Measuring RPM of Propeller

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Hi,

I am looking into how to measure the RPM's for my double AXI5330/20 motor with propellers. I have a Hacker Master 77-O-Heli ESC. Thank you.

 

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https://www.amazon.com/Laser-Photo-...hvlocphy=&hvtargid=pla-4583932702310568&psc=1
I've also converted a tachometer for brushed to measure prop speed.many use reflective tape or magnet