Duncan,
>
> What is your ratio(s) of inlet area to outlet area?
What really matters is the ration of outlet area to RADIATOR area. As
already stated on Contrails, too large an opening won't flow more air
than a correctly proportioned one, since it is the radiator that governs
the flow in the duct.
It may help to figure the radiator duct system as a duct with 3 valves
in series. The middle valve is the radiator. As it is partly closed,
opening the other valves more than necessary won't change the overall
throughput.
There is no use opening the outlet more than about 1 time the radiator
frontal area, you would only add drag without increasing air flow
through the radiator. Drag decreases as you reduce outlet area by means
of the cowl flap. In cruise, outlet area should be smaller than the
inlet area to achieve minimum drag. It is advisable to construct the
adjustable cowl flap with a range of openings from 1 time the radiator
area to near zero (power descent on a cold day).
In our setup, inlet area is 25% of the radiator frontal area, whereas
exit cowl flaps can be adjusted from 70% to 0% of the radiator area (410
cm). We currently cruise with flaps adjusted to 18% of the radiator
area, or less on cold days. On summer hot days, best cruise (75% power)
is with the exit adjusted at about 28% radiator area. Of course we open
more in a prolonged climb.
During flight tests I installed a data logging system to check adequacy
of the pressures and temperatures.
Adjusting CHT and oil temps to within 5C of desired temperatures is a
breeze, and we found no special limitations concerning ground run time.
A thorough cooling study is really worth the time and hassle.
Hope this helps,
Regards,
Gilles
http://contrails.free.fr
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