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Engine Mounts

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mstull

R.I.P.
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
I have a question about engine mounting, specifically for my direct drive MZ201. How tightly can or should the engine be mounted?

If I mount the engine with soft rubber mounts, its vibration cracks the muffler, and makes the carb splatter a little premix fuel into the air filter, probably at some low RPM, like idle. The gasoline evaporates, leaving the premix oil. And after enough hours, the filter gets saturated enough to restrict the air flow and choke the engine. It's not a big deal to clean it periodically. And it's not too big a deal to have the muffler welded periodically, although I'd sure prefer not to have to worry about that.

If I use harder rubber mounts to reduce engine and carb vibration, the whole airframe vibrates. I've learned from previous planes, that airframe vibration can/will cause airframe failures. The vibration is uncomfortable in the cockpit, and somewhat disquieting too.

My particular application is unusual. The MZ201 is a very large displacement (626 cc) twin cylinder engine. Mine is stripped, making it very light... no electric starter, no fan cooling, no reduction drive, single ignition, wood prop. And I'm operating it at unusually low RPMs: full throttle = 3,450 RPM; cruise = 3,100 RPM; idle = 1,000 RPM. So it vibrates a lot more than other twins. This engine vibrates in all axes: roll; pitch; and yaw. But yaw seems predominant.

Should I isolate the engine from the airframe with soft mounts, and live with the resulting carb splatter and muffler cracking? Or should I mount the engine firmly with harder rubber mounts, and live with the uncomfortable cockpit vibration and eventual airframe failures? Or should I seek a compromise?

Adding weight directly to the engine would help stabilize it, like an electric starter or gear box. But this is an U/L, where low weight is critical.

One solution, I used years ago on a previous design with the extremely high vibration Hirth F-33, was to solidly mount a long, heavy bar to the engine, to stabilize it. It worked well, but was heavy. I designed that plane, so the bar was span-wise inside the wings. On this plane, it would add weight and significant drag.

Another idea might be to add flywheel mass. There's no room for a flywheel where the recoil starter lives. I could add a large diameter, heavy prop plate on the front, but I'm not confident predicting its ramifications, or how much it would help.

I hate vibration.
 
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