is a tool for high rigor and numerical results.

To explain things (or even understand them,) you first have to grasp the pure concepts. Example: if you can run a SPICE program, does it mean that you understand circuit physics? No. What if you memorize the systems of equations that model each component, then do you understand circuit physics? Nope, you've just learned to run the simulation using wetware. A system of equations does not confer understanding.

OK, Airfoils:

A thin flat plate, if tilted and moved horizontally through the air, will produce lift. Why? Perhaps it would help to know that whenever a thin tilted plate is moved horizontally through thick syrup, it produces zero lift. In syrup, the flow pattern around the airfoil is very different than the pattern in air. Something about the viscous syrup will cause the lifting force to vanish.

It also helps to know that airfoils work just fine when submerged in incompressible water ...so the lifting force has nothing to do with compressibility and changes in air density.

It turns out that the lifting force only appears when flowing air (or water) is able to follow the top and bottom of the flat plate ...and also it must depart from the tilted trailing edge and keep on flowing down. When this occurs, the air above the tilted plate will speed up, and the air below the tilted plate will slow down. The flow-pattern near the leading edge is very different than the pattern near the trailing edge: near the leading edge it resembles the flow being sucked into a pipe. But at the trailing edge, the flow resembles an air jet which is directed diagonally downwards. In other words, inertia of air is crucial to the production of lift ...and if the air was viscous enough that inertia effects became insignificant, then lift could not exist. To produce lift, a wing has to "throw air," and the air must keep moving in the "thrown" direction after the wing has passed by. Yet at the same time, the speed of the air above the flat plate is much faster than the speed below, and this produces a pressure-difference that drives the airfoil upwards. Deflected air causes pressure difference, which creates a reaction force: "lift."

(ASIDE: at the scale of bacteria, lift doesn't exist. At the scale of bacteria, water behaves more like thick tar. Friction dominates, and fluid jets become impossible: the pattern of "sucking" looks just like the pattern of "blowing." A spinning bacterial propeller doesn't throw off any jet of moving fluid, instead the fluid just flows in a closed circular path around the propellor, and therefore it experiences zero reaction force. Instead, bacterial motors behave as "corkscrews" which drive bacteria along in the same way that a screw can push into or out of wood.)

Next:

A thin flat plate, if it is bowed up in the center( "cambered,") will still produce lift whenever it's moved sideways. But it's not tilted. It's just bowed upwards. This does make some sense, since because air has inertia, the trailing edge of the curved plate determines how air departs. It tilts downward, so air moves diagonally downward off the trailing edge. The lifting force appears because the trailing edge is tilted downwards and producing the same phenomenon as the tilted flat plate. But ...why do airplanes use cambered airfoils? Ah, that's the next stage upwards in understanding. It's because of some important "nonlinear" or "2nd-order" effects: turbulence and flow-detachment. If all wings were tilted flat plates, then the air flow would "detach" at the leading edge, and it would refuse to follow the upper surface. The lower surface would still push air downwards, but the upper surface would not. Whenever this flow-detachment occurs, the problem is called "wing stall." This problem can be cured by bending the leading edge in a downward curve, so it meets oncoming air head on. The rest of the airfoil is then shaped in a smooth curve to avoid flow-detachment in any other spots.

So one solution to explaining airfoils is to ignore the need for streamlined cambered airfoils and "stall." Instead explain only the tilted flat airfoil.
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Re: 0803: "Airfoil"

Postby dwasifar » Fri Oct 08, 2010 7:07 am UTC

ThomasS wrote: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.


Great, now we'll have the creationists joining the debate.

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Re: 0803: "Airfoil"

Postby wbeaty » Fri Oct 08, 2010 7:08 am UTC

X9fx1cZ wrote: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.


Dr. Klaus Weltner discovered the probable source: Prandtl got it wrong in 1921. Those lines I II III IV shown below are incorrect. In fact they only appear as depicted whenever the airfoil is greatly tilted down and adjusted to give zero lift. (Yep, the two flows only meet at the trailing edge if the lifting force is exactly zero!)

Image
Prandtl, L.: Applications of modern Hydrodynamics to Aeronautics, in: NACA Report, 1921, 116, pp. 161-182

(That's from Weltner's 2nd paper listed at http://user.uni-frankfurt.de/~weltner/)
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Re: 0803: "Airfoil"

Postby imantodes » Fri Oct 08, 2010 7:11 am UTC

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


There are a bunch of complicated answers, but here's a really simple one: lift doesn't even matter if you have enough thrust.

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Re: 0803: "Airfoil"

Postby Sundew » Fri Oct 08, 2010 7:14 am UTC

As my physiology professor once put it, this is the general key to answering a student's question:

1) "That's a great question!"
- The professor is 100% sure he or she knows the answer, was waiting for a student to ask that very question (sometimes by going as far as omitting certain points), and is going to spend the next five minutes talking about the answer and how it relates to either the subject at hand or his or her research.

2) "That's a good question."
- The professor has a generally good idea of the answer, but he isn't completely certain. Often also used when the professor knows the answer but can't bring it to mind at that very moment, or when the professor can make a generally educated guess and come up with reasonable speculations. Sometimes followed up with "I'll get back to you on that."

3) "That's an interesting question."
- The professor has no idea how to answer the question and is about to feed you a pile of bull****.

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Re: 0803: "Airfoil"

Postby wbeaty » Fri Oct 08, 2010 7:19 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.


VERY TRUE! Remember that the lifting force created by flapping insect wings wasn't understood until recent years, after a team at Cambridge built a large robotic scale model of moth wings, then observed the surrounding 3D flow submerged in water filled with tiny bubbles:

http://www.google.com/search?q="Charles+Ellington"+moth
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Re: 0803: "Airfoil"

Postby Marlayna » Fri Oct 08, 2010 7:27 am UTC

arbivark wrote: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.


Of course you were supposed to mark it wrong. It was wrong.
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Re: 0803: "Airfoil"

Postby Algrokoz » Fri Oct 08, 2010 7:31 am UTC

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

Yes, it's quite easy actually. A lot of the lift from a planes wings comes from the fact that they are tilted downwards, i.e. like the blade of a ceiling fan or your hand when you stick it out your car window. If your engine is strong enough it can provide enough lift from this alone, with no pressure differentials. This means even if the plane is upside down as long as the pilot keeps the right angle between his wings and the ground he will stay in the air.

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Re: 0803: "Airfoil"

Postby wbeaty » Fri Oct 08, 2010 7:36 am UTC

And then it gets WORSE:

It's impossible to understand airplanes by only employing 2D flow diagrams. These diagrams depict an odd sort of "flatland flight" where no work is performed on the air, and an airfoil could ideally coast along forever.

In the real world, this type of flight is called "infinite wing" flight. It's also the same as "ground effect" flight, where the aircraft altitude above ground is << than one wingspan. During ground-effect flight there is a venturi effect, and an instant reaction-pair exists between the wing and the ground. In 2D airflow diagrams, there are instant reaction forces between the airfoil and the floor/ceiling. (These forces remain the same even if the floor/ceiling are pulled away to infinity.) In other words, 2D diagrams depict a type of venturi effect.

To understand where lift comes from, you have to move into the 3D world where there is no venturi-force between the wing and the Earth. All is not lost though. We soon discover that wings are very similar to smoke-ring launchers. Wings produce lift in the same way that an "Airzooka" experiences a kick of reaction force each time it launches a vortex-ring. A mass of air is entrained within the "separatrix" surface surrounding the vortex ring. (And if you could throw a water-filled sphere while immersed underwater, you'd experience a reaction force!) Wings aren't impossible to grasp. It's just that airplanes launch a downward-moving vortex ring continually. This takes the form of a vortex-pair. The vortex-pair entrains a mass of downward-moving air. There are many photos of this phenomenon, since the clear air in the vortex-pair becomes visible when it moves down into a layer of fog...

http://www.google.com/images?q=downwash
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Re: 0803: "Airfoil"

Postby melladh » Fri Oct 08, 2010 7:52 am UTC

I had a teacher in jr high who used to complain that I didn't ask enough questions (just to show interest - my test scores were fine), but whenever I asked a question, he wouldn't know the answer, or dismiss it entirely. He was still pleased that I asked the questions though, but I can't for the life of me figure out what this exercise in futility was supposed to be good for, as we must both very early on have understood the pattern there.
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Re: 0803: "Airfoil"

Postby jmorgan3 » Fri Oct 08, 2010 7:59 am UTC

wbeaty wrote:And then it gets WORSE:

It's impossible to understand airplanes by only employing 2D flow diagrams. These diagrams depict an odd sort of "flatland flight" where no work is performed on the air, and an airfoil could ideally coast along forever.

While it's true that most 2D flow diagrams you see are inviscid simulations, they still give a good conceptual picture of how lift is produced. In fact, the wingtip vortices you're so excited about are described by inviscid potential flows. Also, there is nothing preventing one from making a 2D viscous flow diagram to illustrate the true flow around a wing, it's just that 2D simulations are much easier to make.


wbeaty wrote:In the real world, this type of flight is called "infinite wing" flight. It's also the same as "ground effect" flight, where the aircraft altitude above ground is << than one wingspan. During ground-effect flight there is a venturi effect, and an instant reaction-pair exists between the wing and the ground. In 2D airflow diagrams, there are instant reaction forces between the airfoil and the floor/ceiling. (These forces remain the same even if the floor/ceiling are pulled away to infinity.) In other words, 2D diagrams depict a type of venturi effect.

I am not entirely sure what you're saying in this paragraph. Most 2D flow diagrams of airfoils have freestream boundary conditions set an infinite distance from the airfoil. That is quite a different situation than a wall boundary condition a finite distance from the wing.

wbeaty wrote:To understand where lift comes from, you have to move into the 3D world where there is no venturi-force between the wing and the Earth. All is not lost though. We soon discover that wings are very similar to smoke-ring launchers. Wings produce lift in the same way that an "Airzooka" experiences a kick of reaction force each time it launches a vortex-ring. A mass of air is entrained within the "separatrix" surface surrounding the vortex ring. (And if you could throw a water-filled sphere while immersed underwater, you'd experience a reaction force!) Wings aren't impossible to grasp. It's just that airplanes launch a downward-moving vortex ring continually. This takes the form of a vortex-pair. The vortex-pair entrains a mass of downward-moving air. There are many photos of this phenomenon, since the clear air in the vortex-pair becomes visible when it moves down into a layer of fog...

http://www.google.com/images?q=downwash

Lift does not require 3D effects. In fact, 3D effects reduce the lift of a wing. What do you mean by vortex-pair? Do you mean the starting vortex and the bound vortex, or do you mean the two wigtip vortex lines? If it's the former pair, that phenomenon also occurs in 2D. If it's the latter, they don't create lift, the bound vortex creates lift.
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Re: 0803: "Airfoil"

Postby tussock » Fri Oct 08, 2010 8:04 am UTC

The lift generated has more to do with circulation around the wing than anything else.


Well, sort of, if that's how you model it, for subsonic flight. Still, the "lift" is primarily an opposed force, matching the force the airfoil puts on the airmass it passes through, countering the gravitational acceleration of the aircraft. So rather than the aircraft falling, the region of air that the wings pass through is driven downward, conserving the momentum you'd expect of the aircraft in freefall.

Because no matter how fancy your wing, you really do need to conserve momentum, and you absolutely are being accelerated by gravity (relative to the reference frame of the ground beneath and air ahead). All those other equations just show you how big the cross section of air is you can push on, and how hard you'll expect to be pushing, roughly, assuming all sorts of not-exactly-but-nearly-true things.

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Re: 0803: "Airfoil"

Postby Felstaff » Fri Oct 08, 2010 8:16 am UTC

So if I put an aeroplane upside down on a treadmill...
Away, you scullion! you rampallion! You fustilarian! I'll tickle your catastrophe.

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Re: 0803: "Airfoil"

Postby littlelj » Fri Oct 08, 2010 8:21 am UTC

Marlayna wrote:
arbivark wrote: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.


Of course you were supposed to mark it wrong. It was wrong.

I'd have to agree - if the rubric had stated "give your answer in whole centimetres" then 36 was right, but otherwise not, since m/cm/km etc are continuous, not discrete.

Re: planes staying in the air, I agree with Billy Connolly. God did it for a laugh once (to piss off a Wright brother) and now spends all his time keeping them aloft. That's why He is too busy to keep an eye on anything else.
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Re: 0803: "Airfoil"

Postby glow-in-the-dark » Fri Oct 08, 2010 8:26 am UTC

There is an unintentional extra in this cartoon. Teaching something to someone else "elevates" your own knowledge too - at least, that's my experience with coaching and teaching people. There is no better improvement to your own understanding that someone else not quite following you or (even better) challenging you.

So the first pane with wing and title above has to me an extra meaning. I will charitably assume that's intentional :mrgreen:

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Re: 0803: "Airfoil"

Postby Hawknc » Fri Oct 08, 2010 8:36 am UTC

Felstaff wrote:So if I put an aeroplane upside down on a treadmill...

You play a dangerous game, old man.

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Re: 0803: "Airfoil"

Postby libra » Fri Oct 08, 2010 8:44 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. Like that people in Columbus's day thought the world was flat (pretty sure that's what they fed me in elementary school).


I believe some people have already done so:-

Wikipedia list of common misconceptions

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Re: 0803: "Airfoil"

Postby libra » Fri Oct 08, 2010 8:53 am UTC

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

Technically, no. But then the strip is about how the teacher responds to an impertinent question, not about whether the reader can answer that question or not.

One of my maths teachers answered an impertinent question about vulgar fractions, namely "What do we need to learn this stuff for?" with "You'll need to learn it for your exam, so just shut up and do the exercises!" This strip reminded me of that obnoxious teacher somewhat.

Incidentally, the kid who asked that question never learned what vulgar fractions are for, or much else about mathematics at all, and even now he struggles to work out loose change or split the bill in a restaurant. He paints magnificently in oils, but he'd find it difficult to work out that 1/6 is less than 1/3. Don't get him started on VAT.

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Re: 0803: "Airfoil"

Postby Monika » Fri Oct 08, 2010 9:06 am UTC

I am never getting on an airplane again.
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Re: 0803: "Airfoil"

Postby ARTF » Fri Oct 08, 2010 9:34 am UTC

Please tell me i'm not the only one who read 0803: "Airfoil" as NACA 0803: "Airfoil"

Incidentally, this is NACA 0803, it's unlikely to be useful for anything other than low Re.

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Re: 0803: "Airfoil"

Postby Cira » Fri Oct 08, 2010 9:48 am UTC

Reminds me of my middle school science teacher.
Not only would she answer most questions with "It's complicated", when a student was interested and wanted more information on something, they were usually shot down with "You don't need to know that 'till high school."

And of course, at least once per lesson, this would happen:
Teacher: "Does anyone know why/how [something we've never covered in class] happens?"
Of course, nobody ever did. We were all fresh out of elementary school.
Teacher: "No? Okay, I'll let you think about it then."

She always seemed disappointed when we didn't know.
It'd be one thing if it was something she'd been over and over and people just hadn't been paying attention, but she'd just kick off the lesson with "So, today we're going to start learning a new topic! Do you know everything I'm meant to teach you over the next few weeks?"

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Re: 0803: "Airfoil"

Postby geck0 » Fri Oct 08, 2010 10:10 am UTC

Its amazing how people continue to miss the principle of vorticity and angular momentum conservation - for all eternity. I cannot tell how often I heard the stupid entangled-reconnected-air stuff, even of colleagues during my study.

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Re: 0803: "Airfoil"

Postby michelcolman » Fri Oct 08, 2010 10:32 am UTC

I'm an airline pilot, and I was fed exactly this theory (equal transit time, Bernoulli) during ATPL airline pilot theoretical training. It is amazingly common, and I would guess at least 90% of pilots believe it to be true because it's all they were ever taught.

Whenever this theory is given to a class, there's usually one or two students that ask "why does the air have to get to the trailing edge at the same time?". I have heard many different "answers" to this question (like my favorite, "otherwise a vacuum would be created"), all of them incorrect because in reality the air does not get to the trailing edge at the same time. Otherwise, it would be trivial to calculate the amount of lift from an airfoil by just measuring the top and bottom of the profile, and you would get about 2% of the amount of lift that is actually produced. In reality, the air above the wing doesn't just go a little bit faster to get to the trailing edge at the same time, it goes a lot faster and gets there first (under most conditions).

When I asked my professor about this (many years after he first taught me), he first started moaning about people without scientific background thinking they know everything better, but then did some looking up and had to admit that, indeed, the theory he had taught us was oversimplified and did not really agree with reality.

So know I'm an airline pilot with 15 years of experience and I still don't know for sure why airplanes fly. I do have a couple of theories, though:

- Action and reaction: gaining in popularity because of its simplicity and obvious correctness: if the wing makes air leave the trailing edge at an angle downward, it must receive a reaction force somehow. Mass of air per second, multiplied by vertical speed, gives kg/s times m/s which makes kg m/s^2 which is Newton. That's exactly the lift the airplane is getting. The problem is that this is not a very accurate calculation to make, since it's difficult to just determine a mass of air that is affected, and the average speed it is getting, since air is affected to a degree depending on the distance from the wing. Also, it doesn't say anything about pressure distributions which are nice to know for all sorts of secondary effects, and it doesn't begin to explain things like stalls.
- Bernoulli and Coanda together: The Coanda effect basically works perpendicularly to Bernoulli. Bernoulli says that, if air is going faster at some point, something must have made it go faster, and this "something" can only be a pressure gradient. Air parcels are accellerated when moving from high to low pressure, write that down mathematically, solve the equation, apply a Taylor series approximation, and half-rho-v-squared (the famous Bernoulli's law) pops out. Coanda is the same thing for changes in direction: as the air curves over the top of the wing, it needs a perpendicular pressure gradient to supply the centripetal force. Basically the Coanda effect just says that such a pressure gradient will automatically create itself to make air flow "stick" to the surface as it curves (due to boundary layer effects etc.). So now just throw the two together: air has to curve over the top surface due to Coanda, this installs a perpendicular pressure gradient (low pressure above the wing, gradually increasing with distance), but air also has to go through this low pressure area, so it speeds up thanks to Bernoulli, which in turn increases the need for more centripetal force (Coanda) and this feedback loop continues until an equilibrium is reached where both Coanda and Bernoulli are satisfied.
- Circulation theory: basically a more sophisticated version of the Bernoulli+Coanda theory. In fact both the perpendicular and longitudinal pressure gradient are components of the same vector equation. Trow in some more laws, stir and mix, and then give up since it all becomes too complicated. So now try the same for something simpler, like a cylinder. That's a lot easier (well, if you give it a few years of study): a rotating cylinder produces lift, and you can calculate exactly how much. Now try to solve the wing by transforming it to a cylinder and back again. You end up with a flow pattern where air from the bottom curves around the trailing edge to meet the air from the top somwhere upstream. That can't be right... enter the Kutta condition which moves the stagnation point (where the streamlines come together) to the sharp trailing edge. Now go back to the cylinder... bingo, with the cutta condition added, the airflow exactly matches that of a rotating cylinder. Now let the computers figure out the rest, since few humans actually understand what this means, except for "the wing somehow makes air turn, creates a vortex,...". And then it turns out that the Kutta condition isn't always exactly correct after all, but that's another story and shall be told another time.

This is just what I picked up from various sources on the internet. If anyone else has something better, go right ahead and send it!

Michel

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Re: 0803: "Airfoil"

Postby spekkeh » Fri Oct 08, 2010 11:09 am UTC

ndu192 wrote: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


As a person who's flown quite a few glider planes in his day (and also upside down), I'd like to +1 this. Airfoils are shaped that way so that the airflow at the top moves faster than at the bottom, resulting in lift (actually could it be that the wing bulges at the top to make the wind 'stick' to the wing instead of 'vortexing' out? dunno, not a physicist but I heard something along those lines). There's just no law that states that the same airflow needs to keep up and connect at the same time at the back of the wing.

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Re: 0803: "Airfoil"

Postby Taot » Fri Oct 08, 2010 11:11 am UTC

littlelj wrote:
Marlayna wrote:
arbivark wrote:... "give your answer in centimeters" so the smarter kids answered 36, and we were supposed to mark it wrong.


Of course you were supposed to mark it wrong. It was wrong.

I'd have to agree - if the rubric had stated "give your answer in whole centimetres" then 36 was right, but otherwise not, since m/cm/km etc are continuous, not discrete.


I agree that 'give your answer in centimeters' tells nothing of how accurate an answer is required. Just what unit is to be used. I'd like to offer, however, opinion that the right answer to accuracy lies within what was calculated. If it was 7,5 x 4,8 = 36,0 is right, 7,28 x 5 = 36 is right, 6,62 x 5,5 = 36,4 is right etc. given that it was a school setting and the general rule with digits ( if that is the approriate word ) is to round to the least accurate number used if not specified otherwise.

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Re: 0803: "Airfoil"

Postby osmigos » Fri Oct 08, 2010 11:55 am UTC

markchd wrote:
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.


That entire page seems to be one giant misconception. Every article I looked at before giving up spent a long time building a wicker man, then they didn't even disprove the subject. Instead they seemed to like taking some small, vaguely related fact and presenting the idea that because their fact is true the other must be false.

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Re: 0803: "Airfoil"

Postby littlelj » Fri Oct 08, 2010 12:13 pm UTC

Taot wrote:
littlelj wrote:
Marlayna wrote:
arbivark wrote:... "give your answer in centimeters" so the smarter kids answered 36, and we were supposed to mark it wrong.


Of course you were supposed to mark it wrong. It was wrong.

I'd have to agree - if the rubric had stated "give your answer in whole centimetres" then 36 was right, but otherwise not, since m/cm/km etc are continuous, not discrete.


I agree that 'give your answer in centimeters' tells nothing of how accurate an answer is required. Just what unit is to be used. I'd like to offer, however, opinion that the right answer to accuracy lies within what was calculated. If it was 7,5 x 4,8 = 36,0 is right, 7,28 x 5 = 36 is right, 6,62 x 5,5 = 36,4 is right etc. given that it was a school setting and the general rule with digits ( if that is the approriate word ) is to round to the least accurate number used if not specified otherwise.


Good point. Like if you measure angles to the nearest degree there's no point measuring your RSJs to the millimetre!!
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Re: 0803: "Airfoil"

Postby jonadab » Fri Oct 08, 2010 12:35 pm 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.


Well, let's start one...

Like that people in Columbus's day thought the world was flat (pretty sure that's what they fed me in elementary school).


Me too, but that was in the eighties. Can someone verify that it has been taught _recently_?

Here are some other candidates. (Again, we would need to verify that they are _still_ being taught.)

Blatantly Wrong:
*In the dark ages, everyone believed that the sun revolved around the earth. They got this idea from the Bible. (Nobody ever bothers to give a reference for this, unless it's a reference to "sunrise/sunset" like meteorologists still use today.) Galileo (usually; occasionally Copernicus) showed that in fact the earth revolves around the sun, which is what we believe today. (What's that you say? Equal force? Sum of the square of the masses? Both objects traveling together through space? Nonsense. The sun is fixed in place at the center of the solar system, obviously.)
*A rock is, for practical purposes, chemically a closed system on a geological timescale.
*Rome was conquered by barbarians who invaded from outside the empire, pushing the borders back until they eventually reached the capital city.
*The old saw about George Washington and the cherry tree.
*Lincoln issued the Emancipation Proclamation in order to free all the slaves and make slavery illegal, which was pretty much the main point of fighting the Civil War in the first place.

Misleading or Counterproductive:
*Fractions with a numerator larger than the denominator are "improper" and should be converted to mixed numbers.

Pointless and a Complete Waste of Class Time:
*any arithmetic problem with more than four non-zero digits in any of the numbers, including the answer. (We spend *thousands* of hours on such problems in gradeschool, and it taught us nothing except that "math is hard and we hate it".)
*every single gradeschool unit that's ever been done on Johnny Appleseed

elementary school: columbus discovered america. jr high: vikings discovered america.


Both are true. They didn't have the internet back then, so the Vikings didn't post their discovery on Twitter. Consequently, Columbus didn't know about it, so when he found America (err, Haiti or Cuba or whatever he actually found), he had and his compatriots no absolutely idea that the Vikings had already been to Newfoundland. Columbus did a better job of publicizing his results (the man was nothing if not a braggart), so lots more people found out about the Americas after his trip. It's also worth noting that Columbus actually believed he had found a short route to Asia. (He was an idiot. Educated people had a *much* more realistic idea of the size of the Earth, even back then.)

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Thank you

Postby Zak McKracken » Fri Oct 08, 2010 12:44 pm UTC

This comic is a gift. I'll print it out and stick it right to our wind tunnel.
I'm involved in a project where we teach Schoolchildren (and their Teachers!) about aerospace stuff, and this "theory" has been a huge nuisance, alhtough most Teachers don't mind being told how it really works, because in their heart they feel it's not quite right, only they've never been taught
anything else.

The actual principle has been explained in this thread, but I'll try to clarify some things here:
ghjm wrote:"Anything involving Bernoulli" - "So how do paper airplanes fly, since their wings are completely flat?"


Attention! Bernoulli is actually the application of the first law of thermodynamics to a inviscid (no friction) fluid (i.e. kinetic energy being converted to pressure and the other way round), nothing to do with air particles at the leading edge agreeing to have a date at the trailing edge (which they don't. Which would be stupid).

The easiest to understand explanation of Bernoulli (that I know of) is this:
If the flow is like a ball in a minigolf course, then the pressure field is the local elevation of the course. If it goes up, the ball decelerates, if it goes down, it accelerates. Therefore, where the pressure is highest, the flow is slowest (because it had to go "uphill" to get there), where it's lowest, the flow is fastest.
In fact presure and elevation are both potential fields and act exactly the same way.

Fun fact: You could design a minigolf course for every (inviscid, incompressible) twodimensional flow, such that a ball shot at the right velocity and direction will follow a streamline, including velocity and everything!

The other analogy is a water surface (supposed you ignore waves!). Where the local pressure is high, the surface goes upward and vice versa

Now how does the pressure field form? The same way the water surface forms (precisely the same way!):
Imagine a pillar of a bridge. Water going towards it wil be slowed down, becaus it's "hitting" the pillar head-on. Meaning all the kinetic energy is converted to pressure. Meaning the increased pressure is diverting the water left and right to go around the pillar. On the sides of the pillar it has to follow the curvature, which creates a centrifugal force, which again creates a pressure gradient (because the water "wants" to leave the surface). The low pressure "attracts" water, keeps the flow attached to the pillar surface, thus you get accelerated flow, and the water surface right beside the pillar is lower than the regular water level in the river. On the backside (if it is a good pillar) the water closes again and goes back to normal pressure/water level.

So why does a flat plate create lift?
General explanation for anyone who knows some physics: Momentum preservation.
Air has mass, you're pushing air downwards, therefore there must be a force opposite to the direction you're pushing the air into. Anything that does this creates lift. You could use a bent pipe (even a straight one) to produce lift, only not very efficiently so. Any body moving through air will produce lift (or a downward force) if the geometry isn't both symmetrical and at zero angle of attack, because it's pushing air around.

More in detail:
On the lower side, air rams into the plate and is being pushed downwards. You can imagine that air hitting an obstacle will have increased pressure (and reduced speed).
On the upper side, air is "sucked" (be careful with that word, some understand it as meaning "negative pressure" which does not exist. It's just lower than in the rest of the field) downwards. Why that? Air has mass, so it will be going straight unless there is some pressure gradient to change its course. Now, just like at the sides of above-mentioned bridge pillar, there's a centrifugal force which causes a pressure gradient (Mass tries to get away from the surface, thus decreases pressure close to surface, until the reduced pressure is sufficient to help the air around the bend). This will of course accelerate the air on the upper side.
Towards the trailing edge, the pressure above and below the plate will return to "normal"
Attention: If you overdo this, the pressure increase from the suction peak at the front of the airfoil/plate/whatever to the trailing edge (i.e. the return to normal pressure) will be large enough to cause separation, and then the flow won't follow the surface anymore, and a lot (though not all) of your lift will be gone.


For Mathematicians:
Accelerating the air above and decelerating below a certain point is also achievable by rotating a cylinder (in 2D: a circle) in a flow. In fact, from a certain distance, the effects of an airfoil and a rotating cylinder are indistinguishable. That's why engineers also talk about "circulation" of an airfoil, referring to the equivalent rotating cylinder. That's nice, because there's a comparatively simple mathematical model for the rotating cylinder. The circulation of that equivalent cylinder is also found in the wing-tip vortices of a real wing in 3D.
The thing about cylinders is potential theory. With some few simplifications you can get a linear set of PDEs from the Navier-Stokes equations (no friction, no solid body rotation, no compressibility) for which singular superimposable analytical solutions exist. There's constant uniform flow (trivial), sources and sinks (great for modelling displacement volumes) and ..... vortices! If you overlay one source, one sink and one vortex with one uniform flow, you get a rotating cylinder and there you go! There's a simple analytical relation between the strength of the vortex and the produced lift force (and even some drag components).

wbeaty wrote:
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.

VERY TRUE!

Hmm... not quite. I think what's dazzling many is the circulation/airfoil flow thing, but you don't actually need that to explain lift.
All that's needed is actually momentum equilibrium: I'm throwing something downwards, so I get an upwards force (I'm also slowing it down some, so it comes at the cost of drag).

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

Yes, it's quite easy actually. A lot of the lift from a planes wings comes from the fact that they are tilted downwards, i.e. like the blade of a ceiling fan or your hand when you stick it out your car window. If your engine is strong enough it can provide enough lift from this alone, with no pressure differentials.


First part: correct, second part: wrong.
You're pushing air down by means of pressure differentials. The pressure distribution on a lifting body is how the force that we know already must be there is transferred to the body (or: from the body to the air). Looking at the pressure distribution or looking at the mass that's being moved (or at the circulation that is being created) are just different levels of observation, and they must always lead to precisely the same result.


wbeaty wrote:It's impossible to understand airplanes by only employing 2D flow diagrams. These diagrams depict an odd sort of "flatland flight" where no work is performed on the air, and an airfoil could ideally coast along forever.


Sorry, wbeaty, that's not quite correct :(
Aircraft are being designed with 2D methods on a regular basis, only in advanced stages is it necessary to move to 3D.
A 2D analysis shows how an airfoil would perform if it was part of an infinite wing. For a sailplane, this is very close to the truth, for a typical passenger aircraft, it's a good approximation, for a delta wing, it's ... well, not much use. Keep in mind, though, that "2D" doesn't mean inviscid, incompressible etc.. Maybe the 2D analyses you're familiar with are really low-fi models? In case you're referring to NASA's "foilsim" Java applet: That's just a better illustraton, by no means a simulation. XFLR5 (available at sourceforge) uses a potential flow method that is already pretty accurate. You can also make a transonic Navier-Stokes analysis of a 2D airfoil and be extremely close to reality. "full" Navier-Stokes solutions are not feasible yet, though (only for really small scales, like a centimeter of a flat plate or so). If someone today says they're doing them, they usually mean Reynolds-averaged Navier-Stokes, which means the effects of microscopic turbulence in the boundary layer and in shear layers is statistically modelled. No problem in most cases but sometimes a weak point.
In 2D you can very well assess both lift and drag produced by an airfoil and compute from that the power necessary to keep it aloft, and in consequence also the amount of work performed on the air (depending of course which method you are using).
What changes from 2D to 3D for decently slender wings is only that the local angle of attack changes slightly, that a swept wing gets a reduced effective velocity (so you'd better do the 2D computation with an accordingly reduced Mach number), and of course close to the wing tips, there's the wing tip vortex that changes a lot. That is why for a delta wing, 2D is actually not much good, because it essentially is all just a big wing tip. (and some other effects, but let's not get derailed even more).

You're right, though, in saying that ground effect can't be usefully assessed in 2D, but that's only relevant if the ground is closer than the wingspan is wide. And if you're doing "2D" measurements in a wind tunnel, you need lots of corrections for wall effects (similar to ground effect plus ceiling effect, sidewalls are'nt trivial either), unless you go to some lengths in the measurement set-up.

After all this nitpicking: Great work pointing out that drawing from Prandtl, I'll show that around. Didn't know why he of all people would get that wrong. That somehow makes me feel better for myself :o)


*wipes sweat from forehead*

OK, that should answer that ... right?
Everyone got it?

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Re: 0803: "Airfoil"

Postby Karilyn » Fri Oct 08, 2010 1:10 pm UTC

jonadab wrote:
arbivark wrote:the one that got me: elementary school: columbus discovered america. jr high: vikings discovered america. me: wait, they're teaching us lies on purpose?
Both are true. They didn't have the internet back then, so the Vikings didn't post their discovery on Twitter. Consequently, Columbus didn't know about it, so when he found America (err, Haiti or Cuba or whatever he actually found), he had and his compatriots no absolutely idea that the Vikings had already been to Newfoundland. Columbus did a better job of publicizing his results (the man was nothing if not a braggart), so lots more people found out about the Americas after his trip. It's also worth noting that Columbus actually believed he had found a short route to Asia. (He was an idiot. Educated people had a *much* more realistic idea of the size of the Earth, even back then.)

You're both wrong. Asians discovered America about 8000 years (+/- 6000 years BECAUSE I DO NOT REMEMBER) before the Vikings.

Or are you forgetting about Native Americans? They had to get to the America's somehow; they didn't evolve there separately from the Mediterranean humans :P

Thusly Asians colonized America long before the Vikings discovered it.
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Re: 0803: "Airfoil"

Postby Zak McKracken » Fri Oct 08, 2010 1:14 pm UTC

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

That entire page seems to be one giant misconception. Every article I looked at before giving up spent a long time building a wicker man, then they didn't even disprove the subject. Instead they seemed to like taking some small, vaguely related fact and presenting the idea that because their fact is true the other must be false.

I did find some that are somehow OK, some are just nitpicking, but many are exactly what you describe them as.
Skates don't glide on ice because of molten water, because some experiment being conducted to show it is wrong. Instead there might be a tiny layer of water on the surface that acts as a lubricant ... WTF?

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Re: 0803: "Airfoil"

Postby Zak McKracken » Fri Oct 08, 2010 1:27 pm UTC

Karilyn wrote:
arbivark wrote:the one that got me: elementary school: columbus discovered america. jr high: vikings discovered america. me: wait, they're teaching us lies on purpose?

Asians discovered America about 8000 years (+/- 6000 years BECAUSE I DO NOT REMEMBER) before the Vikings.


In my view that's just nitpicking, not demystifying.
Indians got to America, that's for sure and obvious. Next, Vikings visited, but left without any serious consequences for themselves or the rest of Europe.

From the viewpoint of western civilisation, and thus the viewpoint of the majority of schoolbook-makers in America, it was in fact discovered by Columbus, because before that noone that he knew (and noone anyone knew that he knew and so on) knew it existed.

Afterwards, people in Eurasia and Afrika had access to the knowledge of America's existence, and people in America could know about Europe, so either the Native Americans of that time discovered Europe or the Europeans had discovered America. The latter sounds more reasonable. No lies involved.


Columbus didn't go all "damn, I thought I had found it first , but the natives got here first, they beat me to it." Ok, in fact he believed to be in India, but that's a different story.


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Re: 0803: "Airfoil"

Postby Karilyn » Fri Oct 08, 2010 1:44 pm UTC

Zak McKracken wrote:From the viewpoint of western civilisation, and thus the viewpoint of the majority of schoolbook-makers in America, it was in fact discovered by Columbus, because before that noone that he knew (and noone anyone knew that he knew and so on) knew it existed.

The viewpoint of Western Civilization isn't always the right one.

The idea that Columbus (in ignorance of the Vikings) discovered America, in part, is based off of ignorance of the pattern of human evolution, that Native Americas were in fact not native, and that they had to come from somewhere. And specifically, they immigrated to the continent over time from Asia.

The fact that the western civilization textbooks are currently wrong, due to ethnocentrism does not mean that they should be supported as correct "because they don't know any better because they are ethnocentric".
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Re: 0803: "Airfoil"

Postby Larry » Fri Oct 08, 2010 1:56 pm UTC

As an aerosexual, I think this is my favourite xkcd ever.

I have 9 new tabs open from this thread alone, well done everyone! :D

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Re: 0803: "Airfoil"

Postby philsov » Fri Oct 08, 2010 2:20 pm UTC

reminds me of mallrats with the scooner and the easter bunny.
The time and seasons go on, but all the rhymes and reasons are wrong
I know I'll discover after its all said and done I should've been a nun.

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Re: 0803: "Airfoil"

Postby DevilsAdvocate » Fri Oct 08, 2010 2:25 pm UTC

One common wrong explanation that annoys me is, when talking about humidity and evaporation and condensation, "warm air holds more water vapor than cool air." No. Somewhat better would be to say that a given volume of empty space can hold more warm water vapor than cool water vapor. A given volume of space holds the same maximum amount of water vapor at a given temperature regardless of the presence or absence of air in that volume. The temperature of the air enters into it only in as much as any water vapor will quickly come to the same temperature as the air around it.

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Re: 0803: "Airfoil"

Postby TigerHunter » Fri Oct 08, 2010 2:43 pm 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. Like that people in Columbus's day thought the world was flat (pretty sure that's what they fed me in elementary school).

"Lies My Teacher Told Me" by James Loewen. It focuses exclusively on U.S. history, but is still a fantastic read.


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