Then you would remove some trailing edge material from the foam core, and replace it with balsa. You could cut the control surface out, then remove some leading edge material and replace it with balsa. This adds rigidity, and provides a material for gluing the hinges in. Here's an image you've seen many times, showing the profile of a foam wing and aileron.įoam does not provide a good material for hinging, so when you cut control surfaces for a foam core, you must face the trailing edge of the foam core, and the leading edge of the control surface with balsa. Here's a tip for foam wings you may not have thought of. (Use the appropriate type of wire for your power requirements, and use caution.) Here's what this simple wire tool would look like. One neat way to do this is to make a small custom tool, for use with your foam cutter's power supply, or by inserting the tool into a soldering gun. ![]() This could be used for installing a wing socket for a joiner tube, or simply a channel to route servo wires through. Sometimes you'll have a need to cut a channel, or other hole in your core, span-wise. If properly done, the resulting structure is both strong and light, and has the advantage of being relatively fast, compared to cutting, shaping, and assembling a lot of balsa. This can be accomplished with physical pressure on the sheeting/foam/sheeting "sandwich", or by vacuum bagging, which is the superior method. Next, any interior cutouts and/or installations are done in the foam, before it is sheeted and/or fiberglassed. The basic process is to produce templates first, (usually a "root" and "tip" template) then use the electrically-heated hot wire to cut the basic wing shape out of the foam. There are several commercial devices made for cutting foam wing cores and other parts. Making Foam Wing Cores is relatively easy, using styrofoam. A wood structure as precise could have been built, but the time factor was not something I was not happy about, so I went with foam. In one afternoon, I cut foam wing cores, assembled the sheeting, and had this shape, which is a lot more precise, and less expensive. I didn't do a very good job of getting the shape right, (got impatient, because it was "just a test") and correcting all those ribs would've been a very long process, and fairly expensive too. This was made in the traditional way, plotting individual ribs, inserting spars, and finally sheeting the structure. Take a look at the two images below, which were tests for me, when I began building plugs for the "Avanti", and you'll see why I prefer foam at times. One advantage of making foam wings is that it's easy to be both fast and precise. (See the How To Make Plugs For Molds.Differently page.) I've actually poured the two-part expandable urethane foam between bulkheads on a keel structure, let it expand, then sanded it back down to the bulkheads, making a rough shape for a plug very quickly. It is also easily fiberglassed, using either polyester resin or epoxy. Having said that, it has it's place in modeling, especially in the case of making plugs for molds. It should NOT be cut with a hot wire, or otherwise "burned" because it gives off very toxic fumes that are a serious hazard. Urethane foam is also fairly light, easy to sand and cut, and has the advantage of resisting most glues and solvents. Don't use the heavier ("pink") foam for flying model parts, in general, although it's good for making plugs. It has the advantage of being very easy to sand and shape, cuts easily with a hot wire, and, in the lower densities ("white" and "blue") is fairly light. That's why, in addition to the structural strength it adds, foam wings and fuselages are always sheeted and/or fiberglassed, before any finishing work can be done. CA and polyester resin will also melt styrofoam, and should not be used. Almost any other type of glue, solvent, and paint will melt stryrofoam, and obviously should not be used. Styrofoam (extruded or expanded bead polystyrene) can accept epoxy, and certain other types of mild "white glues", with epoxy being the best choice. Foam has some qualities you need to be aware of, as far as how it reacts with adhesives and solvents. ![]() In a composite structure, the foam serves the purpose of providing a shape, while the material that surrounds it serves the purpose of providing the strength. Like most other composites, the use of foam in models has a specific purpose.
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