Abstract: To better understand the aerodynamic reasons for highly organized movements of flying organisms, the three-flapping wing
system in tandem formation was studied numerically in this paper. Different from previous relevant studies on the multiple
flapping wings that are equally spaced, this study emphasizes the impact of unequal spacing between individuals on the
aerodynamics of each individual wing as well as the whole system. It is found that swapping the distance between the first
and second wing with the distance between the second wing and the rearmost wing does not affect the overall aerodynamic
performance, but significantly changes the distribution of aerodynamic benefits across each wing. During the whole flapping
cycle, three effects are at play. The narrow channel effect and the downwash effect can promote and weaken the wing
lift, respectively, while the wake capture effect can boost the thrust. It also shows that these effects could be manipulated
by changing the spacing between adjacent wings. These findings provide a novel way for flow control in tandem formation
flight and are also inspiring for designing the formation flight of bionic aircraft.

Key words: Three flapping wings · Unequal spacing · Aerodynamic performance · Tandem formation