You may recall that Doppler shift is the veeeeerooooom sound you hear when an vehicle whizzes past you and the frequency of the sound drops. Police radar functions using the same physics principle, but the difference is the sound they are using is not audible. Without going into too much detail, as the vehicle approaches you the sound waves are compressed -- or higher frequency and after it passes the sound waves are lower frequency. The amount of frequency change corresponds precisely to the speed with which the vehicle is traveling.
As it turns out, some German folks shared my interest, and have already created a tool that you can use to determine the speed of a low pass with pretty good precision. All you need is a digital audio recording. You can record the sound using any video camera, or probably even a cell phone if it records audio. The trick is to change the recording into a .wav file. If you need assistance with this, email me at firstname.lastname@example.org and I can probably assist you making your recording into a .wav file.
So ... just to restate this -- by recording video or audio of your model airplane you can calculate the speed of a low pass. The closer the model passes to the recording device the more accurate your measurement will be. A clean recording -- without lots of extraneous noises is also a big plus.
Here is the link to the information and to download the program that analyzes the sound and converts it into a speed measurement. http://www.sprut.de/electronic/soft/scope.htm
For those who don't read German .. use this link and read a translation of the website.
If you just want to get after it and download the program that does the conversion click HERE
One final thought -- the program gives speed in Kilometers / hour. To convert to Miles/hour multiply by .621371