The unsteady vortex-lattice method is used to calculate the pressure coefficients on thick and thin airfoils in steady and unsteady flowfields. The parameters which affect the results, such as time step and aspect ratio, are studied. The effects of Reynolds number and thickness of a wing in steady state and in oscillation are investigated. The present computed results for thick and thin wings are in close agreement with the experimental data. The numerical results obtained from a lifting-surface approximation are also in close agreement with the experimental data for a wing as thick as 18%. The lift and moment coefficients are affected by the thickness of a wing in oscillation and this effect is more noticeable for the moment coefficient. But to illustrate this it is necessary to go as high as 27% thickness. A wing in steady flight near a wavy surface, such as in the case of a large transoceanic wingship, is simulated by a wing oscillating in heave near a flat surface. In accord with the wingship, small aspect ratios and slight camber are considered. The numerical simulation predicts that the mean aerodynamic loads on a wing executing a simple-harmonic heaving motion are higher than the corresponding loads on the same wing in steady flight at the mean height and the same angle of attack. The increases are about the same for all heights. Hence, these preliminary results suggest that it would be beneficial to fly near the waves; that doing so would improve the aerodynamic efficiency. Also included in the present results are numerical simulation of the wakes that show the strong influences of the ground and the oscillations on their behavior. The unsteady vortex-lattice method is further used to investigate the effect of trailing vortices from a large leading wing on a trailing aircraft. The aerodynamic response of the trailing aircraft is examined by calculating the lift and drag forces and the pitch and roll moments. Furthermore, the aerodynamic response and the behavior of the wakes of the crossing wings are investigated. / Ph. D.