0803: "Airfoil"

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Grant10k
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Re: 0803: "Airfoil"

Postby Grant10k » Fri Oct 08, 2010 2:59 pm UTC

I remember once, in middle school science class, we were measuring the volume or densities (I forget which) of different objects (marbles, cork, paperclilps, plastic cubes, etc.) and I noticed something fundamentally wrong with the way we were measuring.

We were dropping the objects into water to measure how much the water rose, but if the object was lighter than water, it would float on top, and if it were heavier, it would sink to the bottom. Hypothetically, a cork stopper with a lead core (so it sinks) would give different results than the pure cork, and if it had a uranium core, it would give the same results as the lead core. In essence, we were measuring the density of object lighter than water, and the volume of objects heavier than water.

When I brought this concept to the teacher, as I remember it she realized the flaw, but had the class continue with the experiment anyway. I don't think there was enough time left in the class to void everyone's results and start over, but I was still bummed.

jbo5112
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Re: 0803: "Airfoil"

Postby jbo5112 » Fri Oct 08, 2010 3:00 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).


2nd grade: "Use the prism to shine a rainbow on your paper. See the 7 distinct colors?" No, I see colors smoothly blending. I must be stupid because everyone else says they see it.

3rd grade textbook said that heavy objects fall faster that light ones. It contradicted the statement later, but words fail me.

4th grade textbook said that the water in ponds is clear and ponds just look colored because of the surroundings. "I'm preparing you for next year where you will get even more homework."

5th grade field trip to planetarium (paraphrase): "We calculate how far away a star is based on how big it looks and how big it actually is. We calculate how big a star actually is based on how far away it is and how big it looks." The first time he made these statements were in different sections of his lecture, but I still caught it and called him on it. He then made both statements within a minute or two, apparently not realizing that you can't solve for 2 variables with his 1 pseudo-equation. The explanation from the planetarium employee was reduced to "we also have other methods we use", and he quickly moved on after a 5th grader exposed his stupidity.

I learned genetics in 7th grade from a man who was glad that he had daughters instead of sons so they didn't inherit his baldness.

There is false biology being taught to defend evolution, like a human fetus having gill slits and the human tail bone is of no use, when it's an anchor point for some muscles. Yes, I'm the creationist joining the debate.

I must concede that a NASA employee visited my elementary school, who was quite hilarious and gave quite good explanations for a lot of the physics. 4th graders had a correct understanding of how gravity, orbit and apparent weightlessness works. He included numbers with his explanations, and probably even some formulas.

merlanai
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Re: 0803: "Airfoil"

Postby merlanai » Fri Oct 08, 2010 3:01 pm UTC

Every year from 2nd through 7th grade I asked my teacher why we needed oxygen. The answer was always, "To breath." "Yes," I would reply, "but what does the oxygen do?" They just looked at me like I was crazy and ignored the question. I wasn't trying to be a smart-ass, I honestly wanted to know, and didn't know how to find out myself (google didn't exist yet). It wasn't until 7th grade when I got a science teacher that was fresh out of college who could finally answer the damn question. The one thing I can say is the answer was just as interesting as I hoped it would be.

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Re: 0803: "Airfoil"

Postby jbo5112 » Fri Oct 08, 2010 3:09 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.


The "correct" way that I learned was to round to the least number of significant digits when multiplying and dividing. If it was 7.5 x 4.8 = 36 is right, 7.28 x 5 = 40 is right, 6.62 x 5.5 = 36 is right, 4,000,000 x 1.15 = 4,000,000 is right, etc. (we use '.' to designate fractional parts). We were told to use your method for addition and subtraction. However, I learned this in school, so I should probably assume it was wrong.

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Zak McKracken
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Re: 0803: "Airfoil"

Postby Zak McKracken » Fri Oct 08, 2010 3:20 pm UTC

Karilyn wrote:
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.

I didn't mean to imply that. There is no correct viewpoint. You could say "America was discovered by the Indians in [many years] B.C." and ignor Columbus, but that would'nt tell how everyone else got there. In the sense of the word, everyone born in the American continent is "native" american. It's just used as "The people who were already there when we got there". But this only makes sense if "we" is defined.
Imagine this scenario (completely unrealistic, because Will Smith would not let this happen):
Aliens crash-land on earth, in Australia. They displace the human population there without exchanging any sort of information, then move on to find that somewhere to the east there's another larger continent. By their standard it would be correct to call any inhabitants of that continent "natives", and they would also be correct in saying that they discovered America. Now who's right?

There is no objective truth there, so it's moot fighting over it. Maybe it would be Basically Decent(er) to say that Columbus was the first European to discover America. Or to say that America was discovered by the Indians, later by the Vikings and even later by Columbus. But if you overdo this, you will have to mention everyone else who discovered something that was already known, only not to him (or her, or it (aliens, you know)?).

Karilyn wrote: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.

Neither Columbus nor any of the people he was aware of had any way of knowing about the existence of that continent except by stumbling over it. Columbus did just that, and so I think it is completely justified to say that he discovered it. This would only be different if the knowledge had been somehow available anywhere in their society or any other society that had any sorts of contacts with them, even by proxy. The knowledge must have existed in the viking people but was lost, so if you must insist, you might say that he rediscovered America, but that sounds as if he had merely forgotten were it was.

This does not mean the Indians didn't discover it too, or the aliens won't discover it sometime in the future, but actually ... ah, blast it. I think as long as the context is clear to everyone, there's not much that can be misunderstood here. That doesn't mean people shouldn't take care how they say stuff. I think there's no harm in the word, only in how you say it.

Given all this, I understand the term "natives" as "the people who were here before we discovered the place". Again, this requires knowledge of "we" to interpret. Good luck finding a completely neutral formulation today that will be accepted by the aliens who will invade Earth 3000 years after the other aliens who crash-land in Australia have taken over the planet (supposed there is anyone left then who would be called "native" by todays standard).

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Technical Ben
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Re: 0803: "Airfoil"

Postby Technical Ben » Fri Oct 08, 2010 3:23 pm UTC

It's not the "these are the calculations we use, they are limited, but give a roughly correct answer" thing I have a problem with. It's the "this is how a plane flies" or "dinosaurs were green" kind of "facts" we are fed at school that annoy me.
Also known as "everything your learnt at school is wrong!"
On the subject of inverted flight, I've thought of a logical thought experiment to explain how they can fly upside down.
Think of a symmetrical plane, not just left and right, but also symmetrical up and down. This way, no one can even tell if it is upside down or not. It can now fly through the air, the only thing that makes it fly upwards or downwards, is the angle of attack (where you point the nose). So imagine your standard plane, it can also change it's angle to fly, even when upside down. It would just have to point it's nose further into the sky. As long as it does not stall of cause.
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Re: 0803: "Airfoil"

Postby SBD » Fri Oct 08, 2010 3:25 pm UTC

Despite being an aviation enthusiast for most of my life, I didn't really understand lift and aerodynamics until I started actually learning to fly, and doing aviation now that I'm in uni. For instance, I never knew that the tailplane actually has a downwards-acting force in accelerated flight.
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merlanai
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Re: 0803: "Airfoil"

Postby merlanai » Fri Oct 08, 2010 3:27 pm UTC

jonadab 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.


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.)


Well, I started kindergarten in 1995, so that's fairly recent.

Dark Ages, sun/earth/galileo: check. Though, in middle school it was decided that middle ages was a more PC term.
George Washington/cherry tree: actually that was disproved in middle school but my teacher was progressive.
Emancipation Proclamation: Check
Improper fractions/mixed numbers: check (I never was able to get the hang of mixed numbers)
huge numbered math problems: check
johnny appleseed: HEY I LIKE JOHNNY APPLESEED!

Also: TIMED TESTS. For those of you who didn't have them, they are sheets of paper with 100 simple math problems (addition, subtraction, multiplication, or division depending on your grade) and you get ONE MINUTE to complete them. Then it is graded. In my class you had to read your scores aloud. I was one of the best in my class at math in general, but I had issues with these tests and would generally score in the 20s. I know for a fact that these tests are still widely used.

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Re: 0803: "Airfoil"

Postby X9fx1cZ » Fri Oct 08, 2010 3:35 pm UTC

merlanai wrote:
Also: TIMED TESTS. For those of you who didn't have them, they are sheets of paper with 100 simple math problems (addition, subtraction, multiplication, or division depending on your grade) and you get ONE MINUTE to complete them. Then it is graded. In my class you had to read your scores aloud. I was one of the best in my class at math in general, but I had issues with these tests and would generally score in the 20s. I know for a fact that these tests are still widely used.


I hated those; I always did well, but I hated them. I'd probably fail if I took one today.

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Zak McKracken
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Re: 0803: "Airfoil"

Postby Zak McKracken » Fri Oct 08, 2010 3:48 pm UTC

jbo5112 wrote:There is false biology being taught to defend evolution, like a human fetus having gill slits and the human tail bone is of no use, when it's an anchor point for some muscles. Yes, I'm the creationist joining the debate.


Ok, so when someone explains lift on an airfoil wrong, you don't think airplanes can't actually fly, do you?
When some biologist fakes publications of cloned animals, you probably don't question genetics.
But when biologists fail to interpret some blurry microscope images correctly and later realise the error, you conclude that all of evolution must be bogus?

There are pigs today with a pair of ribs more than boars. They were created within the last century, by nothing but selection. If a pair of ribs can appear in such a short amount of time, there's no reason to think that a completely unused organ couldn't dissappear in a billion years.
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Wildbill
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Re: 0803: "Airfoil"

Postby Wildbill » Fri Oct 08, 2010 3:57 pm UTC

The "path length" explanation has always appeared to be quite wrong, or at best incomplete to me. Why does a kite with an absolutely flat and paper-thin "airfoil" create lift? If you draw a force-free diagram, there can really be only one general explanation that fits all cases, namely Newton's third law. Various airfoil shapes may be more efficient in creating lift with less drag, but they all basically push air downwards, creating an equal and opposing force vector pointing upwards that we call 'lift'. Everything else is details, albeit very important details.

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Karilyn
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Re: 0803: "Airfoil"

Postby Karilyn » Fri Oct 08, 2010 3:58 pm UTC

merlanai wrote:Also: TIMED TESTS. For those of you who didn't have them, they are sheets of paper with 100 simple math problems (addition, subtraction, multiplication, or division depending on your grade) and you get ONE MINUTE to complete them. Then it is graded. In my class you had to read your scores aloud. I was one of the best in my class at math in general, but I had issues with these tests and would generally score in the 20s. I know for a fact that these tests are still widely used.

I remember those, and I remember I aced those fucking things. They generally didn't expect children to be able to complete all 100 problems in 60 seconds.

I wonder if anybody else was taught how to taught how to count to 100 on their fingers, and use the same technique to be able to do a long series of simple addition/subtraction/multiplication/division as fast as a person could read it off to you.

36 divided by 6 plus 5 times 2 - 2 divided by 5 minus 2 minus 2 plus 10 times 5 plus 6 divided by 8 plus 2 EQUALS:

And then everyone raises their hand, hopefully with the number 9.

FYI: You count to 100 on two hands using one thumb as a 5, and the other hand having fingers be 10 with the thumb being 50.

9 on the 100 scale looks the same as 5 on the traditional finger counting scale.
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Re: 0803: "Airfoil"

Postby airshowfan » Fri Oct 08, 2010 3:59 pm UTC

Airplanes can fly upside down (i.e. airfoils can still generate lift upside-down) because the stagnation point changes.

(Unfortunately, this is compatible with the incorrect "the separated air must meet up with itself at the trailing edge, so the air taking the longer path must speed up" explanation).

Say you have an airfoil (symmetric or cambered, doesn't matter) moving through the air at some airspeed. Say that you pitch it to whatever angle of attack is necessary to supply a certain amount of lift (say, the lift force necessary to counter the weight of the airplane).

Some of the air moves over the airfoil. Some of the air moves under the airfoil. Some point on the leading edge of the airfoil will mark the spot where the air is split. That is the stagnation point. On the image, I marked it as a red dot.

Now say you turn your airfoil (e.g. the whole airplane) upside-down. Say that it's still moving with the same airspeed. Say that you pitch it to whatever angle of attack is necessary to supply a certain amount of lift (say, the lift force necessary to counter the weight of the airplane). If you have a symmetric airfoil, it will be the same angle of attack as before. If you have an asymmetric airfoil optimized for non-inverted flight, it will need a higher angle of attack to fly inverted, but that doesn't matter much right now.

inverted-airfoil-stagnation-changes.png


You'll notice that the stagnation point will be further down than it was before. On the image, I marked it as a green dot. The airfoil naturally generates a "hump" out of the upper part of its leading edge when it's upside down (just as it does when it's not upside down).

As for what kinds of airplanes can sustain inverted flight for how long, there are five things to keep in mind:

1) An asymmetric airfoil is less efficient when inverted (i.e. it will produce more drag while generating the same lift at a given speed)
2) The engine is usually mounted so that it produces thrust in the straight-forward direction when the wing is at the angle of attack that, during cruise flight, causes the right amount of lift to be generated to overcome the weight of the airplane. So if you turn the airplane upside down and then pitch the nose up in order for the wing to work, the engine will no longer be pulling straight forward, it will be pulling forward and up. So you lose some component of thrust (since the horizontal component of the thrust is now the engine thrust times the cosine of the angle through which you had to pitch the nose up for the wing to keep working). Yes, there is a tiny component of the thrust pulling upwards, and you'd think that this would help the wings out, but most airplanes have small thrust-to-weight ratios so the vertical component of the thrust during inverted flight is pretty negligible, unless you're flying an inverted high-alpha airshow pass in a ridiculously overpowered airplane.
3) Engines, the fuel tank, the oil system, and other things (and the pilot!) might not function very well when inverted.
4) Some structures will be loaded in reverse during inverted flight. The tops of the wings which used to be in compression will be in tension, the bottoms of the wings which used to be in tension will be in compression (so any struts under the wings might buckle), etc.
5) The horizontal stabilizer, which is usually required to balance the airplane, is mounted at a different angle of incidence than the wing, for stability. So when you turn the whole airplane upside down, the horizontal will be at the wrong angle of attack, and the elevators will have to work very hard in order to make up for this.

All that numbers one and two mean is that you need more thrust to sustain inverted flight. Most airplanes have some excess thrust (i.e. the engines can produce a lot more than is needed for level flight) so no real problem there.

Number three and four mean that either your systems and structures are inverted-friendly (they can work properly in negative gs), or they are not. If they're not, they'll malfunction when you sustain negative gs, even if the airplane can aerodynamically do it.

Number five is the real reason why you can't fly inverted in a typical Cessna or airliner:

inverted-airliner.png


The CG is ahead of the wing (for stability), so the horizontal stabilizer needs to push downwards (for balance), so it is set at a negative angle of incidence (i.e. it's like a little wing, but upside down). If you flip the plane upside down (and then pitch the nose up so that the wings still lift upwards), the horizontal stabilizer will now be at a very high upwards angle of attack, and will try very hard to push the tail up and bring the nose back down. The only way to overcome this is if your elevators can overcome this stabilizing force. Cessnas and airliners have big stabilizers set at a significantly negative angle of incidence (which give these airplanes great stability) and small elevators (so the pilot can't maneuver too wildly), but fighters and aerobatic airplanes have the horizontal at almost the same angle of incidence as the wing (so they're not very stable) and big, powerful elevators, sometimes the whole stabilizer is one huge elevator (so that they can pitch up and down very fast, make tight turns, etc). So the elevators in fighters and aerobatic airplanes is powerful enough, and the stabilizers are weak enough, that when you roll inverted, the elevators can overcome the force of the stabilizers and keep the airplane at that nose-up angle that it needs for the wings to work while inverted. But airliners and general-aviation airplanes, not so much.

So if you roll a fighter or an aerobatic airplane inverted, and then push the stick, the nose will point upwards and the wings will work and you're golden. But in a typical general-aviation airplane or an airliner, if you somehow manage to roll it inverted, the nose will sink, no matter how hard you push that yoke, because the stabilizers are just too big and set at too great an angle and you don't have very powerful elevators to overcome the stabilizers' tail-up force.

Note that all this is in regards to airplanes "pulling negative gs", i.e. where the wing generates lift in the direction from the pilot's head to their butt rather than in the direction from the pilot's butt to their head. Many inverted maneuvers, like properly-executed loops and barrel-rolls, are positive-g maneuvers, i.e. the wing always works the normal way, and the pilot is always pressed against the seat and never hangs from the straps. (Whether this should be understood as a "centrifugal" effect or just as an airplane rolled inverted and pushing downwards against its own inertia... you can ask Mr Bond). The only real challenge in doing these positive-g rolls and loops is that the maneuver will have a "bottom" and a "top", and that difference in altitude means a difference in potential energy that comes right out of your kinetic energy, i.e. the airplane goes slower at the top than at the bottom (just like a roller coaster or a projectile - the engine will help a little but not much, since you have to pull off these maneuvers fairly quickly if you hope to keep the gs positive). But the range of speeds that an airplane can operate at is relatively narrow: There is a minimum stall speed below which the wings don't work, and a maximum speed dictated by engine thrust or by structural considerations. Most non-aerobatic airplanes can't do a loop because if you're flying at your maximum speed and you pull up, you'll get to your minimum speed before you get to the top of the loop, and the airplane will stall. (If you're inverted when this happens, i.e. if you make it more than 1/4 of the way around the loop and your nose has gone past the vertical, then stalling can put you in an inverted flat spin that some airplanes can't get out of. So don't try this at home). But almost any airplane can do a barrel roll, even (famously) a 707. You have to pull a couple of gs when you transition from level flight to going up the barrel roll, and then you have to pull a couple of gs again when you transition from going down the barrel roll back into level flight, but pretty much any airplane can handle that, and the altitude change will be relatively small so it won't take most airplanes from max speed to below stall speed.

EDIT: I think I was wrong in my "the elevators are probably not powerful enough to overcome the stabilizers' alpha-reducing nose-towards-the-horizon force" estimation. In my defense, I'm a structures guy. I can tell you how the airplane components are loaded very differently during -1g flight than they are during normal flight, and how they're still strong enough to take -1g flight, plus a 50% safety margin (just like the famous 50% safety margin on the 2.5 positive gs that they're supposed to withstand), and which flight conditions load which components most heavily (not ALL of them, but most of the structurally important ones). I can give you numbers about stress levels, strengths, material thicknesses, how shear beams are configured, when buckling happens, how stringers and frames are shaped and why, and how far apart they are, and so on. (Well, I'd be fired if I did, but I could). But when it comes to stability & control, what I know comes from a few classes in college, some experience flying Cessnas, some experience in Flight Simulator, and going to lots of airshows. It's a lot, but it's not as precise as what I know about structures. Anyways, over the weekend I got talking with someone who does deal with stability and control and airplane handling for a living, and he told me that an airliner should not only be able to survive a -1g manouver, in practice an airliner can actually be pushed over that hard on command. At the right speeds, with nose-down trim, etc, the elevators should be able to overcome the horizontal stabilizers' stabilizing force, and hold the airplane at the negative angle of attack necessary to sustain -1g flight. What I told him is that I've never been able to pull it off in Flight Simulator, but he insisted that it's doable. The only way to know for sure (other than taking an airliner up and flipping it, which is a bit beyond our means) is to try it in a REAL flight simulator. Now I really want to try it! But in any case, for the record, what I wrote above in this post (about how the elevators will not push down hard enough to keep the horizontal stabs from raising the tail and dropping the nose, so the wings can't be kept at enough of an angle to generate lift while inverted) is probably wrong.

EDIT 2: Yep, I was wrong.
Last edited by airshowfan on Tue Oct 12, 2010 2:28 pm UTC, edited 10 times in total.

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Re: 0803: "Airfoil"

Postby jpers36 » Fri Oct 08, 2010 4:01 pm UTC

Karilyn wrote:FYI: You count to 100 on two hands using one thumb as a 5, and the other hand having fingers be 10 with the thumb being 50.

9 on the 100 scale looks the same as 5 on the traditional finger counting scale.


Forget that, I can count to 1024 on my two hands.

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ubikuberalles
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Re: 0803: "Airfoil"

Postby ubikuberalles » Fri Oct 08, 2010 4:02 pm 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?


The U.S. Air Force Academy? :P

Seriously, though, I was taught that in elementary school.
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Re: 0803: "Airfoil"

Postby Grant10k » Fri Oct 08, 2010 4:15 pm UTC

jpers36 wrote:
Karilyn wrote:FYI: You count to 100 on two hands using one thumb as a 5...

Forget that, I can count to 1024 on my two hands.

It's all fun and games until you get to 132.

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Re: 0803: "Airfoil"

Postby negative_one_rad_theta » Fri Oct 08, 2010 4:24 pm UTC

It's good that the equal transit fallacy was finally addressed in this comic.
It's bad too, because the mere existence of such a popular non-scientific explanation for a physical phenomenon makes me feel dead inside.
It is probably the second best/worst (depending on you look at it) perversion of physics that has ever been tried.
(The worst being those gut-wrenching abuses of anything with the word "quantum" in it).

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Re: 0803: "Airfoil"

Postby Pterosaur » Fri Oct 08, 2010 4:37 pm UTC

HonoreDB wrote:"This is the way it will appear on the test."


“Will this be on the test?” is always more important than “Is this accurate?”

Steve the Pocket wrote:Someone needs to compile a list of things they actually still teach in schools.


Besides those previously mentioned, the one that I recall is the “tongue taste bud map.” I learned that in high school 15 years ago.

jonadab wrote: 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.


Incidentally, I discovered a new land yesterday in the woods behind my house where the deer like to nap. (The Parks Dept. probably knew about it, but they never told me.)

I shall call my new land Deervania.
Until you stalk and overrun,
You can't devour anyone.
--Hobbes

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Pesto
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Re: 0803: "Airfoil"

Postby Pesto » Fri Oct 08, 2010 4:57 pm UTC

cs24 wrote: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

Huzzah! Vindicated! The explanation that the two air streams would meet up at back of the wing never made sense to me, although I never openly questioned it.

Cool to see that the upper air stream does indeed travel faster than the lower air stream.

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Karilyn
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Re: 0803: "Airfoil"

Postby Karilyn » Fri Oct 08, 2010 5:25 pm UTC

jpers36 wrote:
Karilyn wrote:FYI: You count to 100 on two hands using one thumb as a 5...
Forget that, I can count to 1024 on my two hands.

Binary? Yeah I never learned do to binary quickly in my head. Though admittedly I still count with my thumb as a 5 if I'm trying to tally up some whatever I'm counting. Kinda weird how it stuck with me.

Zak McKracken wrote:
Karilyn wrote:
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.
words

Image
Last edited by Karilyn on Fri Oct 08, 2010 5:36 pm UTC, edited 3 times in total.
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Re: 0803: "Airfoil"

Postby charonme » Fri Oct 08, 2010 5:28 pm UTC

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

A kid could be convinced/silenced (even though perhaps not educated) by this: The planes you saw flying upside down were not using this principle we just discussed here. They fly because their engines are so powerful they could even fly straight up. The primary function of their wings is not to provide lift, just to balance them and provide coordination.

ghjm wrote:"Anything involving the engine" - "So how do gliders fly?"
"Anything involving Bernoulli" - "So how do paper airplanes fly, since their wings are completely flat?"

depends on your definition of "fly". If gliders and paper airplanes fly, then also rocks fly. As a crude simplification - gliders and paper planes don't fly, they just fall slowly :twisted:

I believe the equal distance explanation is wrong, but not because we can find examples of planes that fly despite having different wings. This only proves that the equal distance explanation could not be the only reason why something with wings flies, it does not prove that the explanation is incorrect in case of the particular wing shape that it is applied to.

Grant10k wrote:
jpers36 wrote:Forget that, I can count to 1024 on my two hands.
It's all fun and games until you get to 132.

What's so unfunny about 132? Remember you have to start with all fingers in the 0 position to count ONE if you want to get to 1024 ;)

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Re: 0803: "Airfoil"

Postby airshowfan » Fri Oct 08, 2010 5:36 pm UTC

charonme wrote:The primary function of their wings is not to provide lift, just to balance them and provide coordination.


You know that this is incorrect, right? Just checking.

charonme wrote:This only proves that the equal distance explanation could not be the only reason why something with wings flies...


Actually, the incorrect equal-distance explanation (or, rather, the incorrect different-distances-but-supposedly-same-time explanation) CAN hold up as the only reason that something flies, even inverted, if you take into account the fact that the stagnation point changes when you turn the airfoil upside down.

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Re: 0803: "Airfoil"

Postby Karilyn » Fri Oct 08, 2010 5:37 pm UTC

charonme wrote:
Grant10k wrote:
jpers36 wrote:Forget that, I can count to 1024 on my two hands.
It's all fun and games until you get to 132.

What's so unfunny about 132? Remember you have to start with all fingers in the 0 position to count ONE if you want to get to 1024 ;)

Look ma, I can count to 132!

Image

(the 1024 is implied. Otherwise you could count to 1025 ;) )
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Re: 0803: "Airfoil"

Postby HonoreDB » Fri Oct 08, 2010 5:40 pm UTC

Pterosaur wrote:Besides those previously mentioned, the one that I recall is the “tongue taste bud map.” I learned that in high school 15 years ago.


Ooh, me too, around the same time. My parents neatly refuted it that evening by sticking out their tongues and asking what the theory predicted about their respective preferences for salt.

(America was discovered by utahraptors).

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Re: 0803: "Airfoil"

Postby libra » Fri Oct 08, 2010 5:46 pm UTC

Zak McKracken wrote:
jbo5112 wrote:There is false biology being taught to defend evolution, like a human fetus having gill slits and the human tail bone is of no use, when it's an anchor point for some muscles. Yes, I'm the creationist joining the debate.
If a pair of ribs can appear in such a short amount of time, there's no reason to think that a completely unused organ couldn't disappear in a billion years.

Case in point: the human brain.

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Re: 0803: "Airfoil"

Postby charonme » Fri Oct 08, 2010 6:00 pm UTC

Karilyn wrote:(the 1024 is implied. Otherwise you could count to 1025 ;) )

You don't "count" to zero. If something is implied, it could be zero. But you could maintain that you could count to 1024 on your fingers by indeed "implying" the 1024 by having all fingers in the 0 position :)
I still fail to see why two middle fingers stop fun, while one does not :?

airshowfan wrote:You know that this is incorrect, right? Just checking.

The factual correctness of the answer to the kid is irrelevant as I pointed out before I wrote it. The only matter is whether it is capable of silencing the kid and maintaining the appearance of an "all knowing teacher" :twisted:

Super_jambo
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Re: 0803: "Airfoil"

Postby Super_jambo » Fri Oct 08, 2010 6:17 pm UTC

You kids can't hold your fingers in more than one position?
Trinary will give you 59049... ;)

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Re: 0803: "Airfoil"

Postby sableye22 » Fri Oct 08, 2010 6:27 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).


They- they didn't?
Everything I know is a lie! (I feel like I'm only half-joking....) I have no idea how planes fly upside down but I know I'm in love with the teacher's little stick fists of rage in the last panel.
You don't have to spend, you just have to pretend.

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Re: 0803: "Airfoil"

Postby Grant10k » Fri Oct 08, 2010 6:28 pm UTC

charonme wrote:
Grant10k wrote:
jpers36 wrote:Forget that, I can count to 1024 on my two hands.
It's all fun and games until you get to 132.

What's so unfunny about 132?...

...

I still fail to see why two middle fingers stop fun, while one does not :?


Ok, I don't like doing this because it ruins the joke. The double middle finger that you get when you count to 132 in binary is actually far from unfunny. One could argue that it's actually the MOST funny number when counted on one's hands (585 can be funny depending on which direction you're holding your hands.)

For your entertainment: Kirby counting to 132: t('.'t)


libra wrote:
Zak McKracken wrote:
jbo5112 wrote:There is false biology being taught to defend evolution...
...there's no reason to think that a completely unused organ couldn't disappear in a billion years.
Case in point: the human brain.

Zing. Nice.

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Re: 0803: "Airfoil"

Postby charonme » Fri Oct 08, 2010 6:41 pm UTC

you're right, exactly my point :)
I was being an ass, sorry

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Mike_Bson
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Re: 0803: "Airfoil"

Postby Mike_Bson » Fri Oct 08, 2010 6:43 pm UTC

This one is the best one in a long time, I love it!

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Re: 0803: "Airfoil"

Postby ddxxdd » Fri Oct 08, 2010 6:52 pm UTC

Alright, I've had a relevant question burning the back of my mind for YEARS.

We've all been taught that the force of friction equals Normal Force times coefficient of friction, right? And that the height, width, and length of the object don't factor into that calculation, right?

Well then why is it that wider tires on cars and trucks provide better braking power? Or is the fact that "wider tires procures better braking power" a myth?
I'm waiting for someone to say something worth sigging...

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Re: 0803: "Airfoil"

Postby jshilliday » Fri Oct 08, 2010 6:55 pm UTC

Speaking of preparing your children, here's another opportunity. Sooner or later, a teacher will tell your child's class that "The moon does not rotate on its axis; it always has the same side facing the earth." If the teacher will allow it, your kid should say "OK, you be the Earth, just stand there." And then she faces the teacher and moves in the appropriate circle around her, saying "Now I'm facing the blackboard, now I'm facing the windows, now I'm facing the back of the room, and now I'm facing the door. See, I'm turning around my own axis like the moon does." It's amazing how many people will watch the demonstration and STILL insist the moon doesn't rotate.

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Monika
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Re: 0803: "Airfoil"

Postby Monika » Fri Oct 08, 2010 7:06 pm UTC

Grant10k wrote:I remember once, in middle school science class, we were measuring the volume or densities (I forget which) of different objects (marbles, cork, paperclilps, plastic cubes, etc.) and I noticed something fundamentally wrong with the way we were measuring.

We were dropping the objects into water to measure how much the water rose, but if the object was lighter than water, it would float on top, and if it were heavier, it would sink to the bottom. Hypothetically, a cork stopper with a lead core (so it sinks) would give different results than the pure cork, and if it had a uranium core, it would give the same results as the lead core. In essence, we were measuring the density of object lighter than water, and the volume of objects heavier than water.

When I brought this concept to the teacher, as I remember it she realized the flaw, but had the class continue with the experiment anyway. I don't think there was enough time left in the class to void everyone's results and start over, but I was still bummed.

You realized this in middle school? Most adults answer this riddle wrong:
You take a lump of gold, get into a boat on a closed lake and row a bit. Then you throw the gold into the water. Does the water level of the lake sink or rise?



Karilyn wrote:I wonder if anybody else was taught how to taught how to count to 100 on their fingers, and use the same technique to be able to do a long series of simple addition/subtraction/multiplication/division as fast as a person could read it off to you.

36 divided by 6 plus 5 times 2 - 2 divided by 5 minus 2 minus 2 plus 10 times 5 plus 6 divided by 8 plus 2 EQUALS:

And then everyone raises their hand, hopefully with the number 9.

FYI: You count to 100 on two hands using one thumb as a 5, and the other hand having fingers be 10 with the thumb being 50.

9 on the 100 scale looks the same as 5 on the traditional finger counting scale.

I don't get it. Can you make a drawing?
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airshowfan
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Re: 0803: "Airfoil"

Postby airshowfan » Fri Oct 08, 2010 7:07 pm UTC

Floating objects displace their weight in water (which is a lower volume). Sinking objects displace their volume in water (which is a lower weight). Right?

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Re: 0803: "Airfoil"

Postby Monika » Fri Oct 08, 2010 7:09 pm UTC

airshowfan wrote:Floating objects displace their weight in water (which is a lower volume). Sinking objects displace their volume in water (which is a lower weight). Right?

Right. When you throw out that gold the water level sinks, because gold has a higher density than water. Most people say "unchanged".
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airshowfan
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Re: 0803: "Airfoil"

Postby airshowfan » Fri Oct 08, 2010 7:17 pm UTC

Yeah, that makes sense. While on a boat, the gold is displacing a lot of water (its weight), but once in the water the gold is displacing less water (its volume).

Once the gold goes overboard, the boat hull displaces less water corresponding to the weight of the gold, while the gold itself displaces some water (not as much, i.e. not enough to make up for the decreased displacement of the boat hull) corresponding to its volume.

I had to think about that for a minute. (But I would not have said "unchanged"!)

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Re: 0803: "Airfoil"

Postby davebrown42 » Fri Oct 08, 2010 7:40 pm UTC

ddxxdd wrote:Alright, I've had a relevant question burning the back of my mind for YEARS.

We've all been taught that the force of friction equals Normal Force times coefficient of friction, right? And that the height, width, and length of the object don't factor into that calculation, right?

Well then why is it that wider tires on cars and trucks provide better braking power? Or is the fact that "wider tires procures better braking power" a myth?


Basically a myth. Wider tires do increase rolling resistance, so if you have no brakes at all, wider tires will slow you down slightly faster. :P

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Karilyn
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Re: 0803: "Airfoil"

Postby Karilyn » Fri Oct 08, 2010 7:58 pm UTC

Monika wrote:
Karilyn wrote:I wonder if anybody else was taught how to taught how to count to 100 on their fingers, and use the same technique to be able to do a long series of simple addition/subtraction/multiplication/division as fast as a person could read it off to you.

36 divided by 6 plus 5 times 2 - 2 divided by 5 minus 2 minus 2 plus 10 times 5 plus 6 divided by 8 plus 2 EQUALS:

And then everyone raises their hand, hopefully with the number 9.

FYI: You count to 100 on two hands using one thumb as a 5, and the other hand having fingers be 10 with the thumb being 50.

9 on the 100 scale looks the same as 5 on the traditional finger counting scale.

I don't get it. Can you make a drawing?

It's basically roman numbers.

Finger = I
Thumb = V
Other hand finger = X
Other hand thumb = L

I
II
III
IIII
V
VI
VII
VIII
VIIII
X
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Kline-Fogleman airfoil, longer path underneath.

Postby wbeaty » Fri Oct 08, 2010 7:59 pm UTC

Be aware that Kline-Fogleman and supercritical airfoils fly just fine. Their upper pathlength is the short one.

airshowfan wrote:Actually, the incorrect equal-distance explanation (or, rather, the incorrect different-distances-but-supposedly-same-time explanation) CAN hold up as the only reason that something flies, even inverted, if you take into account the fact that the stagnation point changes when you turn the airfoil upside down.


Nope, that completely underestimates the value for lift (besides misleading everyone about the nature of lift-production.) If you track the flowing air using smoke-pulses (or using a CFD simulation,) you find that the parcels divided by the leading edge will never rejoin. The upper parcels greatly outrace the lower ones. I.e. the air velocity is everything, while the surface path lengths are basically irrelevant.
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