Helicopter Crew Seat Vibration: Criteria, Design Approaches and a Comanche Case Study

Helicopter vibration is a key factor in crew and passenger comfort. Considerable efforts have been made to reduce vibratory rotor forces and airframe responses. The structural dynamic characteristics of the seats play an important role in the ride comfort equation as well but this is often all but ignored. Examples of measured vibration levels on the airframe structure and on the seats are presented for cases where the seats degraded ride quality. Laboratory test results are shown which identify the structural modes which influence the seat response to main rotor induced n/rev vibration. Finite element analysis has been used to predict seat vibration characteristics but results have not correlated well with test results for reasons that are partly understood. Improvements to such analyses could be of great value, allowing vibration criteria to be addressed in early design of the seat. Several design approaches are discussed which allow some control over seat vibratory response. The challenge of meeting vibration requirements without compromising other important design criteria is discussed. The specification for the crew seats for the RAH-66 Comanche includes stringent vibration transmissibility limits. A series of vibration tests and several design modifications were made to assure that the seats would not be resonant. As users of future rotorcraft demand still lower vibration levels along with improved crash safety and low cost and weight, the opportunity will continue to exist for innovative approaches to crew and passenger seat design.