Wing spar, review of AN3 bolt scheme
I am considering whether to accept the current wing spar AN3 bolting scheme with no washer under the head, or to go to a longer AN bolt, introducing a washer under the head.
Indications are that the torque setting has been increased as well, to 37 in-lbs (including the nylock torque). This figure was obtained from Pascal Latten's blog and will be verified with TAF.
The main spar thickness is 23.65mm, or 0.931 inch. The AN3 bolts currently used are AN3-12A, with a grip of 0.875 inch. No washer is placed under the head, so the grip falls 0.056" of spanning the spar. A single AN3 washer of 0.0625" is placed on the thread of the bolt.
Considering the use of AN3-13A bolts with a 1.000 inch grip, and one washer under the head. The grip will therefore extend 1.000-0.0625-0.931 = 0.0065 inch beyond the spar surface. Then two AN3 washers on the tail, under an AN365 nylock nut will result in the same number of turns of thread protruding past the nut. This use of 3 washers (one on the head, two under the nut), is right at the limit of acceptable usage. I will verify that this is acceptable to TAF.