Electric Duct Fan (EDF) Self-Launch Glider?

At 21:45 17 January 2011, Martin Gregorie wrote:
On Mon, 17 Jan 2011 11:59:05 -0800, CLewis95 wrote:

No numbers, but:
- multiple impeller blades destroy efficiency due to interference
between the blades. Its similar to the inter-plane drag than makes
biplane less efficient than monoplanes. As a result, the fewer blades
the better, hence the superiority of the two blade propeller provided
speeds are low enough to avoid tip compressibility problems.
- a bigger diameter impeller is better because moving a given mass of
air slowly is more efficient for generating thrust than moving it much
faster as is required by the smaller impeller.

Against that, about a ducted fan can offer is reduced tip losses.

That has to make an Antares-style pop-up system that turns a large, two
blade prop a better bet than a ducted fan system.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |


The duct does do a bit more than reduce tip losses - there's an
additional thrust component from the duct lip, which in the long run comes
from an increase in effective capture area. The airship people like them
because they are easier to vector for take-off ... plus there's the
reduced noise (acoustic shielding) and increased safety (blade
containment).

The big question for a self-launcher is how you retract a ducted fan - if
it's producing the same thrust as a prop, it's going to have a
similar(ish) frontal area, or else be really inefficient.

Doug