New respect for the CT

[FastEddieB]

You will see RPM restrictions in other engines with crankshaft driven propellers, so it's not just a gearbox issue.

 

My Tiger had a yellow arc (avoid continuous operation) on the tach with the prop I got it with (McCauley), but that went away with the installation of a Sensenich.

 

BTW, my McCauley lives on as a wall clock in my living room!

 

[Roger Lee]

"So, clearly, no one actually knows.  This is all based on the belief "

 

They do know it just isn't in writing. They have 40 million flight hours on the Rotax. They have seen the damage. I have tons of damaged engine pictures from one thing or another, but you'll never see anything in writing on the causes.

 

It just isn't in writing. Everything is not in a book for damage causing  problems. It isn't for us, it isn't for cars, motorcycles or boats.

The old gearbox's had a 15 degree dog set their new one has 30 degree. Pre 2003 gearboxes didn't have a clutch either except as an option.The low compression on a Cont., Lycoming  or even the Rotax 912UL doesn't pose the same issue as the 11:1 compression for the 912ULS. You guys need to compare apples to apples and other engines are oranges.

The other issue is vibration harmonics at low rpms on the system that a pilot will never feel.

Engineer's built the engine with a specific rpm range for running in mind. Running at lower rpms for short times like landing or sight seeing isn't an issue because of its short duration and varying throttle settings during such events and compared to the life of the engine.

A training engine that uses a lot of varying throttle is not at risk as much as the guy who constantly flies in the mid 4k's.

 

Just don't make a steady diet of it or for excessive times at low rpms. The same thing goes for the first start of the day at warm up. You should be above 2K rpm. Somewhere between 2000-2300 is a good place. Below that in a ULS engine causes problems and even then when sitting waiting to take off for an extended time 2000 is far better than 1600-1700.

 

Remember the UL engine is different than the UL just because of compression and the UL has no clutch in the gearbox. Apples to apples. remember that the Maintenance manual is written for all three engines and does a poor job at separating them for different scenarios. You can complain all you want about the manuals, but it is what it is and we have to work with what we have.

[Roger Lee]

Hi Andy,

 

The engine design has its hot point in the mid 4k's to slightly upper 4K rpms. Below that they are cooler.

[FlyingMonkey]

Hi Andy,

 

The engine design has its hot point in the mid 4k's to slightly upper 4K rpms. Below that they are cooler.

I figured there was an rpm band where heat generated was more than airflow, I just didn't know if I was going to hit it.

[Doug G.]

Roger, this is not an issue of faith, it is mechanics which means it is based in analysis and data. I know we have this mysterious Austrian entity with its mystery engines, but they are known for empirical study. If this comes from Rotax I am being they have data, so why isn't there any data to support this? Why doesn't Rotax put it in writing if it is so bad? What is the increase in gearbox issues in planes used in training aircraft? This is data that should be available.

If "Rotax says" Rotax must have a reason. Who in Rotax says? I want to ask them where their data comes from.

Not a personal issue - I cruise at 5k rpm, but I also taxi, land, and practice at a lower rpm.

Oh, and is it vibration, gearbox or something else that is at issue at 4300.

(I have a German friend who is a European international contact lawer who could transcend the language barrier if that is the problem.)

[Ed Cesnalis]

Low RPM at a low power setting is a different issue than low RPM at a relatively higher power setting.

[Anticept]

The early gearboxes used with the IO520s were upgraded and that solved most of the problems of GTSIO engines as used in the Cessna 404 and 421. I don't remember any RPM restrictions on the 421 I flew, but it's been a few years. I know a guy who still flies them and I'll ask him next time I see him. Slower props are also quieter. The IO520 in my old T210 maxed out at 300 hp for 5 minutes max but pretty much the same engine design in the 421 would max at 375 hp. Turned much faster.You will see RPM restrictions in other engines with crankshaft driven propellers, so it's not just a gearbox issue. As I recall, Stearmans have an RPM restriction and I think the old C172 with a six cylinder Continental and the Stinson 108a with the Franklin had such a yellow range just below normal cruise rpm, but again this has been a few years so don't hold me to it. As Anticept says, sometimes it's the prop combination with the engine at certain speed ranges that induces torsional harmonics that damage engines. Change the prop (if legal) and the problem may go away.My understanding is that most of the time gearboxes are used so the engine can develop horsepower by turning quickly and the prop can operate in an efficient rpm range.Running the Rotax at 3900 rpm gives a prop rpm of about 1600 or a little more. Not very efficient but that was not the point in Andy's flight.An engine that can run 5800 is at about 55% power at 3900 per the graph at 5-2 of the 912 operating manual. The next page shows constant speed prop info which is a little different. I'm not sure where on the power curve that engine would be at 3900 if pitched to run something like 5200 WOT.

Yes, gearing exists to slow the props. The closer the tips are to the speed of sound, the less efficient they become, and each blade interferes with one another in the prop wash.

 

Right around 4500 rpm, it gave me 1850ish on prop tach from my balancer.

[WmInce]

. . . "Right around 4500 rpm, it gave me 1850ish on prop tach from my balancer." . . .

Please further clarify.

Exactly what is meant by "1850ish."

[FlyingMonkey]

Engine rpm issues aside, the reason for the title of the thread is that I am impressed at how the CT can cruise with great efficiency at 125kt all day long, yet be equally adept at 68kt cruising. It even did pretty well out of a grass strip in the mountains with no drama using just the standard roller skate wheels and tires.

[FlyingMonkey]

On the main point...

 

Sorry I could not make it - the Fly-In had been on my calendar, but of course my little Sky Arrow is still grounded awaiting modules.

 

Oddly, I have not yet been into that Brasstown strip. I've been meaning to and thought maybe I could go there as part of my upcoming BFR.

 

If you want company on your next trip over, let me know.

 

I was going to ask you Eddie, but then remembered your plane was still down, and that just seemed like rubbing salt in the wound.

 

You would like Brasstown, the runway is like a golf course. Rising terrain at both ends, so best to take off when the DA is not terrible.

[Anticept]

Please further clarify.

Exactly what is meant by "1850ish."

Prop RPM. It naturally drifts a little bit, so +-10 rpm is pretty accurate

[Roger Lee]

4500 engine rpm reduced by the gearbox at 2.43 is 1851 rpm at the prop.  Cruise engine rpm of 5200 rpm reduces to 2140 prop rpm.

 

"Why doesn't Rotax put it in writing if it is so bad? "        I don't know. My guess would be company image?

   

I'm not them and they do as they want. Even the master distributors which there are 23 worldwide don't always have enough influence.

 

The manuals have a fairly good amount of knowledge for maint., but if you want to get more info that isn't out there in writing, in a manual or just public knowledge you have to go to school, repeatedly. Each time someone says something new, different, more in depth or just new info from class to class.

 

Whether you want to go to school and dislike it that is where the non published info is. That is the system in place like it or not. I learned early on you can sit and complain about the system and it won't change or suck it up and do what needs to be done and work within the system. Making good friends with top Rotax people gets you even more non publicized information.

 

That's the super secret society. LOL

All you have to do to get there is make the effort.

 

 

Just an FYI as people visit other forums:

Remember the saying about you can catch more flies with sugar than salt.

Well that is in play on all forums and in the Rotax world. Criticize everyone and everything (i.e. forum owners, Rotax techs, Rotax instructors, aircraft MFG techs, ect...) and you will get little help and knowledge when you need it.

People are people and like to be treated as you would.

 

I don't make the rules, but I have learned to navigate them.

[Jim]

All I'm gonna add is listen to your engine and feel it in your ass.  You can tell if you hit a resonance.  Mine has a couple of them between 3700 and 5500.  I never fly at those RPM and try to transition through them quickly.  That said, I really enjoy puttering along at 3800 RPM and a thousand feet. The drivetrain is smooth as silk at that speed.

[CT4ME]

From another thread... lockwood video 

http://ctflier.com/index.php?/topic/507-100ll-and-rotax-912uls 

[WmInce]

I usually only fly 3800 RPM in downwind reducing speed to land.

 

I cruise at 5000 to 5300. (I am never at seal level).

 

I notice vibration a bit more in the 4000 range.  I have never seen 5500 rpm unless descending to land from a long final straight in at lower altitude controlled airports.

 

I don't use 100LL.  And I just switched to Mobil 1 racing oil (all syn).

Feeling a little insecure?

[josjonkers]

"So, clearly, no one actually knows.  This is all based on the belief "

 

They do know it just isn't in writing. They have 40 million flight hours on the Rotax. They have seen the damage. I have tons of damaged engine pictures from one thing or another, but you'll never see anything in writing on the causes.

 

It just isn't in writing. Everything is not in a book for damage causing  problems. It isn't for us, it isn't for cars, motorcycles or boats.

The old gearbox's had a 15 degree dog set their new one has 30 degree. Pre 2003 gearboxes didn't have a clutch either except as an option.The low compression on a Cont., Lycoming  or even the Rotax 912UL doesn't pose the same issue as the 11:1 compression for the 912ULS. You guys need to compare apples to apples and other engines are oranges.

The other issue is vibration harmonics at low rpms on the system that a pilot will never feel.

Engineer's built the engine with a specific rpm range for running in mind. Running at lower rpms for short times like landing or sight seeing isn't an issue because of its short duration and varying throttle settings during such events and compared to the life of the engine.

A training engine that uses a lot of varying throttle is not at risk as much as the guy who constantly flies in the mid 4k's.

 

Just don't make a steady diet of it or for excessive times at low rpms. The same thing goes for the first start of the day at warm up. You should be above 2K rpm. Somewhere between 2000-2300 is a good place. Below that in a ULS engine causes problems and even then when sitting waiting to take off for an extended time 2000 is far better than 1600-1700.

 

Remember the UL engine is different than the UL just because of compression and the UL has no clutch in the gearbox. Apples to apples. remember that the Maintenance manual is written for all three engines and does a poor job at separating them for different scenarios. You can complain all you want about the manuals, but it is what it is and we have to work with what we have.

On this note; I fly with a VP prop and with that you can pretty well chose any rpm for any speed. When doing so after 1300 hours in my CTSW I know the optimal range for rpm is around 5200 rpm.or higher. Between 5000 and 5200 you can tell and feel the engine not being as smooth. Below 4800 definitely not optimal especially when on load versus descending with that rpm. So since 2005 I have been running it around 5200 or higher during t/o. The smoothest range. I can hear my gearbox working extra at 4800 or below.

I often f;y with my buddy in a super cub with balloon wheels and he cruises around 85 kts. I fly similar speed full fine pitch and about 30% power at 5200rpm and the 912 is happy!

[Jim Meade]

This thread is deja vu all over again. We're back to the recurring themes. One view is that this is all physics and mechanics and it should be explainable, reproducible and understandable. And public. Another view is that only by learning secret lore from persons who are not willing to share their experiences with the public does one become a master mechanic.

 

There is some parallel in old-school aviation. Until recently, Lycoming and Continental seemed loath to accept that running their engines lean of peak could have any positive effects. George Braly and a few brave souls did a lot of testing and published data that a few pilots tried and found to be useful. Some articulate mechanics like Mike Busch began writing about LOP and explained the physics of why it worked. The more one understood about how the engine operated the more one could make up their own mind about whether LOP was something one wanted to try in one's own airplane. Now, lo and behold! LOP is being accepted publicly by Lycoming and Continental and by many aviation mechanics as an acceptable practice - even a preferred method of operation - in some flight regimes.

 

The idea that someone should not challenge what is written and said about Rotax engines because one will make someone angry and then the questioner will not get help when needed is outrageous, or maybe just silly. I made comments about Rotax Owners group in Van's Air Force and in the Rotax blog just as I've made them in this and other forums. The search for clear, consistent information to be found in carefully edited documents and the desire to see the credentials of persons who purport to be experts in a field should be rewarded by answers that directly address the questions. That is the way to avoid developing a field where local knowledge, inevitably tainted with limited scope and personal bias, is the only way to become somewhat competent in a field. If Rotax pursues the avenue of withholding information except to the cognoscenti it will find itself like Lycoming and Continental eating it's words.

 

I own and enjoy flying an Aeronca Champ off my farm. I've very seriously considering a replacement that I am legal to do all the maintenance on, as I'm not an A&P and don't choose to become one. When I first got my CTSW, I'd have thought of using a Rotax engine in this new farm plane. Now, I am having very serious reservations about Rotax and would look at other engines so I would not have the feeling that I was being held hostage to unreleased information, ignition coil parts backup and so forth.

 

There is getting to be increased discussion of Rotax and LSA in Van's Air Force, Pilots of America, AOPA forum, Kitfox and others. I wish I could find more overseas forums on Rotax. Some of these forums freely discuss modifications, solutions, learning experiences and views from a different perspective so that adds texture and context to the discussion on this forum. There is no doubt that a broad view of the Rotax engine will continue to shed light on it's best practices and maintenance. If the Rotax response is indifference, then the consequences of not responding to it's customers will eventually redound to it's disadvantage.

 

They know but they won't put it in writing? This is not the middle ages. We don't need priests to explain the Rotax religion to us - we need a bible in plain language. "Pay no attention to that man behind the curtain." The example of George Braly, Mike Busch and the availability of the internet will reach even into the hinterlands of Austria.

 

See you here next year for the same discussion if nothing changes.

[Doug G.]

Amen!

[Doug G.]

Roger, you are in the same place as the rest of us. You have not heard "Rotax" say anything, you have heard someone say "Rotax says..." with no data to give substance or verify it.

I find it hard to believe Rotax has not tested these things, and recorded data on vibration and wear. And, if so, I also find it hard to believe they will not share that data, or even the concern with owners since it is their actions, not the mechanic's that are most affected by this if it is a serious factor.

[WmInce]

I'll go with whatever Roger says over anyone else in the FD universe except Armando.  Even California Power Supply refers owners to Roger.

What makes you think we are interested in who you "go with?"

[Doug G.]

That is not what this is about. I think Roger is great, I think he is honest and much more helpful than he would need to be. My issue really isn't with Roger, it is with Rotax for making information important to owners either inside info, or unverified (and seemingly unverifyable) word of mouth, back channel, info they charge for via their classes.

This is not esoteric info, damage because of low rpm cruise is essential flight and consumer information.

[Roger Lee]

There are few people in the world that has access to inside info. These aren't my choices and it's the way it has always been. I just tell you like it is and I have no idea why Rotax doesn't publish some info. They have tested and plotted vibration on their engines. The one big factor is the hundreds of ways these engines are mounted and used. You can't test for all of that. You can't test for all the poor maint. Or the people who want to do their own thing. The engine at Rotax was on an optimal test stand and conditions.

Everyone is free to use or not use what ever info is published on any forum or article by any author. Everyone is free to pursue any avenue they want to obtain info or maintain their plane, but after 40 million run hours I think a few people have a reasonable handle on what's going on.

 

What I try to do is pick as many top brains in the field as I can, get as much info from all different kinds of MFG's I can, do independent research and then pass on all this info plus something from the 30 aircraft I service a year.

If the info helps you great if you don't like what I have to pass on then take the other path.

 

 

I tell it like it is and rarely have any sway over the authorities.

 

Like it or not, dismiss the info or not isn't a big deal and probably no one cares, but verbally attack any author, form admin or member and your information highway dries up quickly and the word gets around.

 

My motto is to pay it forward and help others and it doesn't always have to cost money. Pay it forward is free.

 

Standing on the sidelines and throwing stones at anyone trying to help isn't the best use of your time, money or brain.

 

 

I started out just like everyone else many years ago with my first Rotax and I made it my mission to learn more, complaining and not pursuing just handicaps you in any endeavors.

[FredG]

"but verbally attack any author, forum admin or member and your information highway dries up quickly and the word gets around."

 

Roger, please say what you mean rather than dance around it.

 

As I see it, people are frustrated by the perception that there is a lot of Rotax "wisdom" floating around but few verifiable Rotax facts.  The complaint is that the management of a mechanical system (ie, the 912 engine) should be based on a set of empirical facts rather than word-of-mouth.  It is not an unreasonable request.

 

Eddie had it right with his analogy to evidence-based medicine.  

[Roger Lee]

Remember to not shoot the messenger. I can't do anything about it. I have to take my answers on faith too sometimes, but the source is usually beyond reproach and I geta better detailed info when I travel out of country for my schools.

 

I'm a normal everyday guy just like everyone else here. I like help when I can. Since people have helped me in my life it's now my turn to pay it forward.

 

I don't have any answers for you about why Rotax does some things. It is what it is and I have learned to work within the system as it is. The info for the most part is out there you just have to learn how to mine it out.

I don't have any magic answers. I started off like everyone else.

 

I have been lucky enough to go to many Rotax schools, learn from some of the top people, read all the manuals and get the opportunity to work on lots of Rotax engines which is leaps and bounds more educational than only having one to work on.

No matter what, it is a slow long process because you have to get the experience and that takes years.

If a person went to 2-3 Rotax schools over 5 years, actually read all 5 manuals cover to cover and had the opportunity to work on at least 15 engines a year it would take about 5 years to really have a good understanding.

 

Becoming good friends with the top people and attending special schools gets more info than you can imagine. It opens doors that are normally closed.

 

 

Hi Fred,

 

I think I did say what I mean.

Don't beat the people up who try to help (especially since it's on their own time and dollar) or they will stop helping you. They worked hard and spent quite a bit of money at getting their knowledge base and most of the better techs like to help others in need. The people who like to help will turn a deaf ear when you need it most if all you do is criticize them. I have seen this happen many times over the last 15 years.

[ctfarmer]

I find it extraordinary that the entire www can be searched, that there are tens of thousands of these engines in long term service and nothing, not a word is mentioned about possible damage as a result of medium rpm continuous cruise as we are hearing here.

 

I can find warnings from Rotax et al suggesting a minimum idle of 1800 to 2000 rpm will reduce the wear in the dogs even though a lower minimum idle is permitted.

 

I can find information that the induction and balance tube system is designed to optimise and smooth out pulses and read that the 912ul is optimised for a 5200RPM continuous operation suggesting this is where the engine will be smoothest.

 

No mention of any operational condition other than idle speed in relation to premature gearbox wear have I managed to find.

 

There will no doubt be node(s) of natural frequency occurring at certain rpm but those points, as has been stated here earlier can be felt in operation. That to me is important to understand and avoid. I would agree that hearsay may be heeded, especially if there is no cost but in this case there is.

If I want to spot or muster cattle on our property, I do not want to be running at 5200rpm regardless of flap setting to get slow flight for example. As well, there are countless times when slow flight is required and to suggest you ad drag to archive it is unacceptable.

 

I have listened to the arguments and but will now go and listen to my engine in minute detail to find out those nodes.