Flapping rotary wing: A novel low-Reynolds number layout merging bionic features into micro rotors

Since the birth of bio-inspired flapping-wing micro air vehicles, a controversial topic, i.e., whether and to what extent a flapping wing can outperform conventional micro rotors, has existed in the field of micro-to pico-scale unmanned aircraft. However, instead of answering this debate, an alternative idea that combines the flapping-wing and rotary-wing layouts was proposed and has been extensively studied over the last ten years. By merging bionic features of flapping wings into micro rotors, this novel layout, i.e., flapping rotary wing (FRW), can maintain autorotation with no driving torque and achieve both a superb lift generation and a moderate efficiency at a Reynolds number between 10³ and 10⁴, presenting an additional choice for micro air vehicles when facing a task to balance the payload and energy cost. As the first review of FRW, this paper overviews the concept, bionic features, aerodynamic principles, and development of flyable prototypes since 2010, from fundamental aerodynamic mechanisms to key points in prototype design, including wing structure, actuator, transmission system, energy source, etc. The advantages and disadvantages of this novel layout over conventional flapping wings and micro rotors are discussed. Four challenging directions are then suggested to improve the flight performance of this layout and thus boost its application in military and civilian fields.