[sdiy] Synthesizing helos

[Tim Ressel]

To all insaniacs:

For anyone out there who wants to synth a helocopter
sound, I just ran across this info from my time a
century ago ;-) when I was working with the beasts.
This is for a CH-47D. That is one of those big
choppers with two main rotors and no tail rotor. I
only included the major noise makers:

Rotor RPM              225 RPM
Engine N1 at 100%      18720 RPM
Combiner XMSN blower   9739 RPM  (12 blades/15
stators)
Aft XMSN blower       10517 RPM  (11 blades/11
stators)
Forward XMSN blower    6912 RPM  (14 blades/30
stators)
APU shaft             58228 RPM

Divide RPM by 60 to get hertz. The main rotors consist
of two sets of 3 blades that are 60 degrees out of
phase. Don't be tempted to assume a single six-rotor
model; it will not be the same. Remember the rotors
are at different distances, and the difference
changes.

I don't have info on blade counts for the turbines, so
guess. As far as the fans go, the important thing is
the blade pass frequency. That is how often a moving
blade passes a stationary blade. So you get:
(Shaft RPM / 60) * #blades
(Shaft RPM / 60) * #stators
Keep in mind for turbines there are multiple sets of
fan blades. I would just use the blade counts from the
three fans listed and use them at the turbine RPM.

Cheers ya loonies,
--TR

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