Seakeeping dynamics of a single cushion, peripheral cell-stabilized air cushion vehicle

Abstract

A study of air cushion vehicle (ACV) motion in waves is presented for a single cushion ACV having a cellular, peripheral cell-type skirt system. The craft is considered to be traveling at constant speed while encountering regular waves of arbitrary heading. The dynamic equations for pitch, heave, and roll motions are derived using the cushion and cell air flow equations. These equations are solved numerically using a digital computer. The results are shown as frequency response curves giving steady-state motion response amplitudes as a function of encounter frequency or wavelength for fixed craft speed and wave steepness. The theoretical predictions are then compared with experimental data taken from scale model, towing tank tests in head seas. The comparison shows good agreement for pitch motion, while heave motion damping is overpredicted. Nomenclaturearea of orifice between loop and vertical cell AQ -equilibrium leakage area b = beam C n = orifice coefficient / = cushion leakage fraction of gap height hi = depth below origin of fully extended skirt at /th jupe H = unit step function I x = craft moment of inertia about the x axis I y = craft moment of inertia about the y axis K = roll moment k = wavenumber k ]t k 2 = increase in cell width on inside, outside per unit change in height / = cushion length M = pitch moment m = mass of craft m c = mass of air in cushion P = cushion gage pressure P L = loop pressure Q = volume rate of flow Qo, 1,2 -f an performance parameters AS/ = length of skirt seal around cushion periphery T = effective width of skirt jupes U 0 = craft speed V c = cushion volume W -craft weight x c =x coordinate of cushion centroid z c = depth of top of cushion from origin Zo = heave coordinate Z = force on craft in the z direction /3/ = angle of outward normal at the /th cell with respect to the x axis 7 = angle of wave propagation direction with respect to the* axis y c = ratio of specific heats ( =c p /c v ) 77 = wave height B -pitch angle A = wavelength p = density <t > = roll angle u e = encounter frequency oj = wave frequency Subscripts a = refers to atmosphere ij = refers to /th,./th seal (jupe) L = refers to loop 0 = refers to equilibrium conditions