Dynamic Behavior of Ground-Effect Machines in Motion Over Waves

Abstract

Dynamic motion in heaving and pitching of a ground-effect machine traveling over a train of sinusoidal waves is formulated on the basis of the small-disturbance theory. The equations of motion are derived as two third-order ordinary differential equations with coefficients which depend on the characteristics of the machine and the mass flow into or out of the cavity under the machine. The coefficienis are linearized at the equilibrium height; then, the solutions are obtained for the natural frequencies, the damping, and the steady periodic responses to motion over waves. It is found that a ground-effect machine with peripheral and central jet curtains is generally stable for heaving and pitching, but the damping is rather small. The effect of coupling due to the discontinuity of mass-flow coefficients is also shown to be very weak and thus may be neglected in this linearized analysis. A program, prepared for the IBM 1 620 digital computers at Hydronautics, Incorporated, allows study of the dynamic stability of a machine through variation of pertinent parameters, and also investigations of its dynamic response to various wave-encounter conditions. Theoretical predictions for practical machines, which have in model tests experienced dynamic motions over waves, are presented here and compared with experimental results. Satisfactory agreements are indicated in the natural frequency and response both for heaving and pitching.