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Subject: [FlyRotary] Re: Thick or Thin? The debate continues
Perhaps the reason there is such diversity of opinion on this issue is
because of the complexity of making an optimum determination; there are
a large number of variables. To mention a few; flow rates, tube
thickness, tube wall thickness, fin density, thickness and design, air
speed, inlet diffuser design, blah, blah . . . So there is a tendency
to make qualitative and intuitive judgments which ignore all but a few.
The *only* way to discuss a technical factor is to discuss it assuming all
other factors are equal. To assume otherwise is a total waste of time.
The criteria are size, weight and drag. The Nascar and other racing
folks have done a lot of optimizing that applies for us. I think there
is good reason why most the race radiator vendors make rad cores in the
2 - 3.5" thick range. I still have the plots from some fairly extensive
studies that Fred Moreno posted on the "other" list a couple of years
ago. Using values for the rad design that are "typical"; his studies
show a fairly broad optimum for core thickness between about 2.5 and
3.5".
I found Fred's study very interesting and accepted it as further proof that
thick is better than thin. Can anyone seriously clasify 2.5" - 3.5" as
thin? For my RV-8 I bought the thickest off the shelf rad I could find
(2.625") and would have gone an inch thicker had I been willing to spend 3
or more times the price for a custom rad.
Perhaps one factor that Tracy's argument may have slighted in the
thicker cores is the decreasing efficiency of heat transfer as the temp
difference between surface and air temp gets lower going through the
rad. Thicker may be fine if the fin density is a bit lower the fins are
a bit thicker. It's difficult to say that there is one right way
because each installation is different; so it may be best to go with the
body of evidence that says somewhere in the range of 2 - 3.5 inches.
To the contrary, I specifically addressed this issue and redily admit that
using a thick rad sacrifices rad efficiency (defining effeciency here as
disipated BTUs per sq." of fin area) in order to gain aerodynamic efficiency
(less drag). Gaining a pound or two for a significant reduction in cooling
drag is a good deal.
One thing is for sure; you have to have the pressure recovery needed to
overcome the pressure drop in the core. So if you are going to hang the
radiator out in the free stream air, it better be very thin. If you
have an effective diffuser design the slows the air by a factor of 4 - 5
then be happy with your 3" thick core. If you want to minimize drag on
the plane, have an outlet duct that efficiently accelerates the air back
to an area of about 1.5 - 1.8 times the inlet area.
FWIW; Al Gietzen
Absolutely agree. If you are flying a Pietenpole with a totally exposed rad
hanging out in the breeze, use a thin rad.
Tracy Crook
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