Bat Flight Generates Complex Aerodynamic Tracks

TLDR

Flapping flight creates a time‑varying vortex wake whose momentum change represents aerodynamic force. The study aims to revise modeling strategies for membranous‑winged flapping flight based on unsteady aerodynamic performance. We found that small bats generate wakes distinct from birds: each wing forms its own vortex loop, and at moderate and high speeds the outer (hand) wing and arm wing exhibit opposite circulation during the upstroke, producing negative lift on the hand wing and positive lift on the arm wing.

Abstract

The flapping flight of animals generates an aerodynamic footprint as a time-varying vortex wake in which the rate of momentum change represents the aerodynamic force. We showed that the wakes of a small bat species differ from those of birds in some important respects. In our bats, each wing generated its own vortex loop. Also, at moderate and high flight speeds, the circulation on the outer (hand) wing and the arm wing differed in sign during the upstroke, resulting in negative lift on the hand wing and positive lift on the arm wing. Our interpretations of the unsteady aerodynamic performance and function of membranous-winged, flapping flight should change modeling strategies for the study of equivalent natural and engineered flying devices.