بررسی تأثیر دینامیک آشوبناک بالشتک صندلی پلی‌یورتان بر سطح راحتی خلبانان بالگرد بل 214

This study investigates the chaotic dynamics of nonlinear seat cushions and their influence on pilot ride comfort during helicopter operations. Pilot comfort represents a critical aspect of rotorcraft vibration research; hence, it is necessary to investigate both the seat suspension and the cushion system as the principal contributors to passenger comfort. Among these, the seat cushion holds particular importance due to its cost efficiency and significant influence on low-frequency vibration attenuation, which strongly affects perceived comfort. Conventional linear models of polyurethane energy-absorbing cushions not sufficient to accurately capture their real dynamic behavior. Improper specification of stiffness and damping characteristics can lead to inadequate vibration isolation and, in the long term, potential health risks for pilots. The main innovation of this research lies in systematically evaluating the effect of nonlinear stiffness parameters on the onset of chaotic behavior and assessing their effects on vibration isolation performance. Therefore, the seat cushion is modeled as a mass–spring–damper system, where the nonlinear stiffness coefficient is determined experimentally. Because the cushion model is nonlinear therefore, they have nonlinear dynamic behavior, and this study investigates the relationship between such nonlinear behaviors and ride comfort. The cushion model is included into a 4-DOF biodynamic system and subsequently extended to a 5-DOF model for improved fidelity. The nonlinear dynamic behavior is analyzed through bifurcation diagrams and maximum Lyapunov exponent, and then it is confirmed by other tools such as, time response, phase-plane trajectories, Poincare maps, and FFT spectra. Next, in the periodic, multi-periodic and chaotic ranges, the passenger comfort parameters, including transmissibility and mechanical impedance and apparent mass, have been investigated, and the optimal ride comfort ranges have been identified. The results showed that the stiffness propertie of the cushion, which cause the dynamic behaviors to be in the chaotic range, significantly increase the seat-to-head transmissibility and increase the mechanical impedance, indicating that the cushion no longer absorbs vibrations but instead amplifies irregular forces. These findings emphasize the importance of carefully select the nonlinear properties of materials in the periodic range to maintain dynamic stability, ensure effective vibration isolation and maintain pilot safety and operational comfort during helicopter missions.