How to vacuum bag foam wings with balsa skins [Archive]

Ralph Weaver

May 01, 1997, 01:00 AM

Applying balsa sheeting to a white foam core doesn't have to be a daunting
task. Just follow this step-by-step guide starting with a freshly cut core.

Step 1. Cut any slots for control linkage or spars.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/Core1.jpg" width="443" height="78">

Figure 1. Spar slots.

Step 2. Glue in the linkage or spars. Be sure that any hard material is below the top
surface of the foam, then fill with balsa or filler.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/Core2.jpg" width="157" height="77">

Figure 2. Spars and filler.

Step 3. Trim the edges of the balsa sheets. It is important that the edges are straight
and square. I use a balsa stripper and a long straight edge.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/trimming.jpg" width="319" height="292">

Picture 1. Trim the edge of a balsa sheet.

Step 4. Tape the sheets together. The tape will be on the finish side.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/joining.jpg" width="236" height="244">

Picture 2. Taping the sheets.

Step 5. Glue the balsa sheets together. Let the first sheet hang over the edge of the
work surface. Apply a thin bead along the seem. Water based glues are best because they
will cause the wood to swell slightly, filling any gaps and they are easy to sand. After
all the seems have been glued, lay the sheets out flat and use a damp cloth to wipe off
excess glue. Place a few weights on the sheets so they will dry flat.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/gluing.jpg" width="176" height="244">

Picture 3. Gluing the sheets.

Step 6. After the glue has dried, remove the tape. Sand the balsa smooth on both sides.
It is easier to sand now that after it's on the wing.

Step 7. Lightly sand the foam and vacuum well to remove any dust. Wipe the balsa with a
tack rag to remove dust.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/dusting.jpg" width="373" height="135">

Picture 4. Foam core ready to bag. Note spar and control cable.

Step 8. Cut the balsa to size. The grain should run parallel to the training edge. The
sheeting should extend just past each end and about 1/8" past the leading edge of the
foam. The trailing edge of the sheeting extends about 1/2" past the foam core. Lay
the core on the sheet and double check before any cuts are made! Cut both top and bottom
sheets the same size.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/Core3.jpg" width="435" height="125">

Figure 3. Core and sheeting.

Step 8. Draw a line on the sheeting where the trailing edge of the foam
will go. Also mark where any glass or carbon fiber will be placed on the sheeting.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/marking.jpg" width="202" height="286">

Picture 5. Marking the foam trailing edge on the cut sheet.

Step 9. Lay the sheets together with the inside (marked) surfaces on the
inside. Tape the trailing edges together. A 1" piece of tape every 6" is usually
about right.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/taping.jpg" width="251" height="294">

Picture 6. Tape the trailing edges together.

Step 10. Optional - spray the inside surface of the sheeting with a couple
light coats of laquer. This prevents too much epoxy from soaking into the wood later.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/spraying.jpg" width="381" height="183">

Picture 7. Spraying laquer on the inside of the sheets.

Step 11. Prepare the fiberglass trailing edge and any other glass or
carbon fiber. Spray them with a light coat of 3M 77.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/tacking.jpg" width="136" height="292">

Picture 8. Spraying 3M 77 on the fiberglass and carbon fiber.

Step 12. Apply the trailing edge and carbon fiber.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/reinforce.jpg" width="377" height="233">

Picture 9. Applying the carbon fiber spars.

Step 13. Use a brush to apply resin to any fiber glass or carbon fiber.
Don't go overboard, but the fiberglass needs to be completely saturated to provide a stiff
trailing edge.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/brushing.jpg" width="408" height="302">

Picture 10. Applying the resin.

Step 14. Apply the resin to the rest of the sheeting. Drip a little at a
time onto the sheeting from the mixing cup and spread it out immediatly. Work a little at
a time or the epoxy will soak in before you can spread it. Very little epoxy is needed
when bagging, just barely wet is OK. I usually add just a bit more on the leading edge
area.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/Core4.jpg" width="339" height="94">

Figure 4. Lay core on Sheeting.

Step 15. Lay the core on the sheeting, lining up the trailing edge with
the line drawn earlier.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/foam.jpg" width="270" height="238">

Picture 11. Laying the core on the sheet.

Step 16. Carefully fold the top over and add a couple of pieces of tape
along the leading edge so it's easier to handle. Make sure the foam is in position and
smooth out the sheeting.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/finishing.jpg" width="314" height="228">

Picture 12. Folding the sheet over.

Step 17. Cut the bag so there is at least 3-4" on each side and about 6" on
each end. Close one end with caulking tape. Put breather felt or braided rope at the end
and along each side so all the air can escape.

Put a peice of caulking tape on the bottom side of the open end. Place the air tube on
the tape and put another piece of tape on top of the tube. There must be some felt under
the end of the tube or it will suck in the bag.

Slide the wing into the bag. Place the bottom foam shell under the wing and smooth out
the bag. Place the top shell on top of the wing and place a little weight on it. Make sure
everything is lined up and seal the open end of the bag.

<img src="http://static.rcgroups.com/articles/ezonemag/1997/bagging/bagging.jpg" width="407" height="331">

Picture 13. In the bag.

Step 18. Turn on the vacuum pump. Smooth out any large wrinkles at the bag is pulled
tight. Press down on the tape at the ends to ensure a good seal. Check for leaks. Leave
until the resin cures.

Done!

Visit Ralph Weaver's home page at <a href="http://www.iquest.net/~weaverr">http://www.iquest.net/~weaverr</a>