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Belt Re'drive Mount

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mstull

R.I.P.
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
I have a question for y'all. I'm designing my next plane, considering different engines and engine mounts, trying to save weight and complexity. One idea came up that I don't have any experience with, or heard of anyone trying with a belt reduction drive:

Can you solid mount the prop pulley part of the reduction drive to the airframe, rather than to the rubber mounted engine?

Twin cylinder engines seem to vibrate most in yaw and roll, and mostly at low RPM, at and near idle. Neither of those would affect belt tension with the prop pulley mounted directly above or below the crankshaft. Obviously the engine would need to have rubber mounts that provide enough support to withstand belt tension. And the exact angle and position of the prop pulley would have to be adjusted with the belt tensioned, so the pulleys are properly aligned when belt tension pulls the engine on its rubber mounts. I'm using a multi-V J-belt on my Kawasaki 340.

Here's the consequences I've thought of:

The mass, of the re'drive and prop bolted to the engine, normally helps reduce engine vibration. The gyroscopic stabilization of the prop probably helps even more. So the engine could vibrate significantly more. People talk about the difference in engine vibration from just the difference in gyroscopic stabilization between 2 and 3 blade props. I'm not sure if prop vibration could cause structural issues with the airframe, as long as the prop is well balanced. Normally airframes are heavy duty near the engine anyway.

Mounting the prop pulley to the airframe would allow more flexibility in design, since the prop could be located farther from the crankshaft. And it could take less bracketry, saving weight. I had to mount my engine inverted to allow room for wing flap deflection. With the prop mounted to the airframe and a longer belt, the engine could be mounted lower, right side up.

I'm using what J-Bird calls a (plate mount) Micro-V drive. The brackets that connect the prop pulley to the engine have to be very heavy duty to withstand the vibration. I've had a few failures of those brackets over the years on different engines. I made most of the parts of the ones in these pictures myself.

The other part of the question is: Will the engine vibrate so much more without the prop and re'drive attached to it, that the carb won't work and the muffler might break?
 

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