Gessow, Alfred & Crim, Almer D. A Method for Studying the Transient Blade-Flapping Behavior of Lifting Rotors at Extreme Operating Conditions, report, January 1955; (https://digital.library.unt.edu/ark:/67531/metadc56061/m1/1/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .