AN ANALYTIC MODELING AND SYSTEM-IDENTIFICATION STUDY OF ROTOR FUSELAGE DYNAMICS AT HOVER

A combination of analytic modeling and system identification methods h ave been used to develop an improved dynamic model describing the resp onse of articulated rotor helicopters to control inputs. A high-order linearized model of coupled rotor/body dynamics with literal coefficie nts is compared to flight test data from single rotor helicopters in t he near hover trim condition. The identification problem was formulate d using the maximum likelihood function in the time domain. The dynami c model with literal coefficients was used to generate the model state s, and the model was parameterized in terms of physical constants of t he aircraft rather than the stability derivatives, resulting in a sign ificant reduction in the number of quantities to be identified. The li kelihood function was maximized using the genetic algorithm approach. This method proved highly effective in producing an estimated model fr om flight test data which included coupled fuselage/rotor dynamics. In terpreting these results, it is shown that blade flexibility is a sign ificantly contributing factor to the discrepancies between theory and experiment shown in this and previous studies. Addition of flexible mo des, properly incorporating the constraint due to the lag dampers, res ults in excellent agreement between flight tests and theory, especiall y in the high frequency range.