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Bob, that is going to be one sweet looking
aircraft. If I judged accurately from the photo your radiator is
horizontal and I presume in the duct/scoop I see forward of the lower Fire
wall. If I visualize it correctly the air enters the front of the scoop,
makes a 90 degree turn to go through your horizontal radiator and then makes
another 90 degree turn and goes out the top rear of the duct work - please
confirm or correct.
I don't need to tell you that ideally, you would
not want to make the air do two 90 degree turns - but, we all face compromises
with our installations. I presume you already have your
radiator? if not there are possibly some significant things (if you are
having a custom one made ) regarding the orientation of the cooling fins to the
air flow that might be done. IF you had the core constructed such that the
air channels/fins were oblique to the plain of the radiator this
would greatly help the entry and exiting of the air through the core. But,
I will assume the air core channels are perpendicular to the plane of the
radiator as is commonly done.
Given that is assumption is correct, then there are
two approaches I can think of.
One is as you so correctly surmise
is to guide the air through its turns and the core with a
minimum of perturbation. However, if it is not too late, I would strongly
recommend that you incline your core in the duct as much as you can. If
appears from the photo that you have sufficient depth to incline the radiator
(assuming its width is the same or smaller than the ducts) 20-35 degrees.
If you could do that it would help the air flow a great deal.
By inclining your radiator (even a little bit would
help quite a bit ), I've attached a crude drawing of what I think your
airflow/radiator situation is and one that would be better. Let me know if
this change is feasible? Otherwise, we will work with what we
have.
Ed
----- Original Message -----
Sent: Thursday, May 03, 2007 4:00
PM
Subject: [FlyRotary] Re: Cooling area
drag
Ed,
Give me some pointers on my cooling system before I get started. You
surely will save me some mistakes. My air
intake will be below and behind my radiator. I imagined just building some
deflectors to "s" the air up to the radiator. Aft of the radiator the air will
exit out the back half of the air scoop. the radiator will be laying flat,
under the motormount. the oil coolers are under each wing...as it should be in
s spitfire.
Thanks,
Bob Mears
Supermarine Spitfire
-----Original Message-----
From:
eanderson@carolina.rr.com
To: flyrotary@lancaironline.net
Sent: Thu, 3
May 2007 11:49 AM
Subject: [FlyRotary] Re: Cooling area drag
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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