  Hovering insects are limited by their physiology and need to rotate their
wings at the end of each back and forth motion to keep the wing's leading edge
ahead of its trailing edge. The wing rotation at the end of each half-stroke
pushes the leading edge vortex away from the wing which leads to a loss in the
lift. Unlike biological fliers, human-engineered flapping wing micro air
vehicles have different design limitations. They can be designed to avoid the
end of stroke wing rotation and use so-called water-treading flapping
kinematics. Flapping wings using conventional flapping kinematics have a
designated leading and trailing edge. In the water-treading mode, the role of
the leading and trailing edges are continuously alternated throughout the
stroke. Here, we compare velocity field and force measurements for a
rectangular flapping wing conducting normal hovering and water-treading
kinematics to study the difference in fluid dynamic performance between the two
types of flapping kinematics. We show that for similar power consumption, the
water-treading mode produces more lift than the conventional hovering mode and
is 50% more efficient for symmetric pitching kinematics. In the water-treading
mode, the leading edge vortex from the previous stroke is not pushed away but
is 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 for
longer. This makes the water-treading mode a promising alternative for
human-engineered flapping wing vehicles.
