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Wed Feb 21, 2007 9:55 pmby SWR
From what L.W. says,he designs the ports to be small enough to be in the "sonic" area for longer. Another interesting thing is that as soon as a flow goes sonic, a well-arranged expansion after the critical point actually reacts the opposite of a subsonic gas,it speeds up the gas furthermore...thereby creating a bigger PD before and after the "nozzle".
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Thu Feb 22, 2007 2:48 amby jsa
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Thu Feb 22, 2007 8:24 amby Tony
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Thu Feb 22, 2007 4:04 pmby larrycavan
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Thu Feb 22, 2007 4:52 pmby 86rocco
It's been a long time since I last studied these types of problems but iirc, the speed of sound in air is related to the density of the air, that's why it changes with temperature, in the case of the plane at 40,000 feet, the increase in density due to cooler temps is more than offset by the decrease due to altitude so the mph value of the mach number increases with altitude.
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Thu Feb 22, 2007 7:39 pmby 1960flh
Rocco;
Actually as we go up in altitude the gas density goes down air is thinner, but temperature also plays a big role in the calculation. Look here and try some different numbers.
I think the next time I butcher a set of head pipes I will put a pitot tube centered on the exhaust port and then see if I can measure the gas speed and temp at WFO.
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Thu Feb 22, 2007 7:45 pmby jsa
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Thu Feb 22, 2007 7:59 pmby larrycavan
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Thu Feb 22, 2007 8:09 pmby larrycavan
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Thu Feb 22, 2007 8:16 pmby larrycavan
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Thu Feb 22, 2007 8:37 pmby jsa
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Thu Feb 22, 2007 8:38 pmby larrycavan
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Thu Feb 22, 2007 10:18 pmby SWR
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Thu Feb 22, 2007 10:31 pmby bruce
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Fri Feb 23, 2007 4:03 amby 115-1170124124
For ref sake, the International Standard Atmosphere is defined as starting at sea level and 59*F. Pressure is ~2116psf and the speed of sound is ~1116fps. At 10000 feet, temp, pressure, and "a" are 23*F, 1455, and 1077 respectively. 20,000 feet: -12, 972 and 1036. 30,000 feet: -48, 628, and 994. At 36,089 feet (the tropopause): -70, 473, 968. From 36,089 to somewhere around 65,610ft (another "pause" which I don't remember right now), the temp remains constant at -70 AND "a" stays constant at 968fps while the pressure drops from 473 to 114psf. At 75,000, the temp increases to -65 and the speed of sound also increases to 974 while the pressure continues to drop to 73psf. This pretty much removes pressure from the speed of sound calculation. Just so we have a consistent reference. -- Doug