Rotax’s powerful new 915iS

At a journalists-only event at Rotax Aircraft Engines, several of my fellow writers and I got the opportunity to fly the new 915iS.

As we took turns evaluating the brand-new Rotax 915iS, company leaders told us that besides official Rotax pilots and selected airframe builders, we were among the first to experience the powerful new engine from the world’s leading producer of engines for light aircraft.

First Impressions of 915iS

I flew in two 915-powered aircraft with Rotax pilot, Christian Sixt, an American flight school-trained aviator with an impressive list of FAA certificates. Naturally, he is also intimate with the 915iS.

Starting was as with any Rotax 9-series engine I’ve ever flown. The engine immediately burst to life.

My flight evaluation was conducted in an Aquila light aircraft. This European design was equipped with the “Stock Box” instrument — more formally, Stock Flight Systems Engine Monitoring Unit or EMU developed by Michael Stock in collaboration with Rotax.

The Stock Box proved most helpful in observing many factors about the new engine as it provides, among other information, a percentage of throttle, fuel burn, and engine revolutions, plus prop speed expressed via manifold pressure.

Soon after advancing the throttle, I noticed greater acceleration but two other parameters were more obvious. The climb angle was very steep — nothing like a 40% boost in power to launch an aircraft into the sky.

Fuel consumption was higher than I expected, but speed was also higher. When we adjusted power and prop to drop below three gallons an hour — a surprisingly low consumption rate — speed reduced to 100 knots true. When zipping along approximately 150 knots true, consumption rose to the 7-9 gph range.

The 915iS fuel injected, turbocharged, and intercooled engine can produce 141 horsepower up to 15,000′, then sustain 135 horsepower indefinitely.

A couple of my fellow aviation journalists felt the 915 started “softer” but they also felt it ran slightly smoother. Rotax engine experts felt most of this was attributable to engine control software and that same code can also be applied to the 912iS.

Quite clearly Rotax’s 915iS is well suited to aircraft that are larger and heavier than most Light-Sport Aircraft, although operators of LSA seaplanes or those flying from high elevation fields can benefit from the added power.

Fuel requirements for the 915iS and all 9-series Rotax engines are essentially the same. Thomas Uhr, head of Rotax’s Austria plant and a longtime engine expert and enthusiast who is also a pilot, advises the highest octane auto fuel, preferably without alcohol, to get the most power and best results.

Uhr also explained that — perhaps surprisingly to some readers — the 915iS compression ratio is lower to allow for the turbo boost.

“Stress is actually a little less, therefore, with the turbo engine,” he observed.

Rotax’s 915iS uses the same displacement as the lower power 912iS. In a weight-to-power comparison — grams per kilowatt hour is the metric Rotax engineers use — fuel consumption is only about 6% higher in the more powerful engine.

At the journalist event in April 2018, Rotax Aircraft Engines manager Marc Becker indicated that 200 915iS engines had already been delivered around the globe.

Of these, “120 are now with end customers,” he noted. “About 20 different airframes are flying today.”

Some 46 different manufacturers are working to prepare for the new powerplant.

“Our expectation is to have 400 engines out in the field by year end,” he added.