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From: "Ed Anderson" <eanderson@carolina.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-2499980@logan.com>
Subject: Re: [FlyRotary] Re: Naca Report on Radiator Thickness
Date: Wed, 21 Nov 2007 19:25:42 -0500
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----- Original Message ----- 
From: "Ron Springer" <ron2369@sbcglobal.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Wednesday, November 21, 2007 6:17 PM
Subject: [FlyRotary] Re: Naca Report on Radiator Thickness


>> > The drag increase of 58% sounds way too low. You
>> > increased surface area by 300%. Unless mass flow
>> > decreased a lot (it didn't) or drag coefficient
>> > dropped a lot (it shouldn't), then this can't be
>> > right.
>>
>> Well, there is no change in frontal area between the
>> radiators, so the old 1/2pV^2*A drag factor remains
>> essentially the same for all  - discounting the
>> small 5% decrease in mass flow which would (by
>> itself) help decrease the frontal drag some.  So the
>> question is would the increase in skin friction be
>> proportional to the increased internal surface area
>> ( I would presume it is)?  And if it is? Then what
>> is the absolute amount of drag per square inch based
>> on.  Is the  internal core drag  a small part or a
>> large part of the overall core drag.????  I know -
>> it probably depends........ {:>)
>
> Ed,
>
> This is a very good point. I am thinking flat plate
> drag equations and infinitesimally thin fins. That is
> not true. The drag is probably dominated by the
> stagnation of air on the frontal area of the fins and
> anything else that produces frontal area. That frontal
> drag will probably dominate the internal skin friction
> drag. So, even though the internal skin friction drag
> should go up 260% for the thicker case (see other
> email), the frontal drag will be only be slightly
> reduced due to the lower mass flow. The net effect
> could very well be only a 58% increase as your
> equations show.
>
> I've been schooled!
>
> Thanks,
>
> Ron
>

Ron,

I really appreciate you taking the time to educate me!.  I can read and I 
can work the math (most of the time), but what I am truly lacking is the 
overall education and body of knowledge in the aero and thermo areas.  That 
means that I can easily focus on one factor to the exclusion of other 
perhaps conflicting and equally valid factors.  That plus just trying to 
integrate all (some?) of this  into a coherent picture just really gets my 
head to hurting at times.

So having  you point out the areas where my conclusions (or assumptions) may 
be questionable helps me know where to focus a bit more studying.

I really hope you have time (and inclination) to read that report and I 
would be interested in your assessment.  Having written reports for the 
Government myself in a different life, I know that just because its in a 
report does not necessarily make it so.  But, it seems that most of the 
stuff that is available and of interest in our hobby was written 5 or 6 
decades ago {:>) and they really used a lot of analytical math - a number of 
the more recent studies I thought would be informative turned out to use CFD 
tools and had lots of pretty pictures (and some were really interesting), 
but little in the analytical derivation area.

Thanks again and have a Happy Thanksgiving.  I'll be over at my daughter's 
house playing with my three grandsons.

Best Regards

Ed