Biomimetic flapping-wing air vehicles are Unmanned Air Vehicles (UAV) mimicking the exceptional flight capabilities of insects and birds in the natural world which can delicately control posture and position during flight by flapping wings. These air vehicles have great potential for high-performance applications due to their unique biomimetic shape and flight characteristics, enabling them to achieve high maneuverability and aerodynamic efficiency in low Reynolds number environments. Typically, biomimetic flapping-wing air vehicles adopt lightweight flexible-wing structures. However, the Fluid-Structure Interaction (FSI) problem is a prominent research challenge regarding the periodic flapping motion. This article provides an overview of the current research status of fluid-structure interaction in flexible-wing flapping, introducing the common methods for fluid-structure interaction and important dimensionless parameters involved in numerical simulation analysis of flexible-wing flapping. The latest progress achieved in both domestic and international research is analyzed, and the tools used in computational analysis are shared. Furthermore, the article identifies the problems and potential directions for further development in current research, offering prospects for future studies on fluid-structure interaction in biomimetic flexible-wing UAVs.

Certain insect species have been observed to exploit the resonance mechanism of their wings. In order to achieve resonance and optimize aerodynamic performance, the conventional approach is to set the flapping frequency of flexible wings based on the Traditional Structural Modal (TSM) analysis. However, there exists controversy among researchers regarding the relationship between frequency and aerodynamic performance. Recognizing that the structural response of wings can be influenced by the surrounding air vibrations, an analysis known as Acoustic Structure Interaction Modal (ASIM) is introduced to calculate the resonant frequency. In this study, Fluid Structure Interaction (FSI) simulations are employed to investigate the aerodynamic performance of flapping wings at modal frequencies derived from both TSM and ASIM analyses. The performance is evaluated for various mass ratios and frequency ratios, and the findings indicate that the deformation and changes in vortex structure exhibit similarities at mass ratios that yield the highest aerodynamic performance. Notably, the flapping frequency associated with the maximum time-averaged vertical force coefficient at each mass ratio closely aligns with the ASIM frequency, as does the frequency corresponding to maximum efficiency. Thus, the ASIM analysis can provide an effective means for predicting the optimal flapping frequency for flexible wings. Furthermore, it enables the prediction that flexible wings with varying mass ratios will exhibit similar deformation and vortex structure changes. This paper offers a fresh perspective on the ongoing debate concerning the resonance mechanism of Flexible Flapping Wings (FFWs) and proposes an effective methodology for predicting their aerodynamic performance.

The flapping motion has a great impact on the aerodynamic performance of flapping wings. In this paper, a surging motion is added to an airfoil performing pitching-plunging combined motion to figure out how it influences the lift performance and flow pattern of flapping airfoils. Firstly, the numerical methods are validated by a NACA0012 airfoil pitching case and a NACA0012 airfoil plunging case. Then, the E377m airfoil which has typical geometric characteristics of the bird-like airfoil is selected as the calculation model to study how phase differences φ₁ between surging motion and plunging motion affect the aerodynamic performance of flapping airfoils. The results show that the airfoil with surging motion has comprehensively better lift performance and thrust performance than the airfoil without surging motion when 15°< φ₁ < 90°. It is demonstrated that surging motion has a powerful ability to improve the aerodynamic performance of flapping airfoil by adjusting φ₁. Finally, to further explore how flapping airfoil improves lift performance by considering surging motion, the flapping motions of E377m airfoil with the highest lift coefficient and lift efficiency are obtained through trajectory optimization. The surging motion is removed in the highest lift case and highest lift efficiency case respectively, and the mechanism that surging motion adjusts the aerodynamic force is analyzed in detail by comparing the vortex structure and kinematic parameters. The results of this paper help reveal the aerodynamic mechanism of bird flight and guide the design of Flapping wing Micro Air Vehicles (FMAV).