Resumo:
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.
