HeliTach is an acoustic tachometer for radio controlled helicopters. It works by recording an audio sample of the helicopter and using an algorithm to analyze the sound of the helicopter's blades. HeliTach uses an FFT algorithm written by Stephan M. Bernsee of The DSP Dimension.
Use HeliTach by first setting a rough estimate of your helicopter's headspeed so the algorithm knows where to look. Then place your iPhone on the ground 5-10 feet from your helicopter and pressing the "Measure Headspeed" button. Walk away at least 20 feet from the helicopter for safety, spool up the helicopter, and wait at least 10 seconds with the rotor at full RPM to make sure you capture a good recording. Hit throttle hold, retrieve your iPhone, and read your headspeed off the analog tachometer or digital readout.
HeliTach records and processes audio samples continuously until the user tells it to stop and displays the highest headspeed recorded. So far, it has been tested with 450 and 500-size electric helicopters with success.
I can't guarantee accurate results on all helicopters, nor can I be held responsible for personal injury or damage to your iPhone or helicopter.
Version 1.1 has support for multiblades
O pessoal do Helifreak testou e aprovou. Link