A review of membrane-wing aeroelasticity

Recent developments in micro-technology have been the driving force behind the scientific interest in micro air vehicles (MAVs), as these became feasible in the recent two decades. However, the unique requirements of these palm-sized air vehicles of high maneuverability in the low Reynolds number flow regime, in addition to the demand of fast adaptation to unsteady flow conditions, are practically impossible to attain with rigid wings. The massive flow separation that dominates the upper surface of rigid wings in this flow regime significantly reduces the aerodynamic efficiency of the wing in both steady and unsteady flow, thus presenting harsh limitations on the aircraft's agility. Seeking a solution for this conundrum brought vast attention to membrane wings, inspired by the wings of bats. Membrane wings are distinguished by their ability to passively adapt to flow conditions, whether these are steady or unsteady by nature. Several review papers have addressed the static aeroelastic response of such wings, focusing on the implementation of such wings in MAVs, with additional details on the dynamic response of membrane wings. In this review paper an overview of recent developments in the understanding of membrane wing aerodynamics is presented, focusing on the dynamic aeroelasticity of membrane wings in steady flow conditions. Special focus is paid to the physical mechanisms that drive membrane oscillations and the aerodynamic benefits of such oscillations.