Plenum Air Cushion/Compressor-Duct Dynamic Interactions

An analytical model for the static and dynamic behavior of plenum-type air cushion suspensions, including compressor and feeding duct interactions, is developed and experimentally verified. Conditions for cushion instability (flutter) are identified and stable and unstable operation is demonstrated both analytically and experimentally. Step response tests conducted by dropping experimental scale model suspensions from heights up to 1.75 times the equilibrium hover height show that a linearized model accurately predicts the experimental response, including sprung and unsprung mass natural frequencies and damping ratios. Criteria are given for selection of low-order dynamic models, based on product expansions, for the distributed parameter feeding duct. It is shown that cushion-duct dynamic interactions may strongly affect cushion stability unless the lowest duct resonant frequency is substantially higher than the suspension sprung and unsprung mass natural frequencies.