by Chappy » Wed Feb 04, 2004 11:27 am
John,
The KFM's are nice engines, and the electric start is the way to go on 2-place machines. The KFM cranks are much thicker than some other engines, so that's good. Still, using large, heavy direct driven props to limit engine output and prop noise, as was our practice way back when, is probably not the best way to go. Not only does it beat up the crankshaft from excessive torsional twisting, it doesn't allow the carburators to function properly. My personal feeling is that smaller props are better, and the output should be limited by restricting the throttle bore. Afterall, these engines were designed to operate at fairly high RPMs. Longer props can still be driven fast enough to go supersonic at the tips, or can fatigue near the hubs.
As you probably all ready have noticed, I'm a "Re-drive Guy" when it comes to two strokes. But other people have worked on coming up with solutions for crank problems on the KFM when used in the direct drive mode. The KFM was also used in the Monet Moni motorglider. Someone came up with a neat prop hub assembly that incorporated a shock absorbing material within it. If you search the Moni archives, you should be able to find info on it. Also, I have a friend in Chantilly (also a John) that purchased one of these hubs (probably 15 years ago), so you can probably take a look at it, just let me know. I thought about building something like this too, but I would have done it differently.
Chappy