# In an unpressurized cabin, why is the cockpit ambient pressure lower than the outside pressure?

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?

## 1 Answer

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.

• 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. youtube.com/watch?v=eHnhRrbyEIE Apr 21, 2017 at 0:08
• 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. Apr 21, 2017 at 0:24
• 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. Apr 21, 2017 at 0:26
• Diagram would help here. Apr 21, 2017 at 7:02
• 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. Apr 21, 2017 at 11:31