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How do some aircraft designs avoid carburettor icing?

Reportedly, some types don’t suffer from it at all.

However, to achieve this, one would need to warm up the incoming air by some 20-30C, which is quite a big loss of efficiency.

Is there some other way to get around the problem?

Administrator
Shoreham EGKA, United Kingdom

Hi,
On Rotax the body of the carburettors are heated enough to prevent ice to form in the inlet. The incoming air is not heated very much. Maybe that is the trick in some installations, the carburettor is kept warm enough from the engines heat.

/Patrik

ESG..., Sweden

Three ways of heating the carb body itself are electrically, with liquid coolant or through conductive heating by the engine block.

It’s not 100% clear to me why some types get carb ice in an appropriate climate and other types don’t. For engines without active carb body heating the difference could well be the strength of the conductive path from carb body to engine, as @mcrdriver wrote. That sounds plausible. See photos below – Lycomings are reputed to have far less carb ice issues than Continentals. I don’t have have any useful personal experience, the last time I experienced carb ice when airborne was when flying with my father in the late 1970s although I’ve had it more recently when taxiing.

Lycoming Carb Installation (Carb Bolted Directly to Oil Sump)

Continental Carb Installation (Carb Bolted to ‘Spider’, so Mounted Remotely to Crankcase)

Last Edited by Silvaire at 12 Jun 17:46

I was taught to do a carb heat check every 10 minutes. I’m now careless on that, but do it more often than every 10 minutes when suspicious. An O200 in a Jodel DR1050 has carb ice on many flights, and several times in the flight.
I had a share in a Pa28-161 and never had carb ice.
I can’t recall having it with a C152. All flown in the same area, north. Scotland.
I’ve never had it with an O200 in a C150, but that’s because I’ve only flown it in West Colorado, Utah, and Arizona.

Maoraigh
EGPE, United Kingdom

On a carb, the butterfly valve must be after the fuel injection spot. This, it’s the butterfly valve that gets iced up. It’s possible to design the carb without the butterfly valve, and this will eliminate carb ice for all practical purposes.

The elephant is the circulation
ENVA ENOP ENMO, Norway

The rotax microlights I trained on had rightly cowled engines and the air intake was from k&n style filters inside the cowl. POH basically said they don’t suffer except in extreme conditions.

I then bought a jabiru which had conventional AND electric carb heat. It had crashed 2 years earlier landing short of runway with suspected carb icing and after that event the electric heat was added. I left it full on always, and never had any ice.

I then moved to another jabiru without electric heat and it iced up regularly.

I then moved to a Tecnam with rotax which as well as the tight cowl had carb heating jackets fed from the water cooled heads. It never even considered icing up.

My conclusion is that tight cowl with air drawn from within works well on rotaxes and seemingly costs Nothing power wise. For the jabiru the electric heater worked very well, and again no apparent loss of power.

EGKL, United Kingdom

We get noticable carb icing in the Robin several times a year.
It’s really not a big deal for anyone with a couple of brain cells to spare………. you notice the revs are a little lower…….you may tweek the power a bit like every other time. You notice it again and pull the Heat on and the revs increase 200ish RPM……..ah it’s an Icing day……..little extra care today end of.
We definitely get it more severely and perhaps more often if flying in company with a little less power. I Know that’s expected but I mean we have a noticable difference. Again 15yrs in and no complaints from me.

Back to the original question…..I wonder about airflow or lack of it in and around the cowling. Perhaps some cowlings cause swirling around which incorporates some very hot manifolds etc which warms just enough to help, where others ‘breathe’ in a way which is unhelpful. Or some don’t really pass much air other than where its well baffled for cylinders etc then out and gone allowing a little warming in the convenient areas.

Just my 2p worth.

Pete

United Kingdom

I occasionally get carb heat (maybe once every 50 hrs) with the O-320-B3B on my aircraft. The same engine installed on the Apache that I did my multi rating in would get dreadful carb ice at the merest hint of visible moisture.

The worst time I got carb ice was halfway across the Irish Sea. Of course the engine got even rougher with the carb heat on but I just had to leave it and wait till it cleared. It was the longest 30 seconds of my life, even though I didn’t lose so much as 1ft of altitude…

Last Edited by alioth at 13 Jun 09:34
Andreas IOM

carlmeek wrote:


My conclusion is that tight cowl with air drawn from within works well on rotaxes and seemingly costs Nothing power wise.

Not just Rotax. The VW based Rectimo in my little machine breathes from within the cowling under the engine. In 21 years I have never had carb icing.

Many years ago I recall Ivan Shaw of Europa fame claiming that the engine did not suffer carb ice because the Rotax was so frugal with fuel that there was so little of it passing to contribute much to evaporative cooling within the carb. That may have been sales bull, but I also recall extensive ground running of my Rectimo, sans cowl, in humid weather when condensation was abundantly visible on the outside of the carb yet no ice formed within the throat, so maybe low fuel flows do help.

The RV-4 that I was flying for the last 4 years has a curburetted Lycoming O320-B2B and has never had a hint of carb icing.

That uses the Lycoming system of running the intake pipes through the sump and pre-warming the air. I think that’s reckoned to cost about 5HP in lost efficiency.
It’s also been mainly operated in a dry climate, which no doubt helps.

The worst that I’ve had was a VW powered Taylor Mono which had a feeble carb heat system that once put me down on Santa Pod on a January flight when I couldn’t recover power.

KHWD- Hayward California; EGTN Enstone Oxfordshire, United States
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