Thread: Yippee! Doppler me Don!
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Old Feb 09, 2010, 09:06 PM
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How to make a good doppler pass:

To make a good doppler pass you need a few things, here are some pointers on how to make good doppler passes.

1) First you want to keep the background noises as low as possible. Flying with a loud gas engine in the background, or adding music to a soundtrack, or talking close to the recording camera can make it difficult to extract the audio needed for doppler.

2) You must fly at a constant throttle setting. For doppler to work, the "source" frequency must remain constant. This means your rpm must be constant during the doppler pass. If you rpm drops (due to voltage sag or moving the throttle stick) then your "source" frequency changes as well and you will not get a valid doppler reading.

3) You MUST fly in each direction at least 5 to 8 times the distance from the microphone to the plane. If your plane passes 100 feet from the microphone, then you must fly at least 500-800 feet towards the microphone and 500-800 feet past the microphone. So the closer you are the better. I shoot for 50 feet and I often place an MP3 player in the flying field, set it to record audio, then fly over it as low as I can. This way you are not flying too close to your camera man. If you do not fly the required distance ratios, then you do not get a complete doppler shift, and you will rob yourself of speed.

4) Fly in a straight line coming to and going from the microphone. Do not pull up or turn. Always fly in a straight line.

5) Do not gun the throttle, you need to be at full throttle as you enter your doppler pass, reach the microphone, then go past the microphone.

6) Use the "rule of 3". This means your approach should be about 3 seconds and your depart should be about 3 seconds. Count in your head each way.

7) Know the outside air temperature. The speed of you model depends on the frequency shift and the local speed of sound. The speed of sounds varies with air temperature so make sure you know the air temp within a few degrees.

When you follow these guidelines you will get fast and complete doppler shift and the reading will be very easy to do.
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Last edited by The Don; Apr 03, 2011 at 07:46 AM.
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