Tuesday, December 1, 2020

Wing Repair/Reconstruction

November 26, 2020:

And so begins the repair, or rather, reconstruction, of my wings, after the left wing departure incident during trailer transport a month earlier.

To begin, I used a heat gun to soften the 3M 30NF contact cement adhering the rib finishing tapes to the wing surface. I only needed to do the top side, because the rib stitching is cut as the tapes are pulled up, freeing the bottom surface.

All 12 rib tapes removed in about 30 minutes.

All that work 5 years ago to glue, shrink, prime, and paint my wings is gone in an instant when I cut the fabric along the LE, TE, root rib, and tip spars.

Here's a good look at the gently upward bent TE, sustained by the traffic sign impact. Surprisingly, the outboard rib was not damaged in the least. The wing tip spar tube didn't fair well at all. It was broken in two, right where it was riveted, top and bottom, to the TE socket that is all crushed up into a pancake.

These six pictures show the internal state of my wing fabrication after five years (roughly 30 hours) of flight. The closeups of the ribs show how the plywood underlayment diagonal webbing of the 3 inboard ribs have bowed and delaminated under fabric tension and aerodynamic stresses. The 1st inboard rib was the only one that had a complete failure of one of its web diagonals. The central and outboard ribs, being progressively less high than the inboard ribs, and thus stronger, remain fully intact and undamaged. 

The last two photos show the warpage of the root rib and the bowing of the nearly 6' long compression spar to which the rib was directly attached via vertical wood blocks and U-bolts. I imagined such reinforcement would have been more than sufficient. How wrong I was. Needless to say, this rib will be completely redesigned to be a 1-5/8" thick double rib, per other proven wood-built wing designs.

Another partial design failure:  Several of the LE "D-cell" styrofoam bays warped from heat of the day and, in two bays, shrunk and separated from one of their rib pairs. This design will be replaced with a standard aircraft plywood skin D-cell. Well, partial D-cell, since it won't wrap all the way around to the under surface.

Good news! ALL of the 3M 5200FC polyurethane adhesive, bonding the ribs to the LE and TE spars, are fully intact! Even the tip rib that rotated a good 15 degrees or more when the TE spar bent upward after impacting the road sign. In other words, the adhesive maintained a strong, flexible, elastic bond to the bare (scuffed) aluminum. It probably compares well with 3M's 2216 epoxy that's also flexible, but without the mess of mixing two agents together. I call this a design SUCCESS!

November 28-30, 2020:

Now for the repairs:

The root compression spar has been unbowed. The photo appears to show it as "wobbly," but this is just an optical illusion (differing shadings/reflections and background lines).

The TE wing tip socket for the wing tip spar was rebuilt using a 1-1/8" OD tube, telescoped in and riveted to the 1-1/2" OD TE. The smashed up original, that was cut off, is sitting on the wing tip rib.

To mend the bowed rib webbing and the one that was broken, I glued ordinary half-round pine trim molding on one side of the rib, alternating each side, to give each side the maximum surface area for gluing. For the broken web diagonal, two stiffeners were glued to both sides of the web.

Close up of a half-round stiffener glued and clamped to one side of a bowed web diagonal.

In preparation for the construction of the plywood skin LE D-cell, all the styrofoam blocks and vinyl roof flashing, making up the originally-designed D-cell, were removed and notches cut into the ribs to accept a full-length 1/2" x 3/4" pine stringer.

I used an angle grinder with a metal cutoff wheel to cut the notches.

A view of all the notches in a rather straight row. The grunge on the right is the remnants of the styrofoam blocks and Tightbond III glue forming the original D-cell.

A nice bead of Tightbond III in each notch (and buttered on the corresponding area of the stringer), ready to accept the stringer.

1/2" x 3/4" pine molding stringer glued and clamped in to the notches. This will be the top edge for gluing the 3/32" plywood skin D-cell to. The bottom edge will be contact cemented to the front edge of the LE spar, wrapping around to about the 4-o-clock position.

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