This is a Momentous Technology milestone.
The paper provided the theoretical foundations of induced and profile power analysis for rotors, and introduced blade element theory for performance and hub loads of flapping rotors in forward flight. The paper is referenced below,
Note: The exact day the paper was published is unknown - only the month is known.
Source: "Milestones in Rotorcraft Aeromechanics", Johnson W., , NASA/TP-2011-25971, May 2011, Page 2