A theoretical and experimental investigation has been made of the two-dimensional, aerodynamic characteristics of flexible airfoils or sails with no thickness. Thin-airfoil theory has been used to calculate the shapes of the camber lines as a function of angle of attack and excess length of material over the chord length. It is found that for specific eigenvalues of a nondimensional chordwise tension parameter, specific shapes arise for which there is no stagnation point. Some eigenmodes also correspond to boundaries between stable and unstable airfoil shapes. Lift, center of pressure, shapes, and pressure distributions are determined for the entire practical range of angle of attack. An experimental investigation of the lift, drag, and moment of quasi-two-dimensional flexible airfoils has been made, and the measurements have been compared with theory. Certain of the aerodynamic characteristics are in good accord with theory, but certain others are not, principally because of fabric porosity and boundary-layer separation.