Non-linear oscillations of a simple flexible skirt air cushion

Intrinsically non-linear aspects of the heave dynamics of a flexible skirted plenum chamber air cushion are investigated analytically and experimentally. The phenomenon used for this purpose is the tendency of dynamically unstable cushion motion to grow to a stable limit cycle oscillation. Two non-linearities are included in the mathematical model: these are the quasi-steady orifice law applied to the cushion air escape and other flow processes, and flow shut-off associated with skirt-ground contact. Numerical procedures are applied to the differential equations to predict frequencies, amplitudes and waveforms of the oscillations. These results are compared with experimentally measured values, and good qualitative agreement is obtained. This includes confirmation of a prediction that, for equilibrium values of cushion pressure and volume flux close to linear stability boundaries, two stable limit cycles exist, one associated with skirt-ground contact and one without.