Leonardo Sanches

Robustness Analysis of Helicopter Ground Resonance with Parametric Uncertainties in Blade Properties

This paper presents a stability robustness analysis of the helicopter ground-resonance phenomenon. By using the lifting procedure the uncertain linear-time-periodic model of the helicopter is transformed into an augmented uncertain linear-time-invariant model that allows the application of μ-analysis tools. The lifting procedure involves a periodic switching linear-time-invariant piecewise model computed using oversampling of the system period. The representativeness of the lifted model for various oversampling period values and methods is discussed and compared with a Floquet analysis for several parametric discretization configurations. A μ-analysis is then applied to find the worst-case parametric configuration for a given rotor angular rate. The parametric uncertainties taken into account are the dynamic characteristics (stiffness and damping) of each blade hinge. A significant advantage of the proposed approach is that it enables performing ground-resonance analysis for a rotor with dissimilar blade properties due to aging effects. Considering uncertainties on the four blade hinge stiffnesses and damping factors, the μ-analysis performed on the lifted model leads to the conclusion that the worst case for degraded rotor stability corresponds to the symmetric perturbation of all the blades.

Experimental Analysis of a Model Based Crack Identification Approach for Rotating Machines

In this paper an experimental application of a crack identification methodology based on combinational resonances is proposed. The technique uses external applied diagnostic forces at combinational frequencies, together with a pseudorandom optimization code, Differential Evolution, in order to characterize the signatures of the crack in the spectral responses of the flexible rotor. A finite element model is used to obtain the dynamic responses of the rotating machine. In this system, the additional excitations are applied by means an electrodynamic shaker. The method of multiple scales is used to determine the frequency of the excitation force under which combinational resonances appear. The breathing crack is simulated according to the Mayes model, in which the crack transition from fully opened to fully closed is described by a cosine function. The additional flexibility introduced by the crack is calculated by using the linear fracture mechanics theory. The results show that the methodology is a promising tool for estimates the crack location and depth.© 2014 ASME

Helicopter Ground Resonance Phenomenon With Blade Stiffness Dissimilarities: Experimental and Theoretical Developments

Recent works study the ground resonance in helicopters under the aging effects. Indeed, the blades lead-lag stiffness may vary randomly with time and be different from each other (i.e.: anisotropic rotor). The influence of stiffness dissimilarities between blades on the stability of the ground resonance phenomenon is determined through numerical investigations on the periodical equations of motion, treated by using Floquet’s theory. Stability chart highlights the appearance of new instability zones as function of the perturbation introduced on the lead-lag stiffness of one blade. In order to validate the theoretical results, a new experimental setup is designed and developed. The ground resonance instabilities are investigated for different types of rotor configurations (i.e.: isotropic and anisotropic rotors) and the boundaries of stability are determined. A good correlation between both theoretical and experimental results is obtained and the new instability zones, found in asymmetric rotors, are verified experimentally. The temporal responses of the measured signals highlight the exponential divergence at the instability regions.Copyright