Why are the fan blades uneven?
 

Just curious, does anyone know why the spacing of the fan blades round the hub on Mercedes cooling fans is non-regular?

I imagine it has something to do with the noise generated. I wouldn't try to even them out;)!

Probably balance as well.
Best not try & improve what MB spent time & $$ optimizing.

I believe most cars with larger fans are like this. I think it's to cancel some of the roar and harmonics.

I've always thought it was to allow room to get to the crankshaft bolt.

combination of both, allows room to work and reduces the blade pass frequency noise by changing the seperation distances. it give you # of blade different frequencies at low amplitude where an evenly spaced blade pattern gives one frequency at a high amplitude. They also use prime number of blades to eliminate multiple orders of frequencies

For the same reason the tread on tires is not regular around the circumference. Even spacings generate a few specific frequencies while the uneven spacing gives some variation to the frequencies at any constant rpm.

Think how vibrato 'improves' a single musical note.

What would be an example of a non-prime number of blades?

6 is non-prime

I thought the distribution of blades had something to do with balancing the loads on the water pump?

If that was the case the fan would have to be "keyed" to the pump and fixed.

How would uneven spacing improve balance?

So is it really non-prime numbers that are a problem? Or is it even numbers? Or is it both?

Maybe you should be counting the ' spaces ' instead of the blades...
both in number and spacing...

http://en.wikipedia.org/wiki/Lockheed_YO-3

http://www.yo-3a.com/yo3a6918007.html

number of blades determines number of spaces, but its the trailing edge of the blade that makes the noise

primes are 3, 5, 7, 11, 13, 17 etc

heres why.


take an 8 blade fan, it will generate frequencies at multiples of 8, 4, 2, and 1

a 12 blade fan will generate frequencies at multiples of 12, 6, 4, 2, 3, 1