Here It Is! True-Beam Foam Wing Construction

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.

New True-Beam Foam Wings

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 goes over our SLC covering followed by primer and Rustoleum(tm) paints for a quick gloss finish, or many other paints can be applied as a final finish  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 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 spar from light, quarter grain, 1/16 in. balsa. Glue the cross grain stiffeners about 3 in. long to the main spar at the root, 1 in. wide strips at intervals along the wing, stopping about 3 in. from the tip. Trim the root of the main spar, top and bottom, to allow for hardwood(spruce or bass) joingers that connect the wing halves through the fuselage.  The joiners can either stand up vertically, or lay flat.  Bevel the ends to make it easier to slide into the space between the main spar and the spars when assmebling the wing to the fuselage.

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 3/8 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 the glue on towards the wing surface.  Work back and forth to ensure the glue is evenly spread. The spar caps should protrude about 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.  The spar should be flush with the root end of the wing.

4) Apply glue in the gap between the top and bottom foam auxiliary spars.  You can run a bead of glue along a piece of 1/16 in. music wire and use it to spread glue into the slot.  If needed, put a thin bead of glue onto a piece of 1/16 in. square balsa to fill in between the top and bottom foam spars.

5) lay the wing flat on the bench or in a piece of foam cut from the outer 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.  The wings are cut with a slight dihedral angle at the root, so the top of the wing will be flat after assembly.  Make sure the panels are allowed to dry with the same surface up so no warp is introduced.

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.

You will need some light weight spachtling compound(DAP, Red Devil, or a housebrand), also some water-based acrylic/polyurethane varnish such as Carver-Tripp, Varathan, or Min-Wax.  For filling and leveling the wing surface mix a couple of tablespoons of varnish into 1 cup of filler and mix thoroughly.  Mix every time before use.  The varnish will settle to the bottom after the mix sits for a day or too.

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 filler/varnish.. This stabilizes the surface for easier sanding. Let the varnish dry at least 24 hrs. so it firms up solidly.
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)  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. Periodically 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 10gr. per panel.)

6) Apply SLC covering, or other very light weight plastic film that can safely go over foam).  The simplest finish is to stop here and go to step 9, final finishing. When applying the SLC covering start with the bottom. Cut a piece large enough for the panel. Start ironing it down in the at the high point at the wing root, working towards the tip and edges. Try to stretch and smooth the covering as it is ironed to prevent  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 reapplied at least once if you are careless and iron in a wrinkle. Overlap the top covering by wrapping around the leading edge.

7) A slightly heavier, sturdier cover can be had by applying a second layer.  This can be either SLC or medium silkspan. Dampen 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. I've found that a decently smooth finish directly over foam weighs pretty much the same regardless how you get it done. It takes a certain amount(read weight) of finish to harden the foam and build a durable finish. One layer of SLC covering is by far the lightest.  One layer of heavy silkspan, two layers of light silkspan, or alternating film and silkspan end up the same. One solid coat of unthinned w-b varnish only weighs slightly more than multiple coats of thinned varnish with the same final finish. It is possible to greatly ADD excess weight with too much filler or multiple layers of paper.

8) Seal the silkspan with w-b varnish. Let dry and sand. If you use two layers of OO silkspan apply a thin coat of w-b varnish to the first layer. Directly apply the second layer, and seal with w-b varnish. After sanding examine the surface. If the foam still shows through, apply another thin coat of filler. Sand thoroughly and vacuum the wing clean. Try not to go through the paper. Keep touching up any spots that need it as long as you can stand it and the weight doesn’t build up too much.

9) buff the covering with a scouring pad to knock off some of the shine.

10) Apply a thick, dry layer of automotive primer and sand virtually all of it off.  The surface will have a mottled grey/white appearnce.  Continue with your favorited finish.  Just be very careful with solvent-based paint(dope, etc) so the stuff doesn't get under the covering and start dissolving the foam.  The SLC covering can be wrapped around the root rib face and sealed down.  Also you can apply a small amount of w/b varnish on the edges of film and around the root to help protect the foam.

11) Apply the base coat of color.  If you use spray enamel(Rust-O-Leum(tm) or similar) be careful with white.  It seems to stick less well.  Make sure the surface has been thoroughly wiped at least twice with a lint-free cloth and alcohol.  Apply the enamel with a very light mist coat.  Allow to dry 15 min. until tacky and then apply a finish coat.  Use just enough to get a solid color.  Allow to dry at least overnight.  Mask any trim colors using a good quality, relatively low tack masking tape.  Remove the tape carefully so the finish won't pull up.

Target Weights:

Bare Cores 150-160 gr.

Filled with spachtle, sanded, with spars, 3/16 in. TE, and ribs 175-180 gr.

2 layers of OO Silkspan 90 gr.

SLC Covering 30gr.

Rustoleum Paint 2 coats 60 gr.