Abstract: A flexible, hollow, lift-developing airfoil has a frontal air inlet, and a valveless air-filled chamber between upper and lower surfaces. The upper surface is impermeable to air. The lower surface has a permeable section which permits air to freely flow therethrough in either direction depending on differential atmospheric pressure inside and outside the chamber. When the airfoil is in horizontally advancing flight, the permeable section is not exposed to the relative wind. Under such conditions air enters the chamber through the frontal opening, and excite through the permeable section. When the airfoil is descending in a substantially vertical direction, the permeable section is exposed to relative wind, and air enters the chamber therethrough, flowing out through the frontal opening, to automatically maintain a desired aerodynamic profile.