Why does the Varieze need a metal wing connector?

Question

Why does the Varieze need a metal wing connector?

If the Varieze has Fibreglass wings and a center spar, why is the connector made of cast metal?

Answer 1

For a concentrated load like a spar fitting where the bending load of the spar is transferred to the fuselage structure through bolts, you need a bearing surface (that is, the bore of the bolt hole) that can handle such a concentrated load in a small package.

Epoxy resin doesn't do that well at this, being "plastic" and you generally want a metal interface if you want the connection to be compact, with nearly all the loads being transferred through two bolts. To do the connection using fibreglass plates say, you would need a much larger and bulkier bolt interface to achieve the same compression/tensile strength with an epoxy/glass matrix.

So, for a connection where you want a single connection point with two bolts that have to bear massive stresses, like Rutan wing roots, it's sometimes preferred to use a metal alloy plate, and distribute the load from the plate to the composite spar with many fasteners. If the structure is carbon fibre, use of aluminum becomes a problem (galvanic differences) and you may replace the aluminum plate with stainless steel or plated steel and live with a bit of extra weight.

Gliders get around this problem by having the wing spars cross each other in the fuselage and pin themselves together and to the fuselage with a single large pin on each side. Instead of large plates, a metal bushing in the fibreglass in the center of the spar overlapping joint, on each side, to provide a bearing surface for the pins/bolts that is harder and tougher than epoxy resin, is sufficient.