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Jan 07, 2011, 05:46 PM
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Discussion

Low range airspeed measurement


I am looking to measure rather low air speeds (>50 mph) and am wondering what people are using for airspeed measurements in this range and how accurate they are?

From my math I need a sensor with a full span of 0.07 psi (~0.5 kpa). Sensors with that low of a range are all rather expensive compared to the higher range ones such as Freescale MPXV5004DP give accurate measurements in the lower speed ranges?
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Jan 07, 2011, 06:07 PM
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I use the mouser 841-MPXV7002DPT1 from 125 mph down to 14 mph. Less than
that is sort of noise. Use a real tiny pitot tube, and good flush static port,
not by the props.
Jan 07, 2011, 10:34 PM
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Tom Harper's Avatar
What is the application?

Tom
Jan 07, 2011, 10:49 PM
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More accurate airspeed indication on a Lazair ultralight. Stalls around 20 mph cruise at 35-40 mph Vne is 55 mph.

My uncle owns and fly's one and the steam gauge it is currently equipped with is pretty useless, the needle does move off the stop till you are at 25 mph and the rest of the range is mostly unused as it goes all the way up to 90 mph.

I am starting to research a digital gauge cluster for it and it happens to have a flight envelope that is very similar to R/C aircraft. Looking to do altitude, airspeed, and RPM EGT CHT for both engines. Probably drive a sunlight readable LCD using an Arduino.
Jan 08, 2011, 08:02 AM
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Tom Harper's Avatar
If you are into electronics you could make an accurate device with a pair of ultrasonic transducers.

Another possibility is the vane units used for testing air conditioning systems. They accurately measure low flow rates. They're available from companies that sell surveying instruments. Worth looking into.

Tom
Jan 08, 2011, 08:10 AM
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At low speeds you can look into meteorology measuring instruments,
the most relevant would be ultrasonics, or there is one that uses air passing over a precision heat source.

Or you could go the laser doppler option!
Jan 08, 2011, 09:39 AM
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I suppose I could make a transit time ultrasonic flow meter but it still needs to be light as well as inexpensive (a Lazair is worth about $3000 in flying condition)

I would still prefer to use a pitot static system and if the MPXV7002DPT is able to give a good signal down to 14 mph then it should be fine. That very start of the curve is always going to be finicky because it is using the square root.
Jan 08, 2011, 05:33 PM
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Cheapest, simplest, reliable, is the Blowback Vane as used on light aircraft for, believe it or not, nearly a hundred years. At one time standard equipment on aircraft such as DeHavilland "Tiger Moth" biplane. I have used myself, learnt to fly on Tigers. The vane was mounted on a wing strut further out than the prop slipstream, which would have given a false reading of course. The Lazair being a twin with wing struts, no problem. You could make one from scratch, hang it out the car window to calibrate it, mark where the vane was at different speeds. I have made a few for research on big UAVs, with a camera normally looking ahead, pivoted to look at the exterior mount vane when needed. The picture is of a vane on a Tiger Moth wing strut, the pic says it all. They can work down to very slow speeds, two or three mph if needed, a case of having the right spring pressure and vane area for the speed range. In passing, a mechanical system beats an electronic one on reliability and cost every time!
Last edited by macboffin; Jan 08, 2011 at 06:13 PM.
Jan 08, 2011, 05:48 PM
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Vane type is a pain in the ass on a lazair. You have nowhere to mount it where it is visible and where it is not affected by the propwash. Alot of people use rotometer air speed indicators but they have similar issues.

With a differential pressure system a pitot can be mounted in clear air for increased accuracy. The idea behind an electronic gauge cluster is to allow for far more measurements that you can fit in the tiny pod on a lazair.
Jan 09, 2011, 10:47 AM
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Another possibility might be the "Micro Gust Thermal Anemometer" project article featured in the February 2006 issue of Nuts and Volts magazine.
Unfortunately the complete article is NOT online but it is briefly described at the following URL. They also sell a fully assembled board for $39.95.
http://gellerlabs.com/MGTA.htm

The anemometer project uses an LM399 Zener diode as a sensor to measure tiny air currents. Although the article does not provide any exact numeric specification for how sensitive it is, the author mentions that he can puff several feet away from the sensor and see the output respond. The board requires 15 Vdc and draws 20 ma at no wind.

-John Elliott


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