This is a prop with hub developed in Openscad. I did this to find a method by which a prop could be designed in Openscad. It is still a work in progress and I do not pretend to be an expert in prop design. There might be another way to do this, it is rather complex, but it works and is a manifold. The X axis iterates from the center out. At each Xn paired with Xn+1 verticies are calculated. The trick is to make sure the next iteration has the inside vertices exactly the outside of the previous. That way Openscad recognizes the polys as a manifold. The assign statements clean up the inside loop dependent variables. The hub is a seperate part where the prop sits in a cylindrical cutout for stress relief. Update - Working the math the way I did with this it is tough to keep it manifold. I just uploaded another project that could be applied to this that gets around the problem. http://www.thingiverse.com/thing:856452