by Gord Dyck, Director of UPAC and member of the Ultralight Working Group
Basic ultralight aircraft (BULA), with their low gross weight of 1200 lbs, have primarily used non-certified engines as powerplants. The majority of certified engines are too heavy for the ultralight aircraft, cost too much and burn expensive aviation gas (100 low lead). Ultralight aircraft builders have, until recently, been reliant on two-stroke engines, predominantly from Hirth and Rotax. These engines offered high thrust to weight; albeit,at the expense of fuel consumption (a typical Rotax 503 or 582 burns four gallons an hour of automotive fuel at cruise power), and they offer less reliability than four-stroke engines. A Rotax 582 at 65hp and just over 100 lbs provides excellent power to weight. However, all two strokes are very sensitive to air/fuel/oil mixture and require constant carburetor maintenance and attention to their exhaust gas temperatures if they are to be flown safely. Having flown many hours behind a Rotax, my new ultralight project required a powerplant. With the announcement that both Hirth and Rotax were no longer manufacturing two-stroke engines, my latest BULA build needed a new powerplant. Ideally, I was looking for a four stroke to reduce fuel consumption, enhance safety and eliminate carburetor maintenance.
Original engine bracket
I found a new engine that seemed to fit the bill: a two-cylinder V-twin 800cc, 60hp, four-stroke fuel injected engine with electronic ignition that weighed within a few pounds of a Rotax 582. The engine is an all-terrain vehicle (ATV) engine that has been in production for years, with the manufacturer making thousands of engines a month for the recreational vehicle market. As the US distributor LSA Aeromarine said, their engine was “red neck proven.” So, I proceeded to purchase the engine and install it into the airframe. The distributor was in the process of installing the new engine on his personal sport aircraft Merlin concurrently with us installing the engine on my build. Much to our chagrin, we were leading the distributor in developing the installation procedure. People in the ultralight community are much more pilot/builder types, unlike the majority of the general aviation pilot community, who are limited by regulations with their certified aircraft. Assisting with the installation was an Argo design engineer, who routinely works with these engines and their wiring harnesses. Together, we decoded the Delphi computer harness, determined the fuel pressure the engine was happy with and installed the Ace aviation belt reduction drive that replaced the continuously variable transmission (CVT) the engine normally was attached to.
The engine started with the turn of a key, just like your ATV. The initial engine run ups indicated that the engine mount as designed for the ATV/Merlin wasn’t fit for purpose on my application. There was too much vibration in the mount. The diagonal upper engine mount was replaced with a pair of diagonal mounts, one on each side of the engine. These shock mounts provided the rigidity necessary. The Merlin exhaust system wouldn’t work with my cowl configuration, so I elected for a pair of straight pipes. The Mk1 stacks sounded like an angry Harley-Davidson, so these were modified to a single pipe feeding a cylinder with baffles and three stacks which made the sound acceptable. With these fixes in place, the first test flight was flown. The engine ran fine but required a pitch adjustment to reduce engine rpm and maximize thrust. With propeller pitch adjusted, subsequent flights demonstrated good performance from the propellor/re-drive/engine combination. A stabilizing bracket was added to each muffler stack, as it was found that the two attachment bolts were loosening after each flight.
Re-designed bracket incorporating a bearing
After five hours of orbits around the field, the aircraft was performing well. During a slow flight practice, the engine hesitated on being asked for power, and a precautionary landing was undertaken. Inspection revealed a ring of oil inside the cowl in line with the redrive. Further investigation then discovered that the engine crank seal was leaking oil. The spinner, prop, cowl and redrive were removed. The front crank cover was removed where it was confirmed that the crank oil seal had failed. The failed oil seal was a symptom of a bigger problem: the design of the crank case cover. Instead of having a sealed bearing, this design used a bushing that supported the crankshaft. This bushing was in the process of failing. Investigation with LSA Aeromarine engineer, our own engine chap and our Argo guru indicated that the bushing was not designed to take a lateral load that the belt drive had imposed, as opposed to the ATV CVT. Clearly, this was a major engineering problem that required a fix.
A two-stage fix was developed by LSA Aeromarine. First, working with LSA Aeromarine engineer, Ben Bosma, the Ace Aviation redrive was redesigned to incorporate a bearing to take the side load from the belt drive without touching the existing engine design. A mounting bracket holds the upper pulley. This bracket was modified to incorporate a bearing that sleeved onto the engine’s crankshaft. This would eliminate the side load on the engine’s original bushing. A new bracket was manufactured and incorporated into their aircraft and my aircraft. This modification has entered production for the companies Merlin aircraft. The proposed long-term fix was to get Gaokin, the engine manufacturer, to change the crank cover and incorporate a proper bearing for future engines being used in this application.
With the new redrive tower in place, I flew the a/c for another two hours before the new oil seal began leaking again. The engine was removed and returned to the distributor. The distributor has gone into production with their new redrive variant. The redrive manufacturer continues to do product improvements to their design and continued to reach out to our builders’ group.
So, what did I learn going through this process? Firstly, folks in the industry are solid and want to solve problems. Although not required in basic ultralight aircraft, keeping a detailed log for airframe/engine was very useful in both discussion with the distributor and tracking development, and I would consider it a best practice. Secondly, one must be prepared to work constructively with the distributor and the design engineer to solve problems. It was essential to communicate what we discovered and what they had discovered to work towards a common goal, whether it was puzzling through the wiring harness, working on the ATV engine manual or brainstorming a fix for the crank seal and bearing. The ultralight community is one of companies, builders and flyers who combine non-certified equipment with best practice build techniques to meet ultralight aircraft requirements. Innovation has always been part of the community. It is enhanced by the sharing of information to improve the best practices, whether this is better build techniques or a more reliable engine.
From my experience, one of the roles of Transport Canada and the Ultralight Safety Working Group is to promote communications on technical best practices as the ultralight community continues to innovate, whether that is because of the retirement of a range of engines or the arrival of new technology, such as electric power.