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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.


Acoustic Antenna: Part 1 


Well folks, don't hold your breath waiting for Acoustic Antenna: Part 2! Its taken me 2 years to get this far already. If you are not into computers, the following may not be to your taste, but do give it a read.

If you have taken an interest in using Doppler to get the airspeed and RPM of your model, then this is for you. I know the F2A guys are using this, and really its essential to optimise performance and even see what the other guys are doing. Have a look at my other articles for more info.

The sound from a model airplane engine is comprised of a set of tones known as harmonics. The tones have an apparent higher pitch when the model is approaching you, and a lower apparent pitch when it is going away from you. This is a large effect, easily measurable, and was discovered and named after Herr Doppler. By measuring the pitch change, one can determine the speed of the model and the RPM of the engine using some very simple equations.

You need a tape recorder to store the sound, a computer with a sound card and Richard Horne's wonderful free program called Spectrogram to extract the tones. That's all, and it works a treat. You can even get lap times, acceleration and heat times, even with more than one model up. Good for F2C, you get speed and RPM for all 3 models.

Now after doing this for a while, I wanted more. Surely with all that racket going on, there is more to be had from a sound analysis. Would it not be desirable also to find the airplane trajectory? In R/C pylon race, a tightly flown course can win races. But if you go too far passed the pylons, you lose out badly, but may not even know you are doing it. Replaying the course you have flown on a computer screen would  let you know your mistakes. So what we need is a bunch of microphones stuck up in the air and configured in such a way as to reveal the position in space of the model as it flies along. I call this device an "acoustic antenna". It is going to need at least 4 microphones grouped in a bunch to find the direction of the model in 3 dimensions, so right away we have a hardware problem. Our previous Doppler method only needed one mike, but now we need 4! How do we get the sounds into the computer? Thought you might ask that!

Well it turns out that, not only can you connect a mike to the  mic-in port on the sound card, but with the right circuitry you can put 2 more into the line-in port. Not only that, you can install a second sound card into the computer, giving another 3 mikes! That ought to be enough! It may also be possible to add 2 more mikes to the sound card CD port, and even 2 more to the AUX port! So what about software? Clearly Spectrogram won't handle this lot, you are going to have to write your own code. The programming language BASIC is widely accessible to those with a computer bent, and the easiest of all languages to learn. I use a version called Power Basic, available from www.PowerBasic.com. You have to pay for this version, and its harder to use than say Quick Basic, but its very fast and is a fully compiled language. This means you need to be able to write code that will read and control your sound card. Here is an example of code that will read a sound value from the microphone port of just about any sound card in any PC.

OUT &H220 + &HC, &H20 
do loop until INP(&H220 + 14) and &H80
value = INP(&H220 + &HA)

There, wasn't that fun! Similar commands are available to read the other ports and control the sound card. Now, what do we do with the values from all those mikes we have stuck up in the air? Well, the idea is this. Because the mikes are separated from each other, a given sound wave from the model will reach the different mikes at different times. Since sound is really quite slow at 340 m/s, these times are easily measured. In fact, the mikes really only need to be about 6" apart, so our acoustic antenna will be quite small, about the size of a 
loaf of bread.

The quantity to be measured is not really time, but is called phase. If 2 mikes are square on to the model, then the sound will reach both mikes at the same time, and the phase is zero. If the 2 mikes are pointing at the model, then the phase is at a maximum, as the sound reaches one mike much sooner than the other. This means we can get an indication of the position of the model from the phase difference. With 2 more mikes arranged in a square, we can in fact find the exact direction in which the model lies. Determining its trajectory is then just a heartbeat away. The phase difference is obtained by determining the Fourier transform of the signals read from the mikes. These transforms yield the tones as seen in the Spectrogram program, and also the phase values. 

This really works. Sitting here tonight in my computer lab I have a tone generator (built from a Jaycar kit, price $50) in the corner of the room. I have 2 mikes, about 6" apart, going into 2 sound cards in my 486 DX4-100. Moving the mikes around each other, I can watch the phase change on my computer screen.  

So that is Part 1 of this development program; the feasibility study. It all works and cost me $40 for the extra sound card, $5 for the extra mike, $50 for the tone generator, and nothing for the computer, as Big Norm Kirton, the super-fit bus driver and scramble flier gave it to me! Cheers Norm! 

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