LIGHTWEIGHT WING SKINNING
by Mark Atwood
This article covers the process of bonding the wing skin to the foam. It assumes you have already equipped the wing (with servo trays, spars, retract plates, etc.) and made and prepared the balsa wing skins.
The main goals when sheeting a wing (or stabilizer or other surface) are to make it straight, light, and strong. Strength usually comes from the shape and structure of the materials you use. With the possible exception of pylon racers, which endure very high loads, most wings need little more than a small single spar or, for a two-piece wing, a well-secured wing tube. The phenolic type is sufficient to meet the strength requirements of the aircraft in this article, which is a 2-meter F3A RC aerobatics (pattern) model. Given that, the main focus should be on lightness and straightness.
Some builders labor intensively to core out foam material. They do this not to save the weight of the foam (which weighs practically nothing), but to remove material where glue would have to be located. The process is time-consuming, and if done improperly it can critically reduce the structure’s stress-handling ability.
If the builder has properly selected the wood materials and the foam cores are made from the lightest material possible (0.75–1 pound in density), this method can be used to obtain the lightest structure possible. The weight of the glue is the enemy when building light. The following is a simple, low-tech, low-skill method for producing an extremely light, straight wing with a minimal amount of glue and fuss. In addition, this construction method is fast because you won’t have to labor over hollowing out delicate wing cores.
Preparing and marking the foam
To start, identify all the places on the wing core that will eventually be cut out or cut through during the building process. This typically includes the ailerons, flaps, servo openings, landing-gear openings, etc. The wing shown will have an aileron cutout as well as a servo opening on the bottom.
Use a permanent marker (they write best on the foam) to outline every place that will be cut. When marking the aileron cutout, keep in mind you’re likely cutting an inner and outer boundary around the aileron so there is room to cap the wing surface with balsa and to cap the edges of the aileron.
Draw accurate centerlines on the foam wing’s root and tip. Measure the center point of the wingtip (or root) at various points from the leading edge (LE) to the trailing edge (TE), and connect the points to form the centerline. Take your time; your wing will be only as straight as your centerline.
Once that is complete, test-fit the wing skins and foam cores into the foam shucks from which the wing was cut. Trim the balsa skins so that there is approximately 1/4 inch of overhang on all sides. This will make it easier to line up the wing in the shucks after the glue is applied.
Setting up and aligning
Lay the entire assembly on a flat surface. A piece of old pool-table slate is ideal: flat and sturdy, and you can often get a slab for free from a local billiards repair shop. It may be slightly chipped and unusable for a table but perfect for a level building board.
Use a height gauge (a simple homemade version is sufficient) to measure the distance from the tabletop to the centerline. The centerline of the wingtip should be parallel to the board, as should the centerline of the root. They will probably not be at the same height because most wings have some dihedral, but both centerlines must be parallel, or you’ll build a twist into the wing.
Do not assume the wing shucks are straight. Internal stresses in the foam often cause it to twist slightly upon cutting. Check to make sure you can get the position close. You’ll spend the time to get it perfect after you apply the glue.
Materials Needed
- Foam core and shucks
- Balsa skins (made to fit)
- Height gauge (homemade or professional)
- Glue syringe (for example, from SIG Manufacturing)
- Two- or three-hour slow-set epoxy (adhesive—not resin)
- 1/2- to 1-inch plywood board (slightly larger than the panel being sheeted)
- Sharpie marker (various colors recommended)
- 100–150 pounds of weight (cinder blocks work well)
- Hard, sturdy, flat surface (pool-table slate is ideal)
Applying epoxy and sheeting
Once you're satisfied the wing can be positioned correctly, mix some epoxy. I use roughly 1 ounce to sheet both sides of a 2-meter pattern model’s wing panel (approximately half of a 900-square-inch wing).
I like a two-hour or three-hour slow-set epoxy—preferably one that’s not too thin. This is contrary to many sheeting methods that spread a thin epoxy across all the foam. A thin epoxy spread too thin will be quickly absorbed into the foam, and you risk getting poor adherence.
If you lay a bead of thicker epoxy on the foam, you're guaranteed to get a great bond where glue is located. The trick is knowing where to put it. Any of the various brands of glue syringes works well to distribute a consistent bead of adhesive onto the foam. Fill the syringe with glue (it will hold roughly 1/2 ounce of epoxy) and place a small bead (approximately 1/16 inch in diameter) on all the areas marked on the foam.
When placing glue on the perimeter, a line within 1/2 inch of the edge is sufficient. The glue will spread when the skins are placed and weighted. Begin making a checkerboard of glue lines, leaving roughly 2-inch squares. Draw lines of glue back and forth and then up and down until there are no open areas larger than 2 x 2 inches.
Once you have distributed the glue on the foam, place the wing skin carefully over the foam, flip the wing over (I place it in the foam shuck), and repeat on the other side.
With glue on both sides and the wing carefully positioned in the shucks, place a 1/2- or 3/4-inch piece of plywood or particleboard over the wing to weight it down. Using the height gauge, check to make sure the centerlines on the root and tip are parallel to the building surface.
The line at the root should be the same height from the board all the way from the LE to the TE. The same goes for the centerline on the tip, although the tip height may vary from the root height. If both lines are parallel, you are sure to have a straight wing.
If the lines are not parallel, make small adjustments to move the wing in the shucks to straighten it out. Do not be afraid to have the wing slightly out of alignment with the shuck if that's what it takes to get the measurement straight.
Weighting, curing, and final notes
(Editor's note: Those who build foam wings designed with wash-in should follow the plans and measure to make sure the correct incidence is maintained. The author described following the plans to ensure the wing was built without incidence in the wingtips.)
Once you have the measurements straight, add weight on top of the plywood board to apply pressure while the glue cures. Somewhere between 100 and 150 pounds is necessary to ensure a good bond. Cinder blocks weigh nearly 50 pounds each, and they are inexpensive and easy to find. Allow the glue to cure for a full 24 hours before removing the weight.
The result will be a solid, straight wing. And by using less than an ounce or so of epoxy, it will be a light one.
—Mark Atwood [email protected]
Transcribed from original scans by AI. Minor OCR errors may remain.
