From: "Marvin Kaye" <marv@lancaironline.net>
Subject: Re: [FlyRotary] Re: Calculation of Rotary Power and Cooling
  Capacity
To: flyrotary
Date: Thu, 09 Oct 2003 21:19:55 -0400
Message-ID: <web-2630907@logan.com>
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Posted for "Ed Anderson" <eanderson@carolina.rr.com>:

Hi Mark

Happy Birthday!

No, you are correct, it is limited at this point to the two GM evaporator
cores. However, that can be fairly easy to modify and I agree that offering
the ability to change the number of the rotors and not the cooling capacity
isn't fair {:>).  I could easily modify it to permit you to put any number
of GM cores in the cooling system, but that would probably not be
satisfactory to anyone.

What I plan to do shortly is come up with a section to let the user specific
the broad parameters of a radiator.  It will be assume to be a simple
one-pass radiator and the cooling again will be based on the air flow mass
through the radiators. Using the well known heat transfer equation  Q =
Wm*(Tout-Tin)*Cp(air).  Wm being the airmass flow calculated based on air
speed and radiator cross sectional area.  It does assume all the air that
can flow throught that cross section, does flow throught that cross section.
So not totaly realistic - but, my flying results and a couple of other folks
have found it fairly close.

We all know that thicker radiators have some drawbacks, however, they also
have some advantages - especially if you have higher dynamic pressure to
push the air throught.  The thin automobile radiators are designed that way
primarly due to the low dynamic pressure available.

What I am working on is taking a 1" radiator as the ideal "thiness" and then
derating thicker radiators.  For instance a couple of rules of thumb (by the
cooling business folks - not just my idea) is that doubling a radiators
thickeness only buys you a 25% increase in cooling effectiveness due to the
increase thickenss (because of the Delta T fall off).  So if a 1" radiator =
100% then instead of a 2" radiator = 200% increase in cooling effectiveness,
it will equal only 125%. A three inch only 1.185% and a 4" only 1.125%.  Now
that factor will be multiplied by the total surface/volume to derate the
radiator.  A 4" thick radiator would still end up with more heat rejection
area than the one inch even derated to 1.125. Don't know how accurate that
appraoch really is, but that is more realistic that giving it a 400%
advantage over the 1" radiator.  However, these are just some thoughts about
how to provide a meaningful (if not totaly accurate) and simple cooling
capacity toy.  Always looking for suggestions.

  Similar a doubling of thickenss reduces airflow by 10% so if the 1"
radiators flow = 100% , then a 2" radiator would only have 90% of that flow,
a 3" only 80% and so on.

So taken together I think those two rules of thumb should provide results
close to what we are observing. I am currently planning on permitting inputs
of length, width, depth and perhaps fin spacing (it turns out what really
counts is the amount of surface area the air has to make contact with and
thick radiators have more of than).  I don't won't to make the data input
labor intenstive - then it wouldn't be a fun toy.

Probably would have the user input radiator once and have it remain that
until they change it.

In any case, enjoy your birthday present


Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com

> I found that I would be 50% deficient with two GM a/c coolers.  (big
> surprise, huh?)  What I would like to ask (remember, it is my birthday) is
> this, is it possible to input different radiator and oil cooler sizes to
> reflect what radiators we have chosen?  Being limited to two GM a/c
coolers
> and one RX-7 oil cooler seems to negate the ability to input 1, 2, or 3
> rotors.  Am I missing something here, or is there another way to do this?