Donald A. Howard
naca-tn-2395
Jul 1951
An autopilot sensitive to yawing acceleration was used to control four simulated airplane (mass-spring) configurations and a series of bench tests were conducted to determine the response characteristics of the combinations. The occurrence of an unstable mode of motion was predicted in previous theoretical analyses, NACA TN 2007 and TN 2005, in which the autopilot was assumed to have a constant time lag and a constant amplitude ratio.
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http://naca.central.cranfield.ac.uk/reports/1951/naca-tn-2395.pdf