PIV results from a kinematic study of robotic bird wings

A faster cruising speed increases drag and thereby the thrust (T) needed
to fly, while the weight and lift (L) requirement remains constant. Birds
can adjust their wingbeat in multiple ways to accommodate this change in
aerodynamic force, but the relative costs of different strategies remain
largely unknown. To evaluate the efficiency of several kinematic
strategies, I used a robotic wing (Ajanic et al., 2023) and quantitative
flow measurements. I found that, among the tested strategies, changing the
mean wingbeat elevation provides the most efficient solution to changing
T/L, offering insight into why birds tend to beat their wings with a
greater ventral than dorsal excursion. I also found that although
propulsive efficiency (ηp) may peak at a Strouhalnumber (St, measure of
relative flapping speed) near 0.3, the overall efficiency of generating
force decreases with St. This challenges the expectance of a specific
optimal St for flapping flight and instead suggest the chosen St depends
on T/L. This may explain the variation in preferred St among birds and why
bats prefer flying at higher St than birds (Taylor et al., 2003) since
their body shape imposes relatively higher thrust requirements (Muijres et
al., 2012). In addition to explaining flapping strategies used by birds,
my results suggest alternative, efficient, flapping motions for drones to
explore aiming to extend their flight range.