Nick,
I understand your concern about high rigidity, and the efforts that are applied in attempting to tune components about a selected RRPM. I hope that the following response will clarify my 'brute strength' approach.
Graviman,
Dave, i think you need to find a good aerodynamic efficiency justification for 4 blades per rotor.
For a quick justification, how about Sikorsky's coaxials? The
S-69 (XH-59) ABC had three blades per rotor. It did not quite achieve the anticipated maximum forward velocity due to excessive vibration. Note that the
X2 coaxial shows four blades per rotor.
For a longwinded justification; I did a crude and error prone comparison of a
3-blade rotor and a
4-blade rotor for the twin main rotor UniCopter, a couple of years ago. It was done to evaluate the lateral dissymitry of lift, but I suspect that its conclusions can be applied to drag.
The addition of a 4th blade provides a reduction in the drag of the individual blade, as mentioned by Nick. In addition, it appears that the moment about the craft's X-axis, from the advancing side of an 4-blade 'theoretical' absolutely rigid rotor is a constant, irrespective of what azimuth(s) the (one or two) advancing blades(s) are at.
What I surmise from this is that the individual blades will be subjected to varying in-plane shear as they rotate through the 360º. However, if the rotor is 'absolutely' rigid, the
total rotor will not experience any cyclical moment about the Z-axis or cyclical forces along the X and Y-axii.
This makes me think that the closer the rotor can come to 'absolute' rigidity the less the vibration (from this source) should be. In other words; the very high frequencies will be well above what could be detrimental to the structure, components and occupants of the craft.
Dave