Hi Gang,
Sure enough, the temperature rise we see just after the airflow goes away reveals very close to what the battery core temperatures were during flight. You want, actually need, about 100F during flight; preferably 120F. the batteries don't "wake up" until 100F. for this reason, having the battery and motor/ESC cooling paths segregated is a good idea: you can partially bock off the battery path in cool weather.
Anywhere from 120 to 140 is okay, but above 150 you are aging the batteries more than you need to. A lot of the F3A stuff comes down 1t 140 to 150 on a warm day. The latest generation batteries have brought that down a solid 10 F from what we saw a year ago, but those batteries weigh a couple of percent more.
So how much does the air expand? Assuming that the air, at most, reaches the same temp as the battery core (it won't) then it leaves the plane at 140F or just under 600 Rankine. (absolute scale with Fahrenheit sized degrees) Entry air is at about 530 R on a 70F day. Since volume at atmospheric pressure is proportional to absolute temp, we find that, as an upper bound, the air only expands 13% in this example.
The question is how to measure inlet and exit areas(?) Scoops facing directly forward or directly aft are good, but big holes in the bottom of the plane just cause drag and have to be at least 50% larger than the inlet, as John notes. One of the Japanese F3A pilots (one who builds!) had a clever solution: rear facing exhaust scoops built into the trailing edge of the wing fillets. Semi-Scale jets anybody?
later,
Dean P.