Fast Build Straight Wings Simple Construction
Foam wings have long been used for wing construction in RC, Stunt, and Combat. Conventional sheeted construction requires lots of light, expensive balsa and somewhat tricky sheeting. Make one mistake and the wing gets ruined! Sheeted foam wings are also pretty heavy for Stunt use unless they are extensively cored out or cut out, introducing even more opportunities for mistakes and increasing building time. Combat wings have often used bare foam wings covered with heat shrink covering( a technique pioneered by the Core House, by the way). Most of the Core House combat planes use this construction- quick, easy, and light, but the finish can leave something to be desired for pretty airplanes. True-Beam construction takes building foam wings a step into the future.
The True Beam wing depends on a number of innovations to deliver a light, strong, straight wing with a very quick build time. First, the CNC cut foam wing cores act as both a jig for assembly, the ribs and sheeting, and provide the overall shape. Second, the foam is stiffened and protected by the composite covering. You can’t get much lighter. We use a precise, computer-controlled foam cutter to shape the wings. The outside is smooth and precise with a smooth foam surface that makes finishing much easier and more uniform. Internally the foam is hollowed out to reduce weight. Slots are cut to position the spars and align the spar webs between the upper and lower surfaces. A couple simple 1/8 in. ribs complete the structure.
The finish on a True-Beam wing becomes part of the composite structure. We've done a LOT of experimenting over the years. Modern materials such as water-based varnishes combined with tried and true Silkspan make an easy to apply finish that strengthens and stiffens the wing. The Silkspan can be topped with our SLC covering and Rustoleum(tm) paints for a quick gloss finish, or many other paints can be applied to the Silkspan conventionally.
Giles Construction Notes ` 12/02
Please read these notes through in order to get a good idea of how the wing is put together. This is still a work in progress, so the exact construction sequence you use will depend on you......
Prebuild Steps:
1) Prepare the spars- the spars are reinforced with fiberglass and epoxy at the root. Prepare a 3 in.sheet of 1/8 in. Medium weight A grain balsa. Score the sheet with the grain for about 3 in at one end. Use approx. 10 scores per inch. Lay a piece of 2 oz. Glass about 4 in. Long on the sheet. Glue down with 30 min. Epoxy. Smear the epoxy on and warm it with a heat gun so it penetrates the glass and wood. Soak up any excess by blotting with a paper towel. Strip the sheet into six pieces just under ½ in. wide.
2) Lightly sand the wing cores with 100 grit paper on a large block to remove any "fuzz". Don’t try to remove all imperfections at this time.
3) Cut the main and auxiliary spars from light quarter grain balsa. Glue the cross grain stiffeners to the main spar at the root, and at intervals along the wing, stopping about 3 in. from the tip. Glue five cross grain siffeners on the rear spar near the root, mid and tip. Leave 1/8 in. clearance along the top and bottom edges to clear the grooves in the foam.
Assembly:
1) Glue the leading edges into the front of the wing. Apply a very thin coat of polyurethane glue to both sides of the leading edge. Insert it in the wing with 1/4 in. Protruding at the root.. Use masking tape to pull the foam snugly square against the wood. Take care that the foam lines up evenly along the length of the panel to avoid introducing warps and that the leading edge is straight.
2) When dry, test fit the spars in the wing. They should slide in easily. Sand as needed for a tight slip fit. Apply a very thin coat of polyurethane glue to the bare surface of each spar cap. Slide the caps into the wing with the glass to the center and work back and forth to ensure the glue is evenly spread. The spar caps should protrude about ½ in. at the root.
3) Run a very thin bead of polyurethane glue along each edge of the main spar. Carefully slide the spar into place, holding is slightly cocked vertically and spreading the cores slightly so the glue does not get wiped off. Once in place work the spar back and forth to spread the glue.
4) Do the same procedure on the auxiliary spar. You can also run a bead of glue along a piece of 1/16 in. music wire and use it to spread glue into the slot.
5) lay the wing flat on the bench or in a piece of foam cut from the block and lightly clamp it by laying several magazines on top.
6) Repeat the procedures for the other wing. Lay it down as it will fit on the plane when clamping so both wings will be symmetrical.
7) Check the spars for solid bonding. Glue can also be applied by running a thin bead along a piece of music wire and laying it in the slot in the foam or corner of the spar and foam and working back and forth to apply the glue.
Finishing:
I prefer to completely finish the wing before assembling the plane. You can also assemble the plane and finish it. The basic procedures will be the same. The goal of finishing is to build up a smooth, stiff surface to support the final color and resist dings and hangar rash. The basic process is to stabilize the surface of the foam, fill in the pores, and finally, either cover and paint, or just paint.
I use four types of coatings on the wing- light weight spachtling compound(Red Devil, Dap, etc) water-based acrylic varnish(MinWax Polycrylic, etc.), a filler for the foam made by blending equal volumes of light weight spachtle and 50% thinned w-b varnish, and a sanding filler made by adding corn starch to an equal volume of 50% thinned w-b varnish.
1) Sand the wings, with spars and other wood installed, thoroughly with 100 grit paper on a large block and get them smooth and even. Work slowly with light pressure so the foam does not get deformed. Take extra care around the leading edge, the ribs, and the trailing edge to sand lightly so the wood is sanded evenly down with the foam. Too much pressure will cut the foam lower than the wood.
2) Seal the foam with a coat of w-b varnish thinned 50% with water. This stabilizes the surface for easier sanding. Let the varnish dry at least 24 hrs. so it firms up solidly.
3) When dry, sand lightly smooth and fill the surface with light weight spachtling compound. Use a spreader and work the filler into the holes. Try and leave a coat of ~1/32 in. Let dry thoroughly. If the filler is not completely dry it will tend to pill and roll when sanding and make grooves in the foam. When dry, block sand thoroughly. Try to remove all the filler on the surface without sanding through into the foam.
4) Make up the foam filler coat of w-b varnish and light weight spachtling compound. In a plastic kitchen container blend w-b varnish into an equal volume of filler. The mix should have the consistency of whipped cream. The filler will probably tend to separate while you are using it. Keep it mixed up every time before you dip the brush. Work the filler well into the foam and try for an even thin coat. Let it dry completey and block. Different brands of filler have different characteristics. The cheaper store brands tend to be coarse and sand out of the foam. Dap and Red Devil are a little harder and leave a better film on the surface after sandinding block
5) Sand virtually all the filler off, leaving just a slight film. Try not to sand completely through the filler into the foam. If you do, try applying another coat of filler to the area and resanding. When done sanding vacuum the wing thoroughly. Keep touching up any spots that need it as long as you can stand it and the weight doesn’t build up too much. ( I like to weigh each panel before and after applying the filler. Keep sanding until the weight gain is no more than 5 gr. per coat.)
6) Apply a thin coat of w-b varnish, thinned 50%. and block sand again. The finish should be evenly smooth and shiny.
7) Apply a layer of OO silkspan. Wet the paper so it expands. Lay it on the wing and brush thinned w-b varnish through it to hold in place. Cover both sides of the wing and then allow to dry. Watch the panels when drying. If the covering tries to pull up along the edges smooth it down. It will be much harder to smooth out when dry.
8) Apply a coat of sanding filler. Let dry and sand.
9) A second layer of silkspan will make the wing surface sturdier and further cover up any blemishes. It will also add significant weight. If you use two layers of silkspan don't apply sanding filler to the first coat. Directly apply the second layer, and a coat of sanding filler when dry. After sanding examine the surface. If the foam still shows through, apply another thin coat of filler. Sand thoroughly and vacuum the wing clean. Keep touching up any spots that need it as long as you can stand it and the weight doesn’t build up too much.
10) cover the wing with the clear covering supplied. Start with the bottom. Cut a piece large enough for the panel. Start ironing it down in the middle, working towards the edges. Try to stretch and smooth the covering as it is ironed to eliminate any wrinkles. Use the lowest heat you can to just get it to stick so the heat won’t raise the foam texture. The covering can be heated and repplied at least once if you are careless and iron in a wrinkle. Overlap the top covering by wrapping around the leading edge.
11) Carefully seal the edges of the covering by wiping with the covering iron to eliminate any bubbles and seal the covering down tightly.
12) buff the covering with a scouring pad to knock off some of the shine.
13) Apply a mist coat of Rustoleum Gloss Protective Enamel. Folllow up with a second coat about 15 min later. Apply just enough paint to cover completely. It is best to paint the surfaces one side at a time. When the first side is dry, turn the wing over and paint the other.
14) Mask any trim colors using a good quality, relatively low tack masking tape.
Target Weights:
Bare Cores 150-160 gr.
Filled, sanded with spars, 3/16 in. TE, and ribs 175-180 gr.
2 layers of OO Silkspan 60 gr.
SLC Covering 30gr.
Rustoleum Paint 2 coats 60 gr.