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I've started a new project using large molded balsa shells. Some of this is experimental, but much of it is based on prior efforts and proven techniques. I thought it might be useful, or at least entertaining, to post a few photos for those who are interested in scratch building. I think it will all work out as planned, but this is my first time to mold shells this large, so who knows?
The project is a fairly large jet fuselage. Overall length will be 90" with molded shells 78" long. I carved a 1/6 balsa model to get a handle on the shape and appearance. The basic fuse structure is circular in cross section with just a bit of camber to the central axis.
< Message edited by Bob R2 -- 2/24/2008 1:27:01 AM >
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I've done a bit glass molding in the past, but somehow working with balsa is more satisfying to me. I suppose that goes back to my beginnings in modeling when I built with balsa and ambroid. That dates me big time.
Over the years, I have used formed balsa shells with glass/carbon fiber applied to both the inside and outside surfaces. This allows me to work with balsa but achieve many of the advantages of molded construction - strength and rigidity in a shape with compound curvature.
The reason for applying glass/carbon fiber to both sides of a balsa shell is to dramatically increase strength and rigidity (particularly in compression) as compared to either a pure balsa structure or a pure molded glass structure. This is pretty much common knowledge, but it is always a lot of fun to see a relatively flexible piece of balsa become as rigid as plywood.
In the first pic below, a piece of 1/8" x 4" x 14" balsa (10# density) is suspended in a vise with a 3.5# load applied in the middle of one end. The bowing is evident. In the second pic below, the other end of the balsa has been laminated with 1 layer of 2.3 oz glass on one side and 2 layers of 2.3 glass on the opposing side. With the same 3.5# load, there is very little deflection.
< Message edited by Bob R2 -- 2/23/2008 5:37:32 PM >
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I've fabricated shells in a variety of ways, but I think the best for shapes with heavy compound curvature is balsa molding. The video by Bob Hunt and Bill Werwage is an excellent reference, particularly the second half of the video. Balsa molding may look complicated, but it is actually fairly simple, and it is very rewarding if you like scratch building and balsa construction.
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The first step was to fabricate the forms over which the balsa will be molded. The method I used is very similar to that in the Hunt video. The base is fabricated from 1/2" mdf. The formers are made from 1/8" plywood. When making the formers, cut two of each, one for the form and one for a glue up frame (fabricated later). The easiest way to get identical copies is to glue pieces of ply together with 3M spray and then cut.
The formers were glued in place and 2# eps blocks were glued between the formers. A hot wire with a stand off riding the formers was used to get the eps blocks within about a 1/4" of the formers. This saved a lot shaping effort.
The best tools I have found for rapid final shaping are a Stanley sure form plane and a flexible sanding block. The plane is available at most hardware stores. I fabricated the flexible sanding block out of 1/4" ply in about 10 minutes and applied 60 grit paper, but similar tools are available from auto body supply houses. The length of flexible block should be such that it can span two formers. Shaping using this approach goes very quickly. I got each form to final shape in about 30 minutes.
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The next step is to glue up the balsa sheets. Sorry - no photos, but the sketch below shows the general idea. The blue lines represent a fuselage half surface unrolled flat. Because the balsa shell will be 78" long, it is necessary to create the shell from a forward balsa piece about 32 inches long and a rearward balsa piece about 48" long. Each piece is created much the same way as a wing skin for a foam wing. 1/8" thick x 4" wide sheets are cut up and then edge glued using your favorite technique. The glue needs to be waterproof as the skins are going to be saturated with water prior to forming. "A" grain balsa is best for molding.
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I also find this thread very interesting. In boat construction the laminates are done with edge grained balsa. Do you do the same? Do you also vacuum press your balsa composite?
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Thanks for the encouragement.
bipeaddict - end grain balsa would theoretically be best, but I've never figured out how to use it with thin shell structures that have complex curvature. End grain works great for flat surfaces however. No vacuum will be used, just ace bandages.
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The forward and rearward balsa sheets will ultimately get glued together. The strongest way to glue the sheets together is with a scarf joint. The pic below shows the basic approach (viewed from the side edge). A scarf joint 6 times the thickness of the material gives a good strong joint. The easiest and quickest way to accomplish this with 1/8" material is to lay the pieces on top of one another, place at the edge of a hard flat surface (edge of a workbench or a piece of mdf), slide the upper piece over 3/4", then remove material with a rigid sanding block with coarse paper. When the upper piece is rotated over, it will fit the lower piece very tightly. This is a quick and easy process, but a little practice on scrap wood is worthwhile before attacking a large balsa sheet.
I decided to glue the pieces together after molding as I was concerned that a glued up scarf joint might interfere with the molding process.
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Now it is time to soak the balsa sheets. I've used pans and sinks for smaller pieces, but buying or building a pan large enough for this project would be a pain. I've found the best approach is to use four large bath towels. If you are married, you might want to use old ones (although no damage is done to the towels). Flop two towels down on the shop floor as shown below. Now begin carefully pouring water on the towels. Fully saturate the towels until water begins to pool and almost (but not quite) runs off the edge of the towel. This will be a fair amount of water, probably several quarts. Now wet out the balsa sheets in a sink or with a garden hose and then lay the balsa sheets on the saturated towels. Now take the other two towels, lay them on top of the balsa sheets, and fully saturate as before. The balsa is effectively submerged in water and will fully saturate. I've waited as little as an hour, but just to be safe, I like to wait several hours before I remove the balsa sheets from the towels. If you are in a hot, dry climate, you may have to wet out the towels once or twice. However, this time of year in my shop, the towels stayed fully saturated over night.
Some folks recommend the addition of ammonia to soften the balsa, but I have not found it necessary and just use straight tap water.
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Large composite shell using molded balsa - 
