Design and Performance Validation of a Three-Segment Passive-Folding Flapping-Wing Aerial Vehicle with a Two-Stage Elastic-Joint System

To address the limited passive-folding amplitude and insufficient aerodynamic gains of large-scale flapping-wing aerial vehicles (FWAVs), this study proposes a three-segment passive-folding FWAV incorporating two elastic joints equipped with embedded torsion springs. Each wing is divided spanwise into inboard, middle, and outboard segments. During the upstroke, the two elastic joints enable the wing segments to fold sequentially, whereas during the downstroke, elastic restoring action returns the segments toward their unfolded configuration. This asymmetric deformation reduces adverse aerodynamic forces during the upstroke and enhances positive vertical aerodynamic-force generation during the downstroke. A carbon-fiber framework with graded spar diameters and an integrated servo protection and support structure are also employed to achieve lightweight construction and improve structural protection. A coupled aerodynamic–structural model is established, and the joint locations and folding-response parameters are optimized. Bench-top tests conducted at a flapping frequency of 3 Hz and a flapping amplitude of 100° show that the peak vertical aerodynamic force and peak thrust of the three-segment configuration reach 1.87 N and 1.08 N, respectively. The peak vertical aerodynamic force is approximately 19% and 12% higher than those of the one-segment and two-segment configurations, while the average vertical aerodynamic force increases by approximately 21% and 10%, respectively. Comparative free-release trajectory tests show that the cumulative altitude loss of the three-segment configuration is approximately 1.5 m under the tested conditions, demonstrating good short-term altitude-holding capability. These results indicate that sequential passive folding can effectively improve the aerodynamic performance of large-scale FWAVs.