Investigation into Wake of Flapping Wing of Robotic Bird

The Robird is a drone developed by Clear Flight Solutions (CFS) that was designed to mimic a Peregrine Falcon in appearance during its flight. The drone has the same dimensions and weight as the real falcon, and produces lift and thrust by flapping its wings. Birds instinctively sense that a falcon in flapping flight is on the hunt, making the Robird very suitable for pest bird control. Though this robotic bird proves that flapping flight by a robot is possible, the theory behind flapping flight is not fully understood. The goal of the present study is to contribute to a better understanding of flapping wing propulsion of a robotic bird like the Peregrine-Falcon based Robird. To that aim a set-up has been designed, realized and utilized for the Aeroacoustic Wind Tunnel of the University of Twente. The semi span-wind tunnel model corresponds to the port side of the full-scale robotic bird, capable to operate at realistic conditions of flapping flight in which the wing carries out a combination of a plunge and a pitch motion. The present study explores the use of Particle Image Velocimetry (PIV) in the analysis of the aerodynamic/propulsion performance of the Robird, derived from the measured flow in the wake of the robotic bird. For the present exploratory study, a single measurement plane is considered. This plane is parallel to the symmetry plane of the Robird and reveals the wake downstream of the mid-wing section. Utilizing a control-volume analysis gives an estimate of the time-averaged thrust produced by the robotic bird. The results show that for Strouhal numbers above 0.1, the wake of the flapping wing has, time-averaged, a jet-like profile, associated with the generation of thrust. For increasing values of the Strouhal number, up to St = 0.45, the maximum value of the jet velocity increases. For Strouhal numbers smaller than 0.27 a jet-like profile is found that is symmetric about the mid position of the flapping wing. For higher values of the Strouhal number the jet-like profile becomes asymmetric. The PIV results are compared to the results of an unsteady, 2D, potential flow model of a pitching/plunging airfoil section, corresponding to the mid-wing section of the robotic bird. The results of the model show results very similar to the ones obtained from PIV.