Aerodynamics of permeable membrane wings

This study addresses a thin wing section, built with a solid impermeable forward part and permeable membrane aft part, in a steady flow of incompressible fluid. Closed-form expressions are found for the lift and pitching-moment coefficients of the wing, relating them with the width of its forward part and permeability of its aft part. It is shown that the lift slope coefficient uniformly decreases with increasing permeability, changing from the familiar value of 2π to 2π times the relative width of the forward part. Concurrently, the aerodynamic center moves from its familiar position at the quarter-chord point of the wing to the quarter-chord point of the forward part, but its movement is not uniform. When the width of the forward part is less than half of the chord, the aerodynamic center initially moves backwards, and only then forwards.