Jay,
The writer makes two contradictory statements: First, he says that more blades
are less efficient. Then in the next sentence he says that a fan with less
blades has to turn faster to move the same amount of air, which implies that a
fan with fewer blades is less efficient.
Fan blades only move in the previous blade's wake when the fan is stalled, i.e.
the airflow is not moving relative to the fan. In an airplane propeller
situation, and for a radiator fan when there's a place for the air to go, the
blades move through a helical path relative to the airstream, and each blade
sees 'new' air. The tip noise comes from air spilling around the tip and causing
turbulence to itself, not to the next tip (if that makes any sense). You can fix
that by shrouding the fan or by running one of those fans that has the perimeter
ring attached to the blades, which is effectively a perfect shroud.
I read a book a LONG time ago about the testing they did in the late 40's on
supersonic-tip propellers; they ended up with a scimitar-like blade shape,
effectively increasing the 'sweep' angle as the diameter increased, to keep the
shock wave on the tips under control. They did testing on a P-47 and got it up
well above 500 MPH.
Me? I run a six-blade Ford fan, in the Tiger shroud. Says 'made in Canada' on
it, and the blades are made of aluminum. It's got the little hole for the water
pump shaft.
Theo Smit
tsmit@home.com
B382002705
> At 08:04 AM 7/1/99 -0700, you wrote:
> >
> >
> >
> >Here is something I'm sure Bob and my dad will not leave untouched. In
> >today's LA Times, in the "Science" section they had the following:
> >
> >"Q: Is a fan with five blades more effective than one with three blades?
> >A: The more blades a fan has, the less efficient it becomes because each
> >blade is moving in the turbulent wake of the blade preceding it. But a fan
> >with fewer blades has to turn faster to move the same amount of air, so it
> >will have a higher tip speed and thus be noisier. The same principles
> >apply to airplane and ship propellers. The optimum efficiency is achieved
> >with a single blade attached to a suitable counterweight, and some planes
> >have actually flown with such a design. The primary reason for multiple
> >blades is to reduce the diameter and the noise."
> >
> >
> >Jay
> >
> >
> >
> James Barrett Tiger II 351C and others
>
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