The fan blades are asymmetrically spaced so that they don't generate a
single frequency. This is important to keep the fan from self-destructing,
and to keep the noise and vibration under control. If the blades were evenly
spaced, the fan would tend to create a loud single-note hum. The same thing
is done on the six-blade "canadian" fan - the three pairs of blades are not
equally spaced.
If you run tires like the old Bridgestone RE-71s, you can clearly see that
the lateral tread sipes are not equally spaced around the circumference
either. If they were, the hum from the tires would be very loud.
Theo
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