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Hovering insects are limited by their physiology and need to rotate theirwings at the end of each back and forth motion to keep the wing's leading edgeahead of its trailing edge. The wing rotation at the end of each half-strokepushes the leading edge vortex away from the wing which leads to a loss in thelift. Unlike biological fliers, human-engineered flapping wing micro airvehicles have different design limitations. They can be designed to avoid theend of stroke wing rotation and use so-called water-treading flappingkinematics. Flapping wings using conventional flapping kinematics have adesignated leading and trailing edge. In the water-treading mode, the role ofthe leading and trailing edges are continuously alternated throughout thestroke. Here, we compare velocity field and force measurements for arectangular flapping wing conducting normal hovering and water-treadingkinematics to study the difference in fluid dynamic performance between the twotypes of flapping kinematics. We show that for similar power consumption, thewater-treading mode produces more lift than the conventional hovering mode andis 50% more efficient for symmetric pitching kinematics. In the water-treadingmode, the leading edge vortex from the previous stroke is not pushed away butis captured and keeps the newly formed leading edge vortex closer to the wing,leading to a more rapid increase of the lift coefficient which is sustained forlonger. This makes the water-treading mode a promising alternative forhuman-engineered flapping wing vehicles.

To tread or not to tread: comparison between water treading and conventional flapping wing kinematics