David CL Francis wrote in message ...
On Mon, 16 Aug 2004 at 21:04:45 in message
, Roger Long
wrote:

The three blade prop will be less efficient per unit of area than the two
blade where it counts, near Vx with trees in the windshield. Given a
limitation on length however, the extra blade area of the three blader can
easily offset the efficiency loss by a substantial margin.

Just curious, but how does this fit with the 6 bladed props on the
latest C130s? The Herk has gone from 3 to 4 to 6 bladed props it seems.
Short take off and good climb out is a major requirement for the C130 I
would have thought?

Still curious but how does the extra blade area compensate for a loss of
efficiency? Depends how you define efficiency perhaps? If the 3-blade
prop loses something does the extra blade area restore the efficiency?

When the airframe manufacturer more powerful engines in an existing
airframe, he has to be able to use that increased power or it's a
waste of money. Increased power will have to be absorbed either by
turning the propeller faster (which wastes much of the increase, since
drag increases by the square of the increase of propeller blade
speed), by using a prop with longer blades (but then ground clearance
becomes a problem), or by installing a prop with more blades. More
blades works for most installations.

With regard to the single-bladed prop someone suggested: there
was such an animal created by an American inventor about 30 years ago
(maybe more) and installed on his T-Craft. It was an automatic
constant-speed affair, with the blade mounted, with an opposing
counterweight, on an angled transverse pivot on the hub. Thrust and
centrifugal forces worked together to move the blade fore-and-aft a
bit to change blade pitch angle, and that old T-cart showed improved
performance. Didn't sell because it looked so strange.

Dan