Dave Acklam wrote: > 1) What is the key force or mechanisim providing strength to the WAF joint? > Is it the clamping pressure imparted by the bolts? The shaft of the bolt > itself? > > 2) Is there a preferred method to 'adjust' a set of wing fittings that > were drilled wrong? Would welding up the hole & re-drilling work? Bushings? > Just make a new set?
The accepted method to this (and you are certainly not the first) is to epoxy wooden dowels into the previous holes and redrill to fit the new fittings. Soak both hole and dowel in T-88 (or other similar) epoxy first. The main mechanism for how the WAFs work is through shear, in the case of the bolt-to-WAF joint, but the bolt-to-wood interface is through compression of the bolt against the hole in the wood. Regardless of where the WAF is, the bolt is only capable of pushing against the hole in the wood...not so good at pulling on a hole, so compression is what makes the bolts work in the wood. Given that, the dowel method makes perfect sense, and how it's been done in the past. It's all well documented in the old newsletters at http://www.krnet.org/newsletter/ . Another thing to realize is that when the plane is flying with positive G's (almost always), the upper spar cap is in compression, and the lower cap is in tension. That's why the edge distance of the bolts to the spar cap can violate the minimum distance that common sense would normally dictate...think of the upper spar as being pushed into the fuselage, and the lower spar cap being pulled outwards from the fuselage, rather than supporting plane, and the above paragraph makes more sense. And as Chris wrote earlier, friction doesn't count in the calculation, mainly because the wood will likely "creep" over time so that the tension is relaxed, although friction is always nice to have to keep the wings from wobbling up and down on the ground. Mark Langford ML at N56ML.com http://www.n56ml.com
