The Instability of Pitching Oscillations of an Airfoil in Subsonic Incompressible Potential Flow

A theoretical study is carried out to determine the conditions under which steady-state pitching oscillations of an airfoil in one degree of freedom are possible in two-dimensional subsonic incompressible potential flow. The results show that such oscillations can occur if the airfoil moment of inertia is sufficiently great and if the axis of rotation is located at a point that is forward of the airfoil quarter-chord but not too far forward of the airfoil leading edge. The practical significance of these results with respect to flutter and airplane stability problems is briefly examined.