Response From the REAL Ryan Mackey
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:
Originally Posted by Cap'n Bob
I still see he hasn't learned the definition of supercritical either and that .70 - .72 Mach is no where near Mcrit for a 757, therefore,
"compressibility" is not a problem for the 757 at such Mach speed, after all Mmo is .86 on the 757.
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
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
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.