Effects of wing flexibility on bumblebee propulsion

This study provides a comprehensive analysis of the effects of flexibility on bumblebee propulsion. The unusual stiffness distribution of bumblebee wings caused by the absence of veins on their outer one-third region is analyzed for the first time, with two flexible-wing models and one rigid-wing model tested in two-way Fluid-Structure Interactions (FSI) simulations. The two flexible-wing models are the uniform stiffness model in which the whole area (100%) of the wing is flexible and the reduced tip-stiffness model in which the tip region (30% of the total wing area) is more flexible than the rest of the wing. The rigid wing has been found to have the highest power economy (ratio of lift to input power), with the uniform- and reduced tip-stiffness wings being 7% and 15% less economical, respectively. The rigid wing produces drag, while both flexible-wing models produce thrust with a difference of approximately 3%. In forward flight, the positive thrust clearly highlights the importance of flexibility for the aerodynamic performance and propulsion of a bumblebee which will be unable to fly forward if its wings are modeled as rigid bodies.