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

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?

Take a look at this link, I didn't read it but the first graph can help you: https://visaya.solutions/en/article/wishiknew-static-pressure-measurement — Acsed., Feb 18 '21 at 12:36
@Acsed: Thanks. Unfortunately, that image depicts how to measure dynamic vs. static pressure, while I wonder how that static pressure is obtained in the first place. — Zsolt Szilagyi, Feb 18 '21 at 16:53
"both effects (Bernoulli and ram, so to speak)" – Those are one and the same, aren't they? The "Bernoulli effect" is the "ram effect." — Tanner Swett, Feb 18 '21 at 22:20
Does this answer your question? Why does the faster-moving air over the static port not result in a lower static pressure? — Peter Duniho, Feb 19 '21 at 00:43
Does this answer your question? Does the pressure at the static ports drop as the aircraft's speed increases? — Peter Duniho, Feb 19 '21 at 00:45
@PeterDuniho: That question has responses stating that "Zero static pressure error on the fuselage exists". My question asks for how/why are these points not shifted by changes in airspeed, so I start where the other question left off. But I begin to see that this should be rather asked in a physics exchange, as it might be beyond the scope of an aviation group. — Zsolt Szilagyi, Feb 19 '21 at 10:37
"My question asks for how/why are these points not shifted by changes in airspeed" -- i.e. you start with a false premise and ask why it should be so. That's not a useful question. The duplicates both rebut your premise and go on to explain how real airplanes deal with the actual effects of airspeed on static pressure. That's the best you can hope for, when you ask the wrong question. — Peter Duniho, Feb 19 '21 at 17:16

score 5 · Answer 1 · edited Feb 18 '21 at 11:27

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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.

edited Feb 18 '21 at 11:27

Bianfable

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answered Feb 18 '21 at 10:16

Abdullah is not an Amalekite

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Thanks for your answer! "Freestream air flowing undisturbed past a flat surface will have the same static pressure": How does that relate to a straw sticking together when air as blown through? (Or my shower curtain hugging me)? In both cases, it seems that a constant flow is present. – Zsolt Szilagyi Feb 18 '21 at 16:52
You will find sources that tell you that it's because fast air simply has low pressure. But the truth is this: fast fluid drags slow fluid, which drops in pressure. This pressure drop is more intense when confined, so fluid jet is attracted to surfaces – Abdullah is not an Amalekite Feb 18 '21 at 17:25

Jabirhee · Answer 2 · 2021-02-19T00:20:26.713

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.

This would explain how any remaining pressure change is accounted for, even after placing the port on a suitable location — Abdullah is not an Amalekite, Feb 19 '21 at 01:15

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

2 Answers2