This is exactly how the AR-5 tail cone was done, except sans vacuum (see "How it's Made" video above).If the turtledeck has straight tapers (like the Cassutt), the carving can be done with a hotwire saw and a pair of templates for the outside, and another pair indexed to the outside for removing the inside.
The added weight of the core is negligible but the stiffness goes way up. Next time you're around some homebuilts poke on the cowls and see how flexible they can be without a core. "Stiff enough" is probably in the eye of the beholder for a non-structural part like your turtledeck.
Let's put it in perspective. There are several braces and or ribs in the various plans I have seen for turtledecks, and they all weigh somthing. If you put in the braces, you can go very thin on a coreless carbon and maybe even on a coreless fiberglass turtledeck. But, if you make it with the cores that BoKu and I have been describing, skipping the cores at the edges of the parts, you can skip ALL of the bracing and ribs above the fuselage truss, and then maybe you will save some weight. The thin stuff with ribs and braces and building like metal is called black aluminum thinking. Doing it with cores makes it internally braced and takes advantages of the composites...
To expand on our previous replies: It's hard to appreciate how stiff yet light composite sandwich construction can be until you handle pieces made with it. If there is a glider repair shop near you where you can pick up a piece of composite sandwich sailplane wing skin, that might really help you understand better than all these mass and area numbers we've been throwing out.
That's not even remotely what you've been doingMy experience actually doing that exact thing suggests that that assessment is considerably optimistic. The big bugaboo is inescapably concentrated loads that require both reinforcement and adequate load transfer--things like wing and control system attachments, control surface hinges, walkways, and assembly bondlines. Based on the tapered carbon fiber RV-6-7-8 wings we're building now, I'd say that the weight savings is more along the lines of about 25-40% depending on various factors.Airframe wise, I think that with a CFRP infused structure you can easily cut the structural weight by 50 to 65%. By structural I mean anything that's riveted on say an RV8.
That turtledeck has a substantial bulkhead (rolloverr protection) at the rear of the cockpit, tube braces at five places along its length, and is mostly fabric covered. Pretty light to begin with, simple, and straightforward to build. That makes it tough to beat. As I understand it, the entire turtle deck is non-structural, that is, it floats and the fuselage truss below carries the loads.Older Cassutt but shows turtle deck from a nice angle. I think I'll stick with aluminum for the TD and composite for the tips, pants and cowl.
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From what I've learned on this thread a basic aluminum sheet of .020" can't be replaced by composites without being heavier. I'm excited to learn composites, hence this thread, but I'm not willing to completely alter the TD to do it.