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Note: Support for nitro helicopters was added in version 2.0, so reviews of older versions of the app do not account for this fact

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 or engine. HeliTach uses an FFT algorithm written by Stephan M. Bernsee of The DSP Dimension.

HeliTach has an "Electric" mode that listens to the sound of the blades and a "Nitro/Gas" mode that listens to the engine RPM to measure headspeed. The "Electric" mode can work with nitro/gas helicopters but not vice versa. Set the settings for the applicable mode, press "Measure Headspeed," and set the iPhone on the ground near the helicopter. Spool up and hover a few feet away from the iPhone to get a couple good measurements. The "History" tab shows all measurements taken in addition to the measurement displayed on the main tab.

HeliTach records and processes audio samples continuously until the user tells it to stop and displays the measurement with the highest value or strongest signal. It has been tested with 250, 450 and 500-size electric helicopters with success. The results have been verified by comparison to an optical tachometer.

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.

Requires external microphone for iPod Touch.