Interesting thread. It seems that the old debate about plank or heli is being replaced by the debate about tiltrotor or counterrotor. My own view is that both will eventually become cost competetive, but that different strengths will be inherent to each. Clearly weight minimisation will be key to either design, with rigid rotor performance.
Tiltrotor will never be as efficient in hover, but counterrotor is unlikely to be as efficient in high speed flight. Once variable RRPM establishes itself as the next enabling technology the lines will become tighter, since counterrotor will offer lower rotor drag. The smaller pusher prop will however never match the tiltrotor for prop loading, so speed may become the next competetion. As emerging rotor technologies for active twist become available, higher speeds become possible with counterrotors.
The interesting thing will be to see how tiltrotor responds to this challenge. Maybe swept tip blades for higher tip, thus overall, speeds. Maybe increased coning for more spanwise flow componenent (similar to swept fixed wings). These technologies can again be fed across to counterrotors.
How far will this competition take rotorcraft development? Dare i ask: Is a supersonic rotorcraft possible?
Mart