Advancement of prop blade in flight, new information

On Thu, 19 Aug 2004 at 08:11:24 in message
, Todd Pattist
wrote:
True, but so what? The same thing is true of a wing. You
never hear of downwash or wingtip vortices *above* the wing,
yet it's still true that most of the lift from the wing
comes from the top surface, not the bottom surface. The
pressure difference relative to static is still greatest
above the wing. You're confusing where the air comes from
with the force produced by deflecting that ai

Right. The only way forces can be applied to a wing or a prop is by
differential air pressure on the component itself.

The generation of such forces can only occur when a momentum change is
produced on the air.

The only way you can jump off the ground is by applying an equal force
to the ground and to the bottoms of your feet. You also have to provide
energy to create the change of momentum that gives you a vertical
velocity. This is not quite the same as the wing or prop but I am trying
to reinforce the principle that forces that move or support objects must
be applied directly to the object.

Some things may seem different (like magnetic levitation), but it is
just that the force is supplied by a force field.
--
David CL Francis

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.

"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

Corky

Computer dropped my add on to your posting so will try again.

Besides the round engines, the V's also had gearing. The Merlin in the
P-51 had a two to one (ie, engine ran 3000 rpm on take off and prop
turned 1500 rpm).

Big John

On Tue, 17 Aug 2004 08:03:06 -0400, Corky Scott
wrote:



You don't see that so much anymore in

Karl

You are looking at the data wrong.

The rear engine on a 337/0-2 'sucks' air over the wing center section,
increasing lift that the front engine does not generate. With this
increased lift, the bird will climb faster and in general perform
better on rear engine when on single engine.

Data I was given on check out when I flew the 0-2 .

Big John
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On Wed, 18 Aug 2004 12:14:12 -0700, "kage"
wrote:

The rear engine supplies more thrust on a Skymaster.

See:

http://www.skymaster.org.uk/perform.asp


Karl


"Peter Duniho" wrote in message
...
"AJW" wrote in message
...
I agree with some of your observations, but re efficiency -- the
airflow into the loow pressure area around the prop comes from
pretty much everywhere, but the exit flow is directed backwards.
I think props don't get much thrust from 'suck' as opposed to 'push'.

I never said they did. However, an airplane flying 100mph through the air
WILL necessarily have significant flow through the prop from the front.
If
an airframe is in the way of that airflow, it affects the airflow and in
turn the prop.

[...]
Didn't the Skymaster do better with the rear prop, and the Rutan around
the
world airplane?

I don't know much specific about Voyager. I'd say the fact that it was
the
rear engine they used in cruise, not the front, says something about that
particular design. Note, of course, that the rear engine of Voyager was a
smaller engine; it was the one used in cruise for fuel efficiency reasons,
and its location may have been dictated by CG issues or something else,
rather than efficiency per se.

Only Rutan could answer for sure why exactly the lower horsepower engine
was
put at the back, and whether that was a significant issue or not.

As far as the Skymaster goes, everything I've heard about the 337 was that
the rear engine/prop was always a problem. Thrust was worse and the
engine
had cooling problems.

In any case, as I said before, it's not like rear engines are impossible.
There are numbers of aircraft out there flying with rear engines. It's
just
that a rear engine is not the miracle worker one might think it is.

The other issues re having the engine visit the cockpit during a crash
surely bear thinking about.

Certainly a concern, but I'm not aware of any data that indicates
rear-engine aircraft are significantly less crash-worthy. In a crash
where
the engine is likely to actually shift all the way into the cabin, the
cabin
is not likely to have survived the crash in any case, whether the engine
is
in front or the rear.

It's also true that propwash does a good job of keeping the Mooney's
windscreen clear during rain.

This is only a concern during ground operations. In flight, and in fact
quite early in the takeoff run, the relative wind due to the aircraft's
movement is sufficient for keeping the windscreen clear.

Pete