Resistance hysteresis and pitch instability mechanisms of an air-cushion vehicle during unsteady acceleration

Unsteady acceleration can amplify resistance and destabilize the pitch of air-cushion vehicles through delayed hydrodynamic adjustment. This study investigates these mechanisms using an LCAC numerical model combining overset meshes, volume-of-fluid interface tracking, and coupled surge, heave, and pitch motions under rigid-skirt and prescribed-pressure assumptions. Comparisons with quasi-steady baselines reveal clockwise resistance-pitch hysteresis and resistance overshoots of up to 67.1% under light loads. Two load-dependent instability modes emerge: aerostatic pitch divergence under light loads and thrust-drag deadlock caused by severe bow immersion under heavy loads. In the heavy-load case, abrupt thrust cutoff triggers a surfing effect, releasing accumulated bow-wave energy and recovering approximately 7.8% of the kinetic energy lost during deadlock, enabling resistance-hump crossing. A case-specific dynamic safety envelope in the Froude number-acceleration plane demonstrates that quasi-steady predictions overestimate stability margins, highlighting the importance of acceleration history in assessing longitudinal instability.