Pitch–Heave Symmetric Stall Flutter of a NACA0012 at Transitional Reynolds Numbers

Abstract

Large-amplitude limit cycle oscillations (LCOs) in one degree of freedom (1DOF) in pitch and 2DOFs in pitch–heave induced by stall flutter are explored experimentally on a NACA0012 wing. Because stall flutter is mainly understood from the point of view of dynamic stall, it has been generally agreed that the oscillations associated with negative aerodynamic damping occur about a large angle of attack. In contrast, symmetric oscillations about the origin are reported in this work, where the amplitude of oscillation, ranging from 30° to about 50°, is limited by flow separation. The elastic axis is fixed at . The airspeed is varied over a range of Reynolds numbers extending from to . For experiments in 2DOFs, the stiffness in pitch is kept constant, whereas the heave stiffness is changed to cover a range of frequency ratios, . Except for cases with frequency ratios close to one, the introduction of the DOF in heave does not significantly affect the LCO dynamics. It is determined that the stall flutter observed is fundamentally a 1DOF problem in pitch, whereby the pitch motion drives the heave motion. Nonetheless, the work done by the lift force (on the heave) is larger than the work done by the aerodynamic moment (on the pitch).