Published September 4, 2018
The Revolutionary Rotax 915
The Austrian engine company's newest model could be downright revolutionary.
By Dave Unwin
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It would be fair to say that, since the Rotax 912 arrived onto the general aviation scene 29 years ago (in 1989), the Nine-series of Rotax engines have transformed GA. At first it was an engine used solely in homebuilt aviation, as the LSA category was still 15 years in the future. In Europe and amidst a busy U.S. light sport scene, the engine was a godsend.
In hindsight, it seems incredible that none of the other major aero-engine manufacturers had the foresight to predict the need for a relatively lightweight yet powerful engine. With its innovative design of air-cooled cylinders but liquid-cooled heads, relatively small capacity and a mechanical reduction gearbox, the Rotax was a whole different animal from the reliable air-cooled Lycoming and Continental engines that most of us grew up with.
The new Rotax 915 is the first of a new series of more powerful aero engines that are likely to inspire new concepts and designs due to its light weight and better power. Photo by Dave Unwin
Thanks to its good power-to-weight ratio and low fuel consumption, the 912 rapidly gained acceptance, even if the way it started and stopped—all at once with a shudder—made most of us wince. However, it was the emergence of the Light Sport Aircraft class in the USA and its strict 600kg MAUW that really secured the 912’s place as the engine of choice for most light aircraft designers.
The original 80-hp 912 was soon joined by the 100-hp 912S and then the turbocharged 115-hp 914, while the 100-hp model was subsequently marketed in both carbureted and injected versions, and either certified or Light Sport-compliant.
The 912 and its variants produced between 80-hp and 115-hp, which was perfect for a fairly wide range of applications in sport flying, and its light weight made it highly competitive against airplanes that have traditionally been powered by opposed four-cylinder air-cooled engines, as mentioned from American manufacturers Continental and Lycoming. A lot of folks have been asking for a more powerful Rotax to compete with popular air-cooled models putting out between 125-hp and 160-hp.
This is the year that Rotax made those wishes come true. And after this year’s AERO at Friedrichshafen, I—along with several other aviation journalists—was invited to visit the Rotax factory at Gunskirchen in Austria to hear more about the latest engine, the 141-hp 915, learn how it’s made and sample it in flight.
Before going flying, I enjoyed a visit to Rotax’s ultra-modern factory. It’s quite an easy place to find as it’s based on Rotaxstraße. If you’re flying into the lovely local airport of Wels, you can’t miss it as “Rotax” is written in big red letters on the factory’s roof.
Wandering around the factory was very interesting. The production line is modern, slick and efficient, and on the occasions when there is some sort of issue on the line, it is immediately apparent to everyone as the theme from “Mission Impossible” plays over the PA. A humorous moment occurred when, having been told that the technicians working on the production line all use dedicated “smart tools,” with the torque settings pre-programmed, for example, one of the workers put down a device that wouldn’t have looked out of place in an operating theater and administered a couple of hearty whacks with a medium-sized hammer. I laughed and couldn’t help but ask if it was a “smart hammer?”
One facet of the factory that I found particularly interesting was the nitriding process, which hardens some components. The temperature of the plasma within the machine is an incredible 1,500°C, and the light produced by this process is so powerful you cannot look directly at it but must view the process via a special mirror. Visits to the areas where the aero-engines are built and tested were particularly interesting.
While watching the motors being run on the dyno, I asked if there were any big differences between the way the certified and ASTM-compliant engines are built and tested—and there isn’t. Each engine is built using exactly the same materials and to the same specifications, and then tested in exactly the same way. Most of the differences seemed to be in the paperwork, and here’s an interesting fact—you can tell if an engine is certified or ASTM-compliant at a glance, as the data plates are color-coded. Many people know you can tell the power produced by any given nine series Rotax from 50 meters away, as the heads are color-coded. Black for 80-hp, green for 100, red for 115 and blue for 141. However, what you may not know is that the data plates are also color-coded. If it’s a certified engine, its data plate is red, while if it’s black, it’s ASTM compliant.
So, what exactly is a 915iS? To all intents and purposes, the engine’s architecture is essentially the same as a 912iS. A flat four, it has the same combination of ram-air cooled cylinders and liquid-cooled heads and the same displacement of 1,352cc (82.5ci). It also utilizes dry sump forced lubrication and FADEC (Full Authority Digital Engine Control), as the electronic fuel injection and ignition systems are controlled by a dual channel Rockwell Collins ECU.
There are also some significant differences, such as a reinforced crankshaft, new pistons and a redesigned gearbox with a new reduction ratio of 2.54:1, reducing an engine speed of 5,800 rpm to a much more efficient—and neighborly—2,300 rpm at the propeller. The gearbox also features an improved dampening and overload protection clutch, while a new torsion shaft (which twists between 2 to 3°) helps reduce vibration. It has been designed from the outset for constant speed propellers.
The big difference, though, is the turbocharger installation. This has a compression ratio of 3.5:1 and not only increases the power available to 141-hp for up to five minutes, with a METO, or max continuous of 135-hp, but also ensures the power stays constant up to the engine’s critical altitude of 15,000 ft. This turbocharger is a very cleverly designed piece of kit. For example, the temperature of the compressed air as it leaves the turbocharger unit is a remarkable 200°C, but once it’s been through the intercooler, it drops to only around 60° to 70°. The entire installation is neat, compact (except for the turbo, externally it’s very similar to the 912iS) and only 15 kg heavier.
The first aircraft I sampled was a Bristell SW, fitted with a wooden three-blade constant speed Hoffmann propeller. The prop was quite interesting in its own right, being of the slightly curved “scimitar” type and having really quite a broad chord, bearing in mind the horses available.
Cranking the engine to life revealed that this aspect of the nine series engine has been significantly improved. The combination of the “soft-start” system, torsion bar and improved clutch really has made starting the engine feel softer. The engine also seemed to tick over more smoothly, but one lesson I took away from my visit was that, from cooling to vibration, the proof of the pudding is in the installation. Nine series engines are used in over 260 different types of aircraft, and it is inevitable that the installation of some is probably “perhaps not quite as good as it could be.”
As I taxied into position, I briefly reviewed our weight and the ambient conditions. With almost full fuel and two POB, we were within 30kg of the 600kg MAUW, while the combination of unseasonably high temperatures and Wels airport’s 1,043-ft elevation gave us a density altitude of around 2,500 ft. The wind was 7-10 kt straight down runway 27’s 1,390 meters of concrete.
Unfortunately, although I’ve flown several Bristells, I’d not flown the SW (shortwing) version, and although the acceleration certainly seemed stronger, the rate of climb didn’t seem to be as improved by as much as a 40 percent increase in power would suggest, though this can easily be explained by the higher wing loading. Even before getting in, I’d noticed that the engine is offset slightly, so that the thrust line isn’t exactly straight down the fuselage centerline. Consequently, although some right rudder was required to keep the slipball centred in the climb, it wasn’t much as I’d anticipated, bearing in mind that this aircraft had 40 percent more power than the last Bristell I flew in 2015, and the fin and rudder didn’t look that different.
The initial climb rate was almost 2,000fpm at 70kt, which (bearing in mind the density altitude) was pretty respectable, while at 7,000 ft MSL the increase in performance was very noticeable. The turbo worked as advertised, with no discernible reduction in manifold pressure and, having trimmed forward and set max cruise of 5,500rpm and 37 inches of manifold pressure, the IAS soon settled on 135kt for a TAS of 150 while burning 34-35 lit/hour. Pretty impressive numbers, and although if you’re a long-term Rotax pilot you might be thinking that 35lit/hour is quite thirsty, I’d counter that 150kt TAS is quite fast! Pull the power back a long way to, say, 4,800rpm and 17 inches, and the engine is now just barely sipping 10lit/hour at 80kt TAS, while a good compromise (Bristell call it the ECO setting) of 5,000 rpm and 36ins MP of manifold pressure still give a TAS of around 145kt at 7,000ft AMSL.
Every engine is set up to exacting standards in Rotax's Austrian factory by the company's technicians. Photo by Dave Unwin
As the primary purpose of this flight was to evaluate the engine, I didn’t get the opportunity to explore the envelope completely, but it certainly seemed as if the slightly heavier engine had (as you’d expect) shifted the CG slightly further forward. Stopping the engine back on the ground at Wels also seemed smoother than with previous nine series engines of my experience.
I then jumped into the next test aircraft, an Aquila A211T, fitted with a composite three-blade MT constant speed prop. This machine is not an Aquila project, but is being used by Rotax as a testbed. Consequently (and unlike the Bristell), Aquila does not plan to offer the 915 as an option. Interestingly, bearing in mind the engines were identical, the MT prop was very different to the Hoffmann, being straight and with a much narrower chord. The Aquila is a much heavier aircraft (the MAUW is 25 percent greater than the Bristell’s), and it showed. Both the initial acceleration and rate of climb were, as you’d expect, not as good as the SW’s. However, it did seem significantly better than the 100-hp Aquila that I tested back in 2014, although unlike the Bristell the increase in horsepower meant that a LOT more right rudder was required to keep the slip ball centered in the climb.
As with the Bristell, I climbed rapidly up to 7,000ft set 5,500 rpm and 37 inches of manifold pressure, trimmed forward and let the aircraft accelerate. Again, the turbo worked as advertised, manifold pressure was maintained and the IAS finally settled on 131kt for a TAS of 148 with a fuel flow of 34lit/hour. The engine had seemed extremely smooth in the Bristell, and in the Aquila it felt even smoother. This may be due to the dissimilar propellers or even a product of the different materials used in the manufacture of the two airframes (the Aquila is predominantly of composite construction, while the Bristell is mostly made of metal). From a quantitative perspective, I’m reasonably confident about the veracity of the data gathered, as both aircraft were fitted with a Stock Flight Systems Engine Monitoring Unit. Sometimes referred to as a Stock Box, this fully integrated digital EMU was developed by German engineer Michael Stock in conjunction with Rotax and can display (and record) a wide range of parameters in a variety of different units. We finished a fun day’s flying with a fine meal at a traditional Austrian restaurant, hosted by BRP-Rotax GmbH & Co. KG General Manager Thomas Uhr. Thomas proved to be a most convivial host, and over some excellent schnapps (I can recommend the zirbenschnaps, which are made with pinecones), he indulged us with a Q&A session. Bearing in mind both test aircraft were fitted with C/S props and that the 915 was specifically designed with C/S props in mind, an obvious question was “would fixed pitch propellers be an option?” He replied that we should “have a look at all of our other products—there are none where we have let our fixed-pitch customers down. But official announcements are only possible if a product is available. So: no comment."
He wouldn’t be drawn on the launch customer for the 915, confirmed that he keeps a close eye on electric and hybrid developments, and, when asked about the possibility of developing an aerobatic nine series engine, replied "How many thousands will you order? From an engineering point, we would love to do so, but we don't see the market yet."
He was then asked—bearing in mind parent company BRP makes several vehicles (such as Sea-Doos, Ski-Doos and Can-Am ATVs and SSVs)—has Rotax ever considered building an aircraft? “Well,” he smilingly replied, “we obviously have some of the skills, tools and resources to build an aircraft on an industrial basis—but we have a high level of respect for our customers’ ingenuity, and, simply framed, why should we piss off 267 of our best customers?”