[Normy]

The reason for the horizontally-opposed engine? It allows better air-cooling without increasing frontal area drastically. This is why the majority of aircraft flat engines use a pushrod valve train- it interferes less with cooling air flow. The 911 motor with its' SOHC heads and cam chains works because it has a cooling air fan, something that aircraft engineers want to avoid.

Radials are very light for their power, and quite rugged, but they have enormous frontal area. The V-12 engines used in WWII fighters were superior from an aerodynamic standpoint, but they were heavy and they had the "achilles heel" of liquid cooling.