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

[Tman[_2_]]

Somebody posed that seemingly simple question to me, but kept coming
back to the point that they stumped me.... And I am stumped. What do
you see wrong with the logic in this dialog?

Q: Why do I need to lean out my carb when I climb?

A: Ahem, seems you forgot your PPL ground school. The air is less
dense. Fewer air molecules per unit volume. Therefore, you need less
gas, so you lean it out!

Q': Um, ok. Well I looked at my ground school text, and it shows how a
carby operates. Apparently, avgas is kept at a constant level in a
float bowl, which is vented upstream of a venturi. Air flows through
the venturi, and creates a lower pressure, the resulting differential
pressure forcing the avgas across an orfice and into the airstream,
where it mixes it all up in a nice and precise ratio.

A': OK, go on.

Q'': Well, as you climb, I understand the air gets less dense. Let's
assume for simplicity that the volumetric efficiency of the engine
remains fixed, therefore the velocity in the venturi remains the same.
Now the air is less dense, and from the previous chapter in ground
school 101, the differential pressure "p" is related to the density "r"
given a certain velocity "v" like this:
p = 1/2 r v^2
So given a constant velocity, and a decreasing density, won't the
differential pressure decrease, effectively metering less avgas across
that orfice?

A'': OK; I'm sure you're simplifying assumptions are too simple, you
missed something there.

Q''': OK, let's get a little more precise. The mass airflow rate, m,
through a carby is m = c v , where c is a constant for a certain
throttle setting, v is the velocity. Substituting that into the eq's
above , we see that p = 1/2 r m^2 / c^2. Now we know that the mass flow
rate for a liquid across an orfice is very close to proportional to the
square root of the pressure drop [ I actually had to check up on this
one, but it appears to be so:
http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm ] --
and of course the density of the avgas doesn't change appreciably[!].
Therefore, the avgas flow rate is proportional to "r^(1/2) m". From
this point of view the carby at a constant air density can be viewed as
a device that meters a constant mass proportion mixture of avgas and
air, across a range of mass airflows -- ignoring the effects of
accelerator pumps, full-throttle enrichers, idle circuits and all that.
But note that as the density decreases, the fuel proportion to air
decreases -- suggesting that one would need to ENRICH the mixture when
climbing into less dense air. Assuming that the desired mass proportion
of fuel/air is approximately the same across varying densities (which
seems very reasonable to both of us).

A''': OK, I do follow that (after some work)... and I'm stumped.

Granted, some simplifying assumptions here, but no convincing
explanation of why you would need to lean that red know when climbing...
(and I don't question that you in fact do)....

Anyone see what is amiss?

T

[Tech Support]

Let me try a simpler explanation.

14.7 to one is the normal fuel air ratio in gasoline engines.

As you climb the air (14.7) gets less and mixture goes rich so you
have to lean (reduce) the fuel flow to keep the14.7 to one ratio that
engine likes.

Big John

************************************************** **************************

On Mon, 19 Jan 2009 14:23:57 -0500, Tman >
wrote:

>Somebody posed that seemingly simple question to me, but kept coming
>back to the point that they stumped me.... And I am stumped. What do
>you see wrong with the logic in this dialog?
>
>Q: Why do I need to lean out my carb when I climb?
>
>A: Ahem, seems you forgot your PPL ground school. The air is less
>dense. Fewer air molecules per unit volume. Therefore, you need less
>gas, so you lean it out!
>
>Q': Um, ok. Well I looked at my ground school text, and it shows how a
>carby operates. Apparently, avgas is kept at a constant level in a
>float bowl, which is vented upstream of a venturi. Air flows through
>the venturi, and creates a lower pressure, the resulting differential
>pressure forcing the avgas across an orfice and into the airstream,
>where it mixes it all up in a nice and precise ratio.
>
>A': OK, go on.
>
>Q'': Well, as you climb, I understand the air gets less dense. Let's
>assume for simplicity that the volumetric efficiency of the engine
>remains fixed, therefore the velocity in the venturi remains the same.
>Now the air is less dense, and from the previous chapter in ground
>school 101, the differential pressure "p" is related to the density "r"
>given a certain velocity "v" like this:
> p = 1/2 r v^2
>So given a constant velocity, and a decreasing density, won't the
>differential pressure decrease, effectively metering less avgas across
>that orfice?
>
>A'': OK; I'm sure you're simplifying assumptions are too simple, you
>missed something there.
>
>Q''': OK, let's get a little more precise. The mass airflow rate, m,
>through a carby is m = c v , where c is a constant for a certain
>throttle setting, v is the velocity. Substituting that into the eq's
>above , we see that p = 1/2 r m^2 / c^2. Now we know that the mass flow
>rate for a liquid across an orfice is very close to proportional to the
>square root of the pressure drop [ I actually had to check up on this
>one, but it appears to be so:
>http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm ] --
>and of course the density of the avgas doesn't change appreciably[!].
>Therefore, the avgas flow rate is proportional to "r^(1/2) m". From
>this point of view the carby at a constant air density can be viewed as
>a device that meters a constant mass proportion mixture of avgas and
>air, across a range of mass airflows -- ignoring the effects of
>accelerator pumps, full-throttle enrichers, idle circuits and all that.
> But note that as the density decreases, the fuel proportion to air
>decreases -- suggesting that one would need to ENRICH the mixture when
>climbing into less dense air. Assuming that the desired mass proportion
>of fuel/air is approximately the same across varying densities (which
>seems very reasonable to both of us).
>
>A''': OK, I do follow that (after some work)... and I'm stumped.
>
>Granted, some simplifying assumptions here, but no convincing
>explanation of why you would need to lean that red know when climbing...
>(and I don't question that you in fact do)....
>
>Anyone see what is amiss?
>
>T

[BT]

If he does not believe that the engine is not operating at peak efficiency..
Climb above 5000MSL in a full throttle full rich condition and watch the
RPMs (fixted pitch) decrease.
Then lean the engine and watch the RPMs increase. Leaner air, lower the gas,
same fuel to air ratio is achieved and the RPMs pick up.

BT

"Tman" > wrote in message
...
> Somebody posed that seemingly simple question to me, but kept coming back
> to the point that they stumped me.... And I am stumped. What do you see
> wrong with the logic in this dialog?
>
> Q: Why do I need to lean out my carb when I climb?
>
> A: Ahem, seems you forgot your PPL ground school. The air is less dense.
> Fewer air molecules per unit volume. Therefore, you need less gas, so you
> lean it out!
>
> Q': Um, ok. Well I looked at my ground school text, and it shows how a
> carby operates. Apparently, avgas is kept at a constant level in a float
> bowl, which is vented upstream of a venturi. Air flows through the
> venturi, and creates a lower pressure, the resulting differential pressure
> forcing the avgas across an orfice and into the airstream, where it mixes
> it all up in a nice and precise ratio.
>
> A': OK, go on.
>
> Q'': Well, as you climb, I understand the air gets less dense. Let's
> assume for simplicity that the volumetric efficiency of the engine remains
> fixed, therefore the velocity in the venturi remains the same. Now the air
> is less dense, and from the previous chapter in ground school 101, the
> differential pressure "p" is related to the density "r" given a certain
> velocity "v" like this:
> p = 1/2 r v^2
> So given a constant velocity, and a decreasing density, won't the
> differential pressure decrease, effectively metering less avgas across
> that orfice?
>
> A'': OK; I'm sure you're simplifying assumptions are too simple, you
> missed something there.
>
> Q''': OK, let's get a little more precise. The mass airflow rate, m,
> through a carby is m = c v , where c is a constant for a certain throttle
> setting, v is the velocity. Substituting that into the eq's above , we
> see that p = 1/2 r m^2 / c^2. Now we know that the mass flow rate for a
> liquid across an orfice is very close to proportional to the square root
> of the pressure drop [ I actually had to check up on this one, but it
> appears to be so:
> http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm ] --
> and of course the density of the avgas doesn't change appreciably[!].
> Therefore, the avgas flow rate is proportional to "r^(1/2) m". From this
> point of view the carby at a constant air density can be viewed as a
> device that meters a constant mass proportion mixture of avgas and air,
> across a range of mass airflows -- ignoring the effects of accelerator
> pumps, full-throttle enrichers, idle circuits and all that. But note that
> as the density decreases, the fuel proportion to air decreases --
> suggesting that one would need to ENRICH the mixture when climbing into
> less dense air. Assuming that the desired mass proportion of fuel/air is
> approximately the same across varying densities (which seems very
> reasonable to both of us).
>
> A''': OK, I do follow that (after some work)... and I'm stumped.
>
> Granted, some simplifying assumptions here, but no convincing explanation
> of why you would need to lean that red know when climbing... (and I don't
> question that you in fact do)....
>
> Anyone see what is amiss?
>
> T

[Darkwing]

"Tman" > wrote in message
...
> Somebody posed that seemingly simple question to me, but kept coming back
> to the point that they stumped me.... And I am stumped. What do you see
> wrong with the logic in this dialog?
>
> Q: Why do I need to lean out my carb when I climb?
>
> A: Ahem, seems you forgot your PPL ground school. The air is less dense.
> Fewer air molecules per unit volume. Therefore, you need less gas, so you
> lean it out!
>
> Q': Um, ok. Well I looked at my ground school text, and it shows how a
> carby operates. Apparently, avgas is kept at a constant level in a float
> bowl, which is vented upstream of a venturi. Air flows through the
> venturi, and creates a lower pressure, the resulting differential pressure
> forcing the avgas across an orfice and into the airstream, where it mixes
> it all up in a nice and precise ratio.
>
> A': OK, go on.
>
> Q'': Well, as you climb, I understand the air gets less dense. Let's
> assume for simplicity that the volumetric efficiency of the engine remains
> fixed, therefore the velocity in the venturi remains the same. Now the air
> is less dense, and from the previous chapter in ground school 101, the
> differential pressure "p" is related to the density "r" given a certain
> velocity "v" like this:
> p = 1/2 r v^2
> So given a constant velocity, and a decreasing density, won't the
> differential pressure decrease, effectively metering less avgas across
> that orfice?
>
> A'': OK; I'm sure you're simplifying assumptions are too simple, you
> missed something there.
>
> Q''': OK, let's get a little more precise. The mass airflow rate, m,
> through a carby is m = c v , where c is a constant for a certain throttle
> setting, v is the velocity. Substituting that into the eq's above , we
> see that p = 1/2 r m^2 / c^2. Now we know that the mass flow rate for a
> liquid across an orfice is very close to proportional to the square root
> of the pressure drop [ I actually had to check up on this one, but it
> appears to be so:
> http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm ] --
> and of course the density of the avgas doesn't change appreciably[!].
> Therefore, the avgas flow rate is proportional to "r^(1/2) m". From this
> point of view the carby at a constant air density can be viewed as a
> device that meters a constant mass proportion mixture of avgas and air,
> across a range of mass airflows -- ignoring the effects of accelerator
> pumps, full-throttle enrichers, idle circuits and all that. But note that
> as the density decreases, the fuel proportion to air decreases --
> suggesting that one would need to ENRICH the mixture when climbing into
> less dense air. Assuming that the desired mass proportion of fuel/air is
> approximately the same across varying densities (which seems very
> reasonable to both of us).
>
> A''': OK, I do follow that (after some work)... and I'm stumped.
>
> Granted, some simplifying assumptions here, but no convincing explanation
> of why you would need to lean that red know when climbing... (and I don't
> question that you in fact do)....
>
> Anyone see what is amiss?
>
> T


My knowledge is limited to small engines (all two cycle), which the vast
majority I specialize in are diaphram/crankcase pulse type carbs. Are you
sure that the carb is regulated by the venturi flow through the carb? Are
carbureted aircraft engine not run off crank case pulse?

[Darkwing]

"Darkwing" <theducksmail"AT"yahoo.com> wrote in message
...
>
> "Tman" > wrote in message
> ...
>> Somebody posed that seemingly simple question to me, but kept coming back
>> to the point that they stumped me.... And I am stumped. What do you see
>> wrong with the logic in this dialog?
>>
>> Q: Why do I need to lean out my carb when I climb?
>>
>> A: Ahem, seems you forgot your PPL ground school. The air is less
>> dense. Fewer air molecules per unit volume. Therefore, you need less
>> gas, so you lean it out!
>>
>> Q': Um, ok. Well I looked at my ground school text, and it shows how a
>> carby operates. Apparently, avgas is kept at a constant level in a float
>> bowl, which is vented upstream of a venturi. Air flows through the
>> venturi, and creates a lower pressure, the resulting differential
>> pressure forcing the avgas across an orfice and into the airstream, where
>> it mixes it all up in a nice and precise ratio.
>>
>> A': OK, go on.
>>
>> Q'': Well, as you climb, I understand the air gets less dense. Let's
>> assume for simplicity that the volumetric efficiency of the engine
>> remains fixed, therefore the velocity in the venturi remains the same.
>> Now the air is less dense, and from the previous chapter in ground school
>> 101, the differential pressure "p" is related to the density "r" given a
>> certain velocity "v" like this:
>> p = 1/2 r v^2
>> So given a constant velocity, and a decreasing density, won't the
>> differential pressure decrease, effectively metering less avgas across
>> that orfice?
>>
>> A'': OK; I'm sure you're simplifying assumptions are too simple, you
>> missed something there.
>>
>> Q''': OK, let's get a little more precise. The mass airflow rate, m,
>> through a carby is m = c v , where c is a constant for a certain throttle
>> setting, v is the velocity. Substituting that into the eq's above , we
>> see that p = 1/2 r m^2 / c^2. Now we know that the mass flow rate for a
>> liquid across an orfice is very close to proportional to the square root
>> of the pressure drop [ I actually had to check up on this one, but it
>> appears to be so:
>> http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm ] --
>> and of course the density of the avgas doesn't change appreciably[!].
>> Therefore, the avgas flow rate is proportional to "r^(1/2) m". From this
>> point of view the carby at a constant air density can be viewed as a
>> device that meters a constant mass proportion mixture of avgas and air,
>> across a range of mass airflows -- ignoring the effects of accelerator
>> pumps, full-throttle enrichers, idle circuits and all that. But note that
>> as the density decreases, the fuel proportion to air decreases --
>> suggesting that one would need to ENRICH the mixture when climbing into
>> less dense air. Assuming that the desired mass proportion of fuel/air is
>> approximately the same across varying densities (which seems very
>> reasonable to both of us).
>>
>> A''': OK, I do follow that (after some work)... and I'm stumped.
>>
>> Granted, some simplifying assumptions here, but no convincing explanation
>> of why you would need to lean that red know when climbing... (and I don't
>> question that you in fact do)....
>>
>> Anyone see what is amiss?
>>
>> T
>
>
> My knowledge is limited to small engines (all two cycle), which the vast
> majority I specialize in are diaphram/crankcase pulse type carbs. Are you
> sure that the carb is regulated by the venturi flow through the carb? Are
> carbureted aircraft engine not run off crank case pulse?
>

I guess I need to add this, I realize that airplanes have fuel pumps, but
are they ran off the engine belt or pulse? I would think the smaller LS
engines like the Rotax might be a pulse carb with float bowl.

[[email protected]]

On Jan 19, 3:23*pm, "Darkwing" <theducksmail"AT"yahoo.com> wrote:
> "Darkwing" <theducksmail"AT"yahoo.com> wrote in message
>
> ...
>
>
>
>
>
> > "Tman" > wrote in message
> ...
> >> Somebody posed that seemingly simple question to me, but kept coming back
> >> to the point that they stumped me.... And I am stumped. *What do you see
> >> wrong with the logic in this dialog?
>
> >> Q: Why do I need to lean out my carb when I climb?
>
> >> A: *Ahem, seems you forgot your PPL ground school. *The air is less
> >> dense. Fewer air molecules per unit volume. *Therefore, you need less
> >> gas, so you lean it out!
>
> >> Q': *Um, ok. *Well I looked at my ground school text, and it shows how a
> >> carby operates. *Apparently, avgas is kept at a constant level in a float
> >> bowl, which is vented upstream of a venturi. *Air flows through the
> >> venturi, and creates a lower pressure, the resulting differential
> >> pressure forcing the avgas across an orfice and into the airstream, where
> >> it mixes it all up in a nice and precise ratio.
>
> >> A': *OK, go on.
>
> >> Q'': *Well, as you climb, I understand the air gets less dense. *Let's
> >> assume for simplicity that the volumetric efficiency of the engine
> >> remains fixed, therefore the velocity in the venturi remains the same.
> >> Now the air is less dense, and from the previous chapter in ground school
> >> 101, the differential pressure "p" is related to the density "r" given a
> >> certain velocity "v" like this:
> >> p = 1/2 r v^2
> >> So given a constant velocity, and a decreasing density, won't the
> >> differential pressure decrease, effectively metering less avgas across
> >> that orfice?
>
> >> A'': OK; I'm sure you're simplifying assumptions are too simple, you
> >> missed something there.
>
> >> Q''': *OK, let's get a little more precise. *The mass airflow rate, m,
> >> through a carby is m = c v , where c is a constant for a certain throttle
> >> setting, v is the velocity. *Substituting that into the eq's above , we
> >> see that p = 1/2 r m^2 / c^2. *Now we know that the mass flow rate for a
> >> liquid across an orfice is very close to proportional to the square root
> >> of the pressure drop [ I actually had to check up on this one, but it
> >> appears to be so:
> >>http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm] --
> >> and of course the density of the avgas doesn't change appreciably[!].
> >> Therefore, the avgas flow rate is proportional to "r^(1/2) m". *From this
> >> point of view the carby at a constant air density can be viewed as a
> >> device that meters a constant mass proportion mixture of avgas and air,
> >> across a range of mass airflows -- ignoring the effects of accelerator
> >> pumps, full-throttle enrichers, idle circuits and all that. But note that
> >> as the density decreases, the fuel proportion to air decreases -- *
> >> suggesting that one would need to ENRICH the mixture when climbing into
> >> less dense air. *Assuming that the desired mass proportion of fuel/air is
> >> approximately the same across varying densities (which seems very
> >> reasonable to both of us).
>
> >> A''': *OK, I do follow that (after some work)... and I'm stumped.
>
> >> Granted, some simplifying assumptions here, but no convincing explanation
> >> of why you would need to lean that red know when climbing... (and I don't
> >> question that you in fact do)....
>
> >> Anyone see what is amiss?
>
> >> T
>
> > My knowledge is limited to small engines (all two cycle), which the vast
> > majority I specialize in are diaphram/crankcase pulse type carbs. Are you
> > sure that the carb is regulated by the venturi flow through the carb? Are
> > carbureted aircraft engine not run off crank case pulse?
>
> I guess I need to add this, I realize that airplanes have fuel pumps, but
> are they ran off the engine belt or pulse? I would think the smaller LS
> engines like the Rotax might be a pulse carb with float bowl.

Aircraft engines like the Lyc or Continental often have NO fuel
pumps if the aircraft is a high-wing type with the tanks in the wings.
Gravity does the job. If there's a pump it's driven off a cam in the
accessory section, like an old automobile engine-mounted fuel pump,
and there'll be a backup electrical pump in case the mechanical pump
quits.
A four or six cylinder engine's crankcase doesn't have pulsation like
a two-strokes because the air/fuel mix doesn't travel through the
case.
The float carb is mostly velocity-sensitive. It doesn't care very
much what the air density is, so as the density decreases the fuel
flow won't decrease as much and the mixture will get richer.
I'm no physicist, but I still have to lean my engine as I climb.
That tells me more than any number of formulae.

Dan

[Ash Wyllie]

Tman opined

>Somebody posed that seemingly simple question to me, but kept coming
>back to the point that they stumped me.... And I am stumped. What do
>you see wrong with the logic in this dialog?

>Q: Why do I need to lean out my carb when I climb?

>A: Ahem, seems you forgot your PPL ground school. The air is less
>dense. Fewer air molecules per unit volume. Therefore, you need less
>gas, so you lean it out!

Because the carb measures volume, and adds enough fuel for that volume. So, if
the air is less dense, there is less mass of air entering the cylinders, and too
much fuel.

To correct that, you lean the mixture.


-ash
Cthulhu in 2012!
Vote the greater evil.

[Todd W. Deckard]

"Tman" > wrote in message
...
> Somebody posed that seemingly simple question to me, but kept coming back
> to the point that they stumped me.... And I am stumped. What do you see
> wrong with the logic in this dialog?
>
> Q: Why do I need to lean out my carb when I climb?
>
I'll take a stab at this one. Its a very good question.

A Stromberg carb does not require a mixture adjustment (at least below
8000'). It diverts low pressure air from the back of the venturi into the
fuel float bowl. In this way it is "self regulating" just as you describe.
The amount of
fuel drawn in is proportional to the air pressure.

Older classic airplanes will use this type of carberator system and thus
have no mixture.

I believe more "conventional" systems use a mixture simply becuase the
logistics of balancing all the
jets is difficult and because slight misadjustments in the orifice that
tunes the ratio might result in
a catastrophically lean mixture. A carburetor can have four jet circuits
for idle, midrange, main and accelerator (I am quoting more Rochester
Quadrajet than Lycoming -- so aircraft mechanics please jump in).

Your mixture valve is in front of all of these and so restricts the fuel
thru all of them. If you have a higher performance
engine, or operate at a higher range of altitudes, I suspect you cannot
build a mechanical metering system that covers the range without regions of
overly rich or overly lean so we have the man in the loop.

I am pretty sure of my answer but I'd invite any clarification.

Todd

[[email protected]]

On Jan 19, 7:05 pm, "Todd W. Deckard" > wrote:
> "Tman" > wrote in message
> > Q: Why do I need to lean out my carb when I climb?

> I'll take a stab at this one. Its a very good question.
>
> A Stromberg carb does not require a mixture adjustment (at least below
> 8000'). It diverts low pressure air from the back of the venturi into the
> fuel float bowl. In this way it is "self regulating" just as you describe.
> The amount of
> fuel drawn in is proportional to the air pressure.
>
> Older classic airplanes will use this type of carberator system and thus
> have no mixture.

The Stromberg carb's bowl is vented to a dead airspace behind
the venturi just like all the other carbs and has the same rich
problem with altitude that the others do. The Stromberg was built with
a mixture control valve cavity in the upper casting and many were left
empty and capped off to run full rich, or had the valve installed and
the lever wired to the full rich position. Most of the population, at
least years ago, lived near the coasts and flew puddlejumpers that
didn't fly very high. Fuel was cheap, too. So the makers didn't see
another control as having much value, expecially the back-suction
mixture type that the Stromberg uses and which will not act as an idle
cutoff for shutdown.
I have one of those old carbs on my airplane. I operate off a
strip that's around 3000' ASL. I machined the necessary parts for the
mixture control, they being very rare now, and installed them. It
works fine. It's a homebuilt and so such doings are permitted.
Air from behind the venturi where the air, being still, is at or
near ambient pressure, and this air passes into the cavity I
mentioned. There's a much smaller port in the cavity that leads to the
venturi itself and has considerably less pressure when the throttle is
open. Air is sucked out here. A third port into the cavity leads to
the bowl itelf. As long as the port from behind the venturi is wide
open, air can come in from behind the venturi and get sucked into the
venturi proper without exerting any negative pressure on the bowl.
When we lean, the mixture control simply starts choking off the air
supply from behind the venturi and allows the lower venturi pressure
to lower the bowl pressure, which reduces fuel flow through the jet
into the nozzle. Because the venturi's pressure drop is about zero
when at idle, it won't suck back on the bowl to act as an idle cutoff
at shutdown.

> I believe more "conventional" systems use a mixture simply becuase the
> logistics of balancing all the
> jets is difficult and because slight misadjustments in the orifice that
> tunes the ratio might result in
> a catastrophically lean mixture. A carburetor can have four jet circuits
> for idle, midrange, main and accelerator (I am quoting more Rochester
> Quadrajet than Lycoming -- so aircraft mechanics please jump in).

The typical light-aircraft carb like the Marvel Schebler/
Precision Aeromotive/Tempest carb has one jet. The mixture control is
in the bottom of the float bowl and is a small valve that varies fuel
flow directly, from max to nothing at all.

> Your mixture valve is in front of all of these and so restricts the fuel
> thru all of them. If you have a higher performance
> engine, or operate at a higher range of altitudes, I suspect you cannot
> build a mechanical metering system that covers the range without regions of
> overly rich or overly lean so we have the man in the loop.

Yup, you can, and it's done, too. It's just not cheap or
simple. There are various aneroid actuators that adjust fuel flow for
altitude, along with power valves that increase fuel flow for takeoff
and climb and other overriding devioces that prevent overly lean
conditions. Some pressure carbs (no float bowl) have these systems and
are similar in some ways to the fuel controllers used on fuel
injection systems. These types measure ambient air pressure, air
velocity through the carb, fuel supply pressure and fuel delivery
pressure and so forth and make the adjustments constantly.
This is diagram of a pressure carb, without the aneroid mixture
control. I can't find one on the 'net with it:
http://www.navioneer.org/riprelay/The%20Navion%20Files/CarbFlowChart.jpg


Dan

[Stealth Pilot[_2_]]

On Mon, 19 Jan 2009 14:23:57 -0500, Tman >
wrote:

>Somebody posed that seemingly simple question to me, but kept coming
>back to the point that they stumped me.... And I am stumped. What do
>you see wrong with the logic in this dialog?
>
>Q: Why do I need to lean out my carb when I climb?
>

like most arguments in aviation this one arises because a question is
asked with a misleading part in it.

"why do I lean my carb when I climb" is a bull**** question open to
much misinterpretation.

the fact is that most airfields are near sea level, say under 1,000ft
elevation anyway. there is actually no need to lean a carby at these
altitudes.
in fact the mixture is left rich so that as you climb the over rich
mixture aids in engine cooling.
continental's pilot notes will tell you this.

"why do I need to lean my carburettor at higher altitudes" is probably
a better wording of the question.

that is simple. the air is less dense so the amount of fuel it needs
to achieve full combustion is reduced. the density of the fuel doesnt
decrease so you need less of it.

learn to think in more precise terms and a lot of the confusion
vanishes.
Stealth Pilot

[Tman[_2_]]

Couple followups to the responses on this thread.

First, I'm not doubting that you have to lean at higher altitudes; that
is empirically most true. I'll try to restate the question below...
Stealth Pilot wrote:

> "why do I lean my carb when I climb" is a bull**** question open to
> much misinterpretation.

> "why do I need to lean my carburettor at higher altitudes" is probably
> a better wording of the question.

Sure, that's what I was asking in the original post.

> that is simple. the air is less dense so the amount of fuel it needs
> to achieve full combustion is reduced. the density of the fuel doesnt
> decrease so you need less of it.

That I understand. It's also effectively a handwaving explanation to
the original question.

Here's the original question restated. Can somebody explain, why at
higher altitudes, the less dense air does not automatically meter less
fuel through a carb venturi, automatically making up for this
difference. A carb venturi operates taking advantage of the decrease in
pressure associated with the increase of velocity through the venturi --
a phenomenon which is dependent on the density of the air.

This question doesn't apply to a fuel injected engine, where as I
understand the fuel metering volume does not depend on air density, or
to any carbs with a compensating device.

[Tman[_2_]]

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.

[Stealth Pilot[_2_]]

On Tue, 20 Jan 2009 05:44:01 -0500, Tman >
wrote:

>Couple followups to the responses on this thread.
>
>First, I'm not doubting that you have to lean at higher altitudes; that
>is empirically most true. I'll try to restate the question below...
>Stealth Pilot wrote:
>
>> "why do I lean my carb when I climb" is a bull**** question open to
>> much misinterpretation.
>
>> "why do I need to lean my carburettor at higher altitudes" is probably
>> a better wording of the question.
>
>Sure, that's what I was asking in the original post.
>
>> that is simple. the air is less dense so the amount of fuel it needs
>> to achieve full combustion is reduced. the density of the fuel doesnt
>> decrease so you need less of it.
>
>That I understand. It's also effectively a handwaving explanation to
>the original question.
>
>Here's the original question restated. Can somebody explain, why at
>higher altitudes, the less dense air does not automatically meter less
>fuel through a carb venturi, automatically making up for this
>difference. A carb venturi operates taking advantage of the decrease in
>pressure associated with the increase of velocity through the venturi --
>a phenomenon which is dependent on the density of the air.
>

I think what actually happens in an aircraft carby is much cruder than
the elegant descriptions would have you believe.
in the marvel schebler ma3-spav there are a number of jets that all
need to be tweaked just so for the damn thing to work.

there is an accelerator jet which is pumped when the throttle goes in.
it also unpumps when the throttle gets pulled out but that just
refills it's barrel. the arm to this has three settings increasing or
decreasing the pump stroke and once you have this just sweet for your
engine dont change it. the outlet pipe needs to point just so into the
inner venturi ring.

there are a number of idle jets (actually just holes in the side) in
the wall of the carby that keep it running around idle when the
butterfly is almost closed. once this is adjusted for steady idle dont
change it.

then in the main jet comes in a whole number of sizes both in position
and size of the apertures that allow fuel to escape. there is also an
insert restricter in the base which is tweaked for each particular
engine type. dicking around with this to get a sweet running engine
can take ages.

as for the changes in velocity or density actually changing the fuel
flow well hmmmmm. if an engine can be run from sea level to about
6,000ft at full rich and still run and produce power, well that doesnt
seem like too much of a flow change with density.

jabiru use a bing altitude compensating carby. this has some sort of
altitude sensing compensation mechanism in addition to the usual
metering but that is a different animal.

the old style carbys have a lot of tinkering behind the steady smooth
metering in usual operation and have a simple adjustment that pilots
can use to keep the mixture in the runnable range at other times.

so my explanation is that if your surmise is correct I think it has
only about a tenth of the effect you think it does.
lots of tinkering and tweaking go behind making you think it does.

Stealth Pilot

[[email protected]]

On Jan 20, 6:32 am, Stealth Pilot >
wrote:

> as for the changes in velocity or density actually changing the fuel
> flow well hmmmmm. if an engine can be run from sea level to about
> 6,000ft at full rich and still run and produce power, well that doesnt
> seem like too much of a flow change with density.

Combustible mixtures range from 8:1 (air:fuel by weight) to
18:1. At sea level at full rich it'll be around 10:1. Pretty rich. As
you climb it'll head for 8:1 as the air's density decreases and the
engine will start to run rough and make black smoke. The mixture is
changing and you need to do something about it. We have airplanes here
at 3000' ASL that won't run nicely at all at full rich on a standard
day and we have to lean them for takeoff if it's any warmer than that.

Dan

[Todd W. Deckard]

Tman,

I've been pondering your question quite a bit. I believe I have it. In
deference to Dan my Camaro never idled correctly.

Ultimately your engine depends on the mass flow of air divided by the mass
flow of fuel. But the amount of fuel drawn up is a function of the pressure
difference in the carbureture venturi. So here goes:

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).

Bernoulli got the idea from Newton thats why the 1/2 m v squared. Now
discouragingly this has the density
in front of it, which is why you posed the question. The difference in
pressures is directly proportional to the density.

Now the low pressure in the neck of the venturi is what is drawing the fuel
up (or properly the difference between the neck and ambient). Again we use
Bernoulli to describe the forces acting on a particle moving along a
streamline -- and this time it is properly incompressible.

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

Again, Bernoulli copped it all from Newton and was just telling us that
kinetic energy + potential energy = a constant.

If we apply this to your fuel being drawn up we get:

(#) p (ambient) - p (neck) / density of fuel = .5 * (velocity of fuel) ^2 +
(gravity * vertical distance from bowl to jet)

However mass flow is the density * the size of the pipe * the velocity.

So the mass flow of air = density of air * carb barrel size * velocity
(opening)

But from (#) the mass flow of fuel is being determined by the pressure
difference (which also carries the air density)

so Air over Fuel cancels your density term.

An altitude compensating carburator puts a small vacuum on the fuel to
prevent the rho from dividing out.

Mr Wizard could have explained this better that as the air gets thinner it
sucks on the straw with less force but it takes less force to slurp up the
gas becuase of the reduced pressure. So the gas drawn up stays about the
same, however the mass flow of air drops off with density so the mixture
richens.

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

[Stealth Pilot[_2_]]

On Tue, 20 Jan 2009 08:30:34 -0800 (PST),
wrote:

>On Jan 20, 6:32 am, Stealth Pilot >
>wrote:
>
>> as for the changes in velocity or density actually changing the fuel
>> flow well hmmmmm. if an engine can be run from sea level to about
>> 6,000ft at full rich and still run and produce power, well that doesnt
>> seem like too much of a flow change with density.
>
> Combustible mixtures range from 8:1 (air:fuel by weight) to
>18:1. At sea level at full rich it'll be around 10:1. Pretty rich. As
>you climb it'll head for 8:1 as the air's density decreases and the
>engine will start to run rough and make black smoke. The mixture is
>changing and you need to do something about it. We have airplanes here
>at 3000' ASL that won't run nicely at all at full rich on a standard
>day and we have to lean them for takeoff if it's any warmer than that.
>
> Dan

thanks for the mix ranges dan. couldnt recall them.
do you have an answer for the guy's original question?

[Allen[_1_]]

I think you are making this waaaay to complicated. The volume of air going
through the venturi remains the same as you climb but the amount of oxygen
(the component needed to burn the fuel) decreases. The volume of air
remains the same so the fuel drawn out of the float bowl remains the same.
There is less oxygen so the mixture become rich.

That's my story and I am sticking to it.

--

*H. Allen Smith*
WACO - We are all here, because we are not all there.
"Todd W. Deckard" > wrote in message
...
> Tman,
>
> I've been pondering your question quite a bit. I believe I have it. In
> deference to Dan my Camaro never idled correctly.
>
> Ultimately your engine depends on the mass flow of air divided by the mass
> flow of fuel. But the amount of fuel drawn up is a function of the
> pressure difference in the carbureture venturi. So here goes:
>
> 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).
>
> Bernoulli got the idea from Newton thats why the 1/2 m v squared. Now
> discouragingly this has the density
> in front of it, which is why you posed the question. The difference in
> pressures is directly proportional to the density.
>
> Now the low pressure in the neck of the venturi is what is drawing the
> fuel up (or properly the difference between the neck and ambient). Again
> we use Bernoulli to describe the forces acting on a particle moving along
> a streamline -- and this time it is properly incompressible.
>
> { Pressure / density } + .5 * { velocity ^ 2 } + gravity *
> change_in_height = a constant
>
> Again, Bernoulli copped it all from Newton and was just telling us that
> kinetic energy + potential energy = a constant.
>
> If we apply this to your fuel being drawn up we get:
>
> (#) p (ambient) - p (neck) / density of fuel = .5 * (velocity of fuel) ^2
> + (gravity * vertical distance from bowl to jet)
>
> However mass flow is the density * the size of the pipe * the velocity.
>
> So the mass flow of air = density of air * carb barrel size * velocity
> (opening)
>
> But from (#) the mass flow of fuel is being determined by the pressure
> difference (which also carries the air density)
>
> so Air over Fuel cancels your density term.
>
> An altitude compensating carburator puts a small vacuum on the fuel to
> prevent the rho from dividing out.
>
> Mr Wizard could have explained this better that as the air gets thinner it
> sucks on the straw with less force but it takes less force to slurp up
> the gas becuase of the reduced pressure. So the gas drawn up stays about
> the same, however the mass flow of air drops off with density so the
> mixture richens.
>
> Q.E.D. Good question. If I ever become a physics teacher I am going to
> put this one on the final!
>
>
>
>
>
>

[[email protected]]

On Jan 21, 7:19 am, "Allen" > wrote:
> I think you are making this waaaay to complicated. The volume of air going
> through the venturi remains the same as you climb but the amount of oxygen
> (the component needed to burn the fuel) decreases. The volume of air
> remains the same so the fuel drawn out of the float bowl remains the same.
> There is less oxygen so the mixture become rich.
>

More properly, the weight of the air decreases. Oxygen still
makes up 21% of the air.

Dan

[[email protected]]

On Jan 21, 1:16 am, Stealth Pilot >
wrote:

> thanks for the mix ranges dan. couldnt recall them.
> do you have an answer for the guy's original question?

I think Todd Deckard answered that well. I'll leave the math
to those good with it.

Dan

[Tman[_2_]]

Allen wrote:
> I think you are making this waaaay to complicated.
I know I am. I'm an engineer, it's what we do...
Anyways.

> The volume of air going
> through the venturi remains the same as you climb
I believe this is a good approximation -- volumetric efficiency should
not change (much) as an aircraft climbs into thinner air, and throttle
setting and RPM's approx constant.

> but the amount of oxygen
> (the component needed to burn the fuel) decreases.
The mass flow decreases of air (and oxygen), and therefore the amount of
fuel required decreases; I agree.

> The volume of air
> remains the same so the fuel drawn out of the float bowl remains the same.
This is the part I don't agree with. The mass rate of fuel drawn outta
the float bowl is driven by the pressure decrease in the venturi, which
depends on both the density and the velocity. Density goes down as you
climb. Amount of fuel drawn should not remain a constant.

If I can give a rough and poor analogy. As you climb into less dense
air, you don't achieve the same aerodynamic performance from the
airplane with a given airspeed. You will stall at a higher airspeed,
your airplanes gear and flaps can handle higher airspeeds, Vx and Vy are
higher airspeeds, and Vne is a higher airspeed. The air has to go
faster to have the same effect on the wings and other devices on the
bird. It is because of the decreased density... I'd propose that is the
same effect as is happening in the carb venturi. The air has to go
faster to have the same effect in pulling gas out.

Oh yeah, before somebody corrects me I'm particularly talking about TAS
above, I know these speeds stay roughly the same at IAS (just because
the ASI is also thus affected ;) ).

>
> That's my story and I am sticking to it.
And most people are satisfied with that, I'm wondering why it doesn't
stand up to a little deeper analysis.
>

[Tman[_2_]]

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 :)

[Bertie the Bunyip[_24_]]

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

[Maxwell[_2_]]

"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.

[Bertie the Bunyip[_28_]]

"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

[Ron]

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

:-)

[[email protected]]

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

[Tman[_2_]]

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!

[[email protected]]

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

[ah]

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

[Bertie the Bunyip[_28_]]

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

[ah]

Bertie the Bunyip wrote:
> 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..

Do you need synchromesh?
--
ah

[ah]

Bertie the Bunyip wrote:
> 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..

Can you do synchromesh?
--
ah

[John \C\]

"ah" > wrote in message
anews.com...
> Bertie the Bunyip wrote:
> > 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..
>
> Can you do synchromesh?
> --
> ah

Can you quit Slurping "Some Gay Guy", a.k.a. Art Deco ??
--
HJ

[Bertie the Bunyip[_28_]]

ah > wrote in news:49791d81$0$57669
:

> Bertie the Bunyip wrote:
>> 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..
>
> Can you do synchromesh?

They're OK, just need to do the spigot bearings.


Bertie

[Stealth Pilot[_2_]]

On Wed, 21 Jan 2009 19:25:39 -0500, 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!

totally correct. you richen the air in the mixture by reducing the
fuel.

[Maxwell[_2_]]

"Bertie the Bunyip" > wrote in message
...
> ah > wrote in news:497d9da3$0$57661
>>
>> He actually had sum?
>
> Well, presumably enough to navigate his way out of the womb.
> OTH being an ass baby would explain a lot..
>
> Bertie


Heckle and Jeckle, identical twin retards.

[[email protected]]

On Jan 19, 1:23*pm, Tman > wrote:
> Somebody posed that seemingly simple question to me, but kept coming
> back to the point that they stumped me.... And I am stumped. *What do
> you see wrong with the logic in this dialog?
>
> Q: Why do I need to lean out my carb when I climb?
>
> A: *Ahem, seems you forgot your PPL ground school. *The air is less
> dense. *Fewer air molecules per unit volume. *Therefore, you need less
> gas, so you lean it out!
>>lots of really interesting stoichimetric ratio stuffage snipp-ed)....
>
> Anyone see what is amiss?
>
> T

Yes, the obvious answer to any Texas Aggie: "because it's easier than
leaning out the window".

[Bertie the Bunyip[_28_]]

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

>
> "Bertie the Bunyip" > wrote in message
> ...
>> ah > wrote in news:497d9da3$0$57661
>>>
>>> He actually had sum?
>>
>> Well, presumably enough to navigate his way out of the womb.
>> OTH being an ass baby would explain a lot..
>>
>> Bertie
>
>
> Heckle and Jeckle, identical twin retards.

Oh good one maxie.



I love heckle and jeckle, BTw Old chum!


Bertie

[mbrosch]

Q: Why do I need to lean out my carb when I climb?

Stealth Pilot is rude but he is also correct.
I think your confusing volume with density. As the aircraft climbs into less dense air the amount of oxygen for a given volume decreases. That IO 360 engine (ie) is still breathing in 360 cubic inches of air every 2 revolutions, but that air is less dense and carries less Ox at altitude than it does down low. A rule of thumb is that 50% of the Ox in the atmosphere is below 18k'.