There is a range of temperatures which are most propitious to carb icing given the humidity. IIRC it is +5° to -30°. (don’t quote me it’s from memory.
When air enters the carb, the venturi effect can lower that temperature by as much as 30°. If the warmth from the engine or carb heat increases temp by 15° to 20° this can put you smack in the middle of the most conducive temperatures for carb icing.
Eg it is 0° with high humidity. The air entering the carb could be -30° carb heat adds say 20° so air entering carb is -10°.
Having both heat due to position of carb and carb heat might well increase air entering another 15° and you are still in the carb icing range.
That doesn’t stop me using carb heat if it is there but I am aware that position of carburetor can also do the job. So if an aircraft designer has positioned the carb so that it is in a heat flow, personally I wouldn’t dream of adding it without further testing.
YMMV.
Also important that the application of carb heat reduces power which in the climb can be significant.
You mean it gets heated to a point where the moisture is able to condensate and consequently freeze at the carb walls whereas otherwise the moisture would just pass the carb which is possibly warmer than the (very cold) air itself? I can see that. The situation I had in mind was the typical summer Sunday afternoon bimble which I consider the overwhelming use case of Rotax installations.
[posts overlapped, gallois’ post already answered the question]
Clipperstorch wrote:
You mean it gets heated to a point where the moisture is able to condensate and consequently freeze at the carb walls whereas otherwise the moisture would just pass the carb which is possibly warmer than the (very cold) air itself?
What the exact mechanism is I don’t know, but many POHs advices against using partial carb heat unless you have an indicator for carbutettor air temperature.
Most Robin POHs say never use partial carb heat, it’s either all or nothing.
Exactly, and for good reasons (part carb heat can produce icing).
I don’t understand how partial carb heat can make carb ice possible. I understand the carb ice diagrams, but an increase in air temperature will be accompanied by a reduction in humidity unless water vapour is added.
In the Super Guépard has fuel tanks behind the seats in the fuselage as @Vic says but there is no problem climbing at 1000ft per min and more on UL95. As we have lots of similar aircraft here, doing the same, I can’t see how the fuel pump is the full answer.
@Maoraigh a mass of warm air can carry more water vapour than a mass of cold air so doesn’t necessarily become less humid with a rise in temperature. That water vapour is turned to water droplets or ice as the temperature drops. Think dew or frost on grass. In the carb this ice or dew forms very rapidly at the mouth of the carb just at the butterfly and that can’t move.
Partial carb heat often does not produce enough heat to melt this frost or evaporate this dew. Again think how strong the sun has to get to melt the frost on the grass on a clear day. In the carb those water droplets when hit by -30° air is more likely to turn to hard clear ice than soft frost and to do it very quickly. You need quite a bit of heat to remove that.:)
Most ULs actually have wing tanks and an aux pump. Like the B23, one of the aircraft types that seems to suffer from the issue that we are discussing.
