On Jan 18, 4:13*am, 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
*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

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