This report, by Robin Gray, explained the structure of the rotor wake in hover. This led to a much better ability to model and predict the spanwise air loads and performance of rotors in hover.

Details of the report are provided below,

Gray, R. B.: On the Motion of the Helical Vortex Shed From a Single-Bladed Hovering Model Helicopter Rotor and Its Application to the Calculation of the Spanwise Aerodynamic Loading. Princeton University, Aeronautical Engineering Department Report No. 313, Sept. 1955.

(Note: Only the month and year of this milestone are accurate)

Source: "Milestones in Rotorcraft Aeromechanics", Johnson W., NASA/TP-2011-25971, May 2011, Page 24