I have looked it up from the forum and couldn't find what the proper way one should tach their engine/prop is.
Where should I point my laser tach to measure the rpm?


Thanks in advance
helirecago

From a safety point of view, approach the prop from the rear. You will need to be 100-150mm from the prop to get a good reading. You can look at the middle all the way out to the tip of the blade.

Must have bright day. No artificial lights unless pure DC incandescent.

The non-contact tach I got is similar to this one
http://abqindustrial.com/tachometers...-cdt-2000.html

If I haven't mistaken your words, I should point to the blade from the back of the prop but I can't think of any 90 degree surface on the blade that can reflect the laser to the tach for measurement

Sorry if i sound too dumb

It appearse this tach is for measuring electric motors? But it can be used on a model glow engine.

You can use it by standing behind the prop and shine it on the prop blade. The RPM measurement will be twice the actual rpm because there are two prop blades.

According to my understanding the tach need a perpendicular reflective surface for it to work. But I am afraid the pitch of the blade will reflect the laser elsewhere and the tach can't catch it...

It should be OK to just look at the back of the prop. Try it and see. That's the way all the other tachs are used.

Let's see, Will report back if it works

Thanks a lot for your advice, W8ye

Wave your hand with fingers separated and if get a reading it does not need reflection.

If it needs reflection put something shiny behind.

Try the moving-fingers method, no reading recorded unless i brought my hand real close to the tach. So i supposed my fingers are not shiny enough[:@]
my tach comes with some reflective adhesive tape , so I will stick them to the back of the blade and see if it works tomorrow.

Thanks everyone again

I hope the reflective tape works with your laser tach, but if not, the optical tachs made for our hobby use are much less expensive than the laser tach you are trying to use.
Regards,
Pete

I suppose that reflective object does not have to rotate. So if laser beam reflects from object behind the propeller or fingers the tachometer gets pulses to count. Of course if the pulse widht matters and in this case on/off are reversed if compared to a tiny shiny tape on propeller.