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Date:  3-24-2010
Number of Hours:  3.00
Manual Reference:  
Brief Description:  Drill spar cap extruded angle for rivets

1. Pretty much following the procedure shown in the Zenair videos, I back driilled the angle, first using a 3/16" bit, following with a #11. I used cutting oil to lube the bit, so I need to clean up the parts before riveting.

2. Several outboard rivets were originally 5/32" rivets. Those I back drilled 1st 5/32, then 3/16, and then #11, except the outboard most rivet which will take an AN3- bolt. That one I drilled 5/32, then #14, and finally reamed to 3/16.

3. The inboard 6 rivets also get AN3- bolts. I consulted with Caleb from Zenair whether to further widen those to #11 or #10 so that the holes in the extruded angle would be about the same as the holes in the spar cap & spar cap doubler, or ot leave them 3/16 which is a tight fit for the AN3-'s.

Caleb said that either way would be OK. I expressed my concern that the extruded angle and the doubler plate could end up bearing the entire shear load, and thus be subject to fatigue failure, ultimately leaving a much weakened joint. One could conceive of successive fatigue failures leading to a joint with less strength than the design. Caleb also suggested that changing the AN3-'s to AN4-'s was another alternative.

I consulted with an aeronautical engineer. He was much concerned about fatigue failure resulting from loose holes. His first recommendation would be to use oversize bolts. I was unable to locate a good source for oversize 3/16" bolts. One vendor offered to attempt a special order, but they required a minimum of $100/item for special order. The engineer also mentioned oversized Hy-Lok fasteners, but I am unable to find a good source for thm, and they require special tools for installation.

The engineer opined that AN4's in reamed holes would be OK if the holes were careully reamed to 0.250. On balance, I think that is the best way to go.
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