Carburettor help

[peanutter]

i was asked a question that i couldn't properly answer regarding carby icing yesterday - thought i might seek help here.

whilst trying to explain that we need to put the carby heat to HOT when reducing power to outside of the green power range i stated that this is too protect against the chance of getting carby icing due to venturi effect. The student then said: but if the fuel / air mixture remains the same, and we reduce the amount of fuel (closing the throttle) wouldn't that mean that there is also less air intake and therefore a reduced venturi effect and thus no more risk than in normal flight?

please help. :confused:

[Dave Incognito]

The reason is due to the butterfly valve. When you set the throttle for a low power setting, the butterfly valve restricts airflow through the carburettor (hence the term throttle).

However, as the distance between the valve and the wall decreases (in order to restrict the air flow), you are in fact setting up a venturi.

If this type of icing becomes severe, it can freeze the butterfly valve into it's current position, or even stop air flow entirely. :eek:

Hope this helps.

[peanutter]

so would i be right in thinking that at higher (normal) power settings, when the butterfly valve is open there is not really much venturi effect, and the venturi effect only occurs at the lower power settings when we close the throttle?

[Dave Incognito]

Yes, that is applicable to icing around the butterfly valve (throttle ice).

However, don't forget that the other types of carby ice can form over a much wider range of power settings (e.g. impact ice and fuel/refrigeration ice)

Cheers, Dave.

[ 04 December 2001: Message edited by: Dave Incognito ]

[peanutter]

But they are only a concern in sub zero temperatures, unlike throttle icing, correct?

[Tinstaafl]

Not so.

Impact icing can occur in +ve temps eg flying through freezing rain.

Venturi/throttle butterfly icing can occur in temps up to around 30 deg C

Humidity, temperature & power setting are all factors involved with carb. icing.

The general trend is that carb. icing is more likely as temps lower, power setting reduces & humidity increases.