It had proved very difficuly to predict lateral flapping using the analyses of the time. The data gathered by Harris enabled the development of more sophisticated analytical models that improved the ability to design control systems correctly.

Harris, F. D.: Articulated Rotor Blade Flapping Motion at Low Advance Ratio. Journal of the American Helicopter Society, vol. 17, no. 1, Jan. 1972, pp. 41–48.

Source: Johnson W., Milestones in Rotorcraft Aeromechanics, NASA/TP-2011-25971, May 2011, Page 27