One of the big advantages was that the rotary motion assured a
substantial flow of cooling air across the cylinders and cylinder
heads, even when the aircraft was flying slowly, or the engine was
instlled in a pusher configuration. This was very important since the
engineering knowledge and manufacturing technology available at the
time was not really sufficient to achieve satisfactory air cooling
with static cylinders. Most early attempts at air cooled aircraft
engines with static cylinders had limited success. At best, very rich
fuel-air mixtures had to be used to help with the cooling. In other
cases, such as the disasterous ABC Dragonfly (of which over 10,000
were ordered off the drawing board), the result was an engine that was
completely useless.

Robert