SplineDrive: Thanks for your comprehensive review / assessment in post #16. A fair bit to absorb!
I had overlooked you already mentioned X-2 in your post #11 (as 60FltMech did in the original post!) and have only now realised it formed the principles of their SB-1 Defiant bid. You made the comment "
It [rigid contra-rotating rotors] might even lose to a simple winged helicopter in a few years". Do you mean something like the Sikorsky S-67 (
https://en.wikipedia.org/wiki/Sikorsky_S-67_Blackhawk)!!
As you have obviously thought carefully about overall rotorcraft design, you might also like to comment on whether there could be a future in a modern-day version of the Fairey Rotodyne gyroplane (
https://en.wikipedia.org/wiki/Fairey_Rotodyne)? I am sure design techniques are now available to substantially reduce the noise generated by the rotor tip jets. Perhaps this isn't so much a technical challenge anymore, rather one of overcoming protest over use of such rotorcraft in built up areas (where they might otherwise still attract a commuter market).