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Old 03-22-2016, 08:45 AM   #31  
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The 787 carbon fiber fuselage allows higher cabin pressure not so much because of increased strength but because of increased stiffness.

An aluminum fuselage stretches like a balloon when it gets inflated, and then comes back to close to the original when the pressure is released. Multiple inflation and deflation cycles cause fatigue cracks, and the higher the pressure, the faster the cracks would form.

Even if the 787 fuselage were no stronger than the aluminum fuselage, its resistance to stretching would allow a higher pressure inside.

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Old 03-27-2016, 07:51 PM   #32  
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Originally Posted by Adlerdriver View Post
The 787 carbon fiber fuselage allows Boeing to pressurize the aircraft to a lower cabin altitude than a similar aluminum design. Since that aircraft also uses an electrical pneumatic system rather than the traditional bleed air system on most aircraft, they can accomplish this increased pressurization with no additional demands on the engines. Nothing come for free, so I would imagine the electrical demands are considerably higher. This may demand more from engine driven electrical components but obviously that was deemed to be an acceptable tradeoff by the designers. From what I understand, in actual practice, this electric-centered aircraft design has had some growing pains in terms of system reliability.

Cabin pressurization is a fairly simple concept and the physics involved with actually accomplishing it aren't going to change. You have to pump enough air into the fuselage to sustain life inside a tube that spends hours above 30,000 feet. It seems like Boeing was able to make some minor "breakthroughs" in fuselage structure and system architecture that allow them to get cabin altitude lower without major sacrifices in efficiency. They were able to drop it from typical ranges of 6000-7000 feet down to 5000-6000.
The main benefit to the 787 being carbon fiber (aside from strength at lighter weight) is that it does not have corrosion issues like aluminum skin aircraft. So this allows Boeing to increase the humidity which is a major factor in helping you feel better after a long flight.

The A380 has a lower cabin altitude on average than the 787. The A380 runs about 5,000 feet, but this is due to the fact that the designers at Airbus had to substantially increase the fuselage strength for the 380 just to hold its own structure of two full main decks. Being that the structure was already very beefy by necessity, Airbus was able to pressurize the vessel to a lower altitude - while this means by nature that there is an increase in humidity to a certain extent, it is not up to the humidity levels of a 787. So the 787 general runs about 6,000 feet cabin altitude with higher humidity and the A380 runs about 5,000 feet cabin altitude with lower humidity.

The combination of both is what helps you feel better after a 12 hour flight.
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Old 08-28-2016, 11:20 PM   #33  
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You'll have to forgive my ignorance, but I always thought that one of the reasons aircraft were pressurised to 7-8000 feet was weight.

Air at sea level is 14.7psi, what is it a 8000ft, around 11psi? Does that not make a big difference to the weight of the aircraft and therefore fuel use?
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Old 08-29-2016, 10:46 AM   #34  
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Originally Posted by Wheelnut69 View Post
You'll have to forgive my ignorance, but I always thought that one of the reasons aircraft were pressurised to 7-8000 feet was weight.

Air at sea level is 14.7psi, what is it a 8000ft, around 11psi? Does that not make a big difference to the weight of the aircraft and therefore fuel use?
The weight of the airmass inside the fuselage in flight is inconsequential, and has no impact on the flight planning for fuel consumption. Were that the case, long range aircraft might focus on oxygen use for the limited number of crew members and depressurize the aircraft.

Pressurization serves multiple functions aside from comfort and life support. It's part of the rigidity of the aircraft structure

The difference in eleven vs. 14 psi in terms of weight of the airmass inside the fuselage is negligible and not a part of structural planning for fuel consumption, strategic planning, or inflight performance or other calculations. The only time pressurization comes to bear on performance is when considering the effects of pack and bleed usage on engine efficiency, particularly during the takeoff and initial climb.
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Old 08-29-2016, 12:30 PM   #35  
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PSI is a measure of pressure. What you're thinking is weight. You'd need to measure it in a unit like slugs per barrel. Comes out to about .01 slug/bbl at sea level on a standard day.
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Old 08-29-2016, 12:53 PM   #36  
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Thanks guys - you live and learn
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Old 01-17-2019, 03:01 PM   #37  
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Quote:
Originally Posted by Wheelnut69 View Post
You'll have to forgive my ignorance, but I always thought that one of the reasons aircraft were pressurised to 7-8000 feet was weight.

Air at sea level is 14.7psi, what is it a 8000ft, around 11psi? Does that not make a big difference to the weight of the aircraft and therefore fuel use?

You're exactly correct - it's all about the weight of the aircraft. Every pressurized aircraft has a maximum pressure differential (between outside pressure and cabin pressure) that reflects the maximum load the pressure vessel can withstand. For instance, in a 747-400 it's 9.4 psi, which is enough pressure to fly at its max certified altitude of 45,100' with an 8000' cabin altitude (the highest cabin altitude allowed by law on commercial aircraft).

Could they pump it down to sea level? Sure, all they'd have to do is add a bunch of structural weight and either lose around 75 seats or 3 hours worth of fuel. Not gonna happen. Most people have little or no discomfort with an 8000' cabin, and it'll rarely be that high in flight, as they've got to burn off most of their fuel before they can get to their max altitude anyway. The pressurization system is designed to keep the cabin at the lowest possible cabin altitude (and thus, the max allowable pressure differential) automatically.

Last edited by redeyed; 01-17-2019 at 03:19 PM.
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Old 01-17-2019, 04:37 PM   #38  
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I'll just put these here again.....

Straight from Boeing (they know a thing or two about airplanes): Controlling Nuisance Moisture in Commercial Airplanes

The rain in planes | The Economist
Big difference between moisture problems and humidity. They may or may not be linked, depending on the situation.
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