Prediction of UH-60A Structural Loads Using Multibody Analysis and Swashplate Dynamics

Abstract

The first part of this paper compares three rotor blade structural dynamic formulations: a finite element formulation with modal reduction, a full finite element formulation without modal reduction, and a multibody-based full finite element formulation for arbitrary large deformations. The second part of this paper studies the effect of swashplate dynamics on blade loads and servo-actuator loads. In all cases, measured airloads, damper force, and control pitch angles from the UH-60A flight tests are used to predict and analyze the structural loads. In the first part, the emphasis is on the validation of a multibody formulation, which is first verified with analytical solutions for beams undergoing hypothetical large deformations (elastica), then validated with the Princeton beam large deformation tests, and then finally used to predict the UH-60A structural loads. Two flight conditions are considered: a high-speed, high-vibration flight and a highly loaded dynamic stall flight. Predictions from the multibody analysis are compared with the full finite element and finite element based modal methods. It is observed that the predicted blade loads do not show any significant difference between the three formulations. In the second part, the four-bladed multibody rotor model is coupled to a swashplate-servo model to predict servo loads and to study the effect of swashplate dynamics on blade loads. It is observed that the higher frequencies of servo loads, 8/rev and 12/rev for this rotor, require modeling the swashplate dynamics. The low-frequency component, which is a dominant 4/rev load for this rotor, is less affected by swashplate dynamics and is determined primarily by the accuracy of the 3, 4, and 5/rev pitch-link loads. The 3-5/rev pitch-link loads, and in general the structural loads on the rotor blade, are not affected by swashplate dynamics.