Investigation flapping-flight aerodynamics of a robotic bird

The Robird is a bird-like drone or ornithopter that generates both lift and thrust by flapping its wings. Its appearance, size and weight, but specifically its flapping flight performance resemble that of a Peregrine falcon. This paper describes an extension of Prandtl’s Lifting-line method from steady flow to unsteady flow to predict the unsteady lift and thrust of the robotic bird. The extension comprises the derivation of the Kutta-Joukowski Theorem for unsteady flow, an unsteady Kutta condition and the representation of the wake as a stationary transpiration surface carrying a time-dependent dipole distribution, which instantaneous strength is solved from the spanwise distribution of the circulation of the lifting line at earlier times. For the cases considered, the numerical method predicts that both the section-lift and section-thrust of the wing vary with time. The cycle-averaged section-lift and section-thrust, as well as the cycle-averaged overall lift and thrust, are mostly positive during the flapping flight. Cycle-averaged distributions of sectional circulation, lift and upwash, as well as cycle-averaged overall lift depend on Strouhal number, not on pitch amplitude, while the distribution of cycle-averaged sectional thrust, as well as cycle-averaged overall thrust depend on Strouhal number and on pitch amplitude. The topology of the wake as predicted by the unsteady-lifting-line method, is compared to results of experiments performed with 2D3C PIV applied to a wind-tunnel model including a full-scale port-side wing of the Robird. For the downstroke, the near-wake of the flapping wing is a free shear layer, which vorticity distribution has the orientation as predicted by the vortex distribution determined by the unsteady-lifting-line method The results of the measurements and of the predictions show topological similarities.