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As I study for my IRA written, this (VFR) question resurfaced and continues to irk me. The annotations cite the Venturi effect as the sole reason, but I don't follow.

From Wikipedia: "The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe"

Do we consider the parcel of air in the cabin to be the fluid moving through the constricted section of some imaginary/abstract pipe? If so, why? Can anyone shed more intuition on this?

David DeVine
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The air moves around the fuselage, which causes a localized increase in velocity around the aircraft. This increase in velocity causes a slight decrease in the outside air pressure, which effectively sucks air out of the fuselage, making the cabin at a slightly higher pressure altitude.

The effect is localized.

While the reduction in pressure, and increase in pressure altitude inside the cabin is small, many GA aircraft display a 40 foot or so difference in pressure altitude between the alternate static port (inside the aircraft) and the pressure altitude with the factory static port.

mongo
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    Opening the air vents can change the pressure inside to be higher than outside. Barry Schiff gives a good demonstration in a Wonderful World of Flying clip. https://www.youtube.com/watch?v=eHnhRrbyEIE – JScarry Apr 21 '17 at 00:08
  • The original poster may be helped by imagining the fuselage as a carburetor venturi turned inside out. That is to say that the constriction is on the outside of the aircraft going through the air, rather than inside the cabin. – mongo Apr 21 '17 at 00:24
  • The opening and closing of air vents is a different effect, largely the introduction of ram air into the cabin. A window could increase the pressure with a slip into the open window, and decrease the pressure with a slip towards the side with the closed window. – mongo Apr 21 '17 at 00:26
  • Diagram would help here. – Notts90 Apr 21 '17 at 07:02
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    Sorry, @Notts90, I am not a good artist. If it helps visualize, think of a wing, somewhat symmetric along the chord, with "lift" on the top and on the bottom and in this example, on the sides. That "lift" is created by a small pressure differential (Re: Bernoulli Principle) caused by a small increase in air velocity as the air moves around the fuselage. I will see if one of this term's aerodynamics students would do a diagram with calculations for extra credit. – mongo Apr 21 '17 at 11:31
  • @mins I am sorry you are misled. The velocity of the air inside and the external flow are not really relevant. Where you are on the money is with your analogy to wings. This problem is similar to where there is a port to the top of the wing, and when that port is open, air leaves the cabin to match the pressure at the top of the wing. In the question instance, the entire fuselage is an airfoil, and the differential in pressure is integrated over the fuselage through leakage to the cabin, and this results in a lower absolute pressure in the aircraft. – mongo Apr 21 '17 at 14:42
  • The molecules of air in the aircraft remain there, unless part of the effluent "sucked" from the cabin through leaks in the doors, etc. The acceleration of the cabin air due to the fuselage velocity changes is not a relevant factor in this problem. If the plane fuselage were 99% lead would the problem be different? The lead having substantially more inertia than air. I am just trying to understand our dissonance. – mongo Apr 21 '17 at 16:13
  • The point is that inertia of the air inside the fuselage is not a factor in this problem. – mongo Apr 21 '17 at 19:14