# Pressures in and around an aircraft

• alandala
In summary, when considering pressures in and around an aircraft, there is a static pressure inside the cabin and a static pressure and dynamic pressure outside the aircraft. In the event of a rapid decompression, the static pressure is the one that matters for determining flow. For a bottle shaped object flying through the air at a constant velocity, the pressure difference between the outside and inside would be the dynamic pressure. This concept can be applied to an airspeed indicator in aircraft. Static pressure can also be referred to as thermodynamic pressure, as it is a measure of energy per unit volume. The location of a leak in the aircraft fuselage can affect the static pressure and the location of a static port is important for accurate readings.
alandala
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
alandala said:
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.

alandala said:
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?

alandala said:
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.

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?

alandala said:
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.

## 1. What is air pressure and how does it affect an aircraft?

Air pressure is the force exerted by the weight of air molecules in the Earth's atmosphere. It affects an aircraft in various ways, including lift, drag, and stability. The movement of air over and under the wings creates a difference in air pressure, which generates lift and enables the aircraft to fly.

## 2. What is the difference between cabin pressure and atmospheric pressure?

Cabin pressure is the pressure maintained inside the aircraft cabin to provide a comfortable environment for passengers and crew. It is usually lower than atmospheric pressure at cruising altitude, which is the pressure exerted by the Earth's atmosphere at a specific location and altitude. Pilots use cabin pressurization systems to regulate the pressure inside the aircraft cabin.

## 3. How does altitude affect air pressure in and around an aircraft?

As altitude increases, the air becomes less dense, resulting in lower air pressure. This decrease in air pressure affects the performance of an aircraft, including the engine, lift, and stability. Pilots must adjust the aircraft's speed and angle of attack to compensate for the changes in air pressure at different altitudes.

## 4. What is the role of airspeed in managing pressures in and around an aircraft?

Airspeed is the speed at which an aircraft moves through the air. It plays a crucial role in managing pressures in and around an aircraft because it affects the airflow over the wings and other control surfaces. Pilots must maintain a certain airspeed to generate enough lift and control the aircraft's movements effectively.

## 5. How do changes in temperature affect air pressure in and around an aircraft?

Changes in temperature can significantly impact air pressure in and around an aircraft. As air temperature increases, the air molecules expand and become less dense, resulting in lower air pressure. This decrease in air pressure can affect the aircraft's performance, and pilots must make adjustments to maintain safe and efficient flight.

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