Sport Pilot Talk

The Tecnam Bravos I'm learning in do not have carb heat installed, and I noticed in the POH that carb heat is an option. Is there a reason the Rotax 912 engines don't need it?

Have you tried the Rotax owners forum?

http://rotax-owner.com/en/

Many like the RV-12 have the air filters attached directly to the carbs. In that configuration the engine is constantly getting warm cowl air...essentially it has carb heat all the time.

In my CTSW, the carb heat is a flapper that opens the intake tract to warm, under cowl air, accomplishing the same thing, but only when activated. I think either is fine, but if there was an intake that took in cold outside air and no carb heat control, I'd want to know why...these engines can ice.

Cluemeister wrote:Have you tried the Rotax owners forum?

http://rotax-owner.com/en/

Thanks. Did a little searching there but didn't learn much. There are a couple of ways to install carb heat, like electric or by running engine coolant through, but I didn't find any great explanations for why it's not needed.

I guess since the air intakes are under the cowl, the air is pretty hot already, which makes icing unlikely.

Unlikely is good, but impossible would be better. It seems that if icing does occur, there won't be much I can do about it.

MrMorden wrote:Many like the RV-12 have the air filters attached directly to the carbs. In that configuration the engine is constantly getting warm cowl air...essentially it has carb heat all the time.

Yeah, I guess that's the case. Air filters are cones (look like K&Ns) attached to the carbs under the cowling, so the air should be rather warm.

Although the 912 is resistant to carb icing, no engine is immune - so having carb heat is a Good Idea, even if you never use it. The carbs sit on top of the jugs (unlike a typical Lycoming or Continental installation, where the carb hangs below the engine). Since the jugs get warm, there is always essentially some heat to the 912 carbs. Doesn't mean they can't ice up, but it helps.

If you check the POH of a Cessna or Piper product (except for the PiperSport), you'll see you are advised to pull on full carb heat when downwind, every time. Most Rotax powered LSAs don't say this, because (1) the carbs are in warm air to begin with, and (2) there are two of them, so the chance of both icing up at the same time is rather slim (in fact, if one starts to ice up, the engine runs rough, on just two cylinders -- giving you lots of vibration and PLENTY of warning to pull on carb heat).

That said, if you don't have carb heat to pull on, then in the unlikely event of carb ice, you could be flying a glider. At the first hint of engine roughness, if you don't have carb heat, advance the throttle to heat up the engine environment -- carb ice is most persistent at or near idle.

Another thing to remember is that carb heat essentially routes warm air (typically from around the exhaust system) directly into the air box. Since these are air-breathing engines, when you pull on carb heat, you lose power (this because warm air is less dense than cold air -- less air means less oxygen, which means less fuel/air mixture to burn, which means RPM goes down). This has gotten more than one pilot in trouble. Consider that you're getting engine roughness and losing RPM. So you pull on carb heat, which makes the engine run even rougher and RPM to drop even more! So, you figure "OMG, that was the wrong thing to do," and push carb heat back in. Engine speeds back up for a few seconds, before it quits completely...

If you do use carb heat, then, please remember that things have to get worse before they can get better.

Well, these planes aren't equipped with carb heat, so applying it isn't an option. I assume that's one reason for using partial power during landing. My CFI tells me not to go to idle until landing is assured. I'm going down to about 3000 or 3200 rpm when I'm abeam the landing point, so I guess that preserves some engine heat.

Of course, at this time of year in FL, air temps outside the cowling are running over 95F, so no immediate worries, but that won't always be the case.

There is really very little correlation between outside air temperature and carb ice. In fact, you are probably more prone to carb ice on a hot summer day than on a cold winter day. That's because it is moisture content that dominates carb ice formation. On a hot, muggy summer air there's more water vapor to condense and then freeze in your carbs than there is on a cold, dry winter day.

Formation of ice is caused by the venturi effect. A carb condenses the fuel/air mixture when atomizing the fuel, and then expands it again. Expanding gasses absorb heat, so 95 degree summer air can, if humidity is high enough, quickly chill the water vapor to freezing. That's essentially how a refrigerator works.

Yes, I understand that, but I didn't think the Venturi effect could produce enough delta T to induce freezing when the inlet air was above about 75 or 80. How much temperature drop is there? I thought it was around 40 degrees.

Half Fast wrote:Yes, I understand that, but I didn't think the Venturi effect could produce enough delta T to induce freezing when the inlet air was above about 75 or 80. How much temperature drop is there? I thought it was around 40 degrees.

I think it's about 60° F, meaning one can get carb ice at 90° F OAT.

Why don't you Google it quickly and let us know what you find?

FastEddieB wrote:
Why don't you Google it quickly and let us know what you find?

Hmmmm...... Interesting. See chart below (anyone know how to re-size this thing?). When OAT is above 95F or so, the probability of icing is low, but the probability goes to zero at higher humidity. The probability at 90F is zero for relative humidity greater than 60%. At near 100% humidity, carb ice won't form at temps above about 83F.

At >90F and dewpoint >75F, there doesn't seem to be much chance of carb ice.

It seems to me, though, that it should also be a function of the actual carb Venturi design and how much pressure differential (and thus temperature change) is produced by that particular carb.

Same information, presented in a slightly different format:

http://avsport.org/docs/carb_icing.pdf

Half Fast wrote:

MrMorden wrote:Many like the RV-12 have the air filters attached directly to the carbs. In that configuration the engine is constantly getting warm cowl air...essentially it has carb heat all the time.

Yeah, I guess that's the case. Air filters are cones (look like K&Ns) attached to the carbs under the cowling, so the air should be rather warm.

Yup, exactly like the RV-12. Basically "all carb heat, all the time". Plus the air filters are right over the exhaust pipes, so plenty of convective warming. I don't think you'd ever get carb ice with that setup in anything resembling normal conditions.

I was skeptical of this setup at first, it seems like it would lose some power. But my buddy's RV-12 makes very similar RPM and TAS numbers to my CT with the fancy flapper carb heat thingie, so now I'd say simpler is better!

The most common Rotax 912 carb ice scenario, from what I have read, is about 60-65°F OAT and higher than average humidity.

Best policy, if the engine is running normally and gets suddenly boggy, rough, or loses RPM, add the heat and keep it in! Usually if I have feelings that "these are good carb ice conditions" I will just put in the carb heat and leave it on until landing. In my airplane I don't even see a noticeable RPM drop using it, so why not?

MrMorden wrote:The most common Rotax 912 carb ice scenario, from what I have read, is about 60-65°F OAT and higher than average humidity.

Best policy, if the engine is running normally and gets suddenly boggy, rough, or loses RPM, add the heat and keep it in! Usually if I have feelings that "these are good carb ice conditions" I will just put in the carb heat and leave it on until landing. In my airplane I don't even see a noticeable RPM drop using it, so why not?

I was taught temps in the mid 50's, and visible moisture.