Rotax 912S/ULS carb heat control

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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AviatorCrafty
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Rotax 912S/ULS carb heat control

Postby AviatorCrafty » Thu Jun 25, 2020 5:01 pm

Tomorrow I'll be checking out the second of two P2002s that my school has. It has some really cool avionics and is barely ever rented (N535TA is the tail number if you're curious). I've already flown one of two and that one, like the P92s I've flown, have the 912ULS and no carb heat control, however from the few glances I've had with this one it has an actual carb heat control, like the Cessna 162 I trained in. Why do some Rotax's have carb heat controls while others don't? Should I use it like normal carb heat or only when I suspect icing?
Nick Schenkelberg
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Re: Rotax 912S/ULS carb heat control

Postby Warmi » Thu Jun 25, 2020 5:04 pm

AviatorCrafty wrote:Tomorrow I'll be checking out the second of two P2002s that my school has. It has some really cool avionics and is barely ever rented (N535TA is the tail number if you're curious). I've already flown one of two and that one, like the P92s I've flown, have the 912ULS and no carb heat control, however from the few glances I've had with this one it has an actual carb heat control, like the Cessna 162 I trained in. Why do some Rotax's have carb heat controls while others don't? Should I use it like normal carb heat or only when I suspect icing?


I don’t have any carb heat control on my Sting - the plane comes with what the call a “ full time carb heat ” ...
Flying Sting S4 ( N184WA ) out of Illinois

chicagorandy
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Re: Rotax 912S/ULS carb heat control

Postby chicagorandy » Fri Jun 26, 2020 5:26 am

So, to the uninformed (namely me-lol) , it leads to the questions:

#1 Why carb heat in some planes must be switched on and off? and
B - Is there any 'harm' in just leaving it on all the time?
"Don't believe everything you read on the internet" - Abraham Lincoln

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Re: Rotax 912S/ULS carb heat control

Postby drseti » Fri Jun 26, 2020 6:57 am

chicagorandy wrote:Is there any 'harm' in just leaving it on all the time?


To understand the answer to that question, you have to consider what happens during a runup when you apply carb heat (on a plane that has that control). Notice that the engine slows down - you can hear it, and see it on the tachometer. Why? Because internal combustion engines breathe air. Warm air is less dense than cold air (basic chemistry). So, carb heat robs the engine of power. And it slows down. So, if you had carb heat on all the time, the engine would run at reduced power - all the time.

If this is true, then why would you ever use carb heat? The answer has to do with what air is made of: mostly nitrogen, some oxygen, a bit if carbon dioxide, and various trace elements. One of those trace elements is water vapor. Now, under the right (actually the wrong) conditions, that water vapor can freeze inside your carbs, and cut off or severely restrict the flow of the fuel-air mixture. When this happens, the engine can stumble, or even quit.

The solution? Use carb heat to melt the ice. Now, if you have carb heat on all the time, you won't have carb ice - but you'll also not have full engine power.

When do you need maximum engine power? Why, on takeoff at high density altitude. High density altitude results not just from high field elevation, but also on hot, muggy summer days - the very conditions most conducive to the formation of carb ice. * So, you are unfortunately most likely to need carb heat under the same conditions as those for which you need maximum power!

Here's where pilots get into trouble. Your engine starts to run rough. You lose power, and feel vibration. You figure maybe you're getting carb ice, so you pull on carb heat. Immediately, you lose more power, and get more vibration. Figuring you must have done the wrong thing, you push the carb heat knob back in - and then the engine quits! So, remember, if you have carb ice and have to use carb heat, do so. Put in full carb heat, increase throttle if you can, and realize things will have to get worse before they get better.

In a separate post, I'll discuss why not all engines have carb heat controls, and why the Rotax 912 ULS in particular is relatively immune to carb ice. But I gotta go fly now.

* the reason carb ice forms on hot, humid days and not on cold winter days is that the warmer the air, the more water vapor it can hold. Counterintuitive, I know, but carb ice is largely a summer, not a winter, phenomenon.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
fly@AvSport.org
AvSport.org
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Re: Rotax 912S/ULS carb heat control

Postby drseti » Fri Jun 26, 2020 2:17 pm

chicagorandy wrote:So, to the uninformed (namely me-lol) , it leads to the questions:

#1 Why carb heat in some planes must be switched on and off? and
B - Is there any 'harm' in just leaving it on all the time?


Randy, while answering your question with my earlier post (above), I got to realizing that you're not the only one unsure about the hows and whys of carb heat. So, I was inspired to schedule a future EAA Webinar on the subject. It's not going to air for quite a while (my queue is rather long already), but I invite all forum members to register next year at http://EAA.org/webinars, and tune in on 12 May 2021. Here are the title and summary:

Avoiding Carburetor Icing - a cool pilot's guide to carb heat

Carburetor icing can rob your engine of power, and if not corrected, will quickly turn your airplane into a glider. In this FAA Safety Team WINGS an AMT award presentation, Prof. Shuch tells you everything you always wanted to know about carb heat, but were too cool to ask.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
fly@AvSport.org
AvSport.org
facebook.com/SportFlying
SportPilotExaminer.US

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Re: Rotax 912S/ULS carb heat control

Postby drseti » Fri Jun 26, 2020 2:51 pm

chicagorandy wrote: Why carb heat in some planes must be switched on and off?


I already explained in an earlier post why you really don't want to keep carb heat on all the time. But, not all planes even have a carb heat control - and among those that do, there are different requirements for when and how you use it.

For example, some engines are particularly vulnerable to carb ice. A notorious case in point are the small-bore Continental engines with Marvel Schebler carbs used in many antique aircraft (including the legendary J3 Cub built right here on my airport in Lock Haven PA). In those installations, the carb attaches to the bottom of the engine, and is hanging right out there in the airflow. Those planes tend to be landed with power all the way at idle - which basically closes off a butterfly valve in the carb throat, maximizing pressure differential in the venturi, which then makes the carb extremely sensitive to blockage from ice crystals. So, we are taught that, upon power reduction on downwind, we pull on full carb heat for every landing. This certainly protects against carb ice, but in the event of a go-around, carb heat will prevent us from getting full power (as mentioned in my earlier post), so climb is anemic. For that reason, one has to remember to set full carb heat on downwind, and then turn carb heat completely off on short final (in case it becomes necessary to convert the landing to a takeoff).

Other engines are relatively immune to carb ice. A good example here is the Rotax 912 ULS that powers many of the LSAs we all fly. So, unlike the 65 HP Continental in the Cub, we don't automatically pull carb heat on downwind. We don't have to. This is partly because the carbs in a 912 sit on top of the number 2 and 4 cylinders, where they are always subject to a bit of residual engine heat. (Yes, that does indeed slightly reduce the maximum power available from this particular engine.)

But, there's another reason why the Bing 64 carbs used on the 912 ULS rarely ice up. That is because there are two of them (the left hand driving cylinders 2 and 4 through a 2-port intake manifold, and the right hand carb similarly driving cylinders 1 and 3). Now, the odds of both carbs icing up at exactly the same rate are pretty close to zero. So, if the aircraft is being flown in conditions conducive to carb icing. one carb will tend to ice up first. Now the engine doesn't quit outright, because it will still run (sort of) on just two cylinders. Yes, it will lose power, RPM will drop, and it will shake your teeth loose, but it doesn't quit. And, all that vibration gives you lots of warning that you're picking up carb ice (you'd have to be pretty inattentive to miss it). So, as soon as the plane starts shaking, you just pull on full carb heat, push the throttle, and wait for the ice to melt. When the engine smooths out, you can bring back the power, turn off the carb heat, and go on your merry way.

Now, if you happen to be flying a Rotax 912 iS engine (or the new 915), you don't ever have to pull on carb heat. In fact, you can't, because you don't have a carb heat knob. Why not? Because these are fuel injected engines -- they don't have carbs! Probably one of the best reasons to prefer fuel injection over carbs.

One final Rotax note - the turbocharged 914 is still a carbutreted engine -- so of course it can be equipped with carb heat. You'll still very seldom have to use it, thought, because - well, two carburetors, mounted above the back jugs.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
fly@AvSport.org
AvSport.org
facebook.com/SportFlying
SportPilotExaminer.US

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Re: Rotax 912S/ULS carb heat control

Postby fatsportpilot » Fri Jun 26, 2020 5:07 pm

drseti wrote:But, there's another reason why the Bing 64 carbs used on the 912 ULS rarely ice up. That is because there are two of them (the left hand driving cylinders 2 and 4 through a 2-port intake manifold, and the right hand carb similarly driving cylinders 1 and 3). Now, the odds of both carbs icing up at exactly the same rate are pretty close to zero. So, if the aircraft is being flown in conditions conducive to carb icing. one carb will tend to ice up first. Now the engine doesn't quit outright, because it will still run (sort of) on just two cylinders. Yes, it will lose power, RPM will drop, and it will shake your teeth loose, but it doesn't quit. And, all that vibration gives you lots of warning that you're picking up carb ice (you'd have to be pretty inattentive to miss it). So, as soon as the plane starts shaking, you just pull on full carb heat, push the throttle, and wait for the ice to melt. When the engine smooths out, you can bring back the power, turn off the carb heat, and go on your merry way.

I don't think the Rotax 912ULS can run AT ALL if one carb fails. It can run just barely with 3 cylinders but not at all with 2. It will just stop.

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Re: Rotax 912S/ULS carb heat control

Postby drseti » Fri Jun 26, 2020 5:47 pm

fatsportpilot wrote:I don't think the Rotax 912ULS can run AT ALL if one carb fails.


True. But we're not talking here about a carb actually failing. I was emphasizing that, at the first hint of ice in one carb, the engine will become so unbalanced that it will start to run rough. The vibration will get your attention long before the engine actually quits, giving you ample warning to pull on carb heat well before an actual problem develops.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
fly@AvSport.org
AvSport.org
facebook.com/SportFlying
SportPilotExaminer.US

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Re: Rotax 912S/ULS carb heat control

Postby chicagorandy » Fri Jun 26, 2020 6:35 pm

Thanks for the concise explanations - much appreciated
"Don't believe everything you read on the internet" - Abraham Lincoln

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Re: Rotax 912S/ULS carb heat control

Postby 3Dreaming » Fri Jun 26, 2020 11:45 pm

Paul, the Stromberg NA-S3 carburetor is far more common than the Marvel Schebler for the A-65 in the J3 Cub. It to is susceptible to carburetor icing.

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Re: Rotax 912S/ULS carb heat control

Postby drseti » Sat Jun 27, 2020 6:33 am

Thanks for that info, Tom. I'll incorporate it into my webinar.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
fly@AvSport.org
AvSport.org
facebook.com/SportFlying
SportPilotExaminer.US

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Re: Rotax 912S/ULS carb heat control

Postby 3Dreaming » Sat Jun 27, 2020 8:24 am

As a side note the Stromberg carburetor typically doesn't have a mixture control, and when they do it is different than the mixture control of the Marvel Schebler. The Marvel has a mixture control like most that have been in aviation a while is used to, it actually uses a valve to shut off the fuel to the main jet. This is called a mixture with an idle cut off. The Stromberg mixture controls the venting of the float chamber. By restricting the vent you can restrict the flow of fuel. It will not typically cause the engine to die when placed in the full lean position. It is of marginal help when cruising at altitude.


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