To mark certification of the 915, BRP-Rotax invited Pilot to visit its production facility in Austria and fly a selection of aircraft powered by the new 141hp engine | Words Dave Unwin | Photos: Dave Unwin & BRP-Rotax

It would be fair to say that since the Rotax 912 exploded onto the general aviation scene a scant 29 years ago in 1989 the ‘nine-series’ of Rotax engines has transformed GA.

With 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 combination of air-cooled cylinders but liquid-cooled heads, relatively small cylinder capacity and mechanical reduction gearbox, the Rotax was a world away from the air-cooled Lycoming and Continental engines that most of us grew up with.

With its good power-to-weight ratio and low fuel consumption the design rapidly gained acceptance, even if the way it started and stopped made most of us wince!

It was the emergence of the Light Sport Aircraft class in the USA, with its strict 600kg maximum all-up weight (MAUW) limit that really secured the nine-series Rotax’s place as the engine of choice for most light aircraft designers.

The original 80hp 912 was soon joined by the 100hp 912S and then the turbocharged 115hp 914, while the 100hp model was subsequently marketed in both carburetted and injected versions, either certified or, for LSA, ASTM-compliant.

However, you can never have too much power, especially for four-seaters, and after this year’s AERO at Friedrichshafen aviation journalists were invited to visit the Rotax factory at Gunskirchen in Austria to hear more about the latest engine?the 141hp 915?see how it is made and sample it in flight.

Before going flying, our party enjoyed a visit to Rotax’s ultramodern factory. It’s quite an easy place to find as it’s based on Rotaxstraße, while if you’re flying in to the lovely local airport of Wels you can’t miss it, as ‘Rotax’ is writ large in red letters on the factory roof.

Rotax make many engines of different types?around 300,000 a year in fact, although only around one per cent of these are aero engines. The rest are used in a bewildering variety of applications, from skidoos and seadoos to ATVs, SSVs and even as stationary engines. (I got to sample some of the other applications in various off-road vehicles at a sporty little test track. This was so much fun that I hoped the Editor, who’d had to stay at home, manning the office, wouldn’t find out!)

Indeed, at a glance you might think that building aero engines is almost a hobby for Rotax. However, quite apart from the significant contribution the aero engine division makes to parent company BRP’s bottom line it’s also important from a branding perspective.

It’s a given that aero engines must be reliable, so when a prospective customer is considering their next purchase of a snowmobile or wetbike, it doesn’t hurt if the salesman can point out that “we also make aircraft engines”.

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 is played over the PA!

A humorous moment occurred when, having been told that the technicians working on the production line all use dedicated ‘smart’ tools (i.e. the torque settings etc are all pre-programmed) one of the workers laid down one of these devices?a thing that wouldn’t have looked out of place in an operating theatre?and administered the assembly he was working on a couple of hearty whacks with a medium-sized hammer. “Was it a smart hammer,” I asked. Answer came there none.

One facet of the factory that I found particularly fascinating 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, we had to view the process via a special mirror.

Visits to the areas where the aero engines are built and tested were particularly interesting. The first motor Rotax made for an aircraft was an air-cooled, two-cylinder two-stroke, and for over a decade the company dominated the market with such well-known engines as the 277, 503 and 582.

In fact, for many years the production of two-stroke and four-stroke engines was split around 9:1 in favour of the two-stroke, whereas these days that relationship has been completely reversed.

While watching engines being run on the dynamometer, I asked if there were any big differences between the way fully certified and ASTM-compliant engines are built and tested?and was told 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 colour coded. Many people know you can tell the power produced by any given nine series Rotax from fifty metres away, as the heads are colour coded. Black for 80hp, green for 100, red for 115 and now blue for 141.

However, what you may not know is that if it’s a certified engine, its data plate is red, while if it is black it’s ASTM compliant. See? Every day’s a school day!

Proof of the pudding…

The next day it was off to the historic Wels airport to fly some aircraft fitted with the new powerplant, but before going flying we were thoroughly briefed by Head of Flight Test Siegfried ‘Seigi’ Heer, who also revealed one further fascinating fact: all small GA airfields in Austria are designated as nature reserves, and this can help defend them against voracious developers. What a brilliant idea!

Back to the 915iS: to all intents and purposes the engine’s architecture is essentially the same as that of the 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 (82.5 cubic inches).

It also utilises ‘dry sump’ lubrication, oil being fed from, and returned to, a separate reservoir, and FADEC (Full Authority Digital Engine Control), the electronic fuel injection and ignition systems being controlled by a dual-channel Rockwell Collins ECU (electronic control unit).

There are also some significant differences, such as a reinforced crankshaft, new pistons and a redesigned gearbox with a higher reduction ratio of 2.54:1, reducing an engine speed of 5,800rpm to a much more efficient?and neighbourly?2,300rpm at the propeller.

The gearbox also features an improved vibration damper/overload protection clutch, while a new quill shaft (which twists between two to three degrees) also plays a part in reducing torsional vibration. The drive has been designed from the outset for constant-speed propellers.

The big difference though is the turbocharger installation. The impellor has a pressure ratio of 3.5:1 and not only increases the power available to 141hp for up to five minutes, with a METO or ‘max continuous’ of 135hp, but ensures the power stays constant up to the engine’s critical altitude of 15,000ft.

This unit 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 sixty to seventy degrees. The entire installation is neat and compact?except for the turbocharger, it is very similar externally to the 912iS and only fifteen kilograms heavier.

Talking with Siegi before we went flying, I opined that as much as I admire the 912 series (I have several hundred hours being either pulled or pushed by them) I’ve never liked the way they start and stop with such harshness. “I think you will see a significant improvement in that area,” he replied.

The first aircraft I sampled was a Bristell SW (‘SW’ standing for short wing), fitted with a wooden three-blade constant speed Hoffmann propeller. The prop was quite interesting in its own right, being of the curved ‘Scimitar’ type and having really quite a broad chord, bearing in mind the horses available.

Cranking the engine into life confirmed Siegi’s claim that this aspect of the nine series engine had indeed been significantly improved. The combination of the ‘soft-start’ system, quill shaft and improved clutch really has made starting the engine feel much less harsh.

The engine also seemed to tick over more smoothly but?as Siegi was at pains to point out to me several times during our conversations?from cooling to vibration, the quality of the installation is a significant factor. Nine series engines are used in over 260 different types of aircraft, and it is inevitable that some installations are “perhaps not quite as good as they 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 thirty kilograms of the 600kg MAUW, while the combination of unseasonably high temperatures and Wels airport’s 1,043ft elevation gave us a density altitude of around 2,500ft. The wind was seven to ten knots, straight down Runway 27’s 1,390 metres of concrete.

Unfortunately, although I’ve flown several 912-engined Bristells I’d not flown the SW version, and although the acceleration certainly seemed stronger, the rate of climb didn’t seem to be as improved by as much as a forty per cent increase in power would suggest?but this could easily be explained by the higher wing loading.

Even before getting in I’d noticed that the engine was offset slightly so that the thrust line wasn’t exactly straight down the fuselage centreline, and consequently although some right rudder was required to keep the slip ball centred in the climb, it wasn’t as much as I’d anticipated, bearing in mind that this aircraft had so much 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 7,000ft MSL the increase in performance really was very noticeable. The turbocharger 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/hr.

Pretty impressive numbers, and although if you’re a long-term Rotax pilot you might be thinking that 35 lit/hr is quite thirsty, I’d counter that 150kt TAS is quite fast! Pull the power back a long way to say 4,800rpm and seventeen inches, and the engine is now just barely sipping litres per hour at 80kt TAS, while a good compromise (Bristell call it the ECO setting) of 5,000rpm and 36ins of manifold pressure still give a TAS of around 145kt at 7,000ft AMSL.

As the primary purpose of this flight was to evaluate the engine I didn’t get the opportunity to explore the flight envelope completely, but it certainly seemed as if the slightly heavier engine had (as you’d expect) shifted the C of G slightly further forward.

Stopping the engine back on the ground at Wels again seemed smoother than with previous nine-series engines of my experience.

A second bite…

After a quick cappuccino, I 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 flying test bed. In fact, and unlike Bristell, Aquila Aviation 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 Bristell SW’s Hoffmann, being much straighter and having a much narrower chord.

The Aquila is a much heavier aircraft (the MAUW is 25% 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 100hp 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 centred in the climb.

As with the Bristell, I climbed rapidly up to 7,000ft, set 5,500rpm and 37 inches MP, trimmed forward and let the aircraft accelerate. Again, the turbocharger worked as advertised, manifold pressure was maintained and the IAS finally settled on 131kt, a TAS of 148 with a fuel flow of 34 lit/hr.

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. The Bristell also had a pair of G600s.

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 zirbenschnapps, which is 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!”

Nor would he be drawn on the launch customer for the 915. He did however confirm that he keeps a close eye on electric and hybrid developments, and when asked about the possibility of developing an aerobatic nine-series engine, his response was “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 make several vehicles (such as Seadoos, Skidoos and CanAm 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 put simply, why should we piss off 267 of our best customers?”

It’s all in the numbers

Comparisons are odious, sometimes unavoidable?and doubtless by now many readers are thinking “Rotax make many engines, but which one would best suit my needs?”

To answer that particular conundrum we need numbers, not words. The installed weight of the 100hp 912iS is 75.4kg and it costs around £20,000, while the 115hp 914’s installed weight is 74.5kg and it costs £27,000.

Both have 2,000hr TBOs. Finally, a shiny new 141hp 915iS has an installed weight of 90kg and costs around £30,000. Currently the TBO is only 1,200hr, but it is a very new engine and Rotax is notoriously conservative. I would expect the TBO to rise as operational experience is gained, and it is worth noting that although early 912s had a relatively low TBO of 1,200 hours, this has been progressively increased to 2,000.

All prefer the same fuel?basically the highest-octane mogas you can find, with as little alcohol in it as possible. I suspect that for most LSA and UK homebuilt applications the 912iS is just fine, but for operators of seaplanes and amphibians, aviators who are based where it’s ‘hot ’n’ high’, tug pilots and?well, people like me, who never turn down a few more ponies?the 915iS will be very tempting.

As for any further development of the nine series, I suspect there’s not going to be any. As the 915 produces in percentage terms a creditable 41% more power than the original 912S, it seems reasonable to assume that this is probably as powerful as these engines are going to get.

Of course, you could definitely still get a lot more out of the same basic unit, but as power produced increases, longevity invariably decreases. It’s worth noting that many classic American aero-engines typically only churn out around half a horsepower per cubic inch.

For example, the typical O-360 produces 180hp, O-320s are usually good for 160, while O-200s are normally around 100. The 915 is only 82.5ci, and it already produces 141hp (with much better fuel consumption), although a lot of this efficiency is obviously attributable to high running speed allowed by the cleverly engineered reduction gearbox.

Anticipating a considerable amount of interest in this new engine (and it appears to have been very well received by the industry) Rotax has already produced over 200 915 units, approximately 120 having been delivered to manufacturers. Twenty or so of these are installed and flying.

I was impressed by the 915iS. Will it be as influential as the original 912? You know, it just might.


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