Study on multi-modal dynamic vibration absorbers applied to helicopters

Abstract

Vibration control is a critical challenge in helicopter engineering, where high-amplitude structural vibrations accelerate component wear and fatigue while degrading crew performance and comfort. Conventional Dynamic Vibration Absorbers (DVA) are widely used in helicopters, as an evolution of the DVA, there is the Multi-Modal Vibration Absorber (MMDVA), which can suppress several rotor harmonics simultaneously with a single device. This study implements a finite element method (FEM) based design methodology for MMDVAs tailored to the main and tail rotor frequencies of a Brazilian Navy Lynx helicopter, utilizing carbon fiber leaf springs integrated with steel inertial masses. Experimental vibration analysis using flight test accelerometer data informed target frequency selection, revealing that while the dominant excitation frequencies are governed by main rotor and tail rotor harmonics, their relative amplitude distribution varies significantly between pilot and co-pilot locations and across flight conditions. From this analysis, four target frequencies were selected for each MMDVA to attenuate. A two-stage optimization routine based on a genetic algorithm was implemented: the first stage aligns the isolated natural frequencies of the MMDVAs with the target frequencies using modal analysis, and the second stage refines its geometry through coupled harmonic analysis to account for damping and interaction with the host structure. Two candidate geometries were proposed, taking advantage of multiple vibration mode shapes like flexion and torsion. The first geometry comprises two mass blocks, each contributing with two vibration modes, generating two antiresonances per block. The second geometry comprises three mass blocks, whose combined motion contributes to four vibration modes of the structure, generating four antiresonances. By generating multiple antiresonances with a single device, the proposed MMDVAs achieve broader vibration suppression than conventional single-mode absorbers, making more efficient use of absorber weight compared to adding multiple independent devices to the structure.