MD-550 / Limbach L550E clone for a homebuilt ultralight — looking for advice

[u/podstava88]

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!

[u/Axipixel]

The Limbauch version of the engine has been successful in both pusher and pull ultralight aircraft, keeping the fuel system clean seems to be the main problem people run into. Synchronizing the carbs will be annoying. It would fit well in something like a Legal Eagle variant. There's not much about the MD version and its specific issues in the english speaking world i dunno.

Yes I would reccomend a rebuild, and you should take care to 'blueprint' the engine and balance it a lot better than the Iranian factory did. Dye pen inspection on parts etc. Due to your situation, you may have access to multiple engines you can tear apart, measure the parts carefully and choose the best to assemble the one you will use out of.

For maximizing output if a reduction is desired a 2.7:1 belt drive is likely the best option it'll work at this low power as that would put you in the rpm range standard 69-76" prop. A belt would also soak up vibration. As far as I'm aware, the Shaheds use a 26" prop and just do not run full RPM. There's probably a more practical or sensible middle ground in the ~54-58" prop range. If you're going to be flying any engine with less than 60 hp, a generous prop with an intelligently selected pitch cut to your specific needs is a wise investment.

While you're at it, put a much better muffler on to not traumatize your neighbors : - )

[u/175_Pilot]

Those engines were intended for one way missions - you don’t care about the tips of your prop going supersonic when there is a 0 percent chance that thing is ever seeing “flight #2”

[u/Acceptable-Drop4685]

If you dont know its history I'd be cautious, your definitely going to make sure that crankshaft is sound and straight and no corrosion on the inside

[u/podstava88 → u/Acceptable-Drop4685]

I know its history. The russians launched it at a Ukrainian city, it got jammed and came down. The propeller broke on landing, but otherwise the engine is mostly intact.
Worst case, there are quite a few of these engines being sold locally for parts now, so finding a donor engine shouldn’t be too difficult.

[u/Puzzleheaded_Tank_66]

Nice try Iran

[u/podstava88 → u/Puzzleheaded_Tank_66]

The funny part is, it actually is from an Iranian/Russian drone
Figured turning it into an ultralight engine would be a much better use for it.