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New respect for the CT


FlyingMonkey

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Today I took a trip to a private strip in Brasstown, NC for a fly-in of Highlander owners. I went with a friend who flies an Avid Flier powered by a Rotax 582 and with bush tires. On a good day his WOT speed is 80-85mph. We wanted to fly up together, but we were not sure how well the CT would fly that slowly for the roughly one hour flight.

 

The answer is: Great! I left the flaps at 0 and we flew up there at 68-70 knots the whole way. I was turning about 3900rpm in cruise (I can hear Roger cringing...). The airplane was stable, easy to fly, and not behind the power curve. There's no telling what my fuel burn was, but I'd be surprised if it was much more than 3.5gph.

 

On arrival I made a 30 flaps approach, and was high because I wanted to make sure my buddy in the Avid ahead of me had plenty of room to get off the runway. I was about to call a go around when he got off the runway, so I slipped it hard and it came right down to the 2200ft grass runway with no problems. I hit the flare at 48 knots and made a decent landing on the slick, wet grass.

 

The takeoff was uneventful. Earlier we had watched a 152 get dangerously close to a stall/spin after he ate up 3/4 of the runway getting airborne, then had to make a steep turning climb to clear terrain. The CT had no problem, I got the nosewheel off immediately and held it off until rotation, nosed over to build a little speed, and then climbed out at 60kt with no drama.

 

What a machine!

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No specific in writing authoritative source. Just repeated over and over in class and from instructors for more years than I can count. This comes from instructors that have spent weeks at the factory in Austria and some with over 30 years Rotax experience and many viewings of damage caused by owners. With the high compression and pulse from the engine and the 30 degree movement within the gearbox from the dogs the lower rpm allows this to move. Not good on the gearbox. If you want to do a 180 turn or two at low rpm no problem as it is shortlived in the grand scheme of things, but long and repeated low rpms will take its toll on the gearbox.

Technically Rotax would like to see the engine run at 5500 with the capability to achieve 5800. This isn't realistic and especially for either fixed pitch or a ground adjustable prop. 

 

Training is harder on an engine than a steady cruise, but is better than a long cruise at repeated low rpms because in training the rpm varies constantly. The UL can do better here with its lower compression over the ULS.

The plane will fly for quite a while at any rpm for hundreds of hours, but remember the idea is to get to TBO and beyond without an expensive service.

 

What's the worst thing that can happen to a Rotax engine?

 

 

 

 

 

 

 

 

 

 

It's owner.

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Not everything is about credentials and scientifically proven or even exact wording at times. Some things are just taken on faith or we wouldn't still be debating the Bible for the last couple of thousands of years. And if you wanted all the documentation and credentials then the Vatican should open all the guarded archives to the world.

 

 

Some things we just have to take at face value from a trusted source. I guess it's up to you to decide who that trusted source is for you Rotax info. Not everything is in their manuals and many thing have been lost in the translation to English.

 

(The languages of Austria include German, the official language and lingua francaAustro-Bavarian, the main language outside Vorarlberg; Alemannic, the main language in Vorarlberg; and several minority languages.)

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So, those of you who do flight training, have you seen noticeable wear to your gearboxes due to constant takeoff, landings, pattern practice,etc.? Just because a group of people believe something does not make it a fact. Toilets don't swirl the other way south of the equator. :-)

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You'll need hundreds of hours to see any extra wear and and each engine would have its own wear patterns. The difference may be that a 1000 hr. gearbox inspection may need extra parts over all the rest that just get new Belville washers and new shims. There are a couple of parts that may take an extra beating and need replacement over a gearbox that was cared for better. Where the extra wear would happen is anybodies guess because every engine would have been flown differently.

 

It's like many things. Sometimes your lucky and sometimes your not.

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At those revs you would also experience about half the power transmission through the gearbox so that fact on its own should see a greatly increased life of the gears and bearings.

 

Roger, What parts are we expecting to show increased wear!

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CTFarmer: the problem is that the firing impulses are farther apart, so the gear lash and slippage become a factor. At higher RPM, the firing impulses are more consistent, and the resistance on the prop is higher, so it helps to keep the gears meshed together.

 

As I was told by Dean @ lockwood: at high RPM, the engine wears a little faster, at lower RPM, the gearbox wears a lot faster.

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So, clearly, no one actually knows.  This is all based on the belief that the crankshaft, at 4000 rpm, speeds up and slows down, during each rotation, due to the power stroke of individual cylinders.  The propeller, having no such tendency, rotates against air resistance at a fixed speed.  Hence, at some slow speed, during each rotation of the engine, sometimes the gearbox shaft is driving propeller and sometimes the propeller is driving the gearbox shaft.  When that happens, wear will increase.

 

As I calculate it, a 912 (or any 4 cylinder 4-stroke engine), has 133 power strokes per second at 4000 rpm (4000/60x2).  Will the engine slow sufficiently in the eight thousands of a second duration that separates power strokes for the propeller to get ahead of it, only to catch back up to it during the next power stroke?  Or, due to the speed the crank is rotating (thereby diminishing the acceleration that occurs each power stroke) and the drag of air slowing the propeller, does the gear train remain in constant contact?  

 

I'm going to guess that 4000 rpm is sufficient to maintain constant contact, but I, like everyone else here, am guessing.  

 

BTW, I have heard Dean Vogel (Lockwood) say that partial throttle operation of a 912 ULS is no problem of any kind.  So, it kind-of comes down to what we mean by "high" and "low" and what me mean by "partial".

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It sounds like primarily the low rpm operation is a wear factor on the gearbox.  Since the gearbox gets a 1000hr removal and inspection, I'm okay with potentially having to replace some parts at that time, as long as I'm not doing damage to the actual engine.  This will only be an occasional thing to fly with my slower friends, normally I'm at 5000-5400 in cruise.  The flight yesterday (in the first pic you can see the Avid in the very upper left corner):

 

10633560_10152625863833286_4652918225750

 

10626254_10152625863828286_9012526921936

 

And the airfield:

 

10659087_10152625863733286_4068778746180

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This is a problem that isn't exclusive to Rotax engines. Low RPM also sets up harmonic vibrations, that, in certain airframe and prop configurations, could concentrate in different parts of the engine. This is why many standard aircraft have yellow arcs, but can change based on what prop is used, or what airframe it is mounted to.

 

This is especially true of GTSIO engines. They will often place a heavy restriction on certain RPM ranges, because it tears the gearboxes apart from the harmonics.

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This is a problem that isn't exclusive to Rotax engines. Low RPM also sets up harmonic vibrations, that, in certain airframe and prop configurations, could concentrate in different parts of the engine. This is why many standard aircraft have yellow arcs, but can change based on what prop is used, or what airframe it is mounted to.

 

This is especially true of GTSIO engines. They will often place a heavy restriction on certain RPM ranges, because it tears the gearboxes apart from the harmonics.

 

If this is really a wear issue for the Rotax engines, I would love it if they would put a yellow arc on the engine...that would be better than having to hear about the issue secondhand and then have to decide if we trust the source (I trust you, Roger!   :)  ).  I like to abide by manufacturer recommendations, but it's sometimes hard to know what is a real recommendation when it's not published. 

 

BTW, I was worried the engine might run hot at the lower speed, but CHT and oil temp never got above 190 at those speeds.

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Engineers build and design the products, but it's us mechanics that maintain the engines, see the effects of operation, and give them the real world data to refine their products. But, sometimes engineering departments don't recognize a problem, or are shushed by management. Manufacturers live in a different environment than the field, and sometimes don't see (or choose to see) the problems that mechanics see in the field. This is why it is a personal belief that field experience should be REQUIRED of engineers, much like residency for doctors.

 

In addition, with how crazy liability is these days, a lot of manufacturers would prefer to sweep problems under the rug by silently releasing updated versions of their products, hoping that the old ones fall out of use before too much liability crops up. Pre-1980's, you could call up a manufacturer and they would practically walk you through unwritten procedures because "that's how someone else in the field figured out how to do it and it works". Now, it's hush hush, just replace the part, and shut up.

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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.

 

 

On the secondary point...

 

I listen to a couple of "skeptic" podcasts. One is medical in nature. The infectious disease specialist who hosts it (Mark Crislip, the podcast is "Quackcast") says words to beware of from a doctor of any sort are, "In my experience...". IOW, experts in all fields are subject to the same biases as the rest of us - confirmation bias, post hoc reasoning, cherry picking data, etc. 

 

We've seen this in mechanics saying LOP operations will "burn valves". 

 

Anyway, there's a trend to "evidence-based medicine", and that would be a good idea here, but related to maintenance.

 

It's very possible that low-rpm operation is detrimental to ROTAX gearboxes, but it seems like the "evidence" is anecdotal, along the lines of "In my experience...". It seems like it would be trivially easy for ROTAX to provide hard data, by putting engines on test stands and running them at various RPM and then documenting engine and transmission wear at set intervals.

 

BTW, it is intuitive that high rpm operation causes more engine wear than low rpm, but that is not a given. My understanding is that in normal operation all major engine parts are separated by an oil film, and not actually touching - if they were touching, the engine would screech to a halt in seconds. If an engine is rated for a continuous operation at a given rpm and to a certain TBO, its not certain that operating it more slowly would result in less wear and/or more longevity. And in some engines, slower speed operation (possible through propeller control) may actually lead to higher combustion chamber pressures and conceivably more wear.

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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.

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