By using a Doppler Reading tool such as WaveOsScope which can be downloaded for free at the end of this page, one can measure the speed of their airplane or aircraft by a simple sound wav file.
The Doppler effect changes the pitch as a function of the airspeed of the model. If one measures this pitch shift, one can exactly determine the airspeed up to few km/h.
WAVEosSCOPE can read WAV files which are sound files recorded with a camcorder or any device which records sound i, and analyze the frequencies of the signal.
In simple terms, here is how this works, you place your sound recorder next to you while flying your plane, let the recorded record while you do a flyby, this fly-by should be a perfect one, starting from distance, fly your plane as close as you can to your recorder and do a pass the same distance where you started the point of start, all in one straight line. Now this sound file should be taken to a sound application such as sonly sound forge, or Microsoft sound recorder, and converted to 8.000kHz mono since this is the best frequency for more accurate results.
after you have your sound file, it should sound like this -> click here to download this sample flyby wav
Now open your WaveOsScope application, choose the sound file you just created, and you will get a screen like this:
Now from the screen, you will see a few red S shaped graphs, which really is your flyby in a graph format, now pick the most clear and visible S that has the highest peak like the image above, right click on the start of the left S and right click on the end of the S as shown above, you will notice now that on the top of the application, you get your Km/h results, which can then be converted into mph.
If you have problems and do not understand, please comment on the bottom and i will gladly help you.