Bird feathers inspire aerodynamic flaps

Covering the upper and lower surfaces of a bird’s wings are a set of feathers, called covert feathers, which improve flight aerodynamics. Flexible flaps inspired by covert feathers have been attached to aircraft wings to examine their effects, but most of these studies were limited to either suction-side flaps or pressure-side flaps — flaps attached to the upper and lower surfaces, respectively. Kim et al. analyzed how a dual-flap system compares to these single-flap efforts.

“Because the flaps are passive and flexible, they may not need motors or active control,” said author Young Dal Jeong. “They can move naturally with the flow, similar to feathers on bird wings.”

The researchers conducted three sets of simulations examining flaps attached to airfoils. They started with an unaltered airfoil, then added a single suction-side flap, and lastly an additional pressure-side flap.

The flaps, it turns out, do not work in isolation; the two affect one another synergistically. On its own, the suction-side flap suppresses reverse flow and separation to keep lower pressure above the wing, achieving lift. The pressure-side flap, on the other hand, increases the pressure beneath the wing. However, the pressure-side flap also affects flow over the top surface, and together, the two create a strong suction system above the wing and higher pressure beneath.

“This showed us that the two flaps create a new flow mechanism together, rather than simply adding their separate effects,” Jeong said.

In the optimal configuration, the dual flaps increased lift by about 73% compared to the unaltered control case, an improvement impossible to achieve with either type of flap alone.

Though the group hopes their study can help lead to simple, lightweight aircraft flow control, their interest in bioinspired design goes beyond its applications.

“Bioinspired design is not only about copying nature’s shape,” Jeong said. “It is about understanding how nature works.”

Source: “Lift enhancement of an airfoil using covert-inspired dual-side flexible flaps,” by Gun Wook Kim, Young Dal Jeong, and Jae Hwa Lee, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0331722 .