Heaving and Pitching Response of a Hovercraft Moving over Regular Waves

Consideration is given to the pitching and heaving of a hovercraft whose plenum air cushion is divided into two compartments by a transverse barrier. Linearized equations of motion are developed, subject to the assumption that the skirt hem does not contact the surface beneath; the coefficients can be calculated in terms of craft geometry, weight and lift-fan characteristics, or in terms of natural frequencies and damping ratios for pitch and heave. In considering the response to regular head and following seas, particular attention is given to accelerations experienced at the bow and to the position of the pitch centre, where the vertical accelerations are least. The behaviour of a realistic craft is examined for waves of various lengths and for different craft speeds, and consideration is given to the effects of moving the mass centre and of modifying the barrier between cushion compartments. A relatively vigorous response is predicted for certain distinct ratios of craft length to wave length, as was the case for the pure heaving response studied earlier. Once again, it is possible to relate this maximum response to conditions at which rapid changes occur in cushion volume and escape area, but now the changes for the individual compartments must be considered as well.