Odd numbered fan blades. Why?

I've asked a few people this, noone seems to know the reason.

Pretty much all fan blades for computers (and in a lot of other applications too) have numbers of fan blades equal to a prime number. So: 3, 5, 7, 11 or 13 blades, or sometimes more (yes I've seen a fan from a PSU with three blades lol!)

Why the prime numbers?! Increased efficiency, aesthetics, or something else?


I await with trepidation for the Bit-Tech boffins to come forth with an answer!


Chris

Edit: There as also nine bladed fans, as pointed out by Will
So the question is now "why an odd number of fan blades?"

 

I think it's more to have an odd number. A fair few fans have 9 blades too. My guess is so that they're not symmetrical, but I'd have no idea why.

 

Intriguing, I never seen one with nine. I'll have to pay more attention to worklog pics :O
Must be more than a coincidence? Something to do with blade tip vortices perhaps?

 

Aerocool, Asus, Gelid, Noctua, maybe the Fesers (I can't see - clear plastic), Revoltec, Scythe, all 9 blades, all here

 

Wiki to the rescue:

Ducted fans usually have an odd number of blades to prevent resonance in the duct.


Wasn't able to find anything else about it though...

 

I really ought to stop using stock case fans lol
I probably haven't noticed 'cause 9 is the only prime number in the 1 to 13 range.

I probably need a greater understanding of fluid dynamics and rheology to find out why. Maybe I can email New Scientist


Thanks for the help chaps

 

I think its to prevent users from fixing a fan with a broken blade. See, with an even number of blades, if one blade snaps just snap off the opposite blade then file down the nubs remaining.
Now with a prime number of blades if one blade breaks, well ur kinda screwed cause super glue and epoxy arn't really gonna hold to well. Solution, go buy another fan and the manufacturers win cause they get your money.

 

I actually never thought of that And a few times I've tried to fix a fan by removing blades or pieces of blade, trying desperately to balance it, removing more and more material till I've ended up with a smooth, bladeless hub that while balanced, is utterly useless.

 

Well a bladeless hub could have uses like if you were to, wait, ya, your right. they are useless.

 

Merely coincidence? All prime numbers are odd, except two. It would make sense to design a fan with an odd number of blades to allow easier spacing, especially given their curved shape?/guess/

Did Santa Claus not leave you any decent toys, so that you were forced to stare at fans to pass your time

 

Odd or even won't make any consistent difference to spacing until you get deep into the crazy fluid dynamics, as I understand it the odd nature reduces the resonance from buffeting and back pressure, although that's just a theory from a clever friend.

 

I think that it is to do with symmetry or rather the lack of it.

When something that is unbalanced is spinning it can damage the axle and creates vibrations and therefore noise. With a symetrical fan, when one blade is passing over the bit of plastic that supports the hub/motor the opposite is over 'open' air. There is a difference in pressure and the axle becomes unbalanced. However with odd numbers of blades you can't have an opposite blade to unbalance the spinning

I don't know how significant the effect is with computer fans but that is the reason that wind turbines have 3 blades...

 

+ rep

 

Of course Santa brought presents!
He brought fans! lmao

Nah he didn't really, but I have a draw full of them I've pulled from various bits of equipment.

Yeah a Wiki article says it reduces resonance, and what Dave says about buffeting with the hub supports makes a lot of sense.

Thanks very much!
Back at ya!

 

Wait, what? There is only one. And that's "2". If any other number can be divided by two it's not a prime number, is it?

My god, I'm stupid...

Good suggestion. It could also be that the motor have to have pairs of coils. I guess that could also be a source for resonance.

 

think it's more like, if a fanblade breaks or chips the fan is a gonner.. more money to be made in replacements..

look at a prop plane and they have both kinds

 

Although if a nine bladed fan had a broken blade, one can just remove every third blade.
It'd make for a piss poor fan, but at least it'd be balanced.

 

I think it have something to do with simplifying the manufacturing process.
360/9=40

 

No, they're all made by injection moulding and designed on CAD software. No difference if it's even or odd at all during the CAD or CAM.

The resonance explanation sounds good to me.

CC

 

Sounds reasonable or close to the truth to me!

FWIW, I have a few four-bladed fans, but they're not enclosed - they attach to a shaft via a locking collar.