David Massey wrote:
>
> Message text written by J&E
> > I know one of the ways to increase 'air flow' on ceiling fans is to
> change the pitch. 10 degree pitch will Not move as much air as say a 18
> degree pitch, keeping all thing equall. Therefore has anyone tried
> twisting the fan blade to a greater pitch to increase a greater draw of
> air to cause a greater "air flow" through the radator? -Cosmo Kramer
>
> <
>
> Cosmo,
>
> Fan design is not a trivial thing and I would advise against significant
> modifications to a fan that will be turning at as much as 5,000 RPM. Yes
> you can twist the fan blades to get a greater pitch and "grab" more air but
> if you twist one more than the others or if you twist one in such a way
> that it is out of radial alignment from where it once was you have an
> imbalanced ending to your crankshaft. Furthermore, if you twist it too
> much you will fatigue the metal and a blade can fly off causing untold
> damage. (This happened to a friends Austin Healey. He was luck in that
All the above is true, and there's a further consideration. Blade pitch
is designed into a fixed blade as a compromise for the widest range of
operation, but this does not mean that the blade operates at the highest
efficiency at all speeds of rotation. Ideally, the blade pitch should
change with rotational speed (this has to do with the effective total
angle of attack, because the angle at which the blade "sees" the air
changes the faster it goes). This lack of efficiency at all speeds is
the principal reason why so many cars today do not run a mechanical fan.
With an electrical drive and a single speed, the fan blade can then be
optimized to run most efficiently at that speed.
In short, adding pitch to a fixed blade probably would make it more
efficient at low speeds, but would likely make it very inefficient and
much more power-consumptive at high speed (keep in mind that the speed
range of the ceiling fan is small compared to that of the fan on a car
engine). Nevertheless, the added possibilities of fatigue failure
mentioned above should be kept foremost in mind.
Cheers, Cosmo.
--
Michael D. Porter
Roswell, NM
[mailto: mporter@zianet.com]
`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)
`80 TR7 (3.8 liter Buick-powered)
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