Investigation of Helicopter Seat Structural Dynamics for Aicrew Vibration Mitigation

Helicopters are known to expose the aircrew to whole body vibration that leads to undesirable occupational health issues. This paper presents the investigation of helicopter seat structural dynamics to indentify solutions to mitigate the vibration levels transmitted to the aircrew. A flight test was conducted on a Bell-412 helicopter to measure the aircrew body vibration levels in order to characterize the vibration transmission path through the seat structures. Modal analysis was conducted on a representative seat configuration to investigate the seat/aircrew dynamics. The critical vibration modes that contributed to the aircrew body vibrations have been identified. In an effort to mitigate aircrew vibrations, several candidate cushion materials and their combinations have been evaluated on a Bell-412 helicopter using typical flight conditions. The combination of an original in-service Bell-412 seat cushion together with a commercially available urethane pad was found to demonstrate promising results on both the pilot and co-pilot, indicating that helicopter seat vibration isolation performance can be improved through the optimization of seat cushion impedance properties. Active structural modification of the helicopter seat structure was also investigated. Two stacked piezoelectric actuators have also been retrofitted on a Bell-412 helicopter seat frame to function as active struts to provide control authority to reduce critical vibration modes experienced by the mannequin. Closed-loop experimental results demonstrated simultaneous suppression of the critical mannequin vibration modes to achieve significant global reduction of body vibration levels of a mannequin. The test results verified the effectiveness of the adaptive helicopter seat mount concept for helicopter aircrew vibration mitigation applications.