or you can do it the hard way. set up to small flags(referance points) like 10-20ft apart and have your dad handy with a stopwatch. the have him start it at the first flag then stop it at the second flag. then lets say for example your pop wing goes 20 ft in 0.5 seconds. then divide 20 by 0.5 and get 40. you pop wing goes 40mph.that was an example!
That would be 40 feet per second, which is not equal to 40mph. 1.46ft/s = 1mph, so that's actually around 27mph. More importantly, though, this method is surprisingly difficult to do accurately. At any reasonable distance between points, it's tough to tell exactly when the plane has passed the marker.Best I've tried was with an onboard camera... limited success.
Dan, if you can get a video of the plane passing close to the camera on a straight, level path with maybe ~3 seconds time coming and going and good, clean audio, someone here on RCG can get a Doppler reading and tell you with plenty of accuracy how fast it's going. I'll even do it if I can get Wav-O-Scope working.