Has anybody built a pitot tube AS indicator?

[longshot 0]

Hi All,

I was wondering if anybody has built an pitot tube air speed indicator or AOA indicator based on the MPX pressure sensors.

Shot, out

[Mr.RC-Cam 123]

Related projects:

http://www.chrisgood.com/uav/avionics/airs...d_slide0024.htm

http://www.electronic-engineering.ch/micro...html#components

http://www.freescale.com/files/sensors/doc...note/AN1573.pdf

http://www.grc.nasa.gov/WWW/K-12/airplane/pitot.html

[Mluvara 0]

Some quick advice -

AOA indicators using pressure are VERY difficult and require special calibrations of the system, including a specially designed probe. They are commonly referred to as five hole probes, because they use airspeed, AOA and AOS measurements (requiring 5 holes), plus a set of static ports. This type of probe also needs to be designed for a specific flight envelope as it is difficult to design one that operates over a wide range and still expect accuracy at the lower end. Simply put, pressure is not linear with airspeed.

If you do a pitot-static system for IAS, it is essential you get a good static port measurement. Without it, it's a basically a cr-AT-p shoot trying to reference to fluctuating pressures inside a fuselage.

Regards,

Michael

Edited April 22, 2005 by Mluvara

[Temporary Insanity 0]

Will somebody set me straight here? Every time I look at the equations, I see that the frontal area of the pitot tube has no bearing on pressure at the sensor, yet, when I think about it, it seems that it would have a direct influence. How about a simple explanation?

[Terry 5]

The frontal area will not change the pressure of the air only the volume of air. You may well be able to get a increase in pressure with a funnel shape as it will cause a ram effect but this is different. You may also get losses using very small tubes.

Terry

[Temporary Insanity 0]

Can you explain the small tube losses?

[Terry 5]

Air is like a liquid, it will stick to the walls of the tube. The air close to this moves less easily until you get about 1mm away from the side of the tube and then there is no drag or restriction in flow. I guss you should not use a tube smaller than 3mm bore to be sure.

Terry

[Temporary Insanity 0]

I'm a little lost since this is a static application. Know what I mean?

[Terry 5]

Yes I know what you mean !...I used the phrase MAY get losses as I am not at all sure if it will effect a static application...I think the only way to be sure is try it. Do you intend on using tubes smaller than 3mm ?

Terry

[Temporary Insanity 0]

I was thinking that a 1/8" O.D. brass tube would be good.

[Terry 5]

Well thats 3.2mm so my guess is that it will be ok.

Terry

[MikeDD 0]

A commercial unit;

http://www.emsjomar.com/SearchResult.aspx?CategoryID=3

[Temporary Insanity 0]

Looks like 1/8" brass tubes to me.

[Mluvara 0]

Pressure is pressure. Size of the port shouldn't matter once you build up pressure in the tube. There really is no flow since the pressure sensor has a sealed device at the other end. The biggest issue is to get proper static port placement and make the front of a combined pitot-static probe have a good radius.

I use 1/8" pitot-static probes all the time. The dynamic port is 1/16" tubing whereas the outer shell and inner tube form the boundary for the static port.

Michael

Edited May 13, 2005 by Mluvara

[dilbert 0]

The frontal area will not change the pressure of the air only the volume of air. You may well be able to get a increase in pressure with a funnel shape as it will cause a ram effect but this is different. You may also get losses using very small tubes.

Terry

So I gather that the pitot tube opening should match the pressure sensor opening, so that there is no ram effect. Is this right?

[Terry 5]

No, on reflection I dont think you can get ram effect on a static application.

Terry

[radiohound 0]

Here is one more airspeed indicator with pic basic pro code:

http://www.steveonweb.com/index.php?id=29,0,0,1,0,0

If that link does not work, you can get to it from here with a bit of navigation:

http://www.steveonweb.com