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Old Mar 18, 2012, 08:29 PM
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Measuring Speed

I know its spread throughout the many threads here but, is there a single source of the different methods to get the speeds, which are bogus, cost effective, etc. Radar vs doppler vs GPS? Stopwatch and pylons or pitot boom reading? It would be nice to know if the method(s) I'm using are reliable or, if not, how inaccurate they may be?

ciao,
Mike
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Old Mar 19, 2012, 04:32 AM
"The Judge"
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All I can say, when measure with doppler CORRECT WAY, you get like 1% fault , could be 2% but then you have to have all faults on your side, this method never failed me if you fly CORRECT, and analyzy it carefull..

GPS for me did not work so good, sometimes the speed could differ like 10-20% and one time I got 356km/h in a 3s Turnleft 600 watts of power , But Im sure the most expensive GPS around maybee is a step better..

From what Ive read, real radar gun shoule give you right speed as it is for the moment of time. But I wouldnt bet my life on guns like bushnell, even if they can show real speed often, thet also seems to be incorrect sometimes, ive seen better results with the pocket radar..

Measure speed with stopwatch do get the accurate speed over a given distance, but to do it manually with a stopwatch cant get you the precsion you want, if you have an inputdevice that could start/stop the watch when you fly by, then it would be more correct..

My conclusion: Doppler for sure when you do it you self and know you fly right and measure it right, to do doppler for someone else could be wrong in many ways, the person could fly totally wrong giving a much higher speed, so you could always cheat there... But for your self you cant go wrong with doppler, and its for free, just record sound of a lowpass with your cellphone, and analyze in your computer later

Sebbe
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Old Mar 19, 2012, 05:02 AM
310mph Kolibri T25 Swist
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Funny enough I just orderd a pocket radar this morning, even more funny it turned out to be in stock in Sweden at the same price that the one I got of ebay. Hopefully the customs will have a busy day when my parcel drops by haha
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Old Mar 19, 2012, 05:58 AM
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GPS is accurate but not reliable for our models. They don't work beyond 4G, a pylon racer will exceed 20G easy.

RADAR - they have problems with range and acquiring the fast small approaching model, locking and getting a measurement before it passes. Also quite hazardous for the guy holding the gun.

Doppler - is a post analysis tool. It's difficult to get good readings and there many factors that could make it inaccurate. Difficult to trust as no 2 people will give you the same answer.

Pitot - generally works well but measures airspeed and not ground speed. Not sure about calibration. But they are affected by where and how they are placed. And as they use pressure sensors they are affected by G loads.

Point to point and stop watches is the only real way to get really accurate answers. But if you are using humans, they become a problem after 300kph (according to the fai) and electronic triggering is required. (see German speed cup).
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Old Mar 19, 2012, 06:21 AM
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if you fly correct and record sound, doppler will get you really good accuracy..
Of course, if someone send me a file, I have no clue how he has been flying when he recorded.... BUT for your own porpuse, it is very accurate if you know what you are doing..

I trust a clean doppler pass(when I have flown myself) more then a GPS or pitot tube reading(calibration can be off).

Radar is ok if measured 3 or more times and you are getting same speed each time, then I would trust it..
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Old Mar 19, 2012, 06:34 AM
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The speed trap with electronic triggering is certainly the most accurate method for ground speed and required for the recoginition of FAI world records, valid passes from both sides are required to compensate for the wind. Of course that method is eleborate and not practical for everyday flying. For that, pitot tubes are the way to go. Airspeed is exactly what you want to be able to determine if your model is getting better (faster) or not. Guys who compete in F3s, F5B and F5D use pitot tubes to test new setups. Most of them use the Unilog speed sensor here in Europe but eagletree is also good as far as I know. GPS is no good for this purpose.
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Old Mar 19, 2012, 07:21 AM
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Kwahn, but you can dive how much you want before measuring the speed ?
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Old Mar 19, 2012, 07:52 AM
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Yes, I think they can. It's apparently quite difficult to max out the model's speed by diving from 600m or so and still get a clean pass through the sensor gates. I've never seen it live but some of the competitors don't even get valid measurements so it's also about flying skills, not just the potentially fastest plane.
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Old Mar 19, 2012, 08:37 AM
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I have seen and flown the speed cup (not german). Most will dive but, the model must be below 35m height 100m before the entry timing gate and above 5m at the 200m exit gate.

Because of the distances involved not that easy, dive or no dive.
The German system has problems triggering so the model needs to be close to the safety line and with some wing on show to get it to trigger. This increases the difficulty factor.

Btw radar is pretty accurate, more than using a Doppler pass for sure.
It's just about range and acquisition time. For example if you want to beat a speed camera you just need a car with a low radar signature and speed above 300kph (top gear proved that)
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Old Mar 19, 2012, 03:17 PM
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My neighbor is trying a new GSP/speed logger. He got it on Ebay for 20 bucks or so. Its been showing, as he gets closer to his models, his speed increases to a point inwhich they are now being packed for the RC field hitting a high of 108 mph. As his wife gets closer to see what hes up to, his speed approaches that of light, creating a black hole effect which she now can't see what hes doing!!
Simply amazing as he came over and showed me his new device. Why he was walking around with it on his head I didn't even ask!!!
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Old Mar 20, 2012, 09:23 AM
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I don't want to knock any of the above but the only way to get a true airspeed is with an on board pitot tube using air pressure to measure the airspeed..
All the other methods measure ground speed which means nothing unless it is flat calm.
I have just bought an on board pitot tube sensor from Winged shadow and it works great.
It measures speed during vertical dives as well.
Radar is ok but difficult to fly exactly down the beam and unless you are dead on it is not accurate and only measures the speed over the ground not airspeed.
Jim
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Old Mar 20, 2012, 09:32 AM
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Quote:
Originally Posted by Fox-composites View Post
I don't want to knock any of the above but the only way to get a true airspeed is with an on board pitot tube using air pressure to measure the airspeed..
All the other methods measure ground speed which means nothing unless it is flat calm.
I have just bought an on board pitot tube sensor from Winged shadow and it works great.
It measures speed during vertical dives as well.
Radar is ok but difficult to fly exactly down the beam and unless you are dead on it is not accurate and only measures the speed over the ground not airspeed.
Jim
Get outta my brain! Was just about to type the same thing.
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Old Mar 20, 2012, 09:38 AM
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Nice to know we agree Alien. Thanks for the back up.
Jim
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Old Mar 20, 2012, 11:10 AM
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Quote:
Originally Posted by Fox-composites View Post
I don't want to knock any of the above but the only way to get a true airspeed is with an on board pitot tube using air pressure to measure the airspeed..
All the other methods measure ground speed which means nothing unless it is flat calm.
I have just bought an on board pitot tube sensor from Winged shadow and it works great.
It measures speed during vertical dives as well.
Radar is ok but difficult to fly exactly down the beam and unless you are dead on it is not accurate and only measures the speed over the ground not airspeed.
Jim
Jim it's not like you to state the obvious. The OP was requesting some information on the different ways to measure speed, pros and cons. But also remember that you can't use a pitot static systems for speed records. The point to point measurement is performed in both directions in a single flight and the average used thus negating wind. Although not true airspeed it is a measure of real'ish' speed. - Now we just need to find a location where the wind changes direction 180degs to make our speed record attempts.

BTW - the OP has not returned so I would suggest the thread is dead.
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Last edited by Darron; Mar 20, 2012 at 11:23 AM.
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Old Mar 20, 2012, 11:13 AM
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I'm here..been reading every post and looking elsewhere!

Dead thread in two days? Maybe a little life left as new ideas are still being posted. Still haven't found another location that covers the pros / cons of the various methods.

MIke
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