Friday, December 11, 2020

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.

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