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What is the best (least expensive) way to determine airspeed/groundspeed

Gazoo

Active member
#1
I see posts about how fast someone's plane flies. 50MPH, 115MPH... I'd love to figure out how fast some of my planes can fly. Problem is, radar guns are a little expensive.

At a park close to me, the police set up one of those radar trailer doo-hickeys on the road. I figured I would make a run for it. On my third pass at it, I actually got clocked but then promptly exploded into the side of a tree because I was not looking at the plane...and I still did not see the speed.:mad: I only hope that there are cameras on the radar trailer and that the cops play the video at their Christmas party.:black_eyed:

So, I have seen radar guns, airspeed indicators (pitot/static types), GPS and Doppler effect used.

Please give me some feedback on the best methods and thanks for your input.
 

PsyBorg

Wake up! Time to fly!
Mentor
#2
Simple maths? If you have a way to mark out exact distance and some one with quick reaction times you can figure it out from there without taking your focus off flying safely. If you know a long stretch of road not traveled you could have some one on a motor cycle or in a car pace you but that's probably not the best way either. GPS is not really as accurate as it could be. The radar gun option is best.

I know when I was a kid we used to ask the local cops with their handheld radar guns to clock us at pitching practice and they were more then happy to spend a few minutes on occasion for us doing so. It does not hurt to ask one politely. The worst they can say is no. The best is they can be cool and do it for you.
 

Piotrsko

Active member
#3
Traditional way from stone age: 2 sticks about 200 ft apart, someone else with a stopwatch. Fly over sticks, start & stop timer. Do math for speed.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#4
Traditional way from stone age: 2 sticks about 200 ft apart, someone else with a stopwatch. Fly over sticks, start & stop timer. Do math for speed.
Well you got it mostly correct but the actual requirements are that the measured distance must be in the same direction as the prevailing wind. 2 flights or passes are required. One into the wind and a second a short time later in the opposite direction..

The airspeeds are calculated for each pass and the airspeeds are then averaged to give what the actual ground speed would have been if there was no wind.

A very simple and very accurate method if used properly. Still used for speed records around the world.

Have fun!
 

b-29er

Active member
#5
Doppler effect is reliable and cheap if you can do math and are already recording your flights.
Basically you compare the sound of your propeller coming towards you to the sound of the propeller going away from you. You compare the change to the speed of sound and viola. Uses free software (audacity) and a little bit of calculation work, but its more precise, as the two-stick method is susceptable to viewer angle issues and reaction time (pressing a button on a timer.)
 

Grifflyer

WWII fanatic
#8
Doppler effect is reliable and cheap if you can do math and are already recording your flights.
Basically you compare the sound of your propeller coming towards you to the sound of the propeller going away from you. You compare the change to the speed of sound and viola. Uses free software (audacity) and a little bit of calculation work, but its more precise, as the two-stick method is susceptable to viewer angle issues and reaction time (pressing a button on a timer.)
I wonder if that would still work with an EDF...
 
#11
Why not a little GPS flight recorder?
They're not too expensive (I think I got mine for ~$50), and they are fairly accurate.
You'll have to wait to find out what the speed was after you pull the data, so it's not immediate satisfaction.
 

kpixels

Antigravity or bust...
#12
For the "least expensive" I agree with PsyBorg. A trick to measure a distance between two landmarks is to use the measuring tool in GoogleEarth. If zoomed all the way in, I've found it to be accurate to within a foot or so based on actual measurements taken from construction sites. I make passes the length our local RC air srtip in both directions and average the two times and then apply the "simple maths".