Jim, Thanks for clearing that up. I somehow knew you'd be the one to provide the answer. ;)
Lux, James P wrote: > ...On Thu, 9 Apr 2009, Prentice Bisbal wrote: > >> I disagree with the sonic barrier wall analaogy. Is it that clearly >> technical barrier the slowed down jet research, or did the nuisance of >> sonic booms to people on the ground just make supersonic R&D less >> convenient? I've heard that supersonic travel over land is restricted in >> the US. > > > .....Yes, indeed, supersonic flight over land in the US is restricted. > Exceptions for military training flights in designated areas (e.g. desert > wasteland) and things like shuttle landings (characteristic double sonic boom > from leading and trailing shock waves.. the shuttle is big and fast). Kind > of put paid to useful Concorde flights except from New York and DC (not to > mention that Concorde was just plain old LOUD on the ground) > > ....Sonic booms are also used as a tactical weapon of sorts in, e.g., Gaza. > > > Actually, historically, it was absolutely the technical barrier, which > was profound. Pilots in WWII not infrequently went into a dive, and of > course diving one can approach the sound barrier quite easily. > > They died. With very few exceptions, and they were lucky ones. One of > two things killed them. At near-supersonic speeds, the equations that > govern airflow and lift completely and nonlinearly change form. All of > a sudden, the pilots discovered that they were unable to actually move > the yoke of their aircraft against the enormous forces that locked them > in, and they discovered that the lift they were counting on to pull them > out of the dive (in particular the lift generated by the aircraft tail) > suddenly disappeared. A few clever pilots thought to put on their > airbrakes, slowed to subsonic speeds, and managed to pull out. The rest > didn't. The other problem that plagued the deliberate attempts to break > the sound barrier were harmonics that appeared and were nonlinearly > amplified as the aircraft approached the barrier.\ > > ...I don't know that they're harmonics, per se, but just the phenomenon of > aerodynamic flutter, which was very poorly understood and is basically a > resonance thing. (the nonlinearities in the state equation for air doesn't > help with understanding, of course). Flutter can occur at lower speeds, too > (it's what sets Vne on some small planes), but at lower speeds, how to deal > with it is easier to solve empirically AND it's testable by gradually > creeping up on it and waiting for the evil moment. When you get close to > sonic speed (and actually, it's localized sonic speed over part of the > airframe that's troubling), then you get very fast changes. One needs to be > a manly man test pilot. > > BTW, demonstrating sonic flow is easy. Your run of the mill air nozzle is > probably in "choked flow" with a shock wave in the throat. If you have a > usual shop air compressor, and the air is damp (bad for the tank, but good > for the demo), and vent it through something like an 1/8" hole, you'll > probably see a nice Mach cone, as the rapid change in pressure causes the > water to condense. An old style non-OSHA safe air blowing nozzle does > nicely, so does a 1/4" or 1/2" ball valve feeding a pipe cap with a hole > drilled in it. > -- Prentice _______________________________________________ Beowulf mailing list, Beowulf@beowulf.org sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
