How can static pressure be obtained over a range of airspeeds? [duplicate]

Question

Multiple instruments rely on static air pressure, i.e. pressure unaffected by airspeed.

To my understanding, no such air should exist around an aircraft. Air in motion exhibits a lower pressure measured laterally, and a higher pressure measured head-on (ram air).

Switching to an in-cabin alternate static source usually decreases measured pressure, as the cabin altitude is above outside altitude, due to its equalizing with accelerated outside air.

The only solution I could think of was to measure pressure at an angle, where both effects (Bernoulli and ram, so to speak) cancel out. Now such an angle would be airspeed-dependent. How can a reliable pressure be obtained, without computational aid, over a range of airspeeds?

Answer 1

I used to have that misconception too.

It turns out, to get that pressure drop, you need to accelerate the air. On aircraft, this is done by changing its direction and causing it to flow around convex curvature. This occurs at the tail, on top of the wings, and often at subsonic speeds, on the nose just past the tip high pressure point, and beneath the wings. (Yes, subsonic flow is weird)

Freestream air flowing undisturbed past a flat surface will have the same static pressure - relative to the surface or absolute -, regardless of airspeed.

Answer 2

Airspeed indicators on older aircraft are often directly connected to static and pitot pressure through tubing that runs to the static ports and pitot probes.

Those indicators pretty much indicate the difference between the pitot pressure and static pressures. That type of indicator shows Indicated Airspeed.

Calibrated Airspeed is generally Indicated Airspeed corrected for instrument, installation, position, and configuration errors due to incorrect air pressure at the static port.

Aircraft Operating Handbooks for older aircraft often provide tables for determining Calibrated Airspeed at different speeds and configurations, (like flap extension) if pressure at the static ports are significantly affected.


True Airspeed is Calibrated Airspeed adjusted for temperature.

Modern aircraft have static ports and pitot tubes connected to an air data computer that also get inputs from other aircraft sensors (like air temp, and flap/slat extension) to accurately indicate a True Airspeed that is calibrated specifically for the the aircraft.

I've answered this as these terms relate to aviation because it's an aviation group. If the interest is a more general instrumentation and measurement oriented, maybe a company like Omega still has instrumentation handbooks online. A good avionics text book will walk you through the principles and calculations too.