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