Effects of wing loading on the aerodynamic characteristics of autorotating maple seeds

Abstract

Natural maple seeds can fall into stable autorotation when descending. Their excellent aerodynamic performance can be applied to biomimetic aircraft design. Wing loading plays an important role in flight performance. To make clear the effects of wing loading on the aerodynamic characteristics of autorotating maple seeds, experiments integrated with numerical simulation were performed. In the experiment, the free fall test and the wind tunnel test were conducted on maple seeds with variable wing loadings. During the free fall process, four typical stages can be divided for the maple seed according to different kinematic characteristics. In the numerical simulation, the Moving Reference Frame method was used to simulate the fluid of autorotating maple seeds. Both experiment and simulation results show that descending velocities and spinning rates rise almost linearly with the increase in wing loadings, and descending velocity is closely related to spinning rate. Obvious leading-edge vortexes were observed on seed wing, which are the flow mechanism of high lift. The pressure differences between the top and lower surfaces of seeds grow with increasing wing loadings. Larger wing loadings lead to more serious separation of leading-edge vortex. The parameter influence analysis demonstrates that smaller conning angles or larger spinning rates are beneficial for high-lift flight. The spinning rate has a more significant effect on the autorotating maple seeds. However, when the spinning rate is larger than 200 rad/s, the lift does not increase anymore. The spinning rate should be controlled smaller than 200 rad/s when applied to biomimetic aircraft.