Crude (cheap) airspeed sensor using Mic?

[AnthonyRC]

Ok, so maybe this is a crazy idea, but might be worth a little discussion.

Does anyone know why one couldn't use a microphone as a crude airspeed sensor?

The idea is simple, just hang the thing out in the wind, possibly on a wingtip to get it away from the motor noise (which on electric models is minimal).

Could be calibrated on a calm day (no wind) using a GPS.

I'm sure that it wouldn't be as accurate as other techniques, and might require more than one mic with a pitot-like assembly to handle side-winds, but it seems that for low-end UAV use it might be adequate.

[Blue Sky]

An interesting idea. You could put the mic in the end of a small tube, making a resonant cavity. Then use a band-pass filter and integrate the output to get a
voltage level that reflects the air speed.

-Dave

[AnthonyRC]

What would the advantage be of the resonant cavity?
I was thinking more of a small tube with the back-end open, and the Mic in the airflow.

[Blue Sky]

My thought was it would reduce noise and increase sensativity.
Perhaps not necessary.

-Dave

[AnthonyRC]

Might make sense, my fear would be that a resonant cavity has to resonate at some frequency, but in this application we're really interested in white noise.
Might be interesting to high-pass filter the mic output (with a fairly low cutoff) to remove the energy contributed by the motor and its vibration of the airframe.

Time to give this a try I think, could just squirt the raw audio down the audio channel of a 2.4GHz video transmitter and capture/process/analyze on the ground.

[Hovertime]

I don't see how it would work... Microphones are lousy airspeed sensors ;)
Then its better use my old cheap crude idea - use old non-optical mouse components-optical encoder wheel with small prop mounted on its axle...

[coafin]

Use force sensor (spring) inside hull, between nose and tail. More speed = shorter plane.

[AnthonyRC]

I don't see how it would work... Microphones are lousy airspeed sensors ;)
Then its better use my old cheap crude idea - use old non-optical mouse components-optical encoder wheel with small prop mounted on its axle...

I'm not sure why it wouldn't work. Ask any motorcyclist what helmet wind-noise does with increasing speed.

[Unterhausen]

might work. I'm actually trying to figure out how to use microphones in flight for a different purpose. If I ever get around to it, I'll report back.

[Blue Sky]

My idea was to use a fixed frequency, the amplitude of which was determined by air speed, not white noise, which is, well, noisy!
Until someone tries your idea this is all just angels on the head of a pin.

-Dave

[Tuner]

Being an audio engineer I can help!

An Omni Directional Microphone capsule would be the best.
You can purchase small Electret capsules that are used in Cellphones.....
Make sure its Omni Directional
Omni Microphone have a diaphram strecthed over a vented cavity like a pot. These microphone are called pressure gradient microphones so they are sensitive to pressure variations.

You could use an average voltage output and this would increase as the noise levels increas BUT with this technique keep in mind High frequecies have less of an effect than low frequencies so a quite low pitch noise would register more than a loud high pitch noise.
I would recommend using a Shelf filter (looks like a mesa or plataeu) cut out all frequecies below and above a certain frequency. My recommendation would be to allow the frequencies of 200-700hz.
This frequency range stears you clear of bumps and wobbles(0-80hz) and high frequencies associated with motors(800hz or greater).

If you leave the microphone in the plane it could pickup more motor noise than wind noise at low speeds this could be a problem. Even at higher speeds the higher motor power may still create noise that overpowers the wind especially with Gas Motors.

Me personally I would start with the mic in a small sealed plastic box with a tube. Have the tube exiting the wingtip and pointing to the Rear of the plane. This is similar to a Pitot used for airspeed but we are not messuring air pressure because although Omni capsules are pressure sensitive they are made to equalize with the ambient airpressure they only sense rapidly changing aipressure. So using a tube pointing to the rear will cause turbulance at the end of the tube. The end affect of the turbulence is a changing pressure from a vacume to normal ambient pressure. Cutting the tube at a 45 degree angle or in a V shape may help increase the turbulence at the tip of the tube, a good thing.

The problem with this technique is it would create a low frequency oscillation (around 1-700hz) that could be confused with other low frequencies but Im not sure of what maybe bumps in the air will rattle the airframe... Isolating the box in foam will help but some vibration will be transmitted to the tube and can be amplified inside the tube so the tube should be very flexible like silicon/rubber fuel tubing to reduce this effect along with putting foam inbetween it and where it touches the frame.

One problem with this is if the tube has to big an inside diameter and/or if the box is to small the microphone will max out and it would reduce the speed and speed range it would work for. So youd have to experiment.

Me I need a device like this but I only need to know if I am above the Stall Speed.
I had considered using a spring loaded, hinged flap on the body of the plane attached to a Potentiometer and as the airspeed increases the flap will rotate until it is flat against the body of the plane. This should give me a good idicator of stall speed if I adjust the flap size and spring tension appropriately.


Just my 8 cents.
Scott

[kd7ost]

I've pondered this but don't see how it could work. At least not easily. The microphone puts out electrical signals based on sound "waves". Variations in pressure against the diaphragm or whatever it uses. It depends on variations in that pressure to change output. Put a microphone in the wind and it gets a half cycle of motion. It gets compressed. You might use a device that has a whistle and the microphone listens to the whistle. Like a variation on an old siren or the Little Orphan Anny whistle ring. Maybe you could use an Ultra Sonic deer whistle sold as a gadget for automobile use. Then use an Ultra Sonic transducer and calibrate the pitch of the whistle with air speed. You would have to keep it clean though.

I've seen an article or two where guys used a Mouse optical encoder wheel coupled to a small bearing supported prop, more like a boat prop, sticking straight ahead into the laminar flow of wind out on a wing end.

Dan

[AnthonyRC]

If you blow into your microphone does it generate white noise?
If you blow harder does it get louder?

I think this would work, my only concern is that a cross-wind would generate as much noise as a head wind.

[kd7ost]

If you blow into your microphone does it generate white noise?
If you blow harder does it get louder?

I think this would work, my only concern is that a cross-wind would generate as much noise as a head wind.

I don't know if thats white noise but you are right. It does get louder. You just need an audio amp and then an A to D converter to translate the Amplitude of noise into something that can be read by the processor. It still seems easier to use a digital source to begin with though.

Dan

[Hovertime]

It only gets louder to a point. Please take a few minutes and watch a couple aerial RC videos that have audio....