Wblane proclaimed:

> Aside from the cold why do carbureted airplane engines freeze up at high
> altitude? I thought at high altitude there was very little moisture?

There is enough moisture to condense due to nothing more than the
turbulence from passing aircraft...aka contrails or more formally
condensation trails.

The absolute humidity will drop off as you go up in a standard
atmosphere, but the relative humidity may still be near 100%.

The planes most in danger of carb icing don't fly very high...

 

Wblane proclaimed:

> Aside from the cold why do carbureted airplane engines freeze up at high
> altitude? I thought at high altitude there was very little moisture?

There is enough moisture to condense due to nothing more than the
turbulence from passing aircraft...aka contrails or more formally
condensation trails.

The absolute humidity will drop off as you go up in a standard
atmosphere, but the relative humidity may still be near 100%.

The planes most in danger of carb icing don't fly very high...

 

Wblane proclaimed:

> Aside from the cold why do carbureted airplane engines freeze up at high
> altitude? I thought at high altitude there was very little moisture?

There is enough moisture to condense due to nothing more than the
turbulence from passing aircraft...aka contrails or more formally
condensation trails.

The absolute humidity will drop off as you go up in a standard
atmosphere, but the relative humidity may still be near 100%.

The planes most in danger of carb icing don't fly very high...

 

Ted Azito proclaimed:

> Mike Romain <romainm@sympatico.ca> wrote in message news:<413E09BB.307E2038@sympatico.ca>...
>
>>Now you are the one full of ----!
>>
>>I have been in two different vehicles and had the throttle freeze at WOT
>>or 70+ mph!
>>
>>These didn't even have propane, they were carbs.
>>
>
>
> Hardly the fault of propane then,was it? (Shades of Monty Python's
> stunned parrot....)
>
> Yes, it happens, more often a linkage problem than anything else. But
> this was about propane specific throttle freezeups.

Expansion of any gas [including the atmosphere in a venturi] causes
adiabatic cooling. You can get propane freeze-up at fairly high
temperature if the humidity is high enough and the combination of
pressure drop in the airflow and in the propane expansion is
high enough.

Don't remember any of the Suburban Propane trucks having freezeover
problems in winter, but then never tried driving one in Montana or
Alaska grade winters...just down to maybe 0 degrees F.

 

Ted Azito proclaimed:

> Mike Romain <romainm@sympatico.ca> wrote in message news:<413E09BB.307E2038@sympatico.ca>...
>
>>Now you are the one full of ----!
>>
>>I have been in two different vehicles and had the throttle freeze at WOT
>>or 70+ mph!
>>
>>These didn't even have propane, they were carbs.
>>
>
>
> Hardly the fault of propane then,was it? (Shades of Monty Python's
> stunned parrot....)
>
> Yes, it happens, more often a linkage problem than anything else. But
> this was about propane specific throttle freezeups.

Expansion of any gas [including the atmosphere in a venturi] causes
adiabatic cooling. You can get propane freeze-up at fairly high
temperature if the humidity is high enough and the combination of
pressure drop in the airflow and in the propane expansion is
high enough.

Don't remember any of the Suburban Propane trucks having freezeover
problems in winter, but then never tried driving one in Montana or
Alaska grade winters...just down to maybe 0 degrees F.

 

Ted Azito proclaimed:

> Mike Romain <romainm@sympatico.ca> wrote in message news:<413E09BB.307E2038@sympatico.ca>...
>
>>Now you are the one full of ----!
>>
>>I have been in two different vehicles and had the throttle freeze at WOT
>>or 70+ mph!
>>
>>These didn't even have propane, they were carbs.
>>
>
>
> Hardly the fault of propane then,was it? (Shades of Monty Python's
> stunned parrot....)
>
> Yes, it happens, more often a linkage problem than anything else. But
> this was about propane specific throttle freezeups.

Expansion of any gas [including the atmosphere in a venturi] causes
adiabatic cooling. You can get propane freeze-up at fairly high
temperature if the humidity is high enough and the combination of
pressure drop in the airflow and in the propane expansion is
high enough.

Don't remember any of the Suburban Propane trucks having freezeover
problems in winter, but then never tried driving one in Montana or
Alaska grade winters...just down to maybe 0 degrees F.

 

They don't - unless you get into clouds or high relative humidity.
Icing at altitude is usually restricted to areas of visible moisture
and below 18,000 feet - with the exception of those foolish enough to
fly into higher storms. The carb icing problem occurs at a most
inconvenient place: in the landing pattern at low altitude. That's
why carb heat is on the before landing checklist. Of course, that
also depends on what you call 'high altitude' - to me, if I don't
require oxygen I'm not very high.

On Wed, 8 Sep 2004 20:24:42 UTC wblane@aol.combotizer (Wblane) wrote:

> Aside from the cold why do carbureted airplane engines freeze up at high
> altitude? I thought at high altitude there was very little moisture?
>
> >Given the right weather conditions (cool and humid) the previous engine in
> >my `79 CJ-5 would occasionally ice up the throttle butterfly. It was
> >always a little exciting when it would happen, because I'd only notice
> >when I was trying to take _off_ speed and the gas pedal would stay where
> >it was. I'd have to give it more throttle to get it unstuck.
> >
>
>
> -Bill (remove "botizer" to reply via email)


--
Will Honea

 

They don't - unless you get into clouds or high relative humidity.
Icing at altitude is usually restricted to areas of visible moisture
and below 18,000 feet - with the exception of those foolish enough to
fly into higher storms. The carb icing problem occurs at a most
inconvenient place: in the landing pattern at low altitude. That's
why carb heat is on the before landing checklist. Of course, that
also depends on what you call 'high altitude' - to me, if I don't
require oxygen I'm not very high.

On Wed, 8 Sep 2004 20:24:42 UTC wblane@aol.combotizer (Wblane) wrote:

> Aside from the cold why do carbureted airplane engines freeze up at high
> altitude? I thought at high altitude there was very little moisture?
>
> >Given the right weather conditions (cool and humid) the previous engine in
> >my `79 CJ-5 would occasionally ice up the throttle butterfly. It was
> >always a little exciting when it would happen, because I'd only notice
> >when I was trying to take _off_ speed and the gas pedal would stay where
> >it was. I'd have to give it more throttle to get it unstuck.
> >
>
>
> -Bill (remove "botizer" to reply via email)


--
Will Honea

 

They don't - unless you get into clouds or high relative humidity.
Icing at altitude is usually restricted to areas of visible moisture
and below 18,000 feet - with the exception of those foolish enough to
fly into higher storms. The carb icing problem occurs at a most
inconvenient place: in the landing pattern at low altitude. That's
why carb heat is on the before landing checklist. Of course, that
also depends on what you call 'high altitude' - to me, if I don't
require oxygen I'm not very high.

On Wed, 8 Sep 2004 20:24:42 UTC wblane@aol.combotizer (Wblane) wrote:

> Aside from the cold why do carbureted airplane engines freeze up at high
> altitude? I thought at high altitude there was very little moisture?
>
> >Given the right weather conditions (cool and humid) the previous engine in
> >my `79 CJ-5 would occasionally ice up the throttle butterfly. It was
> >always a little exciting when it would happen, because I'd only notice
> >when I was trying to take _off_ speed and the gas pedal would stay where
> >it was. I'd have to give it more throttle to get it unstuck.
> >
>
>
> -Bill (remove "botizer" to reply via email)


--
Will Honea

 

On 08 Sep 2004 20:24:42 GMT, Wblane wrote:

>Aside from the cold why do carbureted airplane engines freeze up at high
>altitude? I thought at high altitude there was very little moisture?

When induction air is pulled through the venturi it increases density
at the top of the restriction. This causes an increase in heat, but
this heat is given up when liquid fuel is added to the air stream. at
the bottom end of the venturi, where the throttle butterfly lives, the
cooled airstream decreases in density, and cools to a temperature
lower than it was at the beginning of the journey. Any water vapour
that was in the air condenses onto the throttle butterfly, and if the
temperature is low enough, it will turn to ice.

This process can occur at any altitude, in any season. All that is
needed is a set of temperature, humidity, and throttle conditions that
are conducive to icing, and you've got it. Typically though, it occurs
on a set throttle which is only partly open - exactly what occurs in
aircraft. In extreme conditions though, icing will occur during
changing loads and throttle openings, such as in an automobile.

The freezing typically begins on the engine side of the throttle
butterfly. Ice gradually accumulates there and eventually closes or at
least restricts the gap between the butterfly blade and the barrel of
the carburetor. The result is loss of power and a stuck throttle, not
nice in an aircraft. Corrective action is to pull the carburetor heat
lever, which brings hot air either into the induction airstream or
else directly to the venturi. This causes a further reduction in power
until the ice has gone, then power increases to a point where
carburetor heat can be reduced, which further increases the power up
to full power without heat.

You don't need to be at high altitude for this to happen, but it is
certainly just as common an occurrence at high altitude as it is at
sea level in, say, equatorial New Guinea.

--
GW