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Old Dec 11, 2005, 09:14 PM
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I'm a late-comer to this thread. A couple of thoughts on this thread--

Airspeed--
Calculating true airspeed from indicated airspeed is useful for navigation, e.g. time * speed = distance. How the plane flies, e.g. stall speed, depends on indicated airspeed so there is no need to make the calculation to compensate for air density.

Total energy--
The TE probe provides a static source to the variometer so that the vario doesn't follow dives and pullups. As a result the vario shows a glider pilot the true lift as he/she enters a thermal even though the stick may be pulled back (converting kinetic energy into altitude).

Some years ago I flew sailplanes (fullsized) and we used Braunswcheig tubes for the static source for the vario. It is a closed off tube bent 90 degrees with several slots cut into the trailling edge. It looked similar to the one in the following url--
http://www.tek-variometer.de/englisch/index.html

An even earlier scheme used a diaphragm/valve arrangement that connected to the pitot tube and static source and fed the vario with a compensated pressure--much more complicated and expensive than the simple Braunswcheig tube. This was all before microprocessors became powerful & cheap.

My thinking for model airplanes is to do the total energy computation in the micro, using the pitot-static pressure as the input. One should be able to add some calibration points so that the compensation is improved. The basic scheme is based on the kinetic energy being proportional to the square of the speed and the polar of the plane is parabolic. With the newer high performance airfoils the polar departs a little from parabolic so that the basic TE compensation is not perfect (but vastly better than none).

Altimeter--
Intersema Sensoric, MS5534A has an interesting part--

http://www.intersema.ch/site/technical/ms5534.php

It is 27.15 Swiss France, about $20US, and has a built in 15 bit ADC. The shipping on samples is 30 Swiss Franc which doubles the price to the door (for one unit).

Given the trouble and cost of what needs to be added to get good resolution on the Freescale parts, the cost may well be worth it.
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Old Dec 11, 2005, 11:12 PM
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Silicon Valley
Joined Sep 2003
42 Posts
Quote:
Originally Posted by HLGNut

http://www.rcatsystems.com/index.php

that use a simple brass tube for the pitot tube and ambient air around the sensor for the static.
FYI - The rcats unit does not use a simple brass pitot tube. We have a very precise combined pitot-static probe that is designed for giving good measurements. Using ambient static inside a fuselage is not very accurate at all and in the majority of cases, will skew your data one way or the other and is dependent on aircraft speed/attitude,etc.

http://www.rcatsystems.com/accessories/pitotprobes.php

Michael
RCATS
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Old Dec 17, 2005, 08:09 PM
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Sterling Heights, Michigan, United States
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Finally got the boards and had some time to stuff it. So far it looks good. Now comes the programming part. It will be a while, anyone interested in one of these boards let me know, based on this design http://www.kapelec.com/altivie1.htm

Kin

http://www.embeddedtronics.com/
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Old Apr 24, 2006, 03:12 PM
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Los Angeles
Joined Aug 2004
383 Posts
Looking at this thread posts on the Pitot tube. I didn't see any specific measurements for length of the tube the width of the static holes and the tube bend radius.Any information on how to choose those parameter?
Thanks
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Old Nov 12, 2006, 01:27 PM
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Quote:
Originally Posted by rocky79
Looking at this thread posts on the Pitot tube. I didn't see any specific measurements for length of the tube the width of the static holes and the tube bend radius.Any information on how to choose those parameter?
Thanks
That is exactly what i am looking for... any hint much appreciated
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Old Nov 13, 2006, 01:08 PM
AndyKunz's Avatar
Illinois
Joined Sep 2001
24,040 Posts
Quote:
Originally Posted by Mr.RC-CAM
I definitely will be interested in how the AD7714 performs for you (looks like a nice part). It will take a serious PCB layout (faultless grounds and signal guarding) if you need all 24 bits.
We did one with it shooting for 22 bits, and grounding was a MAJOR issue. It does work as advertised, providing you get the layout right. We spent a lot of time on the phone to the chip design team trying to get it resolved. Eventually we cancelled the product.

We're using an MCP3551 now on a related product and having far less trouble all around. Not as capable, but 18+ bits of precision was easy. Getting test equipment just as accurate was fun - company politics

Andy
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Old Nov 15, 2006, 07:32 PM
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Redmond, WA, USA
Joined Oct 2000
752 Posts
Quote:
Originally Posted by rocky79
Looking at this thread posts on the Pitot tube. I didn't see any specific measurements for length of the tube the width of the static holes and the tube bend radius.Any information on how to choose those parameter?
Thanks
There was another thread that had a link to a german site (http://home.germany.net/100-173822/d_logger.htm) which had a relative-dimensioned pitot tube, didn't talk about your specific parameters, but I think the dimensions noted are the critical ones.:
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