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A thought on the loading of the wing bolts:  These kind of joints work because
the bolt only provides compressive force between the two pieces being attached
(spars in this case).  The bolt must not see any shear itself.  The shear forces
are carried by the friction between the parts.  I once designed hubs for
Chrysler and the number of wheel studs was determined by engine torque, bolt
circle and clamping force of the nut/stud/hub system.  We made sure the hub and
wheel would not "clock" relative to each other or failure would happen quickly.


It's safe to take the same approach with the spars and attaching bolts.  However
it complicates analysis because you have to guess at the structural shear
forces, bolt/nut clamping force and the contact pressure between the two spars
which will vary by distance from the bolt and the friction coefficient between
the spars.  Adding spring members like washers won't help because they won't
come close to compensating for loss of clamp force.  I think there are aluminum
bushings in both spars that probably come close to matching the temp. expansion
coefficient of the bolt which would minimize temperature concerns.

Best bet is to torque to spec., safety wire, and retorque occasionally if
several washers were needed.  Key is that there is adequate thread engagement.

Regards,
Ed de Chazal
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