Aerodynamic optimization of a two-segment flapping wing via kinematic timing design

Flapping-wing aircraft often experience pronounced negative-lift excursions during stroke transitions, which degrade their aerodynamic stability and energy utilization, thereby hindering practical deployment. To address this issue, an albatross-inspired two-segment flapping-wing configuration was investigated with a dwell (waiting) phase introduced at stroke reversal as a kinematic timing design strategy. A three-phase kinematic model (upstroke-dwell-downstroke) was established and simulated using the XFlow lattice Boltzmann solver. Numerical simulations were conducted at flapping frequencies of 3-5 Hz with varying dwell-time ratios, and the resulting aerodynamic loads and vortex structures are analyzed. The results showed that incorporating a dwell phase reduced negative-lift fluctuations; the peak negative lift decreased as the dwell-time ratio increased up to 1/6 of the cycle and then approached a plateau. At a dwell-time ratio of 1/6, the peak negative lift was reduced by approximately 37% relative to the no-dwell case, whereas further extension of the dwell phase induced oscillations in the lift history. Overall, properly designed dwell timings provide quantitative guidance for motion sequencing and aerodynamic optimization in multi-segment flapping-wing systems.