Fan design 101

["Graydon D. Stuckey" <graydon@apollo.kettering.edu>]

On Thu, 6 Aug 1998 MSV96@aol.com wrote:

> got a 7 blade fan ready to install on the 5kcstq (thanks to Dave Head) and it
> has curved scythe-like blades.  It seems the more blades a fan has, the higher
> its frequency pitch, which it more pleasing to the ear IMHO. >>

That's true.  Take the fan rpm and divide by 60 to get revolutions per 
second.   Then multiply by 5 (for the # of blades) to get the blade passing 
frequency.

> On this subject...does anyone know if there was a change in fan motor max RPM
> when Audi changed from 5 to 7 blades? Reason I ask...in my 911 days, I recall

I would guess there was.  Cooling fans in general are designed to do 
their job and no more because typically, they are very noisy and a 
tremendous source of vibration. Without knowing the exact geometry of the 
fans, I can't say for sure though.

> fan). The reason I was told is that the fan with more blades actualy pushed
> less air when turned too fast (by the fan pully intended for fewer blades).

That's possible.

BTW, I deal with a company that now makes engine cooling fans that used to
make submarine propellers.  They are doing well in the automotive market
because they _really_ know how to make quiet, efficient fans.  :-) Take a
look at the fans on the new Cadillacs, Park Avenue/Aurora/Riviera/etc. 

They also did the HVAC fan in the new Seville.  This fan is
state-of-the-art!  It was a major contributor in making the new Seville
the quietest car in the world. 

Later,
Graydon D. Stuckey

"There's alot more to Jazz than just wrong notes"