Advancement of prop blade in flight, new information

[Corky Scott]

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

[AJW]

>
>
>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
>
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.

[Roger Long]

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
>
>
>
>
>
>

[DanH]

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

[AJW]

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

[Dale]

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.

Cessna 175

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

[Mike Rapoport]

Any airplane with a "G" in its engine designation.

Mike
MU-2

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

[Thomas Borchert]

Ajw,

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

All Thielert Centurion driven aircraft - way over 100 and counting.

--
Thomas Borchert (EDDH)

[Dylan Smith]

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.

--
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"

[Dylan Smith]

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"

[Paul Sengupta]

"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

[tscottme]

"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

[Corky Scott]

On 16 Aug 2004 22:13:49 GMT, (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

[Ron Natalie]

"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.

[G.R. Patterson III]

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.

[Thomas Borchert]

Corky,

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

But you will, again.

--
Thomas Borchert (EDDH)

[Peter Duniho]

"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.

[Corky Scott]

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

[AJW]

>
>>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?

[David CL Francis]

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

[Peter Duniho]

"AJW" > wrote in message
...
> 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?

Pusher props are only *theoretically* more efficient. Since the airframe
disturbs airflow into the prop, you just wind up with a different kind of
inefficiency. Also, there's the problem of how to protect the prop from
ground strikes, engine cooling, CG and the like. All solveable, of course,
but the engine/prop in front configuration actually works pretty well in
most cases.

Pete

[Janne Blomqvist]

In article >, AJW wrote:
> I'd have thought we'd see more pusher props, but
> that's not a common SEL configuration either. I wonder why?

If you crash, it's better to hide behind a big heavy engine instead of
being sandwiched between said engine and ground. ;-)

--
Janne Blomqvist

[Paul Sengupta]

"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

[AJW]

> 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?
>
>Pusher props are only *theoretically* more efficient. Since the airframe
>disturbs airflow into the prop, you just wind up with a different kind of
>inefficiency. Also, there's the problem of how to protect the prop from
>ground strikes, engine cooling, CG and the like. All solveable, of course,
>but the engine/prop in front configuration actually works pretty well in
>most cases.
>
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 suppose my model's example would be to stand a
few feet in front of a fan, then a few feet behind it.

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

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

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

[Peter Duniho]

"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

[kage]

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
>
>

[AJW]

>
>
>> 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.

[Capt.Doug]

>"Peter Duniho" wrote in message> 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.

You've heard wrong. The overheating problem was quickly corrected.
Single-engine climb-rate was 100 fpm better at sea-level on the rear engine.

D.

[Peter Duniho]

"Capt.Doug" > wrote in message
...
> You've heard wrong. The overheating problem was quickly corrected.
> Single-engine climb-rate was 100 fpm better at sea-level on the rear
engine.

Thank you kage and Doug. I stand corrected.

Still, that doesn't mean that a rear-engine is the perfect solution for
every airplane, and of course it's similarly not true that no airplane
should have a rear engine.

[Capt.Doug]

>"Peter Duniho" wrote in message > Still, that doesn't mean that a
rear-engine
>is the perfect solution for every airplane, and of course it's similarly
not true
>that no airplane should have a rear engine.

Which is the reason Rutan doesn't put pusher engines on all of his designs.
It's my understanging that the C-337 performs better on the rear engine
because the rear prop sucks air in behind the blunt end of the fuselage thus
decreasing drag. On a slick low-drag design, I imagine the difference
between front and rear drive would be much less pronounced.

D.

[David CL Francis]

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

[Dan Thomas]

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

[G.R. Patterson III]

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.

[Dan Thomas]

"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

[Big John]

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

[Big John]

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
>>
>>
>