[Beowulf] Moores Law is dying

Prentice Bisbal prentice at ias.edu
Fri Apr 10 07:17:13 PDT 2009


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.


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