Adam asked:
3. A big rotor diameter means that the rotor can turn slower (since the tip speed limits the design of any rotor). Slow turning rotors create much less noise. Why isn't this done more on helicopters?
I am working on a large and slow rotor.
The Boeing Co. has a slow rotor design called Hummingbird A-160 (search google for info)
But mostly, the interest seems to be about increased speed.
If your design is slow, then wood might work OK. I think wood was abandoned because wood absorbs moisture from the air and the blade can go out of balance. Also, the blade should have chordwise balance at 25% from the Leading Edge. This requires a large steel bar in the nose of a wood blade. Why not just make the nose section out of metal? That way you have the balance and strength in one part.
The blade design and material would depend on the size and use of helicopter.