Escape and Normal Flight Modes Revealed by Schlieren Imaging and force measurements of live tethered Dragonflies

We employed high-speed Schlieren imaging in combination with direct force measurements to investigate the near-field aerodynamics around live, tethered Pantala flavescens dragonflies flapping in quiescent air. Analysis of the Schlieren images reveal the evolution of vortical structures during a typical flap cycle. Light-path-averaged velocity vector fields, extracted using the optical flow method, revealed two predominant flow regimes termed as Escape Flight Mode (EFM) and Normal Flight Mode (NFM). Simultaneously, we analysed natural variations in wing kinematic parameters, namely hindwing–forewing phase difference, stroke-plane inclination, and flapping frequency. For EFM cases, we observe minimal wing phasing and stroke-plane inclination, combined with elevated flapping frequencies, with wings generating a vertically aligned momentum jet. The flow was characterised by coherent leading-edge vortices and strong constructive interactions between their wake. For NFM cases, larger wing phasing and inclination was observed, with the momentum jet fragmenting and orienting horizontally due to detrimental wing–wake interactions. The force measurements help deduce a frequency based criterion to distinguish bursts as EFM and NFM, which supplements the interpretations made using Schlieren imaging. Our results highlight that dragonflies actively adjust the aerodynamic behaviour by subtly modulating wing-kinematic parameters for rapid transitions between desired flight modes.