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| A Physicist's Take on the Plane Crash | |
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| Topic Started: Oct 6 2009, 04:56 PM (783 Views) | |
| shure | Oct 6 2009, 04:56 PM Post #1 |
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Hi, (I leave it up to you to quote chosen excerpts of this elsewhere, but don't be afraid to quote a bit more and/or to explicitly state that the quotes are taken from a somewhat longer piece. Otherwise the readers may find my points lacking and the whole argument will be all the less credible. Disclaimers for the win.) _______________ It's odd how people keep referring to the planes as "prevailing" in the impacts. The towers stood through the impacts and would have stood forever if it wasn't for their improbable "collapse". As for the planes, they were quite thoroughly destroyed and quite effectively halted in the process. I mean, look at that Phantom jet crashing into a solid wall ( http://www.jokeroo.com/video/extreme/plane-vs-wall.html )... Clearly the plane doesn't "prevail" in this case: it may appear to "pierce" through the wall "like knife through butter", but what it actually does is "splash" against that wall, pulverized inch by inch as it goes. The incoming parts of it remain virtually intact for longer than one could expect, but for every section of the plane there comes a moment where it reaches the wall, and from then on that part of the plane is history. Halfway through the impact, for example, it would be wrong to look at the tail and infer the position of the nose as being deep inside the wall: the nose simply doesn't exist at that point, it's already been blown to smithereens. So in this case, clearly the plane doesn't penetrate the wall in any way, instead it is thoroughly destroyed, "disappears into dust" as they put it. To be fair, the "dust" we see is just the lighter and faster kind of debris: the impact may have produced somewhat larger chunks, but they travel slower than the dust and so can't be seen in the video. As for the wall, it essentially stood its ground, although it may have to be replaced after such an incident. Note that it's a "smart wall", so it is actually designed to give way a bit, just like the structure of modern cars and trains and such. So it's not like it's intact or merely dented, either. (The most relevant difference between that video and 9/11 is that the towers had a lot of solid obstacles such as slabs of concrete, but also a lot of hollow spaces in between. This means that the debris had much of an opportunity to continue flying in the general direction of the initial impact, whereas a solid wall would have deflected all the junk sideways or backwards. So the planes were still "atomized" upon impact, gradually, just like the F4 here - but unlike here the debris mostly went into the building, plowing through the cluttered office floors, possibly hitting the core, and eventually either coming to a halt or breaking out the other side, like the infamous engine of the South Tower plane.) Back to your fundamental question about speed. First of all, the plane was definitely NOT "somehow harder because it was moving fast and the building wasn't". The relevant parameter of the impact is the RELATIVE velocity of both objects: in the big scheme of things, maybe the plane was moving backwards at 200 mph through the solar system and the building was chasing it at 700 mph. Thus the velocity can be distributed differently between the objects, depending on the frame of reference, and so the relative 500 mph figure has no reason whatsoever to specifically harden the plane with respect to the building, or vice-versa. On a more important note, neither the plane nor the building are homogeneous solid objects, and so it's not just about the raw mechanical properties of either steel and aluminium, such as hardness or tensile strength. There are complex structures colliding: both the plane and the building are engineered assemblies of metal beams, designed with certain operating conditions in mind. The detailed behavior of these structures during the impact will be determined not only by material properties, but also by the size, shape and orientation of the structures. It is with all these characteristics in mind that the role of the velocity can be determined. In other terms, it's not as clear-cut as "a big chunk of steel VS a big chunk of aluminum": in order to analyze the impact you have to take a closer look at the colliding structures. I've addressed this in my writeups before, and the key point is that stress and deformation propagate through the materials and structures at a certain velocity (it's not as simple as the speed of sound in either material; there are structural considerations too). At low impact speeds, the stress has the time to propagate from the point of impact throughout either object: either it concentrates in some kind of weak spot, causing irreversible deformation (mechanical damage and failure); or it ends up stored in the whole structure, reversibly, in the form of elastic energy - this elastic energy can then be released when the impacting objects bounce off each other, or as they vibrate/resonate in the aftermath of the collision. Conversely, if the impact velocity is high enough, then the stress and deformation will fail to propagate through the structure much or at all - in this case, material or structural failure can only occur in the close vicinity of the point of impact, whereas on a larger scale the structures will remain intact. From here on, the 9/11impacts have two very distinct features that should be addressed separately. First, on a large scale, the rear part of the plane sustains no visible deformation as the front part impacts the tower and is supposedly shredded to bits. Second, on a small scale, it appears that the fuselage and wings have (respectively) punched and cut through the perimeter columns. As for the first point, that Phantom-jet-VS-wall video is enough to provide a basic understanding. Whatever happens upon impact (the wall being dented or the plane being crushed), the tremendous stress and deformation do not have the time to propagate through the plane backwards like they would for a slower impact velocity. Thus the collision is all the more violent as it's localized at the current point of impact, whereas the incoming rear part is virtually intact and mostly doesn't even "know" that something unspeakably horrible is happening up front. It is fair to think of stress and deformation as elementary "information" about the progressive impact with the wall - this information fails to travel across the structure of the plane, or rather is beaten to it by the progressive impact itself. As for the second point, we must have a closer look at just what kind of structures are colliding, on the small scale. As an example, let's consider a wing impacting the perimeter columns - and for the sake of the argument let's assume that the plane is a regular jetliner, i.e., that the wings were not reinforced in any way, nor were they fitted with shaped charges or such. Above we noted that stress and deformation essentially fail to propagate through the plane (or building), in regard of the large velocity of the impact. This means that the outcome of the impact between the wing and the columns is decided locally, on the scale of no more than, say, a couple of feet. At every point of impact we basically have a one-on-one encounter between a single aluminium beam and a single steel beam, both of them a couple of feet long (technically those foot-long beams are an integral part of larger structures, but since they don't have the time to share the stress around, they're essentially on their own). Now what can we say at this point? A vertical steel beam and a horizontal aluminium beam are hurled at each other at 500 mph, who wins? The answer is, we don't know. The material properties of both steel and aluminium are relevant, but the outcome of the impact also depends on the size and type of either beam. What if the steel "beam" is a guitar string? clearly it doesn't have much of a chance against the wing of a plane (although technically it's harder than aluminium, and will thus leave a scratch on the wing). So, structure matters just as much as material here, and steel can definitely "lose the fight", i.e., it can fail as a structure, despite its higher "hardness" and tensile strength. Like I said before, the problem is more subtle than two large chunks of metal crashing together. In the case of 9/11, as far as I know, the perimeter columns were not regular beams but essentially boxes, with a square-foot hollow section surrounded by inch-thick walls (roughly). This is well-adapted to the task of supporting and redistributing top-down stress, in the normal operating conditions of the building. However, it is possible that the rather thin walls of the box make such a beam quite vulnerable to being hit from the side, in a cutting motion. Note that even though the perimeter columns are multiply redundant (i.e., in a hurricane they will work together very effectively, redistributing the stress among them as needed), the wing hits them one by one, andvery quickly. So what really matters here is how strong a single column is, and how much of a cut it can take from an aircraft wing. On the other hand, we have an airplane longeron (that's what the thicker structural beams of a plane are called), which is responsible for the stiffness of the wing, in a variety of operating conditions; when the aircraft is grounded, the fuselage is propped up and the wings would fall off if it wasn't for the longerons; when airborne, the role is reversed, because the wings generate lift and it's the fuselage that weighs them down; the longeron must also cope with the engine pod pulling forwards, and the rest of the wing being subject to drag. All in all, the aluminium beams used in wings are apparently quite resistant to all kinds of bending stress, and by inference they'd do well in "cutting" situations too. For example, suppose such a wing hits a road sign pole or a metal-tube lamp post, which is designed to support just its own weight and then some (safety margin), in an upright position, with moderate wind. I expect the aircraft wing to do quite well in a high-velocity collision against such a pole. This is where intuition fails me, as far as 9/11 is concerned. I have no clear idea of the structures used in the construction of the planes (be it the Boeings from the official story or whatever they really were). I also don't know much about the material properties of the actual flavors of steel and aluminium that we are looking at. Therefore I can not make a definitive statement about what really happened to the wings and columns (and fuselage) upon impact. Maybe a regular airframe could do the trick, and maybe it couldn't. Since there is no evidence that the planes were regular Boeing 767s, that's a moot point. It was certainly possible for SOME kind of plane to puncture the perimeter, cutting every column across the wingspan and punching a bigger hole in the middle where the fuselage hit. Then again, it was not necessary for the wings to consistently cut through every column, neither have I seen thorough evidence that they did. Suppose that some column or other was merely dented and bent but not broken by the impact; or that it DID break, but that the eventual gap was, say, a couple of inches wide, in other words not enough to let the whole wing pass. What then? nothing special, really; the wing would take a lot of damage itself, and then it would fly past the column, left and right rather than through the gap, and into the building. At any rate, there are trusses and corrugated steel and concrete right behind the columns, and that's something the wings just can't keep winning against. So it's not like the aluminium has to beat the steel beam "hands down" here; it's OK if a wing section is cut by a column and flies in through the adjacent windows, merely denting the column in the process; on the pictures of the aftermath, you can't really tell the difference. Keep in mind that the fuselage impact is the only place where you can see anything like a "gaping hole" (and even so it's not really gaping because the floors are still there, mostly). Across the wingspan, what is seen from a distance (and often mistaken for a hole) is the outline of the missing aluminium cladding that fell off the steel beams shortly after the impact - the beams themselves are mostly intact and the actual impact points (dents and gaps and such) can only be resolved on a few extreme closeups shot from helicopters. There, hope this helps, and maybe I'll forward some more emails to you if you still have questions afterwards. _______ Serguei Edited by shure, Oct 7 2009, 01:11 AM.
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| achimspok | Oct 6 2009, 11:49 PM Post #2 |
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Wow, thanks Jeff, that's a very good article. One remaining question (or two): The Perdue University tested an aluminum projectile filled with water a shot against a steel plate. The result of that test was used as an input for a computer simulation. Perdue says that the result of the simulation including the bowing and bending was very much the same as the observable damage. I conclude the data Perdue used were precise. The experiment (e.g. projectile, steel plate) used a very different "structure". Therefore I guess that the structure doesn't play any or just a minor role. May be the described propagation of energy through the structure was not 1 foot but zero. In other words, I could imagine that at a certain velocity the destruction is faster than the propagation of energy through the material. The effect I have in mind would be comparable with breaking through the sound barrier - some kind of stationary summation of energy like an "energy wave" that passes through the material. May be that's why the Perdue University described the result of the experiment as a "fluid" behavior of the steel. ... I (and may be no non-physician) would have expected such a behavior. But if that behavior occurred then possibly the material (as well as the structure) is irrelevant as long as enough material (as a source of energy) meet each other above some threshold speed. Is such behavior known? On the other hand, when I look at the photographs of the impact zone there is no exterior damage that looks like "fluid" behavior. Nevertheless the Perdue simulation allegedly was very precise. What's the problem here that I apparently missed? Edited by achimspok, Oct 6 2009, 11:53 PM.
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| broken sticks | Oct 7 2009, 12:42 AM Post #3 |
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Yeah, seriously - great post jeff. right on. the more i think about trying to model the impact in a series of images, the more i think "this would look a lot like purdue's if it was in 3D". You're right. I mean, the velocity of the destruction can totally be faster than the energy wave of the impact. I think the force of the impact affects the amplitude of the wave, not the velocity of the wave (which probably depends on the materials involved). d'you mean like, at a high enough speed, anything could be made to disintegrate in a collision, or rather anything could be made to appear as if it was melting into something? you're right i think if you mean either of those cases - i might have misunderstood though lol what sort of signs are you thinking about? Edited by broken sticks, Oct 7 2009, 12:44 AM.
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| elephant room | Oct 7 2009, 03:54 AM Post #4 |
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I though this was a good point made by genghis: nice sound track ... that couldnt be the real entry path, could it? Edited by elephant room, Oct 7 2009, 03:55 AM.
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| broken sticks | Oct 7 2009, 04:47 AM Post #5 |
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i think that simulation is for the 1st impact, but i thought the tail looked as though it had dropped slightly on one or two frames of the 2nd impact somewhere... maybe the last frame or two of spiegel, i'll go have a look. anyway, i guess what purdue are saying is that there was enough structural integrity in the rear half of the plane for it to hold together as it hits the floor at an angle, whilst inside the building. it seems improbable to me, plus i'm not sure the fuselage would have remained so intact as far into the building as it does in the purdue simulation. purdue appear to have the nose making the large hole straight away, and then the whole fuselage going through this hole. but i'd have expected the hole to start about much smaller that that, and be made bigger progressively as the whole length of fuselage causes more and more damage around the hole. are purdue the guys that put the weight of the plane in a sausage-skin shape for their model? that would explain this little issue i guess. once the wings hit, then there's no lift on the back, so it would have nothing to make it defy gravity from that point, technically making it drop. but i'd expect this amount to be very small lol i'll see if i can do the maths at this late hour... gravity is 9.81m/s/s we're talking about probably 3 frames of 30fps video, which is about 0.1 seconds. so does that mean it would have been accelerated vertically to a rate of about 0.981m/s by the time its all in the building? i think so.... alright, that would mean the tail would only drop due to gravity by an amount around 10cm before its all gone. purdue's model has a lot more than a 10cm drop for flight 11's tail. i guess they must really think the plane rubbed up against the floor like that, causing the tail to drop. doesn't sound right to me though. |
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| achimspok | Oct 7 2009, 12:29 PM Post #6 |
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If the steel behave fluid the cuts should look like molten. So I can Imagine the effect probably due to the crystalline structure and some increasing amplitude of energy that might reach a point to cause fluid behavior. These cuts should look very different to broken columns. I'm not sure at the moment but I think that NIST allegedly collected some impacted columns.
I think it could. You should have a look at the view from the side. The plane was on a downwards path. I don't know the angle used in the simulation but the upper part of the fuselage skids on the floor and was forced into a horizontal path. That would turn the tail down (if enough time for the force to propagate to the tail). Purdue Sim Hmm, ... what did the Purdue University? ...fluid behavior or enough time to turn the tail down? Edited by achimspok, Oct 7 2009, 12:41 PM.
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| achimspok | Oct 7 2009, 02:50 PM Post #7 |
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 I was wrong! This is what the National Geographic anchor says: "To model what this fuel load will do upon collision Purdue launched fluid filled aluminum cans to simulate an airplane wing impacting a steel column. A thermal camera captured the impact and fluid behavior. This raw data was also added into the sequence." So it seems she talks about the "fluid behavior" of the fluid fuel. Interesting anyway: the steel at this point in time shows the imprint of the aluminum projectile as if there isn't any deformation to the aluminum. So that steel really acting in some unexpected way. Any idea?   |
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| RasgaSaias | Oct 7 2009, 03:19 PM Post #8 |
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We would need to know how were those images obtained. What sort of equipment was used to record that. Maybe there's something surrounding the projectile we don't see. Because the steel, apparently, and I'm guess this is just because we're not looking at a normal image, starts deforming before being touched by it. But the steel also seems to recover its shape by sealing the hole when the projectile is already on the other side. Which is strange. We're missing something to understand what we're looking at. Edited by RasgaSaias, Oct 7 2009, 03:32 PM.
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| achimspok | Oct 7 2009, 07:20 PM Post #9 |
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  For me it looks like one of those projectiles shot against steel. That's what they say. If the shot it with a thermal camera and the projectile had air temperature then it would be invisible. Nevertheless, the hole in the steel has the shape of the aluminum projectile. Hence, the steel indeed acting like fluid. The aluminum acting like solid. Something is strange. |
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| broken sticks | Oct 7 2009, 09:52 PM Post #10 |
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hey, if the projectile is making a hole in something, we're seeing the steel on the other side of the hole maybe. |
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| RasgaSaias | Oct 7 2009, 09:57 PM Post #11 |
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That's why I believe this is some kind of special imaging technique. I don't know what it is though. But there seems to be something with the same color as the background around the the projectile. Edited by RasgaSaias, Oct 7 2009, 09:59 PM.
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| achimspok | Oct 8 2009, 12:50 PM Post #12 |
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same color means same temperature. The projectile is invisible. You see the steel and the fluid inside of the projectile. But you are right, they seemingly used something like tomography to visualize just one thin layer trough the material. |
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| broken sticks | Oct 8 2009, 01:57 PM Post #13 |
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a couple of regular videos from either side of the hole would be good. |
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| acebaker | Oct 9 2009, 04:49 PM Post #14 |
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Thermal imaging does not allow you to see inside of an object, it is not an Xray. I have a fair amount of experience looking at high quality thermal images from a FLIR thermacam. I have never seen a solid object become invisible. Chairs, tables, etc., room temperature things absolutely show up. Air temperature is irrelevant, you do not see the air in a thermal image. You still need an object off of which to reflect the infra-red waves, just like visible light. |
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| achimspok | Oct 9 2009, 04:59 PM Post #15 |
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Hmm, sounds plausible. The anchor said that they used thermal imaging. But as I mentioned, it looks more like some kind of tomography like magnet spin tomography or something like that. |
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