Rotax leading

[eidolon45]

On a previous topic (X-air for training), Dr. Seti reminded me that I can fly my Tecnam at a reduced power setting if I want to "enjoy the scenery" on the way to the next training session (or at least find some useful landmarks before I blow by them!). I had to reflect on why we all seem to fly so fast in this neighborhood (except the Cessna guys). It occurred to me that some of our instructors I have flown with are convinced that running the R-912 at less than 5000 RPM on 100LL hastens lead fouling substantially. I haven't read anything to confirm that yet, but am wondering if anyone has any insight on this topic? Of course, we don't do pattern work at that power setting, but it seems to be the norm to be at 4800+ once we reach cruise. (Yes, we do use the lead abatement additive, in case anyone is wondering.)

[rfane]

Rotax recommends minimum cruise rpm of 4.900+ to eliminate vibrational issues. With 100LL you should be at 5,200+ for the lead fouling issue, ideally 5,300. Decalin is a must if you are using leaded fuel.

[eidolon45]

Ok, so much for "leisurely" cruising. At 5300 RPM, Tecnams are zipping along at least 113 Knots - more if the DA is low!

[eidolon45]

Ok, so much for "leisurely" cruising. At 5300 RPM, Tecnams are zipping along at least 113 Knots - more if the DA is low!

[rfane]

This doesn't mean that you have to run those RPM's 100% of the time. Slow down as needed for the situation, but don't run it slow as a normal thing.

[drseti]

Rotax recommends minimum cruise rpm of 4.900+ to eliminate vibrational issues.

I find that most vibrational issues are related to carb synchronization. If the carbs are well synched, you can get away with economy cruise for prolonged periods. Best way to tell if your carbs are out of synch: pinch off the rubber part of the compensating tube between the two intake manifolds. If RPM doesn't change, you're good to go.

Decalin is a must if you are using leaded fuel.

Absolutely! Actually, you have two choices: Decalin Run-Up, and Alcor TCP. I prefer the former, because the latter is corrosive, hence can't be carried in the plane.

[roger lee]

The Rotax 912 was designed to run between 4800 -5200 rpm for your normal cruise with 5000 rpm at the max torque. If you attend enough Rotax schools that are taught by the right people you will hear this over and over since it seems to be a very common question. As rpm increase past 5000 rpm the torque begins to drop, but HP increases. It is recommended and taught in Rotax classes that if you can use at least 91 oct. or more auto fuel you are far a head of 100LL leading problems which are multi faceted. It is also taught to use more rpm during cruise if using 100LL to help keep down the leading. It will build up no matter what you do, but using the "Best Practice" techniques will help keep the leading down at a reduced rate. Using Decalin (a lead scavenger)(no more TCP, it caused problems) and higher rpms 5200-5300 rpm during cruise will help keep the leading rate down and blown out the exhaust. Running lower rpm's will cause more leading (4800 rpm) and unwanted vibration. You may not feel it, but it is there. A good cruising rpm is 5000-5200 for 91 oct. and 5200-5400 for 100LL use. These aren't numbers pulled out of the hat, but real numbers from 21 years of experience with fuels by Rotax. You are better off too setting up your engine to run at 5500 rpm WOT flat and level. Setting the 912 up to only achieve a WOT setting of 5300 or less is counter productive and several levels and actually harmful to the engine by overloading it and has caused stress cracks in crankcases. No one is saying you can't slow down or pull the throttle back during approach or for sight seeing because it is short term use, but your normal cruise rpm should be in the proper range. You won't see the problems now. It's like smoking, it won't kill you today or tomorrow and you'll feel fine, just give it enough time and it will cause problems in the long run. If you use 100LL all the time then you should not wait for a gearbox tear down at 1000 hrs. because at around 800 hrs. your clutch in the gearbox is so clogged with lead it most likely isn't working. I have seen what the leading does to the internals of the gearbox and engine. That is why I use only 91 oct. and 100LL with Decalin and higher rpms when I travel and can't get auto fuel.

The Rotax 912 is a very good engine until an owner gets hold of it. Half my business is fixing those owners issues and usually cost the owner a lot more money in the long run because they new more than Rotax, didn't follow the Rotax recommendations and poor maint. intervals. I love those guys. The other half are the ones that follow the Rotax recommendations and maint. intervals. I make a lot less money on them, they have less down time, inspections are easier and I have to replace or fix less parts. I may make less money on this owner, but these guys are a lot happier. I have been doing this for a few years now and when some things come into the shop you just have to shake your head and wonder what they were thinking. Most of the issues and myths surrounding the Rotax are in print some where and it is so easy to do the right thing. If all else fails follow the manuals and SB's.

I would recommend that everyone should take a Rotax class or two and not listen to their neighbor. Then they could be in the happy group and have more money to take the wife out more often.

R. Fane can tell you that there is are two forums that address these subjects on a regular basis with Rotax Techs. Okay Roger F. your up.


p.s.
Hi Eidolon45,

We have Tecnam's and other SLSA with Rotax at our field and they fly the above rpms all the time. I do their maint. and many others from the 5 surrounding states so I see a lot of Rotax 912's every year. They're all healthy because they follow very simple proven rules.

[drseti]

If you use 100LL all the time then you should not wait for a gearbox tear down at 1000 hrs. because at around 800 hrs. your clutch in the gearbox is so clogged with lead it most likely isn't working.

If memory serves, Roger, the Rotax SB that ups the 912S TBO to 2000 hours also requires that the gearbox pull be done every 600 hours, when operating with 100LL.

[zdc]

Roger, how does the Rotax compare to a conventional aircraft engine, like the O-200, in terms of maintenance cost, longevity, and the ability to tolerate user abuse. Does the use of 100 LL have as much adverse effect on an O-200 as a Rotax?

[roger lee]

Hi ZDC,

Typically leading is worse on air cooled engines and engines with more tolerance among parts. The Rotax will tolerate lead just like the 0-200 and both will accumulate lead. But after seeing 0-200's and Rotax 912's after 1200-1500 hrs. of time with 100LL the leader is the Rotax on leading issues by far and the cost of replacing parts trashed by 100LL. Both engines can run 91 oct. auto fuel and it is absolutely in your best interest to do so. Maint cost are more for each engine over time because of 100LL use too. Rotax wants 25 hr. oil changes with 100 LL, but will let the oil change be as high as 100 hrs. on auto fuel. I still change my oil at 50 hrs on auto fuel. The gearbox on a 912 will need more attention over time with 100LL use. Typically air cooled loose toleranced engines use oil where the Rotax 912 does not. There is a weight penalty to for the 0-200 over the Rotax of approximately 40 lbs. give or take depending on other engine components. The Rotax parts are more sturdy as the Rotax has armored valves and Gilnisil coated cylinders just to name two. The Rotax cylinders have the original factory hone marks after 1500 hrs. They will almost never wear out. You can't even hurt them with a regular honing stone. It takes a special honing stone to even touch them. Valves never need to be ground as they are too hard. I have friends with a Rotax 912 that have over 3000 hrs and have never touched them.
So I would say the very close tolerance water cooled Rotax could out live a much looser toleranced air cooled 0-200 depending on its care and feeding. Any one can screw up an engine by failing to educate themselves on their expensive engine.

As far as owner abuse coming in to play well that could be a very larger area to cover. You may see less abuse in the 0-200 because more of those are worked on by A&P's. Many of the Rotax 912's are on experimental's and not maintained as per the Rotax manual like an 0-200.
Now throw in the SLSA certified mechanic maintained Rotax engine and all that for the most part changes because they need an RLSM-A or an A&P to work on and inspect them. The problem has been though A&P's with little to no Rotax training making mistakes that cost big bucks in some cases. i.e. opening the oil system and not doing an oil purge and cooking a hydraulic lifter and bending a rod and or valve that doesn't work on air that the A&P introduced into the system, big money. A Rotax 912 isn't any harder to work on than an 0-200 it has just been that mechanics don't take the time to learn the differences, don't look at SB's, don't bother with obtaining the maint. manuals and don't care because they think it is just like the 0-200 or their motorcycle engine. A Lycoming and a Continental aren't the same engine why does a mechanic expect a Rotax to be any different.

To me the bottom line is all engines with a few exceptions are probably good until an owner or uneducated mechanic works on them and think they know more than the factory's proven "Best Practices" program that was developed over many years and hundreds of thousands of dollars.

So I won't bash an engine as being bad, just some are better suited for certain applications.
I do have my personal like for the Rotax 912 over the 0-200 and for my own reasons and any one here may absolutely like the 0-200 over the Rotax 912. I would just say get all the real and true facts before you think one or the other is better and the true facts don't always come from your neighbor or mechanic. If engines were so much different and terrible we wouldn't have Chevy's, Ford's, Honda, Harley, Cummings, ect...
They are just different and each person gets to make up his/her own mind for its intended use.

I hope this answer isn't too vague in some areas and too pointed in others. We all have biases.

[zdc]

Thanks for your thoughtful response. I I know Cessna did consider the Rotax for the C162. It would be interesting to know why they didn't select the Rotax.

[drseti]

I know Cessna did consider the Rotax for the C162. It would be interesting to know why they didn't select the Rotax.

My understanding is that Cessna's long-term plan is to use the Lycoming IO-233, when it becomes available in production quantities. That makes perfect sense, since Textron owns both Cessna and Lycoming My guess is that migrating from the O-200 to the IO-233 will be much easier than if they had started off with the Rotax.

If I were running the project, not only would I use the Lycoming engine, I'd also go with a Macauley prop (another company owned by Textron). It's all about vertical integration.

[zdc]

One LSA engine that seems to get left out when the discussion turns to powerplants is the Jabiru. I have no idea how well that engine has faired out in the field. That's the only airplane manufacturer that I know of that also makes its' own engine.

[ArionAv8or]

One LSA engine that seems to get left out when the discussion turns to powerplants is the Jabiru. I have no idea how well that engine has faired out in the field. That's the only airplane manufacturer that I know of that also makes its' own engine.

I personally love mine (Jabiru 3300) and it cruises at 120 Knots @ 2850 RPMS in my Lightning with ease. The EAB versions do 145 Knots and the engine is rated @ 3300 RPMS. I would never take anything away from any other engine manufacture out there, including the Rotax, it just boils down to personal preferences. I like the lower RPMS of the Jabby and the quiet smooth running.

[roger lee]

It's like ArionAv8or says they are just different.
Like Chevy's and Ford's. You tend to like one or the other, but they both get you where you're going. So long as the owner is happy who cares other than that.