Investigation of the Effect of Chaotic Dynamics in Polyurethane Seat Cushion on Ride Comfort of Bell ۲۱۴ Helicopter Pilots

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 helicopter vibration research; hence, it is necessary to investigate both the seat suspension and the cushion system as the principal contributors to pilot comfort. Among these, the seat cushion holds particular importance due to its cost-effectiveness and significant influence on low-frequency vibration attenuation, which strongly affects perceived comfort. Conventional linear models of polyurethane energy-absorbing cushions are 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, it has nonlinear dynamic behavior, and this study investigates the relationship between such nonlinear behaviors and ride comfort. The cushion model is incorporated into a ۴-DOF biodynamic system and subsequently extended to a ۵-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, Poincaré maps, and FFT spectra. Next, in the periodic, multi-periodic, and chaotic ranges, the pilot 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 properties 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 selecting nonlinear properties in the periodic range to ensure stability, achieve effective vibration isolation, and maintain pilot safety and operational comfort during Bell ۲۱۴ helicopter missions.

کلیدواژه ها:

Ride comfort ، Nonlinear ، Helicopter ، Chaotic behavior ، Polyurethane cushion