"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/fluid..._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.