Fans

[Anonymous]

Offbeat question...

Why are the blades on the fans (belt driven) not evenly spaced in a perfect radius? One would think that it would cause a balance problem.

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[xctasy]

It's all to do with resonance.

A number of equally placed blades causes a resonance to occur. By stopping the symetery, the broken air from the pitch on the blade is varied enough to stop resonance being severe. I don't know if it relieves the formation of an eddy, or just why they do it exactly. Its more than just simple harmonic motion, its apparently a real curly issue.

Most heating and ventilation engineers are familiar with the issue. I came over it in a mag article in the early 90's.

 

[aussie7mains]

Yes Kiwi is right, its to reduce noise, especially with those awfull steel bladed thingies which drag heaps of power. The later plastic blades with some shape to them work MUCH better, but everybodies gone to electric these days anyway.
www.daviescraig.com.au
for good electric ones.
A7m

 

[SuperMag]

They're balanced. Look closely and you'll see some paddles are bigger than others, and they are arranged so that the center of mass when calculated is directly in the center.