Fly with a Snowmobile Engine?
Rethinking the Rotax 912
By Ed Downs
Long used in U. S. experimental aircraft and many certified European aircraft, the Rotax 912 has become the almost-standard engine of choice for S-LSA aircraft designers. But, ask most American pilots about flying behind a Rotax, and you get the “What, me fly with a snowmobile engine?” question followed by a wide-eye look of horror when told that the 912 delivers its maximum rated 100 horsepower at 5800 rpm. Most simply equate the name Rotax with the famed two stroke engines of the Ski-Doo snowmobile and Sea-Doo watercraft. Many conclude that the Rotax 912 is simply a variation of its well-known two-stroke counterparts. Nothing could be further from reality.
To be sure, Rotax (located in Wels, Austria) began its long history in1920 by producing industrial grade, two-stroke engines. Many U. S. consumers first met Rotax when the two-stroke Ski-Doo snowmobile was introduced in 1962. In 1970, Rotax joined the aerospace giant, Bombardier (yep, airliners and Lear Jets as well as many other aerospace products) and by 1974 was producing its first certified aircraft engine in Europe.
Development of a dedicated four-stroke aircraft engine, the Rotax 912, began in 1984 with early versions in use by 1989. This writer first flew behind the Rotax 912 in early 1991. Rotax began building BMW motorcycle engines in 1993 and added ATV engines to their repertoire in1998. By the turn of the new millennium, Rotax owned the lion’s share of the world’s recreational engine market, celebrating delivery of their 5-milionith engine.
Today, Rotax cranks out over 200,000 engines per year (70 percent four-stroke), delivering product into a worldwide sales and service network. While the Rotax 912 has been certified under a variety of international (including the FAA) standards for many years, ASTM standards were met in 2005, and the “multi-fuel” (mogas and avgas) 912 qualified for a 2000-hour TBO in 2010. Okay, can we finally put the “snowmobile” issue to rest?
Let’s briefly talk technical. How would one describe the Rotax 912? I have borrowed a paragraph from the web site (www.Lockwood-Aviation.com) of Rotax experts, Lockwood Aviation. According to Lockwood, “The Rotax 912 is a 4-cylinder, 4-stroke liquid/air cooled engine with opposed cylinders, dry sump forced lubrication with separate oil tank, automatic adjustment by hydraulic valve tappet, two carburetors, mechanical fuel pump, dual electronic ignition, electric starter, propeller speed reduction unit, engine mount assembly, air intake system and exhaust system with muffler.” Wow, that is a mouth full! But how does this brief description relate to operations?
First, the use of liquid cooling to the cylinder heads means less engine mass is needed to absorb and disperse heat. This, coupled with high RPM to obtain horsepower, adds up to light weight, coming in at about 140 pounds. That is 60 to 80 pounds less than contemporary air-cooled 100/115 horsepower engines. The short stroke, couple with high rpm (auto and motorcycle technology) contributes to fuel flows that are up to 30 percent less than what most of us have seen in the past. The propeller speed reduction unit is pressure lubricated and designed for a reduction ratio of 2.43 to 1. This equates to less that 2400-propeller rpm at 5800-engine rpm. Engine cruise rpm’s range from 5200 to 5500, with the prop turning as slow as 2100 rpm. All of this lends itself to efficient prop designs and nearly stealth noise levels, especially when one considers the use of a muffler system. This prop reduction technology results in an engine that accelerates as if driving in first gear. Be ready when you slap the power to a 912 as it will wrap up to max thrust very fast. Small throttle movement can result in large power changes.
Preflighting a 912 discloses the use of auto fuel and high-grade, synthetic automotive oil. The separate oil tank requires some special pre-flight considerations. Starting is simple, with most installations using a typical “off/left/right/both and start” switch, except we are talking electronic ignition, not magnetos.
The electronic ignition is powered by an internal generator (PMG) system dedicated to ignition. There is no reliance upon the alternator or battery, other than for cranking power. A choke, not primer, is commonly installed and used, primarily, on cold days. A mechanical, engine driven fuel pump works all the time and most installations incorporate a back-up electric fuel pump. There is no lean mixture to deal with, as the dual carburetors automatically adjust for altitude, all the time, at all power settings. Simply turn the “mag” switch to “start” and the 912 will bark to life, quickly reaching a normal idle speed of about 1800 rpm.
One unusual operating consideration is an oil temp gauge with two red lines. There is a minimum and maximum limit to watch. While sometimes referred to as “cylinder head temperature,” depending upon the airframe manufacturer and installation, you are actually observing coolant temperature. Rotax defines specific approved coolants and water mix ratios that must be carefully observed. The radiator is often cleverly hidden within the cowling, giving the plane a contemporary “air cooled” look. The small size of the 912, coupled with the high-mounted prop reduction unit and use of an extended prop shaft, allow designers to archieve some great looking cowling designs.
A typical run up and “mag” check is done at about 4500 rpm. Your first take off in a Rotax powered S-LSA will be surprising, as the engine and plane accelerate very fast. You will note an almost turbine like smoothness and tone that takes getting used to. The 5800-rpm setting is only allowed for five minutes, after which you can use 5500 rpm for the rest of the flight, if you want to.
Most prop configurations actually limit max rpm to around the 5500-rpm range, meaning there is no limit to monitor other than engine temps. Well designed Rotax 912 installations, combined with liquid cooling, result in coolant and oil temperatures that stabilize quickly and stay in the green. Typical cruise fuel flow is between 4.8 and 5 gph, and economy cruise settings fall into the 3.5 to 4 gph range. Economy is the name of the game and that includes a well-organized, worldwide service network. And, yes, your local A&P has full access to Rotax maintenance information and can acquire a formal Rotax “certified” label, after completing a short, factory approved training program.
To be sure, there is a lot more to this story, including various levels of certification, different “dash” variants of the 912 and a detailed owner’s manual. But, the intent of this article is not to make you a Rotax expert, but simply to set aside preconceptions held by many who have not had the opportunity to experience the 912. Now, get out there and try one, first-hand, as more S-LSA’s appear on America’s flight lines!
