Measuring speed

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Measuring speed

Postby RCModelReviews » Fri May 21, 2010 1:16 am

With the arrival of affordable telemetry, the need for radar-guns to measure a model's speed will probably disappear in the next few years.

However, since there's possibly going to be an attempt on the world jet-powered RC model speed record around here in the not-too-distant future, I figure we'll probably need two different ways of confirming the model's speed.

At speeds of 600kph (400mph), a radar gun becomes somewhat difficult to use (many don't even read that high). In order to remain "in range" long enough, the pilot of the model has to fly a very precise line, fairly close to the ground (where the gun will be).

So, I was contemplating doing something a little different.

I figure that if I use a couple of doppler radar models aimed at right-angles to the flight-path, I should be able to use them to act as triggers for a simple timer.

In theory, the radar units should produce a tone with a pitch that drops in frequency as the aircraft approaches a position exactly in line with the radar beam. At that point, the frequency of the tone will be 0Hz. The frequency will decay/rise in accordance with a sine-function as it approaches and flies away from the imaginary line the radar will paint across the flight path. So, if I space the radar sensors apart by a suitable distance (perhaps 300 yards) and use the 0Hz point to trigger the start and stop of the timer, the speed can be measured pretty accurately.

What I'll have to experiment with is exactly how accurately I can detected the null-frequency which indicates that the plane is exactly opposite the radar units. This will be an interesting project and I welcome input from others who might have other ideas of how to measure the speed of a model moving this quickly.
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Re: Measuring speed

Postby fernandez » Sat May 29, 2010 8:20 pm

A pressure sensor would be the easiest or NMEA VTG speed message from gps module.
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Re: Measuring speed

Postby hillflyer » Mon May 31, 2010 7:05 pm

Hi Bruce,
a few years ago I found yet another approach to measuring the speed of a powered Model plane:
Audio doppler measurement and analysis.
There is this amazing little program that does it all for you:
http://www.sprut.de/electronic/soft/vspectrum.zip
Just play with it a little. You can simulate a real model flyby by whistling into the PC mike. Go from a high pitch to a lower one, then use the cursor to mark the 2 frequencies. Pronto, there is your speed.
The program was intended for electric powered models, but maybe it also works with jets.
Have fun!
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Re: Measuring speed

Postby RCModelReviews » Mon May 31, 2010 9:49 pm

Yes, I've used doppler myself however, the noise from a jet engine is pretty much white-noise so it would be hard to measure the doppler shift -- since it's effectively an equal sound level across a wide range of frequencies.

Good thinking though!
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Re: Measuring speed

Postby ergocentric » Tue Jun 08, 2010 11:31 pm

Modulated infrared beam is very common for long range security sensing, and infrared curtain is common for machine guarding, could a fast jet hit a small 'curtain' twice in a straight line?

Largest curtain I've used was about 1m by 2m so the technology would have to be stretched to the limit.

How large an area would you need? How tall would a practical tower be?

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Last edited by ergocentric on Tue Jun 08, 2010 11:34 pm, edited 1 time in total.
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Re: Measuring speed

Postby ergocentric » Tue Jun 08, 2010 11:33 pm

Another thought, modulated laser on the aircraft pointed down, and receiver bars across its flight path on the ground.
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Re: Measuring speed

Postby pkt » Fri Jul 02, 2010 12:26 pm

Two high speed video cameras, mounted a measured distance apart, with marker rods mounted vetically in front of them. The plane flies along in front of the two cameras, and the time taken to fly between the two marker rods is measured. Needs high-speed cameras though. The measurement could be made either by image processing software or manually by watching the slowed-down video. The advantages are that the model height/line accuracy is not critical, and you can easily confirm the model is flying level between the measure points.

I believe this is the method used in post-war speed trials of early jets such as the Meteor, using high speed film cameras :-)
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Re: Measuring speed

Postby RCModelReviews » Fri Jul 02, 2010 9:02 pm

A good idea -- but since my existing camcorder is already starting to fail (white-balance keeps shifting) and I can't afford to replace it, the chances of getting two expensive hi-speed cameras is pretty remote :-(
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Re: Measuring speed

Postby ergocentric » Fri Aug 20, 2010 10:00 pm

We bought a new house and we were standing around chatting with our inspector when it hit me, the garage door beam was too high, then it hit me that garage door companies tear out and scrap this stuff and it could easily sense an aircraft flying through.

If you were to build a wall of separate beams, you would have to modulate the signals, but I think it would work.
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Re: Measuring speed

Postby RCModelReviews » Sat Aug 21, 2010 3:13 am

I think the easiest way for *me* to measure the speed of *my* model is with the GPS-based telemetry.

Problem solved!

However, users of most other RC systems (such as Futaba) have no such option at present, short of installing a 3rd-party stand-alone telemetry setup.
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