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I have several questions concerning carb icing. My Cessna had carb heat via a muff heater, which caused significant loss of power but prevented engine stoppage due to ice. I’ve read everything I could find on Rotax 912/Bing carb icing and concluded that there are strong opinions that it doesn’t occur and strong opinions that it does occur, and others that maybe it can, the whole range. Rotax and Bing are remiss for not addressing this directly. But I understand, it is like saying “do not drink the contents of the cooling system” (legal worries) - if they say carb icing doesn’t happen and someone says it did, then there is a lawsuit. So I’ve decided to install carb heat.

I like the hot airbox/muff heater style best, but am thinking about something less expensive and easier. There are two popular systems, the coolant AFTER downstream of where ice would form, presumably conducing heat through the carb body and heating up the ice formation point to where it does not happen. The second one uses electricity to heat the carb body directly by bolting to the unused, untapped holes at the point of ice formation in the venturi. Well, ok, there is a third model that heats air with electricity inside the air filter, but I doubt this tiny heat source would do much of anything given the volumes of air that flows here.

Looking at the second type, the one using the untapped holes in the side of the carb, I am thinking of routing a 3/8” coolant line through a copper pipe heat sinked and connected to the side of the carb at this very point. This would be very easy to fabricate and install. If I do this I will post pictures. But this brings me to my questions.

Could too much heat cause vapor lock with the horrible corn gas (ethanol)many of us are foced to use? Or could the heat cause other issues inside the carb? I could make the heat sink to carb mount less effective in an attempt to regulate how hot it might get. But then it might not prevent icing. Keep in mind that the coolant might approach 250 degrees, but hopefully be down around 200. These temps are not what the carb body would see by the time the heat reaches it, just something to keep in mind.

Any thoughts?

Thanks,
Dennis Urban
717-349-7756
  • Re: Carb Icing - again still

    by » 10 years ago


    The Rotax statement is clear: the Installation Manual says carb heat must be installed. It can happen and does happen. I had it myself 2 weeks ago in Oregon. Fortunately on a ground run.
    The Con-Air carb heaters work very well; they do not seem to heat the air at all, just the carb body and throttle butterfly so ice cannot form. I have not heard of any performance or vapor lock issues even when left on all summer. On the contrary the owners love them and do leave them on all year round.
    It would be interesting to hear the results about your heater idea. Maybe Conrad can comment with his vast experience testing this concept?
    Rob

  • Re: Carb Icing - again still

    by » 10 years ago


    i assume i had carb ice on a landing in central ILL in the summer. when i pushed in the throttle for the 'go' part of a 'touch and go' she died. never happened previously, and never since. but now i use carb heat, even in central AZ.

    AZ Keith

  • Re: Carb Icing - again still

    by » 10 years ago


    Thanks. You saved me. I also have looked it up and see “icing” covered in the installation manual. I feel stupid.

    Luck saved me so far given I have been flying in small dew point spread days. I also see Rotax shows an intake air manifold used to mix heated air, if I chose to go that route. Probably I will make the coolant heat to the carb body as I described. My plan is to use the 3/8” bypass line from my Thermo-Bob to permanently go to each carb body on its route. A consideration is what happens inside the water pump with all the backpressure caused by this restriction, which I am further restricting until the thermostat opens. I am thinking this is not a problem, but I don’t know for sure. Is the pump made in a way that allows it to be restricted – and this goes for the stock Thermo-Bob installation as well. I could put Ts into the lines to avoid the additional restriction (as with the Con-Air), but it seems so simple using the existing 3/8” line.

    Here you can see two different electric carb heaters that put heat at the spot on the carb that I want to do so using coolant in place of where they use electrical resistance.

    This is supposed to be for Rotax:

    [url=http://]http://www.aircraftgraphix.com/jabiru%20carb%20heat.htm[/url]

    This says for Bing 64 but “does not fit Rotax”:

    [url=http://]https://www.motionaero.com/dual-heat.html[/url]

    These links show where I would apply coolant heat.

    Thoughts?

  • Re: Carb Icing - again still

    by » 10 years ago


    just a caution that the coolant pump cannot handle any back pressure. It is a simple displacement pump. We tried using a flow meter and the coolant almost stopped!

  • Re: Carb Icing - again still

    by » 10 years ago


    I wonder if someone needs to get word out to all the Rotax 912 owners with Thermo-Bob thermostats. The way Thermo-Bob works is that, when cold, it fully closes off the radiator and only allowing a bypass flow to continue through a 3/8 inch line. I haven't heard of any failures.

    A few "thermobob" results come up on a search here, but there is also a lot of chatter on other forums about this bypass thermostat.

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