On Feb 15, 4:35*pm, Dana M. Hague wrote:
On Fri, 6 Feb 2009 07:28:04 -0800 (PST), bildan
wrote:

A glider tug is one of the few airplanes where the 0 - 60 acceleration
time matters since you have to get the glider up to an airspeed where
the ailerons are effective enough to balance on a single wheel. *30
knots is in the middle of that range.

A glider tug looks like a nearly perfect application for a ducted fan.

I think not. *Ducted fans are horribly inefficient at low airspeeds.
For that matter, they're not that great at higher speeds, either.

-Dana

--
"Next year in Galt's Gulch!"

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.