props: tractor v pusher, q tip, ducted?

On Aug 7, 7:34 am, Bertie the Bunyip wrote:


The drag is induced by the cooling itself.

How can the drag be induced by the cooling itself? If I understand
you, the same cowling with the same air flow shows significantly
different drag depending on whether the engine inside is hot or cold.

I honestly don't understand that.

"Charles Talleyrand" wrote in message
oups.com...
On Aug 7, 7:34 am, Bertie the Bunyip wrote:


The drag is induced by the cooling itself.

How can the drag be induced by the cooling itself? If I understand
you, the same cowling with the same air flow shows significantly
different drag depending on whether the engine inside is hot or cold.

I honestly don't understand that.

The air entering the engine compartment, and flowing past the running
engine's hot cooling fins expands at LEAST double.

That is why the exit opening is much larger than the intake.

That is why there have been claims that the P-51 has a positive cooling
drag, that is the heated air exiting actually gives more thrust than the
drag of air entering the radiator passage and going past the radiator.
--
Jim in NC

"Morgans" wrote in message
...

"Charles Talleyrand" wrote in message
oups.com...
On Aug 7, 7:34 am, Bertie the Bunyip wrote:


The drag is induced by the cooling itself.

How can the drag be induced by the cooling itself? If I understand
you, the same cowling with the same air flow shows significantly
different drag depending on whether the engine inside is hot or cold.

I honestly don't understand that.

The air entering the engine compartment, and flowing past the running
engine's hot cooling fins expands at LEAST double.

That is why the exit opening is much larger than the intake.

That is why there have been claims that the P-51 has a positive cooling
drag, that is the heated air exiting actually gives more thrust than the
drag of air entering the radiator passage and going past the radiator.
--
Jim in NC

Double? Naah. It expands, but doesn't double. Boyle's law (PV=NRT) is
based on absolute temperature, which is measured in kelvin. Kelvin is
Celsius plus 273 degrees. Guessing now, if the air temp went from 300k to
350k (or about 80F/27C to about 160F/70C) you'd see volume increase by
(350/300 -1) = 1/6th = about 17%...

KB

"Kyle Boatright" wrote in
:


"Morgans" wrote in message
...

"Charles Talleyrand" wrote in message
oups.com...
On Aug 7, 7:34 am, Bertie the Bunyip wrote:


The drag is induced by the cooling itself.

How can the drag be induced by the cooling itself? If I understand
you, the same cowling with the same air flow shows significantly
different drag depending on whether the engine inside is hot or
cold.

I honestly don't understand that.

The air entering the engine compartment, and flowing past the running
engine's hot cooling fins expands at LEAST double.

That is why the exit opening is much larger than the intake.

That is why there have been claims that the P-51 has a positive
cooling drag, that is the heated air exiting actually gives more
thrust than the drag of air entering the radiator passage and going
past the radiator. --
Jim in NC

Double? Naah. It expands, but doesn't double. Boyle's law (PV=NRT)
is based on absolute temperature, which is measured in kelvin. Kelvin
is Celsius plus 273 degrees. Guessing now, if the air temp went from
300k to 350k (or about 80F/27C to about 160F/70C) you'd see volume
increase by (350/300 -1) = 1/6th = about 17%...

KB


It's certainly standard practice, at least on airplanes where min
cooling drag is desired, that the outlet be conderably larger than the
inlet...


Bertie

"Kyle Boatright" wrote

Double? Naah. It expands, but doesn't double. Boyle's law (PV=NRT) is
based on absolute temperature, which is measured in kelvin. Kelvin is
Celsius plus 273 degrees. Guessing now, if the air temp went from 300k to
350k (or about 80F/27C to about 160F/70C) you'd see volume increase by
(350/300 -1) = 1/6th = about 17%...

OK. I have always heard the approximate rule for good cooling is double the
intake size for the exit size. Guess it doesn't transfer to expansion.
--
Jim in NC

"Morgans" wrote in message
...

"Kyle Boatright" wrote

Double? Naah. It expands, but doesn't double. Boyle's law (PV=NRT) is
based on absolute temperature, which is measured in kelvin. Kelvin is
Celsius plus 273 degrees. Guessing now, if the air temp went from 300k to
350k (or about 80F/27C to about 160F/70C) you'd see volume increase by
(350/300 -1) = 1/6th = about 17%...

OK. I have always heard the approximate rule for good cooling is double
the intake size for the exit size. Guess it doesn't transfer to
expansion.
--

Maybe the velocity of the exit gasses are less. This would cause them to
need more area.

Danny Deger

Jim Morgans wrote

OK. I have always heard the approximate rule for good cooling is double
the intake size for the exit size. Guess it doesn't transfer to
expansion.

Did you mean to write, "... double the exit size for the intake size?" If
not, you lost me.

"Jon Woellhaf" wrote in message
...
Jim Morgans wrote

OK. I have always heard the approximate rule for good cooling is double
the intake size for the exit size. Guess it doesn't transfer to
expansion.

Did you mean to write, "... double the exit size for the intake size?" If
not, you lost me.

Difficult to communicate that clearly, I guess. Try this.

If the cooling intake is 50 square inches, the exit for the cooling air is
100 square inches. For planes with moveable cowl openings, that would be
with the cowl open full.
--
Jim in NC

Doh! I get what you wrote now. Ain't English great. So easy to be almost
mostly not misunderstood. g

"Morgans" wrote in message
...

"Jon Woellhaf" wrote in message
...
Jim Morgans wrote

OK. I have always heard the approximate rule for good cooling is double
the intake size for the exit size. Guess it doesn't transfer to
expansion.

Did you mean to write, "... double the exit size for the intake size?" If
not, you lost me.

Difficult to communicate that clearly, I guess. Try this.

If the cooling intake is 50 square inches, the exit for the cooling air is
100 square inches. For planes with moveable cowl openings, that would be
with the cowl open full.
--
Jim in NC

"Morgans" wrote in message
...

"Kyle Boatright" wrote

Double? Naah. It expands, but doesn't double. Boyle's law (PV=NRT) is
based on absolute temperature, which is measured in kelvin. Kelvin is
Celsius plus 273 degrees. Guessing now, if the air temp went from 300k to
350k (or about 80F/27C to about 160F/70C) you'd see volume increase by
(350/300 -1) = 1/6th = about 17%...

OK. I have always heard the approximate rule for good cooling is double
the intake size for the exit size. Guess it doesn't transfer to
expansion.

Thanks for the info on the air expanding. Does this translate into more
drag if the engine is running?

Danny Deger