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Paul,

That helicopter is neither weird or old.  It is a Kaman KMax.  It is a
single seater designed strictly for sling loads and it is highly efficient.
It is so good at what it does that it has been put back in production and
the Marines use a variation of it that is unmanned for resupply missions.  I
am very interested in it because it uses servo tabs to control blade pitch
angle.  It's a very efficient technique that eliminates the need for
hydraulics in the control system.  The intermeshing rotors eliminate the
need for a tail rotor which means all the available horsepower goes to
producing lift.

Steve Brazil

What about the ring vortex state?

Paul Lamar