# Pressures in and around an aircraft

I am a bit confused about pressures in and around an aircraft.
Inside the cabin we have a static pressure, since we assume the air is standing still. (I know it's not, due to the Environmental Control system, but let's ignore that).
Outide the aircraft we have the static pressure = p ambient, and the dynamic pressure, summed up to the total pressure.

My question is. When we have a rapid decompression, which pressure from the outside is the one that matters (the one that decides the flow)? The total, the static or the dynamic?

• anil chauhan

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Outide the aircraft we have the static pressure = p ambient, and the dynamic pressure, summed up to the total pressure.
That isn't entirely true. If the plane is moving fast enough for compressibility to be important, then static and dynamic pressure do not sum to total pressure.

My question is. When we have a rapid decompression, which pressure from the outside is the one that matters (the one that decides the flow)? The total, the static or the dynamic?
Static pressure. The dynamic pressure is really just a measure of the kinetic energy in the flow and the total pressure is the total energy in a flow. The static pressure is the thermodynamic pressure.

• K41
Thank you!

Hm, based on this the train of thoughts took me to a different question.
Say we have a bottle shaped object flying through the air with a constant velocity (so one inlet, no outlet). What will then be the pressure difference between the outside and the inside of the bottle?
I assume that inside of the bottle we have the stagnation pressure, and outside we have the ambient, and pressure difference between them is the dynamic pressure (if we ignore compressibility effects). Is this true?

SCP
Say we have a bottle shaped object flying through the air with a constant velocity (so one inlet, no outlet). What will then be the pressure difference between the outside and the inside of the bottle?
I assume that inside of the bottle we have the stagnation pressure, and outside we have the ambient, and pressure difference between them is the dynamic pressure (if we ignore compressibility effects). Is this true?
Yes, this is correct, and what you've described is a simplified version of an airspeed indicator (which actually measures dynamic pressure, not airspeed) used in aircraft.

K41
That isn't entirely true. If the plane is moving fast enough for compressibility to be important, then static and dynamic pressure do not sum to total pressure.

Static pressure. The dynamic pressure is really just a measure of the kinetic energy in the flow and the total pressure is the total energy in a flow. The static pressure is the thermodynamic pressure.
I just want to add, this, in my opinion, is a very good way to appreciate what pressure is. Rather than thinking of it as Force/ area, I try to view it as "Energy/ Volume", an energy term as it were.

If I may, can I ask, if we refer to static pressure as "thermodynamic pressure", does this refer to the molecular motion of the gas molecules when the (bulk) velocity is zero or something else?

rcgldr
Homework Helper
When we have a rapid decompression, which pressure from the outside is the one that matters (the one that decides the flow)?
Depends on the location of the leak in the aircraft fuselage, which in turn determines the static pressure of the air flowing across the leak. If the leak is located on a convex relative to flow surface of the fuselage, then the static pressure will normally be lower than if the leak is located on a flat relative to flow surface. This is why the location of a fuselage flush mounted static port as used on a civilian aircraft is important.