Aerodynamics of a Flapping-Perturbed Revolving Wing

At low Reynolds numbers, revolving wings become less efficient in generating lift for hovering flight due to the increasing adverse viscous effects. Flying insects use reciprocating revolving wings that exploit unsteady aerodynamic mechanisms for lift augmentation. Here, the aerodynamics of an alternative that introduces unsteadiness to the revolving wings through vertical flapping and its potential to improve aerodynamic performance are investigated. The force production and the flow pattern of such flapping-perturbed revolving wing are analyzed via combined experimental and computational investigations. The results show that drag reduction can be produced consistently by a flapping-perturbed revolving wing at zero angle of attack. The reduction is linearly dependent on Strouhal number, and the critical Strouhal number at equilibrium rotating state is similar to that of two-dimensional heaving plates. At positive angles of attack, the flapping perturbation leads to substantial lift augmentation, accompanied by relatively small increase of drag or even minor drag reduction, depending on the Strouhal number and flapping amplitude. Though slightly less efficient at these angles of attack in terms of power loading, flapping perturbations can be used to improve the maximum lift coefficients attainable by revolving wings and thus have potential applications in micro air vehicle designs.