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Abstract Flapping flight of animals has captured the interest of researchers due to their impressive flight capabilities across diverse environments including mountains, oceans, forests, and urban areas. Despite the significant progress made in understanding flapping flight, high-altitude flight as showcased by many migrating animals remains underexplored. At high-altitudes, air density is low, and it is challenging to produce lift. Here we demonstrate a first lift-off of a flapping wing robot in a low-density environment through wing size and motion scaling. Force measurements showed that the lift remained high at 0.14 N despite a 66% reduction of air density from the sea-level condition. The flapping amplitude increased from 148 to 233 degrees, while the pitch amplitude remained nearly constant at 38.2 degrees. The combined effect is that the flapping-wing robot benefited from the angle of attack that is characteristic of flying animals. Our results suggest that it is not a simple increase in the flapping frequency, but a coordinated increase in the wing size and reduction in flapping frequency enables the flight in lower density condition. The key mechanism is to preserve the passive rotations due to wing deformation, confirmed by a bioinspired scaling relationship. Our results highlight the feasibility of flight under a low-density, high-altitude environment due to leveraging unsteady aerodynamic mechanisms unique to flapping wings. We anticipate our experimental demonstration to be a starting point for more sophisticated flapping wing models and robots for autonomous multi-altitude sensing. Furthermore, it is a preliminary step towards flapping wing flight in the ultra-low density Martian atmosphere. 

We numerically examine the use of Gurney flap to modify the two-dimensional wake dynamics for lift enhancement on NACA 0000 (flat plate), 0006, 0012 and 0018 airfoils. Incompressible flows over the airfoils at different angles of attack are considered at Re = 1000. It is observed that the attachment of the Gurney flap at the trailing edge is able to enhance the lift force experienced by the airfoil appreciably with increase in Gurney flap height. The lift-to-drag ratio of the airfoils is also observed to increase at lower angles of attack. The lift spectra and airfoil wake are examined to reveal the effect of the Gurney flap on the formation of different characteristic wake modes and the associated change in the aerodynamic forces exerted on the airfoils. Based on the observations, we classify the resulting wakes into four distinct modes. The emergence of these modes (steady, 2S, P and 2P) are mapped over a wide range of angles of attack and Gurney flap heights for all four airfoils in consideration.