general question: pressure inside planes and submarines (1 Viewer)

[Giant Lobster]

this isnt related to the course but i figured some physics pro's might know the answer hehe

You know on planes, how during landing and takeoff, your ears hurt and sometimes even burst and then blood trickles down your face and you yell in agony at the person next to you but they can't hear you cos for some reason the air pressure is way too low for sound to propagate?

Why is the air pressure (im assuming my ears hurt because of the air pressure) inside the plane affected? and only during times when altitude is changing? Shouldn't the plane be perfectly sealed? hence under all reasonable circumstances, the pressure within should remain nice and good...?

And also, in submarines, esp. those exploration ones which go up to 11km deep, the people coming out of those need to spend half a day in some depressurisation chamber? Why is that? again shouldnt the pressure inside be kept good?

 

[George W. Bush]

The cabin pressure in the plane cannot be 'normal' pressure, as the outwards force on the fuselage would be too great at cruising altitude. Instead, it's pressurised at the air pressure of a low altitude, about 2000m. The 'ears popping' is when the air in the ear is affected by the change in pressure from 0m -> 2000m. I don't really know about the submarine.

Interesting fact: If you had a helium filled baloon on the plane, during take off it would move forwards, rather than backwards. Why?

 

[Giant Lobster]

air is denser than helium so the inertia of air displaces the baloon forward?

hmmm thats pretty shitty then, why don't they make the fuselage strong enough to take on the outward pressure...

engineering limits pfft.

 

[Xayma]

Yes planes dont need to be pressurised till 10 000ft, and they arent till 8000ft they are kept at 8000ft pressure because it causes less stress.

Inside submarines the pressure increases an interesting trick was to place a bit of string taught between two sides of the galley, when it went down deep you would see the string very visibly curve downwards, reducing volume and hence increasing pressure (as the walls cave in)

 

[Giant Lobster]

yeah i figured that, but the shrinking due to pressure would only decrease the available volume by a bit

it doesnt really require the person to spend half a day in some chamber after their trip just cos their cabin was at 1.2 atm or something...

 

[George W. Bush]

Originally posted by Giant Lobster
air is denser than helium so the inertia of air displaces the baloon forward?

Yes, the air is heavier than the helium, so the air moves towards the back of the plane 'pushes' the helium forwards, so to speak.

 

[Xayma]

Originally posted by Giant Lobster
yeah i figured that, but the shrinking due to pressure would only decrease the available volume by a bit

Umm on Diesel Submarines it is a very big difference, if you put the string at a mans height it would occasionaly touch the bottom as you move nearer to crush depth.

And they also increase the pressure themselves to minimise the stress, however it will always be less then 3 ATM as at that oxygen in normal concentrations begins to become poisonous.

And 11km down you wont want to be at only 1.2atm you will want a highly pressurised environment.

And in case your wondering the corridors on most subs are small enough already, I dont want to see them near crush depth.

 

[Giant Lobster]

fascinating.... i wouldn't have figured that the metallic hull of the sub can be compressed so much. And im still doubtful to be honest.

hmmm I wonder how itll feel like breathing at 3 atms. A small puff will last me 3 normal breaths!

 

[Xayma]

The experimental subs normally have much thicker hulls then military subs, simply due to the depth they are travelling at and the fact that they dont need much speed.

And it would probably be very hard breathing at 3atm considering the extra effort you would need to expel the air.