Originally Posted by
scifi
Hi FDR, you mentioned that the 1770 hz is not a multiple of a 400 hz supply. If the blower motor is an induction motor then it will have a slip percentage. if this is 12% then that gives a possibility of 2000 hz, which is the 5th harmonic.
Could you calculate the engine spooling down speed from the descending pitch tone..? It must have something to do with the number of blades on the rotors.
Normally the spectra will relate to the fan or to the shaft.. On one accident, it was possible to determine the acceleration/deceleration of the engine from the momentary overspeed/underspeed of the CSD, as it maintained control, those happened to be tied to the f of the cockpit ventilation system fans, that was the shaft, never caught higher harmonics. .. Your thoughts on the slip is an interesting point, the design would be worth some research. I am not particularly familiar with the ATR, but assume that the system is a DC gen, as a starter generator, so the fans would probably be decoupled from any variations in a drive to the gen itself. That is a WAG, based on similar designs.