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Date: Mon, 12 Nov 2007 15:01:00 -0500
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Diffuser Configuration Comparison
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Al Gietzen wrote:
>  
>
> The most detailed, sophisticated analysis of radiators for our application
> that I have seem was done by Fred Moreno back in ’99; but unfortunately it
> also was done for a specific case of 220 KTAS and 10 fins/in. What it
> suggests is there is an optimum thickness (minimum drag), and that is
> roughly 3”.  That’s for 10 fins/in.; I concluded that my 16 fins/in rad
> should be thinner, and went with 2.5” thckness.  It works exceptionally
> well.  The 2.5” to 3” thickness seems common for rads made by Ron Davis and
> Griffin for racing applications.  That seems consistent.
>   
Was that straight, waved, or lanced fins? I just don't want anyone make 
generalities from a single data point 8*)

On a serious note, this makes a BIG difference. Straight fins offer the 
least resistance to the air, but the waved fins will transfer more heat 
per unit volume. The lanced fins (fins with lots of holes) are as 
efficient as the wave, but has no more backpressure than the straight.

My main radiator is about 3.5". I'm not flying it yet, but I had room to 
make a textbook streamlined duct. I reasoned that with more efficient 
pressure recovery, I should be able to use a thick core, and I made sure 
that it had lanced fins.