First of all, I'd like to thank Speedracer1407 for starting this thread, it's my favorite subject and I love genuine curiosity. Second, a thank you to everyone else, I never realized that ya'll respect what I had to say, per se.
|Quoting Jush (Reply 6):
Might have something to do with the fact that he flew the bloody gorgeous thing.
Nope. I did mention here once that I had a classmate that claimed to be a retired BA
Concorde pilot back the mid-90's. He told me some pretty cool stuff, though I was too young to fully appreciate it.
|Quoting EGTESkyGod (Reply 7):
I think GDB might know a helluva lot more when it comes to Concorde.
Well he worked with her for years, which is admirable enough; he's the first member on my RU
list when I found these network forums. But he can be a stick in the mud when it comes to his pessimism regarding a next gen product...but then I'm the one with the relentless optimism, so that has got to be annoying for someone.
Sorry for being late; I had to sleep, wake up for class and go on with my day. Anyway...maybe I should have said I be back in 22 hours...? The following took me 5 hours to type as it is from memory and half are part of my own theories on sonic boom mitigation - a subject so new that there are no books on it, I've had to teach myself via thought experiments over the past 6 years. Even 2 years ago, I postulated on ideas about killing off the sonic boom in a thread here in the techops, I think it was April 2004? Here's the link, knock yourselves out: http://www.airliners.net/discussions...ech_ops/read.main/87606/6/#ID87606
well first it's not like subsonic wakes, per se, the definition changes (more than likely others will know it by a different term). Subsonic wakes are primarily wing induced (partially fuselage) and are strongest during low-speed like in and around airports. Imagine a sheet of cardboard, the amount of air you can disturb increases as you angle the sheet up. When the aircraft speeds up however, the 'wake intensity' drops because the wing's angle to the wind drops down. Again think of the cardboard sheet as you angle it almost flat. You have to move it faster to support the force you did before.
Now we go even faster.
Supersonic/hypersonic wakes have little to do with the induced flow; in fact they eventually get cut off. Their wakes root from a different source: ram-air. I'll get back to that later.
First: the subsonic effect of the traditional SST.
Delta wings, like those on Concorde, produce what is known as vortex lift. At this point the wing is like a giant version of an engine cowling fin. Those swirls that flow around most subsonic wing tips instead flow around and over the wing's entire leading edge as shown in photo below:
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Photo © Joe Pries - ATR Team
The way a wing lifts is due to the low pressure on top and high pressure underneath. The effect of that higher pressure air wanting to go around and into the lower pressure region creates those swirls.
When a plane goes past the speed of sound, it must push air out of the way that cannot move faster than the speed of sound, so they form a wave, many in fact, like from a boat. At Mach 2, a plane like Concorde spends 1/10th of one second traveling its own length. In that time, what was air below freezing spikes up past the boiling point of water and the ambient air pressure multiplies by 8 in places. That is the condition of the wake just behind Concorde: hot and thick air, but it doesn't stay that way. It drops in pressure and cools rapidly but not all is recovered. Perhaps two or three lengths of Concorde behind her, the air returns to ambient, for the most part. But the shock wave is expanding away from the plane.
Third: The boom
When a boat travels it pushes all of the water underneath and to the side, I refer to this volume change as displacement, it probably has another name. For the boat, it would be the distance from the keel to the water line times the area of the boat, but this would be an initial volume as it doesn't take into account how fast it's moving. Ever stand on a street corner and a bus or large truck passes you by and feels this gust of air? I refer to that gust as a displacement; I mean if you were underwater and a large boat passed over, you will be pushed down, I guarantee it.
Well a supersonic airplane also displaces air, and those air pressure ripples travel down while still expanding, push at your eardrums; you hear a sonic boom. Luckily though, you are not hearing the entire boom, maybe about 30% or less.
This is a fact: Air pressure acts normal to a surface, which is science jargon for perpendicular. On an sonic airplane, while it flies in a volume of hot-thick air, that pressure reflects away from the plane, in all directions from all surfaces including wings (both sides), fins, fuselage, engines -- everything. Imagine a series of shells shaped like Concorde expanding away from the plane, these shell waves ripple out and expand backward, upwards, and downward and all shape the same point, the nose. I guess if you took a model of Concorde and dipped it into a pool of water; you could see the 2D
version of all these shells in the water ripples. The fuselage would have a circular expansion, the wings/fins would start off with flat wall expansions, the leading edge of the wings would reveal half-moon expansions, etc.
But see we the people of Earth only care about what goes down, so for planes like Concorde, her 3800 sq. ft. wing against her nearly 11,000 total surface areas, comes out to about 30%. That pressure starts out at approximately 1500psf (the weight of the plane plus the pressure after the oblique waves times the total surface area all divided by the total area) and depressurizes while it expands. But like I said, it doesn't go to ambient, by the time that wave hits your ears, and you experience 2 psf of 'boom pressure' for a plane like Concorde.
In more than 10 miles of altitude the pressure dropped by over 700 times. From my own calculations, I've fit an approximate curve for the pressure change from altitude tables as natural logarithms, for simplicity. Based on that, I have concluded that the sonic pressure boom decays on a similar curve. That if you were at FL300, you would experience approx 80 psf boom from a M2 plane flying @ FL528. But because you may be in a large airplane, you would not notice it. There isn't enough of an energy transfer in terms of momentum from a thin shockwave washing over another airplane, you wouldn't jolt. The reason windows and houses shake is because they are smaller and absorb some of the energy. Even if it were another Concorde passing by, the shockwaves go through each other; the planes will not be affected.
If the separation was within a mile...I'll theorize a subtle crosswind situation.
NOTE: the above conclusions are assuming the observer is directly below the flight path. Deviating from this will drop the pressure more as the wave would travel farther.
NOTE: I'm not done; I haven’t even got to the good part.
I have a test on Friday and I want to spend all day tomorrow studying. I hope this will be entertaining, I hope ya'll have questions, comments, corrections if any, complaints, etc. Well, I don’t hope for complains but we cannot please everybody. OTOH, it is my life goal to create a cheap (fare-wise, not a piece of sh*t) SST. I should be back here Friday afternoon (local time here is 200am Pacific Standard in USA)
[Edited 2006-03-09 10:05:37]
The meaning of life is curiosity; we were put on this planet to explore opportunities.