Paul
I am building a Bear Hawk which has a aluminum wing.
I was thinking if you attached .375 O.D. .250 I.D. tubing to the under
side of the wing skin running from the front spar to the rear spar
and coupled the .375 O.D. to a 1" I.D. tubing running parallel with the
front and rear spar to make a large heat sink and let the aluminum wing
radiate the heat. How hot do you think a wing would get? The Bear Hawk
has 222 sq ft. of wing area. I figure it would require 16 .375 O.D.
tubes, 30 inches long (8 for the upper skin and 8 for the lower skin)
for each wing. The heat exchanger would weigh 17.25# if it were made
from "M" cooper tubing for each wing. Plus a little over 1.6 gal of
coolant per wing. If the wing didn't get to hot and cause problems from
expansion, and heating the fuel takes, the cooling drag could be reduced
considerable.
Just an idea and thought I would throw it out to see what every one
thinks of the idea.
Would the boundary layer prevent any significant cooling?

Here is ahow you can compare. Calculate the surface area of
radiator exposed to the air and the surface area exposed to the
coolant. That will give you a rough idea if a skin surface radiator
will work. The Schneider cup people did this surface rad trick in
1930 with and had a lot of trouble. Also Lockheed tried it
with the P38 for the intercoolers and also gave up
and went to a conventional inter coolers.