carburetor heat recommendation for 912 ULS?

[hallschuch]

Does anyone have any recommendations for adding carb heat to a 912 ULS? thanks in advance

[jiott]

If you really think you need it, I think the best and easiest to install is the coolant collar kit that installs on the carb inlets using hot engine coolant to heat the carb throats. It is ON all the time heating only the carb throat, not the air so there is no loss of power. Sold by most of the Rotax parts dealers, ACS, Lockwood, CPS, etc.

[Dave S]

In my opinion, Jim is spot on.

There is probably a good reason why Rotax providers use that system.

I am a bit hesitant about the original carb heat on the IV . Aside from unfiltered air, the fabricated heat muff is potentially a source of loose parts/rivets/etc that could potentially mess up the carbs operation or get sucked onto the the engine due to the design/construction of that particular part - you want to inspect it frequently enough to assure that vibration has not worn and dislodged the spacers/rivets between the outer tube and the exhaust pipe.

[hallschuch]

thanks both of you - Jim, you said "if I think I need it"... so I'm new to the kitfox world and my build is almost complete, but is it the case that many do NOT install carb heat? , I ask because all the other small craft I've flown have had it. I'd rather not install it if its not necessary...?

[Dave S]

HC,

I don't know how many do vs how may don't; however, I suspect that a lot of 912 operators do not have carb heat.

I studied the issue quite a bit when our kitfox was built and decided against it. I don't know that the 912 is immune to carb ice, but it certainly is resistant to the problem. A rotax is designed differently compared to traditional aircraft carbureted gasoline engines. A C150 with a continental will find 12 molecules of water within a 5 KM radius, put it in the carb and make ice; but, the carb is under the engine and completely insulated from the engine heat, except for the carb heat. A 912 has the carbs on top and a certain amount of heat will be conducted and radiated upward - then you have two carbs so the temp drop due to fuel vaporization is split between two places with each carb evaporating only half of the fuel and consequently each carb makes only half of the "cold" necessary to make ice. The 912 engine has variable venturi carbs rather than a butterfly system which behaves a bit differently too.

At any rate, over the time we have flown our kitfox, never a hint of carb ice, admittedly it is not flown through clouds with lots of moisture - but I think each builder needs to figure it out for themselves.

It would be nice if there was some sort of survey to find out how prevalent the use of carb heat is with the 912 carbureted engines.

[jrevens]

Drawing air from the warmer rear, upper area of the engine compartment, as opposed to your average Lycoming or Continental that draws air directly from outside (when carb heat is "off") is a big difference in my mind. I'd like to instrument that area with a thermocouple some day and measure what the temperature is compared to the outside air temp. It would be useful to do that several different times under various ambient conditions. For the record, I don't have carb heat for my 912 ULS either.

[HighWing]

Much of my flying has been alongside a bunch of Model IVs. My kit came with the carb heat box and like the vast majority of others the carb heat box eventually went into the recycle bin. Lots of talk about in the early e-mail lists and experience resulted in very few installations and virtually no reports if issues. Of course, these were mostly the 912 UL version.

[jrevens]

Sturdee,

So, you are drawing air directly from outside the engine compartment when not using carb heat. It is a very different situation when the air is drawn from the relatively warmer top rear area of the engine compartment.

[Dorsal]

As John mentions, for many "standard installation" is filters directly on the back of the carbs or on an airbox located inside the top rear of the engine compartment. I believe this is a bit like running medium carb-heat all the time. I have not heard of anyone having carb-ice with this setup. If I were drawing outside air directly to the carbs I would include a heat option.

[sturdee]

Sturdee,

So, you are drawing air directly from outside the engine compartment when not using carb heat. It is a very different situation when the air is drawn from the relatively warmer top rear area of the engine compartment.

Morning jrevens,
the air box and filter are inside the top cowl as per the build Manuel Mk4 912 1993.
Believe me ice is forming inside the carb. Have had manifold drop due too it before carb temp probe installed.
the amount of air flowing through the round cowl in principal you would expect to be warmer after flowing over the engine however someone should measure that. As we know the carb temperature will be colder due to the mechanics of fuel vaporizing.
I think most of the airflow travels down and out the bottom cowl as there is no baffles inside the cowl or need for them as water cooled heads, only air cooled barrels.
I was sceptical until I had seen the probe temps so believe carb heat is nessecary especially in this climate.
I can select on or off. Stay safe
Sturdee