Take a liquid cooled
motorcycle engine, run dual drive belts, one out each of the pylon mounts,
and with the smaller diameter props, you could run them faster to take
advantage of the high revving engine.

Engines run better when fast, props work better when slow. If you're
using a belt drive anyway, there's no point in running the props fast.
Let the belt apply the high engine speed to a large, slow prop.

Jose
(I left r.a.homebuilt in, though I don't follow that group)
--
Math is a game. The object of the game is to figure out the rules.
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