A comprehensive vibration analysis of a coupled rotor/fuselage system

A comprehensive vibration analysis of a coupled rotor/fuselage system for a two-bladed teetering rotor using finite element methods in space and time is developed which incorporates consistent rotor/fuselage structural, aerodynamic, and inertial couplings and a modern free wake model. A coordinate system is developed to take into account a teetering rotor's unique characteristics, such as teetering motion and undersling. Coupled nonlinear periodic blade and fuselage equations are transformed to the modal space in the fixed frame and solved simultaneously. The elastic line and detailed 3-D NASTRAN finite element models of the AH-1G helicopter airframe from the DAMVIBS program are integrated into the elastic rotor finite element model. Analytical predictions of rotor control angles, blade loads, hub forces, and vibration are compared with AH-1G Operation Load Survey flight test data. The blade loads predicted by present analysis show generally fair agreement with the flight test data, especially blade chord bending moment estimation shows good agreement. 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. Pylon flexibility effect is essential in the two-bladed teetering rotor vibration analysis. The pylon flexibility increases the first lag frequency by about 14%, and decreases 2/rev longitudinal and lateral hub forces by more than half. Rotor/fuselage coupling reduces 2/rev vertical and lateral vibration levels by 60% to 70% and has a small effect on 4/rev vibration levels. Modeling of difficult components (secondary structures, doors/panels, etc) is essential in predicting airframe natural frequencies. Refined aerodynamics such as free wake and unsteady aerodynamics have an important role in the prediction of vibration. For example, free wake changes the phase of 2/rev vertical vibration by 55 degrees and increases the magnitude of 4/rev vertical vibration level by five times at 67 knots. Main rotor pylon roll mode has a significant contribution (more than 30%) on the 2/rev vibration. Accurate prediction of airframe natural frequencies up to about 40 Hz appears essential to predict vibration in airframe.