Pressures inside and outside the aircraft

[RipCurl]

With regard to unpressurized aircraft, can anyone explain to me why the pressure inside the cabin is lower than the pressure outside? Seems to me that if I believe what Bernoulli had to say, the acceleration of the air around the cabin should cause a lower pressure outside. People tell me that it is "due to the effects of the relative wind moving past the airframe." OK. Fine. What are those effects?

 

[five-alive]

The air outside is stagnant and the air inside the plane is moving. More velocity pressure means lower static pressure (conservation of energy).

 

[Gutenberg]

Check section 5 in your POH. The relative pressures change with ventilation/cabin heater configurations. There's a table for newer aircraft for calibrating the IAS while using alternate air. There's no one answer, but air rushing over the outside of the cockpit tends to reduce pressure inside the cockpit, much like if you put a wadded up piece of tissue in a TP tube, hold it vertically, and shoot air across the opening at the top. The tissue rises upward due to the lower pressure.

The same happens in meterology. Winds in the jetstream being accelerated around a bend cause a lower pressure aloft and subsequent lifting of the air below. A good source of kindling for thunderstorms.

 

[GravityHater]

You are getting very little ram air to pressurize most common nonpressurized light airplane cabins from either cabin vents or leaks at the front of the airplane (glass and firewall prevent this).
But, because these airplanes are so leaky you are getting a lot of air being sucked out the sides and rear of the cabin.

This is why the cabin pressure might be lower than outside.

 

[troy]

Bernoulli was correct, except that you (and the air with you) are what is being accelerated. The air outside doesn't move (in principle)

 

[VNugget]

five-alive and troy: Bernoulli's principle is irrelevant in this case, as it only applies to a closed system. Here, you have energy being added by the engine.

troy: Remember all motion is relative. From the ponit of view of the outside air, the air in the plane is moving, but from the point of view inside the plane, the air outside is moving, and it is just the same looking both ways.

 

[MauleSkinner]

With regard to unpressurized aircraft, can anyone explain to me why the pressure inside the cabin is lower than the pressure outside? Seems to me that if I believe what Bernoulli had to say, the acceleration of the air around the cabin should cause a lower pressure outside. People tell me that it is "due to the effects of the relative wind moving past the airframe." OK. Fine. What are those effects?

As much as anything, it depends upon what pressure you're measuring outside the aircraft. You're right that the acceleration of the air around the cabin creating the lower pressure...the pressure inside the cabin MUST be higher than that in order for it to be "sucked out", right? On the other hand, if we use the alternate static source, we generally see an increase in altitude and airspeed, which implies a LOWER pressure in the cabin.

The difference here is that the static ports are positioned to best indicate STATIC (unaccelerated) air pressure. They found a place to locate them on the airframe where the velocity of the airflow is relatively unaffected by the shape of the airplane. They couldn't do this for the whole airframe, so there is a lot of air rushing around creating lower pressure just outside the cabin, drawing air out of the cabin, resulting in cabin pressure being lower than static pressure.

Thus, cabin pressure is lower than the static pressure outside, but not lower than the local pressures immediately outside the cabin.

Hopefully that makes sense.

Fly safe!

David