Why does one need to LEAN OUT a CARB when climbing?

Thanks, finally somebody on my nerdy wavelength, and a really thoughtful
reply, but...

Todd W. Deckard wrote:


The carb throat is a double venturi and a manometer between the opening and
the neck would show a theoretical pressure drop of:

p(opening) - p(neck) = .5 * density of air * { velocity(neck)^2 -
velocity(opening)^2 }
(Lets ignore carb ice for a second and say that the air is
incompressible).

agreed.


{ Pressure / density } + .5 * { velocity ^ 2 } + gravity * change_in_height
= a constant

I think you are speaking of the velocity of the gas in the orfice system
and the density of the gas, relative to the pressure differential that
is driving the fuel flow. If so, I agree (caveat below). Note that
this is equivalent to saying that the fuel flow is proportional to the
square root of the pressure differential, same assumption I made.


so Air over Fuel cancels your density term.

Check your math carefully, are you sure that you are not confusing the
density of the fuel (constant) and density of the air (decreasing)
terms. I gave this the quick and dirty back of the napkin verification,
and it seems I still had both density terms in the final equation
relating mass airflow to mass fuelflow. If you think you're right...
I'll do a little more rigorous playing with the terms.

My current hunch on this: The mass fuel flow is not proportional to the
square root of the pressure differential, but more or less directly
proportional to the differential. This is because of the viscous
friction effects of the avgas in going through the metering orfices. If
those effects predominate, (not surprising given the very small orfice
sizes), I'd say Bernoulli has little to say about the mass flow rate of
the avgas, and it is more linearly related to the pressure differential.


Q.E.D. Good question. If I ever become a physics teacher I am going to
put this one on the final!

I think I'm going to forward this to one of my old fluid dynamics profs

Tman wrote in news:-
:

wrote:
On Jan 19, 7:05 pm, "Todd W. Deckard" wrote:
It diverts low pressure air from the back of the venturi into the
fuel float bowl.
Never taken a close look at an airplane carb, but I understand that the
standard config (not this stromberg, but rather something like in a
basic 172N) has a fuel-bowl vent upstream of the venturi, making the air
pressure in the fuel bowl pretty much equal to the air intake
pressure.... Kind of like a standard auto carb from a few years ago when
they still had them. Let me know if that's not right.


They're basically a float bowl and a tube. not a lot to complicate them,
really. Probably the simplest carbs in operation today.

Bertie

"Bertie the Bunyip" wrote in message
...
Tman wrote in news:-
:

wrote:
On Jan 19, 7:05 pm, "Todd W. Deckard" wrote:
It diverts low pressure air from the back of the venturi into the
fuel float bowl.
Never taken a close look at an airplane carb, but I understand that the
standard config (not this stromberg, but rather something like in a
basic 172N) has a fuel-bowl vent upstream of the venturi, making the air
pressure in the fuel bowl pretty much equal to the air intake
pressure.... Kind of like a standard auto carb from a few years ago when
they still had them. Let me know if that's not right.


They're basically a float bowl and a tube. not a lot to complicate them,
really. Probably the simplest carbs in operation today.

Bertie

You're an idiot.

"Maxwell" #$$9#@%%%.^^^ wrote in :


"Bertie the Bunyip" wrote in message
...
Tman wrote in news:-
:

wrote:
On Jan 19, 7:05 pm, "Todd W. Deckard"
wrote: It diverts low pressure air from the back of the venturi
into the
fuel float bowl.
Never taken a close look at an airplane carb, but I understand that
the standard config (not this stromberg, but rather something like
in a basic 172N) has a fuel-bowl vent upstream of the venturi,
making the air pressure in the fuel bowl pretty much equal to the
air intake pressure.... Kind of like a standard auto carb from a few
years ago when they still had them. Let me know if that's not
right.


They're basically a float bowl and a tube. not a lot to complicate
them, really. Probably the simplest carbs in operation today.

Bertie

You're an idiot.




Nope.

But please don't let that stop you maxie.


Bertie

On Wed, 21 Jan 2009 12:29:21 -0500, Tman
wrote:

Thanks, finally somebody on my nerdy wavelength, and a really thoughtful
reply, but...

Todd W. Deckard wrote:


The carb throat is a double venturi and a manometer between the opening and
the neck would show a theoretical pressure drop of:

p(opening) - p(neck) = .5 * density of air * { velocity(neck)^2 -
velocity(opening)^2 }
(Lets ignore carb ice for a second and say that the air is
incompressible).

agreed.


{ Pressure / density } + .5 * { velocity ^ 2 } + gravity * change_in_height
= a constant

I think you are speaking of the velocity of the gas in the orfice system
and the density of the gas, relative to the pressure differential that
is driving the fuel flow. If so, I agree (caveat below). Note that
this is equivalent to saying that the fuel flow is proportional to the
square root of the pressure differential, same assumption I made.


so Air over Fuel cancels your density term.

Check your math carefully, are you sure that you are not confusing the
density of the fuel (constant) and density of the air (decreasing)
terms. I gave this the quick and dirty back of the napkin verification,
and it seems I still had both density terms in the final equation
relating mass airflow to mass fuelflow. If you think you're right...
I'll do a little more rigorous playing with the terms.

My current hunch on this: The mass fuel flow is not proportional to the
square root of the pressure differential, but more or less directly
proportional to the differential. This is because of the viscous
friction effects of the avgas in going through the metering orfices. If
those effects predominate, (not surprising given the very small orfice
sizes), I'd say Bernoulli has little to say about the mass flow rate of
the avgas, and it is more linearly related to the pressure differential.


Q.E.D. Good question. If I ever become a physics teacher I am going to
put this one on the final!

I think I'm going to forward this to one of my old fluid dynamics profs


You all are making this much too complicated. The reason you lean the
mixture as you climb is.... if you don't the check airman will flunk
you on your check ride.

Ron

:-)

On Jan 21, 10:29*am, Tman wrote:
Thanks, finally somebody on my nerdy wavelength, and a really thoughtful
reply, but...

Todd W. Deckard wrote:

The carb throat is a double venturi and a manometer between the opening and
the neck would show a theoretical pressure drop of:

p(opening) - p(neck) = .5 * density of air * { velocity(neck)^2 -
velocity(opening)^2 }
* * * * * * *(Lets ignore carb ice for a second and say that the air is
incompressible).

agreed.

{ Pressure / density } + .5 * { velocity ^ 2 } + gravity * change_in_height
= a constant

I think you are speaking of the velocity of the gas in the orfice system
and the density of the gas, relative to the pressure differential that
is driving the fuel flow. *If so, I agree (caveat below). *Note that
this is equivalent to saying that the fuel flow is proportional to the
square root of the pressure differential, same assumption I made.



so Air over Fuel cancels your density term.

Check your math carefully, are you sure that you are not confusing the
density of the fuel (constant) and density of the air (decreasing)
terms. *I gave this the quick and dirty back of the napkin verification,
and it seems I still had both density terms in the final equation
relating mass airflow to mass fuelflow. *If you think you're right...
I'll do a little more rigorous playing with the terms.

My current hunch on this: *The mass fuel flow is not proportional to the
* square root of the pressure differential, but more or less directly
proportional to the differential. *This is because of the viscous
friction effects of the avgas in going through the metering orfices. *If
those effects predominate, (not surprising given the very small orfice
sizes), I'd say Bernoulli has little to say about the mass flow rate of
the avgas, and it is more linearly related to the pressure differential.



Q.E.D. *Good question. * If I ever become a physics teacher I am going to
put this one on the final!

I think I'm going to forward this to one of my old fluid dynamics profs

I think your problem is the assumption that there's a linear
relationship between air density and pressure differential. The same
pressure differential is the force that lifts our airplanes off the
ground, and as they gain altitude the density decreases. The stall
speed will rise, but not linearly with the decrease in density; it's
the square root of the decrease in density that we're looking for. In
the carb, one-half the density should then cut the pressure
differential and therefore fuel flow by one quarter, which will give
us a mixture twice a rich as when we took off. We find half the
density at 18,000 feet, incidentally.

Dan

wrote:
In
the carb, one-half the density should then cut the pressure
differential and therefore fuel flow by one quarter, which will give
us a mixture twice a rich as when we took off.
At the risk of dragging on the subject ... Wouldn't that actually lean
it out, requiring one to compensate by richening the mixture at higher
altitudes? One-half the density i.e. one half the mass airflow at
constant velocity, fuel flow by quarter... sounds like that mixture is
leaner!

On Jan 21, 5:25*pm, Tman wrote:
wrote:

* In the carb, one-half the density should then cut the pressure
differential and therefore fuel flow by one quarter, which will give
us a mixture twice a rich as when we took off. *

At the risk of dragging on the subject ... Wouldn't that actually lean
it out, requiring one to compensate by richening the mixture at higher
altitudes? *One-half the density i.e. one half the mass airflow at
constant velocity, fuel flow by quarter... sounds like that mixture is
leaner!

No, the fuel flow drops by a quarter, to three-quarters of what
it was at sea level. With half the density, that gives us a mixture
that is half again as rich as it was at sea level, requiring leaning.

Dan

Bertie the Bunyip wrote:
"Maxwell" #$$9#@%%%.^^^ wrote in :


"Bertie the Bunyip" wrote in message
...
Tman wrote in news:-
:

wrote:
On Jan 19, 7:05 pm, "Todd W. Deckard"
wrote: It diverts low pressure air from the back of the venturi
into the
fuel float bowl.
Never taken a close look at an airplane carb, but I understand that
the standard config (not this stromberg, but rather something like
in a basic 172N) has a fuel-bowl vent upstream of the venturi,
making the air pressure in the fuel bowl pretty much equal to the
air intake pressure.... Kind of like a standard auto carb from a few
years ago when they still had them. Let me know if that's not
right.


They're basically a float bowl and a tube. not a lot to complicate
them, really. Probably the simplest carbs in operation today.

Bertie

You're an idiot.




Nope.

But please don't let that stop you maxie.

What's new on teh gasket-front, Cap'N?
--
ah

ah wrote in news:4977e445$0$35423
:

Bertie the Bunyip wrote:
"Maxwell" #$$9#@%%%.^^^ wrote in :


"Bertie the Bunyip" wrote in message
...
Tman wrote in news:-
:

wrote:
On Jan 19, 7:05 pm, "Todd W. Deckard"
wrote: It diverts low pressure air from the back of the venturi
into the
fuel float bowl.
Never taken a close look at an airplane carb, but I understand
that
the standard config (not this stromberg, but rather something like
in a basic 172N) has a fuel-bowl vent upstream of the venturi,
making the air pressure in the fuel bowl pretty much equal to the
air intake pressure.... Kind of like a standard auto carb from a
few
years ago when they still had them. Let me know if that's not
right.


They're basically a float bowl and a tube. not a lot to complicate
them, really. Probably the simplest carbs in operation today.

Bertie

You're an idiot.




Nope.

But please don't let that stop you maxie.

What's new on teh gasket-front, Cap'N?

Not much. Need a gearbox though..

Bertie