Large composite shell using molded balsa (Full Version)

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Bob R2 -> Large composite shell using molded balsa (2/23/2008 5:18:13 PM)

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.



Bob R2 -> RE: Large composite shell using molded balsa (2/23/2008 5:35:22 PM)

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.



Bob R2 -> RE: Large composite shell using molded balsa (2/23/2008 5:47:08 PM)

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.



Bob R2 -> RE: Large composite shell using molded balsa (2/23/2008 6:07:16 PM)

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.




Bob R2 -> RE: Large composite shell using molded balsa (2/24/2008 1:22:42 AM)

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.



Ed Smith -> RE: Large composite shell using molded balsa (2/24/2008 1:27:26 PM)

Bob,

I see there have been no responses to your posts. I hope you do not take this as a lack of interest. I am very interested. Keep them coming.

Thanks,

Ed S



TrueScale -> RE: Large composite shell using molded balsa (2/24/2008 1:49:35 PM)

Same here Bob, Plenty of glass experience but never have done this.

Really interesting, Thanks[:D]

Chuck



soarrich -> RE: Large composite shell using molded balsa (2/24/2008 2:21:07 PM)

I second what they said.[sm=thumbup.gif]



bipeaddict -> RE: Large composite shell using molded balsa (2/24/2008 2:45:20 PM)

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?




Bob R2 -> RE: Large composite shell using molded balsa (2/24/2008 3:15:29 PM)

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.



Bob R2 -> RE: Large composite shell using molded balsa (2/24/2008 3:28:53 PM)

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.



Bob R2 -> RE: Large composite shell using molded balsa (2/24/2008 4:58:40 PM)

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.

Time to go fly. I'll post more tomorrow.



majortom-RCU -> RE: Large composite shell using molded balsa (2/25/2008 9:35:15 AM)

Putting my subscription on this thread. Thinking of using this technique on a pattern fuselage.



Bob R2 -> RE: Large composite shell using molded balsa (2/25/2008 4:09:35 PM)

Considerable force and torque is applied to the form while wrapping. Smaller forms can be held by hand and wrapped, but bigger ones need solid support. The bench vise and a stand worked fine for this project.



Bob R2 -> RE: Large composite shell using molded balsa (2/25/2008 4:23:09 PM)

I've got no pics of the actual wrapping process as things get busy once you start. The Hunt video shows the process quite well. Pull a balsa piece from the wet towels and lay on the form. Align and wrap loosely with masking tape at the ends and in the middle to make sure everything is in the correct position. I started wrapping circumferentially with ace bandages at the scarf joint end. Wrap very firmly and overlap each wind slightly. The balsa sheet will stretch and conform very nicely....up to a point. If there is considerable compound curvature as with this shape, the balsa sheet will stop cooperating - you'll know when. Although the solution looks messy, it works very well, and is shown in the pic below. Slice the remaining (unwrapped) balsa sheet lengthwise. Overlap the two sides and tape down temporarily with masking tape. Make another lengthwise slice and remove the excess triangular pieces. Then continue the wrap.

Expect to use several ace bandages on a large shape. Old bedsheets work well also. Simply tear into 4" wide strips and double over into a two inch wide band.



Bob R2 -> RE: Large composite shell using molded balsa (2/25/2008 4:26:13 PM)

Wrapping completed on one end.



Bob R2 -> RE: Large composite shell using molded balsa (2/25/2008 4:29:36 PM)

The second balsa piece can now be wrapped on the other end of the form. Be sure to align so that the scarf joints match up. Allow to dry overnight - longer if your shop is cool and/or humid.



Bob R2 -> RE: Large composite shell using molded balsa (2/25/2008 4:36:59 PM)

Now for the fun part. After drying, unwrap the bandages. The balsa pieces will relax slightly, but they will retain most of the shape of the form.

I laid plastic wrap over the form and then glued the forward and rearward balsa shells together at the scarf joint with epoxy (with the shells on the form). I also wicked thin CA in the slice cuts made during the wrapping process. I covered the epoxy joint with plastic film and then used ace bandages to hold things in place during cure.

I'm very pleased with the results.



Bob R2 -> RE: Large composite shell using molded balsa (2/25/2008 5:49:38 PM)

Balsa and glass selection

The next step is glass the inside surface of the balsa shell. I used 2.3 oz glass and West epoxy slightly thinned. Use epoxy judiciously. Allow epoxy to cure to the point that it is no longer tacky, but still soft. Then lay on the form and wrap with ace bandages. Allow to fully cure. Plastic film over the form is good insurance against sticking.

This shell is going to be on the beefy side as jets see high G loads and occasional rough landings (flameouts etc.). Also, structural rigidity is important to reduce the likelihood of flutter. However, you can tailor balsa and glass selection to your needs. I've done lightweight shells using 6# 3/32" balsa with lighter glass (.75 oz or 1.5 oz).



Bob R2 -> RE: Large composite shell using molded balsa (2/26/2008 8:26:31 PM)

Time to trim the shell edges. This procedure is pretty much dealers choice. I had previously routed a grove in the side of the form. I used a dremel cutoff with a spacer/standoff on the shaft. Works OK. By the way, masking tape is your friend with a project of this sort.



Bob R2 -> RE: Large composite shell using molded balsa (2/26/2008 8:35:37 PM)

In past projects, I've glued shells to a lightweight frame of formers and stringers. For a variety of reasons, I've decided to glue these shells together on a temporary frame.

The additional set of formers (duplicates of those made for the forms) are drilled for a support tube and notched for stringers. The formers are placed on a piece of 1" steel tube with the same spacing as on the original forms. The stringers are doubled up on each side, but only one will stay with the final shell. Everything is lightly tacked in place with CA.



Bob R2 -> RE: Large composite shell using molded balsa (2/26/2008 8:52:24 PM)

Epoxy is carefully applied only to the upper and lower outer stringers. The shells are placed on the assembly frame and held in place with a few wraps of masking tape. Ace bandages are then wrapped around the shells at the former locations to pull the shells against the formers. At the very ends, use multiple wraps of twine instead of bandages to avoid localized distortion (which I got).

Next, I ran #4 screws into the stringers, 2 or 3 between each former. This helps pull the edges of the shells into position. Note the plywood squares (with packing tape applied to the underside) under each screw to help with the process.

The distortion at the ends from using bandages was not significant, but it was noticeable. I was able to perform minor surgery with a razor saw and epoxy and removed the distortion.



Bob R2 -> RE: Large composite shell using molded balsa (2/26/2008 9:19:07 PM)

The bandages are removed after cure. Examination with a light source in a darkened room shows a shape that is pretty darned fair. A bit of lightweight spackle and careful sanding should produce good results.

Ultimately the temporary frame formers will be removed through the wing saddle and cockpit opening, but for time being they must stay in place until the outer layer of glass is applied.

FWIW, each half shell with glass inside weighed 16 oz. This gives a projected total shell weight with one layer of 2.3 oz on the outside surface of 42 oz. Additional localized reinforcement will bring that weight up - perhaps another 12 to 16 oz. Not super light weight, but pretty reasonable given the size of the shell and the rigidity of the structure.



brokenshale -> RE: Large composite shell using molded balsa (2/27/2008 6:33:07 AM)

This is very interesting. I love to build planes and have built six or seven foamies my design already. I would love to learn how to fabricate and form like you are describing above. It sounds so fulfulling. I will try to locate the viedo and see if I can understand the process better. Thanks for stirring my interest. Jerry Norton



Bob R2 -> RE: Large composite shell using molded balsa (3/2/2008 3:42:58 PM)

You are welcome Jerry.

Carbon fiber applied at wing saddle. The shell was carefully sanded. A layer of 2.4 oz cloth was applied.



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