I. INTRODUCTION: Why use foam core wings? When I first got into R/C, I swore that I'd never use foam; it was to be balsa sticks or nothing for me. Then came the fateful day that necessity forced me to cut a foam wing for a scratch-built project, and I was instantly sold. Since then, around 90% of the models I've built have had foam wings. Foam cores are light, straight, fast, cheap, and tough. Once, a plane of mine suffered a midair in which it met leading edge to leading edge with a built-up model. My plane trimmed a foot off the other plane's wing and kept right on going. I used to own a trainer that was flown through trees three times before I sold it to a student. Like I said, foam is tough.
II. MATERIALS: The materials for foam wings can be exotic and costly, but this doesn't have to be the case. I make most of my foam wings using white foam purchased from a local drywall supplier, balsa wing skins from the hobby shop, and contact adhesive purchased at the hardware store.
A. Foam: There are two main foam plastics used for R/C airframes:
1. Styrene Foam: Polystyrene is the traditional material used for foam structures in model airplanes, and there are two principle types:
a) White Foam: Also called beaded foam, this is what's used to a make cheap beer coolers. It's molded in giant blocks called billets, so nearly any size block is possible. White foam weighs around 1 pound per cubic foot and cuts readily with a hot wire. When shopping for it, make sure you ask for cured foam, rather than the cheaper economy foam. Economy foam is not suitable for making foam wing cores.
b) Blue, Pink, and Gray Foam: These foams are extruded rather than molded, so they have a finer grain structure and are only available in sheets. At about 2 pounds per cubic foot, they're denser than white foam, but they also have very high compression strength. This makes them a popular choice for models that will have to withstand very high G-loadings like pylon racers. The razor-thin wings of these racers have very little volume, so the weight penalty for the heavier foam is slight. Like white foam, extruded foam cuts well with a hot wire, although higher temperature settings are typically used.
2. Expanded Polypropylene Foam: This material, called EPP for short, was originally intended for packaging applications, but it's a modern miracle for modeling. Like styrene foam, its lightweight and can be cut with a hot wire, but it has the additional property of being nearly indestructible. EPP is used to make slope gliders that can stand up to repeated mid-air collisions and crashes. An EPP plane will simply bounce when subjected to an impact that would demolish a balsa model.
B. Skins: There are almost countless materials that can be used to skin foam wings. In selecting the skin material, bear in mind that while foam has high compression strength, it has poor tensile and flexural strength. It's the skin material's job to provide those properties.
1. Balsa: Good old balsa wood is something we're all used to working with. Balsa makes a very good skin for foam wings in that's it's light and has very high tensile strength. Once laminated to a good foam core, the result is a very stiff wing that can be finished with nearly any covering system. Of course, balsa can also be expensive on a large project.
2. Obechi: Obechi is a hardwood veneer that is sometimes used as a substitute for balsa. It doesn't require being glued up into sheets, and it's cheaper than balsa. Unfortunately, it's also heavier, and I normally don't use ii.
3. Kraft Paper: For small models, a surprisingly tough wing can be made with the regular brown Kraft paper that's used for wrapping parcels. White glue or laminating epoxy can be used for the adhesive, and if the wing is vacuum-bagged, a very smooth wing will result.
4. Fiberglass: Sailplane and racing pilots have long been vacuum-bagging their wings with wet lay-ups of fiberglass, carbon fiber, Keviar and other advanced materials. This isn't the black magic that it seems, and there are now some excellent books on the market that explain all the mysteries.
C. Adhesives: Because foam is attacked by most solvents, special adhesives need to be used for laminating foam wings.
1. Contact Adhesives: There are many contact adhesives on the market, but my fayorite for attaching the skins to foam wings is 3M Super 77, an aerosol adhesive that weighs next to nothing and is very simple to apply. Simply spray a light coat on the core and the skin, lay the skin on the core, and smooth it down. The bond is instantaneous and very strong. The problem with contact adhesives is that there's no second chance to reposition the skins if you get it wrong.
2. Laminating Epoxy: Laminating epoxies allow you to slide the skins around while laying up the wing. Of course, the lay-up must be firmly clamped with weights or a vacuum bag until the epoxy cures completely. When using laminating epoxy, try to use as little as possible, as it adds a great deal of weight. Squeegee the epoxy over the surface of the skin, scraping it out very thin. Some builders like to spray a light coat of lacquer or shellac on the skin so that the epoxy won't soak into the wood.
3. Odorless CA: Odorless CA can also be used on foam. I generally use it only for attaching the leading edges and joining the panels, but some also like to use it to attach the actual skins.
D.Template Materials: The ideal material for the cutting templates needs to have several properties. It needs to stand up to high temperatures so the hot wire won't melt or bum it. It needs to be possible to sand the edges very smooth so that the cutting wire will glide over it with minimal friction. It needs to be easy to cut to a very accurate profile. Finally, it needs to be cheap and readily available; the best material in the world is of no use if you can't find the stuff.
1. Plywood: Because most modelers already have it lying around the shop, birch plywood is the first material that usually gets pressed into service for foam wing templates. It holds up to the heat of the wire reasonably well, but it's impossible to get the edge smooth enough to cut really first-class cores. Remember: the rougher the core, the worse the wire will drag during the cut.
2. Mat Board: Artist's mat board can be used for temporary templates, provided you harden the edges with CA and then carefully sand them smooth. Of course, that means it's almost as easy to make a set of permanent templates that you can save for future jobs.
3. Aluminum: I've made many templates from thin gauge aluminum sheeting purchased from the hardware store. It's easy to cut with a set of aviation snips, and the edges can be sanded very smooth in short order. However, the templates can be bent if not stored and used carefully.
4. Phenolic: Phenolic sheet, PC board and other thermoset plastics make excellent templates. They cut readily with a scroll saw, their edges can sanded to a very smooth surface, and they're stiff and tough enough to stand up to years of use.
5.Formica: To me, this is the ideal material for templates. It has all the advantages of board, with the added attractions that it is: a) readily available and b) free. Nearly any cabinet shop that builds counter tops has scrap Formica lying around getting in the way. A polite request will almost always get you a free lifetime's supply, along with an invitation to come back for more.
III. EQUIPMENT: While there are complete commercial cutting Systems available, a very practical foam cutting system can be assembled for just a few dollars.
A. Power Supplies: Modelers can spend a tot of time and energy trying to locate an inexpensive power supply for their foam cutting set-up. Here are a few options:
1. Auto Batteries: For cutting one or two wings, you can use a simple car battery, but bear in mind that you might have to adjust the temperature by adding extra wire length. Since this is hard on the battery, I don't recommend it.
2. Battery Chargers: High output battery chargers can also be used as a power supply, but since most chargers today have peak detection, a battery will need to be added to the circuit to provide a load to keep the charger from shutting off. Again, a problem is the difficulty of adjusting the wire temperature.
3. Variac Transformers: Variac transformers make excellent power supplies; they allow extremely accurate adjustment of the output and will last for years. If you can find one cheap, go for it. But since they normally cost over $100, I chose to look for a lower cost solution.
4. Custom-made Power Supplies: A very practical variable power supply can be built for about $20. My present power supply is made from a 125V 3A toggle switch, a light dimmer switch and a 12V 3A transformer, all purchased at Radio Shack. For safety and convenience, I installed the assembly in a project box and installed Powerpole connectors so that I could disconnect the power leads. In practice, the power is turned on with the toggle switch, and the temperature is adjusted with the dimmer knob. At the end of the cut, the toggle switch is turned off, so that the output doesn't have to be set for each cut. The proper power setting will remain very consistent for a bow of any given length, but if a longer bow is used, the power setting will have to be increased.
IV. CUTTING WING CORES: There are two stages to cutting a set of cores; cutting the blanks to the proper plan view and cutting the actual airfoil in the blanks.
A. Prep Work: Before cutting the actual cores, you have to make the cutting templates and the foam blanks for the wing.
1. Cutting Templates: Again, I prefer scrap Formica high pressure laminate (HPL) for my wing templates. I use an airfoil generation program like Winfoil or Compufoil to print the foil sections out on paper, and then I make separate templates for the top and bottom surfaces. Both templates are drilled with the same hole pattern, and these holes will be used to index the templates to their proper position on the foam blank. After cutting the templates to shape, careflilly sand them as smooth as possible with extra fine sandpaper.
2. Cutting Foam Blanks: Some builders like to cut their foam blanks to shape with a table saw, but I can't imagine why. It makes an incredible mess, and the noise is appalling. I made a pair of perfectly perpendicular uprights that can be clamped on my work bench so that I can slice the blocks to size with the hot wire bow. The uprights have tail pieces that extend below the surface of the table and stop pegs that prevent the hot wire from running off the end. These simple guides make it easy to cut out blanks of any size in short order.
B. Cutting the Cores: There are two main methods for cutting cores with a hot wire:
1. Manual Cutting: A very serviceable foam cutting bow can be made from a piece of 1 X 2" lumber, some 1/4" music wire for the uprights, and nichrome wire purchased from your local hobby shop. You also need to put some heat-resistant insulators on the wire so that you can guide it with your fingertips. The bow can either be suspended with rubber bands or can simply rest on the operator's forearms. Still another option is to put a tail wheel in the middle of the crossbar so that the bow rolls along the table surface. Many operators like to put index numbers on the templates and make the operation a two-man job; the leader handles the longer root chord, and the assistant follows along as the leader counts off with practice, I got to where I could do better on my own, something I'm sure my long-suffering wife appreciated.
2. Machine Cutting: One of my best hobby investments in the last year was a Feathercut~ foam wing cutting machine. At $150, it's not a cheap device, but it makes up for that by cutting core after core with little or no variation. It's downright entertaining to flip a switch, stand back, and watch the machine cut a perfect core while you drink a Coke. There are a number of designs for foam wing cutting machines, but most of them use the same principle: a system of weights and pulleys is used to pull the hot wire through the foam blank. Because there is no human operator, there's no vibration and the cutting rate can be slower. This results in a perfectly uniform cut. Best of all, these machines can be set up to cut tapered cores just as well as constant chord wings.
C. Hint and Tips: There are a number of finer points in operating technique, and here are some of the more important ones. You'll discover more as you gain experience; no two people do everything exactly the same way.
1. You'll find that there's a narrow "sweet spot", in which the cut is as smooth as possible across the length of the core. If the cutting temperature is too hot, the "kerf" left by the wire is too wide and excessive surface melting will result, especially at the narrower end of the wing. Conversely, if the temperature is too cool, the wire will drag, which will still result in excessive melting and roughness at the narrow end.
2. For long wings or extreme tapers, it helps to cut the cores in sections that are then butt-joined before being skinned. This is also helpful for wings that have elliptical planforms or compound tapers. Since the cores only contribute compression resistance to the wing, using multiple sections has absolutely no effect on strength.
3. A perfectly flat work surface is critical to success; if your bench is not flat, your cores will not be straight. By the same token, you need to use large weights to hold the foam blank flat during the cuts; I use stainless steel plates that were obtained from a scrap yard.
V. CONCLUSIONS: After cutting the cores, they can be prepped by lightly sanding them with fine sandpaper and then vacuumed clean. You can then proceed with attaching the skins of your choice with the adhesive that best fits the application. This primer was not intended to establish set rules. As you gain experience with foam, you'll develop your own preferences and perhaps even help to advance the state of the art. Good luck!