Fluorescent = Tach Strobe

weirdjim

weirdjim

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weirdjim
I'm getting ready to do an article about using fluorescent (sodium/mercury vapor) lights as a tach check. THere was a thread in here in late 2008/early 2009 about this very thing in which
Gismo stated:

RPM 2bladeImages
900 4
1200 3
1800 2
2400 3

RPM 3blade images
600 4
800 3
1200 2
1800 4
1600 3
2400 1
​
and I'm not sure that I agree with his numbers. I have used this technique for at least 20 years with some success.

The technique is that you use the strobe effect of a gas discharge light (fluorescent/sodium/mercury) as a pulse generator to illuminate the backside of the prop. At the standard very accurate USA/Canada/Mexico mains power frequency of 60 Hz (cycles per second) you get 120 pulses per second of strobe light -- one on the positive swing of the cycle and one on the negative swing. This equates to (120 pps x 60 seconds) to 7200 strobe light pulses per second.

Now comes a problem that I've always solved using a simple white tape routine. Rather than try and see a black backside of the prop in the dark, I put a piece of white tape near the prop root (about a third of the way up the blade) and use a SINGLE blade as my reference. It is a lot easier to see that white tape than it is a flat black blade.

Now would somebody please check my math, and note that now it doesn't matter HOW many blades the prop has, you are only looking for the white tape.

900 8 white marks (7200/900)
1200 6
1800 4
2400 3

As Gismo noted, using a fluorescent with an electronic ballast (instead of the old inductive ballasts) will not work; the electronic ballast changes the frequency of the strobe to just above the range of the human ear.

Thanks,

Jim
 
weirdjim said:
I'm getting ready to do an article about using fluorescent (sodium/mercury vapor) lights as a tach check. THere was a thread in here in late 2008/early 2009 about this very thing in which
Gismo stated:

RPM 2bladeImages
900 4
1200 3
1800 2
2400 3

RPM 3blade images
600 4
800 3
1200 2
1800 4
1600 3
2400 1
​
and I'm not sure that I agree with his numbers.
Click to expand...
JOOC, which if any of those are in error?

The technique is that you use the strobe effect of a gas discharge light (fluorescent/sodium/mercury) as a pulse generator to illuminate the backside of the prop. At the standard very accurate USA/Canada/Mexico mains power frequency of 60 Hz (cycles per second) you get 120 pulses per second of strobe light -- one on the positive swing of the cycle and one on the negative swing. This equates to (120 pps x 60 seconds) to 7200 strobe light pulses per second.
Click to expand...
Obviously you meant 7200 pulses per minute.

And IME the effect is easier to see if you view the light through the prop disk rather than having the prop illuminated from behind the plane.

Now comes a problem that I've always solved using a simple white tape routine. Rather than try and see a black backside of the prop in the dark, I put a piece of white tape near the prop root (about a third of the way up the blade) and use a SINGLE blade as my reference. It is a lot easier to see that white tape than it is a flat black blade.

Now would somebody please check my math, and note that now it doesn't matter HOW many blades the prop has, you are only looking for the white tape.

900 8 white marks (7200/900)
1200 6
1800 4
2400 3
Click to expand...
Your numbers look fine. There's a fairly useless 5 marks at 1440 RPM but you would need to take care you didn't mistake that for 6 marks.

But I still think that looking through the prop works better and eliminates the need for putting reflectors on the blades.
 
There are iPhone apps that measure rpm by looking for peaks in the audio spectrum; I wonder if that technique would work for this application.
 
I tried that with a computer program once. Maybe I'm too tarded to make it work but I never could get any good information from it.
 
gismo said:
JOOC, which if any of those are in error?


Obviously you meant 7200 pulses per minute.

And IME the effect is easier to see if you view the light through the prop disk rather than having the prop illuminated from behind the plane.
Click to expand...

I have a hard time resolving 2400 RPM two blade prop to 3 blades. I come up with 1.5 blades. Not sure how to resolve this one.

Obviously I meant 7200 ppm.

My experience is better with the white tape than viewing the light through the blades. YMMV.

Thanks for the math check.

Jim
 
This thing here:
http://www.aircraftspruce.com/catalog/inpages/onetouch.php

Which looks like this:

onetouch.jpg


Can sometimes be found in hobby shops for as little as $20 and is quite accurate. We use one to calibrate our tachs. Settable for two, three or four blades. Doesn't work as well on cloudly, dull days as on sunny days, but you can get someone with a flashlight out front to aim it through the prop (stand back some) and into the device you're holding while you run things up. If you think the tool is haywire, hold the sensor up to an older fluorescent light or neon indicator light on a coffee machine and it will say 3600 RPM on the two-blade setting if it's accurate.

Dan
 
I imagine the photocell ones would be fine, as they don't emit anything. Laser, not so much.
 
gismo said:
JOOC, which if any of those are in error?
Click to expand...
Looks to me like you both have the same numbers when you account for the fact that Jim is only counting one blade so you don't get the 180 degree image of the blade looking like the same image (for the two blade case)???
 
weirdjim said:
I have a hard time resolving 2400 RPM two blade prop to 3 blades. I come up with 1.5 blades. Not sure how to resolve this one.
Click to expand...

1.5 blades? Would one blade be shorter than the other?:D
At 2400 RPM (40 RPS) the prop moves 120 degrees each "strobe" (1/120 sec). So if the first stopped image occurs with the prop vertical, the second occurs 120 degrees CW, the third 240 degrees CW and the next image will have the prop vertical again, hence three distinct images of the two blade prop or 6 individual blades. With a 3 blade prop each image appears to be the same since the blades are 120 degrees apart so there are a total of 3 individual blades visible.

Obviously I meant 7200 ppm.

My experience is better with the white tape than viewing the light through the blades. YMMV.
Click to expand...
That may be true, I've never tried white tape on the prop.[/QUOTE]
 
I purchased a model airplane tachometer from the local hobby shop for about $20. Seems to work fine and you just press the button to get the right number of blades. The readout matches the mechanical tach closely. It works in flight too.
 
weirdjim said:
Look at the range spec. First of all, you have to have reflective dots on the prop. Second of all, the range is roughly two feet. I'm not willing to make external in flight measurements without a parachute.
Click to expand...

Boatloads of reviews saying it works fine with anything that creates a contrast, and tons of folks not using reflective tape, and using it at greater distances.

Sometimes you gotta read past the specs and see what the thing will actually do in the real world. For $11, it's hard to get too bummed out if it didn't work.

Most good mechanics already have the more expensive ones and could come along for a test flight or loan it to you for an hour.
 
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