Hi Paul,
I publish a newsletter directed at canard pusher type aircraft and deal
with cooling issues frequently.
I had the opportunity to read your article.
I have not seen one better anyplace for explaining how to determine
cooling system specifications.
I would like to share it with canard pusher enthusiasts in a future CSA
Newsletter.
Terry Schubert
Central States Association Newsletter Editor
quote
Cooling Drag Draft
Check for OK and use drawing Kays & London wedge
/Paul Lamar (TX)/– While travelling, I was thinking of a better way to
illustrate the cooling drag situation. Having my book, "How to Cool Your
Wankel" will help in understanding the situation.
Let’s assume we have a simple rectangular duct 6 inches high and 12
inches wide. That is 72 square inches or one half square foot.
At 200 MPH the dynamic pressure is .0026 X MPH squared.
200 x 200 is 40000.
Times .0026, or about 104 pounds per square foot.
The non porous (solid) 6 by 12 flat plate drag will be roughly 50 pounds
at 200 MPH. It takes prop thrust to overcome this drag of course.
Hooker eloquently puts drag/thrust into HP context in his book "Not Much
of an Engineer".
"Thrust is a force which can be exerted on either a stationary or moving
aircraft. In the latter case thrust horse power is produced. Thus one lb
of thrust exerted on an aircraft moving at 33,000 ft/min, which is the
same thing as 375 mph, is equivalent to one thrust horse power. "
It takes about 500 to 600 cubic inches in a radiator to cool a Mazda 2
rotor. A 6 inch by 12 inch radiator core 2 inches thick is 148 cubic inches.
Four such cores back to back give about 600 cubic inches.
Now the porosity of a core can be related to its flat plate drag. Lets
say 4 cores back to back in a simple duct look like a 6 X 12 porous
flat plate that generates 25 pounds of drag at 200 MPH. Remember a solid
plate that size would be 50 pounds of drag.
200 MPH is about 300 feet per second. So the net HP consumed is 25 X 300
or 7500 pound foot per second.
If you put pound foot per second converted to HP in Google you get:
_http://www.kylesconverter.com/power/foot--pounds--force-per-second-to-horsepower_
Convert Foot-pounds-force Per Second to Horsepower
Multiply by .0018.
.0018 times 7500 is 13.5 net HP out of the prop and about 17 HP out of
the engine given 80% prop efficiency.
That is going to burn 8.5 pounds of fuel per hour or 1.3 gallons per
hour at a BSFC of .5 gallons per HP hour, typical for most engines.
Let’s say we can cut the cooling drag by converting the 600 cubic inch
core into one core 2 inches thick with a frontal area of 300 square
inches and mounting it at an acute angle.
That will be about 25 inches wide by 12 inches high. We will use a Kays
and London wedge
diffuser to slow the air going through the core to one quarter of the
air speed so porosity would be 4 times higher of the simple thick core.
Actually, more air will flow through the radiator and it will cool much
better and your air plane will be faster.
We have now cut the fuel burn due to cooling drag to about .3 GPH.
end quote.
I don't see how this is going to help you much with an air-cooled
engine. Other than the baffling shape you have no control over the number
of fins per inch or the size and length of the fins. You have to live
with what
you have.
You can publish it with proper credits.
Paul Lamar
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