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Old Oct 08, 2007, 11:34 PM
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efether's Avatar
Atlanta, GA
Joined Aug 2007
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Doppler Speed Acuracy?

Recently I started playing in the high speed RC arena, and I've been using the "Wave O Scope" measuring my speed via Doppler. Well, today I flew my FunJet with a MegaMotor 16-15-3 and a 4 Cell Lipo. I also flew my Speedwing Dart (with a Neu Motor 1105/3Y and 3 cell). According to this tool, my speeds seemed low. So...I pulled up the video on VirtualDub and used it to measure the time it took to go from Soccer Goal Post to Soccer Goal post. The field I fly at is 100 yards long...300 feet. I flew the Dart lower than I did the Funjet...so here's some shots:

First, the Dart passes Goal post #1 at 00:57.299



Second, the passes Goal Post #2 at 00:58.566



That's a time difference of 1.267 seconds. Now, simple math...

300' / 1.267 = 236.78 ft/sec
236.78 ft/sec * 60 sec/min = 14206.8 ft/min
14206.8 ft/min * 60 min/hr = 852408 ft/hr
852408 ft/hr / 5280 ft/mile = 161.4 MPH

Now...the doppler for this pass shows 103.4 MPH.



Now...I gotta ask...What's going on?? That's a speed descrepancy of 58 MPH.

The Funjet was way off too...I can't help but wonder...has anyone ever tested this doppler program for accuracy? All files are available for analysis..and will cooperate with anyone willing to try and find a solid answer.

Any and all comments welcome. I'm trying to find the best way of getting the speed from these birds...and after this little comparison...I'm not sure the Doppler program is the way to go.

Later,
Eric
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Last edited by efether; Oct 08, 2007 at 11:44 PM.
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Old Oct 08, 2007, 11:56 PM
hot air rises...
jfinch's Avatar
Pleasant Grove, UT
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I've checked doppler against radar guns a handful of times. The doppler always came out within a couple mph of the radar. Doppler is best when the plane flys in a straight line and passes fairly close to the microphone. Record a few passes and post them up here (video is even better).

How can you tell when your plane is over the goal posts from that video? I think if you're going to do a timing run you should have someone with flags at each goal watching for the plane to be directly overhead. Not all soccer fields are 100 yards long. Are you sure it's 100 yards?
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Old Oct 09, 2007, 12:00 AM
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PetruSoroaga's Avatar
Bellevue,WA
Joined Jun 2006
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your video analysis is not complete/accurate:
* objects closer move faster than objects farther away - case in point - your plane is closer than the goal posts so I suspect it actually did not travel 300ft between those two frames.
* second, related to the one above, the camera also rotates between those two frames which adds to the problem.

If you want really reliable video analysys do two simple things: keep the camera fixed and focused to some big distance, fly in a straight line far in front of the camera and the two end points references should be inline with the flight path: not above or below.
This should yield you quite some accurate readings.

Have fun,
Petru
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Old Oct 09, 2007, 12:03 AM
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Bellevue,WA
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I might not have perfect 3D vision, but just looking at the seccond picture I can tell you the plane is at least 15-20m before reaching the goal post. Try to redo the calculations with this assumption.
Petru
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Old Oct 09, 2007, 12:04 AM
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First picture - it looks to me like the plane it's about 2-3m after the goal post (past it, towards the other post).
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Old Oct 09, 2007, 12:07 AM
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Bellevue,WA
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Hmm... nice problem
So, taking 25 m off of the distance between those two frames you are at about 130mph. Which looks quite in the ball park with the dopler gun. See the post above about accurate dopler reads or redo the video using the other remarks.

Have fun,
Petru
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Old Oct 09, 2007, 12:25 AM
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efether's Avatar
Atlanta, GA
Joined Aug 2007
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Quote:
Originally Posted by jfinch
Are you sure it's 100 yards?
Yes...I've paced it off...while not a tape measure...it was close enough to assume 100 yards is accurate.

Quote:
Originally Posted by PetruSoroaga
your video analysis is not complete/accurate:
* objects closer move faster than objects farther away - case in point - your plane is closer than the goal posts so I suspect it actually did not travel 300ft between those two frames.
* second, related to the one above, the camera also rotates between those two frames which adds to the problem.
Agreed...but we do with what we have....by best estimates...this is close.

Quote:
Originally Posted by PetruSoroaga

First picture - it looks to me like the plane it's about 2-3m after the goal post

...

looking at the second picture I can tell you the plane is at least 15-20m before reaching the goal post
Actually...those frames are pretty accurate. I'll have to figure out how to post the video and you can check it out. I've had two other sets of eyes looking at the video and evaluating which frame pasted which goal post...and the ones posted were agreed upon. Further...I don't think you're estimate of being off by 25 m (METERS = 81 yards...it's only a 100 yard field)...is accuate. But assuming your speed prediction of 130 mph is correct...and we compare that to the doppler speed of 103.4 mph gives us an error of 20.46%. That's just unacceptable. I'd accept +/- 5...maybe as much as 10%...but >20%....nope. That's just guess work.

Thanks for the input...I'll get youtube figured out and get these vids posted for better analysis.

-Eric
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Old Oct 09, 2007, 12:50 AM
hot air rises...
jfinch's Avatar
Pleasant Grove, UT
Joined Jul 2005
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Do you have access to a radar gun for comparison?

BTW, 25 m is not equal to 81 yards. It's 82 ft.
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Old Oct 09, 2007, 09:21 AM
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efether's Avatar
Atlanta, GA
Joined Aug 2007
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25 m = 985.2 inches = 81.8 feet. yup...you're right...my bad

Here's the video of the Dart on that run (sorry about the crappiness of it...you lose a lot of resolution getting it up to youtube) ....

Dart Video

-Eric
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Old Oct 10, 2007, 12:22 AM
hot air rises...
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Pleasant Grove, UT
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Doppler shows 95-100 mph and watching the video I'd say that is about right. It's definately no 160 mph.
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Old Oct 10, 2007, 02:52 AM
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efether's Avatar
Atlanta, GA
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Quote:
Originally Posted by jfinch
Doppler shows 95-100 mph and watching the video I'd say that is about right. It's definately no 160 mph.
The difference between 160 (1.2 sec) and 100 (2.0 sec) is 0.8 sec on a full field run (BTW..measured it again today...exactly 100 yards). From what I'm seeing on the video times...and comparing it to other videos of fast movers...I'll buy the 130 estimate earlier. Hell...I've even stopwatched it at about 1.5-1.8 seconds from post to post...which translates into 113 (1.8 sec) MPH to 136 (1.5 sec) MPH respectfully. Of course...at those speeds...human reaction time could affect it by a significant amount as well...and the video introduces paralax, which makes it hard to estimate when it passed the posts. And when a single frame of the move moves the plane about 7 feet (30 fps video)...makes it tough.

As for the video...the fastest run is at about 0:58 sec (as shown in the first post)...doppler on that one was at 103 MPH. I know there is the Paralax consideration when viewing from a single point and estimating when the plane flew over the goal posts...but I've got several planes that can do 100...and the dart was faster than that. Hell...even my wife said it was faster than the 4S Funjet..and she guess that at about 130 (granted...what does she really know ).

I still think the doppler is off...I'm not sure if it's because my wife (who shot the video) turns the camera to follow the plane, rather than let the doppler shift hit the mic at a constant angle....or because of Air humidity (I've read about that one...and this is Atlanta...after all )...or the wind noise on the mic screwing up the clean doppler pattern on the program. Regardless, I think the doppler is close...but I think I'm doing something wrong or it's flawed to be off >20%. In all likelyhood...it's me Just need to have the technique pointed out to me so I can do it right.

A thought...I found a camera on Hobby-Lobby that is a slim one and can be easily mounted under the wing. Without the Mirror, it would look straight down. Pass it low over the goal posts and pull the times off the vid via VirtualDub. Also do a Doppler recording at the same time...and compare the two. I think that's my next step. Most accurate, I would think.

Thanks for your time and effort!

-Eric
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Old Oct 10, 2007, 12:27 PM
hot air rises...
jfinch's Avatar
Pleasant Grove, UT
Joined Jul 2005
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You don't have access to a radar gun? Again, my experience has been that doppler is within a few mph of radar. Doppler is best if you can fly close to the camera (try to get lower).

Do a search for "how fast" by pdawg for a recent example of a video of where you can compare Doppler with the how fast device.
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Old Oct 10, 2007, 12:44 PM
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treehog's Avatar
Euroland
Joined Jan 2004
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Funjet at 100mph agrees with my estimates

I use my funjet with a fun 480 FAI 55 motor KV 5500 and 3s Kokam 20c 3200mha and a 4.7*4.7

Its afun fast machine I figure 90 to 100mph level flight and a dive might top 115 mph

Thats based on my s400 voodoo with the mega 16/15/3 turn motor with 4.7*4.7 and 3s 2000mha is easly a lot lot faster like 50% or more faster and makes my funjet look positivly slow

This makes sense as the voodoo is a tiny plane with the same power as the bigger FJ and the FJ has a thick wing foil which is not particaly made to give good results at speeds exceeding 80mph as it is flat bottomed and creates gobs of lift and gobs of drag at speeds exceeding 70MPH


But FJ is still a blast of a plane and hoot for acrobatics which are difficult to do on a 150 mph plus micro missile lijke s400 Voodoo


I must try to figure out how to do this doppler stuff some day instead of guestimating only to find out later that when the same crafts with same power units are Radared my guestimates are within 10% of the radar numbers

Simple realy most planes unless you you use collosial power wont greraly exceed therminal velocity speeds ( speed unpowered version will do in a long freee fall vertical dive ) and the mathaamatical formula exists to calculate this and planes like FJ will therminal velocity dive max brick wall at some 115mph and s400 pylons being sleeker will tend to max out at 150mph

Doubling the weight of the craft will increase terminal velocity by some 20% but often could result in slower level flight speeds from induced drag

But the voodoo costs 3 times the price than a MPX FJ for a 30% to 40%increase in speed and requires a lot of extra costs to get the 50% extra speeds

I also ball park pass two goal posts
2 seconds equals 100mph but you need a caller flag person there to drop the flag signal that you passed the first post and i cant find any more Kamakasie callers to volinteer to be the caller

In fact at most club meeting there seems to be a rush to go sit in the cars to recharge the baterries or something after they see the Voodoo in the air
Usualy mutering something under their breath about living to see tommorow to feed the kids tommorow syndrome takes over

Ralf
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Old Oct 10, 2007, 02:03 PM
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efether's Avatar
Atlanta, GA
Joined Aug 2007
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Appreciate the response...where did you get that Voodoo? Found a thread on it...looks fun

Thanks!

Eric
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Old Oct 10, 2007, 03:44 PM
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LuckyDay's Avatar
Nashville, TN
Joined Jan 2006
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Try setting the video camera on top of one of the goal posts and get as close to straight over it as you can... that will yield the best doppler results.

I'd trust the computer analysis of the sound over a best guess of the naked eye any day - too many times perspective has got the best of me and I have flown over or into something thinking I was not close.
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