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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


Doppler revisited


First a word to the Editors of this fine newsletter. You just keep it coming regularly on the 3rd of each month, and there won't be any riots! My goodness, it was tough going without a January edition, I nearly started listening to the golf! And where was last issues editorial, I'm still looking for it? How about 2 editorials this issue to make up!

Now I hope you liked last issues' introduction to Dr. Doppler, because this is another serve of the same dish, just warmed over a little, so its going to ramble, on and on, somewhat. 

Re-reading that article, I realised I had not really given enough credit to Konstantin Zeldovitch's Oscilloscpe 2.51. This program yields not just a virtual oscilloscope, but a dual channel digital storage oscilloscope with single channel audio spectrum analyser. For just $30 spent on the sound card preamp, you will never see value like that again in this life.

Unless of course you download Richard Hornes Spectrogram
Huh, why?

Well the truth is I exaggerated somewhat the ease with which the coming and going frequencies required to get the airspeed and RPM could be obtained from Oscilloscope 2.51. In fact it took both Rosemary and myself to have enough fingers to push all the scope and tape recorder buttons required to get those numbers. The fact is that the "scope" style of display is tricky to use. It occurred to me that if the spectra could all be stored on disc then I may be able to write a BASIC program that would do that for me; ie, the spectrum analyser would be more useful if it had a data logger facility.

So I hopped onto the Information Supercool highway and found Richard Horne's site at http://home.swipnet.se/~w-41522/ There I downloaded gram4265.zip, a 169K file. What a revelation!

This Spectrogram program was used by radio enthusiasts, so Richard incorporated a data logger into it. Otherwise he was using it to get the audio signature of whales, birds and the like.

So what is a spectrogram? Unlike the "scope" type display which plots frequency against sound level at one instant in time, the Spectrogram output is a "scrolling" display.

The display has time on the horizontal axis, frequency on the vertical, and sound level is represented by a color scale. This is a revelation. The harmonics we need for Doppler appear as wavy lines across the screen, so that the peaks and dips which occur with the model coming and going are laid out as clearly as a Picasso painting.

All one has to do is to halt the scrolling, place the cursor on the peaks and dips and read off the frequencies. That leaves Rosemary free to go back and practise her recorder. One can also read off the time, so that lap times also may be extracted from the tape recording. 

Spectrogram image

Furthermore, the data logger feature permits automatic analysis of these data off-line. I have written a BASIC program which does this; the results accompany this text.
Plot of fundamental harmonics

We have had some more experience operating this system now, with some lessons learned.

Some guys in Sydney wanted their pylon racers (F3D) checked out. So they phoned me from Pitt Town and held up their mobile phone as the model screamed around. Fifteen minutes later I phoned them back and gave them their inflight RPM, airspeed and their lap-times! Pretty neat.

But there were problems. For some of the time, the phone cut out altogether. It appears that if the sound level goes beneath a certain threshold, the phone cuts off. This apparently to reduce background noise in conversations. Otherwise, everything worked fine. The same guys also played a tape recording over the phone, which I recorded at this end using an inductive coupler. The result was a little disappointing with respect to noise level, but still quite useable. Since then I have gone to a capacitive coupler, and this works extremely well.

On the C/L side, Bob Fry brought around a tape of his Bendix with Grant Lucas' speed model in the background. We had some problems getting the signal strength strong enough. It didn't help that Bob was in a hurry, but we got there in the end. It appeared that the Bendix/Nelson model was not showing strong high order harmonics, so we had to go on the weak fundamental. The harmonics may have been there, but I did not have time to hunt for them.

The airspeeds we got were about 8% low compared to a stopwatch. It transpired that Alasdair had placed the tape recorder on the ground, close to the circle. This meant that there was a big angle between the plane of flight and the moments when the model was approaching and receding from the tape.
This gave a large height correction factor, which one is better off without. The tape should be held at the same height as the model. It also helps if the tape is about 20m back from the edge of the circle, as this places the peaks and troughs on the Spectrogram further apart.

We also analysed Grants speed model, which was 100m away in the background. He won't mind me telling you that it was doing 130 MPH at 24000, because it is his Picco 40 monoline trainer and he is just foxing anyway! But watch him go in NSW at next years Nats! 

That's it from the prop Doctor for this month, so keep on circulating!

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