Ground resonance is an aero-mechanical instability in helicopters that use soft in-plane rotors. Traditionally, ground resonance is mitigated by using passive lead–lag dampers that provide sufficient in-plane damping. However, these dampers because of their passive nature cannot adapt to all operating conditions. In this work, a magnetorheological fluid–based semi-active lead–lag damper is proposed to offer controllable damping. Two nonlinear control strategies are reported to operate the voltage to be supplied to the magnetorheological damper. The first strategy is a model-based control using dynamic inversion. The second is a fuzzy logic control integrated with a particle swarm optimization algorithm to optimize the control parameters. Both control strategies are shown to be effective in eliminating ground resonance. Unlike bang–bang control, the prescribed control algorithms can make use of complete voltage level available in the magnetorheological damper with smooth voltage updates. A comparative study of the controller performances is made through appropriate performance indices and system responses. Finally, the most optimum control strategy to mitigate ground resonance is inferred.
It is well known that the tensile properties of cotton fibers are strongly associated with fibrillar orientation. The latter property has been generally-expressed by the . Herman's orientation factory or the 40 or 50% x-ray angles in studies wherein x-rh diffraction was used for orientation measurements. While the fz is an index of 'the overall orientation distribution, the :v-ray angles are single point indices describing the relative number of scattering planes oriented to the reference axis at angles that are related to the x-ray angles measured. Therefore, one may expect that the orientation factor would be a more appropriate index of orientation for use in orientation-property studies. However, it has been noticed in some of our recent studies that this need not be the case. TABLE I. Simple correlation coeflicients of tenacity and elongation values with crystallite orientation parameters. a Significant at 1% level. *' Nonsignificant. _ Table I contains the simple correlation coefiicients obtained between different x-ray orientation indices measured from the (002) diffraction arc and some tensile properties determined using the Stelometer for 23 varieties of cottons drawn from the four major cultivated species (G. arboreum, G. herbaceum, G. hirsulum, and G. barbadense). From, the table itwould appear that tenacity at zero gauge length and elongation at 3-mm .gauge length are highly associated with the 20 and 75% ~-rav angles, respectively. For ach case the correlation is better than that obtained -hen f is used. Incidentally, another recent report 2J also states that the mechanical properties of cotton fibers did not show as good a correlation with Ir as with x-ray angles. In this work, however, fewer samples were studied, and of these, most of the samples belonged to one species, G. hirsulum, the others being G. l~erbaceum cottons. We have no definite explanation now to account for the association of tenacity and elongation with different x-ray angles. Our present research efforts are &dquo;directed towards confirming the findings by studying more samples and then examining the results on the basis of the existing knowledge of cotton fiber morphology and fractography. The results will also have to be considered in the light of the suggestion that the 75% x-ray angle might be the beset estimate of the spiral angle (1~.
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