The influence of the tip sails shape on the wing aerodynamics in ground effect

The function of the wing tip sails is to scatter the concentrated tip vortices into several smaller scale vortex structures, and accelerate the dissipation of tip vortices, thereby reducing the induced drag. In order to study the influence of wing tip sails on the flow fields and aerodynamics of a wing in ground effect, wind tunnel experiment is conducted to measure aerodynamics and tip vortex structures of a NACA4412 wing fitted with three elliptic tip sails and three trapezoidal tip sails respectively, and the reason of the differences between the aerodynamic loads on the two wings is analyzed by comparing the flow fields of tip vortices. The lift and drag forces are measured using a 6-component balance, the velocity distribution of tip vortices is scanned by a 7-hole probe. The Reynolds number based on the chord length of the wing is 1.5×10⁵. The experimental results show that the differences of lift and drag forces between the two wings increase as the wings get closer to the ground, and the trapezoidal tip sails is more efficient in lift augmentation-drag reduction than the elliptic tip sails. The local flow direction and local incidence of each sails are different for the two wings, which result in different contributions in increasing the lift and reducing the drag.