On the Zero Dynamics of Helicopter Rotor Loads

Black-box identification techniques have been recently applied to the helicopter rotor, in order to develop dynamic models of rotor hub loads. Since all the black-box models obtained exhibited non-minimum phase characteristics, which are of major importance in control system design, the need arose for a first principle analysis of the system. The objective of this paper is then to analyse a linear model for the response of the vertical component of the rotor loading to perturbations in the blade control input, in order to investigate its non-minimum phase (NMP) characteristics. The cases of hover and forward flight, with rigid and flexible blades, are discussed. In all the considered flight conditions, the model exhibits a real zero located in the right half plane. In the hover case, when considering an ideally rigid blade, the zero is approximately located at the coordinate 2 root 2 Omega, where Omega is the rotor rotational frequency. The paper ends with a study of the modifications in this NMP property arising when considering flexible blades or forward flight, a different velocities.