No hypoxia from high altitude, only O2 valve off

[SMH]

Tested the MiG-15 for hypoxia when unpressurized, and wasn't able to achieve it even after 50 minutes above 12km, including about 20 minutes above 14km (which is 46,000ft!)

Then I induced hypoxia by turning off the oxygen valve. That worked as expected, things rapidly got blurry and lost color saturation, except I was able to recover just by turning the O2 back on. Didn't need to pressurize or lose altitude. That doesn't sound right to me.

Would be nice to have a reason to need to use the pressurization system. Thanks! (Have a track but it's 15MB, too big to upload here. Can you just trust me? Easy enough to verify, though it takes an hour of flying.)

Edited August 15, 2022 by SMH

[f-18hornet]

On 8/15/2022 at 9:09 PM, SMH said:

Tested the MiG-15 for hypoxia when unpressurized, and wasn't able to achieve it even after 50 minutes above 12km, including about 20 minutes above 14km (which is 46,000ft!)

Then I induced hypoxia by turning off the oxygen valve. That worked as expected, things rapidly got blurry and lost color saturation, except I was able to recover just by turning the O2 back on. Didn't need to pressurize or lose altitude. That doesn't sound right to me.

Would be nice to have a reason to need to use the pressurization system. Thanks! (Have a track but it's 15MB, too big to upload here. Can you just trust me? Easy enough to verify, though it takes an hour of flying.)

 

Hi SMH,

it is not necessary to fly 50 minutes. You can try 12 Km altitude for just less than 2 minutes to get hypoxia with Oxygen Supply Valve off and Cockpit Air Valve off as well.

[portman]

Hypoxia occurs when there is not enough oxygen. However, in your MiG you wear your oxygen mask at all times, so no hypoxia is going to occur (except when you turn off the oxygen valve...).

The main reason for using the pressurization system is the low air pressure at high altitudes, which your body would not survive (your body fluids start to boil etc.).

[SMH]

4 hours ago, portman said:

Hypoxia occurs when there is not enough oxygen. However, in your MiG you wear your oxygen mask at all times, so no hypoxia is going to occur (except when you turn off the oxygen valve...).

The main reason for using the pressurization system is the low air pressure at high altitudes, which your body would not survive (your body fluids start to boil etc.).

Incorrect. At a certain altitude, the partial pressure is such that even on 100% O2, you simply do not breathe in enough oxygen molecules for your body chemistry to work. This is why U-2 and SR-71 crews wear spacesuits. 

[Raisuli]

22 minutes ago, SMH said:

Incorrect. At a certain altitude, the partial pressure is such that even on 100% O2, you simply do not breathe in enough oxygen molecules for your body chemistry to work. This is why U-2 and SR-71 crews wear spacesuits. 

If I recall correctly 4 PSI at 100% O2 is enough partial pressure to get the job done.  Those 'space suits' run just that; at much more than 4 movement becomes too difficult. <edit> Worth noting at sea level the PP of O2 is about 3 PSI; they add one because suits are hard work... </edit>

If you want hypoxia set up an axis on the F/A-18 OBOGS, set it to maximum (I tested a 1-1024 and validated it was at 1024) and you'll die on the ground.  Wags said this is working as expected, which does explain why the Navy is chronically short on pilots.

Edited April 5 by Raisuli

[AJaromir]

As far as I know, in Mig-15 is simulated that you have your oxygen mask always on. So no matter your canopy is open or not, you will not get hypoxia because you wear oxygen mask. But once you turn off the main valve, you will starve oxygen, even on ground because your mask is always on.

[Tiger-II]

The cockpit pressurization is for your ears and comfort, not because it is neccessary at 40000 ft.

Without pressurization, there would be so little cabin air you'd freeze to death, and the large and rapid pressure changes due to vertically climbing and descending would hurt if not burst your eardrums.

Hypoxia is a different phenomenon.

The SR-71 and U-2 suits are for protection in the event of loss of cabin pressure/ejection. Inside a normally functioning SR the cabin altitude is around 30000 ft, but if you eject without a suit you suddenly wind up at 85000 ft, and you have massive problems. It also protects against the wind ripping you to pieces as you could be leaving the aircraft while it is still very much supersonic.

Time to LOC (Loss of Consciousness) at 85000 ft is <1 second.

Edited April 7 by Tiger-II

[SMH]

On 4/6/2024 at 8:49 PM, Tiger-II said:

The cockpit pressurization is for your ears and comfort, not because it is neccessary at 40000 ft.

Without pressurization, there would be so little cabin air you'd freeze to death, and the large and rapid pressure changes due to vertically climbing and descending would hurt if not burst your eardrums.

Hypoxia is a different phenomenon.

The SR-71 and U-2 suits are for protection in the event of loss of cabin pressure/ejection. Inside a normally functioning SR the cabin altitude is around 30000 ft, but if you eject without a suit you suddenly wind up at 85000 ft, and you have massive problems. It also protects against the wind ripping you to pieces as you could be leaving the aircraft while it is still very much supersonic.

Time to LOC (Loss of Consciousness) at 85000 ft is <1 second.

 

40,000 ft is right at the edge of where you can survive on pure O2 at ambient pressure, actually.

"At about 40,000 ft altitude (12.2 km), even breathing pure oxygen does not put enough oxygen in our lungs for us to survive."

https://www.eoas.ubc.ca/courses/atsc113/flying/met_concepts/02-met_concepts/02b-oxygen_physiology/index.html

[Tiger-II]

Yes...that's why the cabin was pressurized to 30000 ft.

If the cabin is at ambient pressure, then you're correct - we can't breathe much higher than that without forced air supply.

Edited April 18 by Tiger-II