A method to optimal design the geometric parameters of a foldable flapping wing based on quasi-steady aerodynamics

Compared to rotorcraft and fixed-wing aircraft, foldable flapping wing aircraft have more significant advantages. Most foldable flapping wing aircraft (FFWA) are designed based on bionics of birds directly. However, the geometric parameters of these aircraft have been analyzed or optimized to improve a FFWA’s flight performance rarely. To improve FFWA’s flight performance and reduce the dependence on the thrust, a method to optimal design a foldable flapping wing geometric parameters based on aerodynamics has been proposed in this paper. First, a quasi-steady lifting line theory of foldable flapping wings was derived, and the expression of its lift in the vertical direction was obtained based on aerodynamics. Then, a typical FFWA model was taken as the studied object and an objective function to obtain the maximum lift in the vertical direction of a cycle was put forward. Third, we analyzed the relationship of some key geometrical parameters of the inner and outer wings of the FFWA model in terms of the requirements and capabilities of the objective function. Finally, we optimized the key geometrical parameters by using genetic algorithm. The angle between inner wings decreased from 132.58° to 16.59° and the angle between the inner wing and the outer wing decreased from 105.60° to 29.56° at the highest position, which were more similar to the flight status of birds. The outer wings were expanded more rapidly and the movement range of the outer wing tip was wider after optimization. The lift of the optimized aircraft increased with 110% by calculation.