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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. 

The long-range migration of monarch butterflies, extended over 4000 km, is not well understood. Monarchs experience varying density conditions during migration, ranging as high as 3000 m, where the air density is much lower than at sea level. In this study, we test the hypothesis that the aerodynamic performance of monarchs improves at reduced density conditions by considering the fluid–structure interaction of chordwise flexible wings. A well-validated, fully coupled Navier–Stokes/structural dynamics solver was used to illustrate the interplay between wing motion, aerodynamics, and structural flexibility in forward flight. The wing density and elastic modulus were measured from real monarch wings and prescribed as inputs to the aeroelastic framework. Our results show that sufficient lift is generated to offset the butterfly weight at higher altitudes, aided by the wake-capture mechanism, which is a nonlinear wing–wake interaction mechanism, commonly seen for hovering animals. The mean total power, defined as the sum of the aerodynamic and inertial power, decreased by 36% from the sea level to the condition at 3000 m. Decreasing power with altitude, while maintaining the same equilibrium lift, suggests that the butterflies generate lift more efficiently at higher altitudes. 

The long-range migration of Monarch butterflies extends over 4000 km. Monarchs experience varying density conditions during migration. Monarchs have been spotted at 1200 m during migration and overwinter at 3000 m, where the air density is lower than at the sea-level. Furthermore, Monarch butterflies have large flexible wings which deform significantly during flight. In this study, we test the hypothesis that the aerodynamic performance of the Monarch wing improves at reduced density conditions at higher altitudes. A design space with air density and stroke plane angle as design variables is constructed to evaluate the effects of fluid-structure interaction at high altitudes in the Reynolds number regime of Re = O(10^3). The effects of chordwise wing flexibility and the aerodynamic and structural response at varying densities are investigated by solving the Navier-Stokes equations, fully coupled to a structural dynamics solver at the Monarch scale. The lift, thrust and power are calculated in the design space. Our results show that lift increases with the stroke plane angle and the air density, whereas the thrust remains close to zero. The mean power required reduces with the altitude, eventually becoming negative at 3000 m. These results suggest that at lower altitudes near sea level, Monarchs can leverage the relatively large magnitude of their lift and thrust forces. At higher altitudes butterflies can fly while minimizing the power.