BMatthews,

I must confess I don't know much about the ultimate effiency or speed limit for a prop driven plane, apart from compressibility issues at the tip. My intuition is that there is no fundamental advantage to a ducted fan, unless you decelerate the flow before it passes through the blades. Generally, my guess would be that there is more drag involved in pumping a bunch of air through the duct with an engine in the way. It does seem that the typical pitch for ducted fans is much higher than for traditional props, with a corresponding smaller diameter. I am not aware of any specific reason that traditional props could not be run at higher pitch and smaller diameter, although there might be some benefit to using a shaft extension to get the prop a little further ahead of the blunt object which is the engine. I will try to do some reading on this topic, and see whether I can learn something more.

Thanks ,

banktoturn