Influence of elastomeric lag damper modeling on the predicted dynamic response of helicopter rotor systems

Abstract

Several modeling approaches that describe the behavior of elastomeric materials as they are used in helicopter rotor lag dampers are examined, and evaluated against laboratory test data. Then, selected models are used in a simulation of a helicopter rotor startup, and compared to one another and to flight test data. The results show that the simple, complex modulus model will yield good predictions of damper energy dissipation, but is inadequate for predicting forced response. While some models perform better than others, all have limitations that make them unsuitable in some situations. It is recommended that more effort be put into acquiring and analyzing damper test data in order to facilitate the development of more robust modeling approaches. a, = Fourier cosine coefficients hi = Fourier sine coefficients c = damping coefficient (Ib/in/sec) Cj = damping coefficient (Ib/in/sec) / = damper force (lb) k = stiffness coefficient (Ib/in) kt = stiffness coefficient (Ib/in) K' = stiffness modulus (psi) K = damping modulus (psi) t = time (sec) u = displacement (in) v = velocity (in/sec) Wdia = damping energy (Ib-in) a> = frequency (rad/sec) * Associate Fellow AIAA, Member AHS Copyright © 1995 by Donald L. Kunz. Published by the American Institute of Aeronautics and Astronautics with permission. Introduction Elastomeric materials are used in a wide variety of parts on modern helicopters. Bearingless rotor helicopters, such as the McDonnell Douglas Explorer, the Bell Model 680, the Boeing/Sikorsky RAH-66, and the MBB BO-108, employ an elastomeric snuboerdamper that serves to both restrain the motion of the pitch housing and to provide necessary lag damping to the rotor system. Elastomers are also used in the lag dampers of aircraft such as the McDonnell Douglas AH-64 Apache, the Bell Model 412, and the Boeing Model 360. Because of the fact that the use of elastomers in critical components of rotorcraft is now so widespread, the subject of elastomeric damper modeling has received increased attention in recent years. For many years, the preferred method of describing the stiffness and damping properties of elastomers has been the complex modulus method. This method models the damper as a linear spring and linear damper operating in parallel. However, it is clear from even the most rudimentary study of elastomeric material properties that linear modeling of elastomeric damper response is entirely inadequate. Reference 1 presents an in-depth discussion of the influence of the nonlinear characteristics of elastomers on the design of lag dampers. This paper discusses the dependence of elastomeric material properties on such factors as amplitude and frequency of motion, temperature, and preload. However, instead of focusing on the effect of these factors on damper response characteristics, the emphasis is placed on how each factor affects the values of the linear complex moduli. It is clear that this investigation was tailored to focus on specific design parameters (e.g., spring rate and energy dissipation), rather than on developing a comprehensive damper model. In an effort to keep the basic structure of the complex modulus method intact, Refs. 2 and 3 propose modifications that account for dual-frequency excitation and thermal effects, respectively. Reference