Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

wavescope question

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Old 01-09-2007, 12:48 AM
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propjets4me
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Default wavescope question

Dont know if this is the right forum to ask this , but I've seen many referrences to wavescope here...if this is the wrong forum I appologise

but with all the talk about prop efficency and slippage etc is the reading/result you get from wavescope possible to contain errors for example could the engine sound like its screaming but due to various ineffencies actually being flying slower than what it sounds like

I know it measures doppler effect so in one sense I would think it would be accurate ..but would like to hear your opinons


thanks
Frank
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Old 01-09-2007, 01:35 AM
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Default RE: wavescope question

An error in Doppler is possible if the angle of the aircraft to the pick-up microphone is great. But any error will result in a lower than true reading of the speed. As long as the angle is less than 7 degrees, then the result should be 99% accurate.
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Old 01-09-2007, 01:40 AM
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Default RE: wavescope question

I have used sound recordings and an oscilliscope to analyse aircraft performance.

By comparing the waveforms before and after it passes you you can calculate the doppler shift and the speed of the aircraft.

By using the fast fourier transform feature of the oscilliscope you will see peaks that correspond to the engine rpm ( and also peaks for the various harmonics).

Initially I recommend making some recordings of your aircraft with a sound recording device.
There is a program I think called Winscope which allows your PC to work as an oscilliscope.
Set up a spread sheet with the required values and enter values for your altitude or air pressure which effect the speed of sound.
If you know how fast your engine really is turning it helps in choosing the correct peaks to compare.
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Old 01-09-2007, 08:03 AM
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Default RE: wavescope question

Wavescope does measure doppler effect. This is the change in frequency of the oncoming plane from approach to departure. Imagine holding a cup of water that is dripping water at a constant rate over a small body of water. If you hold the cup still you will have concentric ripples radiating out from where the water is dripping. Now, move the cup at a constant rate. The ripples now will be compressed in the direction of travel and stretched away from the direction of travel. The difference between the arriving and departing waves is greater the faster the cup moves. The doppler effect programs determine this difference.

As to the in-accuracies by distance. Take a look at the rings. Just to the side of the line of travel, they arrive pretty much in line with the direction of travel. But the farther out you get from the source the greater the angular distance until you reach a point that is so far away you only get the side of the exhaust which isn't compressed at all. Essentially it's like you are flying a control line airplane which from the center of the circle has no Doppler shift. This is why you are supposed to fly within 20 feet of the recording device.

Temperature also affects the speed of sound and needs to be figured into the equation, that is the Lufttemperatur box, in Celsius.
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