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On 20 Apr, 23:22, wrote:
On 19 Apr 2007 10:03:06 -0700, wrote:

I heard, somewhere, that if you apply full carb heat and then
immediately lower the collective (on R22s and 44s)this can cause the
engine to run rough and in extreme cases can even cause engine
failure.I s there any evidence for this or is it just a silly rumour?
I have, up until now, always pulled on full carb heat just before
descending now I wait a few seconds before lowering the collective.

You should always wait to give the heat time to melt any ice that
might be present. If you lower the collective immediately, you might
not have given the ice any time to melt which might lead to a stalled
engine.

Regards.

Jan

Thanks Jan, that makes sense. So what you are saying that the
carburettor is more prone to icing when using less power?

This is so basic that I am surprised that you are in the machine at all
without this knowledge. There is a depression formed between the carb
venturi and the inlet valve. If the butterfly valve of the carb is closed,
the expansion of the air after the valve is greater than if the valve is
completely open and therefore the temperature drop is greater.

The depression is measured with the MAP gauge. ie, anything less than
atmospheric pressure is depression.

The carb air temperature is measured upstream of the butterfly in the R22.
This is why it is unreliable under 18" MAP. It is measuring the air before
it is subject to the cooling effect of being expanded after the butterfly.
The carb air temperature in the R44 is measured after the butterfly.

The only way you are going top conk by cutting the throttle after applying
full carb heat is if you are already iced up and the melting ice is being
ingested by the engine. Or alternatively, the ice has narrowed the venturi
area and dropping the butterfly asphyxiates the engine completely.

Regards
Andrew