X-Mozilla-Keys:                                                                                 
Al Gietzen wrote:

The air flow required can be similarly computed; and as it turns out, for a
50 F temp increase in the air (a seemingly reasonable number that Paul
chose), the air flow in cubic feet/min (cfm) is very close to being equal to
the heat rejected in Btu/min. So for a 20B I'd need about 7000 cfm for a 50
F delta T.  That's a lot of flow (and drag).  Maybe we need to design for
a higher >>air temp increase.

I assume in all of this that the design operating point is a sustained max
power climb.  Is this reasonable?  Climbing at 1000 fpm means that the
power will be dropping after a few minutes no matter what you do.

Upon further reflection it occurs to me that at the design point of 100 F
you could not generate the max power (sea level standard conditions); only
about 91%.  If you're at 2000 ft above s.l.; then reduce that again by about
0.93, bringing the design point down to more like 85% of max power (normally
aspirated, of course); with corresponding reduction in needed air flow.

In considering radiator design, the 10 fps air flow design point used for
autos may not be that meaningful to airplanes.  For a car you need to design
for high-power, low-speed where you have little dynamic pressure and rely on
the fan.  Slowing the large airflows we are computing to 10 fps would result
in very large ducts.  This is consistent with large, thin radiators, and may
yield low drag (only if the air is again accelerated with a large exit duct)
but is not consistent with our space constraints.  My guess is (based on
installations that work well) that we could use much higher air velocity
thru the rad and be OK.  Those of you flying; what area ratios do you have
from duct opening to radiator surface?

Al Gietzen

 
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