"Paul Sengupta" wrote in message . ..
"David CL Francis" wrote in message
...
The extreme of this is the enclosed fan where the
enclosure markedly reduces tip losses. The fan runs nicely along like
this with a high blade area and little daylight visible through the
disc.

Whatever happened to the concept of piston engines running
a ducted fan? That aerocar thing has them, but what about
on other more normal planes?

How efficient is a ducted fan compared to a prop? I seem to
remember hearing in model aircraft settings, a prop is more
efficient.

Paul

Unducted props tend to be more efficient simply because they are
of larger diameter, and it's much more efficient to accelerate a large
column of air to a lower speed that to accelerate a small column of
air to a high speed. The higher RPMs necessary for small props cause
much more drag on the prop and horsepower is lost to turbulence,
noise, heat and so on. A 150 HP lightplane driving a six-foot
propeller at 2700 RPM would never generate more than about 500 pounds
of thrust, no matter what the blade pitch might be. A small helicoper
with 150 HP driving a 27-foot rotor at about 300 RPM will generate far
more thrust, enough to lift the entire helicoper, which might weigh
1500 lbs.
Dan