Strikes me one can measure the RPM of a flying plane by measuring the frequency of the noise it makes (beware of doppler effect) with a frequency counter. If one isn't handy, one could record the sound and look at it with any number of programs on a PC. Plenty of DIY kits and circuits on the net too.
Yup. Formula one teams with sound analysis equipment are able to determine using Doppler and the engine note their competitors speed, maxiumum RPM, and gear ratios, to astonishing accuracy.
On 4 Nov 2006 05:15:40 -0800, snipped-for-privacy@digiverse.net wrote in :
Hmm.
If you also recorded air temperature and altitude along with the sound of the plane, maybe you could calculate the airspeed from the Doppler effect. Or are there too many unknowns? It may be that you'd have to know the RPMs during the recording session to determine airspeed from the Doppler effect.
You're also going to have some noise in the data (pun intended; you may laugh now), aren't you? The carb and the exhaust make noises, too. And the airframe probably acts as a mixer for every vibration coming from the engine.
I think if I really wanted the information, I'd take a tachometer apart, put the sensor up near the prop, and radio the output back down to the ground in real time. It would take more hardware but less calculation.
Maybe you could even market the unit. Some kind of thing that could be strapped on to any plane big enough to carry it. Combine it with an airspeed indicator and you'd have a nice test bed for playing with various props.
Lemme know when it's ready. I'd pay $100 for a think like that.
OH, dang!
The pitot tube has to be out of the prop blast, doesn't it? And the reference tube has to be in calm air. The batteries, processor, and TX have to be hung around the CG, and the unit can't induce drag or it'll ruin the measurement of your top speed for bragging rights.
Complexity happens.
Let us know how your experiment turns out. One observation is worth a thousand expert opinions. ;o)
Marty
Marty
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I have looked at sound waveforms on oscilloscopes and it's actually very easy to tell the frequency if there's not much else going on. With a plane you'd just have to take the frequency/rpm reading when you fly it past yourself.
Most people test their props on the ground, which is no accurate indication of how it performs at normal airspeed.
As for telling the plane's speed, that's a different ballgame. The police use radar on cars. Simplest is to measure the time it takes between two points.
On 4 Nov 2006 08:45:19 -0800, snipped-for-privacy@digiverse.net wrote in :
Yes. It sounds like it would be fun and worthwhile to get some information while in the air.
That only measures ground speed.
The decisive factor for determining that one prop is more efficient than another is true air speed which I think has to be measured at the airframe.
The speed demons among us probably get along OK without the airborne data. It's us pluggers and gearheads who would like to have some more toys to play with. :o)
Marty
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a few years ago a circuit was published in one of the mags that was a variable oscillator feeding a small earpiece speaker, and all you did was tune the note of the speaker to match the note from the engine then read the RPM from the calibrated scale on the adjuster ,IIRC it was a
555 timer project
I thought that all cancelled out if you compare approach sound to sound leaving?
refer to
Engine Noise Reduction & Engine - Propeller combination test results
Doppler Effect - RPM on Ground and in Air Noise reduction - How to by Flying Sites. Noise Reduction - keep your flying sites Noise test results & procedures - Blackridge & District MFC and more on my web page
Regards Alan T. Alan's Hobby Model & RC Web Links
On Sat, 04 Nov 2006 19:40:06 +0000, The Natural Philosopher wrote in :
Beats me. I've never done any Doppler measurements or calculations, so I'm not sure what the requirements are for making good observations. I suppose if you had a tape, you could analyze it on both sides of the peak.
Marty
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On Sun, 5 Nov 2006 08:45:54 +1300, "A.T." wrote in :
Great stuff!
This guy figured it all out:
So, bottom line, it looks like the answer to the original question is "yes."
Marty
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Yup, seems another simple way of doing it.
You can even go to a music store and buy a 'Korg Mini tuner' which will ID the fundamantal or harmonic ...
Earlier than that (Fifties - that I know of) someone made a bank of reeds that were tuned to different frequencies. The appropriate reed (tuned closest to the engine's running rpm/frequency) would become mechanically excited and could be read to reveal the rpm. There were no electronics/electrical power involved. The reeds were acoustically coupled to the engine.
Ed Cregger
| Maybe you could even market the unit. Some kind | of thing that could be strapped on to any plane | big enough to carry it. Combine it with an | airspeed indicator and you'd have a nice test | bed for playing with various props. | | Lemme know when it's ready. I'd pay $100 for | a think like that.
If all you need to know is the RPM of your prop ...
If you need to know airspeed,
Definately fun stuff.
On Sun, 05 Nov 2006 19:04:47 GMT, "Doug McLaren" wrote in :
...
Yes--that looks like the complete deal: airspeed, RPMs, telemetry. The only problem is the price--and I'm sure it's worth what they're asking, just outside my "mad money" range.
I may be able to talk my buddy into a 4M glider with the Eagle Tree system on it one of these days ...
Thanks for the link.
Marty
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Splendid, but I'm happier saving the dosh and doing it a cheaper way.
| Path: be1.texas.rr.com!cyclone.austin.rr.com!news.rr.com!border2.nntp.dca.giganews.com!border1.nntp.dca.giganews.com!nntp.giganews.com!postnews.google.com!e3g2000cwe.googlegroups.com!not-for-mail | From: snipped-for-privacy@digiverse.net | Newsgroups: rec.models.rc.air ..
You should fix your newsreader.
| >
| > though that's a bit more expensive (because it does more.) | >
| > Definately fun stuff. | | Splendid, but I'm happier saving the dosh and doing it a cheaper way.
I didn't say this was the only way of doing it. Martin said he'd buy such a device for less than $100, so ...
The funniest part of the page on determining this by recording the sound is that if you can't figure it out, you can call him up (he gives his number) and play the recording on his machine, and he'll get back to you with the results :)
What the Eagle Tree micrologger is really good for is measuring your amp draw and voltage in flight, but knowing RPM and temperature is useful as well.
It's what was used to give you the dashboard on this video, for example --
(note that this video is worth watching even without that -- there's a big surprise about what happens when you don't secure your battery pack properly in the middle.)
The more expensive Eagle Tree device can keep track of where your plane is, servo positions, airspeed, voltage, amperage, temperature, G-forces ... and can even beam this to you if you buy that upgrade, while storing the information for later perusal.
If all you care about is airspeed and RPMs, it's certainly overkill, but it does much more.
On Sun, 05 Nov 2006 22:17:50 GMT, "Doug McLaren" wrote in :
It even seems to have an extra channel that you can use for a device of your own creation ($99 extra).
I do plan to discuss this with Uncle Dan some day. He's got to finish renovating his house first.
Marty
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The Nitro boat guys have been using an 'Audio' for quite some time. The ones I have seen measure RPM's in the 40-50 k range. They just point it at the boat and it reads the RPM.
How does it do it .....?.......thats the tach's business. :-) Actually , I do know how it works , but I have no idea as to what it takes in the way of electronics to make it work.
They really are a neat piece of equipment. I run Nitro boats some myself and it's a task to get the engine dialed in without one. Just can't rev it up and point a tach like we do with the aircraft. I go 'watch' some of the hot drivers down south race their boats but although I have a couple good runnning boats but can't run with these guys .Just sit and watch. Someone there will always have an 'Audio' tach.
Ken
Since the reed banks were used to control the various channels, you could always tell when your model went into a dive at any given frequency ;-)
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