Thread: Discussion Enlarged Sparky-114 "
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Old Apr 30, 2012, 01:22 PM
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The 'Wack, BC, Canada
Joined Oct 2002
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All I can say is....

I don't recall if I said much about the extension shaft in the other thread but I suspect that you're best off to go with a totally idependant shaft and use a high power RC power boat universal to connect it to the engine. Now boats use a shaft within a stuffing tube to keep things reasonably water tight. In the case of an airplane likely an exposed shaft supported by ball bearings would be a better option. For vibration and impact loading issues I'd suggest that some medium thickness chrome moly aircraft tubing would be a wise way to go. Something in the manner of 5/16 to 3/8 OD with a bearing at each end and one just slightly offset from the middle. At the forward end there should also be a thrust bearing so the radial bearings re not loaded axially. Or if you can find a suitable size angular contact ball bearing for the front then it would provide both the thrust and radial support you want.

Why do I say the middle support should be slightly offset? To avoid the risk of harmonics in the shaft causing runaway vibrations at some particular RPM. If you play guitar or know someone that does they can tell you about plucking harmonics from the string at specific frets. Pick the wrong fret and you just get a dull and short lasting "plump" sound instead of a high long lasting tone. The idea is you want that middle bearing at one of these "dead" points so the materials damp out any vibrations. It doesn't take much either. Something like a 54-46 split is enough.
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Last edited by BMatthews; Apr 30, 2012 at 01:31 PM.
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