Free Falling Fliers Fall Feet First
Blog Author: Professor Adrian Thomas, CSO, Animal Dynamics
Dragonflies are aerodynamically stable. Drop a dragonfly upside down and it self-rights and glides off. So long as the wings are fixed in the normal flight position even a dead dragonfly dropped upside down will rotate to the normal flight position and glide off. A live but floppy anaestheised dragonfly tumbles. The aerodynamic stability comes from the flight posture. A gliding dragonfly holds the left and right wings slightly above horizontal (as you can see in the video and image of a hawker below), with the front wings slightly higher than the hind wings providing roll-stability and some degree of pendulum stability. The gliding dragonflies’ front wings are twisted to a slightly higher angle of attack than the rear wings to provide pitch stability, and the long body extending behind the wings combined with the slight aft wing-sweep provides yaw stability. https://player.vimeo.com/video/571638801?app_id=122963
This is all passive stability – like the stability of an arrow – and it depends on the centre of gravity being slightly forwards of the centre of lift of the wings, and both wings and body providing gentle restoring forces to maintain equilibrium in stable gliding flight. An arrow, a well-made paper dart, a shuttlecock, a parachute and a ball in a bowl are all stable in the same way – no control is required, the system is stable because its geometry balances all the forces acting (in glide, just aerodynamic forces and gravity).
Weirdly, even though they can’t fly, ants, geckos, silverfish and iguanas all also adopt a posture that makes them passively stable when dropped. A great example is this video from Yanoviak, Dudley and Kaspari’s 2005 study in Nature, which shows an ant being dropped from 25m height before gliding 2m to safely land on a nearby tree. All these animals all adopt a characteristic free-fall posture – back arched, legs and arms raised – and holding that posture they turn the right way up if they are dropped or fall, and can even track in free-fall to land on a target.
Being able to land on your feet rather than your back has obvious safety implications for tree-dwelling animals. Being able to direct your fall so you land on a tree’s trunk (rather than the dangerous forest floor) may be even better, and some animals deliberately choose to forage while climbing upwards, then descend in a controlled fall – or a glide – rather than waste energy climbing down. This may be the selection pressure that led to the evolution of flying squirrels, flying possums, flying lizards, flying snakes, flying frogs, and perhaps even the powered fliers: birds, bats and insects.
Fabian Samuel T., Zhou Rui and Lin Huai-Ti 2021Dragondrop: a novel passive mechanism for aerial righting in the dragonflyProc. R. Soc. B.2882020267620202676
Yanoviak, S., Dudley, R. & Kaspari, M. Directed aerial descent in canopy ants. Nature 433, 624–626 (2005). https://doi.org/10.1038/nature03254