Pressure diffrence

[Lump Jockey]

What's the "ballpark" estimate, or exact if you know it, figure for the pressure an aircraft cabin is under during normal cruise altitude? Are we talking in tons?

[Tinstaafl]

Do mean dynamic pressure from the external airflow, or pressure differential from internal pressurisation?

[Lump Jockey]

Internal. Basically need the value if poss, plus any knowledge of doors blowing out, if this is at all heard of!
TIA.

[Intruder]

Start with a ballpark cabin pressure differential of 9 PSI. A 747's cabin is about 200' long, and the fuselage diameter about 20'. Find the surface area of that cylinder and multiply by the differential. It comes out to about 16 million pounds...

[Genghis the Engineer]

Cabin pressure is generally held around 6-8,000 ft equivalent, which would give a pressure of around 81% of sea-level pressure.

I can't think of anything except Concorde cruising above 50,000 ft, which would give a pressure of around 12% of sea-level pressure.

81-12 = 69, say 70% of sea-level pressure as a worst case difference across the cabin.


Sea level pressure is 101,325 N/m², so 70% of that is roughly 71,000 N/m². That's a fair estimate of the worst pressure difference seen across cabin walls, doors and windows.

To put that in more commonly used units, that would be equivalent to 7 grammes per square millimetre, or 10 pounds per square inch. Your car tyres are probably inflated to about 3 times that difference across the tyre walls.

Doors don't burst open in flight, they can't. The door is shaped such that the pressure difference pushes it further into the hole - if you wanted to open a door you'd have to depressurise the cabin first. If anything fails, it's more likely to be the central cabin around the hoop-ribs, and this has happened once or twice but is extremely rare and usually only following a fatigue failure - an overpressure alone wouldn't do it. Modern inspection regimes should ensure that any fatigue damage is identified and checked long before it gets close to that stage - we've all been understandably a little paranoid about the subject since the Comet !

G

[Lump Jockey]

Sorry but could you explain that last bit again for me! How does it work out at 12% at 50,000ft? 12% of what? I thought it would be a higher number due to the higher pressure!? Really no good at this stuff, so help us out here please!
Cheers!
PS: And why did you take 12 from 81? (Daaaa!)

[themoffster]

There was a DC-10 (I think) accident a while ago that was caused by the cargo door opening in flight due to a poor latch and the difference in internal pressure and external pressure caused the floor of the plane to collapse and the plane went down as it was fly-by-wire and the pilots no longer had any control.

It doesn't answer your question at all, but at least it shows the difference in pressure is substantial.

[Lump Jockey]

FBW DC10. Didn't know there was any. Sure it wasn't an MD11, they're newer, but not sure if they're FBW either come to think of it!:o

[Genghis the Engineer]

It's a normal aerodynamicists shorthand to refer to temperature, pressure or density at altitude as a proportion of the conditions at sea level - as you go up pressure drops, temperature drops, and density drops. So, whilst at sea level everything has a value of 1 (or 100%) it all drops as you climb. In a pure vacuum (which doesn't actually exist anywhere) density and pressure would be 0.

When an airliner is cruising at altitude, both the pressure inside and the pressure outside the aircraft are less than sea-level conditions. In pressure terms, inside it can be as high as 81% of sea level pressure, and outside it can be as low as 12% of sea-level pressure. So, the difference between the two (it's always the difference that matters, not any absolute pressure) is 81%-12% = 69% of sea-level pressure.

At that point, I multiplied my 70% (69% rounded off) by actual sea level ABSOLUTE pressure, which gave be the difference in pressure between inside and outside the aeroplane.

The actual values I took from a data book that lives on the bookshelf behind my desk, but it's never let me down yet ! It was "Aerodynamics for Engineering Students" by Houghton and Caruthers, which is about 15 years old and a little crude, but good enough for this sort of crude guesstimate.

Hope this helps.

G

[Intruder]

Themoffster:

The situation you cite shows the difference in whether a structure is designed to withstand a pressure differential or not.

All commercial aircraft are now equipped with "blowout doors" in internal bulkheads and dividers. These doors are designed to equalize pressure quickly in the case where a compartment is depressurized.

In the event of the cargo door opening/failing, the loss of pressure may be too quick to allow equalization, so the internal structure (the main cabin floor) failed. That is because it was not built to withstand the millions of pounds of instantaneous load placed on it.

OTOH, the "pressure vessel" -- the structure enclosing the pressurized part of the airplane -- is designed to withstand the loads placed on it by the air pressure differential, as long as the pressure is introduced at a controlled rate.

[mustafagander]

Intruder,

The reason for the blow out panels and press equalising vents throughout the a/c is the Paris DC10 aft bulk cargo door failure. These panels are certified to cope with a cargo door blow out at max diff, 9.4 psi on the B744. The DC10 was lost because, in short, the rear cabin floor partly collapsed and severed/disabled the flight control cables. no FBW in those days, just good old stainless cables.