measuring speed - RC Groups
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Sep 08, 2017, 05:16 PM
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Discussion

measuring speed


has anybody who flies sailplanes, measured its speed ?gliding-no speeding, just normal glide?

with a radar gun?
and what wind speed?

i ask because recently i began measuring it on my 2 meters, but with another tool, and would like to see how close to the radar gun speed a sailplane glides.
2 meters plane would be the speed that i fly, but any size is better than nothing
Last edited by phil alvirez; Sep 09, 2017 at 02:16 PM.
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Sep 09, 2017, 07:38 AM
Mark LSF # 3792
Span is not the only factor in the speed of a sailplane. Weight, total drag, and airfoil are three others that quickly come to mind.
Sep 09, 2017, 07:54 AM
Sonoran Laser Art
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Last edited by CloudSniffer; Sep 09, 2017 at 03:51 PM.
Sep 09, 2017, 03:40 PM
Registered User
Quote:
Originally Posted by Soarmark
Span is not the only factor in the speed of a sailplane. Weight, total drag, and airfoil are three others that quickly come to mind.
i know. but that is not what i am asking. i just want to know if some1 has measured the speed with a radar gun. and that near ground level, and measuring the wind with an anemometer right there.
Sep 09, 2017, 03:45 PM
Registered User

radar gun


Quote:
Originally Posted by CloudSniffer
I don't have a way to measure speed but I do have a thought. I think we have all experienced that a glider will stand still into some wind. You might google Apps for wind speed. They are available for several types of devices. Over a little time try to get your plane to stand still into the wind, level flight and record the wind speed. The more times you do it I suppose the more accurate it will be. In the end it can't actually measure the wind speed at your exact planes location but might help to get an idea.
your advice is highly appreciated, but, do you mean that the speed of the wind at the place where the plane is flying matches the speed reported by a weather station-at exactly that moment?
i asked measured with a radar gun because it measures exactly the speed.
Sep 09, 2017, 06:02 PM
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GliderJim's Avatar
He means get a wind speed app for your phone. Then use it while flying into the wind and if the wind is strong enough to make your plane stand still, then the wind speed you record with your phone/app will be the speed your plane is flying. Of course the plane will be in the air and the phone/app will be on the ground with you, so it won't be completely accurate. But then if one of us measures the speed of our plane, then it's not likely to match the speed of your plane either, since they're different planes flying at possibly different weights with different airfoils in different air. I don't think there's any reason to believe they would both fly at the same speed.
Sep 09, 2017, 06:07 PM
Registered User

radar gun


hence why i ask for measures with a radar gun
Sep 09, 2017, 06:11 PM
Registered User
Quote:
Originally Posted by phil alvirez
i asked measured with a radar gun because it measures exactly the speed.
I believe a radar gun pointed up at a soaring plane will have triangulation error.

The reading will also be ground speed.
Sep 09, 2017, 06:16 PM
Registered User

for comparison


the idea is to check when the plane is near the ground. the closer the better.
and have an anemometer.
am just trying to find another way to compare the readings am getting with other tools, but if it is not as easy or reliable then never mind.
Sep 09, 2017, 06:43 PM
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mhodgson's Avatar
I think the real answer is on board telemetry. I'm sure there are airspeed sensors and gps can give groundspeed.
Sep 09, 2017, 07:19 PM
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tspeer's Avatar
Phil, I think you posted somewhere else what equipment you were using to measure airspeed and barometric altitude. I think that's a good approach, especially in conjunction with GPS data. You can use a manometer to pressurize the total pressure and static pressure ports with known pressures and build calibration curves. The manufacturer may also be able to supply you with calibration curves for your serial number.

The FAA has a glider handbook with a pretty good chapter on glider performance. You might find the USAF Test Pilot School performance phase text book useful. In particular, the sections on pitot-static calibration (chapter 5.10). Herrington's Flight Test Engineering Handbook is a classic.

Richard Johnson did the kind of flight tests you're talking about on just about every full scale glider in production in his day. His reports are the gold standard for glider flight testing. Some good examples are http://www.harrishillsoaring.org/doc/discus_johnson.pdf, http://www.postfrontal.com/PDF/prove_alianti/ASW19.pdf, http://www.standardcirrus.org/Performance.php, http://members.goldengate.net/tmrent...ocs/135/dj.pdf, http://www.126association.org/docs/126evaluation.pdf.

Notice that each test starts by calibrating the pitot-static system. It's pretty easy to measure the total pressure without appreciable error, so all of the position error is typically attributed to the measurement of static pressure. Static pressure errors affect both the airspeed and altitude measurements. Today, with GPS, you can compare the instrument-corrected airspeed with the GPS ground speed for upwind and downwind runs to get the airspeed error, and from that you can calculate the static pressure error and the barometric altitude position error at different speeds. GPS position data can be used to get distance flown in a time interval for average ground speed. GPS altitude data is not as accurate as barometric altitude, so deducing the static pressure error from speed measurements is probably better than getting it from altitude measurements.

By doing a series of steady glides (trimmed for hands-off flight), you can get the sink speed vs airspeed to do the same plots that Johnson does to show the performance. If you want to correct your data to standard day conditions, the methods in the TPS handbook for climb performance would give you the formulae. If all you are interested in is lift/drag ratio, then you don't need to correct your data to standard day. But if you want to compare minimum sink speed for different configurations, then you will need to do that on the basis of standard conditions.
Sep 09, 2017, 08:07 PM
Registered User

how i do it


thank you very much gentlemen. wonderful data and advice. i dont remember if provided detailed data on what am using. the vario tells me if the plane is climbing or sinking by tones: higher pitch means up and lower down. the faster the more. and flipping a switch the voice tells me the height. and if i start a timer at certain height (like when it tells me the plane is at 100 meters) and stop it at landing i know the flight time from that height.
when i start a stop watch at certain height (lets say 140 meters) and stop it at the next (120 meters in the example), i know the sinking speed.
the other instrument saves the speed with a pitot tube when i flip a switch and allows me to collect 9 readings per flight, and later i print the readings to get an average speed. am measuring speed facing the wind (if any) and getting very close figures so it is working too. so, as i told this when i started the thread, this lets me have all that i need to learn when i do a change in the design if is an improvement or not.
i also asked if some1 has another method that is reliable to do this, but so far only tspeer on the previous post has answered.
Sep 09, 2017, 10:48 PM
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tspeer's Avatar
It would be great if there was some way for you to receive continuous telemetry of altitude and speed, even if you can only record it. Or if you could record the data on a card in the plane and analyze it after the flight.

Here's what I think could be recorded in a model glider:
- airspeed
- barometric altitude
- all control deflections
- GPS latitude & longitude
- GPS altitude
- GPS speed

A flight controller would be able to provide angular rates, linear accelerations and angular attitudes, but these are not needed for performance measurements.

Ideally, you would want the data recorded at, say, 10 times a second for performance measurements but you might be able to get by with 1 hz data.
Sep 09, 2017, 11:02 PM
Sagitta Fanboy
Quote:
Originally Posted by tspeer
It would be great if there was some way for you to receive continuous telemetry of altitude and speed, even if you can only record it. Or if you could record the data on a card in the plane and analyze it after the flight.

Here's what I think could be recorded in a model glider:
- airspeed
- barometric altitude
- all control deflections
- GPS latitude & longitude
- GPS altitude
- GPS speed

A flight controller would be able to provide angular rates, linear accelerations and angular attitudes, but these are not needed for performance measurements.

Ideally, you would want the data recorded at, say, 10 times a second for performance measurements but you might be able to get by with 1 hz data.
That's pretty easy to do with modern telemetry transmitters.

The OpenTX transmitters record all channel outputs by default to SD logs. A vario, Airspeed sensor and GPS sensor will provide the rest of that data, and only the airspeed sensor would be any challenge to install. Not sure if Spektrum TX's record channel state, but the rest they provide.
Sep 10, 2017, 01:10 AM
Registered User
GliderJim's Avatar
I guess I don't understand how someone measuring the speed of their plane is going to help you determine the speed of your plane. That is how I understood your original question.


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