This paper addresses the problem of determining helicopter rotor mode damping from a state-space model of rotor, fuselage, and inflow in which some of the elements of the system matrices are calcualted directly using frequency domain system identification techniques. Simulated flight test data are used to study the fundamental features of a direct state-space matrix identification process, including the sensitivity to output noise and the sensitivity to the initial guess of each matrix element to be identified. An improved formulation of the frequency response matching problem is discussed that substantially reduces the computational cost of the solution. The results indicate that the identification procedure is accurate and robust.
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