Foam Wing Construction
Terry Boston
SPA 698

I’ve built a number of models over the years with foam wings and stabs of all sizes from .25 Pylon racers to 43% models with 200cc gas burners. I have also had the privilege of talking with a number of modelers and taking bits and pieces from each of them to come up with the following way that I’ve found that works the best for me.

Webmaster's note: the wing shown in the photos is for the Deception that Terry built for me in 2019.

Wing Core Preparation

Start by building a servo box for each servo from 1/16” or 3/32” balsa with 1/8” aircraft ply supports. The length should be left oversize so that it can be trimmed off later.
Lightly sand the foam core and lay out the location of landing gear blocks and the servo boxes on the BOTTOM of the wing cores.
Cut out for the servo boxes and landing gear blocks with supports. I use a piece of #12 wire bent to shape installed in a soldering gun. Take the time to make a template from 1/8” lite ply to make an accurate cut.
Fit supports made from 1/8” ply and landing gear blocks in the cores for a good fit. Then glue the landing gear block, inboard support, and torque block together. Drill through tork block into the landing gear block and make sure the 5/32” landing gear wire fits properly. Cap the end of the torque block with balsa. Do not glue into the cores yet. 
Using a formed wire and the soldering gun cut a groove in the TOP of the wing from the servo box area to the root with the help of a guide.
Next make a cap from soft balsa to cover the slot that was just cut - see below. This step will keep the sheeting from sagging into the slot. Now glue in the servo boxes and the slot covers with Titebond or epoxy. When dry sand flush with the core surface. Be careful not to sand into the foam. Test fit the servos and leads.

Before starting the sheeting, make a template of the location of the servo boxes and landing gear slot from an old file folder.
Cap the wing tip with a piece of 1/16” balsa and sand to shape. The reason for this will become evident later on.

Wing Skins

Most kits and plans will tell you to glue a bunch of sheets together and make a diagonal cut to fit the wing. But I feel that by doing this you would end up with end grain trying to conform to the shape of the leading edge especially on a wing with a tapered leading edge. The way I have done it for a number of years is to run a sheet parallel to both the leading and trailing edges. Use softer sheets for the L.E. and a bit harder for the T.E. The center is completed by running sheets parallel to the trailing edge for the top skin and parallel to the leading edge on the bottom skin. This cross graining gives you a ply wood effect and helps prevents warping.
When gluing the sheets together be sure to trim the edge of the sheets and edge sand them before gluing.

Vacuum the cores to remove debris and dust.

I use Gorilla glue to attach the skins to the cores. Spread the glue sparingly on the balsa skins only. Here are my steps.

  • Glue TOP skin on first. Add the top shuck and pin in place through the shucks, sheeting and into the core with T pins so nothing shifts.
  • Flip the wing over. Glue in the landing gear block and supports with Gorilla glue then glue on the bottom skin, add the shuck and pin in place.
  • Weight it down and let it dry overnight.
  • I do each wing half separately instead of stacking two together.

NOTE: Be sure that you have a flat surface to work on. I have a bench with a ½” thick glass surface. Then I place a piece of ¼” glass on top of the core sandwich with lots of weight. I have gathered steel blocks that I use for weight over the years.

Trim the sheeting to the cores. Glue on the leading and trailing edge stock. I use Titebond but epoxy is good too. Wipe off any excess glue and hold everything in place with masking tape while it dries.

When the glue is dry, sand LE & TE flush with the root and tip of the wing. Now draw a center line on the root and tip, and then transfer it to the leading and trailing edges. These center lines help in a number of ways; first they will help when shaping the leading edge and second they give you a straight line on the trailing edge to locate the aileron hinges. 

Now that part is done and it’s time to make the ailerons. Measure the thickness of the trailing edge. Let’s say that it’s ½”; cut four pieces of ¼” balsa wider and longer than the finished size. Cut a piece of 1/16” ply about ½” wide the same length as the balsa stock you cut. Now is the time to use some of the small pieces of 1/16” balsa left over from the wing skins. Cut them so that the grain runs 90°(Perpendicular) to the aileron hinge line. On a flat surface, glue this entire sandwich together at one time using Sig polyester glass resin or slow curing (30 minute) epoxy and add weight. Trim to width, making sure the balsa side is square.

NOTE: if the wood is bowed, put the bowed pieces opposite each other when assembling the aileron - see drawing at right. This will result in a flat aileron. 

Most models don’t have full length ailerons, so mark the location of the area that will become part of the wing. Glue balsa side of that area to the trailing edge and tack glue the aileron area. Use the center lines to align it. Use a block plane and sanding block to shape the aileron. Do not cut aileron free yet. 

Now lay up the wing tips using a 1/16” bass wood core. The basswood is cut to be a guide for later hollowing the completed tip - see photo.

Glue together - thick CA works well here.

Tack glue the wing tip to the wing using the center lines as guides and shape. Break tips off and hollow out. Glue tips to the wing with thin CA. 

Cut the aileron free and trim to get proper spacing. Locate and cut hinge slots in wing and ailerons. Shape aileron leading edge chamfer. To keep a nice sharp edge, cap the ends of the ailerons with 1/32” bass wood.

Using the template that you made earlier, remove the sheeting from the servo boxes and landing gear area on the bottom of the wing. Don’t forget to notch the groove for the servo leads to exit on the top of the wing.

Sand the root to obtain the proper dihedral angle. Glue wing halves together with epoxy. Check the alignment both visually using the center lines and with an incidence meter to ensure that the incidence of both wings is the same. When cured, sand the root area to smooth out any mismatch in the sheeting. Next reinforce with fiberglass cloth. Instead of one piece, I use two pieces overlapping them in the center by about 1”. Sand smooth and fill any small nicks with filler and sand out. If all looks good you’re ready to cover.