At 11:56 18 January 2011, Mike the Strike wrote:
On Jan 18, 4:13=A0am, Doug Greenwell wrote:
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
=A0between the blades. Its similar to the inter-plane drag than
makes
=A0biplane less efficient than monoplanes. As a result, the fewer
blade=
s
=A0the better, hence the superiority of the two blade propeller
provide=
d
=A0speeds are low enough to avoid tip compressibility problems.
- a bigger diameter impeller is better because moving a given mass of
=A0air slowly is more efficient for generating thrust than moving it
mu=
ch
=A0faster 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@ =A0 | Martin Gregorie
gregorie. | Essex, UK
org =A0 =A0 =A0 |

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
come=
s
from an increase in effective capture area. =A0The airship people like
th=
em
because they are easier to vector for take-off =A0... 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

One technique to launch underpowered self-launchers is to auto-tow the
ship until it is airborne and then climb under power. The
acceleration and ground roll can be a significant problem at high
altitudes or on soft fields and the auto-tow is cheap and simple.

Mike


Yes, but you would still need to be able to stow the fan in cruise?

I like the idea of some model airplane fans on a stick waved out of the DV
window :-) ... unfortunately, looking at advertised thrusts for these
units, I don't think they'd be up to it even as a sustainer.