On Jul 12, 11:52 am, Scott wrote:

Do you know what is the mechanism that causes power loss (and ultimately
quitting) due to carb ice? The book drawings show that ice builds up in
the throat of the carb (if I recall) so it looks like the diameter of
the throat is reduced? What does that do to make the engine run at
reduced power and/or eventually die? I suspect that it impedes the air
flow through it, effectively making the mixture richer until eventual
death. Am I close?

Getting warm, but still a ways from the truth. There are two
factors that cause the temperature drop in the carb: The venturi
accelerates the air, thereby dropping its pressure and temperature. We
need that pressure drop to suck fuel from the nozzle, but could do
without the temp drop. The laws of physics being what they are, we
have to deal with it. A similay pressure and temperature drop takes
place at the throttle plate when it's closed or mostly closed; the air
squeezing past the edge of the plate accelerates a whole bunch and
gets cold, however briefly.

The other factor is the fuel itself. As it sprays into the
airflow, it vaporizes. Anytime we turn a liquid into a vapor, we
effect a heat transfer; in this case it's from the air to the liquid
as it turns to a vapor. So the air gets mighty chilly. Same effect you
get if you dip your hand in gasoline then hold it up and let it
dry;your hand gets cold. The fuel sprays from the main nozzles when
the power level is higher, and from the idle ports in the carb wall,
next to the edge of the throttle plate, at lower settings.

So the air, and any water vapor in it as it passed through the
carb, is chilled. The vapor, if there's enough of it, will condense,
and if the temperature drop is large enough, it will freeze to
anything handy. Carb ice. The total temperature drop caused by both
venturi effect and evaporation of the fuel can be as much as 38°C, or
about 71°F. We can expect carb ice at ambient temps of up to 38°C or
100°F if the humidity is near 100%, or the temp/dewpoints are almost
the same.

If I want to go diving off some cliff into the water, I want to know
what the water conditions are, and what's under the surface. If I want
to go flying, I need to know what the atmosphere is up to. That
includes temperture and dewpoint spread; if they're close together, I
will expect carb ice so I'm not surprised when the engine starts
acting up.

If your engine quit on the rollout due to carb icing, either your
system is leaking and the carb isn't getting warm enough air, or
you're not applying it soon enough and getting the carb warmed up some
before reducing power. Or you're making long glides without running
the power up and warming the carb periodically.

What sort of RPM drop do you get when you pull the carb heat in
cruise? When are you applying carb heat? How long was the power-off
glide? What were the temp and dewpoint on the day in question?

Dan