[Wing Ribs]
Making the wing ribs.
Gussets
The original Pietenpol plans call for squared gussets cut from 1/16" 3-ply plywood. I secured 1/16" 3-ply birch plywood from Aircraft Spruce in 1' X 2' sheets. As many other builders are doing, I opted to use round gussets for the wing rib. The round gussets perform as well as the squared gussets, however they are easier to cut and finish, and look more pleasing once assembled within the wing rib. Additionally, it reduces weight by a marginal factor.
To make the gussets, I used a 1 3/8" hole saw with the center drill removed. I simply held the plywood by hand and attempted to stagger the rows of circles cut out to decrease waste. Some builders create jigs for this, but I found that free-handing this was time efficient and still produced a clean, consistent product. I cleaned up the edges on a belt sander.
Fasteners
The gussets are attached to the cap strips of the wing rib via T-88 epoxy. T-88 is the standard in homebuilt aircraft, especially with bonding spruce as this project entails. It is mixed with a 1:1 ratio by volume, according to the manufacturer's specifications.
The original Pietenpol plans call for brass tacks to be used to fasten the gusset to the cap strip. This fastening isn't for structural integrity, but instead used to hold the piece in place while the epoxy sets. Once the epoxy has set, it is the primary bonding force for the joint. For this project, I elected to use stainless steel staples. Staples are more convenient (especially with a pneumatic stapler) and the stainless steel construction means there will be no corrosion over time. The weight of the staples in minimal, and likely less than if brass tacks were used.
Wing Rib Jig
The wing rib jig is constructed on a base of two boards of particle board with a plastic laminate surface. The particle board was chosen becuase of its inherent stability to not contract, expand or warp. The plastic veneer is also a good surface as the T-88 epoxy does not easily bond to it, making for easy clean up. A level was used to create a straight line across the top board to be used as a datum to measure out and draw the wing rib shape. The measurements of the wing rib were transferred from the plans onto the board in pencil. A French curve was used to connect points for the top and bottom camber lines. Then, the smaller 'spars' of the wing were drawn within the wing, and the location for the front and rear spar was established by placing a block of pine the exact size of the spar in its place. This ensures that there will always be adequate room for the spar on each wing rib. Pieces of pine, 1/4" thick, 1 1/4" long and 1" wide were placed along the outer edges of the camber lines. They were stapled in place. Then, a small piece of cap strip (1/4" thick) was used to provide spacing for the inside blocks to be nailed in place. The same process was used for the inner 'spars'.
Finished Ribs
The wing ribs were glued, and the gussets were stapled on on one side while in the jig. The wing rib was then carefully pulled from the jig, placed on a flat and smooth work top, and the gussets for the other side were then glued and stapled. The rib was then allowed to cure while weighed down on the flat surface.
A flush-cutting router bit was used to trim the overhanging gussets, and any excess T-88 was sanded down on the top and bottom cap strips, with sharp edges being broken on the outsides as well. The ribs are stored while weighed down on a flat surface to prevent any warping due to temperature changes.
