What is the shape of aviation engine?

Commercially-made piston engines such as those by Continental (Motors, aircraft engines, not the Lincoln cars) and Lycoming are generally air-cooled Flats. Flat engines such as flat-4 Flat-six and Flat-l2 predominate for private general aviation work, as this is probaby due to both wind resistance in the engine mounts or nacelle (Don"t say Hood, that"s for cars) and ease of maintenance. certain types of private aircraft use radial engines but these are older, pre-war types mostly such as the Stearman Biplane, etc. The Flats predominate- like a Housing complex!