Mailing List Message #40451
From: Ed Anderson <>
Subject: Naca Report on Radiator Thickness
Date: Wed, 21 Nov 2007 16:28:19 -0500
To: Rotary motors in aircraft <>
Ok, here is some additional ammo for the debate (don't you love it how some things just won't die{:>)).
I came across Naca-report-86 "Properties of Special Types of Radiators" which addressed the effects of radiator thickness on factors that are of importance to us.  Mass flow, Drag and Heat removal.  You can down load it from several sites, just type in the above title in Google or other search engine.   But, here is one URL. Click on "View PDF" to get the file.
The report has a lot that makes it interesting reading including the effects of "Yawing" your aircraft on cooling effectiveness - something I know we have all spent nights wondering about, Yawn!  In any case, its the only thing I have found that really addresses the thickness factor and whether you agree or disagree with my conclusions, I think you should add it to your collection of NACA reports.
In any case onward with my interpretation:
Using the same radiator characteristics as in the report but only varying  the thickness of a radiator from 1" thickness to 4" thickness, this is the result I got using their  equations.  (Fortunately,the report had an example of each equation  , so I could plug their values into the equations and check on the accuracy of the formulas I put into the spreadsheet before I used them.    There test cases were radiators with 1 square foot frontal area and ranged  up to 16" in thickness.  They also used a 120 mph input velocity, which is probably a bit higher than for our installations - but I also used that value as well.  Some of the charts in the back show airspeeds down to 40 mph or so. 
But in any case, assuming I did not screw up someplace, here are the results I got when I changed only the thickness of the radiator on drag, mass flow and heat removal:

1. The Drag (R) went from 4.28 lbf/ft2 to 6.77 lbf/ft2 or a 58% increase presumably due to wall friction

2. The Mass flow (M) decreased from 12.33 lbm/ft^2/sec to 11.705 or a 5.13 % decrease 

3. The Heat Rejected(Q) increased from 6.1707 HP/100F to 23.66 HP/100F for a 283% increase

I took each radiator  thickness from 1" to 4"  and calculated the mass flow for each radiator  using their equation.  I then used each radiator's mass flow (it did change, although not a great deal), to calculate the drag and heat rejected. 

 The drag percentage increased presumably due to drag effects inside the core since the mass flow only changed 5.13% which is what would have affected drag caused by the frontal area (which was the same - frontal area that is- for all thickness). 

I knew that the 4" thick core would carry away more heat that a 1" thick one (of the same frontal area), but I was  surprised that it was as much as indicated (again, always assuming I have not screwed up someplace along the way).  The heat rejected increased by 283% between the 1" and 4" radiator of the same frontal area. 

Since the flow rate only varied by 5%, I presume this increase in heat rejection must be due to the fact that the air stayed in the core approx 4 times longer in the 4" vs the1" picking up additional heat.  I would assume the benefit will start to diminish at some point, but apparently not at 4" at 120 MPH. 

I encourage anyone who has an interest to obtain the report and check this out and  to ...err make certain no "subconscious bias" on my part misrepresented this report.   Yes, my spreadsheet is also available to anyone who wants to examine it, use it, find any errors, etc, etc . 

Always the chance I could have screwed up or misinterpreted the equations, but the results are as accurate as I could make it. 

Would appreciate any one finding of errors or misinterpretation of the results please convey it to me (and of course the list).  Spreadsheet is attached.

Lay on! MacDuff!!




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