The forward edge of the blade also contributes to any generated sound. For example, owls fly silently because of the special feathers at the leading edges of their wings. I would imagine that duplicating such a structure in cast plastic is an aeronautical engineer's dream.

BTW, both of my cars have 9-blade fans; 9 is not prime, at least not since I last checked.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970