Flight of Alsomitra macrocarpa

Abstract

Alsomitra macrocarpa is a gliding diaspore which grows on tall trees in tropical Asian forests. It does not rely on gusts nor even on a slight breeze to fly up to hundreds of meters. It is a low aspect ratio wing, which defies the conventional aerodynamic argument that efficient gliders need to be slender. Compared to pappose seeds like the dandelion, Alsomitra macrocarpa has a higher wing loading, yet it reaches a comparable terminal velocity. It also achieves a stable flight in the absence of both a vertical stabiliser and active control. What enables these remarkable flight abilities, which inspired the design of the first manned gliders, are yet to be understood. The investigation herein utilised image analysis, three-dimensional scans and wind tunnel tests. The morphological study highlighted Alsomitra macrocarpa’s uniqueness in terms of dimensions and shape. Through the use of depth cameras, the gliding path of 15 seeds was recorded. While existing literature treats Alsomitra macrocarpa as a flat, two-dimensional shape, drop tests evidenced a preferential flight orientation for every seed sample. Hence, and contrary to previous belief, the membrane wing has an intrados and an extrados. The majority of the seeds presented a helical path, while some moved in a straight oscillatory path that has not been previously reported, with oscillations on the vertical and horizontal plane. This gliding trajectory assumed to be two-dimensional, could be described by a simplified dynamics model. The phugoid style flight coupled the horizontal motion of a tumbling wing with the oscillation of a fluttering wing. Wind tunnel tests revealed how the membrane wing undergoes spanwise deformation under the loads experienced during a glide. This deformation displaces the aerodynamic centre from the plane of the membrane wing. Low-order dynamical models, which included a non-uniform mass distribution, were employed to recreate these oscillations in the vertical plane and showed good qualitative agreement with the experiments. Overall, this work provides new insights into the remarkably stable and efficient flight of Alsomitra macrocarpa. The aerodynamic conditions under which these seeds fly are in the range experienced by Micro Aerial Vehicles (MAVs). Thus, the outcomes of this thesis could aid the design of more efficient MAVs, just as early aviation pioneers were inspired when they saw Alsomitra macrocarpa glide down through the forest canopy.