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Old 19th Feb 2004, 02:02   #1 (permalink)
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Join Date: Nov 2003
Location: Brazil
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Rotax mixture control

I've been looking for an ultralight aircraft to buy, and I found a very odd (for me) feature in the Rotax 912 engines: they have no mixture control lever!

My question is: when ruducing power from takeoff to cruise, is there an automatic leaning? But, how agressive is that leaning? What is the "cruise" mixture automatically set by the carburator equivalent to? Best power, Best economy, or none of those?

In a regular carburated engine, like Archer's O-360, after setting cruise RPM, I had to lean the mixture to the point I wanted. I know that the carburator enriches the mixture automatically at takeoff, so that in other regimes, the rich mixture is not as rich as on takeoff power.

My conclusion is that this regular non-full power rich mixture is still much richer than the best power mixture in a regular carburated engine.

If the Rotax carburator's does just the same, and with the lack of the mixture lever, I will have no choice but to fly at a richer than necessary mixture in all situations but takeoff.

As in Brazil the Rotax 912 is not compatible with autogas (because of the 25% of ethanol added in it), I will have to burn only avgas. I think this would be very bad for the engine.

I also found the CHT's limits of the Rotax too low (300 F), compared to the 400F I'm used to use in my IO-520. Are the materials used in the Rotax somewhat inferior to those used in the regular general aviation engines?
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Old 19th Feb 2004, 04:43   #2 (permalink)
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Rotax 91x series engines have liquid cooled heads which is why the temperatures are lower.
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Old 19th Feb 2004, 15:01   #3 (permalink)
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For detailed information on the BING CD-Carburators installed on the Rotax read this datasheet.

I also think, that you run the Rotax richer than you would run a manually leaned engine in cruise, but as most pilots don“t lean, there is still an improvement.
The CD carburator has the advantage, that it automatically enriches when accelerating, this allows good throttle response although running quite lean at constant power.

As the rotax is a flat 4 with ignition (and therefor intake) order 1 - 4 - 2- 3, always two cylinders at the same intake manifold are filled directly after another in two half rounds of the crankshaft, and then no intake is done for another round. This makes the airflow through the carb pulsating much stronger than for a Conti or Lyco, where all four cylinders draw from the same carb, making airflow nearly constant. Additionally the length of the two intake manifolds for the aft cylinder filled first, and the front cylinder filled second in each cycle are different, this makes the mixture difference between the cylinders even larger than on the Conti/Lycos, and as you can tell if you have 4 EGT probes fittet, is already large for there ones.
Therefor the richest cylinder in the Rotax is running much richer, than the leanest one, requiring an overall richer mixture for smooth running.
Let“s hope Rotax finishes the six cylinder fast, and comes back to the fuel injection for the 912, they already were working on. A few years ago VGS-Motorsport and an italian ultralight manufacturer developed an electronic fuel injection for the 912 independantly from Rotax, and the improvement was enormous. Don“t know why it became no sucess, I believe Rotax doesn“t support the project and did not deliver engines without carbs.
The report is removed from the VGS-homepage now
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Old 19th Feb 2004, 16:28   #4 (permalink)
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The S versions of the 912, do have mixture control.

Depending on what aircraft the engine is fitted to, you can see consumption as low as 10 Lph on a cruise, but enough grunt on wide open throttle to climb at over 1500fpm.

With regards to longevity, all Rotax 912 engines have a TBO of 1500 hrs.
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Old 20th Feb 2004, 02:16   #5 (permalink)
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Cost of overhaul

What would be the cost of a Rotax 912 overhaul?

Is the water temperature equal to the CHT, or are the CHTs hotter than the water?

Anyone has experience with Rotax running continuously on AVGAS?


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Old 20th Feb 2004, 06:43   #6 (permalink)
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This gives a fair idea of Rotax 912 service costs and the effects of running on AVGAS.

For the benefit of non-Brits, it's about £1 = US$1.9 = Eu1.6 at the moment.

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Old 20th Feb 2004, 17:13   #7 (permalink)
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As delivered Rotax 912 and 912S don't have mixture control. The new ones do have a choke for starting rather than a primer.

The heads are water cooled and there are two (diametrically opposed ) CHT probe positions which enter the water jacket. One comes with probe in. My EGTs run about 40 to 60 deg apart but that could also be about leading and trailing exhaust positions as thay are in the airflow. That is pretty well the same amount as my friend's Cessna with injection so I don't think it is a function of carb/cyl. postion affecting mixture.

Yes, the manifolds are seperate (connected by balance tube app. 5mm dia.) and different length for front and back, (mine's backwards anyway) but that should not significantly affect MIXTURE. All the air passes over the same jets.

The mixture is self regulating depending on altitude, (some auto carbs used to do it too,) The only proper way to change the mixture for any given circumstances is to change the jets. Rotax recommend changing the jets for cold weather, ( ie for Winter here!) but most I know just adjust the idle up to about 1800 revs so it doesn't quit.

912 and 912S (and I think 914) are not supposed to need carb air warming. (According to Rotax that choice is down to aircraft manufacturer and they (Rotax) "are not responsible." ) However in some countries the regs say all planes must have carb air hot (Aus for one) and locals have mods to suit. The UK Rotax agents sell a throttle body warming jacket as an alternative. I have had my Rotax 912 quit on going to idle on landing on a 2deg C day with heavy humidity ( Low cloud lying in long strands down both sides of the valley and heavy condensation running like rain off all the metal rooves.) After being allowed to stand for a couple of minutes she ran fine. Experiment repeated twice with same result. I suspect there was icing enough to shut off idle air with butterfly closed but not to clog carb when butterfly open.

Most 912 and 912S run fine and have good reputation but there are a couple of things. 912S not certified and supplied after prop clutch was discontinued can be subject to gearbox vibration at about 70 hrs. The fix is an expensive installation of the clutch. You pay.

There are some advisories out about rockers and cracks in crankcases but they are "inspect, " not mandatory replacements.

I think some serial numbers had some component replacement ADs "at next inspection but I can't remember exactly what. Rotax have a fairly good reputation but their behaviour over the gearboxes has got a few people riled up. I was delivered my engine after the defect ( which they claim happens on only certain airplanes but rumours said up to a third of engines) was recognised but they declined to do anything about it. Go fly it and if it occurs you can pay for the repair. (Approx $700 US for the parts I think,) I was locked in as my plane is only designed to fly with Rotax. Personally I find that a disgusting way of doing business. Imagine a car manufacturer saying " We'll go on selling this car with a gearbox that has a very high chance of failing at 70 hrs and if it does you can pay for an upgrade to repair it."

One owner persuaded the local agents to install the pre- modification set up as a test fix for the problem and has done a good number of hours since without trouble but I don't know if any agents will offer it.

There have been some issues with cold starting kick back but Rotax have upgraged the starter and local agents offered a decent deal for a while, (Well I suppose $200 for an avaition starter, even a very small one, isn't bad.)

Some planes with exhaust above the engine get through exhausts at a famous rate but I think regular 'under' exhausts are fine.

There is a group site for Rotax flyers but it needs upgrading, It includes all the two stroke types. I'll try and get a web site set up as a front end in the next few weeks (Kids get back from Uni!) and hopefully it'll get a few more members. You can find it at Yahoo Groups under "Rotafly" Membership is open and free.

Sorry, More.

Rotax recommend not using Avgas with more than 10% Ethanol or alcahol additives, danger of seals being damaged. Better is Ethanol free. Shell do that here, local gas co offices will give you the breakdown.

Rotax recommend not more than 1/3rd use of Avgas (100LL) or not more than 1 fill in three. Avgas deposits carbon on important surfaces. They also recommend the use of a combined synthetic/mineral oil if you do use avgas. Pure sythetics don't mop up the carbon so effectively. Rotax engines are geared and motorbike type oils with shear additives are recommended though one local operator here (flying school) has had best results with Castrol Syntec.
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Old 20th Feb 2004, 17:36   #8 (permalink)
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Spec of Pegasus Quik, fitted with 912S.


I have flown one of these and can assure you that it does have mixture control.
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Old 20th Feb 2004, 18:46   #9 (permalink)

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Taking the thread (only slightly) off topic... the Rotax 914 also doesn't have a mixture control. The 914 is (AIUI) basically the same engine as the 912, but turbo-charged (with the turbo-charger being pretty much totally automatically controlled).

I had always assumed that the reason for the absense of a mixture control on the 914 was simply because the turbo-charger prevents the manifold pressure from dropping with altutide, and therefore leaning is not required. I'm slightly surprised to hear that the 912 doesn't have a mixture control either, since that dis-proves my theory.

Sorry for the thread diversion!

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Old 20th Feb 2004, 19:48   #10 (permalink)
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Location: Brazil
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Rotax's carburator

What's the difference beetween Rotax's carburator and the regular Lycoming's one?

That's just the altitude compensating feature?

I think mixture management is crucial in any operation. It's not so hard to lear how to manage the red knob. That's why most pilots are afraid of touching it: they have never been taught to!

Simply compensating altitude is not enough - by leaning the mixture during climb, I can get back a lot of the lost power. What about high elevation fields?!

Anyway, in ultralights we have no choice but the Rotax. I hope the overall engine performs well and shows good reliability. That's enough for a 80-100HP engine.

My main concern is about AVGAS. I just read that the manufacturer recomends to cut in half all service intervals for engines running on AVGAS. I think most of that problem could be avoided with a simple mixture control. The main reason for a stupidity like this may be the 2 carburator configuration in the Rotax (to make it more compact). It would require two mixture levers, probably.
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Old 20th Feb 2004, 22:09   #11 (permalink)
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I know of at least one 912 equiped Katana being routinely operated on 100LL and would imagine most of the others used for training would be too....

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Old 20th Feb 2004, 22:29   #12 (permalink)
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Our club Katana DV20 just had its 600h without any complaints on the engine, and we run only on 100LL.
Old 21st Feb 2004, 01:53   #13 (permalink)
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Looks like a lot of engine for a Pegasus. How does the mixture control work 'cos there is no where for it to connect on my curburettors or any of the other 912's I've seen.

I have put 100LL in mine when gassing up at a dock. Runs just fine but I don't do it unless I have too. It's a bit annoying too because my home airfiled has 100LL but I have to cart auto gas in cans.

I can't comment on some people using 100LL full time. Maybe as a manufacturer Katana can make, or have made, mods which are not available to the rest of us.

Since I rely on my engine in mountainous country I follow the recommendations of the manufacturer. I might suggest that a recommendation from the manufacturer that service times be halved if using 100LL full time is a fair indication.

As to whether the mixture control should be manual or auto, is there any evidence that manual or auto is more efficient? Rotax have chosen to go with self compensating, whatever my views about their other attributes, as long as it works, and it does seem to, that is their choice.

Last edited by ChrisVJ; 21st Feb 2004 at 02:09.
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