Advancement of prop blade in flight, new information

"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

"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