# Why is the alternate static pressure lower than the standard static vent pressure regardless of the venturi effect on both?

I understand that the higher relative airspeed around the body of an aircraft in comparison to the air within the cabin causes the cabin to experience a lower pressure.

However, I was just thinking about this concept, and it dawned on me that I have never questioned why this causes a difference in this particular discussion as it relates to standard vs. alternate static ports.

Are the standard static ports on the side of the hull not also experiencing this effect as air passes nearly perpendicular to the standard static port openings as it flows along the body of the aircraft?

Why is the venturi affect stronger around the cabin than it is around the static port holes on the side of the hull at the same indicated airspeed?

• Venturi effect stronger around the cabin? Venturi effect only happens if the space where airflow needs to pass becomes smaller. Around the airframe (and over the static ports) you have the boundary layer. ?? Jun 24, 2019 at 9:43

A venturi effect pertains to a change in pressure caused by a CHANGE in airflow velocity. The static air pressure at the static port is not automatically lower just because the air is in motion there.

After all, the air within the fuselage (and within the piping leading to the static port) is in motion relative to the outside airmass, just as much as the outside airmass is in motion relative to the air inside the fuselage.

However, the overall dynamics of the flow around the fuselage do produce a net drop in pressure inside the fuselage, if it is unsealed. There must be locations on the fuselage that would be bad places to put a static port because they are the planes where this pressure drop is being created by the overall dynamics of the flow.

Another highly related answer-- Does the pressure at the static ports drop as the aircraft's speed increases?

• This answer could be made more complete with additional content but I think it gets at the heart of the matter. Jun 24, 2019 at 14:24
• I think my point of confusion is why is the pressure in the cabin (or wherever the alternate static ports are actually located) lower than in the standard static ports that have the same speed of air (I'm assuming) flowing over their openings? Except the "openings" on the alternate would be things like cracks in the door seals and windows. is it because there is more exposed leak area and the pressure drop is cumulative or what? Jun 24, 2019 at 14:57
• I think the pressure drop in the cabin is caused by the effect of leaks located in places where the airflow is being forced to accelerate. It's a good question and it's possible that my answer could be further improved. Jun 24, 2019 at 15:00
• I wonder if it doesn't have to do with elongation of the opening in the direction of flow such as the top of the door seal. Jun 24, 2019 at 17:03