Helitach

[Vincent LaPorte]

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 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.
Cost $3.99
For Android phone the app is called Heli-Headspeed.
And finally - here is a free program for a windows computer that will calculate rpm from an audio file.
http://www.helifreak.com/showthread.php?t=151698&page=2

Luck,
Vince