Advancement of prop blade in flight, new information

"Dylan Smith" wrote in message
...
In article , AJW
wrote:
I don't know of a SEL airplane in general use that uses reduction
gearing
between the shaft and the prop, Dan.

Many hundreds of examples of Europa aircraft. My friend's Europa runs
the 4-cylinder, opposed, liquid cooled 914S engine at something like
5400RPM in cruise. I think the prop turns less than half that RPM.

Or any Rotax engined plane.

Paul

"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

On 16 Aug 2004 22:13:49 GMT, emove (AJW) wrote:

I don't know of a SEL airplane in general use that uses reduction gearing
between the shaft and the prop, Dan.

They aren't in general use anymore, but ALL the big engined WWII
aircraft used some sort of gear reduction unit to turn the prop, every
single one of them.

You don't see that so much anymore in certified airplanes, but in the
world of experimental homebuilt aircraft where auto conversions are
not uncommon, they are in use.

Corky Scott

"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:

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.

Corky,

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


But you will, again.

--
Thomas Borchert (EDDH)

"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.

On 17 Aug 2004 09:33:10 -0500, Todd Pattist
wrote:

There are two questions here. One is whether the single
blade prop is more efficient than a multiblade prop
producing the same thrust. That's sort of like asking if a
monoplane glider is more efficient than a biplane glider.
Generally, the answer is the monoplane glider is more
efficient, although how much more efficient is dependent on
the details

Todd, doesn't it seem likely that if there were **some** kind of
advantage to using a single bladed prop we'd see a bunch of them being
used?

The second question is whether given a prop diameter limit,
one can produce more thrust with a single or multiblade
prop. That's sort of like asking if you can produce more
total lift with a monoplane or a biplane given a wingspan
limit. Generally, the answer is that the biplane produces
more lift, although not as efficiently as a monoplane with a
longer span.

I refer you again to the WWII fighters (and bombers) which used more
and more blades in order to harness the greater and greater horsepower
ratings of the engines.

Also, biplanes vs monoplanes isn't a clear comparison to two bladed vs
single bladed props. Biplanes always have interference between the
two wings and the wing rigging has to be adjusted to compensate for
this interference, so one wing or the other (or both) are not really
operating at their most efficient angle of attack. Once the airplane
gets moving through the air, the prop blades really aren't interfering
with each other because the blades are describing helical paths due to
the forward motion. There may be some interaction but it doesn't seem
much like interference to me.

Corky Scott

There are two questions here. One is whether the single
blade prop is more efficient than a multiblade prop
producing the same thrust. That's sort of like asking if a
monoplane glider is more efficient than a biplane glider.
Generally, the answer is the monoplane glider is more
efficient, although how much more efficient is dependent on
the details

Todd, doesn't it seem likely that if there were **some** kind of
advantage to using a single bladed prop we'd see a bunch of them being
used?

The second question is whether given a prop diameter limit,
one can produce more thrust with a single or multiblade
prop. That's sort of like asking if you can produce more
total lift with a monoplane or a biplane given a wingspan
limit. Generally, the answer is that the biplane produces
more lift, although not as efficiently as a monoplane with a
longer span.

I refer you again to the WWII fighters (and bombers) which used more
and more blades in order to harness the greater and greater horsepower
ratings of the engines.

Also, biplanes vs monoplanes isn't a clear comparison to two bladed vs
single bladed props. Biplanes always have interference between the
two wings and the wing rigging has to be adjusted to compensate for
this interference, so one wing or the other (or both) are not really
operating at their most efficient angle of attack. Once the airplane
gets moving through the air, the prop blades really aren't interfering
with each other because the blades are describing helical paths due to
the forward motion. There may be some interaction but it doesn't seem
much like interference to me.

Wouldn't you think the more serious interference would be the prop wash beating
against the airplane itself? I'd have thought we'd see more pusher props, but
that's not a common SEL configuration either. I wonder why?

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