Foam for Small Models

FOAM SMALL U Paul Mcllrath FOR ODELS BUILDERS OF commercial homebuilt full-scale airplanes use foam extensively Foam-core wings routine Radio Control Control Line Ready fly models kiddies toys fast-food meals have using plastic foam 20 years acceptance foam builders small models has very slow reasons Some obvious engineering reasons some psychological wont go psychological reasons exception seems widespread feeling using foam model means model must foam After building two-foot Lockheed Vega observers would call foam went back carefully noted different materials used addition foam three kinds wood three different metals two kinds paper three other plastics An balsa model might use nearly types material Thoughtful modelers have always used material does particular job best some cases might foam Information article based personal experience experience limited small models Indoor outdoor rubberpowered Electric spans up 30 inches Builders other types models can expected faced different problems come up different solutions About 20 years ago began look ways use foam food trays coming grocery store After few years experimenting tray foam friend split cost 4 x 8-foot slab foam insulation started investigating ways cut own sheets larger sizes various thicknesses homemade hot-wire sheet cutter successful possibilities foam model construction really opened up Insulating foam food trays foam types have worked Building-supply stores sell insulating foam thicknesses up two inches Food trays available free packaging groceries fast-food meals as-manufactured condition foam has dense glossy skin skin adds considerable weight stiffness sheet thickness skin varies product product suiface extruded hot-pressed cut hot wire will have skin saw-cut sandpapered surfaces will part shaped block will therefore free skinned surfaces part just cut thin sheet might used skin removed intact Some builders use sheets skinned surfaces adhesives finishes use adhere well skin routinely sand off skin thin removing easier might think sheet lying clean flat surface few strokes medium-grit sandpaper block will job discussion article refers material skin removed Characteristics sheets skins left will quite different 46 Model Aviation lage wing model stick-and-tissueall else foam Making 14 cowl bumps balsa would tedious foam its easy Unusual Flying Flea wings bent food-tray stockno need ribs spars dihedral joints tissue covering General characteristics Since foam often used parts normally would make balsa characteristics building practices balsa will usually used making comparisons Sources Some American hobby suppliers beginning offer foam form sheets Two such sources listed end article other sources foam know already mentioned make own sheets using homemade hot-wire cutter scroll saw fitted fine-tooth blade Cost purchased slab insulation cost single sheet model-building size probably few pennies family devours lot groceries food trays can considered free cost sheets hobby sources may vary will probably cheaper balsa Weight Balsa weighs about four 15 pounds per cubic foot Unskinned foam weighs about two pounds per cubic foot Thin sheets skin still might weigh four seven pounds per cubic foot depending thickness skins Strength Its hard make exact comparison strength foam versus balsa sheet applications usually make foam part lV-2 times thickness called out balsa unstressed parts made blocks like headrests air scoops foam can substituted good saving weight harmful consequences An entire fuselage can carved foam blocks high-stress areas like wing landing-gear attachment points will require some internal reinforcement spars longerons stick balsa Cutting shaping use foam starts increase might want make hot-wire cutter cutting own sheets thickness Otherwise common tools used balsa can used foam However tools must sharp dull tool will just mash material just would very soft balsa Foam easy sand sandpaper blocks will quickly become number-one tool foam part can shaped fraction time would take balsa part like cutting tools sandpaper must sharp clean Sawdust foamdust should often brushed part cutting surface sandpaper sandpaper requires regular vigorous cleaning Although foam seems soft dulls sandpaper edged tools rather quickly Blades must sharpened replaced regularly lighter touch required sanding foam balsa sometimes ugly looking scuff mark will unexpectedly appear part sanding happens stop sanding immediately things before resuming Inspect clean surfaces part sandpaper removing loose imbedded material June 1998 47 TIP LAMINATING ISE FORMS CUSTOMDIHEDRAL SANDING BLOCKSSUPPORTS ROUNDED SHA kit ALL EASY TO CARVE AND SAND FROM FOAM AREAS OF HIGH STRESS OR EXTREME WEAR SHOULD BE REINFORCED BALSA PLYWOOD OR STIFF PAPERI GLUE SKIN OFTEN HELPS Foam opens up possibilities experimentationthis biplane Foam profile fuselage saves weightimportant small Electric fuselage formed two foam sheet half-shells models Quick make low cost too Apply less force sandpaper block Go paper finer grit Reverse direction sanding strokes Using sanding block has curved shape roughly similar part sanding also helps object procedures spread contact pressure over larger area little experience will tell what needed will pleasantly surprised see quickly final shaping can done fine sandpaper applied gentle strokes would use applying dope brush Adhesives Common acetate cyanoacrylate CyA glues dissolve polystyrene instantly number white glues adhere well unskinned foam glues intended woodworking other crafts well model work vary little hardness flexibility can pick favorite Epoxy works well heavy high-strength joint often bond parts white glue smear very thin coat epoxy over immediate area have never found perfect glue foam Since foam non-porous white glue doesnt soak does balsa requires much longer dry dry much harder surrounding foam almost impossible sand edge surface Try keep excess glue unseen inside surfaces try scrub off glue smears wet cotton swab before dry Finishing One attractive features foam doesnt have grain surface markings balsa sanded surface has very fine uniform texture accepts paint ink easily brush marks have never used filler foam lightweight spackling plaster found paint hardware stores does good job filling cracks gouges sands easily much like surrounding foam 48 Model Aviation ements traditional model airplane dopes eat foam Aciylic craft paint India ink colored artists ink some felt-tip markers enamel sold plastic models can used unskinned foam Before using new adhesive finishing material try out piece scrap Some combinations glue paint tend cause discoloration cheek too personal favorite finish colored artists ink basic colors added details cut Japanese tissue have seen foam variety colors Light pink red yellow green light blue gray white Light pink blue available area insulation have found no sources other colors except food trays Red orange yellow ink cover pink foam very well White foam will accept color White foam trimmed bit fluorescent enamel used plastic models will knock eye out Some Scale modelers achieve beautiful results airbrushing acrylic paint have used acrylics applied brush trim have never used finish whole airplane Miscellaneous Foams combination low cost ease fabrication makes very useful tryout throwaway parts Patterns laminating forms temporary bulkheads can cut out quickly accurately Cutting out exact dihedral supports easier scrounging around appropriate-size blocks bottles Foam ideal material experimental projects foam surface can bent trimmed down completely replaced fraction time would take build cover conventional surface First test flights often show larger smaller rudder needed Use foam rudder tests replace built-up correct size has determined have made crude test versions entire airplanes try out experimental design determine center gravity CC location investigate spiral-diving tendencies Comfortable old habits have always hard change balsa first coming use Wakefield rules committee dreamed up rule try keep being used competition felt spongy stuff wasnt proper material model airplanes builders small models have about same feeling about polystyrene foam Well just have wait see whether someday spongy stuff will accepted companion balsa tissue music wire 4Paul McI irath 1524 48th St NE Cedar Rapids IA 52402 References Indoor Foam Scale Flying Models An excellent 16-page booklet Photos how-to sketches Available Hannans Runway Box 210 Magalia CA 95954 June 1998 49 Simple low-cost homemade sheet cutter turned out stack foam sheets couple hours Equivalent stack balsa would cost much foam Continued page 58