Coupled rotor/fuselage vibration analysis using detailed 3-D airframe models

Abstract

A comprehensive vibration analysis of a coupled rotor/fuselage system is carried out using detailed 3-D finite element models of the AH-1G airframe from the DAMVIBS program. Predicted vibration results are compared with operational load survey flight test data of the AH-1G helicopter. Modeling of difficult components (secondary structures, doors/panels, etc.) is essential in predicting airframe natural frequencies. Calculated 2/rev vertical vibration levels at the pilot seat show good correlation with the flight test data both in magnitude and phase, but 4/rev vibration levels show fair correlation only in magnitude. Lateral vibration results show more disagreement than vertical vibration results. Refined aerodynamics such as free wake and unsteady aerodynamics have an important role in the prediction of vibration. Main rotor pylon roll mode has a significant contribution (about 26%) on the 2/rev vibration. Accurate prediction of airframe natural frequencies up to about 40 Hz appears essential to predict vibration in airframe.