Actually, thats part of the entire equation. Structurally, blade "thickness" is a result of stiffness targets and material capabilities.
Generally low speed helicopters live with blades designed to be relatively "fat" near the root section since the velocities there are low. Overall profile drag losses on the blade (a big deal in high speed craft) are predominantly defined by the thickness/chord ratio. So aerodynamically, blade thickness contributes to rotor efficiency and cruise speed.
If you had to accommodate the large loads involved with a scaled up aircraft, thickening up a blade is especially penalizing on the ABC concept because the root section experiences reverse airflow on the retreating side and high free stream velocity on the advancing side (in addition to overall drag increasing for the rest of the blade).
In addition, if you are driven to thicken your blade so much to withstand these high loads, you will likely find yourself with a rotor that no longer will dynamically tune, as it will constantly be raising these frequencies with stiffer blades. Not to mention huge weight penalties.
What most people fail to realize is that you cannot simply increase stiffness infinitely to solve problems, as you will possibly detune the rotor and create a rotor that will destroy itself with any destabilizing maneuvers or even gusts.
This is a very similar situation seen on Abe Karem's paper airplane Optimum Speed Tiltrotor. Except it's an even worse dynamic situation in a pylon/wing mounted rotor.
Last edited by SansAnhedral; 8th Oct 2013 at 13:49.