leghorn
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Weight cost of pressurised cabin

Fri Jun 07, 2019 9:42 pm

How much weight does it cost to make an airframe which can be pressurised so that it can fly above FL10.
With the improvements in alloys and composites over recent years surely the weight penalty of having to provide a pressurized tube over what would be the weight of a very strong lightweight frame with a simple unpressurised exterior skin has increased.
 
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Starlionblue
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Re: Weight cost of pressurised cabin

Fri Jun 07, 2019 10:48 pm

As I understand it, the weight penalty has decreased. A composite fuselage is lighter for the same strength. Also, it is less susceptible to corrosion, which is why the 787 and A350 can be pressurised to a lower cabin altitude (higher pressure and humidity).
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
snasteve
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Re: Weight cost of pressurised cabin

Sat Jun 08, 2019 12:58 pm

I wonder why the new carbon fiber airplanes are not certified to go up to 52,000 feet like some of the private jets? Is it really that much more difficult to go 6-7000 ft higher?
 
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Starlionblue
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Re: Weight cost of pressurised cabin

Sat Jun 08, 2019 1:17 pm

snasteve wrote:
I wonder why the new carbon fiber airplanes are not certified to go up to 52,000 feet like some of the private jets? Is it really that much more difficult to go 6-7000 ft higher?


It's not a big issue structurally or aerodynamically. We could theoretically cruise higher than the service ceiling, even in many of the "tin" aircraft. However, if you fly that high, you introduce a whole host of complications involving depressurisation and maximum descent time to a safe altitude. Even at 41000-43000 feet, the time of useful consciousness with an explosive decompression is less than 30 seconds. And if you lose a window at 52000 feet I don't know if the aircraft is able to maintain pressure.

On the business jets that cruise that high, these issues have been addressed. However, this comes at an additional design cost that presumably commercial operators are unwilling to pay.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
mmo
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Re: Weight cost of pressurised cabin

Sat Jun 08, 2019 3:40 pm

snasteve wrote:
I wonder why the new carbon fiber airplanes are not certified to go up to 52,000 feet like some of the private jets? Is it really that much more difficult to go 6-7000 ft higher?


That high you are in the troposphere where there is a constant temp. So, you really don't gain anything by climbing that high as the fuel flow will remain pretty much constant in those altitudes. Depending on where you are and the time of year even going from 410 to 450 doesn't get you anything. You will be towards the end of the flight and the climb fuel will pretty much cost you more than you will save.
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GalaxyFlyer
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Re: Weight cost of pressurised cabin

Sun Jun 09, 2019 1:54 am

Isn’t it the stratosphere where temp remains constant with increasing flight level. True about getting above F450 being a minor advantage at best—end if cruise, so only 90-120 minutes at F470 or F490 at most, KTAS decreases so ANMPP doesn’t improve. The only advantage is losing a headwind.

GF

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