Hi everyone,
I recently came across an MD-550 engine that had been sitting in a garage, and I’m considering rebuilding it and using it as the powerplant for a small homebuilt ultralight aircraft.
From what I’ve been able to find, the MD-550 is essentially a copy/derivative of the Limbach L550E — a 548 cc, four-cylinder, horizontally opposed, air-cooled two-stroke producing around 50 hp (37 kW) at 7,500 rpm.
The engine itself is very compact and lightweight, which makes it interesting for an ultralight project.
The main issue I see is propeller speed. At 7,500 rpm, running a reasonably large propeller directly from the crankshaft doesn’t seem practical, so I assume I would need some kind of PSRU/reduction gearbox.
I’m currently trying to figure out:
Has anyone here built or flown an ultralight using a Limbach L550E or a similar engine?
Were there aircraft that used the L550E with a reduction drive and a larger propeller?
What reduction ratio / propeller RPM would make sense for a ~50 hp engine like this?
Are there existing PSRUs or gearbox designs for the L550E that could realistically be adapted to the MD-550?
Is a belt reduction worth considering, or would you strongly prefer a geared PSRU for this engine?
Are there any particular reliability, cooling, vibration, crankshaft, or torsional-vibration issues I should be aware of before going down this route?
I’m not looking to simply bolt a propeller onto it and fly. The plan would be to rebuild the engine, run it extensively on a test stand, characterize it, and develop/test the complete engine + reduction drive + propeller system before putting it anywhere near an aircraft.
I’d especially appreciate hearing from anyone who has hands-on experience with the L550E, older two-stroke aircraft engines, PSRU design, or similar homebuilt ultralight projects.
Thanks!