Don't fear the Rotax!
Many pilots of normal category aircraft have cast a dim view towards light sport aircraft. One target of their disdain is the predominant engine in use in LSA, the Rotax.
Given Lycoming and Continental have been producing aircraft engines since 1929 and both were major contributors to the war effort in WWII, Rotax is seen as a relatively new kid on the block when it comes to aircraft engines. However, Rotax is no beginner when it comes to engines in general. Rotax, an Austrian engine manufacturer originally founded in Dresden, Germany, has been producing engines since 1920. It has developed 350 engine models delivering its 9,000,000th engine in 2018.
In regards to aircraft engines, Rotax produced its 50,000th 912-series engine in 2014.By comparison; Lycoming had built its 300,000th piston engine in 2001. Rotax’s aircraft engine line has its roots in the Sea-Doo back in 2001. Considering the harsh environment Sea-Doos operate in coupled with high engine demands evolving this into an aircraft engine may not necessarily be bad. By 2012, Rotax launched its Rotax 912 ISC which received European Union Aviation Safety Agency (EASA) certification. What Lycoming has learned over years of experience in regards to aircraft engines, Rotax is gaining over having a large number of units in service in a relatively short period.
Granted, the Rotax engines are no powerhouses.
The original 912 was 80hp, 912S stepped it up to 100hp and the turbocharged 914 gives 115hp. The newest addition, the fuel injected 915iS is a whopping 141hp for five minutes but can sustain 135hp indefinitely. However, considering they power much sleeker and lighter new generation aircraft that are only two place and limited to 120 knots these engines are more than adequate. Where they really shine is in their power to weight ration. The 915 IS’s power-to-weight ratio is 0.72 hp/lb. while the 912 IS is 0.58 hp/lb. The prolific 912 ULS has a power-to-weight ratio of 0.80 hp/lb., using the Rotax dry weight data. In comparison, the comparable Continental O-200-A has a power-to-weight ratio of .56 hp/lb. The 912 IS/ISC Sport weights 140.2 lbs whereas the comparable Continental O-200-A weighs 170 lbs without accessories.
Rotax is continually improving upon its engines. These enhancements in some cases can be extended to earlier versions as was the case with the 912 IS that could upgrade its airbox, ECU software and other components to bring it up to 912 ISC Sport specifications. Upgrades to certificated engines are somewhat limited and certainly not cheap. With a Rotax, when it comes time for an overhaul it might be better to simply upgrade to the latest model given how the product is being continually improved. With the cost of a brand new Rotax being cheaper than what an overhaul costs for a normal category plane engine of equivalent horsepower this is not entirely an unreasonable suggestion.
The reliability of the Rotax is evident in its Time Between Overhauls (TBO). The Rotax 912 IS Sport engine offers a TBO of 2,000 hrs. Certainly comparable to most conventional aircraft engines. The 915 IS/ISC has a 1,200 hour TBO but hopefully with time and experience with this new powerplant that number may and should increase.
One distinguishing feature of the Rotax is that it has no issue with frequent and rapid power changes. Probably an artifact from its Ski-Doo origins.
Besides its rather compact size a couple of other things will quickly catch the attention of pilots more familiar with conventional engines. The Rotax uses a reduction gear to drive the propeller. The Rotax operates at very high RPMs . You will cruise between 4900-5300rpm with max takeoff at 5800rpm. Idle is around 1400rpm. Therefore, the Rotax has a somewhat disconcerting sound to it compared to other aircraft engines. Sort of like taking a 33-1/3 phonograph record and playing it at 78rpm for those who can remember vinyl records. Those used to flying at 2400 rpm can be understandably put out by this, thinking the engine is about to come apart at any second. Though even at this high rpm the Rotax is quieter than conventional aircraft thus helping to reduce issues in noise sensitive areas.
Another obvious difference is the radiator. Rotax engines are a combination of air-cooled cylinders and water-cooled heads. Traditionalists may balk at the idea of a water-cooled engine much like Harley Davidson fans did when the water-cooled V-Rod was introduced. Advantages of liquid cooling is it reduces fuel consumption, is less prone to shock cooling, and it more evenly cools the engine. Certainly cars on the side of the road with steam billowing from an open radiator cap doesn’t inspire much confidence in a water cooled aircraft. But consider the fact the P-51 Mustang, Spitfire, Hurricane, P38 and a host of other warplanes were water cooled and faired quite well under extreme wartime conditions.
Modern technology and a good preflight reduces the risks associated with water-cooling. A good many cars on the side of the road with their hood up might not be there if they had just checked under the hood before departing. Where normal category aircraft have just one carburetor, the carbureted Rotax use two automatic compensating carburetors. No need for a mixture control, however, for starting the Rotax does incorporate a manual choke. With the exception of the choke, starting a Rotax is very much like starting a car and just as reliable.
There are a number of less obvious differences depending on the model. The Rotax uses a dry sump. During preflight you have to rotate the propeller a number of turns to “burp” the oil system in order to check the oil level. The fuel injected models use dual injectors per cylinder. This reduces the diameter of the fuel droplets, resulting in smoother, more stable and efficient combustion.
The turbocharged models incorporate an automatic waste gate control. The 915iS will give you full takeoff power up to 15,000 feet and has a service ceiling of 23,000 feet. Given Sport Pilots are limited to 10,000 ft this is either over engineered or they are anticipating more widespread use by private and above pilots. An engine management system is utilized on some models. Operationally, you fly the Rotax with the electric fuel pumps on continuously. Besides sipping rather than gulping AVGAS, with the Rotax you have the option of using much cheaper MOGAS. Using AVGAS you will have to change the oil more frequently as well.
Rotax makes it easy to identify the model of engine by the cylinder covers.
I guess you have to ask yourself whether it is better to fly with newer engine technology or remain loyal to time tested technology that has remained little changed over decades. As an owner, a lot boils down to how big is your pocketbook where older costs a lot more. One thing for sure, you don’t have to be afraid to fly a Rotax equipped aircraft. It performs as well if not better than its contemporary counterparts do.