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).
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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.
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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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kitplane01
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Re: Weight cost of pressurised cabin

Mon Jul 01, 2019 6:53 am

mmo wrote:
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.


I don't think that's right.

The main advantage of climbing high is to get to thinner air. It's less drag to go fast in thinner air. The engines do work better in cooler air, but that's not the main advantage.
 
frmrCapCadet
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Re: Weight cost of pressurised cabin

Mon Jul 01, 2019 2:58 pm

I don't know the engineering involved but making the fuselage strong enough to hold wings, engine, fuel, tail, passengers and cargo all together needs a certain amount of strength. Intuitively that strength and pressurization strength do not fight each other all that much. Or do they?
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hitower3
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Re: Weight cost of pressurised cabin

Mon Jul 01, 2019 3:12 pm

Fun fact : The mass of the air added through the pressurisation adds up to a few 100kg in a large commercial jet.
 
aeropix
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Re: Weight cost of pressurised cabin

Tue Jul 02, 2019 10:09 am

hitower3 wrote:
Fun fact : The mass of the air added through the pressurisation adds up to a few 100kg in a large commercial jet.


I think this is the actual answer to the OP's question. Of course the added structure, plumbing, and mechanical equipment to create the pressurized environment adds far more to the weight than the compressed air itself, but the added efficiency has proven over decades to be worth the weight tradeoff.
 
StTim
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Re: Weight cost of pressurised cabin

Tue Jul 02, 2019 10:16 am

There are more than just depressurization issues surely. We read about coffin corner where as you fly higher the available airspeed range for safe flight is much reduced. So to fly higher will also require aerodynamic changes to push coffin corner up to whatever new level was required.
 
leghorn
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Re: Weight cost of pressurised cabin

Tue Jul 02, 2019 11:36 am

aeropix wrote:
hitower3 wrote:
Fun fact : The mass of the air added through the pressurisation adds up to a few 100kg in a large commercial jet.


I think this is the actual answer to the OP's question. Of course the added structure, plumbing, and mechanical equipment to create the pressurized environment adds far more to the weight than the compressed air itself, but the added efficiency has proven over decades to be worth the weight tradeoff.

let's just say you have a selection of routes that are under 300km and you can could be flying planes on them day in day throughout the day for 20 years solid. A modern plane similar to a Shorts 360 shed or similar type plane could be flying them profitably never going about 10000metres.
If CO2 tax is weighted based on height above sea level then these planes could be economical.
 
GalaxyFlyer
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Re: Weight cost of pressurised cabin

Tue Jul 02, 2019 5:05 pm

No Shorts is flying at 10,000 meters

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