From Helicopter Vibrations to Passenger Perceptions: A Closer Look on Standards

Abstract

The serial introduction of passive and active anti-vibration means lead primarily to the reduction of the vibration levels at blade passage frequencies Nb/rev. Consequently, other- previously unnoticed- sources of vibration are perceived by rotorcraft occupants. Therefore, a comprehensive vibration assessment metric is required to characterize the impact of different vibration sources of helicopters regarding passenger comfort. Since the advance of industrial/military aerial transport machines, several vibration assessment metrics were developed such as the Intrusion Index (ADS-27A-SP), the overall ride value av (ISO2631-1) and the NASA DISC model. However, these metrics have deficiencies regarding the evaluation of complex rotorcraft vibrations, e.g. the Intrusion Index favors only the rotor harmonics in the vibration evaluation, the overall ride value av is based on uniaxial, sinusoidal oscillations at discrete frequencies and the NASA DISC model considers only vibration measurements on the floor and not on other vibration contact surfaces. Since the rotorcraft vibrations are characterized by the presence of triaxial, multiple vibration sources it is unclear whether one of these metrics is appropriate to assess the perceived discomfort. The work presented in this paper addresses this topic. The suitability of existing vibration evaluation metrics, especially av, regarding typical rotorcraft vibration patterns is investigated. For that purpose, a systematic whole-body vibration campaign was performed, in which human subjects were seated in a helicopter seat on a motion platform and exposed to helicopter specific vibration patterns of which specific frequencies were systematically attenuated or increased. Participants rated the perceived discomfort using magnitude estimation. The campaign reveals, that the overall ride value av is not well-suited to predict and to compare the discomfort of different vibration spectra. Especially in vertical direction, the application of av will significantly overestimate the discomfort. This implies that this metric is not appropriate for evaluation of helicopter specific vibrations and could be improved. The results in this paper are a first step in that direction but more comprehensive analyses of helicopter specific vibrations and their impact on passenger comfort are necessary.