Linear heave dynamics of an air-cushion vehicle bag-and-finger skirt

Results from a linear analysis of the heave dynamics of an air-cushion vehi cle equipped with a bag-and-finger skirt are described. A two-dimensional s ection of the cushion is subject to pure heave or long-wave surface motion inputs. The skirt mass is lumped in the fingers, with the bag being modelle d as a combination of massless inelastic membranes and links. The airflows from bag to cushion and from cushion to atmosphere are assumed quasisteady, and the bag and cushion volumes are modelled as lumped pneumatic capacitan ces. For a configuration representative of a 37 t vehicle, frequency respon se characteristics show the effect of skirt geometry and mass changes, and cushion capacitance. The results suggest that changes in skirt geometry can not be used to radically modify an undesirable heave response, but reducing the skirt mass may be effective. The air compressibility also affects heav e response at high frequencies, with the effect becoming more prominent at the low cushion-flow rates now used in practice.