Asymmetric hovering not as efficient as expected

If hovering were an event in an animal Olympics, dragonflies and hoverflies would be common shoo-ins. Unlike most insects, which hover by flapping their wings along a horizontal plane, dragonflies and hoverflies flap along an inclined plane. Known as asymmetric hovering, some have previously suggested it could be a more efficient flight mode.

But if it’s more efficient, why do most insects flap along a horizontal plane? This question led a team of aerodynamicists to compare the aerodynamics of both hovering techniques. They hoped the work could help zoologists understand why only a handful of species have evolved with asymmetric hovering and help design more efficient nanoair vehicles less than 100 milligrams in weight.

“Despite almost three decades of research, it is still unknown whether flapping wings are better suited than rotors for helicopters at these small sizes,” said co-author Thierry Jardin.

To work out the wings’ aerodynamics, the authors created a 3D numerical Navier-Stokes model to simulate how air flows around flapping wings. To understand the fundamental physics, the model was simplified to a single wing with two degrees of freedom - revolving and pitching motions.

The results showed asymmetric hovering was not more efficient than normal hovering. While other studies have suggested the opposite, the results were somewhat unsurprising to the authors because of the underrepresentation of asymmetric hovering in nature.

“The evolution of flying species probably relies on many constraints, rather than pure aerodynamic constraints,” Jardin said.

The authors plan to continue investigating the optimization of flapping wings, including the role of wing deformation on aerodynamic efficiency.

Source: “Numerical investigation of three-dimensional asymmetric hovering flapping flight,” by D. Diaz-Arriba, T. Jardin, N. Gourdain, F. Pons, and L. David, Physics of Fluids (2021). The article can be accessed at https://doi.org/10.1063/5.0069840 .