Sheeting foam wing cores—completely
Bob Hunt [[email protected]]
In the last installment of the Project Hole Shot build, we attached molded LE caps to the foam wing core. Those are cured by now, so it’s time to make the wing skins.
We are going to use 1/16" balsa for the wing skins. You can use thinner wood (such as 1/32" balsa or custom-cut 3/64" balsa), but it is a bit more difficult to work with and is not recommended for a first try at foam-wing covering. Pick out enough light balsa sheets to do the job, and trim their edges using a sharp #11-blade hobby knife drawn along a metal straightedge. If you don’t already have a selection of good metal straightedges, now is the time to visit your favorite hardware store and pick up a few. Get one that is 48" long and two or three shorter ones. They will be invaluable throughout your building career. While you are in that department, pick up a high-quality combination square.
After you have trimmed the balsa sheets, sand the edges smooth and square using the 24" Perma-Grit sanding bar and the technique I described in the last column, when we sanded the edges of the LE-cap material.
You must cut the balsa sheets to a size that will make four complete wing skins. I do this by laying one piece of trimmed and sanded balsa sheet against the installed LE cap. I tape this piece to the cap and then trim the ends to extend past the end of the foam core on either end by approximately 1/4".
Then I lay the second piece of balsa skin material against the rear edge of the first piece and tape and trim it in the same manner. This process continues until the entire core is covered with the balsa sheeting.
At the TE I trim the sheeting to extend back past the end of the foam by 3/16". I tape four complete skin sets in this manner, and then I join them.
Also included in this column
- Edge-joining balsa sheets
- Wing-skin preparation secret
To edge-join balsa sheets, rip off a piece of waxed paper roughly 4" longer than the longest balsa skin set. Remember that many CL models have inboard wing panels that are 1–2" longer in span than the outboard panels.
Set one of the balsa skin sets on the waxed paper, remove the strips of masking tape that are holding the balsa pieces together, and separate the individual sheets by an inch or so. Position building weights approximately 1/8" back from the edge of the second sheet from the front of the balsa skin set. This holds the edge of that sheet firmly against the waxed paper.
Run a bead of medium-thickness (gap-filling) cyanoacrylate glue along the edge of the weighted sheet. I do this using a thin Teflon tube fitted into the nozzle of the cyanoacrylate glue bottle.
Working quickly, position the first balsa skin piece accurately against the glued second sheet and then turn the building weights across the glued seam to hold the edges of the two sheets in alignment. Attach pieces of masking tape to the surface of one sheet, pull them tightly across the seam, and attach them to the other sheet. Repeat this in three or four places along the span of the skin to ensure the seam is tight. Let the cyanoacrylate cure thoroughly and repeat until all the balsa sheets in a skin are joined.
A custom-made plywood spreader is used to apply epoxy to the wing skins. Grooves in the spreader leave thin epoxy rivulets on the skin. Notice the rivulets’ direction: they run from front to rear on the skin.
No matter how carefully you edge-glue the balsa skin sets, there will be a few places along the span where the balsa sheets will not match perfectly. Although these surface imperfections can be evened out during the final surface sanding of the wing skins after they have been glued to the foam core, it’s easier to block-sand them before the covering process.
I use a Great Planes 12" sanding bar fitted with 220-grit paper for this operation, and I sand at an angle across the seams. This angled sanding technique will shear off wood more quickly and leave a more uniform surface when finished.
Block-sand both sides of the balsa skins until all the seams match in thickness, and then carefully vacuum the skins to remove all sanding dust. If you leave sanding debris on the side of the skins that will contact the core, the result will be a less-than-perfect joint and the skins might eventually delaminate.
Prep the finished skins: spray the bottom surface (the side that will attach to the core) of each wing skin with a thin coat of inexpensive hair spray (such as Suave or Aqua Net). The hair spray is a form of lacquer and will soak into the balsa. When it dries, it will seal the balsa surface and prevent too much epoxy from being absorbed during the skinning process.
I got the idea from Bob Noll. It works like a charm and yields a slightly lighter wing with much better skin-to-core adhesion.
I prefer to use epoxy as the adhesive for covering my foam wing cores. Yes, epoxy can be heavy, but it is actually the lightest method of all if you apply it correctly and prepare the wing skins properly.
In addition, skins applied with epoxy will never delaminate as can those applied with a contact-type adhesive. Most contact adhesives used in modeling are water-based latex types. When the water thoroughly dries out, the contact adhesive loses its gripping properties and the result is delamination.
Contact (materials and tools)
We are ready to apply the wing skins to the core. For this operation you will need the following:
- A box set of Z-Poxy Finishing Resin
- Two graduated mixing cups (each should hold at least 2 oz)
- Two mixing sticks
- Two acid brushes
- An epoxy spreader
- A flat bench
- A few sheets of newspaper
- A masking-tape dispenser
- 75–100 lb of weight (solid cinder blocks work well and are inexpensive)
- A piece of flake board or plywood large enough to cover the complete core/skin assembly
- Paper towels
The epoxy spreader should be made from a piece of 1/32" plywood and needs to be at least 3" long. Round the edges at either end so it cannot dig into the balsa skins while you are spreading the epoxy. Using a three-corner needle file, notch the spreading edge every 5/16"–3/8"; make these indentations approximately 1/64" deep.
Vacuum the cores and the balsa skins one more time—dust is the enemy in a laminating process. Spread out newspaper on your bench. Lay the bottom skin for one of the cores, inside face up, on the paper. Position the core in the top cradle piece so that its bottom surface is exposed.
Mix enough epoxy to coat the surface of two wing skins. For a model the size of our Project Hole Shot, each wing skin will require about 1/2–3/4 oz of epoxy. Mix slightly more than that to be safe.
Pour the epoxy onto the wing skin in a wavy pattern. Using the custom-made spreader, carefully move the epoxy around and evenly coat the entire skin. Once coated, position the spreader at the front of the wing skin at the tip end and pull it toward the TE.
The small notches in the spreader’s edge will leave thin rivulets of glue and remove all but a thin film between the rivulets. The effect is a grid, with the grain running spanwise and the glue rivulets running chordwise. This method ensures that no area of the skin will be unadhered to the core.
Once the skin is coated with epoxy, use an acid brush to apply a thin coat of glue to the forward edge of the wing skin. Position the skin on the core with the glued edge against the back edge of the LE cap piece. Use short strips of masking tape to pull tight the seam between the cap and the skin.
Place the core/skin sandwich in the bottom cradle piece and repeat the gluing procedure on the top wing-skin piece. Once it is glued and attached to the LE cap, place the top cradle piece on top of the sandwich and carefully align all the pieces. Put the plywood board on top of the sandwich and weight it down with 75–100 lb.
If you are covering a wing that has a preinstalled balsa TE piece, clamp the skin to the balsa TE piece. I use Popsicle sticks and clothespins for that procedure.
If you would like a more detailed description of this process and a series of sequential photos, E-mail me and I will gladly forward that information to you.
Next time we will cover sanding the wing halves, installing a bellcrank, and joining the halves accurately.
If you have an interest in CL precision aerobatics (stunt) or sport stunt, you should seriously consider joining PAMPA: the Precision Aerobatics Model Pilots Association.
Sources
- ZAP adhesives — (863) 607-6611 — www.franktiano.com
- Wing-skin photos: [email protected]
- PAMPA — www.control-line.org
Transcribed from original scans by AI. Minor OCR errors may remain.
