Individual blade control with the smart spring – A closed-loop independent harmonic control approach

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Nitzsche, F.
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In steady-state forward flight the cyclic variation of the aerodynamic loads acting on the blade generates forces and moments that are predominantly transmitted to the fuselage at the ??! ?????? harmonic of the rotor frequency, where ??! is the number of rotor blades. The Smart Spring is a semi-active device that allows actively modulating the blade pitch link axial stiffness throughout the indirect action of a piezoelectric actuator. It performs dynamic parametric excitation of the rotor system and introduces Individual Blade Control with the objective of reducing these harmonic cyclic loads transmitted to the fuselage. Previous experimental studies demonstrated that the transmissibility reduction of some harmonics could be greater than 90% for a given combination of the Smart Spring parameters. In this paper, the capability of the Smart Spring to act in the closed-loop control configuration is analytically explored for the first time. The control action results in the realization of an independent harmonic control of the rotor blade system, an improvement in the current state of the art of the technology. The fundamentals for the Smart Spring closedloop independent harmonic control concept are discussed.