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

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 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 has an important role in the prediction of vibration. M/R pylon roll mode has a significant contribution (more than 30%) on the 2/rev vibration. Accurate prediction of airframe natural frequency is essential when the natural frequencies are close to resonance.