System Identification Technique for Active Helicopter Rotors

System identification methodology is developed for a linear time-periodic (LTP) system and applied to an experimental setup of an integrally twist-actuated helicopter rotor blade. Identification is conducted for a controller design, which alleviates vibratory loads induced in forward flight. Since a rotor in forward flight is a time-periodic system due to the aerodynamic environment varying once per rotation, the adopted methodology requires determination of the multicomponent harmonic transfer functions. A simplified identification formula is also derived for a linear time-invariant (LTI) system, such as a rotor system in hover. The latter approach gives another estimate of the primary component among the harmonic transfer functions. The identification experiment is conducted at NASA Langley Transonic Dynamics Tunnel. The magnitude of the higher-order harmonic transfer functions is observed to be small in the frequency range of interest when compared with that of the primary component. This indicates that the present active rotor system may be regarded as a LTI system under the level flight conditions considered. Results obtained in system identification are interpreted in terms of the closed-loop controller design.