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Right now at my university we are busy with a project for designing a conceptual aircraft. Our aircraft is in the CS-23 range, this airplane is a small cargo aircraft with the option for carrying injured passengers (so like an ambulance aircraft).

Our aircraft will have a cruise height at 10,000 ft. We want to make this aircraft pressurised but our question is therefore, is it possible to pressurise the aircraft from inside below the normal 8,000ft. So for example, we are flying at 10,000 ft but we want to have the inside pressure having the same as on sea level if that is possible, or is it only possible to pressurise the inside at 8000ft.

Thank you in advance.

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    $\begingroup$ @GHB, aircraft cruising at 10,000 does not need either pressurization nor supplemental oxygen. 10,000 ft is considered safe. Rare medical conditions where it might be a problem are more easily handled by portable oxygen; such patients are likely to need a ventilator, at least available, anyway. $\endgroup$ – Jan Hudec May 13 '16 at 10:06
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    $\begingroup$ @Simon: If the cabin is hermetically sealed to maintain the pressure when the door was closed, two very bad things will happen: You'll run out of oxygen in a matter of hours, and the humidity from exhaled breaths will fog up all the windows and begin to condense on all the instruments. Pressurized airplanes continually bring in fresh air and cycle out the air already in the cabin. The airplane is not literally sealed tight. It just has a compressor (and heater and humidifier) between the outside air and the cabin. $\endgroup$ – abelenky May 13 '16 at 12:20
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    $\begingroup$ @abelenky I know. I was simply saying that it is entirely down to the designer what happens using all the usual compromises. There is nothing preventing someone from designing an hermetically sealed fuselage pressurised to sea level. $\endgroup$ – Simon May 13 '16 at 12:41
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    $\begingroup$ 8,000' is not a "Normal" cabin pressurization altitude. It is the MAX allowable cabin altitude for a pressurized aircraft. Take any pressurized aircraft and fly it below it's max cruising altitude and the cabin alt will be far below 8,000'. Flying at 10,000 feet you will be able to maintain the cabin much lower than 8,000' with even a very modest cabin pressure differential. One exception to this is when flying to Bogota at 8,361'. Airline SOP requires you to keep the cabin at 8,361' for the entire flight. I would always keep the cabin at 6,000' and then raise the cabin prior to landing. $\endgroup$ – Mike Sowsun May 13 '16 at 12:46
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    $\begingroup$ @JanHudec If you are transporting someone who was injured or became ill while scuba diving, and they are not currently suffering decompression sickness, you might be able to transport them safely without using a decompression chamber if the cabin is pressurized to sea level, but you might not want to expose them to any lower pressure. Oxygen is not the problem here, it is purely and simply (as far as I understand it) a matter of the ambient partial pressure of nitrogen. $\endgroup$ – David K May 14 '16 at 13:33
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If you design the aircraft, shouldn't you be answering that question? If it is part of your aircraft's mission to fly at 10.000ft whilst maintaining sea level cabin pressure, then you should design it that way.

Why would it only be possible to pressurize the inside to 8000ft? Not to 9000ft, 7000 ft or any other arbitrary altitude?

Of course the construction of the aircraft needs to be able to withstand the forces caused by pressure difference between the inside of the cabin and the outside. And the pressurization system needs to be able to maintain the pressure at sea level, so compensating for any air leaks in the pressure vessel.

For an initial feasibility study you could look at the pressure difference on the hull when an aircraft is flying at FL430 whilst maintaining cabin pressure at 8.000ft. Note thate the Boeing 787 maintains the cabin altitude at 6.000ft, while operating at 43.000 ft.

Compare that to the pressure difference between 10.000ft and sea level. Looking at those pressure differences should give you a feeling whether or not it is easy to achieve your objective.

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The SyberJet SJ30 business jet flies way higher, around FL410, and it is pressurized at sea-level. Composite materials help in coping with extreme cycles.

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    $\begingroup$ An even more extreme example would be the International Space Station, which is maintained at sea level pressure while in near vacuum (although of course, I assume pressurization cycles are only an issue for the airlocks) $\endgroup$ – Sanchises May 13 '16 at 11:26
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    $\begingroup$ @sanchises, ISS has advantage of not going through any cycles. $\endgroup$ – Jan Hudec May 13 '16 at 21:26
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    $\begingroup$ @Jan yes, isn't that what the second part of my comment addresses? $\endgroup$ – Sanchises May 14 '16 at 12:17

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