A Closer Look at the Impact of Helicopter Vibrations on Ride Quality

The typical vibration level of helicopters is considerably higher than the levels found on fixed wing aircraft. The main reason for this are periodic airloads on the main rotor leading to elevated cabin vibrations at distinct frequencies equal to multiples of the main rotor speed. Thus, the development of technologies for vibration reduction mostly focuses on solutions that counteract the vibrations at rotor blade passage frequencies. As the reduction of those vibrations is performed efficiently, the question whether non-rotor induced vibrations are relevant for helicopter ride quality arises. The work presented in this paper addresses this subject. Existing vibration evaluation methods are assessed with respect to the capability of including non-rotor vibration sources. These evaluation methods are used for an enhanced comfort evaluation by isolation of vibration signals related to relevant discomfort sources. This approach enables comfort optimization considering arbitrary vibration sources. The application of the presented comfort evaluation procedure to flight test data reveals that non-rotor induced vibrations contribute a relevant amount to the discomfort of occupants of helicopters. This is especially the case if the vehicle is equipped with modern anti-vibration systems. Furthermore, the importance of non-rotor vibration sources is greatly increased on flights in atmospheric turbulences. Hence, existing vibration evaluation methods and procedures have to be evaluated as to how they should be extended accordingly. The provided comfort evaluation method can be used for detailed comfort analysis and the results can serve as a basis for the definition of comfort improvement means.