Continental A-65 engine stall?

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

wrote:


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


In cruise, I see maybe 50 RPM drop. I've always applied carb heat on
downwind before I reduce power from cruise (2150 RPM) to about 1700 RPM
until on final, then to about 1200 RPM about until I roundout for the
flare. If I recall, the temp/dewpoint yesterday morning was 16/12C, so
the air probably did have a fairly high moisture content.

Scott

To All:

Of the two methods of absorbing heat (ie, acceleration of the air &
change-of-state for the gasoline) the endotherm resulting from the
vaporization of the gasoline can be as much as 100 times colder than
the venturi-effect.

As for the amount of water in the atmosphere, we know there is ALWAYS
some... at least, on this particular planet. The FAA provides a chart
that allows you to figure out your probable icing point.

What gets people in trouble is the assumption that the ambient air
temperature plays a SIGNIFICANT role in carb icing. Not so -- at
least not with regard to the 'significant' business. Indeed, when you
have a nice WARM day you are more likely to develop carb or manifold
icing than on a nice COLD day.

Personally, rather than try to figure out the humidity, freezing point
and so forth, I assume icing IS probable and shuffle the knobs
accordingly. Works for me.

As for heat muffs, the FAA expects ours to provide a 90F rise in the
inlet temp to the carb. Truth is, most muffs don't come anywhere near
that amount due to a lack of heat in the exhaust stacks OR
insufficient surface area for the heat to transfer two. Annd if
you've done like most of us and wrapped an over-stretched screen-door
spring around the exhaust stack, odds are it rusted itself into FOD
during the Eisenhower administration. What you wanna do is getta
pound of .035 safety wire and wind it into a coil around a piece of
rod and use that instead of your door spring. OR get yourself about
four of those stainless steel pot-scrubbers and use that. OR get
yourself about fifty feet of .010 stainless steel 'razor blade stock;'
about 1" wide and bend that into angle stock and wrap that around AT
LEALST TWO of your exhaust stacks, because the SIGHNIFICANT factors in
the equation are surface are surface area and thermal flow, so that
while the Choir Girl pads are hands-down winners for surface area they
screw the pooch when it comes to thermal transfer AND cause too much
restriction to begin with.

So give the razor-blade stock a try. Snip it four or five times per
revolution and give each resulting fin a bit of a twist... then go
back and crimp each of the 'fins' with a pair of fluting pliers so the
fin ends up sorta wavy.

Nothing here is carved in stone but a weekend experimenting with a
lawn-mower engine and an IR temperature sensor is equal to a couple of
months sitting in a classroom.

-R.S.Hoover

Bob wrote:
To All:

Annd if
you've done like most of us and wrapped an over-stretched screen-door
spring around the exhaust stack, odds are it rusted itself into FOD
during the Eisenhower administration.



-R.S.Hoover

Yes, I was concerned about the doorsprings corroding and breaking pieces
off to be ingested into the carburetor since there isn't any sort of
filter in the "warm" air path between the heat muff and the carb airbox...

One thing that is different on my system now (since my recent top
overhaul) is the addition of an air inlet on the engine baffle between
the nosebowl and the cylinder (pilot side) with tubing running from it
to the heat muff. My old system just had the inlet pipe of the Aeronca
heat muff unconnected to anything and I noticed that the aluminum heat
muff looked like it had melted a bit where it was contacting the exhaust
pipe so I added the cool air inlet to help cool the heat muff a bit.
Maybe it cools it too much and I'm just not getting enough heat, so
increasing the surface area sounds like a good idea...just unsure of the
method. I know I DO NOT want FOD running into the carb as that is SURE
to cause the engine to react [not so] funny



Scott

Scott wrote:
Yes, I was concerned about the doorsprings corroding and breaking pieces
off to be ingested into the carburetor since there isn't any sort of
filter in the "warm" air path between the heat muff and the carb airbox...

A stainless steel screen should be a standard item over the air intake
on any engine if you are running without a filter. You never know when
something too big to be ingested will get sucked in.

Tony

"Scott" wrote in message
...
Bob wrote:
To All:

Annd if
you've done like most of us and wrapped an over-stretched screen-door
spring around the exhaust stack, odds are it rusted itself into FOD
during the Eisenhower administration.


-R.S.Hoover

Yes, I was concerned about the doorsprings corroding and breaking pieces
off to be ingested into the carburetor since there isn't any sort of
filter in the "warm" air path between the heat muff and the carb airbox...

One thing that is different on my system now (since my recent top
overhaul) is the addition of an air inlet on the engine baffle between the
nosebowl and the cylinder (pilot side) with tubing running from it to the
heat muff. My old system just had the inlet pipe of the Aeronca heat muff
unconnected to anything and I noticed that the aluminum heat muff looked
like it had melted a bit where it was contacting the exhaust pipe so I
added the cool air inlet to help cool the heat muff a bit. Maybe it cools
it too much and I'm just not getting enough heat, so increasing the
surface area sounds like a good idea...just unsure of the method. I know
I DO NOT want FOD running into the carb as that is SURE to cause the
engine to react [not so] funny



Scott

Aha!

I read your post this morning and didn't think much about it at the time;
but but it now occurs to me that the cooling air gains a lot of heat passing
through the cooling fins of the engine--so that you may have created your
own problem by sending cold outside air directly into the heat muff.

Just a thought.

Peter

On Jul 13, 3:33 pm, "Peter Dohm" wrote:

Aha!

I read your post this morning and didn't think much about it at the time;
but but it now occurs to me that the cooling air gains a lot of heat passing
through the cooling fins of the engine--so that you may have created your
own problem by sending cold outside air directly into the heat muff.

Just a thought.

Peter

And since those Aeronca muffs tend to leak a lot, forcing air into
them might cool the pipe too much just when the heat is needed. The OP
needs to seal up those muffs. The connection to the baffling needs to
stay, since a muffed pipe with no airflow (except when carb heat is
on) will overheat and burn out. Piper had plenty of that sort of
trouble on their Cherokees.

The OP gets a 50 RPM drop in cruise, with carb heat. That ain't
much.

Dan

wrote:


And since those Aeronca muffs tend to leak a lot, forcing air into
them might cool the pipe too much just when the heat is needed. The OP
needs to seal up those muffs. The connection to the baffling needs to
stay, since a muffed pipe with no airflow (except when carb heat is
on) will overheat and burn out. Piper had plenty of that sort of
trouble on their Cherokees.

The OP gets a 50 RPM drop in cruise, with carb heat. That ain't
much.

Dan

Actually, my muffs fit fairly "airtight" around the exhaust pipes. I
actually had to cut and trim a bit to make them fit at all, so I think
the majority of the heat produced is captured inside the muff. However,
I just don't think the two halves of aluminum have enough thermal mass
to provide a ton of heat to the carb, so I think I still need to come up
with a safe way to add mass inside the muff.

Scott