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If an airplane is pressurized, why is the cabin pressure still less than on the ground?
Forget the Subway reply earlier. I hope the following link is of some help. http://en.wikipedia.org/wiki/Cabin_pressurization
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It has to do with the structural limitations of the aircraft. For a real world illustration, take a half inflated balloon with you on your next jump. You'll find that on the plane ride up, the balloon expands as the outside pressure becomes lower. Were you to go high enough, the balloon would eventually burst. The same concept applies to pressurized aircraft. You're not so much concerned with the cabin altitude as you are with the difference in pressure between the outside air and the cabin air. That's why pressurized planes have a gauge in the cockpit to measure differential pressure. In the T-6, our cabin altitude would climb with the aircraft until we went past 8k', at which point the cabin altitude would stabilize and your differential pressure would go up. Our limit was 3.9 +/-0.1 psi
If the differential gets too high (too much pressure inside the aircraft), things can start to break, seals can rupture, etc. There are warning systems in place to let the pilot know if this is happening. If you're cruising at a high altitude and the differential pressure gets too low (cabin altitude is too high), there's the potential for hypoxia, DCS, and other nasty things. That's why airliners have oxygen masks available for passengers (don't believe that crap you saw in Fight Club, pure Oxygen DOES NOT make you complacent and lethargic.)
So anyway, the answer to your question is this: the pressure in the cabin during flight is less than on the ground because they don't want to break things by trying to keep sea level pressure inside an aircraft cruising at 35k feet. Make sense?
Edit: Whoops! Didn't see your second post, looks like you figured it out.
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If the differential gets too high (too much pressure inside the aircraft), things can start to break, seals can rupture, etc. There are warning systems in place to let the pilot know if this is happening. If you're cruising at a high altitude and the differential pressure gets too low (cabin altitude is too high), there's the potential for hypoxia, DCS, and other nasty things. That's why airliners have oxygen masks available for passengers (don't believe that crap you saw in Fight Club, pure Oxygen DOES NOT make you complacent and lethargic.)
Well, it looks like you weren't sleeping when you went over for Physiology Training!!! Very nice post btw (dead on) and how's pilot training going? I may have asked this already but, do you know Guennadi Antonov? He's finishing T-1's at KCBM.......
MikeJD 0
Quotethe pressure in the cabin during flight is less than on the ground because they don't want to break things by trying to keep sea level pressure inside an aircraft cruising at 35k feet. Make sense?
Plus, it makes you drowsy so you go to sleep and stop bothering the cabin crew.
billvon 2,990
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>If an airplane is pressurized, why is the cabin pressure still less than on the ground?
1) Takes less energy (i.e. less bleed air, less fuel) to keep the aircraft pressurized to 5000 feet than to sea level.
2) Most people aren't bothered by 5000 feet (but they would be bothered by 10,000 feet)
3) The wider the pressure differential swing the aircraft sees, the sooner it fatigues. The early Comets (first pressurized aircraft) would mysteriously disintegrate after a bunch of pressurization cycles due to metal fatigue caused by all the pressurizing and depressurizing. Metallurgy and construction techniques have gotten MUCH better since then but there's still a good reason to not cycle pressures too much.
>Also deep underwater, is the pressure greater because of the water weight?
Yes.
>So if I put a submarine in my backyard and set a skyscraper filled with
>cement on top of it, would the pressure be greater inside?
No.
1) If you just loaded the top of the sub, you'd crush it.
2) If you had a very clever support that put all that weight evenly over the surface of the submarine, it might withstand the pressure (that's what happens underwater.) However, submarines resist pressure to maintain normal pressure inside. Even at their maximum depth, the sort of submarines you are talking about (i.e. military subs) do not change their internal pressure. Once you go past that depth, the hull can no longer oppose the pressure and the submarine is crushed.
1) Takes less energy (i.e. less bleed air, less fuel) to keep the aircraft pressurized to 5000 feet than to sea level.
2) Most people aren't bothered by 5000 feet (but they would be bothered by 10,000 feet)
3) The wider the pressure differential swing the aircraft sees, the sooner it fatigues. The early Comets (first pressurized aircraft) would mysteriously disintegrate after a bunch of pressurization cycles due to metal fatigue caused by all the pressurizing and depressurizing. Metallurgy and construction techniques have gotten MUCH better since then but there's still a good reason to not cycle pressures too much.
>Also deep underwater, is the pressure greater because of the water weight?
Yes.
>So if I put a submarine in my backyard and set a skyscraper filled with
>cement on top of it, would the pressure be greater inside?
No.
1) If you just loaded the top of the sub, you'd crush it.
2) If you had a very clever support that put all that weight evenly over the surface of the submarine, it might withstand the pressure (that's what happens underwater.) However, submarines resist pressure to maintain normal pressure inside. Even at their maximum depth, the sort of submarines you are talking about (i.e. military subs) do not change their internal pressure. Once you go past that depth, the hull can no longer oppose the pressure and the submarine is crushed.
Coreece 190
QuoteI looked for old threads but they got off track fast and didn't answer my question.
Does anybody use nitrogen to fill their tires?
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wolfriverjoe 1,523
QuoteQuoteI looked for old threads but they got off track fast and didn't answer my question.
Does anybody use nitrogen to fill their tires?
Yep - 78%
"There are NO situations which do not call for a French Maid outfit." Lucky McSwervy
"~ya don't GET old by being weak & stupid!" - Airtwardo
"~ya don't GET old by being weak & stupid!" - Airtwardo
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