Fan question...

[CADsortaGUY]

Why do fans have an odd number of blades? I have a couple with 3, 7& 11. Any reason why they would use odd numbers?

I know we have a Cases and Cooling forum and God for bid Off Topic

but this seems like place to get an answer other than "Uhh, they had an extra fin and needed to put it on something" The question stems from my aquiring a fan (11 blades) that has a damaged blade and I was going to remove it and it's "opposite" for balance but there isn't one if there is an odd number

Thanks,
CkG

 

[heliomphalodon]

I'll hazard a guess: it's because the exact weight of each blade is less critical to the balance if there are an odd number of blades. If the fan has an even number of blades, then diametrically opposed blades must balance each other one-to-one. But with an odd number of blades, each blade is balanced against the average weight of the two most closely-opposed blades.

 

[unclebabar]

This doesn't answer your question, but they do sell ceiling fans with an even number of blades, e.g.

Hampton Bay
52" Polished Brass Chatham? Ceiling Fan with Light Kit
SKU: #261524

at Home Depot. <not an endorsement of Home Depot etc etc>

Since we are guessing, my guess is that the prevalence is due to some vestigial problem concerning fan noise and resonance. The noise (at least the noise due to even numbers of blades) is not a problem any more, but people still make them that way because that's what they are used to.

 

[CADsortaGUY]

Thanks guys,
unclebabar - I know there are a few even numbered blade fans out there but the ones i've seen are usually low rpm ones which i guess i'm not really talking about here but thanks

heliomphalodon - I had thought about that and it makes sense but what I'm looking for is if anyone knows for sure "the reason" or the "main theory" behind it. Yours would qualify though if you/we can find something to back it up. I've scoured the net (google etc) but have yet to happen upon any "good info"

Thanks,
CkG

 

[Evadman]

I remember watching a show on fan development quite a few years ago. heliomphalodon has it right. You do not have to be as exact when there are an odd number of blades. There were other reasons as well though.

<edit>
bas spelling as usual.

 

[KenGr]

It's been a long time since I studied fan and pump design, but the number of blades or vanes is related to the hydraulic vibration frequency. As the blades or vanes pass a discharge structure a pressure pulse occurs. This is called the blade or vane passing frequency. If this frequency coincides with a natural frequency of the hydraulic system or structure, resonant vibrations will occur. For some systems it may be easier to avoid the harmonics with an odd number of blades.

There is really a tradeoff in design for number of blades. The more blades you have the lower the pressure pulses but the more "shading" the blades impose on each other. Fans can easily use either odd or even numbers of blades and it may only be an issue of manufacturing considerations that dictate an odd number of blades for small fans. The balance issue may be a factor, but it doesn't seem to create a need for odd numbers.

 

[Smilin]

Originally posted by: heliomphalodon
I'll hazard a guess: it's because the exact weight of each blade is less critical to the balance if there are an odd number of blades. If the fan has an even number of blades, then diametrically opposed blades must balance each other one-to-one. But with an odd number of blades, each blade is balanced against the average weight of the two most closely-opposed blades.

That sounds like a good guess to me. It would allow a little slop in the manufacturing process.