. The Linear Theory of a Lifting System Moving Close to the Ground
To reduce a nonlinear formulation to the linear theory, one has to assume that the deflections of the surfaces of the wing, its vortex wake, and the ground, respectively, from horizontal planes у — h •  and у = 0 are small compared to the ground clearance h, i. e.,
|2/u, l,w Щ ^ l/g ^ (3.1)
These assumptions lead to the possibility of imposing boundary conditions for the wing and the wake on the plane у = h and boundary conditions for the ground on the plane у — 0. Note that, upon linearization, the quasi-harmonic equation (2.22) is reduced to the Poisson equation. It is obvious that within the linear theory, the pressure is related to the velocity potential through linear differential operators for both the upper and channel flows.