Phil Ethier tried to save me from some embarrassment by mailing me (in
private) on a real whopper that I let creep into a posting; I hope I'm
not violating your trust, Phil, by publishing this piece of email, since
it points out that you were being smarter than I was. :-)
>As a final point, is your fan installed the right way round?
huh?
Uh, yeah, it's confusing to me too... Now I think I see that it doesn't
make any difference. The pitch of the vanes will be the same whether
they're face-in or face out, wouldn't it? Give me an F in 3D visualization
for the day. That's what I get for working on a fax server product
for the last three months instead of on computer imaging systems...
It might make a difference if the fan's blades are curved rather than
merely angled. That is, it's much easier to get a pinwheel to turn
if you blow on the front of it than if you blow on the back of it. The
fan vanes on the black car are flat and will therefore deflect air to
the rear no matter which side I have facing the engine. On the green
car, though, I have a plastic fan with curved blades, and it can be
installed facing either direction. (The black car's metal fan is
clearly installable in only one orientation, which led me to believe
that the direction was significant. :-)
There is probably an orientation there that results in the greatest flow
because you work with the curves or against them; in the best orientation,
the high-pressure side of the airfoil (the underside, if you think of an
aircraft wing creating lift) would face the engine and the low-pressure
side (the top of the wing) would face forward. The resulting pressure
differential applied by this setup would cause flow even if the leading
and trailing edges of the vanes were on a line parallel to the direction of
rotation.
Through some fairly convoluted and not terribly sound imaginings of aero-
dynamic effects, I can imagine the possibility that mounting this fan
backwards would result in turbulence but comparatively little flow and a
large aerodynamic drag on the engine, because the pitch of the blades would
want to push the air backwards but the reverse biasing of the airfoil
surfaces would want to cause flow forwards.
Anyway, I think I'm going to go have a cup of coffee now and see if I
can find my daughter's pinwheel for some experimentation....
--Scott "But Wilbur, your bicycle shop is the wave of the future!" Fisher
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