Effect of transverse gusts on a Red-tailed Hawk*

The inability of smaller-scale uncrewed aerial vehicles (UAV) to navigate gust-laden environments significantly impedes their performance envelope. In this study, the response of a red-tailed hawk to upwards transverse gusts is investigated as a bio-inspired approach to improve UAV gust-resilience. An indoor flight arena instrumented with high-speed cameras was used to quantify the 3D motion of the hawk flying through gusts of two different magnitudes - low and high gust ratios. Although the hawk maintained its flapping motion while flying through the gust, it encountered the gust at different points in the flapping cycle depending on the experimental run and gust ratio. In response to the gust, the hawk executed a continuous downward pitching motion of the wing, decreasing the wing pitch angle to between -20^○ and -5^○. Estimation of coefficient of lift (C_L) across the wing illustrated that the C_L increased at a low rate, to a maximum of around 2 to 2.5 for low gust ratio conditions. At high gust ratio conditions, the C_L initially increased rapidly and then increased at a low rate to a value around 4 to 5. A flight dynamics model accounting only for the wing mechanics was able to better predict the flight path of the bird at low gust ratio conditions when compared to high gust ratio conditions. This points to additional factors (for example, tail pitch and roll modulation), which were potentially used by the bird, that need to be included in the flight dynamics model to improve predictions.