Last week my wife and I were bicycling on the Holmes County Trail north of Millersburg. A little over a mile from the Depot, the Trail crosses a large open field that stretches for about a mile. About half way across, I looked up to see a twin rotor Chinook helicopter coming toward me just a few hundred feet above the ground (I later discovered that my data and little snippets of my recollection do not support the low altitude. See the pages linked below). I have wanted to get a recording of an aircraft to see what information I can pull from the sound. Here was my chance. I have a Canon Powershot G10 camera that has a fine audio recorder built into it. I stopped the bike and fumbled around to get the camera out of my jacket. I turned it on and started punching buttons to make it record. All the time the aircraft was bearing down on me. I didn’t have a lot of time to observe it but I did get a recording as the helicopter went right over head. It was a perfect pass.
I got about 22 seconds of sound. When I listen to the recording it sounds like a helicopter going over. It’s pretty good but I was disappointed when I looked at the spectrogram in Raven Lite. I had hoped for a distinctive Doppler signature brimming with information but it just wasn’t there. I let it rest for a day or two.
Then I listened again. As the aircraft passed overhead and receded away from me, the thumping sound of rotors intensified. When I zoomed in on that part of the time domain waveform I could easily see pairs of impulses. Ahhh! I think there is some information here! I calculated the interval between a couple of pairs and calculated the frequency. This must be the blade passing frequency. The twin impulses are from some phasing phenomenon of the twin rotors. I had been so focused on getting the recording that I didn’t count the number of blades on the rotor. My brother-in-law is a helicopter pilot and instructor in Florida so I sent an email to him with some information and questions.
A little research on the internet in the meantime showed that a Chinook has three blades per rotor and the rotor speed is 225 rpm. I divided the frequency I calculated by three and multiplied by 60 to get the speed…206 rpm. A little low but I assume that the speed changes like it does on a car for instance. Then I got a reply from my brother-in-law. He had forwarded my questions to a friend of his who flies Chinooks. He said the speed is 225 rpm. Evidently the rotor speed is kept pretty constant. Then I realized that the machine was moving away from me…it was Doppler shifted to a lower frequency.
As soon as I have a little time, I’ll add another page ( done ) in the sound analysis section describing this recording and the operating information I am able to pull from it. It should be interesting to see how much I can learn about this helicopter from a 22 second audio recording.