Originally Posted by
tdracer
In-line engines were water cooled - they had less frontal area, but had complex cooling systems and could be taken out by a single hit on the cooling system. Some of that drag advantage was given back due to the need for big radiators, plus you couldn't run them very long on the ground prior to takeoff without overheating.
Radial engines had more frontal area, but were air cooled so no plumbing, radiator, etc. and were far more tolerant of battle damage. Radial engined aircraft were preferred for ground attack due to the better tolerance to damage from ground fire, and since cooling airflow was provided by the turning prop, they didn't generally overheat sitting on the ground waiting to takeoff.
The original S6 racers pumped the coolant through the leading edges of the wings for cooling IIRC
The P51 with clever ducting actually produced thrust from the big radiator bulge on the undersde, not a lot, but more than enough to offset it's drag.
A site well worth a visit and read through.
https://enginehistory.org/