Span efficiency in hawkmoths

Abstract

Flight in animals is the result of aerodynamic forces generated as flight muscles drive the wings through air. Aerial performance is therefore limited by the efficiency with which momentum is imparted to the air, a property that can be measured using modern techniques. We measured the induced flow fields around six hawkmoth species flying tethered in a wind tunnel to assess span efficiency, e i , and from these measurements, determined the morphological and kinematic characters that predict efficient flight. The species were selected to represent a range in wingspan from 40 to 110 mm (2.75 times) and in mass from 0.2 to 1.5 g (7.5 times) but they were similar in their overall shape and their ecology. From high spatio-temporal resolution quantitative wake images, we extracted time-resolved downwash distributions behind the hawkmoths, calculating instantaneous values of e i throughout the wingbeat cycle as well as multi-wingbeat averages. Span efficiency correlated positively with normalized lift and negatively with advance ratio. Average span efficiencies for the moths ranged from 0.31 to 0.60 showing that the standard generic value of 0.83 used in previous studies of animal flight is not a suitable approximation of aerodynamic performance in insects.

Flapping wing aerodynamics: from insects to vertebrates

“…Most of the force data available for insects and hummingbirds only pertain to hovering flight; thus, to make this comparison, we pooled data for hovering insects and hummingbirds with that derived from slow-flying (∼2-3 m s −1 ) bats and birds. Although insects in forward flight generate more lift during the downstroke than the upstroke (Willmott et al, 1997;Ennos, 1989;Young et al, 2009;Henningsson and Bomphrey, 2013), many hovering insects support their body weight relatively symmetrically during the upstroke and downstroke (Dickinson et al, 1999). In contrast, most bats and birds support their body weight primarily during the downstroke across all flight speeds.…”

Computational investigation of cicada aerodynamics in forward flight

“…Although studies on wing models have substantially advanced the understanding of insects' flight, the wing model has intrinsic restrictions such as wing rigidity and simplified kinematics. Tethered real insects were therefore studied to investigate the leading edge vortices [6], the flow [7] and the vortex structures in the wake [8][9][10]. For example, the stereophotographs of flow past tethered tobacco hawkmoths (Manduca sexta) showed the alternating horizontal and vertical vortex rings in the wake structure [11].…”

Optimum hovering wing planform

“…(9) This shows that the elliptic chord distribution leads to a constant induced angle of attack distribution for both parallel and revolving translations, which is, to the authors at least, a delightful result. The constant sectional velocity distribution associated with parallel translation motion (fixed wing) leads in turn to a constant downwash velocity distribution and thus a unity induced power factor, where the induced power factor (denoted as k ind ) is the ratio of the actual induced power to minimum ideal induced power for a given lift [21][22][23][24]. On the other hand, the linear downwash distribution of the elliptic chord for a revolving wing leads to a k ind value of 1.13 (i.e.…”