Unsteady ground effects on the aerodynamic coefficients of thick airfoils

Abstract EN

A model based on the image technique developed previously to compute the unsteady potential flow around cambered plates moving near the ground was extended to handle airfoils with thickness.

The airfoil and its image were represented by panels with linearly varying vorticity.

A wake that grew with time was formed as a result of vortex shedding from the trailing edge to satisfy the unsteady Kutta condition.

The model was used to study the effects of airfoil thickness ratio, position of maximum thickness and thickness profile shape on the relative deviations in the aerodynamic coefficients (from their values far from ground) as the airfoil approaches the ground.

For negative pitch angles, i.e.

nose below trailing edge, the results indicated that thickness effects on the relative deviations are generally very small.

This is in sharp disagreement with the results in steady ground effect, which indicated substantial thickness effects.

When the pitch angle is positive, the relative deviations were observed to drop near ground with increasing thickness ratio in agreement with the trends obtained by using the steady approach.

This trend was attributed to suction on the lower surface of the airfoil caused by a "Venturi effect" very close to the ground, as demonstrated by computed pressure distributions.