Volume Or Mass Of Air In 747
 

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

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.

Actually it is, can't remember the exact altitude but it is pressurised.

From and old 744 tech Manual :8

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

Fair enough, I was trying to keep it simple....

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:

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.

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!)

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

Einstein has nothing to do with it, try Boyle and Gay-Lussac.:ok:

U R right Penco,
but Einstein was this guy with the closed system theory ;-)

But we run out of track.

rgds

"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.

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
:)

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.

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................

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.

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.

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.

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

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
:)

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: