The aircraft is a De Havilland Canada DHC-3 Otter, Single Engine, Seaplane. It is powered by a Pratt & Whitney R-1340 Wasp, 600 hp, air cooled, radial engine with a propeller hub speed of 2250 rpm (37.5 Hz). This aircraft is used for tours and transportation, operated by Harbour Air. The image is extracted from a video taken from Vancouver’s Stanley Park.
I took the video of the takeoff using an Android Smartphone, then extracted the audio track and calculated the waterfall FFT. Audio link.
The spectral peaks experience frequency shifts due to the Doppler effect. The 107 Hz spectral peak is due to the blade passing frequency. The engine has three blades, so this is 3 x 37.5 Hz.
– Tom Irvine
I like your measurements and calculations. I work on a turbo engine design and your work is very stimulative!
Tom — shouldn’t frequency be 37.5Hz?