Pitch-Heave Dynamics of a Segmented Skirt Air Cushion

Abstract

The formulation and experimental validation of a mathematical model of the nonlinear pitch-heave dynamics of an uncompartmented, segmented skirt cushion system for an air cushion vehicle are described. This system relies on surface contact to attain static stiffness in pitch and roll. The formulation includes a dynamic model of the lift air system, the effect of segment flexibility on effective cushion capacitance and on hovergap, and hysteretic skirt-surface contact forces. Predictions of linear stability and of nonlinear response to pitch disturbances are made for two skirt materials, the first generating considerable hysteresis in pitch stiffness, and the second having much greater extensibility but negligible hysteresis. These predictions are compared with experimental results obtained from a 900 kg test model. The basic structure of the system's stability characteristics are correctly predicted; this includes a nonlinear pitch-heave instability associated with coupling between pitch and heave motion, and with the modulation of the cushion volume by pitch motion. However, there remain unaccounted sources of cushion damping. This and other factors, such as sensitivity of the predicted results to the flexure of the panels that form the model air supply plenum, make close quantitative agreement between theory and experiment difficult to achieve.