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papa juliet
25th Mar 2009, 10:57
I have just viewed the answers to the query as to the volume of air in the 747 cabin.
Question , does the mass increase as the cabin is pressurised, does the take off weight vary by any significant ammount?

PJ

Dual ground
25th Mar 2009, 11:18
The simple answer is no the mass of the cabin air will be less. An aircraft cabin is not pressurised to sea level, but normally to the equivalent of around 8000 ft. The cabin is held at a pre-defined differential pressure normally about 8 to 8.5 psi if I recall correctly.The only time the mass of air will be greater due to pressurisation is during maintenance,if the cabin is pressurised at sea level to carry out a leak check.

The cabin is not pressurised during take off and landing so the mass of the cabin air is dictated entirely by outside airpressure and I suppose by the volume taken up by pax and/or cargo.

spannersatcx
25th Mar 2009, 11:54
The cabin is not pressurised during take off Actually it is, can't remember the exact altitude but it is pressurised.

Sir Richard
25th Mar 2009, 12:11
The initial pressurisation, slightly above ambient pressure, begins when the aeroplane reaches 65 kts ground speed.

From and old 744 tech Manual :8

The cabin altitude controller automatically sets cabin altitude slightly below destination field elevation so that the cabin is pressurised slightly on landing. At touchdown, the outflow valves open, depressurising the cabin.

The maximum allowable pressurisation for takeoff or landing is 0.11 psi above ambient.

Dual ground
25th Mar 2009, 12:37
Fair enough, I was trying to keep it simple....

h3dxb
25th Mar 2009, 15:59
Prepressurization during TO and LDG is equivalent to 50 ft to avoid pressure bumps.

And in school I had physic, and I learned something about closed systems.....

or in other words: when we carry birds on board and during TO they start to swing their wings, does the plane become lighter ? because the bird dont stand on the floor or heavier because the wings press the air to the ground ;-))

Neither, in a closed system the weigt is the same.

enjoy :sad:

PENKO
25th Mar 2009, 16:39
But if you pump a gas (air) into a vessel then for sure the weight will increase. Do you scuba? An empty tank is noticeably lighter than a filled one.

Wizofoz
25th Mar 2009, 16:46
The pressurisation of the aircraft during takeoff will indeed lead to a (slight) increase in the mass of the air on borard, and therefore the weight of the aircraft.

In cruise, the fact that the aircraft is pressurised means it weighs more than if it was NOT pressurised, but actually less than it did at takeoff, as it has less than sea-level pressure and is thus carrying a smaller mass of air.

I am not aware of this fact being used in any performance calculation, but if it is, it must be incorporated in the charts, as we simply assume the aircrafts weight is it's takeoff weight minus it's fuel burn (or ZFW plus FOB, which is the same thing!)

h3dxb
25th Mar 2009, 16:48
This is true, but U don't pump Air in , U flow it through...
The pressure in flight is hold to appr. 2000m ASL, so less than on ground, but it is still a closed system, when U look from outside,

sorry I can't override this Albert Einstein

PENKO
25th Mar 2009, 17:21
Einstein has nothing to do with it, try Boyle and Gay-Lussac.:ok:

h3dxb
25th Mar 2009, 17:31
U R right Penco,
but Einstein was this guy with the closed system theory ;-)

But we run out of track.

rgds

Wizofoz
26th Mar 2009, 14:57
"Closed systems" usually refers to the first law of thermodynamics, which predates Einstein by a couple of centuries. The stuff with the fly is pure Newton, and not really applicable to what we are talking about.

Jumbo Driver
26th Mar 2009, 15:18
But if you pump a gas (air) into a vessel then for sure the weight will increase. Do you scuba? An empty tank is noticeably lighter than a filled one.

As we are obviously into pedantry on this thread, PENKO, the tank would not be "full" or "empty" - "charged" or "discharged" it may be - I would suggest it would always be "full" of something, albeit the pressure and density may change ...

JD
:)

muduckace
26th Mar 2009, 15:47
The main reason just about all pressurized aircraft hold a bit of pressure is to keep the doors and windows from rattling or whistling. I forget when the 747 classic did it (someone above mentioned kt) Most aircraft get their pressure bump when throttles are advanced past T/O warning logic switches or positions. Not sure of this but they probably dump when air/ ground switching is made.

Future aircraft will pressurize to greater than 8.5 DP to lessen the altitude effect on Jet Lag, they want to bring the cabin down to less than 5,ft. Modern corporate jets are capable of this to achieve the 40-50k ft cruise altitude. Saves fuel and gets you to your destination faster. Not to mention clear air space high above RVSM.

sizematters
26th Mar 2009, 16:01
what a load of bollox you people do go on about.....................

all I know is that there is indeed a massive volume of air inside a 747................

Machdiamond
26th Mar 2009, 16:50
If I understand the question correctly, is the takeoff prepressurization increasing the weight of the aircraft?

885.9 cubic meters at sea level represents 2392 lb.

To pressurize at -50ft you need to pump up an additional 4 pounds of air, if I get my calculation right.

Closed system or not, you need to accelerate those additional four pounds during the takeoff roll so the effect is real, but totally negligible.

ldo
27th Mar 2009, 07:45
Actually, as negligible as it may be, I believe that when the aircraft is pressurized at altitude, the net effect is that the air inside the aircraft will weigh more.

Reason being that on the ground, no pressurization, the weight of the air inside the aircraft is balanced by the buoyancy effect of displacing an equal volume of air at the same pressure.

At altitude, if the aircraft is not pressurized, then the weight of the aircraft is again balanced by the buoyancy effect (lighter air both inside and outside the aircraft). If the aircraft is pressurized, then the weight of the air inside the aircraft will increase and there will be a net increase in weight.

Of course this is all academic as compared to the weight change due to fuel consumption.

PENKO
27th Mar 2009, 09:25
As we are obviously into pedantry on this thread, PENKO, the tank would not be "full" or "empty" - "charged" or "discharged" it may be - I would suggest it would always be "full" of something, albeit the pressure and density may change ...

JD

Ouch!:}:ok:

Let's keep it simple, I'm no scientist. If a newly pressurized scuba tank weighs more than one that has been used for a one hour dive then for sure any airplane being pressurized will weigh more than an unpressurized one in otherwhise equal circumstances.

Jumbo Driver
27th Mar 2009, 10:27
Ouch!:}:ok:

Sorry, PENKO, I didn't mean to hurt ... ;)

Let's keep it simple, I'm no scientist. If a newly pressurized scuba tank weighs more than one that has been used for a one hour dive then for sure any airplane being pressurized will weigh more than an unpressurized one in otherwise equal circumstances.

I do think this answers the original question very well in non-technical terms.

In summary, when the cabin is pressurised, of course there is more air in the cabin and so there will be more mass (i.e. it will weigh more). However, the increase in mass in proportion to the total mass of the aircraft is proportionately so small as to be negligible.


JD
:)

PENKO
27th Mar 2009, 11:13
You know, I was just getting confused with Einstein, closed systems, birds flying in cabins etc! I have my own question. How much does the mass of a 747 with winglets increase when rain falls upon it? :ok:

Dual ground
27th Mar 2009, 11:17
Light rain or heavy rain?

the rim
27th Mar 2009, 11:44
you guys .....i dont beleive that you actually fly them ...its simple you push the throttles forward and the aircraft moves forward and the packs feed air into the cabin the outflow vales move closed untill there is a pressure increase in the cabin and its all auto after that untill something goes wrong ....right fishey i know you will tell me if its not

fizz57
27th Mar 2009, 14:26
I was intrigued by this, so I ran some numbers through excel.

At sea level, temperature 27 deg C (300K), density of air is 1.18kg/m3 so with a cabin volume of 885.9m3 the mass is 1042kg. The weight of this air is balanced by the weight of air it displaces (Archimedes) which means that there is no effect on the overall weight of the plane. It is however an extra ton of mass that has to be accelerated to take-off speed, over and above what the aircraft would weigh on a weighbridge.

At 2000m, the air density is 1.006kg/m3 and the mass of a cabinful of the outside air falls to 891kg. The air temperature at this altitude is 2 degrees C, so assuming the cabin is heated to 300K and at the same pressure, the air inside is less dense than that outside. In fact its mass is only 817kg, so the aircraft is actually 74kg lighter at this point.

At 10000m, the density falls to 0.417kg/m3 and the OAT is -50 deg.C. The air displaced weighs only 365kg, the pressurised air inside (2000m, 27 deg.C) still weighs 817kg. Although the weight of air in the cabin is less than at sea level, the air displaced weighs even less. So the wings have to produce an additional 452kg of lift - nearly half a ton - to keep all that air in the air, so to speak.

I hope the beancounters at a certain low-cost airline aren't reading this...

papa juliet
28th Mar 2009, 10:42
fizz57 well done :) that has to be the clearest answer ever.

THANKS PJ

Old Fella
28th Mar 2009, 11:22
Muduckace The Classic usually takes off with the packs OFF. Packs introduced after airborne by the old bloke who no longer gets a position in the cockpit. Always set the cabin pressure controller to 1000' above cruise level, to reduce the chnces of getting an overpressure, and ideally the cabin would be fully depressurised by touch-down. All manual, no "electronic magic" involved, just the guy whose presence allowed the MEL to be a lot thicker than it is without him being there.

Landroger
28th Mar 2009, 12:08
Brilliant. One of the best, clearest and most interesting answers I have ever read. :) Well done sir. So interesting in fact, that have save it on my lappie so I can remember the details - someone, somewhere will ask.:8

Roger.

PENKO
28th Mar 2009, 12:47
That must be the best first post ever on PPRuNe. :) Thanks fizz from Malta!

Old Fella
29th Mar 2009, 02:17
H3dxb , sorry to disillusion you, but the air is both "pumped" in and "flowed" through. Pumped in from the engine bleeds and flow controlled by the outflow valves. Outflow is controlled to maintain the required differential pressure such as to have the selected cabin altitude, and at the same time not exceed max permissible differential pressure. Safety valves are fitted to relieve the pressure in the event of a malfunction. Also, the cabin altitude will be determined by the level of pressurisation. Sea level can be maintained up until Max Differential is reached, however it is usual for the cabin altitude to increase at a rate of around 500'/min in the climb. Likewise if the cabin altitude is at, say 6000' in the cruise, it will gradually decrease at a lesser rate than the aircraft descends so that it is at destination airfield altitude for arrival. Ideally, a cabin altitude decrease of around 300'/min is desired to minimize passenger discomfort.

Bushfiva
29th Mar 2009, 03:00
Prompted by Fizz57's excellent analysis, me and the bath have just worked out that I occupy around 0.09 cubic meters. So stick several hundred of me in Fizz57's 747, we'd only displace 30 cubic meters of air or so, affecting Fizz57's calcs by only 3.5%.


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