Klaus Savier kindly sent me this chart from Ducted Fans for
Light Aircraft Analysis Design Construction. by R.W. Hovey

Fact one. If ducted fans were this good helicoptors would
use them.

Fact two. the hover craft from Dover England to France
across the English Channel carrying up to 100 cars,
tractor trailers and about 300 people would also use
them. They don't! They use huge diameter "free" props.

Fact three. From his own chart. Turbo fans ARE ducted fans! 

With all due respect Mr. Hovey is dead wrong.

Dead is what you will be the first time you try to take
off with one of his fans.

I think the mis-understanding comes from the fact that 
a large bell mouth mounted around a prop generates a lot
of thrust. This is because the air flowing into the prop
over the curved surface of the bell mouth generates a low
pressure on that surface hence thrust or lift. The proplem 
comes when you start moving. The huge frontal area of the bell
mouth starts merely looking like drag.

To get more low speed thrust out of a ducted fan make
a bell mouth out of an inner tube. Inflate the inner tube
during takeoff and initial climb and deflate it for cruise
and high speed climb.

Paul Lamar
 
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