The first report is issued that explains the rotor wake geometry in hover

Description

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