Dynamic lift characteristics of a water borne WIGcraft

Abstract

A water borne WIGcraft in principle, generates much higher lift than that of an equivalent free-flight airplane, air borne WIGcraft and planing boats due to the complementary ground effect aerodynamic, hydrodynamic and hydrostatic lift generated by the wings and submerged portion of the hull surfaces. However, the coupling of the ground effect aerodynamics and hydrodynamics of the vehicle, compounded by hull-generated water-spray impinging on its wings present challenges when predicting dynamic forces on the vehicle. Capturing the complex air-water flow about the wings, and its effects on the dynamic lift on the vehicle require advanced and computationally expensive high fidelity nonlinear numerical solution. This paper proposes a semi-empirical model as a simplified alternative to estimate the dynamic lift on the vehicle. The model was developed based on the theories of ground effect aerodynamics and planing watercraft hydrodynamics, as well as physical observations from fully constrained experiments of a multihull watercraft model with and without wings. The various elemental components of the water borne WIGcraft dynamic lift including ground-effect and hull-generated water-spray induced lift on the wings were estimated. To ensure simplicity and consistency in coupling the aerodynamic and hydrodynamic lift coefficients, an equivalent hydrodynamic lift coefficient was proposed for the aerodynamic lift coefficient of the wings of the vehicle. The results of the proposed semi-empirical model and that of the fully constrained model experiments are in satisfactory agreement. The results of the total dynamic lift of the water borne WIGcraft suggest that for a water-borne WIGcraft operating at large draughts with water-spray impinging on its wings, the effect of the wings’ proximity to the free surface water is secondary. The contribution of the hull generated water spray to the total dynamic lift force is significant.