Trimming a moulded balsa shell

I thought I would share with you what I learned from trying this process.

As it turned out, forming the "buck" and doing the moulding was not as difficult as I thought it would be. The shell has to be moulded a little larger than the final piece, for obvious reasons, so it might be a bit tricky to do the final trimming of the shell for a perfect fit. Windy wrote of imbedding a fine wire in the buck that would leave an impression of where to trim, but Tom Morris used an #11 X-acto blade glued to a piece of balsa, and that's the technique I used.

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The base of the mould was a piece of 3/8" balsa, so the trimming tool was fabricated from a single edge razor (with the re-inforcement/handle removed) glued to a piece of 3/8" balsa.

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Here the oversized moulded shell is taped to the mould and the base is clamped down to a smooth work surface. the trimming tool is already to do its' job.

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One careful pass around the perimeter of the mould, and your shell is cut precisely to the correct dimension.

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The trimming process was quick, easy, and precise.

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And...ta..da...the final results. A one piece shell with a uniform thickness of 1/8", and no carving. The finished weight was 0.9 ounces.

Incidently, I could not obtain either the blue or pink styrofoam, and was forced to use the white. It doesn't sand nearly as well as the blue or pink, so try to obtain that material for your mould.

So, if you have been thinking about moulding balsa shells, give it a try. It's all in the technique, you know. <g>

(Leonard...I purposely tried to keep the photo file size small, i.e. less than 22kb)

Cheers,

Then I would trim the cured laminated sheet as you did. The laminated sheets tend to hold their shape very well, and conform a little easier to a slightly convex curvature on the nose end. And they are quite strong, needing no formers to support them inside the fuselage.

At the nose end of the molded shell, you can even add another laminated sheet inside to beef up the nose area a little more.

L.

Leonard Neumann

OK. Here goes.

I just laminated up three wet 1/32 layers of balsa on the molds for my Nobler turtledeck and fuselage bottom in place of the 3/32 for which the molds were designed.

It sounds to me like I could use the two outer 1/32 balsa with a carbon fiber matte inter-layer with some adhesive (dope, resin, thinned epoxy) and end up with a really strong piece and still be lighter than the 3/32 I started with.

As it is, the 3/32 part I ended up with for a turtledeck weighed .175 ox. The 1/32 sheet I started with before cutting the parts layers out weighed .320 oz.

I'm worried about adding weight when putting epoxy between the layers.

Will advise in a week what the results are.

Meantime, there's a spare Nobler turtledeck available to anyone who wants one....

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These are the skins. I used 2 1/32 sheets glued together with Pica Gluit thinned 50/50.

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Here are the wetted skins on the form. I temporary tape them in place.

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Here is what I do to get the nose to taper down. I slit the skins. You can see the Ace bandages too!

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Here is my form.

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The three tops that I have made. The bottom one still needs formers added. The weights are: top one 12 gr. This is a single piece of 1/16 6# stock.
The middle one is 16 gr. 2 pieces of 1/32 glued together. The 1/32 wood was 7 to 8 # stock. Very strong.

Lots of work at first, but then and hour at the most and I have another top block! The only way to go.

Two things. First, let me recommend highly the Hunt video tape on molding balsa. It shows both the simpler technique we have been talking about, and the method for accommodating fairly complex compound shapes. Bill Werwage does a big Bearcat style barrel fuselage, and he uses the technique illustrated by Chris, slitting the material on the ends to conform to the compound curves. There's a clever use of some CF veil material CA'ed on the inside surface to hold the finished shape.

Second thing. Instead of heading down to your Walgreens or other big drugstore outlet to buy 3" or 4" Ace bandages, find a medical supply house. There you will find that you can purchase the bandages in 6", 8", even greater widths, which makes the wrapping job much easier. Although some folks like the "self-sticking" variety of compression bandage, I find them a little bit of a hassle, and the "sticky" part seems to diminish, and they get all crinkly (?) and less useful. The wider, smoother, non-self-sticking bandages work better, IMO. Buy plenty at a time.

As Chris mentioned, 7 or 8 pound balsa is adequate for molded shells, and is actually a little better choice than punk 4-6 pound stuff in the strength/weight department.

I personally like two laminated 1/16" sheets for a top or bottom block, but three laminated 1/32" sheets would be a dandy overkill for a larger ship.

While on the subject of molding balsa, Hunt also illustrated a very nice method of getting nose curvature in the fuselage sides in one of his video magazine issues. I highly recommend that technique as well. I used 3/32" fuse sides with 1/32" plywood doublers in a full sized stunt ship, and I'm convinced that it would work with 1/64" plywood for smaller stunters running smooth little gem engines.

L.

First step is to cut the patterns in the plans by the thickness of the balsa, in this case 3/32) smaller than suggested and then use them to carve the balsa block provided in the kit to the proper shape.

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Note the plan outline for the base of the turtledeck has been cut out leaving 3/32 (or so) in the scrap. This provides the means to trim the block provided accurately to be a plug.

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Note that the plan outline for the formers has been cut out 3/32 smaller than the outline shown. These patterns (under the plug, shown upside down with the molding/trimming platform glued on) are used to make sure that the plug has an accurate profile for the inside of the part to be made.

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Once the basic plug has been carved and checked, a platform of 1/4 balsa is made by sitting the plug on some 1/4 sheet stock, tracing the outline of the bottom of the part and cutting out the platform. The platform is then glued to the bottom of the plug, shifted an inch and a half or so toward the front to provide a consistent inset 1/4 of an inch up from the base of the plug where the actual part starts. This lets the balsa for the part continue smoothly, and marks the edge of the final part.

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The entire plug is given a couple of coats of clear poly poxy finish, to harden and smooth the plug. It now sits on a piece of glass with the bottom of the final part elevated 1/4 of an inch from the plane of the glass.

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Next, a piece of 3/32 balsa is cut at the end where the rudder will stick out, trimmed to end halfway between the lip of the end of the actual part and the base of the plug, soaked in hot water, wrapped in an ace bandage and left several days to dry while I go to VSC 2K2 to look at all the fine Noblers that got done in time.

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When I get around to unwrapping the ace bandage, the final part is snugly formed around the hard poxy coated plug.

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With the ace bandage removed you can see how much excess balsa was formed to be trimmed later. The front of the Turtledeck extends quite a ways, whereas the bottom of the balsa was trimmed roughly by hand at forming time and after coming out of the bandage to allow the base of the plug to run flat on the glass reference plane.

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An X-acto blade is glued to a piece of 1/4 balsa to create the trimming tool.

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I discovered it was easier with this small part to hold the trimming tool down onto the glass and slide the plug/part past it to trim the partline accurately. The tip of the X-acto blade rides under the rim of the mold and slices through the balsa part.

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After rough trimming the front of the final part, a sanding block is used to clean up the front edge to match that of the plug, leaving just the part you need and no excess. Similar procedure is used to trim the tail where the rudder will exit through the turtledeck.

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Once all the trimming is complete, the balsa turtledeck is carefully removed from the plug, which will probably be good for another hundred parts or so. (Anyone want to buy a preformed balsa turtldeck for a Brodak Nobler? I'm in a position to mass produce them...)

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Seen head on, the result is a 3/32 thick balsa shell perfectly shaped so that the outside contour matches that of the Brodak plans.

Slightly more time consuming than carving out the block they give you but with the advantage that the mold is re-useable. I think I will throw the first part away and go mold a 1/32 and 1/16 sheet pair together and then put a layer of carbon fiber and very thin epoxy resin in between them and build a true laminated composite structure as has been suggested. Eric thought I should do three layers of 1/32 balsa with two layers of carbon fiber matte/epoxy between then, but that seems overkill and I begin to wonder about weight.

As it stands the 3/32 sheet weighed .560 oz when it was 36 by 3. The final formed balsa part weighed .120 oz off of the mold.

Clearly I have a ways to go if Warren's entire fuselage top weighs .9 oz.

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The final part resting in place in mock-up on the jigged fuselage sides with the vertical fin and stabilizer in place. It seems like a bit more work, but it is truly satisfying and precise to have the final part in place in my mock up. And, once again, the plug is re-usable, so I think I'll end up molding another batch of balsa to allow layers to provide me the strength I need.

Thanks to Eric Rule of RSM for provoking this burst of parts fabrication effort out of me. I always marveled at the preformed fuselage sides Top Flight had in their Scale and stunt jobs. (I have a pair of their P-51 Mustang kits waiting my attention.)

With this form and a similar one for the bottom of the fuselage, I am prepared to do Brodak Noblers for sometime. I have the plans and parts for a Tom Dixon version, along with some ribs where I went and filled in the 1/4 notch in the leading edge. My plan on the next Nobler is to replicate the Brodak plan parts, mold myself the turtledeck and belly pan, and find some styrofoam to cut into forms for the leading edge and use a molded leading edge with carbon fiber matt, eliminating the 1/4 stick leading edge.

I'd planned to get all of this posted soon, but Warren (the first poster on this thread) beat me to the punch.

Of course, Windy has been going one better nowadays and laying up the whole fuselage in carbon fiber. He uses CF cloth that costs about $100 a yard, and post cured epoxy resin. The result is, of course, just fabulous. I'm not sure how much weight he saves, but the fuselage is super strong.

(I don't know if Windy has actually crashed either Miss Ashley or the Typhoon yet. God forbid that it should happen, but we'll find out if a CF fuselage will stand up to that or not.)

I suspect Windy would need to charge at least $500 for a CF fuselage to be able to sell them, they are still quite labor intensive..

It's all wonderful technology, but..

We can laminate CF veil inside a molded top block, but is it even necessary? My normal fuselage finish treatment is to cover it with .56 oz. fiberglass, nitrate dope binder, cover that with medium silkspan, again finished in nitrate. It comes out incredibly strong and light. Not expensive, difficult or exotic at all, yet more than adequate in strength. (In fact, I can leave out the fiberglass and my low-tech composite still works quite well..)

Just laminating two thin sheets for a molded fuselage top or bottom block will result in better strength than a single thicker sheet. The laminated piece will retain its shape, with the bonus that it is definitely easier to stretch the sheets around the mold.

L.

Expense, for one thing.

Unfamiliarity with the technology of molding CF for another.

>how much wil you charge
>for a couple of Nobler
>turtle backs?? Hmmm.?

Windy charges $25 for one at his site. I had not seriously thought of it until the mold was complete, but I could turn some out for folks that need one.

I'll probably let the first one I made go simply because I think I can do better. If you are building a Brodak Nobler, send me some email. Your address is busted on the post.

One way we could build community is to barter/trade parts so everyone doesn't have to build the same plugs over and over.

I'm clearly doing to do this for every fuselage top from now on.

I'm wondering if using a good coating of dope on the outside of the inner 1/16 balsa and then carbon veil matt and more dope and then replaceing the second formed 1/16 shell would work as a laminating process. Wrap it up with the ace bandage and let it set for a good long time.

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I watched the Hunt video for techniques. I found it interesting the way Werwage cut and spliced his shells. I guess I do everything the hard way. If you don't cut and splice the ends of the shells you will probably get an area around the fat part of the shell where the balsa will bulge and buckle from an excess of wood which wouldn't compress along the grain. I always just worked the buckle into an area like the cockpit or wing cutout and just went ahead and crushed that part of the shell to be cut out later. I also notice several other differences in techniques. I wet the wood in my bath tub with bath water only, no amonia which might affect the strength of the wood. I use strips of bed sheet to wrap my shells. I always mold 3/32" and have no excess material available to sand away bandage wrap indentations. My bedsheet wraps usually leave little or no marks on the shells. If I mark the shells with a really difficult pull, after unwrapping, I rewet the surface of the completely formed shell and wrap it again loosely just to expand the indentations. I try to minimize sanding on my finished shells. By using bedsheet strips and a lot of force, I think I am getting a great deal more compression across the grain of the wood for successful molding of more difficult shapes. I think I am making it hard on myself. I have never had to splice a shell but I intend to try some splicing to ease compression and eliminate buckling at the fat part of the shell. It would also work to simply make an across grain splice at the buckle which would accomplish the same thing.

An example of a full half shell would be this Classic Bearcat. The molds are filled with 20 minute, quick setting joint compound (plaster). It is messy, but cheap and carves beautifully about 20 minutes after being applied.

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A half shell wrapped in bedsheet, drying in the sun. One night an the water heater closet will also suffice.

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3/32" shells usually don't need much internal support.

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Completed shell assyembly. I don't do half shell assemblies any more. Now, I usually mold the shells in pieces (quarters) for better use on jigged fuselages.

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Classic Molded Bearcat.

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Other molded airplanes.

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Although there should be no legal issues, the material is not copyrighted, and is already published on an open public forum, this was not the only issue. Another complaint was that folks had not been expecting their material to be published in print. They would have preferred to edited or otherwise touched up their material..

The very informal, eyeball to eyeball, interactive nature of the forum here which appeals to us might not be completely appropriate to a published article, which is expected to be more polished.

If we take the liberty to EDIT, it is also possible for someone to get his feelers hurt because of the manner in which the editing might be done.

Finally, although I think everyone realizes when they publish a photo or graphic on an open forum here, people can easily download it, copy it, print it, alter it, etc. And I don't think many folks really mind this, but the polite and civil thing to do is always to make sure the owner is properly acknowledged in print.

Just a few things to consider.

L.

One thing I can add. PoLy-U glue for laminating. Almost none required. Actually LIKES the water in the balsa. Pulled laminated shells in one hour.

Great pictures guys...