Aerodynamics of permeable membrane wings. Part 2: Seepage drag

This study addresses a thin wing section, built with an impermeable forward part and a permeable membrane aft part, in a steady flow of incompressible fluid. Assuming that seepage velocity through the membrane is related with the pressure difference across it by Darcy's law, seepage drag of the wing is found in a closed analytical form. It vanishes as the membrane seals up or thins out, and, under most circumstances, increases with increasing camber. It is shown that the seepage drag can be contained by keeping the width of the aft part less than half the wing's chord.