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From: "George Lendich" <lendich@optusnet.com.au>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-2468100@logan.com>
Subject: Re: [FlyRotary] Re: Diffuser Configuration Comparison
Date: Tue, 13 Nov 2007 08:07:27 +1000
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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.

 To all,
That's good info Ernst, but raises the question about fins/ rad set at an 
angle to the air flow.  Disturbed air through the rad (laced/waved) will 
scrub the walls and give better heat transfer and it has been suggested that 
a rad set to angle will achieve better results (angle unknown)/ fins 
unknown).
I know there are those that don't like to generalize, but it seems to me 
that the thinner 2" rads work best for low speed ( cars in traffic/ Taxi, 
run-up, take-off and climb), thicker rads 3.5" to 7" for racing depending 
upon the speed. So my reasoning follows Al's and Ernst suggestions of 
between 2.5" to 3" i.e. in the middle of the two.

Not optimally efficient in either low speed or top speed ranges, but 
adequate for both, using another generalization of 3-3.25 sq ft per HP.

I know that's not true for everyone, but appears to be an acceptable rule of 
thumb for my requirements for a C172 type performance.
George (down under)