Original, graphical version.
The technique of vacuum bagging is used to construct foam core wings, either with a wet layup of fiberglass cloth and resin or to apply a wood veneer skin. Both practices use the same principal: enclose the wing in an airtight "bag", then remove the air from within the bag allowing atmospheric pressure to hold the wing skin against the core while the adhesive cures.
To outline some of the advantages of vacuum bagging, understand the conventional techniques of applying a veneer wing skin to a foam core. After spreading the inside surfaces of the wing skins with adhesive, you place them on the cores, place the cores into their "blanks" they were cut from, and weight the entire bundle to hold the skins firm against the core while the glue sets. If, for example, you were building a slope racer with 500 sq in per wing half, and you applied 50 lbs to the core as it set, you would effectively be applying only 1.6 oz per sq in.--not a whole lot of weight. When you consider the force of the weight (gravity) is acting only in a vertical plane, that 1.6 oz per sq in is even less effective along the leading edge where the surface is not perpendicular to the force being applied.
The use of the vacuum bag eliminates the need to weight the cores while the glue sets. By removing the air from inside the bag, the atmospheric pressure acts against the bag, pressing the wing skin against the core. This pressure is applied evenly, at a perpendicular angles to all surfaces inside the bag. In addition, you can effectively apply a TREMENDOUS amount of pressure. Using our example of a 500sq in wing half, drawing about 20" mercury from the vacuum pump will result in about 10 lbs per sq in applied against the wing surface. That would be the equivalent of stacking 5,000 lbs on your wing core! As you can imagine, you have to be very careful to limit the vacuum inside the bag or you will most certainly crush your core. Two-pound density, blue foam is desirable for vacuum bagging, and is necessary for use with wet lay up, 'glass skinned wings.
Because of the additional pressure applied against the wing, you need to cut your core from 2 lb density blue foam. Used in the "real world" for insulation, blue foam is usually available at a building supply or insulation store. Cut your core as usual, fine sand it as necessary and vacuum and tac-rag it to remove any dust. Do not make any cuts in the core for servos, ballast, etc. Do that AFTER the bagging. Keep the outer "shucks" or "blanks" the cores were cut from handy.
Mylar sheet is used to mold the smooth surface of the mylar to the outside surface of the wing, and does not require a mold release agent. Cut your mylar from .007" sheet to the exact size of the core at the root, leading edge and tip, and about 3/4" beyond the trailing edge of the core. Tape the top and bottom mylar sheets together with packing tape (on what will be the outside surface) along the leading edge, leaving about 1/8" gap between the two sheets.
Multiple layers of lighter 'glass cloth will be stronger than a single layer of heavier cloth. On a 60" or smaller slope wing, I usually use two layers of medium (3.5oz) cloth. Larger slope racer wings will get two layers of heavy (6oz) cloth with an outside layer of light (1oz) cloth to prevent pinholes that would otherwise be caused by corse weave of the heavy cloth. Since I don't fly thermal stuff, you floater guys are on your own to select the right amount and weight of 'glass cloth! ;-) Cut your cloth so that one piece wraps from trailing edge, around the leading edge and back to the trailing edge. Not that we are trying to make the leading edge perfect--it will have to be cut off later--but its easier to apply the resin to the single, large piece of cloth. Cut all layers to the same dimensions.
The bag itself has only two real requirements--it must be airtight and it should be relatively clear so you can see what's going on inside the bag! I use clear .004" visquine plastic, with "strip caulking" to seal it. Both are regular hardware store items. The bag will be a folded sheet of visquine, sealed on the three open sides. Cut the plastic so when folded in half you have about 12" of overlap all the way around the core. Affix the strip caulking in one continuous strip from slightly past the fold, all the way around the perimeter of the bag to just beyond the fold on the other side. Leave the bag open, or the backing (if any) on the caulking until the lay up is done and you are ready to seal it up.
Setup and test your pump, setting the vacuum switch to the appropriate setting--about 16" to 18" for blue foam. (I use a commercial pump intended for R/C use. You can build your own though. At minimum it should have a bleed valve to adjust the initial vacuum, and a vacuum switch which will allow it to cycle on and off rather than run continuously). I like to run an evacuation tube into both ends of the bag, all the way in so they touch the root and tip. You can cut a slot in the end of the tube to prevent it from "sucking" itself to the core. Make the tubes from about 14" of fiberglass arrow shaft or equivalent. Attach the vacuum hoses to the tubes and join them with a "Y" connector inline before the pump. Make sure your hoses are long enough to reach both ends of the bag without pulling on the bag. Keep some packing tape on hand to anchor them to the bench after everything is sealed up.
Now its time to gather everything else that you will need. Epoxy resins, mixing cups and sticks, spreader or "squeegee", rubber gloves, respirator (if necessary), paper towels, etc. You've probably used fiberglass before--gather everything you might need. Choose a resin with a viscosity close to that of warm maple syrup and with a moderate pot life--at least 10 minutes--and working time--at least 45 minutes. Clear your bench of any unnecessary items and lay down a layer of newspaper to catch any spilled resin. Have more newspaper handy to cover the bench with a clean layer before putting the bag down.
Ready? Let's do it. Lay out your mylar sheets, taped together, inside surface facing up. Lay down your 'glass cloth, placing the lightest cloth first and heavier layers after that. Smooth the cloth against the mylar. Mix your epoxy as per the directions, noting the time as you start. Pour a ribbon of resin span wise, about 1/4" wide on each half of the cloth. Use a spreader or squeegee to work the resin into the cloth. Pay particular attention to the color of the cloth. It will turn translucent as it wets out. This is particularly important with more layers of cloth, as you want it thoroughly wetted through to the mylar. Be sure to work out any air bubbles between the layers. Once you are satisfied you have applied enough resin (the surface should be wet, but without puddles), place the leading edge of the core (trailing edge pointing up) on the gap where the mylar is taped together. Carefully lift both halves of the mylar/'glass lay up up against the core--like you were closing a book, or making a big foam-filled, toxic taco! Holding the core/lay up by the trailing edge (leading edge down), put down clean layer of newspaper and place the open bag on the bench. Carefully place the core/lay up in the middle of the bottom half of the bag. Put the appropriate half of the core's "blanks" or "shucks" UNDER the bag to support the core/lay up. Peel the backing (if any) from the caulking. Place the evacuation tubes against the ends of the core/lay up, using a piece of the caulking to hold the tube in place if necessary. Place another 2" piece of caulking over the evacuation tube where it crosses the caulking on the bag. Once satisfied with the tube placement, close the bag, being careful to pull out wrinkles in the plastic as it closes onto the strip caulking (wrinkles WILL leak air into the bag). Pound and roll the caulking all the way around to seal it. The caulking around the evacuations tubes will need some kneading to seal them, but it should seal up tight. Open the bleed valve on the pump and turn it on. Slowly close the valve until you notice the bag beginning to close in around the core. Continue to pull out wrinkles as the bag closes around the core/lay up. Before it closes tight (you can tell by watching as the wrinkles and folds become sharply defined), hold the core/lay up down against its "blank" and roll it chord wise with a small roller to help workout any remaining air bubbles between the layers of 'glass. Be sure that there are no obvious warps or bows in the core/layup--whatever is there now will be there forever!--check it with a straightedge. Once satisfied that all is straight, close the bleed valve until the pump reaches the desired amount of vacuum and the vacuum switch shuts it off. Monitor the pump for a few cycles to ensure that it is working properly, and there are no major leaks. A good pump should be able to hold for a new minutes before kicking on again for a moment. If all looks good, leave it alone until the resin has thoroughly cured--about 9-12 hours.
Once the resin has cured and you remove the wing from the bag, you will have to clean up all the edges. Be very careful while handling the raw edges--ragged strands of cured 'glass and resin make nasty splinters! Use a scroll saw or razor saw with a fresh blade to trim the root and tip ends from the wing. Cut back only far enough to leave a "square corner". Trim the trailing edge about 1/8" beyond the end of the core. Note if your core's trailing edge wasn't straight, do not trim back into the foam. Cut off the leading edge about 1/4" to 3/8" back and replace it with an appropriately sized piece of spruce or hard balsa (I prefer spruce). You can apply masking tape to the 'glass surface behind the leading edge to protect the 'glass from sanding scratches. A towel on the work bench will further protect the surface from dings and scratches. Mark and cut off the ailerons and or flaps, and any servo openings in the bottom of the wing. Surface hinging is open to your preference, although 'glass bagged wings are perfectly suited to the tubular "knuckle" hinges. Shape your leading edges and install the tips as you would any other wing. ;-)
Mix your epoxy resin as directed, and drizzle a small ribbon onto the inside of each veneer--be sure you do a right and left side (top and bottom)!. Use a plastic spreader or squeegee to spread the resin across the entire surface of the veneer. Here's the key: scrape all extra resin from the veneer surface. If your core is clean and has a smooth surface, you surprisingly little resin for a good bond. You will want the surface to take on a satin finish--NOT wet or glossy looking. (Thermal flyers be careful here--this where you can build a lot of extra weight into your wing!) Once finished with the resin, place one of the veneers onto the core. You can pin it with a few straight pins to keep the skin from sliding out of place when you place it in the bag. Just be sure the pin doesn't go through the wing such that it could puncture the bag. Flip the core over and place and pin the other veneer onto the core. Put the wing into the bag and seal it up. Place the core's bottom (or top) "shuck" or "blank" under the bag to support the core as the bag is evacuated. Turn on the pump and monitor the bag and wing as it evacuates, checking the wing for warps. Check if the leading edge is being pushed tight against the core. Sometimes the bag will try to pull into a gap between the foam and veneer at the leading edge. If it does that, gently pull the bag away from the leading edge--as if you were stretching the bag out flat. Once everything is true and the pump is cycling properly, leave it alone to cure--about 9 to 12 hours.
Once the epoxy has cured, pull the wing out of the bag, remove the pins and finish the construction as you would with any other wood veneer skinned wing.
Copyright © 1996, Chris Fouquet.
Revised: September 21, 1996.