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By bookworm
#132464
Pilot H wrote:Partial carb heat can raise the temperature into the icing zone, so should never be used unless a carb temperature gauge is fitted, in which case it can be useful for cruise flight in icing conditions (The partial carb heat can then be used to raise the temp enough to prevent icing as verified by the Temperature gauge, but does not cut back power as much as full heat would)


I remain unconvinced by the physics of this -- though I wholeheartedly agree with your previous comment, BTW.

If air is too cold for icing to start with, I don't see how heating it can make it more likely to produce ice.
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By David Williams
#132499
bookworm wrote:puppies roasting on an open, heated, alternate induction system...

Hmmm....

"Puppies roasting on an open, heated, alternate induction system,
Jack Frost nipping on your nose,
Yuletide carols being sung by a choir,
And folks dressed up like Eskimos."


Well it nearly scans! :D

bookworm wrote:If air is too cold for icing to start with, I don't see how heating it can make it more likely to produce ice.

My understanding is not that the air is too cold to produce ice - but that the ice formed is too cold and "dry" to stick to the throttle or venturi. So warming it will make the ice "stickier". That's just what I've read elsewhere - I'm afraid I can't explain the physics of it either.
By coolflyer1
#134448
Whilst we're on the subject would anyone mind if I popped this question in just about here!
I had a question in a tech exam about the effect of carb heat on the fuel/air mixture.

With carb heat applied the warmer air entering the carb intake is less dense ............ does that make for a leaner mixture or does less dense air mean .. less air ! and therefore a greater fuel to air ratio and hence an enriched mixture.

I'm a bit baffled by it and would appreciate some informed edumacation.
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By AndyR
FLYER Club Member  FLYER Club Member
#134476
less dense air mean .. less air ! and therefore a greater fuel to air ratio and hence an enriched mixture.


The mixture is enriched more or less as you say
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By David Williams
#134487
It's important to understand what we mean by "less air" or "more air". Crucial to this is to know the difference between volume and mass.

As I understand it, the carb delivers a quantity of fuel that depends on the volume of air passing through the venturi - not the mass of air.

With carb heat on, the volume of air passing through is the same, but as you say hot air is less dense. Therefore there is less mass of air.

What matters for the combustion process is the ratio of mass of fuel to mass of air. Therefore, with less mass of air but the same mass of fuel, carb heat leads to a richer mixture.
By ManfredvonRichthofen
#157892
I was told leaving carb heat on risks "detonation"

i smiled politely and continue to wonder what exactly "detonation" means....



personally, i keep my hand on the carb heat switch until i want to turn it off, in recognition of the fact that my sieve like mind will forget to turn it off if i don't do this
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By 2Donkeys
FLYER Club Member  FLYER Club Member
#157896
Detonation is one of a number of forms of "misfiring" in an engine. You could visualise it as the spontaneous explosion of the otherwise unburnt contents of a cylinder. Detonation tends to be associated with:

a) The use of an inappropriately low fuel grade
b) An over-lean mixture
c) Excessive manifold pressure
d) An over-heated engine
e) Excessive carburetor air temperature.

Carb heat use could therefore, under extreme conditions be associated with detonation, but not without some additional provocation.
By Guest
#157914
The worst icing conditions exist in moist air at just below 0 degrees C. You will still get icing below this temperature, but as said before it forms crystals which are less 'sticky'.

Once the temperature is -45 or less the moisture is supercooled and icing will not form.
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By 2Donkeys
FLYER Club Member  FLYER Club Member
#157920
"Guest", I suspect that you are confusing Carb Icing with Airframe icing.
By WhirlyGreyGit
#158732
I couldn't see an answer to the "partial carb heat" question so here it is.

Below a certain temperature ice will not form as the water will already be frozen so will pass through the carb as ice, if you raise the temperature you can then bring it back into the icing range.

Steve

PS For piston heli's there is usually a filter on the hot air so leaving it on only loses power, so long as you have enough in reserve (i.e. probably solo) you'll be ok and you won't normally ingest anything...
...although people that drag fuelling hoses over the dirt can manage to achieve the same effectfor you.... :(
By bookworm
#158795
WhirlyGreyGit wrote:Below a certain temperature ice will not form as the water will already be frozen so will pass through the carb as ice, if you raise the temperature you can then bring it back into the icing range.


Wouldn't I see this ice in the form of cirrus cloud?
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By Ian Melville
#158843
I think WGG is refering to the inside of the carb barrel being raised into the icing range :?:
By Guest
#193604
Anonymous wrote:The worst icing conditions exist in moist air at just below 0 degrees C. You will still get icing below this temperature, but as said before it forms crystals which are less 'sticky'.

Once the temperature is -45 or less the moisture is supercooled and icing will not form.


I think you're thinking of airframe ice and it can form at a higher temp than 0 degrees C
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By wobblyprop
#193614
[swank]
or fly fuel injection like me
[/swank]

Steve :D
By Gpugh
#1855743
Hi I know this is a very old thread but just to correct one statement that all military Chipmunks were wired in heat that is not true navy and I think army ones never were although they were going to be until stopped by my father who at the time was with the naval flight at roborough for the reasons given in terms of ice forming with carb heat selected under certain circumstances and then not being able to do anything about it he also couldnt understand why students were not able to be taught how to use it correctly which was the RAF reason for doing it
Charles Hunt liked this