Calculating wind pressure against a rigid wall

[Cyrus]

Compare to a chamber connected to a static port that is at ambient pressure, or have a really good pressure gauge. The pitot probe is embedded into the center of the wall, so that it's just a flush mounted hole in the center of the wall. Basically it's just a static port mounted to the nose of an aircraft so it acts as a flat plate:

I think you missed what I was saying. That setup won't measure the total stagnation pressure because there is flow perpendicular to the wall face. Just look at the picture I posted and you can see the air flowing transverse to the free stream.

staticports.html

The hole is the end of a pipe connected to a pressure measuring chamber imbeddeded within the "wall".

You would just be measuring the static pressure at the wall face.

The real question would be if I mount a static port on the nose of an aircraft so it's essentially a flat plate, and compare it's pressure reading versus that of a conventional pitot tube (both would have pipes feeding internal chambers as usual), will the sensed pressure be different, and if so, by some approximate ratio? I assume there's some reason that static ports are flush mounted and pitot ports are extended tubes and not flush mounted.

They will both be wrong due to installation errors. They will absolutely be different from each other because the upwash/downwash at the wing will change the reading of the probe at that station. The reason why Pitot tubes are not in the nose is because that is prime real estate on an aircraft. People put things like radar and antennae in the nose.

I know that static ports need to be flush mounted because the end of a tube perpendicular to air flow experiences a vortice that reduces pressure greatly, enough to draw fluid up through a nozzle for the purpose of a spray pump. One of my pet peaves are the web sites or articles that use the end of straws in a cross flow to measure the "lower pressure" of air blowing across the end of the straw by drawing water up the straw to demonstrate Bernoulli. Stick the end of a straw in a spool of sewing thread to make a crude static port and the results are quite different. Drill a hole in a board and stick the end of the straw into the board so it's flush mounted or receded a bit, to make a better static port, and compare to having the end of the straw extended into the wind.

That's right. You don't want any burr from the end of the static port extruding into (or outside of) the boundary layer.

 

[Cyrus]

You might like to read this Jeff:

http://img17.imageshack.us/img17/596/pg1q.jpg

http://img177.imageshack.us/img177/9561/pg2.jpg

http://img10.imageshack.us/img10/7754/pg3p.jpg

http://img41.imageshack.us/img41/8629/pg4g.jpg

 

[rcgldr]

hole in center of wall

That setup won't measure the total stagnation pressure because there is flow perpendicular to the wall face.

Perhaps the center of a flat plate where the perpendicular flow is minimal (or perhaps this isn't possibe).

static pitot at nose of aircraft

They will absolutely be different from each other because the upwash/downwash at the wing will change the reading of the probe at that station.

Wing? I mentioned nose of aircraft.

The reason why Pitot tubes are not in the nose

Yet that's where they are on the Stratoliner in the example shown.

So the issue with the forward facing static port flat plate is that horizontal flow across the hole would be an issue. Since the static port is flush mounted, there is horizontal flow, but the hole "hides" behind a boundary layer that transitions from fuselage speed to free stream speed through viscous layers, and with proper installation it senses the ambient pressure of the freestream flowing across it.

Why isn't perpendicular flow (even if just a small amount) an issue for pitot tubes? Essentially the pitot tube's opening contains a cross section of air that is the equivalent of a tiny flat plate.

 

[Cyrus]

Perhaps the center of a flat plate where the perpendicular flow is minimal (or perhaps this isn't possibe).

I don't know, but that's why I said you would have to vary the probe until you find a maximum reading of the dynamic pressure (if that's even possible).

Wing? I mentioned nose of aircraft.

I misread what you wrote, now I see what you mean.

Yet that's where they are on the Stratoliner in the example shown.

Well, it doesn't have radar in the nose: it's an old airplane. Ideally, you would like to put the Pitot probe on a boom extending out the nose, and many aircraft do this. My point is that sometimes things get in the way that take priority over the probe:

or this:

http://www.totalexperience.co.nz/img/Cessna%20nose.jpg

So the issue with the forward facing static port flat plate is that horizontal flow across the hole would be an issue. Since the static port is flush mounted, there is horizontal flow, but the hole "hides" behind a boundary layer that transitions from fuselage speed to free stream speed through viscous layers, and with proper installation it senses the ambient pressure of the freestream flowing across it.

Yes, that's right. The static port measures the static pressure inside the boundary layer because of the no slip condition.

Why isn't perpendicular flow (even if just a small amount) an issue for pitot tubes? Essentially the pitot tube's opening contains a cross section of air that is the equivalent of a tiny flat plate.

My estimation is that the component is so small it's ignored, but I'll think about this one some more. I want to say it's a matter of "Good enough". I know flows can be tracked using PVIs (http://en.wikipedia.org/wiki/Particle_image_velocimetry" ) if you want very accurate measurements of flow properties. The guys in some of our labs use this method for rotorcraft flow.

 

[rcgldr]

Why isn't perpendicular flow (even if just a small amount) an issue for pitot tubes? Essentially the pitot tube's opening contains a cross section of air that is the equivalent of a tiny flat plate.

My estimation is that the component is so small it's ignored, but I'll think about this one some more.

I was also thinking about the affect of AOA. An F16 fighter AOA can exceed 20 degrees (such as a 9 g turn). Wondering if the pitot tube is on a motorized mount to maintain it's orientation with the airstream or if it's just taken care via math in the electronics.