|
KillHour posted:So... can someone correctly explain how a wing DOES generate lift? Because so far, I've only heard why all the normal explanations are wrong. Air moving along the top of the wing travels faster than air moving along the bottom of the wing. Using Bernoulli's equation, this results in there being greater pressure on the bottom of the wing than on the top. This difference in velocity is caused by both the shape of the airfoil and the angle at which the airfoil points into the direction of travel. Why does the air along the top move faster than the air along the bottom? A common explanation found in textbooks of "the air moves faster because it needs to reach the end of the wing at the same time as the air on the bottom" is an incorrect explanation of a correct observation. The air along the top (the upwash) does move faster than the air on the bottom (the downwash). This change in velocity is due to both the shape and the angle of the wing. Video demonstration: https://www.youtube.com/watch?v=UqBmdZ-BNig
|
#
?
Feb 20, 2015 19:43
|
|
|
|
| # ? Apr 25, 2024 00:25 |
|
|
Insurrectum posted:Well, the easiest way to understand this is to put a plane on a treadmill... You actually have to put the plane on a treadmill that's exactly .9 repeating miles long and then submerge the entire setup in water and have the plane suck water in through its engines while you eat a hotdog covered with bean chili and catsup. V I got caught halfway between writing catsup and ketchup. V Not My Leg fucked around with this message at 20:05 on Feb 20, 2015 |
|
#
?
Feb 20, 2015 19:48
|
|
|
Not My Leg posted:catchup 
|
|
#
?
Feb 20, 2015 19:56
|
|
|
KillHour posted:So... can someone correctly explain how a wing DOES generate lift? Because so far, I've only heard why all the normal explanations are wrong. It's not really an easy explanation to give on an internet forum. Reading about circulation and the Kutta-Joukowski theorem would be a good start. An undergrad aerodynamics class goes into concepts like potential flow theory, velocity potentials, stream functions, etc. Circulation explains why velocities move faster over the top of the airfoil. Bernoulli's equation is perfectly applicable to describe lift for an airfoil in inviscid, incompressible, steady flow. http://en.wikipedia.org/wiki/Circulation_(fluid_dynamics) http://en.wikipedia.org/wiki/Kutta%E2%80%93Joukowski_theorem 
|
|
#
?
Feb 20, 2015 23:59
|
|
|
The explanation I would give to a high school physics student is that the shape of the wing cutting through the air directs air downward, and thus by Newton's third law, the air pushes upward on the wing. Same thing that happens when you stick your hand out of a car window at an upward angle and your hand gets pushed up, except the wing is shaped better to do that more efficiently. Now, that'd be easy to understand if air was a loose collection of non-interacting particles hitting the wing and bouncing off at an angle, but it's not, so exactly how the wing directs air downward is loving complicated as poo poo. Complicated enough to cause an goon argument, apparently.
|
|
#
?
Feb 21, 2015 08:26
|
|
|
The goon argument was actually whether or not Bernoulli's Equation is even applicable when describing lift on an airfoil, to which the answer is a resounding "yes". That having been said, the Newton's Laws explanation is way easier to visualize and is also correct (when one makes pretty normal assumptions), so it's what I'd defer to in most situations where someone might ask "how does airplane lift work?"
|
|
#
?
Feb 21, 2015 09:23
|
|
|
I just got out of a debate with more questions than I started with. It's science, so I consider that a good thing. But I'm hoping you folks can clarify some things for me. On the one side, there's more and more, stronger and stronger evidence appearing that the universe is completely flat. See e.g. http://scienceblogs.com/startswithabang/2012/07/18/how-big-is-the-entire-universe/ and http://en.wikipedia.org/wiki/Shape_of_the_universe . If the universe is truly flat, this apparently means it's also infinite in size. On the other side, the big bang and related theories tell us that the universe started as a singularity, a single, infinitely dense point, and then started expanding at a finite rate. In that case, the universe has to be finite in size. So, what's true? Someone argued that the big bang was a 'physical singularity, not a mathematical one', meaning that while it had infinite density, it was already infinite in size. He argued that "expanding universe" is just physics shorthand for "the matter in the universe is moving away from each other" and doesn't mean the universe actually gets bigger. It doesn't have to if it already is infinite. I personally prefer to consider infinity a purely mathematical concept that does not exist in reality, so I find that argument a bit hard to believe. But it does seem the 'flat universe' evidence and the 'big bang' evidence prove contradictory ideas. What's the general view of physicists on this issue? Carbon dioxide fucked around with this message at 20:25 on Feb 21, 2015 |
|
#
?
Feb 21, 2015 20:20
|
|
|
It would be difficult to reconcile results which suggest an infinite universe and not believing in infinity. For an example of something spreading apart but not getting bigger because it's infinite: take the natural numbers. Double them. They are now further apart, but for every member of the new set almost every member of the natural numbers is greater than it.
|
|
#
?
Feb 21, 2015 20:56
|
|
|
Carbon dioxide posted:On the other side, the big bang and related theories tell us that the universe started as a singularity, a single, infinitely dense point, and then started expanding at a finite rate. In that case, the universe has to be finite in size. That's the trick. The big bang theory does not assert that the universe started as a single point. It asserts that the density was greater and greater in the past, going to infinity at some finite time in the past. But there's no single points involved. Everything expands away from everything. Shamelessly ganked from Wikipedia: 
|
|
#
?
Feb 22, 2015 00:59
|
|
|
Yeah, the term "singularity" is abused a lot in science reporting; the beginning of the universe (in the standard flat lambda-CDM model that seems to fit our observations) is "singular," but it is not a single point. A "singularity" is basically just "somewhere/somewhen in spacetime where the math doesn't make sense." Divide-by-0s, usually. In the flat lambda-CDM model, the universe is infinite for t>0, and singular (meaning we can't say anything meaningful about it, including whether or not it is infinite) at t=0. Also, any claim about the flatness of the universe (or any claim about the universe at all) is necessarily a claim about our local observable universe, which is not infinite. The flatness could be a "local" phenomenon (for a very broad definition of "local"), for all we know. Mathematical models are not reality, and all physical theories are tentative to a greater or lesser degree depending on the available evidence. No one will ever be able to say with any degree of certainty that the universe is infinite. We can only say that our best, simplest mathematical model that fits with all our observations so far is infinite.
|
|
#
?
Feb 22, 2015 21:03
|
|
|
I'm gonna do it; I'm gonna be the retard and ask the retarded babby middle school question: If the universe is literally flat as in 2 dimensional, and if everything necessarily exists inside of the universe then what do concepts such as "above" or "below" actually mean?
|
|
#
?
Feb 23, 2015 13:00
|
|
|
The universe has 3 spatial dimensions regardless of whether it's 'flat' or not. You can think of its 'flatness' as a matter of whether it's curved into a fourth. Wikipedia has an okay illustration of a 2-dimensional universe curving into the third dimension. The same applies to the actual 3D universe, it's just a little hard to draw something with more spatial dimensions than your universe has. HMS Boromir fucked around with this message at 13:25 on Feb 23, 2015 |
|
#
?
Feb 23, 2015 13:21
|
|
|
HMS Boromir posted:The universe has 3 spatial dimensions regardless of whether it's 'flat' or not. You can think of its 'flatness' as a matter of whether it's curved into a fourth. The universe has 11 dimensions 
|
|
#
?
Feb 23, 2015 16:21
|
|
|
Oberleutnant posted:I'm gonna do it; I'm gonna be the retard and ask the retarded babby middle school question: A one-dimensional thing (a line) can be straight or curved, into a circle for example. A two-dimensional thing (a surface) can be flat or curved, into a sphere for example. Take, for example, the Earth. It has a 2-dimensional surface, but it's not flat, it's curved. Consider drawing a big triangle. Put one corner at the north pole, one corner at the equator, and another corner on the equator 90 degrees of longitude away. That triangle has three 90 degree angles. This is weird because if you remember from your high school geometry, its angles should add up to 180 degrees. But that high school geometry is "Euclidean" geometry, where you have assumed you are working on a flat plane. The fact that the surface is curved fucks up the geometry, makes it "non-Euclidean." (A surface can also be curved differently, into a sort of saddle shape, so that a triangle has less than 180 degrees.) The bigger the triangle you draw, the easier it is to notice the discrepancy, and the easier it is to notice that you're on a globe and not a flat thing. A three-dimensional thing (a "space") can be flat or curved, too. That's harder to visualize, since we're used to visualizing things in three dimensions and not four or whatever. When we talk about the universe being flat, we mean that we can draw a big triangle and have its angles add up to 180 degrees. PerrineClostermann posted:The universe has 11 dimensions prove it
|
|
#
?
Feb 23, 2015 16:39
|
|
|
DontMockMySmock posted:
...I'll get back to you on that.
|
|
#
?
Feb 23, 2015 17:25
|
|
|
DontMockMySmock posted:prove it If only this phrase worked on String Theorists.
|
|
#
?
Feb 23, 2015 18:53
|
|
|
I'm sorry to bring aerodynamics back to this thread, but I made an observation yesterday, and I couldn't understand what was going on. My family and I were departing Rochester airport, where weather conditions are best explained by analogy to Hoth as far as my California mind is concerned, and our plane (Embraer 175) had to undergo a deicing procedure before we could flee the frozen Northeast. My daughter and I were seated behind the wing, and we got a great view of the anti-ice fluid streaming off the trailing edge as we took off. But I noticed something really surprising (to me) on the aileron. A pool of green anti-ice fluid persisted on the back half of the top surface of the aileron well after takeoff which as I watched changed from a mostly flat pool of fluid into a standing wave, maybe 3-6 inches tall, almost like a mohawk. It hung around for maybe a minute or two after takeoff. I could not take a picture because my daughter was already very engaged with my phone. Can anyone explain what was going on there?
|
|
#
?
Feb 25, 2015 05:23
|
|
|
Maxwells Demon posted:If only this phrase worked on String Theorists. You can talk, you thermodynamic monster!
|
|
#
?
Feb 25, 2015 16:14
|
|
|
Hermsgervørden posted:I'm sorry to bring aerodynamics back to this thread, but I made an observation yesterday, and I couldn't understand what was going on. My family and I were departing Rochester airport, where weather conditions are best explained by analogy to Hoth as far as my California mind is concerned, and our plane (Embraer 175) had to undergo a deicing procedure before we could flee the frozen Northeast. My daughter and I were seated behind the wing, and we got a great view of the anti-ice fluid streaming off the trailing edge as we took off. But I noticed something really surprising (to me) on the aileron. A pool of green anti-ice fluid persisted on the back half of the top surface of the aileron well after takeoff which as I watched changed from a mostly flat pool of fluid into a standing wave, maybe 3-6 inches tall, almost like a mohawk. It hung around for maybe a minute or two after takeoff. I could not take a picture because my daughter was already very engaged with my phone. Can anyone explain what was going on there? Kinda interested in this one, as I suspect you saw a whole load of physical effects going on at once. When you say 'it hung around', did the stuff freeze and crack away, or did the rippling go away, with the fluid slowly draining? To the best that you could see.
|
|
#
?
Feb 25, 2015 16:19
|
|
|
Hexyflexy posted:You can talk, you thermodynamic monster! I shouldn't exist!
|
|
#
?
Feb 25, 2015 18:24
|
|
|
Hexyflexy posted:Kinda interested in this one, as I suspect you saw a whole load of physical effects going on at once. When you say 'it hung around', did the stuff freeze and crack away, or did the rippling go away, with the fluid slowly draining? To the best that you could see. It definitely did not freeze. I'm not sure what exactly the fluid was, the plane was sprayed twice, first with orange fluid, then with green fluid. The green stuff was still all over the wing when we started the take-off roll. It was dancing and rippling and waving. As far as I could tell, it sort of dripped away upward, perpendicular to the top surface of the aileron. I'd point out that the fluid only acted strangely on the aileron, on the wing surface on either side, and the flaps closer to the wing root, the anti-ice fluid just streamed off as I expected, like rain off a car window. I think I'll cross-post this to the aeronautical insanity thread too, I bet they'll at least know what the liquid is.
|
|
#
?
Feb 26, 2015 06:54
|
|
|
Hermsgervørden posted:It definitely did not freeze. I'm not sure what exactly the fluid was, the plane was sprayed twice, first with orange fluid, then with green fluid. The green stuff was still all over the wing when we started the take-off roll. It was dancing and rippling and waving. As far as I could tell, it sort of dripped away upward, perpendicular to the top surface of the aileron. I'd point out that the fluid only acted strangely on the aileron, on the wing surface on either side, and the flaps closer to the wing root, the anti-ice fluid just streamed off as I expected, like rain off a car window. I think I'll cross-post this to the aeronautical insanity thread too, I bet they'll at least know what the liquid is. iirc, one liquid is sprayed to remove ice already there, and another is sprayed to stick to the wings until the plane can get up and away from the trouble. I imagine you probably caught sight of a vortex or something going over the wing.
|
|
#
?
Feb 26, 2015 15:51
|
|
|
dupersaurus posted:iirc, one liquid is sprayed to remove ice already there, and another is sprayed to stick to the wings until the plane can get up and away from the trouble. I imagine you probably caught sight of a vortex or something going over the wing. That's right. One type is sprayed on while hot to melt existing ice. The others are basically shear-thinning fluids that stick to the wing until a certain airspeed, at which point the viscosity drops and they're blown off.
|
|
#
?
Feb 26, 2015 16:10
|
|
|
DontMockMySmock posted:A one-dimensional thing (a line) can be straight or curved, into a circle for example. But if you curve a line, doesn't that "make" it two-dimensional? Don't you have to curve it into the next dimension?
|
|
#
?
Feb 27, 2015 19:05
|
|
|
You curve it into the second dimension, but it's still infinitely thin in that dimension. If you were "on" the line, you would still only be able to move forwards or backwards. You'd just manage to arrive at the same place you started without changing direction.
|
|
#
?
Feb 27, 2015 19:12
|
|
|
Claeaus posted:But if you curve a line, doesn't that "make" it two-dimensional? Don't you have to curve it into the next dimension? Think about the number of orthogonal directions you could move if you were confined to the line. Incidentally, same reason why, for example, we say the the sphere S^2, even though it "lives" in 3-dimensional space, is a 2-D object: you're restricted to living on the surface of the sphere.
|
|
#
?
Feb 27, 2015 23:43
|
|
|
Claeaus posted:But if you curve a line, doesn't that "make" it two-dimensional? Don't you have to curve it into the next dimension? Only if you find yourself beholden to Euclidean geometry. In non-Euclidean geometry, it's perfectly sensible to talk about n-dimensional surfaces which are curved without needing to talk about n+1 dimensional space. I just used that example 'cause it's more intuitive.
|
|
#
?
Feb 28, 2015 10:08
|
|
|
Ok, it was making sense until DontMockMySmock's answer. 
|
|
#
?
Mar 1, 2015 09:16
|
|
|
Claeaus posted:Ok, it was making sense until DontMockMySmock's answer. He's making the point that when we talk about curved 2D space that doesn't necessarily suggest it's curved into a third spatial dimension. We can define a curved surface without it being embedded in a larger dimensional space.
|
|
#
?
Mar 1, 2015 17:55
|
|
|
Claeaus posted:Ok, it was making sense until DontMockMySmock's answer. Think of dimensions as how many coordinates you need to determine your position in a particular space. For a line you only need one - your displacement from the origin. Hence a circle is also only one dimension. For a plane, you need two - x and y. For a sphere, you can specify your position with latitude and longitude, so it's still two dimensions. For 3-d space, you need three coordinates. You can see the pattern from here.
|
|
#
?
Mar 1, 2015 18:38
|
|
|
Ravel posted:He's making the point that when we talk about curved 2D space that doesn't necessarily suggest it's curved into a third spatial dimension. We can define a curved surface without it being embedded in a larger dimensional space. And indeed our own universe works just that way. Our three spatial dimensions are not curved into a fourth spatial dimension.
|
|
#
?
Mar 1, 2015 18:47
|
|
|
Defining "curved space" without reference to a higher-dimensional space is basically what the "metric tensor" in GR is all about, right?
|
|
#
?
Mar 1, 2015 18:50
|
|
|
Yea, although of course this was not something Einstein invented - the mathematicians (Gauss and Riemann being the big names) has established the mathematical framework a century earlier.
|
|
#
?
Mar 1, 2015 18:56
|
|
|
http://m.space.com/28681-theory-no-big-bang.html?cmpid=514630_20150227_41253006&adbid=10152664471921466&adbpl=fb&adbpr=17610706465 I'm always wary of science and physics "journalism", but beyond the clickbaity headline this sends like a decently written article. Think it has any merit? Or am I looking at armchair-physics-theorist theory #2759026?
|
|
#
?
Mar 3, 2015 15:41
|
|
|
PerrineClostermann posted:http://m.space.com/28681-theory-no-big-bang.html?cmpid=514630_20150227_41253006&adbid=10152664471921466&adbpl=fb&adbpr=17610706465 Yeah, it was discussed in the other Physics thread from a phys.org article. It has merit http://phys.org/news/2015-02-big-quantum-equation-universe.html
|
|
#
?
Mar 3, 2015 18:35
|
|
|
QuarkJets posted:the other Physics thread Other thread on SA? Link, please (using the Awful app).
|
|
#
?
Mar 4, 2015 03:14
|
|
|
The Physics Question Thread
|
|
#
?
Mar 4, 2015 03:34
|
|
|
Thankee much!
|
|
#
?
Mar 5, 2015 01:48
|
|
|
It's been quite cold here recently, but I heard something that i thought was scientifically silly and wanted to get some confirmation. It was 15F out this morning, and then warmed up to 45F. Someone said "the temperature has tripled in only three hours!" From a physics standpoint, this is clearly untrue. If it had been -2F and cooled to -6F, the temperature also tripled. -1C to -3C, same deal. So for the temperature to actually triple, we would have to be measuring in kelvin, right? So if it's -10C out (15F), then to triple the temperature, it would have to get up to 263*3=798K, or about the temperature of red-hot iron, right?
|
|
#
?
Mar 8, 2015 04:49
|
|
|
|
| # ? Apr 25, 2024 00:25 |
|
|
babyeatingpsychopath posted:It's been quite cold here recently, but I heard something that i thought was scientifically silly and wanted to get some confirmation. That statement is not well-defined and can be interpreted in several different ways. If you're asking "What is triple the current temperature in kelvins?" then yes, you've answered your own question.
|
|
#
?
Mar 8, 2015 04:52
|
|