How do some aircraft designs avoid carburettor icing?

A slide.

How does the slide avoid icing, as it’s restricting airflow like the butterfly valve?

I found this video some time ago and found it interesting in terms of understanding what carb icing is and looks like. It’s a slide throttle CV motorcycle carb much like those fitted on a Rotax 912, but I don’t think that makes any difference in the overall principle. There may also be other forms of carb icing, but this one is an accumulation of ice occurring where fuel vaporizes that blocks more fuel from entering the intake air stream.

Keeping the metal warm or alternately heating the incoming air would I imagine prevent this from occurring.

Aircraft carbs fitted to Lycomings and Continentals don’t have a needle, they are ‘fixed jet’ carbs similar to those fitted to a lawn mower. My theory is that Lycomings don’t suffer from carb ice as much as Continentals because the carb is bolted directly to the sump, which is heated to oil temperature and held there. Heat then conducts to the carb body and the fuel jet, which are held at a temperature high enough to prevent ice forming on their surfaces. This would also explain why I get occasional carb ice on my Lycoming while taxiing out, before the oil temp rises, but never in flight in a relatively dry climate.

How does the slide avoid icing, as it’s restricting airflow like the butterfly valve?

It doesn’t completely avoid it, as the video show, but greatly reduce it. The icing is all about fuel evaporating, this cooling the air to below the condensation temp and at the same time below 0 deg. Restriction of the airflow doesn’t account for much temperature difference.

A butterfly valve is aft of the injectors (it’s not in the video, but should be further aft some place if this is a Bing type carb with pressure operated slide/needle, I think at least )

Here, obviously the needle gets cooled by the fuel, and it get iced down. Bing carbs are normally very resistant to carb ice though.

Anyway, the idea is that the butterfly valve is the part that gets cooled down, and it sits at the right place for icing.

In a couple of weeks we are throwing away both Bings in our towplane, replacing them with EFI. This is due to other carb problems, not carb ice.

LeSving wrote:

The icing is all about fuel evaporating, this cooling the air to below the condensation temp and at the same time below 0 deg. Restriction of the airflow doesn’t account for much temperature difference.

Is it? I thought carb icing was all about the pressure drop past the throttle, which is the reason carb ice is more likely to form with partially closed throttle than with WOT. If it was primarily because of fuel evaporating, then carb icing would be more likely to form at WOT because more fuel is passing through the carburetor.

Silvaire wrote:

This would also explain why I get occasional carb ice on my Lycoming while taxiing out, before the oil temp rises, but never in flight in a relatively dry climate.

Right! The only time I’ve seen Carb Ice on my O360 in the Mooney was prior departure on taxi out.

My previously owned O200 in my Cessna was awful with carb ice, until I had a carb temp indicator installed. Then it became obvious that the temps change very different from what conventional training would suggest. Carb Temp practically always was in the yellow range at WOT and high powersettings, while it would be much higher the moment you reduce power. In theory, this totally negated the training that carb heat should be on while on approach, as with the reduced setting temp was mostly in the 10 to 15° C range, well outside the yellow arc.

It became procedure then to keep the needle out of yellow whenever visible moisture was present, which was very easy with the gauge. Mostly carb heat had to be used at high power settings (and C150’s are mostly run WOT anyhow).

I’ve been wanting to install a carb temp indication on the Mooney for a while but don’t have space. I think some engine analyzers also have carb heat as an option, this is something I’ve been thinking to do since I have the plane.

Is it?

Yes. In principle a carburetor is a refrigerator, a liquid evaporator.

Being prone to ice up at lower throttle setting is function of the the butterfly valve position (part closed) and the icing air is moving slower across it, more time to cool it down and hit it with droplets.