A Deep Dive into Rotax LSA Engines

FLYING took a deep dive into all things Rotax with Phil Lockwood, considered by many to be the resident U.S. expert on their powerplants. 

Lockwood runs the Lockwood Aviation Group based in Sebring, Florida, a wide-ranging company which includes an AirCam kit aircraft division, aircraft supply, and service divisions, a repair facility, the Sebring Aviation flight school, and a Rotax mechanic’s school.

Lockwood has been in and around the ultralight business since the early 1980s when many users flew with Rotax two-stroke engines. 

“With the two-stroke engines,” Lockwood explained, “if you knew how to maintain them, and knew when they should be overhauled in a preventative way, they could be run quite reliably. Most of the problems were from people who just didn't know what to do except put gas and oil in them and run them until they blew up. We didn't even know the TBO of the engines because we didn't know how long they should or would last.”

As Lockwood worked on nearly every available model of ultralight engine in his early flying years, he said it became evident that Rotax was making the highest quality product. 

“They came on the scene out of nowhere and very quickly were dominating the ultralight industry,” Lockwood said. “Their engineers were putting in the work to produce different configurations to accommodate pusher-type and tractor-type airframes, very well-configured exhaust systems, and all different types of installations. It was pretty clear to me early on that these guys were going to be the leaders they are today.”

Lockwood said Rotax is the undisputed leader powering the light sport industry today, with an estimated 80 percent market share in new LSA models sold in the U.S. Other estimates put their market share as high as 90 percent. 

That successful run with their reliable four-stroke engine line started in 1989 with their 80 horsepower 912UL engine, which is still available to purchase. A 100 hp version, the 912ULS, is also available. As the company’s market share grew, so did their product line, and now, OEMs and experimental builders can choose not only the carbureted UL and ULS versions of the 912, they can pick the fuel-injected 100 hp 912iS Sport, the 115 hp 914UL, or the company’s largest powerplant, the 141 hp turbocharged 915iS.

That 141 hp turbo Rotax is now being used in the Sling TSi, with excellent results, according to Lockwood. “The TSi is a very good four-place airplane, and Sling is seeing nearly a 1,000-pound useful load in that model by using the 141 hp Rotax engine,” he said.

How Rotax engines came to be the gold standard with LSA manufacturers really comes down to engineering, Lockwood said. 

“Right from the very beginning, Rotax has been focused on making lightweight engines with great power-to-weight ratios,” he said. “They’ve designed engines that can cruise at 5,000 to 5,500 rpm all day long and go to TBO like that. From the metallurgy to the lubricants they use, it is all designed to allow the engine to run at high rpm.”

Lockwood explained that pure physics is behind the success Rotax has found. 

“You look at a 100 to 125 horsepower Lycomings or Continentals, and the Rotax engines are making that kind of power on about half the displacement, allowing for much lighter and more compact engines,” he said. “Many legacy aircraft engines do not have tight control over mixture cylinder to cylinder, so they must run rich of peak. The Rotax engines are very smart, with dual-path [engine control units] controlling the fuel injection. This allows for fully automated precise control over the fuel mixture which improves fuel efficiency, reliability and longevity providing super-efficient lean of peak operation.”

The flight school Lockwood operates has been running “nine series” Rotax engines for a long time, he said, and they typically run to their 2,000-hour TBO. “We go 100 hours on our oil change intervals. And as long as you're using unleaded fuel, we don't do much to the engines in between the 100-hour service intervals.”

While Rotax has factories in China and Mexico that can build the engines for some of their other applications, Lockwood said all their aircraft engines are only manufactured in their Linz, Austria, plant. 

“The build quality of the Rotax engines coming out of Austria is very high. If you ever have a chance to go to the main factory in Austria, do so, because it is very impressive,” he said. 

As to what is in development for the Rotax aircraft engine line, Lockwood only said “they seem to be quite committed to the aircraft engine market. And now they have quite a long history and are continuing to develop new engines. So exactly what is coming? I don't know. But it appears they're moving forward with higher horsepower engines.”

It is hard to write about Phil Lockwood without discussing his AirCam kit airplane. Let’s recap quickly how this open-cockpit, twin-Rotax kitplane came into existence. 

After becoming known for his bush flying in the world of conservation and wildlife photographers and filmmakers, Lockwood was approached in early 1993 by a National Geographic photographer and another conservation photographer who asked Lockwood to fly them on missions in the Congo region of Africa. 

“I had done my previous Africa bush flying in Namibia, which is mostly desert with places to land everywhere,” Lockwood said. “These Congo missions were over dense rainforest in the northern part of the country with 100-foot-high mahogany trees and very little opportunity to land. I made some initial drawings of the prototype AirCam using two 64 hp Rotax 582 engines for redundancy; they liked the safety of the two engines, and we completed it in six months."

Lockwood shipped that first AirCam to the Congo in five DHL shipping crates, and it was so successful, he decided to put more than 22,000 engineering hours into developing the AirCam as a kit. By 1999, he began selling AirCam kits, and the popularity of the design continues today. The first AirCam prototype flown for the Congo missions is now in the EAA Aviation Museum in Oshkosh, Wisconsin.