On Sun, 15 Feb 2009 17:18:10 -0800 (PST), bildan
wrote:

Ignorance!

The high bypass turbofans used on airliners is a ducted fan that is
very efficient at high subsonic speeds. At speeds below 80 knots, a
ducted fan is more than 4 times more efficient than an open prop.

It's just in the middle speed range that the drag of the duct offsets
the gain in efficiency.

A high bypass turbofan is a very different beast than a piston powered
fan.

Max thrust at low speeds is gotten by accelerating a lot of air a
little bit (i.e. big prop, flat pitch). That's not the definition of
a ducted fan.

Also fan to duct clearances have to be REALLY small for best
efficiency... difficult to achieve with the inherent vibrations of a
piston engine without a very heavy structure.

-Dana

--
There are 3 kinds of people: those who can count & those who can't.