Rotax and the Austro-German Philosophy
Second Edition, Vol. 2
Ah, the Rotax LSA engine. Some people like ’em, some people don’t. I find that the people that don’t like them don’t understand them. So, before I get knee deep into the technical aspects of the 9XX series Rotax aircraft engine, we’re first going to have a little history lesson.
The Rotax Company started back in 1920 building bicycle axles in the German city of Dresden. These early axles were revolutionary at the time as they allowed you to “coast” when you stopped peddling. The term ROT-AX came from “Rotating Axle”, abbreviated to ROTAX. In the early ’60’s, Rotax built engines for early snow machines, (snow mobiles). Bombardier, (pronounced Bombar-d-ay, not Bombar-deer) the originator of the snow cat type machine we are all familiar with today, was developed and is headquartered in Quebec, Canada, bought the majority share of Lohner-Rotax in 1970. (Lohner was also a partner with Porsche back in the early days of Germany’s automotive history). This created, Bombardier Recreational Products, later on to included Ski-doo, Evinrude, Sea-doo, Can-Am ATV’s and the Spyder, as well as the Rotax 9XX series of four-stroke aircraft engines. BRP-Rotax, (BRP-Powertrain GmbH & Co. KG) is now located in Günskirchen, Austria. Bombardier Inc., previously Canadair, is the parent company of Rotax-BRP and their aviation division Bombardier Aerospace, which includes the de Havilland “Q” 400 series, the CRJ series and the new CS series jet. So, their heritage isn’t just in the “recreational products” industry. They’re not just manufactures of snowmobile and personal watercraft and related powerplants. Bombardier has an extensive aviation background.
Contrary to popular belief, the 912 and 914 series engines are not derivatives of the two-stroke snow mobile engines that are so popular with the ultra-light crowd. The 9XX series engines were purpose built for the intended application of the “light sport” category of aircraft that were becoming popular in Europe back in the early years of development.
There are two basic models of the 912. There is the original 80 hp (59.6 kW) 912 A/F/UL engine that has a capacity of 1,211 cc’s (cubic centimeters), and 73.9 cid (cubic inch displacement) and a compression ratio of 9.1:1, (black valve covers). The “upgraded” follow on 912 S/ULS is bored out to 1,352 cc’s, 82.5 cu in and has a compression ratio of 10.8:1, (green or blue valve covers), yielding 100 hp (73.5 kW). Both have the same stroke of 61.0 mm. The 912 A and F are used in certified aircraft, such as the Diamond DA/DV series which is quite popular in Europe. The 100 hp (73.5 kW) versions are used in many light sport aircraft, such as the Zenith STOL CH 701, the Tecnam P2002 Sierra, KitFox Model VII and the Flight Design CT line of S-LSA aircraft. The 80 hp (59.6 kW) versions are sufficient to power the new generation of efficient motorgliders, such as the Pipistrel Sinus and the Urban Air Lambada. It is also fitted to some light twins, such as the Tecnam P2006T. The 914, (red valve covers), is a turbocharged version of the 80 hp engine with the same displacement (bore and stroke), but a little lower compression ratio and a power output of 115 hp, (84.5 kW). The 914 uses an electronic control unit that gathers information such as manifold pressure, fuel pressure and RPM for the operation of the turbocharger wastegate.
O.K., now on to the good stuff. First of all, if you are accustomed to Lycoming and Continental aircraft piston engines, you will either have to forget everything you think you know about those engines, or stop reading now. Without an open mind to a different culture and a different way of thinking, you will never be proficient in servicing and maintaining the Rotax 9XX series engine. All three brands may indeed be horizontally opposed, four-stroke, push-rod engines designed for aircraft use, but the similarities really stop there. Some examples of the differences are, One; the Rotax engine has a gear reduction built into the nose case. Now, some Continental engines have an integrated gear reduction built in also, like the GTSIO-520 H series (Geared Turbo- Supercharged Injected Opposed-520 “H”) and if you’ve ever been into one of these engines, you’re already ahead of the game. The Rotax has the same type of gear reduction design as the big Continental. It employs a matched set of spur gears of different diameters. Obviously, the reason for this is that the engine is turning a lot faster to create the intended horsepower, (hp.) but would be too fast for the propeller to be efficient. My October 2017’s blog the “Power Section- Part 2” discusses this RPM vs. horsepower in depth. For a comparison, the Rotax 912 S/ULS churns out 100 hp. at 5800 RPM. The big Continental churns out 375 ponies at 3400 RPM, while still keeping their respective propellers at a reasonable speed. Remember, more RPM = more hp. Two; Like the “E” series Continental in the old Bonanza’s, the Rotax is a “dry sump” engine also. Which means the engine doesn’t store the lubricating oil in a pan attached to the bottom of the engine case. It’s stored remotely in a separate tank. Some big “V” engines and all Radial engines have oil storage tanks like this. Three; Most Rotax engines are carbureted. Although, the Rotax has two, one Constant Depression, side draft carburetor for each bank of cylinders. All carbureted Lycoming and Continental’s have only one, non “CD” updraft carburetor. The “CD” type of carburetor is altitude compensating as it is vacuum operated like the old SU, (Skinner-Union) and Zenith-Stromberg, (Pierburg) carburetors the old British cars used. Ironically, the Limbach Flugmotoren uses a Zenith version of the SU “CD” type carburetors. Another irony is that SU built the carburetors for the Rolls-Royce Merlin and Griffon aircraft engines. Although these carburetors were not of the “CD” type.
Now sit down because here’s where they really differ. Other than the obvious size and weight, and the fact the Rotax is all metric, there are some major differences. One; The Lycoming and Continental still use the ol’ reliable magneto for igniting the air fuel mixture in the combustion chamber. The Rotax does not. The Rotax utilizes a CD (Capacitor Discharge) type of electronic ignition system. There are no moving parts with this type of system. (Just recently, both Lycoming and Continental have started to introduce an “electronic” ignition system, but it still relies on a magneto if the electronics fail). Two; Lycoming and Continental both use a crude, continuous “drip” type of fuel injection system. The Rotax 912 iS does not. The Rotax utilizes a high pressure, electronic sequential type of system of fuel injection, not unlike our modern automobiles. For me, the Continental (TCM) Continuous Flow Fuel Injection is much like the Bosch “K” (konstant) type of fuel injection used on early European cars. Although the Bosch system uses quite a bit higher fuel pressures than the TCM system. Conversely, the Rotax system is much like the Bosch “D” Jetronic (Drück) system used on the VW type 3 and type 4. The “D” Jetronic system uses manifold pressure as a means to assess engine load. The Bosch “K” system used a plate like device that measured airflow to assess engine load. Three; The manufacturing nuances, the machining aspects and the design and assembly processes of the Rotax engine are performed in typical Teutonic fashion. Every surface has been attended to. No casting roughness on any of the surfaces whatsoever. The Rotax engine reminds me of the Porsche 356/super 90/912 engine because of some of the crankcase features and the small push rod tubes. The machining aspects of the Rotax engine also remind me of the Porsche “four banger”. For example, the gearbox magnetic drain plug on the Rotax does not utilize a gasket of any sort. The sealing mechanism of this plug is accomplished by a small raised machined section that seals against the precision machined bore contact surface on the engine case. This eliminates the use of a gasket, which eliminates another part and part number, and the manufacturing costs associated with the addition of this part as well as the logistics in distribution. The design philosophy of the German/Austrians is simple. Chief Designer Rudolph Daub of Wright Aeronautical’s “Tornado” project is noted for stating that “every line on a technical blueprint or drawing is money”. (Read that as production costs). Mr. Daub was an immigrant from the Fatherland, and possessed the brash, no “Blödsinn” attitude typical of a man of his homeland. This attitude was apparent in his design philosophy also. “Produce only enough parts to get the job done”. The Rotax 9XX series engines exhibit this same type of Teutonic philosophy. Another example is the conveyance in which the lubricating oil that is collected at the bottom of the Rotax engine is return to the remote oil tank. Most dry sump engines use collector sumps and scavenge pumps to return the oil in the crankcase back to the remote oil storage receptacle. The Rotax does not use a scavenge pump for this operation. Nope! Those Austrians are pretty ingenious when it comes to using resources that already exist to accomplish this task. To eliminate the complexity and associated costs involved with a secondary oil pump, the Rotax uses the crankcase back pressure to return the oil back to the oil tank. There is no crankcase “breather” so to speak, as the system is a “closed” system back to the oil tank. The oil tank is adequately vented because it is now also venting the crankcase as well. If you have ever “burped” a Rotax engine, then you know what I mean. This mode of oil transfer does create a different set of troubleshooting skills that are particular to the Rotax 9XX series of aircraft engines, and without the proper training in how to understand certain symptoms, you’ll never be able to remedy what’s wrong. The 914 series Rotax engine does employ a scavenge pump for the turbocharger lubricating oil return. But again, in typical Teutonic fashion, this scavenge pump is stacked on the end and is driven through the primary oil pump. No different than the set up used on the Volkswagen “Auto Stick” system in which the torque converter oil pump was stacked on the end of the engine primary oil pump, and driven through it as well.
Unlike its big bore brethren, the Rotax employs a roller type crankshaft similar to the two stroke motorcycle and snowmobile engines. This roller crankshaft set up is similar to the old SPG Roller crankshafts that were available back in the day for the Volkswagen engine. A VW engine with a SPG Roller crank would really rev. Years ago I drove a car with one of these crankshafts installed in the engine, and man, it seemed like that thing would rev forever with no redline in sight. One word of warning when it comes to an engine with a roller crank though. Don’t rev up the engine and then side step the clutch, or you’ll twist the crank out of alignment. The same goes for removing the “flywheel/dynamo” on the Rotax engine.
I had a 914 in here years ago that some hot rod dude in Bakersfield decided that he knew more about the Rotax engine than the “krauts”, (I use this term in an endearing context, as they are my peeps), that he removed the flywheel/dynamo and replaced it with a lighter aluminum disc type arrangement to house the magnetic pick-ups for his aftermarket fuel injection and ignition system. The subsequent removal of the rotating mass the flywheel/dynamo possesses, and the stored kinetic energy it also possessed when shut down, had a detrimental effect on the crankshaft. The propeller end, with the rotating mass as the propeller and gear train, and the absence of the flywheel/dynamo at the other end, allowed the crankshaft to essentially “wind-up” each time the engine was shut down. The mass of the connecting rods, pistons, piston pins and such weren’t enough to keep the crankshaft ends from twisting out of phase from each other. And, eventually, the engine wouldn’t turn over.
I learned a long time ago from Jerry Wood and working on all of those VW’s, Porsches and Merc’s, that the German’s (and Austrian’s) know what they’re doing when it comes to mechanical engineering and design. It’s a different thought process and philosophy than us Americans. It’s a simple, minimalists kind of thinking that screams “if doesn’t need it, don’t’ put it in there” design approach.
O.K., now for the bad stuff. As with anything new to any industry, a certain amount of knowledge must be obtained by selected personnel to get familiar with the new product, and to become proficient at maintaining and repairing said product. I wish this were more true in the aviation industry than it is, but sadly it’s not. The Rotax 9XX series engine(s), just like any other engine whether it’s an automotive engine or an aircraft engine, or a locomotive engine. It demands extensive training and a small capital investment in basic metric and some specialty tools. Unfortunately, it appears that most of my fellow A&P mechanics do not wish to invest in the training or the tooling required to competently service the Rotax engine. Here’s why I say that.
Last year I had a guy come by the shop complaining that the 914 in his aircraft, wasn’t getting the RPM’s it should. I asked him how long he had been experiencing this problem and he said “ever since he got it back from the Annual”. I questioned as to why he hadn’t taken it back to the guy who did the Annual Condition Inspection to correct the problems, after all, you paid him to “fix” it right? This owner expressed that he had already tried that and the mechanic couldn’t do any more for him. During the initial run-up, I noticed that the RPM wouldn’t go much over 3,800. The engine barked and farted at any attempt to run faster than this. It also would occasionally bark and fart at any RPM from idle on up to this magic number of 3,800. The owner was forthcoming with needed information that the carburetors were “just gone through, so it couldn’t be them”. (The first red flag for me). “Why not”? I asked. This seems like the most likely culprit if you ask me. After giving the engine a quick once over, I noticed a few things that made me cringe. Like the wads of silicone pucky globbed on all over the ignition modules. The throttle position sensor connector was barely hanging on to the sensor. The propeller has no resistance in the clutch dogs. After this revelation, I stopped looking. I proceeded to remove both carburetors and once back at the shop, I disassemble both of them for a thorough inspection. Not surprising, I found a plethora of issues regarding the “just gone through” status of these Bing model 64-3 carburetors. First, I find that the floats are not of the latest design per the BRP/Rotax Service Bulletin SB-912-067UL R1/SB-914-048UL R1. Second, the butterfly plate in the 2/4 carburetor was installed upside down. Third, the screws holding this plate in the shaft did not exhibit any upset on the thread side of the screw as required by the Maintenance Manual. And lastly, the TCU (Turbocharger Control Unit), had not been calibrated.
I carried out the 200 hour inspection and service on these carburetors, installed the butterfly plate correctly with new screws and upset the threaded ends as described in the MM. Installed the latest version of the floats, changed the required gaskets and “O” rings and set the float levels. Now, none of this is complicated if you have the book to refer to and the proper tools to carry out the procedures explained in the MM. It became obvious to me, that the guy who had “just gone through” these carburetors, lacked both the knowledge and the tools to properly execute the task which he claimed he had accomplished. It also became apparent the he was not qualified to perform the Annual Condition Inspection based on the condition of the carburetors and the wads of silicone on the ignition modules. Not to mention the condition of the propeller drive system. Technically, this engine is not airworthy according to BRP/Rotax because of these issues. Yet, there it was, fresh out of Annual, all signed off and ready to go.
So, what have we learned from this topic? Well, for starters, we’ve learned that our design and thought process differs significantly from our counterparts across the pond when it comes to mechanical stuff. We also have learned that to be good at aircraft “mechanicking”, we have to be open minded and adventuress in learning new things in order to keep up with new technology and processes. And finally, we need to invest in ourselves through proper training and tool acquisition to be proficient at what we are hired to do.
Hopefully, this month’s topic will inspire other Aircraft Maintenance Technicians, (or airplane mechanic, whichever term you relate to), to better yourself and learn new things that are in our professional world and embrace new technology. When it comes to the Rotax 9XX series engine, go get the required training and subsequent certification that is required to proficiently repair and maintain the Rotax engine. Invest in the tools and equipment required to properly service and maintain the Rotax engine. Embrace the difference of design culture and have fun with it.
But, as I said earlier, if you’re not willing to be open minded and change your line of thinking when it comes to different design cultures and philosophies, you’re never going to be very proficient at the Rotax 9XX series of aircraft engines. Simply, because your heart and mind are not in it. This statement is really true for a lot of things in life.