Advancement of prop blade in flight, new information

"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

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

"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

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.

"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

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

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.

"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.

"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.

"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.