Help - Info for JetHawk II

bildan wrote:
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.


It's efficient - compared to a jet...

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.