SHEETING FOAM WINGS
Revised 4-25-03

Jim Ryan

Introduction: I once swore I'd never resort to foam wings for my models; they were built with sticks or not at all. Now, at least 90% of the models I build have foam wings. Even as a scratch-builder (which means I have to make my own foam cutting templates), foam cores are fast and easy to produce, and they make for a quick-building wing that is straight, strong and durable. One day when I was slope soaring, I had a midair in which my plane and another met leading edge to leading edge. My sloper's spar-less partially sheeted foam wing pruned about a foot off the other plane's built-up wing and kept right on going. I was convinced.

Foam wings are very versatile, and there are a number of materials used for covering them, from balsa or wood veneer at the low-tech end, to S-glass, Kevlar and carbon fiber at the high-tech end. Adhesives range from simple wood glue to various forms of contact cement to two-sided tape to high grade epoxy. Application methods range from simply pressing the skins in place by hand to using weights or clamps to hold them down to vacuum-bagging. Even the foam itself can range from cheap expanded bead foam to extruded blue or gray foam to advanced materials like Rohacell.

Entire books have been devoted to the more advanced processes, and rightly so. But the purpose of this article is to outline a method that is fast, simple, lightweight and which will give the builder a high chance for success. For demonstration purposes, we're using a wing for a small electric model, so we won't address details like spars, landing gear mounts etc. that might be needed on larger models.

1. Here are the materials needed: For your first wing, I recommend using balsa for the skins, since it's easy to work with and all builders are used to handling it. You'll need some specialized adhesives: 1) Glue for making wing skins; some folks like sandable aliphatic glues like Pica Gluit, or you can use thin CA if you're comfortable drawing out long thin beads with it.  Wood glue has the advantage of requiring a little less skill in application technique.  2) Thick odorless CA; this is used for attaching the leading edges and tips to the foam cores. 3) Thin odorless CA; this is used for gluing the trailing edges of the skins and for applying the glass tape to the dihedral joint. 4) Contact cement or laminating/finishing epoxy for bonding the skins to the cores.  For contact adhesives, Southern Sorghum is popular; it's a latex-base adhesive that can be brushed on.  

Technical Alert:  3M Super 77 is no longer suitable for use on foam wings.  The new formulation contains acetone, which will attack the foam.  

2. Trimming and taping the skins: The first step in sheeting foam wings with balsa is to assemble the skins. First, use a long straightedge and trim the edges off all the sheets of balsa. This not only makes the edges clean and square, it also makes them straight (sheets of balsa tend to become crooked as they're dried in the kiln). Next, lay the sheets edge to edge on a clean, flat surface, and after making sure the joint is tight the length of the sheets, tape them edge to edge with masking tape.

3. Gluing the skins: After all the sheets have been taped together (on small wings, I like to make one long sheet to minimize waste), flip the skin over on your work surface and hang all but the last segment off the edge. This flexes open the joint, so that you can run a bead of Gluit sandable aliphatic adhesive the length of the joint. Then, slide the second segment up onto the bench so that the glue joint is squeezed shut. Most of the glue will be squeezed out, so use a spatula or wood chisel to scrape it off. Be careful not to gouge the balsa. Repeat this process for each glue joint until the skin is complete, then cover it with waxed paper and lay books or magazines on it overnight.

If you prefer to use CA, lay the taped-up skin flat on your bench and draw a thin bead of CA the length of the first joint.  Immediately wipe the joint with a wad of paper towel to remove any excess glue, and rub rapidly back and forth as the CA cures (just a few seconds).  Then move on to the next joint.  

4. Sanding the skins: After the skins have dried, carefully peel off the tape. Then, carefully block sand the skins until they're perfectly smooth on both sides.  Use a good flat sanding block for this purpose and 150 grit sandpaper. Be especially careful to sand the joints flush, but don't concentrate in any one place for too long. Rather, work back and forth across the skin uniformly, until it's smooth.

5. Prepping the cores: Cores that are cut on a Feathercut or other automated system require minimal surface preparation. This is doubly good, in that there's less chance of you altering the airfoil shape. The best technique is to use 220 paper on a sanding block (or better yet, a long sanding bar) and to take very light strokes in one direction only. This way, you don't rub the crumbs into the surface. You only want to remove the "angel hair" from the core. Once you accomplish that, stop. Vacuum the cores and the skins to remove all dust. It's also a good idea to vacuum the cradles and the work area. Any dust that gets between the skins and the cores will interfere with adhesion.

6. Mounting the Skins: Lay out newspaper on your work surface.  Place the foam cradle that fits the core you're sheeting on your workbench, and lay the core and the skin on the newspapers. brush or spray both surfaces with contact adhesive - use a light coat only; it's easy to over-do it. Allow the adhesive to fully dry.

7. Lay the core in the cradle sticky side up (it's important to position the cores in the cradles, as otherwise it's too easy to build in a warp that's impossible to correct), and then carefully position the skin over the core and lay it in place. If you're worried about mis-positioning the skin, cover part of the core with waxed paper and then remove the paper after the core is positioned. Once the skin is laid in place, rub down the middle of the skin from root to tip. Make successive overlapping passes working back toward the trailing edge, and then do likewise working toward the leading edge. The object is to smooth the skin down without warping the core. Once the core is completely smoothed down, remove the core from the cradle and trim the skin to approximate size. Then, repeat for the opposite side of the wing.

If using laminating epoxy, the technique is a little different.  Brush the only the skins with a light coat of epoxy.  With this approach, you can skin the top and bottom surfaces at the same time.  With the skins coated with epoxy, lay them up in their cradles in the proper sequence:  Bottom cradle, bottom skin, core, top skin, top cradle.  With everything perfectly aligned, set several heavy weights on the stack to press the skins until they cure.  Alternatively, you can use vacuum bagging to apply the skins, but this technique is beyond the scope of this article.  

8. Installing Trim: Once both wings have been sheeted top and bottom, you're ready to go on to the next step. Trim and block-sand the skins flush with the leading edge, root and tip, and then use a straightedge to trim the trailing edge. Glue on the leading edge cap, and sand it to the proper shape.  Be very careful shaping the leading edge, as this has a great effect on flight performance.  Install the wingtips with odorless CA and sand them to shape. Run a bead of thin CA down the trailing edge to make sure it's properly bonded. You now have two light, straight and perfect wings, ready for joining.

9. Joining the Wings: The easiest and lightest way to join wing panels for small models is to block up each panel and bl;ock sand the root to the proper angle. Then, join the panels with thick CA and block them up at the proper angle to dry. Then, cover the dihedral joint with a strip of glass cloth, which is then saturated with thin odorless CA. After the CA has cured, block sand the tape to feather it into the balsa. That's it! From here, you can proceed with the rest of the construction.

Conclusion: Foam wings are easy to assemble, durable and extremely resistant to warping. Using this technique, you can have a smooth and true wing with just a couple of hours of total work.