Adam Dickson

Second re-bolt of main wing spar with close tolerance bolts

Sling aircraft have indicated that the AN3 bolt holes in current wing spars are drilled to 4.9mm - meaning that there will be plenty of slop with standard AN3 bolts, or even close tolerance AN173 bolts. Clearly the design intention is that the plate-plate friction emerging from the clamping force of the bolts will be sufficient for the design requirement. I noticed this with the replacement spar caps which I considered using (discussed elsewhere), and indeed this was one of the factors tipping me towards retaining the original spar caps.

However my spars are about 5 years old, and these holes were reamed to a smaller size, namely 3/16" = 4.7625mm. Normal AN3 bolts fit snugly, albeit with a little slop, probably about 0.001" or so. This suggested that one could achiever a snug fit without slop if close precision bolts were used. I obtained these and switched out the AN3-13A bolts installed previously. I see only advantage and no downside to making use of the shear-load bearing capability of a close fitting bolt grip.

I have found the AN173-13A bolts are indeed a close fit, sliding in under light finger pressure, with an absence of slop. These were coated with Duralac yellow as before (and under the washers as well where they contacted the aluminium), fitted with a 1/16" thick washer under the head and a 1/16" and a 1/32" thin washer under the nut. This combination left plenty of margin between the grip and the nut as shown while minimising the total washer thickness. 13A bolts were used such that the grip extended entirely though the spar layer stack. These bolts were torqued to about 38 in-lbs, as per Sling advice, then torque-marked.

The AN5 and AN6 bolts were installed with the AN173 bolts and fully torqued just prior to torquing the latter, to ensure perfect alignment of the spar cap layers ensuring smooth mounting of the wings.

The outer left hand spar caps holes were increased slightly to 4.9mm (by mistake, 3/16” intended) when trialling the new caps so this single layer is not a close fit, but given the current approach utilising friction alone this would seem to be a non-issue. Also, even the original outer spar caps had a few holes that were slightly elongated, to 4.9-5.0mm.

This post is from Adam Dickson