Mart,
Dave, what I'm confused by is that you were championing separate lift and thrust rotors. Why the change of view to what to me appears less ideal?
Your confused? So am I.
It's all Nick's fault. He threw down the gauntlet many years ago when he said that there's no such thing as a free lunch.
With a little more seriousness; every concept (configuration) has its pros and cons. Some improvements may be worthy of a free coffee-brake, but none are worthy of Nick's free lunch.
Sikorsky held off further development of the ABC concept for 30 years. Waiting for outside entities to develop new materials and devices cannot be the only reason. The
X2-ABC will have incremental improvements over the
XH-59A ABC . In addition, I sincerely think that the Intermeshing and Interleaving configurations will be better than the Coaxial.
However, in any future battle for the high-speed VTOL market both the ABC-Rotorcraft and the Tilt-Rotor will have their pros and cons.
IMHO, the primary limitation with the ABC craft is its speed limitation. Prouty has said "What is good for high speed flight is bad for hover and what is good for hover is bad for high speed flight." The same might be said about reverse velocity and forward velocity. At this point in time, it appears to me that anything that is done to the blade to minimize or utilize the effect of reverse airflow will create a greater impediment to conventional airflow.
Perhaps this section on Reverse Velocity Utilization will help inspire someone to come up with a solution, get a patent, and make a zillion dollars.
Just my two cents.
Dave.
PS. The UniCopter ain't dead. Far from it. It's just that one should have 'a number of irons in the fire'.