One of the workshops at NARCON 2010 was on "D Helicopters" and it was presented by our current NAR president. He conveyed a great deal of useful information to the large group that attended, which included this total heliroc newbie. It was where I gained the insight about the rotors needing to be low in mass. I also received another interesting nugget there about the rotors, and I do hope that I am summarizing it accurately. He said that due to the relatively slow rotational speed of helicopter rockets, putting airfoils into the rotors does very little (or even nothing) to improve the lift that they generate. He said that as long as the rotors had the proper "twist" (incidence that varies along their length), then flat, thin (as thin as practical) rotors are probably the best option. The effort spent on airfoiling them could be better spent on improving the speed in which they are deployed (lifted up into position), getting them to initiate auto-rotation quickly (putting the right amount of incidence along the innermost third of the blades) and getting them to rotate faster (keeping the rotors light in weight and minimizing the friction in the hub). I think that I have that right, but my apologies in advance if I happen to have misstated anything.
He did show several actual helicopter rockets, including some flown at the Internats. I briefly looked at a rotor of one model up close at the end of the presentation, and if there was any airfoil in it, I sure couldn't see it. The model was designed to boost on a D motor, and the long, narrow rotors were made from, by all appearances, 1/32" thick balsa. I don't believe that it had any reinforcement along most of the length.
Mark K.