Applying Skins to Foam Wings
One of the biggest challenges for the novice is how to apply balsa skins to foam cores. There are many methods, some simple and then some complex. The simplest method is to use a contact adhesive such as 3M™ Super 77 Spray Adhesive. One of the concerns using this adhesive is that it is likely to debond with high temperatures or age. Another alternative is to use epoxy but this is tricky and messy if you are not confident in using this method. The best method we have found was to use polyurethane adhesive found at many hardware stores. Best of all its inexpensive. Here is a procedure for applying the skins.
Once the foam cores have been cut, glue several planks of 1.5mm balsa sheet to form the upper and lower skins of each wing panel. To do this place several sheets on a flat surface. Butt each sheet together firmly and tape together using masking tape. Turn the sheet over and place on the workbench edge. Hinge one of the planks down and run a bead of PVA glue. Repeat for the other planks then place the sheet on a flat surface to dry. The following sketch illustrates this method.
BUILDING TIPS
To glue the skins onto the foam core use the following technique. Obtain polyurethane glue from the local hardware store. (Note: Polyurethane has a honey like consistency. When exposed to moisture it froths and expands filling up voids.) Spread a liberal amount of polyurethane glue over each wing skin. Spread evenly using a spatula. Wet the foam cores using a damp cloth. Place the wing skins onto the foam cores so that the glue comes into contact with the damp foam. Place weights on top until the glue sets. Care must be taken as the wing cores can slide between the skins while setting.
Once the glue has dried remove the wings and trim the excess skins. Glue the leading and trailing edges spars using white glue. Add wing tips as shown on the plan. Cut out trailing edge using stock material. Glue to the trailing edge spar as shown on the plan. Cut ailerons to length as per plan.
Cutting Channels In Foam Cores
One of the difficulties with foam wings is running servo cables or pneumatic air lines to the retracts. This is easily solved by boring tunnels in the foam core to the retract & servo cavities. To do this, use the following method. Place the wing panel on a flat surface and pack it up with weights so it doesn't move. Obtain an aluminum tube, which has the same diameter of the tunnel you wish to create. Serrate or sharpen one edge of the tube. Place guide blocks under the aluminum tube and with a rotational motion bore a hole through the foam core to the desired location. Refer to the following sketch for further details.
Fiberglass Skins
There are many methods for covering models these days but the best is covering in fiberglass cloth. The fiberglass method is durable and allows an adequate surface for priming and painting. The models IJD have built are covered with a light fiberglass (0.73 Oz weighted cloth).
The best method to apply the fiberglass cloth is as follows. Obtain epoxy finishing resin from your local hobby store such as Pacer's Finishing Resin. It's normally thinner than epoxy glue to make it easier to spread. Lay the cloth over the structure to be covered. Cut the cloth approximately 30mm beyond the extremities of the structure. Using digital scales weigh the cloth. Take the weight measurement and add 10%. This new weight is the weight of the finishing resign required to cover the cloth. Mix a batch of the resin using this new weight as a guide. Add a touch of methylated spirits or similar white alcahol to thin the epoxy mixture even further. The final consistency should run fairly easily similar to water. Using a disposable brush, apply the resin onto the cloth. The cloth & balsa will soak up the resin as needed. Don't over do it, if you have shiny patches then there is too much resin. Once done, let the resin cure overnight. Trim off the excess fiberglass and sand back any loose edges. Prime and paint all surfaces. Congratulations that is it !!!
