In Heavy Maintenance class with Eric Tucker as instructor

[Anticept]

For those that don't know who he is, Eric Tucker is the founder of the Rotax Flying and Safety Club, which is the organization approved by Rotax for training technicians and service/repair centers (including overhaul) in the Americas. He often visit Austria for the latest on BRP Powertrain's upcoming changes, as well as works closely with the engineering and service teams at Rotax.

 

If you want the lowdown on any Rotax related questions, I will forward your questions to him for answers. Please note that there are some things he cannot answer due to restrictions put on him by Rotax that can only be answered for iRMTs (rotax is ULTRA PARANOID about the tort system, so they only want to share information with people who have been trained and are current).

 

Before anyone asks, I did inquire about the low RPM issue. First, you need to know that the overload clutch is NOT designed to absorb prop strikes, but as a side effect of the design, it does. What it is actually designed for was the 912S (ULS, and by extension, the 912iS) series engine harmonic issues. Because of the high compression in the cylinders, it was creating idle problems at low RPM, and creating engine harmonics at higher RPM. Rather than design the engine from the ground up, Rotax engineers first sought to see if they can eliminate the problems in the existing design. This is what the overload clutch does. When it was realized the overload clutch could protect the engine in the event of a prop strike, the idea stuck.

 

Part of the design was not just the addition of the clutch. It also changed the design of the dog gears. The dog gears have a ~30 degree slippage in newer produced engines (some other amount in older engines) to help cushion and absorb the pulsing at idle, and smooth out the idle run. Remember, this engine is designed for HIGH RPM operation for long periods of time, and to save costs and engineering, that's what it is centered around. The engine is not designed, nor as thoroughly tested, with low RPM.

 

So, what happens at low flight RPM? The firing impulses are spaced further apart, and the resulting pulsing resonates and excites the dogs and causes them to chatter. The dogs are the only thing transmitting the torsional motion from the prop gear to the prop shaft, the prop gear is freewheeling. Anyways, this chattering is amplified by the propeller, which is basically a giant flywheel on the front of the engine. These cause prop shaft harmonics which are further amplified by the large diameter of the prop shaft gear, and transmitted to the teeth of the gear, causing accelerated wear. Depending on the aircraft and prop combinations, some aircraft are more sensitive than others.

 

The 912 UL and 914 do not experience this problem as severely, because the dogs mesh more closely and they have a lower compression.

 

EDIT: Asked for more detail. The resonant frequency is due to the pulsing between the front and rear cylinders, and reverberates through the engine crankshaft too. So you have all of these resonating parts between 3,500 to 4,300ish. This isn't in the manuals because the expectation is that you will make use of the engine horsepower, and not oversize. The european line of thinking is to size engines for the application and use to the full extent, instead of the american way which is shove as much horsepower as possible under the engine and use a tiny portion of it.

[Jim Meade]

What RPM is considered low flight RPM for the 912ULS?

When the term low flight RPM is used, does it matter if the airplane is set up for WOT at something over 5500 as opposed to WOT at a lower speed, e.g. 5200 or even 5000? In other words, does it matter if the engine is running at a finer pitch versus a coarser pitch for the same RPM? It would seem to me that the amount of load on the prop would be different and thus the effect on the dog gears would be different. What I'm getting at is, is it hard on the engine if you are essentially idling it back as opposed to lugging it?

You might ask him if he'd like to comment on the effectiveness of various manufacturer's oil cooling systems. My reason for that question is since I have an experimental, I would toy with the idea of maintaining a better winter and cooler summer temperature versus what Flight Design has set up.

Ask him why Rotax has not gone with a carburetor that can be leaned, and what effects does introducing a leaning system on a Rotax ULS have.

I have a number of questions on appliances if he's interested in discussing alternators and such.

[Anticept]

What RPM is considered low flight RPM for the 912ULS?When the term low flight RPM is used, does it matter if the airplane is set up for WOT at something over 5500 as opposed to WOT at a lower speed, e.g. 5200 or even 5000? In other words, does it matter if the engine is running at a finer pitch versus a coarser pitch for the same RPM? It would seem to me that the amount of load on the prop would be different and thus the effect on the dog gears would be different. What I'm getting at is, is it hard on the engine if you are essentially idling it back as opposed to lugging it?You might ask him if he'd like to comment on the effectiveness of various manufacturer's oil cooling systems. My reason for that question is since I have an experimental, I would toy with the idea of maintaining a better winter and cooler summer temperature versus what Flight Design has set up.Ask him why Rotax has not gone with a carburetor that can be leaned, and what effects does introducing a leaning system on a Rotax ULS have.I have a number of questions on appliances if he's interested in discussing alternators and such.

From my notes:

 

4,300 to 4,500 RPM is the upper limit of "low RPM". Each engine has a personality, so it's not an exact figure. Rotax designed them to run at 5,000+ RPM, so they chose to focus on the upper RPM band to balance out the mass forces and harmonics.

 

Coarse pitching is not healthy on the engine, especially on engines dated older than 2006 (Rotax beefed up the crankcase in 06). Pre-2006 engines had a condition known as the "smiley face of death", where lugging the engine would crack the crankcase above the cylinders, and it tended to run between two of the case bolts. It looked like a smiley face when the engine ran and the case pressurized and pushed the crankshaft slung oil out. In today's engines, if you run very coarse, the two crankcase halves will vibrate around the bearings. This leads to a misalignment, and the crankshaft will gall the bearings until it becomes difficult to turn the engine. There isn't a timeframe for this as it varies from engine to engine, airframe to airframe, but rotax recieved numerous engines, old and new style, with the smileys and the galling & spalling occurring. I will ask what too coarse would be, but i think I recall 4,900 static WOT at sealevel being a good indicator, since that is where it would be at the highest load.

 

I will ask about oil systems, but I am pretty sure it will be "go with what is needed to cool the engine, but no so cold that the oil doesn't hit a minimum of 190 degrees". The 190 is so it drives moisture out.

 

The carbs lean themselves. It's called a Constant Depression carbeurator. The venturi inside of the carb adjusts on it's own via a plunger, and attached is a jet needle which is inserterted into the needle jet (two different things!) that adjusts with the plunger, changing the mixture. The venturi vacuum is what drives the diaphragm upwards, and as the vacuum decreases (higher altitude or less throttle) the spring will push the plunger down to compensate.

 

I will ask if he might have time to answer you directly. PM me a phone number?

[Ed Cesnalis]

The limitation of the carbs is at WOT (above 92%).  WOT is needed at high altitude (above 8,000') to realize all available power and in this condition no leaning is available.  Is there a reason we are forced to run full rich at WOT?

[Roger Lee]

Hi Ed,

 

You are right and If I was at a high elevation airport and flew high like you I would lean my carbs by adjusting the clip on the needle.

 

 

Looks like I can retire from the forum. We have new meat for heavy Maint.

10 more years and you can join the Ultra Paranoid restrictive info secret society. LOL

[Jim Meade]

Corey,

Is he giving any Rotax publication references for the RPM numbers he is giving out?

 

I must not have asked the carb question correctly. CT asked my question another way so I look forward to your comments. I understand how the carb works - that was not my question.

 

My oil question has to do with Rotax published numbers on max temps as meaasured at the sensor. What does he tell you about what Rotax says is max sustained and max peak oil temps? The question comes up on other and this forum from time to time, so I'm interested in what Rotax says about the high end. I want to know because I want to know if I should modify my operating procedures or the oil system. I am not really asking him to comment on airplane manufacturer oil systsems, although I would welcome it, I am prepared to take the responsibility for that. I was only using that approach as a way to get at what Rotax feels is adequate.

 

Is Tucker connecting the low RPM dog gear issue and the crankcase issue? In other words, he is saying that the pulses feed back through the reduction gear and damage the crankcase? Or are the "smiley face" and the dog gear question two separate issues?

[Jim Meade]

Looks like I can retire from the forum. We have new meat for heavy Maint.

10 more years and you can join the Ultra Paranoid restrictive info secret society. LOL

Roger,

I ask in real sadness why this should have to be? Corey reinforces your comment that Rotax is tort averse.

 

Is that the reason they don't publish data that they are willing to share within the priesthood? You know Rotax will throw you under the bus if there is a discrepancy between what you reported they said and what their lawyers are willing to admit. In other words, like in the spy stories, they will disavow all knowledge.

 

On the other hand, there is the question of whether their publications are simply poorly written? Is it the translation problem? Is it that one pub is supposed to support several different engines? How can that be countenanced? If Rotax were General Motors, they'd be ripped from stem to stern if this is their actual position.

 

As Rotax gets put in more and more airplanes, we see more and more people who ask questions from a basis of healthy skepticism and want and expect factual answers.

 

I'm planning to build another airplane to replace my Champ. Two years ago I'd have unhesitatingly said I'd use a Rotax. Last year, I might have said, well, probably. This year, I am actively looking for an engine with fewer problems that is better, more openly and I'll say more honestly supported by it's manufacturer. I'm not saying I will not use a Rotax, but in the meantime I am every day investigating other engines that will have fewer support problems. This gets back to the question of why Rotax will not publish numbers it's customers can rely on without being a member of the cognoscenti and why Rotax is so reclusive about it's support.

 

Corey's answers to my questions on where Rotax publishes the numbers is very important.

 

Of course, Roger, you know my questions are not directed at you; they are rhetorical. I don't expect you to have more answers than you've already given.

[Ed Cesnalis]

 If I was at a high elevation airport and flew high like you I would lean my carbs by adjusting the clip on the needle.

 

 

That would only result in additional leaning when the needle circuit has control.  Advance the throttle beyond 92% and the main jet takes control and you are then full rich.

[Roger Lee]

Hi Ed,

 

You are correct that at full or close to WOT the main jet is more in control, but since you live and fly so high you do use other lower throttle settings at times and a slightly leaner mixture could benefit you.

[Ed Cesnalis]

Hi Ed,

 

You are correct that at full or close to WOT the main jet is more in control, but since you live and fly so high you do use other lower throttle settings at times and a slightly leaner mixture could benefit you.

 

Rog,

 

Sounds as though you are assuming that when the needle circuit does lean it needs a bit more leaning at altitude?  That is possible but not a given.

 

Corey,

 

If you have a followup I would like to know why we are locked into full rich at high throttle settings.  What is the thinking behind that when cruising at service ceiling?  We are forced to waste fuel and forego power.  Could there be a leaning option just for the main jet only to be used at altitude?  Why did they set up the injected version this way, the computer could handle it.  Is the Rotax unlike a Lycoming where leaning at WOT increases RPM and power at high DA?

[Anticept]

I asked for more clarification.

 

What happens at low RPM is the action of the crankshaft reacting to the firing impulses, and the compression of the next cylinder to fire. This consistent pulsing causes forces to travel back and forth on the crankshaft, causing the resonation and exciting the crankshaft and the gears. The dogs also resonate at certain frequencies, and vibrations occur in the prop gear as a result as well.

 

As a note, approx 5,600 is another resonating rpm, and continuous operation at this RPM is not advised (thats why 5,500 is max continuous).

 

Everything in the engine has a resonant frequency. Engine manufacturers focus on specific operating ranges, rotax included, to elminate vibration problems. The counterweights, case design, etc all revolve around the engine power curve. Below approximately 75% power (~4,300 rpm) the curve drops off.

 

In addition, the european culture is different from America. Engines are sized so that they are appropriate to the application, whereas in America, we tend to shove as much hosepower as possible even though we don't need it. This may be the reason why these RPMs are not mentioned much in the manuals, as in Europe, they fly by the book at recommended speeds for efficiency by airframe manufacturers.

 

The Americas are an anamoly in more than one respect. We have different climates, different formulations in our coolants and fuels, different ways we operate our engines and airplanes. Because of this, a lot of what we discuss here in the Americas stems from our own treatment of our equipment. Some of the "hearsay" items are because the service centers, distributors, and field technicians provide feedback to each other and through RFSC, but each time information changes hands, a little bit is lost. Rotax has an official reporting document called a Service Information Report (S.I.R.). Rotax uses SIRs to evaluate and make adjustments in parts to their engines, but they rarely hear much from this side of the globe; the Americas are TERRIBLE at reporting, and it reflects in our procedures and processes. A large amount of what they do hear, is hearsay, and it makes it very difficult for them to track parts, trends, etc.

 

CharlieTango: Eric said if you want, you can adjust the jet needle and change the circlip from position three to position two, to try and help your power a little. HOWEVER, he warns that you are constraining yourself to your region. If you are going to land at lower altitude airports, you will need to change it back.

 

As for the carb mixture issue: rotax has the airbus philosophy, "Don't trust the pilots." Mostly a litigation thing). They chose automatic carbs because they do not want pilots managing the mixtures, they want to calibrate the carbs for the engine, and that's it, LEAVE IT ALONE. And in a lot of ways, they are right, people are toasting their engines because they are not following rotax procedures because they think they know better, and when their engine drops out after a couple hundred hours, they blame rotax for being a "shitty engine".

 

As for the rich WOT mixture. He said if you wanted to make adjustments, you would need a CO2 and Oxygen sensors, sensitive heat monitoring sensors for the valves, etc to calibrate. Rotax again, does not trust people in the field to do this correctly, and therefore will not support you in the endeavor. Eric did ask though, are you really getting as much power loss as you think? He wanted to know what you are basing it on, and is hoping it is not based on continentals and lycomings, since that is an entirely different engine construction . The 912 series is not designed for high altitude operations, and he said if you want to go high, then you should have gotten a 914. Yes I know, flight design doesn't ship with a 914 over here. Still, Eric is an engine person, and isn't concerned about airframe manufacturer choices. Infact, mentioning flight design makes him cringe a little, because he does not like flight design's installation of the rotax engine, how difficult they are to work on, and the fact that the fuel return line does not go to the wing tanks or a header tank in carbeurated engines.

 

Props: it's pitched too coarsely if it cannot reach 5,200 RPM in cruise. As for the smiley face, this is a result of lugging the engine and putting high stress on the case. It is not the same thing as resonating.

 

i will post more information shortly. We need to get back to work, lunch is over.

[S3flyer]

Corey -- interesting stuff.  thanks for posting.

[Ed Cesnalis]

CharlieTango: Eric said if you want, you can adjust the jet needle and change the circlip from position three to position two, to try and help your power a little. HOWEVER, he warns that you are constraining yourself to your region. If you are going to land at lower altitude airports, you will need to change it back.

 

As for the carb mixture issue: rotax has the airbus philosophy, "Don't trust the pilots to do it right." They chose automatic carbs because they do not want pilots managing the mixtures, they want to calibrate the carbs for the engine, and that's it, LEAVE IT ALONE. And in a lot of ways, they are right, people are toasting their engines because they are not following rotax procedures because they think they know better, and when their engine drops out after a couple hundred hours, they blame rotax for being a "shitty engine".

 

As for the rich WOT mixture. He said if you wanted to make adjustments, you would need a CO2 and Oxygen sensors, sensitive heat monitoring sensors for the valves, etc to calibrate. Rotax again, does not trust people in the field to do this correctly, and therefore will not support you in the endeavor. Eric did ask though, are you really getting as much power loss as you think? He wanted to know what you are basing it on, and is hoping it is not based on continentals and lycomings, since that is an entirely different engine construction . The 912 series is not designed for high altitude operations, and he said if you want to go high, then you should have gotten a 914. Yes I know, flight design doesn't ship with a 914 over here. Still, Eric is an engine person, and isn't concerned about airframe manufacturer choices. Infact, mentioning flight design makes him cringe a little, because he does not like flight design's installation of the rotax engine, how difficult they are to work on, and the fact that the fuel return line does not go to the wing tanks or a header tank in carbeurated engines.

 

 

Ah, another discrepancy between the guru and the docs.  Docs claim ~15,000 service ceiling but guru says its not for high altitude?  Sounds like its capable but not designed / tuned for altitude.

 

I have no idea how much power loss I realize that's why I asked the question.  Here a full rich take off in a plane powered by something like an O-360 will likely result in a crash, the difference is dramatic.  Is the Rotax different?  I have no idea because I have no way to lean for best power.  Best power is one side of the coin, the other being wasted fuel, probably a gallon / hour.

 

More than 95% of the piston powered aircraft that come up here depend on the pilots controlling the mixture.  Its interesting how the rare pilot flying a Rotax powered airplane can't be trusted.  If the issue was addressed at the main circuit only then the majority of pilots  could continue to fly allowing pressure differential to do all of their leaning and in rare cases where leaning at high throttle settings is called for a control for that only could be utilized.

 

I think we have proven that the 912 ULS is a strong performer at altitude, far better than my 0-360 did in my Skyhawk.  The only issue I see is a total lack of leaning at high power settings which becomes an issue at altitudes where all available power is needed or at least desirable.  The lacking feature means wasted fuel and power becoming unavailable where it is most needed.  Protection from dumb pilots should be provided with smart systems, in this case it is provided with a dumb system.

 

California has several 9k-14k mountain ranges that are in immediate proximity to sea level, most trips for me include sea level at one end and high altitude at the other.  I could cross at Mt Whitney and descend directly to Furnace Creek and land below sea level.  Changing needles would do very little because climbing and cruising are done WOT so it would only come into play on descents.

 

I wonder why this is an American issue?  They do have the Alps to deal with.

[Ed Cesnalis]

One follow-up thought:

 

We are denied WOT at best power because pilots can't be trusted, OK.  Why is 100burgers denied the same thing when he doesn't have carbs?  The programmer can't be trusted?

[FastEddieB]

Keep the info coming!

 

The one time I took my Sky Arrow to 13,500'* even at full throttle the engine certainly did not seem to me to be excessively rich.

 

In fact, it felt fine.

 

A "normal" plane with a Continental of Lycoming would definitely not like to be at full rich mixture at that altitude. One can feel the airplane laboring from the excessively rich mixture, something my plane did not exhibit.

 

Are we sure the ROTAX really goes to full rich at full throttle? I was imagining that lower ambient pressure in the float bowls might meter less fuel at altitude for the same volume of air, resulting in a leaner mixture.

 

Let me see if I can find a source for that.

 

*Still had my medical then and stayed for less than 30'.

[Ed Cesnalis]

It might be the same volume of air but at service ceiling O2 is reduced by ~45%.  Pressure differential moving the needle is how the carb adjusts mixture.  The needle circuit has control when the throttle is retarded and the main jet gets full control above something like 92% which means full rich.  Some do argue that it doesn't work as designed and at high altitude the needle still comes into play, I guess we would have to look at how the main takes full control at high throttle settings to see if that is possible.

 

I agree that it runs strong  and smooth WOT at altitude but unless you can lean it how can you know if the RPM would increase?

 

If you check my plugs they look good but prior to landing I descend with the throttle retarded and the needle jet leaning my mixture.  Do my descents clean up any sign of cruising full rich?

[ctfarmer]

Very interesting info Corey.

Notably pitching to coarsely if it can not reach 5200 WOT/cruise - this opens up the envelope for optimising cruise specific applications and improves confidence levels.

 

 

Philosophically, new meat, old meat, new ideas, old ideas, questions, questions, more questions, right answers, wrong answers - all elements necessary for progress.

[Anticept]

Hi All!

 

There are a couple things that I asked that have been cleared up too.

 

First, both the carbs and the injected engine actually do compensate WOT even at high altitude. In the carbeurated engine at lower altitude, differential pressure between the top and bottom of the diaphragm will move the piston upwards and open the venturi completely (thus pouring a lot of fuel out of the discharge nozzle as the needle, which is attached to the piston, retracts). At higher altitude, there may not be enough of a differential pressure to completely move the piston to the full open position, and therefore does not dump tons of fuel into the engine, even at WOT.

 

In the injected version, above 92%, an enrichment circuit kicks in to enrich the fuel a bit to prevent detonation. however, the programming on the iS engine still compensates for altitude so you don't flood the engine!

 

Also, the 912iS being a direct injected engine is a myth. The 912iS is a multi-port injected system, where the fuel is injected onto the intake valve. Rotax is not sure where the idea that it is direct injected is coming from, but if you search their manuals, there is no reference to direct injection. I am on my iPad right now, and the heavy maintenance manual for the injected engine is too big (crashes my browser), but if you look in the line maintenance manual, it shows in a couple of the photos where the injectors are going diagonally right into the intake.

 

As for manuals: feedback in the field is often faster than the publications for manuals. There are changes to the manuals coming down the line that we were made aware of that address some of the concerns and the "super secret society" stuff. The thing is, the mechanics are more in touch with rotax than pilots are. mechanics are also the first to find problems in an engine and establish trends. Mechanics also lean on each other a lot, talking amongst ourselves for advice, and "have you see this problem yet?" Pilots only fly the planes, and most don't concern themselves when something doesn't work, they just complain when something doesn't work (and about the bill when they get the plane back). That doesn't make us engineers, but a lot of us keep our eyes open and since it is our job to maintain the planes. That's why the "super secret society" exists, it's just a side effect of the nature of the business.

 

For oil related stuff: Eric said follow the manual. If the temps aren't higher than what the maintenance manuals call out, then you are fine. Just try to get between the max and 190 degrees in flight. Manufacturers are not always following rotax recommendations for cooling and he says "shame on them", so if you take the time to try and fix the cooling systems, he says your engine will love you for it.

[WmInce]

Thanks Corey.

[Ed Cesnalis]

At higher altitude, there may not be enough of a differential pressure to completely move the piston to the full open position, and therefore does not dump tons of fuel into the engine, even at WOT.

 

We have heard this before.  The Bing's possibly don't work as designed and they might be leaning when they aren't supposed to making everything work out.  When systems fail to work as designed it is usually a bug not a feature.

 

It sounds weak, if its true it would be nice to know.

 

 

On the iS this is how Rotax describes it: The Rotax 912 iS engine automatically improves its fuel efficiency by switching into a lean ECO mode once the throttle is pulled back after the aircraft’s start and climb phases.

 

Notice they don't include that there is full throttle leaning as well.

[Anticept]

Nor do they say it stays at 100% fuel flow at WOT for the injected model. It says it leans it back, but it doesn't say where it leans from. The enrichment only increases fuel flow for the purpose of preventing detonation, but it will compensate for altitude or it would flood the engine at high altitude and stop the engine.

 

This is similar in concept to the economizer valve on lycoming and continental engines with fuel servos. The fuel servos will compensate for lower atmospheric pressure, but the economizer acts as a fuel boost as the risk of detonation is high towards WOT. It will not, however, force full mixture, as I mistakenly stated in the other topic and then you pointed out I was being a fool

 

As for the bing not working as designed: i will ask about more information, but he did mention that at extreme altitude it doesn't work as well, but it should not cause the engine to stop or flood; it is still decently accurate.

[FastEddieB]

At higher altitude, there may not be enough of a differential pressure to completely move the piston to the full open position...

If that's true, then you do not really have full power - a partially closed slide is equivalent to just throttling back with a conventional carb. That would have to leave a lot of performance on the table.

[Anticept]

If that's true, then you do not really have full power - a partially closed slide is equivalent to just throttling back with a conventional carb. That would have to leave a lot of performance on the table.

i will ask about this.

[Ed Cesnalis]

As for the bing not working as designed: i will ask about more information, but he did mention that at extreme altitude it doesn't work as well, but it should not cause the engine to stop or flood; it is still decently accurate.

 

Scenario #1:  As designed, near full throttle and at WOT the main jet gets full control and it has 1 setting, full rich.  There is no mechanism for it to lean at all let alone be decently accurate.

 

Scenario #2:  At altitude pressure differential is insufficient and even at full throttle there is leaning due to the needle circuit remaining in control of the mixture.  As Eddie pointed out this is analogous to a retarded throttle.

 

In either scenario full sustained power is not available.

 

 

Its a good high altitude engine but without a critical feature.  (the ability to lean at high throttle settings)

 

... at extreme altitude it doesn't work as well ...

 

 

At extreme altitudes there is a conflict.  

 

The Bing is set up to lean when the throttle is retarded but at any extreme altitude (above 12,000'?) WOT is needed just to maintain altitude and cruise at 52% power.

 

You need both leaning and WOT at some point, for me its anything above 7,500' DA but the Bing's design requires a retarded throttle to get the leaning so needed.

[Anticept]

There is no mechanism linking the throttle to the jet. It is purely driven by differential pressure. We're done for the day, so I will ask tomorrow.