Aerodynamic Performance of a Multi-Element Flapping Foil Inspired from the Feathered Wings of Birds

Birds utilize feathered wings where individual feathers serve as distinct control surfaces that deform locally under the effect of aerodynamic forces and introduce complex interaction effects. The role of these effects in improving lift generation remains unclear. To investigate this, we analyze a feather-inspired control surface of a flapping foil, composed of three pitching and heaving rigid membranes (referred to as feathers) designed to enhance lift in flapping flight. Two-dimensional numerical simulations are conducted at a Reynolds number of 5000, evaluating the performance of the proposed control surface at three Strouhal numbers ($St$ = 0.08, 0.12, and 0.2), representative of small bird flight conditions. Our results show that specific combinations of feather lengths maximize the lift-to-power ratio for each Strouhal number. The best-performing cases generate up to twice the mean lift force of a single feather for the same power expenditure. At $St$ = 0.12, varying the heave amplitude has minor effects on the peak feather performance. While the upstroke (downstroke) generally produces negative (positive) lift, performance gains are primarily driven by minimizing negative lift during the upstroke. We also quantify the inter-feather interaction effects, which are more pronounced at higher Strouhal numbers. The proposed control surface may be useful in developing efficient micro- and unmanned aerial vehicles.