Research progress on the flight mechanism of butterfly and butterfly-inspired flapping-wing air vehicles

Bionic flapping-wing air vehicles present notable advantages, including high maneuverability, concealment, and efficiency. They hold promising applications in military reconnaissance and exploration search and rescue, rooted in a comprehensive exploration of biological flight mechanisms. Advanced motion observation and experimental techniques have facilitated more convenient and precise recording and analysis of insect flight behavior. Research indicates that common insects exhibit a high flapping frequency, ranging from 25 to 400 Hz, while butterflies, characterized by a lower flapping frequency of approximately 10 Hz, stand out. Despite the unique attributes of butterfly flight, aerodynamic research remains scarce compared to other flying organisms, resulting in an insufficient understanding of their intricate flying skills. Butterflies, distinguished by large forewings and hindwings that flap nearly synchronously on the same side of the body, spanning a substantial range of up to 180°, display substantial pitch swing during flight, with highly coupled wing and body movements. Remarkably, despite these complexities, butterflies demonstrate agile flight capabilities, enabling them to embark on long-distance migrations spanning thousands of kilometers. This exceptional characteristic renders them exemplary subjects for bionics research, capturing the attention of scholars globally. In contrast to other insects, butterflies have a uniquely intricate flight mechanism, complicating the development of butterfly-inspired flapping-wing air vehicles. Current endeavors in this field often simplify the mechanism of butterfly wing–body motion coupling, with only a few achieving controlled and stable flight. Simultaneously, the ongoing advancements in microelectromechanical system technology, aerodynamics, and precision processing are insufficient to support the development of practical insect-scale flapping-wing air vehicles fully. Accordingly, researchers have adopted a bionic perspective, observing butterflies’ free flight to understand their flapping-wing flight mechanism via experimental and numerical analysis methods. By the similarity principle of fluid mechanics, adjusting the scale, lowering the flapping frequency, and emulating butterflies’ distinctive flight motion in engineering, a butterfly-inspired flapping-wing air vehicle with a small aspect ratio and ultra-low frequency flapping was conceptualized and fabricated. Although current prototypes can achieve remote-controlled flight, a considerable disparity persists when compared to the flight behavior and capabilities of actual butterflies. Furthermore, most prototypes suffer from subpar battery life due to energy limitations. In comparison to flapping aircraft mimicking birds or larger insects with a high aspect ratio, butterflies have more intricate flapping movement and tailless posture control. Their unique maneuvering flight control, involving coupled and cooperative wing–body movements, demands further comprehensive exploration. Thus, achieving prolonged, controllable, and agile flight in a butterfly-inspired flapping-wing air vehicle poses a considerable challenge. Consequently, this paper synthesizes the distinctive flight behavior and mechanisms observed in living butterflies, elucidating key technologies for developing butterfly-inspired flapping-wing air vehicles. It also delineates the future trajectory for advancing this aircraft category.