John P. Reeder, James B. Whitten
naca-tn-2459
Jan 1952
Flight-test measurements and pilots opinions are presented of the longitudinal flying qualities and lateral control characteristics of a small single-rotor helicopter. In these tests the damping of the helicopter in pitch and roll was varied by means of a rate-sensitive automatic-control device from the amount present in the helicopter with the device inoperative to nearly three times that amount.
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http://naca.central.cranfield.ac.uk/reports/1952/naca-tn-2459.pdf