Rotaxs 915 iS is Ready to Roll

It's got a great power-to-weight ratio, but also a complex installation procedure. Don't be surprised if it inspires a new design or two.


At 141 hp max for five minutes, the Rotax 915 iS is the lightest, most powerful factory-provided aircraft engine.

As far as powerplants go, the world of experimental aircraft lives in its own creative cloud. The list of alternative engines beyond Lycoming’s dominance is wide even if the volume isn’t deep. In that universe, Rotax’s well-regarded four-cylinder engines have been a contender for builders and designers, especially those looking for the most power for the least weight.

With the announcement of the 135-hp 915 iS, Rotax has upped its game with a powerful, sophisticated engine that’s in a unique power and weight category for factory-provided engines. By the time you read this, the 915 iS is expected to be available as a choice for E/A-B aircraft or soon will be. What drove the development of it was reasonable uptake of the 912 iS in the Light Sport world and demand for more robust engines for the experimental and certified gyrocopters that are popular in Europe.

Marketed as a complete package to builders, the 915 iS will soon be there for the taking at a price point estimated to be about $35,000. It will give builders the option of a sophisticated, electronically controlled engine with impressive high-altitude capability, at least on paper. Of course, at that price point, many builders will likely demure unless weight is a key consideration.

As we go to press this month, Progressive Aerodyne was completing trials of the 915 iS in the kit version of the Searey, and Lockwood Aviation, which sells and supports Rotax engines, was gearing up to accept orders.

While the extra horsepower is notable in the Searey, it’s hardly night and day. It shaves a couple of seconds off the water run and boosts climb rate by about 200 feet per minute.

Evolutionary Engine

Whether the 915 iS is a great leap forward depends on how you define great leap. To be perfectly sober about it, it’s really more of a logical continuation of Rotax’s developmental history dating to the 1970s when the company, then owned by Bombardier, had a famous what-the-hell moment when it realized it was selling far more snowmobile engines than there were snowmobiles. Serendipitously, the company stumbled into the nascent ultralight market, and by sheer happenstance, it became an aircraft engine company.

The company’s two-cycle, two-cylinder engines were found in a range of minimalist ultralight flying machines, but what really put Rotax on the map was the four-cylinder 900-series engines, ranging in power from 80 to, eventually, 115 hp and now jacked up to 141 max for the 915 iS.

Progressive Aerodyne used a Searey as a test bed for 915 iS trials and will offer the engine in its kits.

Like most companies, Rotax has leveraged basic platforms into improved variants of either higher sophistication or greater power, or both. And that’s what the 915 iS is. Its design and manufacturing economics are driven by a fairly ginormous factory in Gunskirchen, Austria, that, by aviation standards, produces a torrent of engines: more than 200,000 a year. The aviation business is a rump on the company’s bread and butter of snow machine, ATV, motorcycle, and personal watercraft engines, and the volume of those powerplants explains why Rotax can produce a 100-hp aircraft engine for under $25,000.

When it launched into four-cylinder, purpose-built airplane engines in 1985—yes, 33 years ago—it leveraged what it knew and that was snowmobile and motorcycle engines.

The 915 iS has a new, redesigned gearbox that extends 50 millimeters farther forward than the 912’s gearing.

The 915 iS shows that heritage. The 915 is not a true clean-sheet engine, but rather a stretched version of the 912. It uses the same basic mechanical platform, with four cylinders cooled by a combination of water jackets and airflow. The heads have been tweaked to improve cooling around the valves, a change that makes sense given that the additional horsepower is coming from turbocharging, not additional displacement or stroking. You would expect Rotax to know that pounding more fuel and air into the jugs would beat up the valves, and they’ve anticipated that.

If you’ve seen a Lycoming crankshaft, you know that it’s a single-piece forging. The Rotax crankshafts aren’t that. Like motorcycle engines, the crank is composed of multiple components pressed together, with one-piece rods slipped onto the crank during assembly rather than being bolted on with big-end caps. You’d have to put a mic on it to notice, but the 915 iS has slightly larger crank journals to accommodate the higher power.

The 915 iS will require a constant-speed prop. It can be electronically or hydraulically controlled.

There are other changes, too. Rotax went to forged pistons rather than cast, and the internal lubrication is set up to squirt cooling oil on the undersides, again as a concession to the potential torture of high-boost turbocharging. And the boost is right up there, at a maximum of 50 inches, according to Rotax’s Marc Becker.

When it introduced the 912 iS in 2013, Rotax also added electronically controlled dual-port fuel injection. The 915 iS has the same system. But in a new cutting-edge engine, why not the direct injection that’s pretty common these days in new cars? Manufacturing efficiency, mainly. Rotax determined that the efficiency gains from direct injection in an engine operating at 75% power weren’t worth the effort and expense of redesigning the cylinders.

Following what has become standard design protocol for electronic aircraft piston engines—like there are that many of them—the 915 iS ignition system is an automotive-type CDI design, but with dual-channel or dual-lane redundancy. Don’t forget, Rotax makes a Part 33 certified version of this engine, and regulators insist on that kind of redundancy.

Fortunately, it’s all automagic and mostly transparent to the pilot in the form of a pair of annunciator lamps in the cockpit that confirm the health of each lane. No specific run-up is required, just a switch check to confirm that both channels are working. In the Searey test article I flew, it took longer to explain the system than it did to run through the preflight checks.

So with essentially the same displacement, the power boost comes from an intercooled turbocharger, while a constant-speed three-blade prop—a requirement for the engine—makes it possible to turn the power into thrust efficiently.

While the carbureted 914 engine gets an additional 15 hp from turbocharging, the 915 represents Rotax’s first foray into intercooled turbocharging for a production aircraft engine. The electronic hooks for turbo controlling are in the ECU with the appropriate software to make it work.

Lockwood Aviation’s Dean Vogel says the engine will require a prop governor for the constant-speed prop, and it doesn’t matter if it’s electronic or manual. The Searey test article was equipped with an electronic prop control with a manual electric backup, but it could also have a manual hydraulic governor.

The 915 iS is loaded with sensors wired into the ECU, and that data is available on an electric bus. How to read it? The Searey I flew was equipped with a prototype version of a device called the EMU915iS. It’s a sophisticated engine monitoring system developed by Stock Flight Systems of Germany. In addition to recording all of the engine’s critical parameters, Peter Stock told me that, in its final production form, the EMU will automatically download recorded engine data for forwarding to Rotax whenever it’s within range of an appropriate data network, at least in production aircraft. Builders, I’m sure, will see this as an option.

Rotax will eventually offer ongoing engine analysis and likely predictive fault and maintenance tracking. Move over Pratt and Rolls-Royce.

Size, Weight and Power

Like all of Rotax’s four-cylinder aircraft engines, the 915 iS relies on impressively small displacement (82 cubic inches) and high RPM—4800 rpm in cruise. That requires a gearbox to step the torque and rpm down to something a prop can digest.

Heretofore, Rotax engines have used a dog-and-gear arrangement to both isolate the prop from torque pulses and to serve as sort of a fuse in case of prop strike. The 915 iS retains a version of that but has a redesigned gearbox and crankshaft-protecting clutch. It also has a torsion bar arrangement to damp prop and shaft harmonics. The gearbox is noticeably longer: 50 millimeters farther forward at the prop flange.

In case you were thinking maybe there would be a free lunch on the weight compared to the 912 iS, sorry. The 912 iS dry weight uninstalled is 140 pounds, while the 915 iS weighs 185 pounds dry uninstalled. Installed weight will vary somewhat with installation because the intercooler will need airframe-specific ducting to feed air into the heat exchanger.

In the Searey, for example, the engine is installed as a pusher and has a substantial water radiator with a scoop and a fan. The intercooler is mounted aft of that, poked up into the airstream on the front of the engine. Other installations may or may not require similar hardware.

Builders will need to find room for both an oil cooler (left) and a turbocharger intercooler. The latter may require ducting, complicating the installation.

So where does the 915 come out in the power-to-weight sweepstakes? The numbers paint a mixed picture.

The best-case power-to-weight ratio for the 915 iS is 0.76 hp per pound, while the 912 iS is 0.71 hp per pound. Looking at other engines in the Rotax line, the stalwart 912 ULS has a power-to-weight ratio of 0.68 hp per pound, using the Rotax dry weight data. The turbocharged 914, with its old-school carburetion, is similar to the 912 ULS at 0.70 hp per pound. Actual installation numbers will change those calcs, but the dry weights show there’s a weight hit when installing intercooling.

Are these numbers better than the typical Lycoming or Continental engine? Generally yes, although engines from the legacy companies have a profusion of models and weights that complicate the comparison. At 125 hp, Continental’s IO-240 is a fair apples-to-apples. It has a dry power-to-weight ratio of 0.49 hp per pound. Lycoming’s 115-hp O-235 is a little worse, at 0.46 hp per pound. The O-360s are typically in the 0.55 hp/lb. range.

It’s not so simple as this, however. When radiators and all the associated plumbing are installed for the Rotax powerplants, the weight and complexity go up and so does cooling drag, variable with the type of airframe. Still, ELSA manufacturers who have used all of the engine choices tell us that the Rotax engines are still the lightest, and either the Continental O-200 or Lycoming O-235 aren’t always easy options.

When compared with heavier Lycomings, say the IO-360 parallel valve, the power to weight still favors the Rotax, but the Lycoming just has more raw power for an airframe that might need it for takeoff or low-altitude climb performance. I suspect builders are going to be writing some spreadsheets to calculate horsepower performance against the lighter overall weight of the 915 iS.

Lockwood’s Dean Vogel told me he did just that with the Questair Venture as a thought exercise. “I did the numbers on the Questaire Venture, and if you subtract out the engine weight Delta, you get down close to Light Sport weight limits. Of course, it has to have a constant-speed prop and it’s way too fast for a Light Sport, but you’re getting the weight down there with this engine,” Vogel said.

The 915 iS is capable of 141 hp with a five-minute limit, then 135 hp well into the teens, meaning its high-altitude capabilities offer intriguing capability for a light, fast-cruising airplane that’s just sipping fuel. Rotax’s software includes performance numbers up to 23,000 feet.

As for efficiency, I haven’t seen reliable data on this yet. The 915 iS Searey didn’t have a power indicator, so I couldn’t estimate brake specific fuel consumption. However, I would expect it to be similar to the 912 iS, which considerably bettered fuel economy over the carbureted 912 ULS.

The carbureted 912 ULS has a typical BSFC of 0.44 pounds per horsepower per hour, which is similar to typical Lycoming engines. According to Rotax’s side-by-side data flown in a Tecnam P92, the 912 iS achieved BSFCs below 0.40, which is among the best efficiency for gasoline engines and rivals diesel performance. We don’t yet know if the 915 iS will turn in similar performance.

But it has the technology to do so. The engine management system has a feature called Eco. Like motorcycles, the 915 iS uses throttle position sensing as a closed loop input. Above 97.6 percent throttle position, the engine runs full rich or nearly. Below that, it runs at what Rotax calls Eco mode, in which case it leans to what European engineers refer to as a Lambda value, specifically Lambda 1.05. In rich mode, it runs at Lambda 0.88. For reference, Lambda 1 is a stoichiometric air-to-fuel ratio of 14 to 1; 1.05 is slightly lean of stoichiometric.

The test aircraft we flew wasn’t equipped with a percent power indicator, so we couldn’t estimate BSFC. We’ll follow up on this data later when the engine is fielded.

Any Takers?

New technology always finds early adopters who will jump right on it, and that may be true with the 915 iS. Wider acceptance is less certain. At the U.S. Sport Aviation Expo in Sebring, I asked a few kit builders what their impression of the engine was. It’s too soon to make much of it, I’m afraid.

Because they’ve done the testing and essentially have sunk work, Progressive Aerodyne will offer the 915 iS as a kit choice for the Searey. It will obviously be an upcharge over the 912 or 912 iS, but the company’s Kevin Oaks said no price has been set yet. And like other kit providers, they know the market is price sensitive.

At Zenith Aircraft, Sebastien Heintz said, “I think they’ve done everything right, but just as we’re seeing with cars and motorcycles and everything, with this technology, it’s just not as accessible to the typical builder.” Like Lockwood’s Vogel, Heintz thinks it will take new, clean-sheet designs to capitalize on the strength of the 915 iS. Zenith has been talking to Rotax about a firewall-forward kit to address installation complexity. “Rotax has shown they’re here to stay, and you can legitimately design an airplane around a Rotax engine. From an efficiency standpoint in these little airplanes, you’d be going at high altitude and burning five gallons an hour. That’s kind of fantastic when you think about it,” Heintz added.


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