Sender: "Marvin Kaye" <marv@lancaironline.net>
To: lml@lancaironline.net
Date: Thu, 21 Oct 2004 02:19:10 -0400
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From: "Jerry Fisher" <jeremyfisher@verizon.net>
Subject: RE: [LML] Re: Keeping Ada Cooling Cool
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"I have looked on the EAA sight and can not find the spreadsheet. Can
you please tell me where/how to locate it."

Lorn,

Sorry, I was away on a business trip when you wrote.  I think that EAA must
want to keep the spreadsheets to themselves, they have hidden them away
really well.  I had to get directions from their technical staff to find
them.  So here goes:

1.  Go to www.eaa.org.  Scroll down to the picture of the latest Sport
Aviation cover at the bottom right of the page and click on it.  You then
get to www.eaa.org/benefits/sportaviation/index.html.

2.  Scroll down to the paragraphs headed January 2004 for liquid cooling, or
August 2003 for air cooling.  Click on the respective spreadsheet heading.

3.  Save it as an Excel spreadsheet.  Note that there are 2 pages for air
cooling, and 4 for liquid cooling.

I have played with the liquid cooling spreadsheet during several long
airline flights, and am really impressed with it.  Not only is the theory
sound, but it does the maths as to intake, radiator and outlet sizes, which
would be way beyond me.  It also covers the desired profile of the intake
and exits, which can be crucial.  I am following this spreadsheet for my
360.  To make these spreadsheets clear, it helps to have the associated
articles.  For anyone who has not got them, I will try to scan and post them
if I can before heading off to the UK on vacation for 2 weeks.  Otherwise I
can fax anyone interested when I get back if you send me your requests
separately to my email address.

I suspect that the basic principles are the same for air cooling as for
liquid cooling, although I have not spent much time on the air problem.  The
principles are to size the intake so that it gets enough air without being
too small (external flow breakaway) or too big (more drag).  The duct shape
needs to be optimized to to keep the boundary layer attached to the duct
walls, and the cross section has to increase until the pressure rises as
high as practical; this equates to low velocity as the air enters the
cooling element (radiator in my case).  For a radiator this can be as low as
10% of the airspeed.  You then have to maintain the duct cross section size
for a given distance behind the cooling element to avoid back pressure.  The
real key though is to then reduce the size of the duct outlet to the
calculated size.  This accelerates the airflow, which decreases the static
pressure, so that the air gets sucked through the cooling area.  This may
mean a cowl flap for the full range of airspeeds.  These work in a
counterintuitive sense.  Fo most applications you want less outlet area as
you slow down, to maintain the ratio of pressures between the air ahead of
the cooling element and behind it.

The figure for cooling drag that someone mentioned of 30% is probably about
right for the average aircraft.  I suspect that for a Lancair it can go as
high as 40%, because the airframe has low drag, and the standard cooling
setup is pretty poor.  Plenums can be effective if designed right, not just
because they clean up the airflow, but because they allow you to control the
duct shapes and the cross sectional areas of the cooling flow.  Finally,
look at the difference between exiting the cooling air downwards or
rearwards.  The increased drag from a down facing exit is amazing.

I hope this helps.

Jerry