AA77 FDR Data, Explained
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Well, looks like my impostor has finally realized that strange taste in his mouth was his own foot. Let me therefore explain, again, for those who are interested, the actual significance of the Critical Mach Number and its relevance to civil transport.
You may recall this all started a couple of pages ago, when I tried to explain how the onset of compressibility in flight does not mean the same thing as "compression," which is one of many idiotic things said by Cap'n Robby Rob. In transonic flow, pressure can go up, and it can go down. This can mean a static probe reads artificially high or low depending on exactly what's going on with the flow. As evidence, I submitted the well-known Prandtl-Glauert Singularity, which is responsible for vapor cone formation at high subsonic speeds, and is evidence of lower than ambient pressure.
The Prandtl-Glauert Singularity arises at the Critical Mach Number. This number has a very specific physical meaning. It is the precise "speed," or Mach number, at which at some point on an airfoil, the air flowing over it is accelerated to exactly Mach 1. Thus, below the Critical Mach Number, there are no shockwaves anywhere. At or any speed above the Critical Mach Number, there will be shocks present. This marks a qualitative change in the flow behavior, hence it is a "Critical Number."
I also remarked in passing that Boeing passenger jetliners frequently cruise at or above the Critical Mach Number. I did not make a mistake. I also, incidentally, did not get that information from Wikipedia, as was alleged on AboveTopSecret by the impostor -- I learned this in my graduate studies, and confirmed it years later working with Boeing. However, it is good to see that Wikipedia has it right. It's not always or even usually wrong, just be sure to check for accuracy if it matters.
This little fact wasn't central to the discussion, so I was surprised to see Cap'n Bob jump on it. But in typical Truther fashion, he indulged in a bit of ad Hominem behavior, specifically the type known as Red Herring -- when shown to be incorrect about the possible effects of compressibility, rather than acknowledge the correct answer or improve his argument, he instead chose to raise a fuss about a nearly irrelevant subject. The reasoning, such as it is, goes like this: If he can convince the readers that I'm wrong about the cruise characteristics of Boeing aircraft, then he can convince you that everything else I say is wrong, and therefore his crackpot theories about September 11th will have a chance. This is completely stupid, of course -- the correct approach would be to find out whether the fact was true, perhaps by contacting Boeing, or if necessary enlisting the services of a transonic wind tunnel. What's at issue here is a simple, isolated fact, no more.
He also could have saved himself additional embarrassment. As noted above, he's confusing Critical Mach Number with other numbers. For example:
This is word salad. Mmo is not the Critical Mach Number. It stands for "Mach, maximum operating" and, depending on the plane, it could be above or below the Critical Mach Number. In the case of the Boeing 757, it is above. (I also note, with amusement, that this Wikipedia page vaguely confuses the two terms, so perhaps it is Cap'n Bob who learned all he knows from that source.)
Mmo is the maximum Mach number at which the aircraft is certified safe to fly. A particularly flimsy aircraft could be limited to well below the Critical Mach Number, although in such a case they probably wouldn't bother to express it. Other aircraft can travel much faster -- an F-15 fighter, for instance, has an Mmo of about 2.5 or so. This is also not limited to military aircraft. The beautiful and tragically retired Concorde, passenger liner, similarly had an Mmo greater than 2.
So what is obvious is that the Critical Mach Number is not, like was thought by the ignorati of pre-1940's, an actual "sound barrier" that cannot be crossed. Whether or not an aircraft can or should be operated beyond this point depends on other effects, like efficiency and safety. So what else can happen?
The appearance of the shock wave at the Critical Mach Number is important in two major respects: First, the shock serves as a disturbance to the airflow, much like placing a tripwire, and this can result in generation of turbulence. Second, the location of the shock can potentially move, maybe even quite a bit, depending on very small changes in the aircraft speed, attitude, and altitude.
The real question is whether the flow behind the shock remains attached, or if it separates from the wing entirely, leaving a region of stagnant air at the wing surface. If the shock is there but the flow remains attached, there's no great change in behavior, only an increase in drag since forming the shock consumes energy. Just because there's a disturbance doesn't mean the flow becomes detached. In fact, many aircraft deliberately create a turbulent boundary layer through little protrusions or wires known as turbulators. This is done because a turbulent boundary layer doesn't grow as quickly as a laminar one, and this can result in reduced overall drag.
As mentioned above, the real question is not the Critical Mach Number, but when the shockwave leads to detached flow. This point is known by a few names, such as the "Drag Divergence Mach Number," and it can be very close to Critical Mach Number, or it can be higher. Sometimes, a lot higher. This depends on wing design.
When the flow is detached, there are a couple of different effects. First, the aircraft wake grows enormously, and this shows up as drag. Lots of drag. Flying beyond this point will burn up lots and lots of fuel. The second is that detached flow leads to a loss of control authority. In the early experiments, diving P-38's and later flying the Bell X-1, this was the real killer, not breakup of the aircraft. In the old P-38's, the flat tail between the booms was in the stagnant region ("stalled") at Mach numbers as low as 0.65 Mach, preventing the pilot from combating the aircraft pitching downward as I'll explain in a moment; this put the aircraft into a dive, it picked up speed, and this frequently led to crashes. The X-1 was a much cleaner aircraft, but it too proved to be nearly uncontrollable as they approached the sound barrier. But these problems were solved. We now build fast aircraft with much larger tails and stronger control surfaces, mounted in cleaner air, enough to counteract these effects.
The other phenomenon, from above, is that the shock location can move quite a bit with minor changes to the flight condition. This can result in the lift force itself "moving" on the wing -- the center of pressure can change radically. When the center of pressure shifts rearward, this causes the aircraft to pitch down, potentially unstabilizing the aircraft. To solve this particular problem, engineers developed swept wings which make the wing appear thinner to the flow and distribute the lift over a broader longitudinal range. Other aircraft play games with fuel redistribution to change center of gravity to match. And, later, they developed supercritical airfoils.
A supercritical airfoil is one whose cross-section is designed to give good performance beyond the Critical Mach Number. Hence the name, "super" == "above" critical. Cap'n Bob seems to think that they increase the Critical Mach Number, and while some of them certainly do, that really isn't what they're for; raising the Drag Divergence Mach Number is what matters. This Dryden whitepaper describes their development, and also the aircraft they benefit. You note that the more radical supercritical wings were not used by any Boeing until the 777 -- the 757 and 767 series use a much more conservative supercritical design.
Most supercritical wings are typified by downward curvature at the lower rear of the wing, and a nearly flat surface on top. The effect on the flow is complicated, but one way to think about it is this: With the flat surface above, the airflow over the top of the wing does not have to change direction at all after it passes through the normal shock. It is therefore more likely to remain attached. Furthermore, the curvature underneath the wing gives rise to what is known as an "aft-loaded" wing, i.e. one where the center of pressure is relatively far back to begin with, and remains far back as speed increases. These are both desirable characteristics if you intend to fly beyond the Critical Mach Number.
Here is the best picture I have found of the 757 airfoil. You will note that it is not terribly bizarre. This is not the kind of wing that will delay shock formation until 0.9 Mach or thereabouts, but it does have aft-loaded behavior.
Boeings, incidentally, have cruised above Critical Mach for a long, long time. For example, the earliest of the modern jetliners is the famous 707, which first flew in 1957 and was derived from the earlier "Dash 80" which dates as far back as 1952. These dates are significant because they are only a few years after Chuck Yeager's fabled flight, and well before research into supercritical airfoils. But surprisingly, the 707 had a cruising speed of 607 MPH, at a service ceiling of 36,000 feet. This speed at that altitude translates to almost 0.92 Mach -- distinctly above the Critical Mach Number.
But how is this possible? We know the problems of controllability are solvable through better control surfaces, which the 707 had. We know we can partly mitigate the effects of compressibility through swept wings, which it also had, even without a supercritical airfoil. But there is still a cost in drag, right? Isn't this flight inefficient, at least?
Well, it's not that bad. You also need to keep in mind that the effect (so called "wave drag") is an induced drag. It only applies to the wings, except for minor other areas where the curvature of fuselage and tail and so on create little shocks of their own. Its magnitude is a function of pitch, and thus lift, rather than a constant cost like the drag off the rest of the plane. So long as the aircraft is only in cruise and not pitched up too steeply, and so long as we don't have detached flow creating a huge wake, the additional drag from compressibility is a fairly minor contributor. We're going really fast so lift also increases, meaning we lower pitch, meaning the wing appears thinner to the flow and reduces the effect. The ordinary drag already scales with the square of velocity, and this won't make it much worse.
There will be a point, of course, where you do get flow separation and drag takes off like a shot, and your aircraft will not be efficient at all, even if it's still safe to fly. But this is not the Critical Mach Number. So long as you're below this other number, the Drag Divergence Mach Number, you can fly just fine, efficiently, safely. And they do.
---
It has been remarked at AboveTopSecret that a pilot wouldn't necessarily need to know these things, any more than your average driver needs to know how a torque converter or recirculating ball steering works. In fact, a pilot really doesn't have to care about the Critical Mach Number. The pilot only needs to know what speeds are recommended and which are safe. Which makes it all the more fascinating that Cap'n Bob appeals so heavily to his "authority" in discussing aerodynamic subjects. He simply has none. As demonstrated by his own comments, he doesn't know this subject and he confuses even very simple terms, like "compressibility" and "compression."
What is more relevant, albeit not surprising, is that his entire club is no better. He claims to have a plethora of knowledgeable pilots backing him up. Well, all it would take is one phone call to Boeing, or any of his gaggle actually taking fluid mechanics courses, and he'd have known that Boeings indeed do cruise above Critical Mach. Or for just one of those pilots to have seen the effect with his or her own eyes, as the picture from WilliamSeger demonstrates. But they didn't do it. The ineptitude is staggering.
Finally, as noted over there, the argument Cap'n Bob put forth is that AA 77 was "well below Mmo," therefore its pressure altimeter could be expected to function. But this is directly at odds with his video, earlier this year, that claims AA 77 could never reach that speed in the first place, and that if it had at that altitude, it would be destroyed. Obviously conditions at that point in flight were either severe or they weren't, they cannot have been both simultaneously.
This kind of confusion is typical in the Truth Movement. If you have no theory and don't care to ever reach a conclusion, you can attempt to claim two completely different things on different days. The goal is to confuse and to include as many alternate viewpoints as possible. This is the exact opposite of science, where noncompetitive hypotheses are discarded, and the solution is as parsimonious as possible.
In closing, it's another banner day for the Truth Movement. This kind of ignorance and obnoxious behavior, to say nothing of the sheer creepiness of pretending to be me while spreading his idiocy, is why Cap'n Bob and his conspiracy theories will never amount to anything.
You may recall this all started a couple of pages ago, when I tried to explain how the onset of compressibility in flight does not mean the same thing as "compression," which is one of many idiotic things said by Cap'n Robby Rob. In transonic flow, pressure can go up, and it can go down. This can mean a static probe reads artificially high or low depending on exactly what's going on with the flow. As evidence, I submitted the well-known Prandtl-Glauert Singularity, which is responsible for vapor cone formation at high subsonic speeds, and is evidence of lower than ambient pressure.
The Prandtl-Glauert Singularity arises at the Critical Mach Number. This number has a very specific physical meaning. It is the precise "speed," or Mach number, at which at some point on an airfoil, the air flowing over it is accelerated to exactly Mach 1. Thus, below the Critical Mach Number, there are no shockwaves anywhere. At or any speed above the Critical Mach Number, there will be shocks present. This marks a qualitative change in the flow behavior, hence it is a "Critical Number."
I also remarked in passing that Boeing passenger jetliners frequently cruise at or above the Critical Mach Number. I did not make a mistake. I also, incidentally, did not get that information from Wikipedia, as was alleged on AboveTopSecret by the impostor -- I learned this in my graduate studies, and confirmed it years later working with Boeing. However, it is good to see that Wikipedia has it right. It's not always or even usually wrong, just be sure to check for accuracy if it matters.
This little fact wasn't central to the discussion, so I was surprised to see Cap'n Bob jump on it. But in typical Truther fashion, he indulged in a bit of ad Hominem behavior, specifically the type known as Red Herring -- when shown to be incorrect about the possible effects of compressibility, rather than acknowledge the correct answer or improve his argument, he instead chose to raise a fuss about a nearly irrelevant subject. The reasoning, such as it is, goes like this: If he can convince the readers that I'm wrong about the cruise characteristics of Boeing aircraft, then he can convince you that everything else I say is wrong, and therefore his crackpot theories about September 11th will have a chance. This is completely stupid, of course -- the correct approach would be to find out whether the fact was true, perhaps by contacting Boeing, or if necessary enlisting the services of a transonic wind tunnel. What's at issue here is a simple, isolated fact, no more.
He also could have saved himself additional embarrassment. As noted above, he's confusing Critical Mach Number with other numbers. For example:
SourceCap'n Bob said:
This is word salad. Mmo is not the Critical Mach Number. It stands for "Mach, maximum operating" and, depending on the plane, it could be above or below the Critical Mach Number. In the case of the Boeing 757, it is above. (I also note, with amusement, that this Wikipedia page vaguely confuses the two terms, so perhaps it is Cap'n Bob who learned all he knows from that source.)
Mmo is the maximum Mach number at which the aircraft is certified safe to fly. A particularly flimsy aircraft could be limited to well below the Critical Mach Number, although in such a case they probably wouldn't bother to express it. Other aircraft can travel much faster -- an F-15 fighter, for instance, has an Mmo of about 2.5 or so. This is also not limited to military aircraft. The beautiful and tragically retired Concorde, passenger liner, similarly had an Mmo greater than 2.
So what is obvious is that the Critical Mach Number is not, like was thought by the ignorati of pre-1940's, an actual "sound barrier" that cannot be crossed. Whether or not an aircraft can or should be operated beyond this point depends on other effects, like efficiency and safety. So what else can happen?
The appearance of the shock wave at the Critical Mach Number is important in two major respects: First, the shock serves as a disturbance to the airflow, much like placing a tripwire, and this can result in generation of turbulence. Second, the location of the shock can potentially move, maybe even quite a bit, depending on very small changes in the aircraft speed, attitude, and altitude.
The real question is whether the flow behind the shock remains attached, or if it separates from the wing entirely, leaving a region of stagnant air at the wing surface. If the shock is there but the flow remains attached, there's no great change in behavior, only an increase in drag since forming the shock consumes energy. Just because there's a disturbance doesn't mean the flow becomes detached. In fact, many aircraft deliberately create a turbulent boundary layer through little protrusions or wires known as turbulators. This is done because a turbulent boundary layer doesn't grow as quickly as a laminar one, and this can result in reduced overall drag.
As mentioned above, the real question is not the Critical Mach Number, but when the shockwave leads to detached flow. This point is known by a few names, such as the "Drag Divergence Mach Number," and it can be very close to Critical Mach Number, or it can be higher. Sometimes, a lot higher. This depends on wing design.
When the flow is detached, there are a couple of different effects. First, the aircraft wake grows enormously, and this shows up as drag. Lots of drag. Flying beyond this point will burn up lots and lots of fuel. The second is that detached flow leads to a loss of control authority. In the early experiments, diving P-38's and later flying the Bell X-1, this was the real killer, not breakup of the aircraft. In the old P-38's, the flat tail between the booms was in the stagnant region ("stalled") at Mach numbers as low as 0.65 Mach, preventing the pilot from combating the aircraft pitching downward as I'll explain in a moment; this put the aircraft into a dive, it picked up speed, and this frequently led to crashes. The X-1 was a much cleaner aircraft, but it too proved to be nearly uncontrollable as they approached the sound barrier. But these problems were solved. We now build fast aircraft with much larger tails and stronger control surfaces, mounted in cleaner air, enough to counteract these effects.
The other phenomenon, from above, is that the shock location can move quite a bit with minor changes to the flight condition. This can result in the lift force itself "moving" on the wing -- the center of pressure can change radically. When the center of pressure shifts rearward, this causes the aircraft to pitch down, potentially unstabilizing the aircraft. To solve this particular problem, engineers developed swept wings which make the wing appear thinner to the flow and distribute the lift over a broader longitudinal range. Other aircraft play games with fuel redistribution to change center of gravity to match. And, later, they developed supercritical airfoils.
A supercritical airfoil is one whose cross-section is designed to give good performance beyond the Critical Mach Number. Hence the name, "super" == "above" critical. Cap'n Bob seems to think that they increase the Critical Mach Number, and while some of them certainly do, that really isn't what they're for; raising the Drag Divergence Mach Number is what matters. This Dryden whitepaper describes their development, and also the aircraft they benefit. You note that the more radical supercritical wings were not used by any Boeing until the 777 -- the 757 and 767 series use a much more conservative supercritical design.
Most supercritical wings are typified by downward curvature at the lower rear of the wing, and a nearly flat surface on top. The effect on the flow is complicated, but one way to think about it is this: With the flat surface above, the airflow over the top of the wing does not have to change direction at all after it passes through the normal shock. It is therefore more likely to remain attached. Furthermore, the curvature underneath the wing gives rise to what is known as an "aft-loaded" wing, i.e. one where the center of pressure is relatively far back to begin with, and remains far back as speed increases. These are both desirable characteristics if you intend to fly beyond the Critical Mach Number.
Here is the best picture I have found of the 757 airfoil. You will note that it is not terribly bizarre. This is not the kind of wing that will delay shock formation until 0.9 Mach or thereabouts, but it does have aft-loaded behavior.
Boeings, incidentally, have cruised above Critical Mach for a long, long time. For example, the earliest of the modern jetliners is the famous 707, which first flew in 1957 and was derived from the earlier "Dash 80" which dates as far back as 1952. These dates are significant because they are only a few years after Chuck Yeager's fabled flight, and well before research into supercritical airfoils. But surprisingly, the 707 had a cruising speed of 607 MPH, at a service ceiling of 36,000 feet. This speed at that altitude translates to almost 0.92 Mach -- distinctly above the Critical Mach Number.
But how is this possible? We know the problems of controllability are solvable through better control surfaces, which the 707 had. We know we can partly mitigate the effects of compressibility through swept wings, which it also had, even without a supercritical airfoil. But there is still a cost in drag, right? Isn't this flight inefficient, at least?
Well, it's not that bad. You also need to keep in mind that the effect (so called "wave drag") is an induced drag. It only applies to the wings, except for minor other areas where the curvature of fuselage and tail and so on create little shocks of their own. Its magnitude is a function of pitch, and thus lift, rather than a constant cost like the drag off the rest of the plane. So long as the aircraft is only in cruise and not pitched up too steeply, and so long as we don't have detached flow creating a huge wake, the additional drag from compressibility is a fairly minor contributor. We're going really fast so lift also increases, meaning we lower pitch, meaning the wing appears thinner to the flow and reduces the effect. The ordinary drag already scales with the square of velocity, and this won't make it much worse.
There will be a point, of course, where you do get flow separation and drag takes off like a shot, and your aircraft will not be efficient at all, even if it's still safe to fly. But this is not the Critical Mach Number. So long as you're below this other number, the Drag Divergence Mach Number, you can fly just fine, efficiently, safely. And they do.
---
It has been remarked at AboveTopSecret that a pilot wouldn't necessarily need to know these things, any more than your average driver needs to know how a torque converter or recirculating ball steering works. In fact, a pilot really doesn't have to care about the Critical Mach Number. The pilot only needs to know what speeds are recommended and which are safe. Which makes it all the more fascinating that Cap'n Bob appeals so heavily to his "authority" in discussing aerodynamic subjects. He simply has none. As demonstrated by his own comments, he doesn't know this subject and he confuses even very simple terms, like "compressibility" and "compression."
What is more relevant, albeit not surprising, is that his entire club is no better. He claims to have a plethora of knowledgeable pilots backing him up. Well, all it would take is one phone call to Boeing, or any of his gaggle actually taking fluid mechanics courses, and he'd have known that Boeings indeed do cruise above Critical Mach. Or for just one of those pilots to have seen the effect with his or her own eyes, as the picture from WilliamSeger demonstrates. But they didn't do it. The ineptitude is staggering.
Finally, as noted over there, the argument Cap'n Bob put forth is that AA 77 was "well below Mmo," therefore its pressure altimeter could be expected to function. But this is directly at odds with his video, earlier this year, that claims AA 77 could never reach that speed in the first place, and that if it had at that altitude, it would be destroyed. Obviously conditions at that point in flight were either severe or they weren't, they cannot have been both simultaneously.
This kind of confusion is typical in the Truth Movement. If you have no theory and don't care to ever reach a conclusion, you can attempt to claim two completely different things on different days. The goal is to confuse and to include as many alternate viewpoints as possible. This is the exact opposite of science, where noncompetitive hypotheses are discarded, and the solution is as parsimonious as possible.
In closing, it's another banner day for the Truth Movement. This kind of ignorance and obnoxious behavior, to say nothing of the sheer creepiness of pretending to be me while spreading his idiocy, is why Cap'n Bob and his conspiracy theories will never amount to anything.
funk de fino
Dreaming of unicorns
I had to sit through hours of this stuff in my tech training. *shudders*
jaydeehess
Penultimate Amazing
Most of us find it nigh on impossible to hold to two mutually exclusive positions at the same time.
My experience is that this does not present the normal logical problem with members of the 911 conspiracy crowd.
Pinch
Critical Thinker
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Last edited:
jhunter1163
beer-swilling semiliterate
R. Mackey, puttin' the E in JREF since 2006.
Furcifer
Guest
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- 13,797
No foolin. It's nice too see someone is actually putting some thought into these birds. You get on one and you can't help but think they've just been slapped together by the lowest bidder
What's funny, or sad depending on how you look at it, is how he tired to hijack a guitar newsgroup with his wackiness.
http://www.ccs.neu.edu/home/will/Music/Jokes/Balsamo/balsamo.html
If he made that claim on the PfT forum you know he deleted that post by now. It would be incredibly stupid of him not to. Making the claim that the Pentagon was hit by a thirty foot long bomb droped by a C-130 packed with 18000lbs of explosives tips the scales in the idiocy department.
Can you imagine the destruction a bomb like that would have done to the Pentagon and the surrounding area? Sorry Bob you are once again guilty of an epic fail!
WildCat
NWO Master Conspirator
- Joined
- Mar 23, 2003
- Messages
- 59,856
Imagine dropping 18,000 lbs of explosives from tree-top level. Your plane would disintegrate in the blast.
When low altitude bombing runs were made the bombs have parachutes or large fins on them to slow them down enough for the airplane to get away before the blast.
A MOAB from 4 feet over your target?
Klimax
NWO Cyborg 5960x (subversion VPUNPCKHQDQ)
Is MOAB = Mother of all bombs?
WildCat
NWO Master Conspirator
- Joined
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- Messages
- 59,856
Yes.
jaydeehess
Penultimate Amazing
an 18000lb bomb dropped on the Pentagon would have also damaged structures as close as the gas station(though as it sits behind an embankment it may be sheltered.
however, those driving along nearby roads would not have been so lucky. A blast like that would have been quite noticiable to say the least.
Of course Cappy Robby may have meant a smaller bomb, perhaps only the Younger Sister of All Bombs, the YSOAB
however, those driving along nearby roads would not have been so lucky. A blast like that would have been quite noticiable to say the least.
Of course Cappy Robby may have meant a smaller bomb, perhaps only the Younger Sister of All Bombs, the YSOAB
D'rok
Free Barbarian on The Land
- Joined
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- 6,399
The PfT FAIL device includes an optional, but very important preliminary component: the Electronic Pulse Initiation Controller.
I have never seen a MOAB explode, but I have seen several 3,000 Pounders explode in dense jungle. The shock wave radius is easily 1/2 mile or more.
FYI, IIRC, the minimum delivery altitude/speed for a Mk 82 Slick 500 lb bomb (no drag device) is about 1,000', 480 Kts +. The MOAB is typically delivered from a C-130 from about 10,000'.
FYI, IIRC, the minimum delivery altitude/speed for a Mk 82 Slick 500 lb bomb (no drag device) is about 1,000', 480 Kts +. The MOAB is typically delivered from a C-130 from about 10,000'.
twinstead
Penultimate Amazing
- Joined
- Apr 8, 2005
- Messages
- 12,374
Well, I'm afraid with the NWO in control of the EPIC FAIL device, we are all doomed
Klimax
NWO Cyborg 5960x (subversion VPUNPCKHQDQ)
I think I have seen a video of its explosion. I think that still today we would see the outcome of its use...