Effects of asymmetrical inflow in forward flight on the deformation of interacting flapping wings

This study investigates the wing deformation of the DelFly II in forward flight conditions. A measurement setup was developed that maintains adequate viewing axes of the flapping wings for all pitch angles. Recordings of a high-speed camera pair were processed using a point tracking algorithm, allowing 136 points per wing to be measured simultaneously with an estimated accuracy of 0.25 mm. The measurements of forward flight show little change in the typical clap-and-peel motion, suggesting similar effectiveness in all cases. It was found that an air-buffer remains at all times during this phase. The wing rotation and camber reduction during the upstroke suggests low loading during the upstroke in fast forward flight. In slow cases a torsional wave and recoil is found. A study of the isolated effects showed asymmetric deformations even in symmetric freestream conditions. Furthermore, it shows a dominant role of the flapping frequency on the clap-and-peel, while the freestream velocity reduces wing loading outside this phase.