Here is an illustrated guide to vacuum-bagged wings for small model aircraft.

I'll be using Jeffco epoxy resin and hardener, with mylar and other bagging materials from Composite Structures Technology (CST) and Aerospace Composite Products (ACP). For those who are interested, I'm using Jeffco 1310 L-6 epoxy resin and Jeffco 3102 "B" hardener. This is a fairly fast hardener, so in warm weather I would use a slower one, such as Jeffco 3156.

Step one (after the design work) is to cut and assemble your foam cores. The left tip had a small surface blemish... oops. ;)

In this case I wanted a shear web. Here's the location drawn on the cores.

Here I've drawn the outline of the shear web on the pre-made web material. This is 1/32" balsa with 1.5oz glass vacuum-bagged on at a 45degree bias. Peel-ply fabric was used to give a rough external surface.

Cut a slot through the wing for the web, using a straight edge and a very sharp razor blade. I carefully cut all the way through the core, using several passes while making sure to keep the blade perpendicular to the wing surface. The web is now glued into the slot in the wing. I used foam-safe CA glue and worked slowly from one wingtip to the other, making sure to get the alignment correct.

Here is a very simple tool for sanding a groove for the carbon spar cap. Just glue a strip of sandpaper to a small wood block.

Use a straight edge as a guide and sand the groove into the core, centered over the shear web.

Cut your spar caps. I used pre-cured 0.007" carbon sheet in this case. Notice I'm using a double layer of carbon near the root of the wing. Also notice the masking tape applied to the wing adjacent to the spar slots. The bottom side of the wing gets the same treatment and spar layup.

Now we're getting somewhere... The caps are glued in place with colloidal-silica-thickened epoxy. Peel-ply fabric is used to absorb excess epoxy and get a rough surface.

Put this in the bag and draw a small vacuum (maybe 5-10" Hg). Don't forget the porous release film and breather. This is CST's GAST pump... it works great and gives very precise control over the pressure in the bag.

When the epoxy has cured, pull the wing out. Remove the peel-ply and masking tape and then fill any surface imperfections with lightweight spackle. Shape the wingtips and leading edge.

Cut your mylars to shape and wax them thoroughly. Make sure the leading edges and tips are trimmed short, to allow the bag to press down on these areas with sharper radii. When the wax is buffed to a shine, apply PVA if you want. When that's completely dry you can spray apint onto your mylars. This paint will be transferred directly to the wing surface, eliminating the need to paint later.

Apply your glass reinforcements, leading edge and any hinge materials. The leading edge and tips get a full wrap-around of glass cut on a 45 degree bias. The hinge is a strip of peel-ply fabric on a 45 degree bias. Use a light spray of 3M Super 77 to tack the reinforcements in place. Take this time to cut all your glass because you won't want to try to do it once you've mixed your epoxy.

Wet out the reinforcements on the wing core with a foam roller and blot with paper towels to remove any excess epoxy. You want this pretty dry, except for right along the leading edge and tips.

Are you ready? Lay out your skins on the mylars and drizzle epoxy over them. Smooth with a foam roller. My skins were 2 layers of 0.70oz glass on a 45degree bias in this case.

Repeat for the second side. Drizzle and roll...

Blot up the excess epoxy with paper towels. Make it pretty dry. Remember that excess epoxy will just make the wing heavier.. not stronger.

Trim the edges of the glass to the edge of the mylars. Add any trailing edge reinforcements at this time. I used a 1/2" wide strip of 3oz glass.

Carefully line up the core and place it on the mylars. Fold the second side over and verify correct placement and allignment.

Insert the "package" into a sleeve of porous release film.

Wrap a couple layers of paper towels over the release film. This will be your breather. You can also use the commercial breather material, but paper towels work great and the price is right.

Carefully insert the whole assembly into your vacuum bag and draw a small vacuum (again, 5-10" Hg). Very gently work out any trapped air with a squeegee. This also ensures better adhesion between the skin and the wing core. Move from the center out and make sure to get both sides! :)

Here you can see how the core extends slightly past the edge of the mylar. Run the edge of your squeegee (gently) in this groove to make sure there's no trapped air.

When satisfied, place the wing in the lower cradle and allign carefully. Make sure the wing is propperly seated and then draw more vacuum (15-20" Hg). Note: If you're using white foam then don't exceed 10" Hg. Add the top beds, a flat board or two and pile on a fair bit of weight. You want to make sure the wing stays flat.

This is the really cool hose-bag fitting which ACP sells.

After a MINIMUM of 24 hours you can shut off the pump and remove the wing from the bag. Remember that epoxy cure rate is variable with film thickness. In the case of a vac-bagged wing, the skin is very thin so the epoxy cures slowly... it will be a week before the wing is fully cured. Handle it carefully to avoid dings and dents.

Remove the wing from the bag and carefully peel back the breather and release film.

Gently work a razor under the lip of the mylar and slowly peel it up. Use a finger to run along the edge once you've got it started.

Peel back the mylar slowly and steadily. Are we excited yet?? ;)

Do the same for the top side of the wing.

Notice the lip where the edge of the mylar was. We'll have to smooth this out.

Use a sharp razor to carefully scrape the ridge down. Do final shaping with a sanding block.

Mark where the trailing edge will be trimmed with a marker.

Carefully trim the trailing edge with a straight edge and a sharp razor. Take your time and make several passes. Don't slip!!

Make a tracing of one tip to make it easier to get the second one identical.

Use the tracing as a template. Cut as before and sand edges smooth with a sanding block.

How light is your wing???

Starting to look like an airplane.....

Use a scroll saw to cut the ends of the ailerons.

Mark the line for the hinge on top.

Mark the lines for the gap on the bottom. Make it about 1/16" wide.

Use a straight edge and a sharp razor to cut just through the bottom glass and foam. Remove the foam by carefully picking it out with the back of a razor blade.

Use a straight edge and a Dremel cut-off disc to score through the fiberglass on the top of the wing.

Gently flex the hinge and scrape hinge line clean with a razor. The hinge should now be able to move freely.

Please e-mail any comments, questions or suggestions to: davidfee@cox.net

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© David Fee 2002
Last Update May 13, 2002