Doppler airspeed measurements - RC Groups
Sep 17, 2006, 10:13 PM
Indoor Free Flight Modeler
Cool

# Doppler airspeed measurements

I was disappointed today that I was unable to get a radar gun speed on my little pylon model. I did some research and found out that unless I am loooking to spend \$700-\$1000 on a radar gun it will be VERY difficult to radar gun a small model like this which has such small frontal area. By the time you get a reading the plane is at too much of an angle and you the error is quite high. The Bushnell gun I got seems to be more suited for models in the 36" span or greater and smaller than that is a real hit or miss and depends on the intestinal fortitude of the guy willing to stand as a model swoops by 5ft above their head. Instead I dug into alternate methods and found the doppler shift method. The premise is quite simple, make an audio recording of your model flying by, then the frequency shift tells can tell you the airspeed.

I found this website which give you the idea behind it:

http://www.sprut.de/electronic/soft/scope.htm
It is free and it is in German, but that will not stop me!

Then I took my daughters MP3 player to the park (it has a audio record feature). Then started recording. You want to get a nice recording of you standing still and the model flying overhead. The nice thing is that you can be a little higher on the speed passes. I then took the recordings and made small WAV files about 4 seconds long, two seconds coming to you and 2 seconds going away from you. Then you open that in the Oscope software. You will see S shaped lines, then just left click on the left side and right click on the right side. You speed on km/hr will be displayed. To convert to MPH just multiply by .6217.

Here are the results I came up with:
My Minispeedwing = 136km/hr = 84.55mph
Bill's Scrambler = 141km/hr = 87.66 mph
My Pylon Model = 193km/hr = 119.98mph
Matt's Overloard = 137km/hr = 85.17 mph
Now the other day Bill clocked me at 83 and I clocked him at 84 with the radar gun. The winds today seemed about the same as the other day when we radar gunned them, so it seesm to corrolate pretty well.

Here is a screenshot showing Matts file and how you do the analysis.

### Images

Last edited by The Don; Sep 17, 2006 at 10:21 PM.
 Sep 17, 2006, 10:17 PM Road-Runnerus Digestus Awesome Don! 119 mph! do not look us in the eyes....we are not worthy! here is the website translated in english.... english bill Last edited by wile_e_coyote; Sep 17, 2006 at 10:24 PM.
 Sep 17, 2006, 10:18 PM Suspended Account you didn't mention the "HOLY @%**!!! that almost ended all testing. that little pylon racer was SCREAMING!!!!
 Sep 17, 2006, 10:25 PM Indoor Free Flight Modeler The audio files are hysterical. I have kept them in full length format. Too funny to delete. Don
 Sep 17, 2006, 10:31 PM Road-Runnerus Digestus hey don can you send me mine so i can try the program? thanks! bill
 Sep 17, 2006, 10:32 PM one game away... I have to say I'm shocked. I thought that your and Bill's wings were at least +50% faster than mine. Maybe the size difference accounts for my misperception. I only wish we had a video of the pylon racer''s near terra firma incident.
 Sep 17, 2006, 10:37 PM Road-Runnerus Digestus heres the video.... bill Last edited by wile_e_coyote; Nov 01, 2006 at 11:24 PM.
 Sep 17, 2006, 10:38 PM Indoor Free Flight Modeler Matt, I think the size is tricking you. That other day we were flying down by the lake and you and me were both up at the same time we were just about dead even. Yes that would have been something to see, I am just glad it did not pancake in the ground! Don
 Sep 19, 2006, 09:29 PM ukiMig29 Cool Anton
 Sep 21, 2006, 08:30 AM How high will it go? I downloaded waveosscope but I am not sure if I am doing all right I got 2 readings, 84kmh and 137kmh Both much less than I expected, but this is my first speed plane. I am adding the video, maybe someone else can do it, and tell me if I am doing anything wrong. It's a projeti running a mega 16/15/3 (I think, can't see label) just bought the plane, and a 5.5x4.3 graupner prop. I would love to get over 100mph with this plane. https://www.rcgroups.com/forums/show...565#post6099545 Latest blog entry: FAA registration
 Sep 21, 2006, 09:02 AM one game away... I forgot to say 'Thanks' Don. I didn't know that you were analyzing mine at all that day since it seemed slow compared to the mini wings (in my eyes anyways). It's nice to have an idea of how fast it's really going. Matt
Sep 21, 2006, 03:25 PM