Advancement of prop blade in flight, new information

"Paul Sengupta" wrote in message . ..
"David CL Francis" wrote in message
...
The extreme of this is the enclosed fan where the
enclosure markedly reduces tip losses. The fan runs nicely along like
this with a high blade area and little daylight visible through the
disc.

Whatever happened to the concept of piston engines running
a ducted fan? That aerocar thing has them, but what about
on other more normal planes?

How efficient is a ducted fan compared to a prop? I seem to
remember hearing in model aircraft settings, a prop is more
efficient.

Paul

Unducted props tend to be more efficient simply because they are
of larger diameter, and it's much more efficient to accelerate a large
column of air to a lower speed that to accelerate a small column of
air to a high speed. The higher RPMs necessary for small props cause
much more drag on the prop and horsepower is lost to turbulence,
noise, heat and so on. A 150 HP lightplane driving a six-foot
propeller at 2700 RPM would never generate more than about 500 pounds
of thrust, no matter what the blade pitch might be. A small helicoper
with 150 HP driving a 27-foot rotor at about 300 RPM will generate far
more thrust, enough to lift the entire helicoper, which might weigh
1500 lbs.
Dan

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

Dan Thomas wrote:

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.

There's at least one motorglider with a counter-weighted single-blade prop on the
market. Part of the attraction of the prop is that it takes less space than a
standard two-blade prop to stow it during glider operations. I don't remember the
brand, but I saw one once at an airshow.

George Patterson
If you want to know God's opinion of money, just look at the people
he gives it to.

I think props don't get much thrust from 'suck' as opposed to 'push'.

Props are like wings - the "upper" surface, i.e. the front
prop surface is critical to good performance. The pressure
differential between the front of the prop and the back is
what produces the force that moves the aircraft forward, and
most of that differential (when compared to static pressure)
is due to the lower-than-static pressure on the front/upper
surface of the prop/wing.
Todd Pattist

Actually, I think the current thinking is change of momentum in the downward
air direction. there was an extensive thead on this in the newsgroup.

Think about this, standing by a fan: you feel more force downstream, where the
air is moving in a fairly well defined column, rather than upstream, where in
fact the air is drawn in from all directions.