All RC Foam Core Wings

Vacuum Bagging Foam Core Wings

Mike has figured out how to strengthen a foam core and make it very durable. The words Foam core wings suggests that foam is the core of the wing. But what is it really the core of?

Mike Waters discovered that there is strength in laminating a sheet of 3/4 ounce fiberglass and a sheet of 1/16″ thick balsa onto the top and bottom of the foam core. Epoxy is used sparingly to adhere the sheets fiberglass and balsa to the foam. Vacuum bagging the whole system while the epoxy cures makes the foam, fiberglass and balsa become one unit and very strong.

Most who vacuum bag foam core wings usually laminate with the wood, usually 1/8″ balsa or thin plywood, on the inside layer and the fiberglass on the outside layer. While this is also very strong and done in the same way, having the wood on the outside makes using coverings very easy. Many, like me at first, may think that 1/16″ balsa would not provide much strength. But I have seen its strength first hand. I also tried to break a piece with my hands but could not break it without a tremendous amount of force.

So, I have watched Mike’s video on sheeting foam core wings several times.

This post is a written version of Mike’s video as I applied it to my first vacuum bagging attempt. I highly recommend watching the video before reading the rest of this post.

I took notes to help me remember the tools and materials needed and the steps in the process. Mike offered a wealth of data in the video. These are my notes albeit edited for posting.

The work table should be large enough to lay the foam block and both balsa sheets side by side. Mine is a piece of 1/2″ x 43″ x 43″ cheap plywood laying on my router table. The 1/16″ x 3″ x 36″ sheets of balsa are taped tightly together at each edge. Then the whole sheet is trimmed to fit the wing core. These balsa sheets are already taped together on the opposite side.

Next cut the 3/4 ounce fiberglass cloth to size. Try not to handle it any more than absolutely necessary or it will lose its shape. Drape it over a chair or stand near the work surface.

Now is the most challenging part. Getting everything together so you can get the laminations all glued in a reasonable time. This is a list of things Mike suggested or used in his video:

  • Foam cores in their shucks
  • 1/16″ x 3″ x 36″ balsa sheets taped together
  • 3/4 ounce fiberglass sheets
  • West Systems 105 Epoxy and 206 Hardener
  • Rit Dye
  • 3M 1 Blue 2090-24EVP, .94 in. x 60 yd. Scotch Painters Tape
  • Scotch Contractor Grade Masking Tape, 1.88 inches x 60.1 yd. (yellow)
  • 4 oz. Graduated Transparent Polypropylene Plastic Cups
  • Stirring sticks
  • Paper towels
  • Denatured Alcohol in a spray bottle
  • Latex gloves
  • 3″ paint roller and handle
  • 36 grit sanding block
  • Exacto knife
  • Vacuum bag and bag seals
  • Burlap
  • 3/4″ MDF plattens

The wing cores I am building have a 1/4″ square balsa spar. So I also needed a small paint brush to put epoxy into the groove in the foam.

I started with some new bagging material. It is a 27″ wide vinyl tube that you can buy by the foot from I cut this one to about 50″ long. The valve was purchased when I got the parts for the vacuum press from I pressed a hole with a hole punch to make a nice smooth round hole for the stem. Be sure to use the rubber washers and O-ring to seal the stem. Place the stem where it will be on a flat area of the platen when in use.

I also have a purple Yoga mat that I lay over the table for the vacuum bag to lay on. Mike uses foam blocks. Protect your vacuum bag! Have it ready before proceeding.

The rest is “Super Easy” if you watched the video.

Mix some epoxy per the label directions. We use West Systems Epoxy and 206 hardener to allow about 30 minutes of working time. I made 3 oz. of epoxy and had to mix a little more at the end. Next time I plan to mix close to 4 oz. Add some dye to the epoxy. It is very difficult to see the epoxy coat if you do not use the dye. Complete coverage is necessary to achieve a strong bond.

Spread the epoxy over the two balsa sheets fairly quickly to keep the heat down. If left in the mixing cup, it will heat rapidly and can even melt the cup. (Ask me how I know.) Roll the epoxy out thinly but with even coverage on both balsa sheets.

Roll epoxy onto the first side of the foam core. Keep this very thin. All it needs is to be wetted with epoxy to ensure a good bond. I need a dropper so I can add more dye. It did not come out dark enough once it was spread out.

Now carefully lay the fiberglass sheets onto the balsa sheets. Try to avoid wrinkles as you lay it down. It will not move very easily after it gets epoxy on it. Roll the fiberglass with the damp roller to remove the air pockets.

Lay the balsa/fiberglass laminate onto the wing core. Carefully align the ends and the trailing edge of the laminate sheet and the foam edges. The trailing edge of the foam block must be made even and square. Place the shuck over the first side while keeping everything aligned on the trailing edge. Turn the block over and do the same to the other side.

Hold everything in alignment tightly and put masking tape on each end as clamps and then along the sides.

Sand the end corners to make sure there are not sharp edges of balsa to puncture the bag.

Now cover the entire mid-section of the foam block to be sure all fiberglass is kept away from the vacuum bag. The hairs become like needles as the epoxy cures. The tape also seals the epoxy in under the foam so it does not adhere to the vacuum bag. Use the Exacto knife to poke holes in the tape along the sides of the foam. This will let air escape.

Tape the block onto a platen at each end and middle of the sides to keep the block from moving while placing it in the vacuum bag. Slide it into the bag. Slide the top platen over the foam block. Add some burlap on top of the platen and a paper towel at the air outlet to filter the air and keep trash out of the vacuum system.

Seal the end of the bag and connect to the vacuum system.

Turn on the vacuum pump. Watch and be sure the vacuum bag does not get pinched while the air is removed.

9″Hg is adequate. If the system cycles up to 9″Hg and down to 8″Hg it will work well. The system should not cycle often. It should be able to hold the vacuum for around 10 to 15 minutes unless there is a leak. Mine is cycling at about 2.5 minutes. So I need to find the leak. It is working but the pump has to start too many times with the leak.

Leave the vacuum system on for about 3 hours or longer. The epoxy has to set before the vacuum is removed.

I left it on for almost 4 hours to be sure the epoxy went off.

I removed the vacuum and removed the foam from the bag.

It took a few minutes to remove the tape but the result looks good.

Notice the yellow tape on the balsa seams. Mike pointed my error. That stuff is hard to remove without damage to the balsa. USE blue painters tape!

The removal of the tape took a while but it did come off. The pressure inside the bag pressed it on really firm. Again, use the blue tape!

Note: I found the vacuum leak after making the first wing. The old style valve I purchased for the bag came with an O-ring. I put it all together the way it was stacked when I got it. Turns out the threads will let air in if the O-ring is not in the correct position.

This is the correct position. The O-ring seals the stem against the metal washer on the inside of the bag. That keeps the threads from leaking.

I tested the empty bag. Now the vacuum press cycles once in 59 minutes for a couple of seconds! I am glad it is now working correctly.

Success! What a relief. I think the next ones will be better now that I have some firsthand experience.

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