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These four pictures show the configuration I used for my first flight with belleville washer 6-stacks under two of the six propeller bolts.
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I put four standard bolts and two belleville stacks holding the prop on, torquing the bolts in opposition until reaching 30 ft-lb on all six. I then safety wired them all (I will admit that I AM getting better at safety wiring - I can probably do all six in about 10 minutes now). I calculated that even if the two belleville washer bolts came off, the four standard bolts would be more than adequate from a strength standpoint to retain the propeller with appropriate compression levels, and a simple Finite Element Analysis of the crush plate indicated that while I might lose ~50% compressive force in the region of the two lost bolts, that would be about 20% of the total area, for a loss of force of ~10%. I decided that was worth the risk - remember, the belleville washer bolts would BOTH have to completely disappear for this to be the case.
Anyway, I took off from Tehachapi (KTSP) and flew down to Bracket Field (KPOC) to return Thomas Kennedy's propeller and prop extension to him - those were the ones I had borrowed to fly the plane back from Desert Center (L64) to Tehachapi. That's about a 35 minute flight. Upon landing, I examined the prop and noted that absolutely nothing had moved or gone out of place or deflected. Good sign.
After lunch, I saddled up and flew the 10 minutes over to Corona (KAJO) to pick up a new spinner dome (Hershey Kiss shape) for about $250. I plan to use this as a starting point for a new spinner attach scheme that uses the crush plate as a mounting point, rather than squeezing the backing plate between the prop and the extension. I'll have more info on THAT when I actually do it. I then departed Corona and flew the 40 minutes back over the San Gabriel Mountains to Tehachapi.
I examined the prop/bolts again after landing, and again, there was no movement of anything. I measured the thickness of the prop hub, and it was within a thousanth of an inch or so of what it was when I departed that morning. Heat transfer through the extension and measurement inaccuracy could easily be the source of the difference.
So far, so good - I'm pleased. I will be leaving the setup as is for another 2 hours or so - 4 hours of flying will get the bolts over one million cycles of whatever stress cycles they're exposed to at 2500 RPM (at 2/rev), and if something's going to fatigue, it'll do it in that time frame. At that point, I'll swap out another two standard bolt installations for belleville washer 6-stacks and fly another 4 hours. If everything's good, I'll then put in the last two and start on the spinner installation.
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Copyright © 2007, All Rights Reserved, Marc
J. Zeitlin
e-mail: marc_zeitlin@alum.mit.edu
