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Actually, Mark, I started pulling together what I
believed to be the major factors without getting too down in the weeds about
rotary cooling, a couple of years ago with the intention of publishing an e book
(pamphlet more likely) . Then I ran into the problem that the seemingly
best diffuser (Streamline duct) was simply too long (in its optimum
configuration) for most of our needs. Yes, you can shorten it but then you
incur more drag. So I scratched my head about that for a while until the
light bulb came on.
After experimenting with a number of duct shapes and
reading more, I came to the conclusion that if my understanding about what
killed effective cooling was correct then I should be able to achieve my
cooling/drag goals with the "Pinched" duct. But, what I wanted to do and
never took the time to do was to go back with Mr. Bernoulli and calculate the
air velocity along each segment of the streamline duct and then do the same for
my "pinched" duct to see if there was any similarity. Also, I have
not paid much attention to the exiting duct - simply because I don't have room
for one. I tired one back almost 8 years ago and decided the zigs and zags
it had to avoid engine/motormount, etc impeded airflow more than helped
it.
But, alas, just as I was recently about to go to
publication, the new "bible" of cooling was published - so how could I possibly
compete {:>). I may still do it as if it passes the gauntlet of folks
on this list (or errors if any {:>)) are corrected,as it may be useful to
some.
Ed
----- Original Message -----
Sent: Thursday, May 03, 2007 12:34
PM
Subject: [FlyRotary] Re: Cooling area
drag
ED,
So, tell us, when is your book on cooling going to be available?
Mark
On 5/3/07, Ed
Anderson <eanderson@carolina.rr.com>
wrote:
Less we forget how important drag is in our hobby, I
took a formula for calculating drag at different airspeeds and the Hp
required to push the given frontal area along at the stated airspeed.
This is for two of our traditional GM evaporator cores
using their combined frontal area of 180 sq inch or 1.25 sq
feet. This assumes that airspeed shown represents the velocity
through the cooling core (which is not really likely to reach speeds above
80 mph if you have any sort of ducting), but that is an assumption on my
part since as Bill keeps reminding me I have not instrumented my ducts
{:>)
|
Air Speed (MPH) |
HP |
|
40 |
0.533333 |
|
60 |
1.80 |
|
80 |
4.27 |
|
120 |
14.40 |
|
140 |
22.87 |
|
160 |
34.13 |
|
180 |
48.60 |
|
200 |
66.67 |
Clearly the faster your cruise speed the more
important it is to minimize cooling drag. Of course the airspeed the
core sees should normally not be over 10% of your cruise speed or 30% of
your climb speed (According to Horners rule of thumb). So slowing down
your cooling airflow to lessen drag is one reason for paying some attention
to your ducting. However, cooling again depends on many other
variables, for instance accepting a high velocity airflow through your core
may permit you to use a smaller frontal area core thereby offsetting
to some extent the higher drag. In fact, space constraints may force
you to his configuration regardless.
Another factor to consider is trade off between
frontal area drag and thermal transfer efficiency. A large thin
radiator is theoretical the most efficient due to that factor.
However, it disturbs a larger segment of air (resulting in higher drag) -
not really important in an auto at 60 mph but very important in a Cozy at
200+ MPH.
A thicker core with smaller frontal area disturbs
less air and while it has more skin drag that is small compared to the
frontal area drag. Tracy refers to the approach of thicker cores as
"... getting the most cooling possible for the smallest column of air
disturbed". So while theoretically the thicker core is less
thermodynamic efficient - it turns out with sufficient dynamic pressure
available it provides definite benefits in our application. The
average thickness of NASCAR radiators is 3" and up to 7" for the longer high
speed tracts. Since they operate in speed regimes close to what most
of us fly - they just might know what they are doing given the $$ they will
spend for even a slight speed advantage.
Ok, back to creating a company - boy, a lot to
learn
Ed
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