Unsteady Aerodynamics Of A Flapping Airfoil PDF Download

Propulsive forces can be generated with flapping or heaving wings traveling through a fluid, as demonstrated in animal flight. The flow field of a bird in flight is one of the most complex aerodynamic problems that can be examined. Many other bird flight theories are based on quasi-steady fluid dynamic assumptions even though the flow field is inherently unsteady. To help examine the time dependency of bird flight, an unsteady three dimensional potential flow panel code was developed. Another problem in studying the aerodynamics of avian flight is the limited amount of quantitative experimental data on flapping wings. To gain further understanding of avian flight and to develop an experimental database for comparison of the fluid dynamic model, a mechanical bird was constructed and placed in a wind tunnel. The mechanical bird, modeled after a pigeon, is capable of flapping at various angles to the freestream flow. For the current study, however, the flapping motion was limited to a plane normal to the oncoming flow. Results of the numerical model were compared to the measured forces on the mechanical bird as well as to the limited data available on real birds. In one case, the model was applied to a flapping wing in a wind tunnel at high advance ratios $(J=4.31)$ where the computed average lift and thrust were within the error bounds of the experimental data. The model was also applied to high frequency flapping flight $(J=0.76)$ of a pigeon flying in a wind tunnel, where the predicted lift matched the weight of the bird. Time histories of the mechanical pigeon agreed well with the numerical predictions for high $(J=5.38)$ and low $(J=0.84)$ advance ratios. The model was also extended to examine the effect of wing twist on flight efficiencies. Flight characteristics of the mechanical bird and a real pigeon were also compared.