Bubble canopy / solar heating

I'm looking for input regarding the amount of air to blast through a
cockpit to offset the effects of solar heating under a large canopy.

This is one of those situations in which I have two little devils
sitting on my shoulders. One (an engineer, natch) is telling me that
three NACA's of a traditional size--say, 2 inchers, is enough for the
Van's guys and therefor good enough for us. The other guy on the
other shoulder (physicist) points out that the Van's guys are in fact
pretty hot most of the time and would like more air. He wants to put
in a pretty big scoop that is going to be hard to physically mount.

My physicist has run a batch of numbers and has got a scheme in which
he calculates that we can do 3 full air changeovers per minute. Now I
think that will keep the cockpit plenty cool, but again his scoop is
very hard to deal with and my gut says his plan is overkill. I think
we want something better than the sweaty Van's guys but less than the
monster mass flow my physicist wants. But I need a number to work
toward.

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?

Thanks--

Steve.