DeTomaso Mailing List: April 1997, Message #81
     



From: MikeLDrew@aol.com

Subject: Re: Meriha fans

Date: Fri, 11 Apr 1997 09:52:53 -0400 (EDT)



Capt Mike Drew

150 Westgate Dr.

San Francisco, CA  94127



Home/FAX:   (415) 334-7860



E-mail:  MikeLDrew@aol.com



Hello all,



First, I wrote,



> He seems to think the stock four-bladed fans work fine when paired with

new

> motors.



Then, Shane wrote,



>>That may be, but as any piston-engined pilot knows, more blades

pull more air, albeit requiring more hp.



To which I respond,



>>>Hmm, that must explain all those airplanes flying around with 10-bladed

props.  For that matter, also all those boats blasting around with 10-bladed

props.



NOT!



I think the actual determining factor would be a combination of fan blade

surface area, blade chord (thickness of the fan blade versus it's length

front-to-back, fan RPM and angle of attack.  The 4-bladed fans have large

blades, while the 10-bladed ones have small blades, total surface area might

be about the same.



Fan blades are essentially wings.  Thin, flat blades that are short from

leading edge to trailing edge will have a very low 'lift' factor, but also

low drag, while thicker, longer blades, and those that are curved (like

various aftermarket conventional engine-mounted fans, as seen in speed shops)

will have much more lift and thus move more air at a given rpm, but put

additional drag on whatever engine is driving them.



Too, for a given fan blade design, increasing the angle of attack will

increase the amount of air it pushes, until the angle is raised to the point

where air fails to smoothly flow across the top of the blade, and turbulence

and separation occurs.  This is how a wing 'stalls' and in an airplane, lift

drops abruptly, followed shortly by the attached airplane.



The stock blades are more or less flat (as I remember; I'm hundreds of miles

away from any Panteras right now) and have a fairly serious angle of attack,

and I suspect RPM is limited by available fan motor torque.  If the GT5 fans

are better (and that's an 'if' since I've only got hearsay to go on), and

deliver more torque, that would in turn increase rpm, and the fans would move

more air.  How much more is anyone's guess.



Here's another thought, fellow amateur aerodynamicists.  The small fan blades

are fairly close to one another, while the large blades have a fair bit of

empty space between them (they're not positioned at exactly 90 degrees apart

for some reason)



I would postulate that the larger blades have more opportunity to take a bite

out of still air and push it rearward.  Any fan blade that moves through the

air leaves an area of low pressure behind it, and high pressure in front of

it.  If the blades are too close together, the subsequent blade is acting

upon less-dense, low-pressure air, and thus has a lesser volume of air to

push.



Since air will always seek equilibrium, a partial vacuum will always be

filled very quickly.  With a 4-bladed fan, as a blade pushes air rearwards it

leaves a vacuum.  I suspect there's a higher probability the vacuum would be

largely filled before the next blade came along, and the blade would be able

to 'bite' 'better' air.



However, for our purposes, the differences are probably slight.  Hey, if your

fans work with 10 blades, go with them!  Since that's what *I've* got, I

certainly hope they do!



Mike