Ram air

On Jun 3, 11:18 am, wrote:
On Jun 3, 10:18 am, "Ken S. Tucker" wrote:

I wonder if ram scoops were ever installed near the
prop tips. Probably too expensive for GA, but a WW2
A/C with tips spinning at what(?) 500-600 mph would
give a nice pressure boost.
I've read dual phase superchargers were used in recon
A/C to get the speed and altitude.

Airspeed off the tips is the same as the speed off the inner
blade areas, due to the pitch washout across the blade span, so
there'd be no advantage to having scoops behind the tips. The
propeller's blades are flying at an AOA of between 2 and 4 degrees in
cruise flight, anywhere between the tips and hub, because of that
pitch variation.
See Figure 6-4 of this page:http://www.allstar.fiu.edu/aero/flight63.htm
Dan

Dan it was a trick question I asked you.
It's the basic aerodynamic physics of the
standard centifugal supercharger compressor
operating principle.
I tossed you a zinger, cuz you claimed to be a
teacher in a college and I couldn't resist :-),
don't worry about it, hardly anyone get's that one
correct, and I hope you get a ha-ha-ah from it.

I mentioned, "dual phase superchargers" as a
hint. Here's the answer: the ram-air pressure
acquired at the prop tips is equal to the loss
of pressure against the centrifugal force pushing
air - via ducting - into the prop center that one
obtains at the prop tips.

Now you know the rest of the story.
Regards
Ken

On Jun 3, 12:56 pm, "Ken S. Tucker" wrote:

Dan it was a trick question I asked you.
It's the basic aerodynamic physics of the
standard centifugal supercharger compressor
operating principle.
I tossed you a zinger, cuz you claimed to be a
teacher in a college and I couldn't resist :-),
don't worry about it, hardly anyone get's that one
correct, and I hope you get a ha-ha-ah from it.

The prop does not throw air outward. We've been over that
one before. The air column actually narrows behind the prop, due to
the lowered pressure caused by the air's acceleration. We can see it
on cool mornings when the dew point is just below ambient temperature,
and the vapor trails off the prop tips outline the periphery of the
air column, showing it to be squeezing inward.
A centrifugal super/turbocharger does it differently. The air
is accelerated outward in the first place, not in an axial manner, and
is directed into a divergent duct known as a diffuser, where it is
slowed and its pressure raised. A different animal altogther, compared
to a propeller.


I mentioned, "dual phase superchargers" as a
hint. Here's the answer: the ram-air pressure
acquired at the prop tips is equal to the loss
of pressure against the centrifugal force pushing
air - via ducting - into the prop center that one
obtains at the prop tips.

"Dual-phase supercharger" is a misnomer. They were two-speed
affairs, going to a higher gear ratio for higher altitudes.

Dan

On Jun 4, 11:41*am, wrote:
On Jun 3, 12:56 pm, "Ken S. Tucker" wrote:

Dan it was a trick question I asked you.
It's the basic aerodynamic physics of the
standard centifugal supercharger compressor
operating principle.
I tossed you a zinger, cuz you claimed to be a
teacher in a college and I couldn't resist :-),
don't worry about it, hardly anyone get's that one
correct, and I hope you get a ha-ha-ah from it.

* * * * * * The prop does not throw air outward. We've been over that
one before. The air column actually narrows behind the prop, due to
the lowered pressure caused by the air's acceleration. We can see it
on cool mornings when the dew point is just below ambient temperature,
and the vapor trails off the prop tips outline the periphery of the
air column, showing it to be squeezing inward.
* * * * A centrifugal super/turbocharger does it differently. The air
is accelerated outward in the first place, not in an axial manner, and
is directed into a divergent duct known as a diffuser, where it is
slowed and its pressure raised. A different animal altogther, compared
to a propeller.

I mentioned, "dual phase superchargers" as a
hint. Here's the answer: the ram-air pressure
acquired at the prop tips is equal to the loss
of pressure against the centrifugal force pushing
air - via ducting - into the prop center that one
obtains at the prop tips.

* * * * *"Dual-phase supercharger" is a misnomer. They were two-speed
affairs, going to a higher gear ratio for higher altitudes.


There are also two stage systems

Cheers

On Jun 3, 5:41 pm, wrote:

The prop does not throw air outward. We've been over that
one before. The air column actually narrows behind the prop, due to
the lowered pressure caused by the air's acceleration. We can see it
on cool mornings when the dew point is just below ambient temperature,
and the vapor trails off the prop tips outline the periphery of the
air column, showing it to be squeezing inward.


More on the narrowing of fluid flows behind propellers:

Marine propeller: http://www.aip.org/pt/feb00/maris.htm

Helicopter rotor: http://www.camrad.com/CAMRADIIresults.html

Behind transport props: http://home.att.net/~polar/130contrail.jpg

Behind an F4U's prop: http://people.bath.ac.uk/ensmjc/Research/corsair.gif

In no case do we see air (or any other fluid) being thrown out
centrifugally.

Dan

"Ken S. Tucker" wrote in
:

On Jun 3, 11:18 am, wrote:
On Jun 3, 10:18 am, "Ken S. Tucker" wrote:

I wonder if ram scoops were ever installed near the
prop tips. Probably too expensive for GA, but a WW2
A/C with tips spinning at what(?) 500-600 mph would
give a nice pressure boost.
I've read dual phase superchargers were used in recon
A/C to get the speed and altitude.

Airspeed off the tips is the same as the speed off the inner
blade areas, due to the pitch washout across the blade span, so
there'd be no advantage to having scoops behind the tips. The
propeller's blades are flying at an AOA of between 2 and 4 degrees in
cruise flight, anywhere between the tips and hub, because of that
pitch variation.
See Figure 6-4 of this
page:http://www.allstar.fiu.edu/aero/flight63.htm
Dan

Dan it was a trick question I asked you.




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