I was invited to Austria by Rotax Aero Engines to sample a variety of planes with their new 915iS engine. The 915 provides a much-needed increase in power that transforms the light aircraft fleet, designed for 100 ponies, into fantastic performers that really put the thrill back into flying.
The new 915iS is Rotax’s biggest step up since the launch of its 80-hp 912UL engine 30 years ago. The ‘900-series’ flat four boxer has become the mainstay of the engine manufacturer’s aero engine business and now accounts for 96% of all its aircraft engine sales, relegating the two-stoke to just sporadic production of the 65-hp 582.
The 915 takes the basic 912ULS engine and increases its power from 100 hp to 141 hp max power and 135 hp continuous. Because of the large increase in power, Rotax has committed enormous time and resources to making sure this engine is just right, particularly after the underwhelming introduction of the 912iS, which suffered from a variety of teething problems, including expensive propeller gearbox replacements. So, for the step up to 141 hp, Rotax was determined to have a trouble-free entry into service.
Rotax first announced the 915iS in July 2015 and it has taken them almost three years to get it to a state where they were ready to release it into the hands, and jaundiced scrutiny, of aviation journalists from Europe and the USA – and me from Africa.
We were hoping to have three different types of planes in which to fly the 915, but the lovely looking French four-seater, the APM41 Simba, which was on the apron, had paperwork issues. So, we contented ourselves with flying the German Aquila and Czech Bristell with the 915iS, and a Diamond DA20 Katana with a 912iS for comparison.
WHAT’S SO SPECIAL ABOUT THE 915?
In piston engine folklore, the dream of getting maximum power from a small and thus lightweight engine was to get one horsepower for every cubic inch of displacement. The 915iS is still just 1,352 cc – or 82 cubic inches – yet it produces 141 horsepower, not far from two hp per cubic inch. So this is a little engine that works hard, and yet has to be as famously reliable as the rest of the four-stroke Rotaxes and make a 2,000 hour time between overhauls (TBO).
When Rotax announced the 915 in 2015, they said that the new engine would have the best power-to-weight ratio in its class, with a service ceiling of 23,000 feet and the strength to provide full power up to at least 15,000 feet. The big changes to the previous models are a substantially strengthened and redesigned core, dual electronic fuel injection, an intercooler and a much larger and revised propeller speed reduction gearbox.
To get that much power from a small displacement it needs to turn fast – 5,000 RPM in the cruise. That means it’s geared, but here Rotax has made a major change. The 915’s gearbox retains a crankshaft-protecting clutch, but it also has a torsion shaft inside the main drive shaft to damp the propeller, piston and shaft harmonics. This makes the gearbox noticeably longer – and can make the cowling more streamlined. However, offsetting the gains from a more streamlined cowl, the engine needs a large intercooler and oil coolers. In some of the earlier installations, most noticeably the Aquila, this has required the addition of a large ‘power-bulge’ air scoop, like a street rod car’s.
Even though the 915 uses the existing Rotax 912’s basic engine, there is no free lunch – the additional 41 horses come at the expense of extra weight. Thus, for comparison, the uninstalled 912iS dry weight is 140 pounds, while the 915iS weighs 185 pounds. Still, an extra 45 pounds for another 41 horses is excellent.
Interestingly, across the range, the best power-to-weight ratio comes from the turbocharged 914, which, with its two carburettors, produces 0.82 hp/lb. The basic 100 hp 912ULS has a power-to-weight ratio of 0.80 hp/lb compared to the 915iS at 0.72 hp/lb. Worst is the complex 912iS at 0.58 hp/lb. These are however all far better than the traditional American lumps: a 125 hp Continental IO-240 has a power-to-weight ratio of 0.49 hp/lb and Lycoming’s 115 hp O-235 is a portly and unfit 0.46 hp/lb.
When the weight and drag of the Rotax’s radiators and plumbing are added, the numbers get a bit worse, but LSA manufacturers who have used all of the Rotax engines say that the engines are still lightest by a healthy margin.
Many of the enhancements of the 915iS are beneath the skin. On our tour of the factory, Marc Becker, the sales head of Rotax Aero Engines, was at pains to point out that the engine crankcase and cylinders have been comprehensively upgraded. Indeed, peering into the state of the art furnaces in the factory, we could see cylinders receiving the latest metallurgical treatments to make them harder and stronger.
As a step-up from the 912, the 915iS has larger crank journals and forged, rather than cast, pistons, whose bottom surfaces get additional cooling through oil spray. Like the 912iS, the engine has electronic fuel injection, but not the direct injection found in car engines. Rotax acknowledged that direct injection would have required redesigned cylinders and so they elected to rather do an incremental upgrade of their existing engine.
There are two entirely independent ‘dual channel’ car-type solid state ignition systems. In the cockpit of the Aquila these two channels (or lanes) are controlled by large switches and not through a magneto selection key, as many pilots are used to. During the engine run-up, you just switch the lanes on and off to confirm that both are working. Another important test is the two electric fuel pumps that provide the high pressure needed for the fuel injection system.
The 915iS gets its performance boost from its large intercooled turbocharger, which is noticeably bigger than the 914’s. The 914 turbo was limited to just 115 hp – and was in fact a boosted 912UL, smaller bore 1,211 cc engine which, when normally aspirated, produces just 80 hp. The 915 uses the larger capacity 912ULS as a starting point and a similar engine control unit as the 912iS, but it now includes the intercooler and a wastegate for the turbo.
IS IT MORE FUEL EFFICIENT?
Small high revving European engines have traditionally been more efficient than a large ‘lazy American lump’. Yet Rotax’s carburetted 912ULS has a brake specific fuel consumption (BSFC) of 0.44, which is similar to typical Lycoming and Continental engines. It was the addition of fuel injection to the 912iS which improved things, to a 0.37 BSFC, which is among the best efficiency for petrol engines – and is not far from diesel performance.
Like all Rotax 900 series aero engines, the 915iS will happily burn Mogas, thus solving Avgas cost and availability issues in Africa. The factory says you should use the highest octane car fuel available, and preferably without alcohol.
It appears the 915iS manages similar fuel efficiencies to the 912iS, but, as we will see in the flight test, the fuel consumption can also be terrible. The electronic engine management system has a feature called Eco. For the technically minded, the 915iS uses throttle position (rather than engine output) sensing as a closed-loop input, but above 97.6 percent throttle position, the engine switches to full rich. In rich mode, it runs at Lambda 0.88 and below that it runs in Eco mode, which leans it to a Lambda of 1.05. For reference, Lambda 1 is a stoichiometric air-to-fuel ratio of 14 to 1. The value of 1.05 is then lean of stoichiometric, but this is not necessarily the contentious lean of peak exhaust gas temperature.
In the cockpit, the 915iS is managed with a nifty, but clearly still prototype, engine management unit (EMU) display which shows all the engine’s critical parameters and throttle position as a percentage, along with fuel flow.
Bringing things completely up to date in the age of the internet, in its final production version, the EMU will automatically download engine data to Rotax whenever it’s within range of an accessible wireless network. Rotax says it will eventually offer ongoing engine analysis and predictive fault and maintenance tracking for all its aero engine customers.
FLYING THE 915iS
Our test flights were arranged by Rotax at their home airfield at Wels in Austria. On the flying club apron they had three planes waiting: an Aquila 210, which looks like a beefed up Samba; a Bristell, which is the latest version of the well-known SportCruiser; and an APM41 Simba, which is an attractive French four-seater, substantially larger than a Sling 4.
I first got to fly the Aquila. This is probably an ideal type for this engine as it is a noticeably more heavily built compared to most light sport aircraft and has an empty weight of 500 kg, with a max all up of 750 kg.
Rotax Flying Club instructor Karl Hamader showed me around the cockpit, particularly the two prominent switches for the two ignition lanes and the fuel pumps. Starting was standard Rotax, but subjectively it appeared to fire up with a particularly pronounced ‘wet-dog shake’ common to Rotaxes. It may, however, just have been the effect of the drum-like interior of the Aquila’s carbon-fibre composite airframe. Notably though, Dave Unwin, SA Flyer’s flight test specialist in the UK, reckoned the 915 started “softer” and seemed to run a bit smoother than the 912 series. Rotax claims this is attributable to the engine management software and that it can also be applied to the 912iS.
Karl handled the ATC communication, which was almost all in English, and left me to fly the plane. There is nothing remarkable about the Aquila’s flying characteristics. Because the 915iS is a turbo, I opened the throttle incrementally, but Karl told me to just shove it forward. Thanks to its electronic management system, the engine responded without missing a beat. Acceleration was brisk and we lifted off from Wels’ 1,100 ft amsl tar runway in about 200 metres. Noticeable was the amount of right rudder required to counteract the P-factor – it would have been nice to have had rudder trim.
The extra power immediately made itself felt in the climb angle. Karl recommended 90 knots and I had to keep pulling the nose up with the ASI settling around an initial 2,000 fpm, and this was with almost full tanks.
At full throttle, I was astounded to see a fuel flow of 44 litres per hour so at about 500 feet AGL, I pulled the throttle back. Manifold pressure was displayed in mb and I had no quick idea what that converted to in inches of mercury (Hg). With the throttle lever pulled back a bit, fuel flow dropped to a still thirsty 34 litres per hour and climb reduced to around 1,700 fpm. Cleared to 7,000 feet, we levelled off above the scatted cumulus and explored the engine’s handling and the Aquila’s cruise numbers.
I pulled the power back to 1,206 mb – which Mr Google tells me is 35.6 inches of mercury – and 5,070 rpm, for a straight and level TAS of 138 knots. The EMU displayed a confusing 94.3%, but this is throttle opening and not percentage power. Fuel consumption dropped to just 26.3 l/h. Pulling the power way back to 1,050 mb (30 inches) halved the fuel burn to an amazing 15.4 l/h and yielded a useful 128 knots. That’s not far from 10 nm per litre and is nearly four times better than the Saratoga at 17 USG/h (64 l/h) for 165 knots. And in motoring terms, 13.5 litres for 130 nm is an excellent 5.6 l/100 km.
What is evident is that the fuel consumption is far from proportional to the throttle position – at 94% throttle we were burning a very modest 15.4 litres, but open the throttle to 98% and fuel consumption doubles to 34 l/h, and open it fully and you are burning an eye-watering 44 litres per hour – thanks to the full-rich function for high power settings.
FLYING THE 915iS IN THE BRISTELL
Czech plane maker BRM Aero makes the beautiful Bristell, designed by Milan Bristela. In its earlier versions it was known as the SportCruiser and so impressed Piper that they made it their Piper LSA.
Realising the potential to really make their sleek Bristell a great performer, BRM was one of the first to fly a prototype with the 915iS. The aircraft I got to fly at Wels was beautifully finished and not nearly as rough and prototype looking at the Aquila. It was also lighter than the Aquila, so I was particularly keen to check out the performance.
Engine start and takeoff were as per the Aquila and this time, forewarned that the engine did not require kid glove handling, I firewalled the power lever. We bounded down the runway and into the air – almost catching me by surprise. The speed soon built, and I had to haul back on the stick to maintain 90 knots. Again, I needed lots of right rudder to keep the ball centred.
We climbed out to 7,000 ft and tried various cruise settings. Unfortunately, the units on the Bristell’s EMU were different from the Aquila’s. The RPM displayed the prop and not the engine speed but at least manifold pressure was in the more familiar inches of mercury. Fortunately for the comparison, it still had throttle opening displayed as a percentage. After conversion of the different units, I get the following figures:
Maximum Cruise at 98% throttle: 149 knots at 34 litres/hour
Economy (Eco) Cruise: 141 knots at 23 litres/hour.
I am blown away – 141 knots in a fixed gear LSA using just 23 l/h takes the wallet busting fear out of just flying for fun.
What is also amazing about the Bristell is that it stalls at just 31 knots, with the Indicated Airspeed showing even less, due to position error from the high angle of attack. To have a speed range of almost five times, from 31 knots to 149 knots, is remarkable and wonderfully safe.
WHAT’S IT LIKE COMPARED TO THE 912iS?
Since I have very limited exposure to Rotax 912 engined aircraft and that is almost all on the Sling 2 and 4, it was arranged that I also fly a DA20 Diamond Katana powered by a Rotax 912iS.
Although the DA20 airframe is lighter and longer winged than the Aquila and Bristell, the differences in performance were still marked. The power of the 915iS may have provided great acceleration and a short takeoff roll and a real sensation of clawing up into the sky, but the 912iS still provided more than sufficient performance. Climb was 1,000 fpm and a similar cruise throttle opening percentage yielded an eight knot slower speed than the Aquila 915.
CONCLUSION – IS IT WORTH THE EXTRA WEIGHT – AND COST?
For those who yearn for more power, or want a cross-country hot rod, the 915iS is a welcome upgrade, even if it comes with a significantly heftier price tag.
Rotax’s Marc Becker sums it up: “The 915iS is about 40-50 pounds heavier when installed than the 912iS. The price is about €uro 3,000 more than the 914 and the 912iS. For that you get an engine with substantially more power and the ability to use that power up to higher altitudes. The 915iS has the best power-to-weight ratio in its class, full takeoff power up to 15,000 ft, a service ceiling of 23,000 ft, better fuel efficiency due to the electronic fuel injection concept, and hence lower operating cost.”
Becker says that to date, 200 915iS engines have been delivered to distributors around the world. Of these, 120 are now with end customers and about 20 different airframes are flying. He reckons around 46 different manufacturers are working to prepare for the new powerplant and expects that there will be 400 engines flying by year end.
For us here in South Africa, I reckon the game changer will be the much-awaited Sling 4 TSi. This is not just a Sling 4 with the 915iS in place of the 914; it has a new faster wing, aerodynamic tweaks to the empennage to reduce drag, and, from a looks and speed perspective, flush rivets. I was hoping to test the Sling 4 TSi as part of this test, but it is still being refined by The Airplane Factory. I am bursting with anticipation.