"Steve S." wrote:
So, the crux of the question: Based on your gut, personal observations
or, in a perfect world, empirical data--how often do we need to have
an air changeover to keep a cockpit at a reasonable temp in hot sun?

Why not perform a test by building a mockup canopy using plastic wrap (e.g.
Saran wrap, Glad wrap and Stretch-Tite)? You don't need to worry about
aerodynamics, so the support frame you lay it over can be anything you can
cobble together. I presume all you need to do is match the exposed
insolation area and cockpit volume of the production canopy. (Maybe use
cardboard or aluminum foil wrapped around some wood posts to mimic the
volume of the shaded lower portion of the cockpit.) Hopefully the optical
properties are close enough to the final canopy material to not throw
things off. Throw in some thermometers, fans, stuff, and a sunny day and
you'll have a full-fledged experiment!

Also, the article "A Futuristic Anachronism" by Bob Fritz in the September
issue of Kitplanes magazine discusses a mechanism used by _experimenter_
Paul Lipps to deliver cool air to his cockpit.