New Root Rib and TE Reinforcements

December 3 - 11, 2020:

Traced out the new root ribs using the damaged CNC routered original and "rebuilt" the broken off trailing edge (TE). Yes, I cut these out by hand with a jig saw.

Dry fitted the new ribs using 1-1/8" x 3/16" pine lattice to determine rib placement with respect to the TE and LE bracket holes. The lattice will become the cap strips and internal webbing. Due to the 1-5/8" thickness of the new root rib, the inboard compression spar needed to move over by about 1". I also trimmed the inboard edges of this and the TE bracket to allow the inner surface of the rib to rest flush against the compression spar.

Fixed the dented LE root by using a large-sized exhaust tail pipe expander. Tightening did not restore full roundness, due to spring back of the aluminum, so I tapped the dented area with a hammer to "retrain" the aluminum where to go. It worked. The tube is now perfectly round, as attested by the ability of a left over length of splice tubing that slipped right in. There was NO damage or visible stress to the horizontal set of root bracket mounting/attachment holes.

Here I'm building the new root rib, using the pine lattice as the top and bottom cap strips. Saved the "best" for last... bending a short length of lattice for the LE.  After soaking in water for 20 hrs, I used a clothes iron on highest setting to heat the wood as I slowly bent it, then found a good location on the wing to keep it bent (plus a bit extra to compensate for spring back) and dried it out with a convective heater.

After the LE cap strip dried, I glued it to the inboard plate and added webbing diagonals. I transferred the locations of the internal webbing to both sides of the outboard rib plate, so that I knew exactly where to apply glue and staples. It's easier and faster to apply a bead of glue to a flat surface than the narrow edges of the lattices. After trimming the LE cap strip, I applied glue to the outboard rib plate and clamped it all together, using staples to hold the plate firmly against the internal lattice webbing. The length and angles of these lattice strips were totally random and unmeasured, just "that looks about right... snip."

Very early on, I discovered that the aluminum V trailing edge was not strong enough to withstand a good push of the wing into their wing carrier brackets. It bent the V, so to prevent that from happening again, I'm reinforcing the area between the ribs where the wing carrier brackets on the trailer engage the wing's TE. Closeup of a lap joint cut into a rib cap strip and reinforcement strip of lattice (same material as used for the root rib).

The back edges of the strips had to be beveled in order to get a rather sharp contact point where the top and bottom meet. Staples temporarily hold the strips to the ribs, because the glue became a lubricant that made them slide out when clamp pressure was applied. Reinforced trailing edge completed. Rinse and repeat for the wing tip area. I will be adding back new aluminum V trailing edges that will extend over and cap these reinforcements.

Completed double root rib, ready to be glued to the leading and trailing edge spars. Although it is over twice as heavy as the original, it's 100 times stronger. I'll take that trade off any day. However, no matter how strong I THINK it is, I'm still going to brace it against bowing and flexing by adding aluminum tubes going from the leading and trailing edge spars to the inboard side.

December 13, 2020

Glued the root rib to the LE and TE spars. The local Walmart now sells Starbrite 8200 Marine Polyurethane Sealant/Adhesive, instead of Attwood 7200 that I used previously. Its specs are similar, so it should perform the same. I added a little bit extra this time and finger "troweled" it for a smooth finish. Uncured PE adhesives clean up with acetone.

Here are the 3/8" OD 6061 T6 aluminum tube (estimated 0.049" wall) root rib braces. Their ends were flattened in a vise and bent, then 3/16" holes drilled into the flats. 3/16" aluminum pop rivets attach the ends to the tube spars, while #12 x 3/4" wood screws with lock washers attach the opposite ends to load-spreading blocks of wood glued to the inboard root rib plate. With these three braces, it's impossible for this rib to twist or bow.

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.