Advancement of prop blade in flight, new information

Corky,

You don't see that so much anymore in certified airplanes


But you will, again.

--
Thomas Borchert (EDDH)

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

"AJW" wrote in message news:20040816181349.12901.00001819@mb-
I don't know of a SEL airplane in general use that uses reduction gearing
between the shaft and the prop, Dan.

My old Navion, Helio Couriers, Republic Seabees, Cessna 175's...
Lots of Rotax powered light planes...


Not overly common, but they are out there.

"DanH" wrote in message
...
AJW wrote:

[snip]
For what it's worth, at 150 kts and 2500 RPM means the airplane advances
about
6 feet per prop rev. A two bladed prop means each balde is in air 3 feet
ahead
of the prior blade.

That's the same number I came up with, but that assumes there's a
one-to-one ratio between engine RPM and prop RPM. Is that true of all
single engine piston aircraft? I'm obviously not an AC mechanic, but I
thought I could see a reduction gear in the cowl.

DanH

In addition, the term "slippage" comes into play. That's the difference
between the theoretical distance the prop should advance with each
revolution and the actual.

And besides a reduction gear creating a difference between engine and prop
RPM, a constant speed prop gives control of the prop speed to the pilot or
the prop governor mechanism.

--
Scott

"tscottme" wrote in message
...
In addition, the term "slippage" comes into play. That's the difference
between the theoretical distance the prop should advance with each
revolution and the actual.

The slippage is only related to a theoretical number based on the prop
pitch. For the purpose of this discussion, the only interesting thing is
the prop RPM versus forward speed.

DanH wrote:

I'm obviously not an AC mechanic, but I
thought I could see a reduction gear in the cowl.

That was probably the flywheel -- they have toothed edges to mesh with the starter.
If gears are used for a reduction system, they are likely to be enclosed in a
housing; you wouldn't be able to see them.

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

If you look at a diagram of the streamlines around a wing, which is all a
prop is, you'll see that the velocity and direction of the air is changed a
surprising distance above and behind the wing. One of your prop blade tips
would be like a wing flying two feet above and behind another. The effect
at this distance would not be significant but would exist.

However, the same prop climbing out at 90 mph and 2800 RPM would put each
blade only 11.15 inches "above" the preceding. This is close enough that
each blade will encounter air that already has some component of motion to
the rear. This reduces the change in velocity (lift) that the blade can
impart.

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.

Another factor in the efficiency equation is the tips. The tip losses and
vortexes are a big factor in wings which is why there is such emphasis on
making tips small (high aspect ratio) and things like winglets. A three
blade prop has an extra tip which will effect the effeciency without any
help from the blades ahead.

--

Roger Long



"Corky Scott" wrote in message
...
Sorry, I got my information wrong when I stated that a three bladed
prop advanced 15 inches during each revolution at 200 mph.

I now have the article in front of me and the exact quote is as
follows: "At 200 mph and 2,800 rpm, the blades on my three-plade prop
follow three distinct helical paths through the air, and each blade is
25" ahead of the previous blade at the same point of rotation."

I repeat that I am not a prop engineer nor do I have any formal
training in aerodynamics but it appears to me that by advancing 25"
during it's revolution, the affect of one blade might have upon the
next one would seem to be pretty inconsequential.

Corky Scott

In article , 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.

Anecdotally, I'd say that the extra blade does easily offset the losses.
You see quite a few glider tugs (lower powered ones especially) like the
Ralleye with a 4-blade prop to improve climb performance (and reduce
noise due to shorter blades)


--
Dylan Smith, Castletown, Isle of Man
Flying: http://www.dylansmith.net
Frontier Elite Universe: http://www.alioth.net
"Maintain thine airspeed, lest the ground come up and smite thee"

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?

Another thought: No matter how many blades there are they are all
subject to exactly the same conditions. There is not a leading blade.
The other blades are in no sense one behind the other. In fact the
rotation of the prop radically changes the velocity vector that actually
meets the blade. 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.

;-)
--
David CL Francis

"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