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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.


Field test: F3D Remote Doppler Recording


The problems of collecting engine/flight data for Doppler analysis have been analysed previously on this website. The placement of the microphone is important in interpreting the data obtained; however its positioning is determined more often by convenience in operation than ideal placement. For this reason, the use of Walkie-Talkie 2-way radios has been proposed by Supercool, with the addition of automatic gain control units to improve sensitivity. These permit ideal location of the microphone, combined with convenient positioning of the tape recorder. But let us take one thing at a time, as we detail the field tests of these units.

In the case of F3D, by far the most convenient, simple and low cost approach is to place a tape recorder in the pits. The RPM and airspeed, thus determined, represent some sort of average coming out of #3 pylon and entering #1. These are useful parameters, although they are unlikely, for example, to indicate the true speed and RPM in the straights.

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Fig 1: Click image to see full size

A sample recording for this position is shown in Figure 1 (172 KB). The tape recorder was a cheap Digitor unit, which suffers from internally generated noise. Despite this, it has very good sensitivity, incorporating an automatic gain control circuit (AGC), with good battery life.

The Spectrogram figure shows a multiplicity of harmonics, mixed in with considerable wind noise. No attempt was made to shield from the 60kph wind, nor the 35degree heat. The data are readily reduced to RPM and airspeed, so things look good for this approach.

For the uninitiated, Supercool operates out of Perth, Western Australia: a location best described as a US Naval Warfare Base. In January, the weather pattern comprises a strong morning Easterly, warming to about 35 degrees by noon and then cooling as a strong South Westerly (the Fremantle Doctor) arrives about 1:00 PM. Perth is, I believe, second only to Auckland as the worlds windiest city. Thus take-offs are rapid, landings short. Wind corrections are required for Doppler data, as the wind alters the speed of sound as seen by the tape recorder.

We then switched to Uniden transceivers fitted with Supercool external AGC's. These latter were placed in woolly socks to cut down the wind noise. This appeared to work quite well, with the units placed out past #1 and past 2 and 3 pylons in an attempt to capture 2 traces for additional information from the one flight. This failed to work, as the wind blew one of the mounting tripods over; my age-imperfect sight could not detect this anomaly from 200 metres away.

In addition, an amplifier was placed between the Uniden units used as receivers and the tape recorder, to give a boost to the signal. Note that 4 Uniden units were in place here, 2 pairs, for receiving and transmitting on 2 different frequencies. The receiver outputs were tied together with a mono-audio 3.5mm T-piece (for want of a better name).

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Fig 2:Click image to see full size

The result is shown in Figure 2 (162 KB). Rather then the set of harmonics seen above, the fundamental frequency was strongly represented, with the harmonics nearly absent. Why this should be, I have no idea, and I'm not running back to that hellish flying field to find out. Analysis of this fundamental harmonic is again straightforward. Interpretation is improved, as being out past #1, these data give a good indication of the speed and RPM in the straights (results which rather appeal to me, as the speed everywhere else will be lower)

Finally, at an earlier testing session, we obtained data from the Unidens without using the rather expensive Supercool AGC's. Again, they were placed out past #1 and pylons 2 and 3, but this time the transmit-keys were held down with rubberbands. The Uniden units appear to have a battery life approaching 10 hours in the transmit-mode; when using 1700 mA/hr LiMH batteries, continuous transmission during a testing session is no problem. This saves all the walking to turn them on and off.

The receiver outputs were again tied together, but no amplifier was added ahead of the tape recorder. Consequently the signal recorded was a little weak, as can be seen if Figure 3 (137 KB)

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Fig 3: Click image to see full size

The traces can be seen to be interlaced. Normally, this figure would represent 2 models up together with one microphone. However, in this case, there is just one model and 2 microphones.

Accordingly, one trace from #1 position gives the speed in the straights, while the second trace gives the speed and RPM rounding 2 and 3. Lots of useful data from just one flight.

I think it fair to say that the field tests show all the methods to be useful, it's just a matter of judging the best approach for your own testing.

And you are quantifying your F3D performances, are you not?

Note that in all these cases the microphone is placed outside the course: this means that the caller cannot operate the tape recorder. The recorder must not be placed inside the course. For Doppler to work, we require both the frequency with the model headed directly at the microphone, and directly away from the microphone: this is achieved automatically when outside the course.

They correspond to the maximum and minimum frequencies in the Spectrogram trace. These frequencies are obtained using the very valuable Spectrogram program from Richard Horne's website.

Until next we meet, earn lots of money so you can afford the very nice Supercool props and AGC's, so that Supercool can have some fun too.


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