Helicopter aircrews are exposed to high levels of whole body vibration during flight. This paper presents the results of
an investigation of adaptive seat mount approaches to reduce helicopter aircrew whole body vibration levels. A flight
test was conducted on a four-blade helicopter and showed that the currently used passive seat systems were not able to
provide satisfactory protection to the helicopter aircrew in both front-back and vertical directions. Long-term exposure
to the measured whole body vibration environment may cause occupational health issues such as spine and neck strain
injuries for aircrew. In order to address this issue, a novel adaptive seat mount concept was developed to mitigate the
vibration levels transmitted to the aircrew body. For proof-of-concept demonstration, a miniature modal shaker was
properly aligned between the cabin floor and the seat frame to provide adaptive actuation authority. Adaptive control
laws were developed to reduce the vibration transmitted to the aircrew body, especially the helmet location in order to
minimize neck and spine injuries. Closed-loop control test have been conducted on a full-scale helicopter seat with a
mannequin configuration and a large mechanical shaker was used to provide representative helicopter vibration profiles
to the seat frame. Significant vibration reductions to the vertical and front-back vibration modes have been achieved
simultaneously, which verified the technical readiness of the adaptive mount approach for full-scale flight test on the
Twin-tail fighter aircraft such as the F/A-18 may experience intense buffet loads at high angles of attack flight conditions and the broadband buffet loads primarily excite the first bending and torsional modes of the vertical fin that results in severe vibration and dynamic stresses on the vertical fin structures. To reduce the premature fatigue failure of the structure and to increase mission availability, a novel hybrid actuation system was developed to actively alleviate the buffet response of a full-scale F/A-18 vertical fin. A hydraulic rudder actuator was used to control the bending mode of the fin by engaging the rudder inertial force. Multiple Macro Fiber Composites actuators were surface mounted to
provide induced strain actuation authority to control the torsional mode. Experimental system identification approach was selected to obtain a state-space model of the system using open-loop test data. An LQG controller was developed to minimize the dynamic response of the vertical fin at critical locations. Extensive simulations were conducted to evaluate the control authority of the actuators and the performance of the controller under various buffet load cases and levels. Closed-loop tests were performed on a full-scale F/A-18 empennage and the results validated the effectiveness of the real-time controller as well as the development methodology. In addition, the ground vibration test demonstrated that the hybrid actuation system is a feasible solution to alleviate the vertical tail buffet loads in high performance fighter aircraft.
Most active vibration suppression approaches have attempted to suppress structural vibration by incorporating active material actuators, such as piezoceramic, within the structure to act directly against vibratory loads. These approaches require the piezoceramic actuators to generate significant force and deflection simultaneously to effectively suppress vibration. Unfortunately, successful implementation of these approaches has been hindered by the limited displacement capabilities of piezoceramic actuators. The Smart Spring concept is an unique approach to actively control combinations of dynamic impedance characteristics of a structure, such as the stiffness, damping, and effective mass to suppress vibration in an indirect manner. The piezoceramic actuators employed in the Smart Spring concept are not used to directly counteract excitation loads but rather adaptively vary the effective impedance properties of the structure. Therefore, the piezoceramic actuators in the Smart Spring are not required to produce large forces and deflections simultaneously. This paper demonstrates the ability of the Smart Spring concept to control dynamic impedance characteristics of a structure through numerical simulations and experimental investigations. Mechanical shaker tests using the proof-of-concept hardware verified the controllability of the impedance properties using the Smart Spring device and its ability to suppress vibration. More importantly, the tests conducted in a wind tunnel demonstrated the performance of the Smart Spring under highly varying unsteady excitation conditions. These tests confirmed that the Smart Spring system is able to actively suppress vibration through adaptive control of structural impedance properties.