Originally posted by wwattles
Unfortunately again, I don't have any kind of a 3-D modelling software available to me, and I don't have a scanner to scan in a hand-drawn image. Anybody out there with an inkling of what I'm talking about who can model something this complex in a 3-D fashion?
OK, I thought about it some more and I think this is what you're trying to describe: Imagine a hollow cone, stood on it's base. You take a hollow cone. Make four equidistant cuts from the top to the bottom. Take each of those segments and rotate through 180 degrees so that the insides are outside and vice versa.
It would look something like the attached pic?
Would it be stable? Well an ordinary cone is stable if the cp is 1/3 of the length from the base and the cg needs to be approx 1/3 of the length from the tip. Playing around with this inside out cone, it seems to me that if you push the material betwen the 'fins' in more and more, then at the limit, it's almost a narrow body tube with four triangular fins running the whole length of the tube. I simmed that up in rocksim and the cp was approximatey 1/2 the distance from the base, and the cg needed to ba a body calibre in front of that.
So as a rough guestimate I would say that the cp for an inside out cone would be between 1/3 and 1/2 of the length from the base. As you go back to the inside out cone from the finned tube, the "effective" body calibre would increase requiring it to farther forwards from the cp, which is itself moving forwards!
If I was making one of these I think I would start by making the cg 1/4 of the length from the tip, and see how it went.
You could make it pretty easily by taking a paper cone, as I did, and then flattening it flding it in half. That gives you a pyramid shape. Open it out and repeat until you get the creases going the right way. Then run thin ca down the inside of the creases and nip together. Once that's dry you could put a stuffer tube down the inside (but not all the way to the end) you would have to centre it somehow at the base end. Glue that in with ca too, and add a thrust ring.
Then cut off the end of the cone and put a piece of coupler in the end to fit back into the stuffer tube. Add a parachute and some nose weight and you're done!