Belleville Washer Flight Test #1

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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.

Rear View of Hertzler Prop with four standard bolts and two Belleville washer stacks

Oblique view of Belleville Washer Stacks

Closer Side View of Belleville Washer Stack

Close-up View of Bolts

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

Last updated: April 15, 2007