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Old 03-17-2018, 10:19 PM   #1 (permalink)
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airspeed indicator

I don't get into the Instrumentation sub-forum much, living in a pre-OBDII world. but I noticed this:


Just A Car Guy: smart way to see if the mass air flow sensor works

Rather than adapting a light-plane pitot tube, couldn't the output from a mass airflow sensor be read via an Arduino input and compared to road speed? The front face of a rear view mirror might be a good monitoring location, with an annular outlet. You could have one on each mirror and DIFF them for crosswinds!

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Old 03-19-2018, 11:19 AM   #2 (permalink)
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That could be a good tool for adding another variable into your fuel mileage calculations. It would give you true aerodynamic speed. for comparisons when you are evaluating changes to the body or running gear. 60 mph (through air) vs 60 mph (through air) regardless of ground speed.

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Old 03-19-2018, 11:28 AM   #3 (permalink)
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Quote:
Originally Posted by freebeard View Post
The front face of a rear view mirror might be a good monitoring location, with an annular outlet. You could have one on each mirror and DIFF them for crosswinds!
I would think you would want them out of the disturbed air near the car, maybe mounted a foot or so in front of the mirrors so they could read ambient wind speed (+ vehicle speed) without the near car squeezing and stretching of the airflow.
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Old 03-19-2018, 12:28 PM   #4 (permalink)
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(FYI: Within five minutes of posting you can Edit your post without accruing an edit notation. Just use the button in the lower right.) But hey, three replies.

An aircraft pitot tube is about a foot long to pierce through the laminar flow and best practice is to place it at a stagnation point. On a car that would be the nose.

I don't know the velocities involved in the intake system. A tube smaller than the throat might make measurement difficult, else a bellmouth could gather more air.
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Old 03-19-2018, 01:08 PM   #5 (permalink)
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Quote:
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(FYI: I don't know the velocities involved in the intake system. A tube smaller than the throat might make measurement difficult, else a bellmouth could gather more air.
I would think intake velocity could be calculated By multiplying displacement by RPM, or, aren't Carburetors rated in CFM? Wouldn't that apply to throttle bodies too?

BTW, thanks for the edit tip.

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Old 03-19-2018, 07:33 PM   #6 (permalink)
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I just want to see a pitot tube installed on a vehicle, that and a wind vane to get wind direction too.

Of course, a malfunction of the pitot tube would likely cause the driver to stall the vehicle with tragic results.
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Old 03-19-2018, 08:08 PM   #7 (permalink)
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Surely, you jest!
Ground vehicle "pilots" have a speedometer and other visual clues to determine their speed. The pitot tube is just for research.
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Old 03-20-2018, 12:08 AM   #8 (permalink)
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Old 03-21-2018, 08:21 AM   #9 (permalink)
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There used to be charts available for airflow vs throttle position. However if you have obd2 you can read it semi directly, if you have a spare light plane airspeed indicator you could just drill a hole, shove a tube into the duct and read direct
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Old 03-21-2018, 12:40 PM   #10 (permalink)
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Sounds like a great idea to me. I work with MAF sensors at work for their intended purpose, measuring airflow. Some quick numbers from an engine I'm working on now:

Idle airflow - 6 g/s
Peak airflow - 660 g/s

Air is about .0806 pounds per cubic foot. Just math to get to speed...

With a 4" tube this equates to about 1 MPH at idle, and about 140 at peak flow.

Seems like an appropriate measurement range. The ones I work with output a frequency, but there are others that output a voltage. Voltage being a bit simpler to measure easily.

Just need some wind tunnel time to calibrate it.

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