Ken,
In plain language, the intuitive spacing of rotor blades... 2=180, 3=120, 4=90, 5=72, etc.
The second idea is a question. Is there ever a situation during flight when the helicopter
would benefit or be smoother by varying those angles? That answer is yes. Anytime a single
blade in the rotor system flies higher or lower than the nominal tip-path plane, it's center of
mass moves closer to the rotational axis and that blade wants to speed up like an ice skater.
It needs to be allowed to do this or the occupants will feel it as a vibration and stress will be
put on whatever component(s) are preventing the acceleration.
The accelerating blade however, needs to have a dampening force on its displacement so it's
always converging on equal spacing instead of diverging. If the spacing between blades were
un-dampened, the difference would quickly destroy the helicopter.
Ground resonance is a serious situation which happens on the ...you guessed it...ground, and
the interaction between the ground and the helicopter causes spacing between blades to diverge
suddenly from equal and it exceeds the damping capability of the lead-lag dampers and the
helicopter ruins itself in a few seconds.
Almost all 2-bladed rotors are rigidly mounted 180 degrees apart and when one blade flies higher
as it swings around to the advancing side because it's developing more lift, the other blade on the
retreating side flies lower the same amount. Their centers of mass move inward the same amount
and both blades want to accelerate THE SAME AMOUNT.
Simplified explanation but it gets the point across.
Real world example. Land a 3-bladed Hughes 269A poorly and when one skid bumps the ground
before the other one, it's likely that will knock the blades away from their 120 Degree spacing. That
makes the helicopter "wobble" or rock on the skids. If timing of that wobble is at the wrong time,
the 125 degrees will become 132 and the 138, and then 140 degrees VERY QUICKLY. Your helicopter
will sustain tens of thousands of dollars in damage unless you react very quickly and pick up to a hover.
It will then go away just as fast. No ground...= no ground resonance.
Moral of the story...KEEP good, serviceable dampeners on your rotor and on your landing gear or
you will regret it.