Flying Fish Sculls to Taxi and Perturbs Wing Lift With Travelling Waves to Land

The top 200 meters of oceans abound in life forms since photosynthesis is possible in that layer. Competition and predator-prey (swordfish-flying fish, 10²–10⁴ to 1 mass ratio) interactions are intense here. Chased by predators, a flying fish (FF) — a pleuston — frantically escapes from the water and becomes airborne. Here we report the visual observations of oceanic surface and body distortions of FF to surmise the mechanisms of propulsion during taxiing and landing. FF leaps, not when it is chased, but when the additional energy required for further increase in speed underwater exceeds that required to leap.¹ The higher metabolic cost of transport of regular flapping flight in air than in water is circumvented by gliding. We examine the BBCTV video² by Richard Attenborough, the noted naturalist. An FF may camber its wings like parafoils and may also twist the outer half of the wings during taxiing and climbing. To produce thrust during taxiing, the FF sculls with the lower lobe of the tail fin to produce a reverse Karman vortex jet; there is rapid flicking of the lower lobe of the tail fin tangentially over the surface. The body acts as a chaotic damped and driven pendulum to produce the high-velocity wide flick. To damp after takeoff, it becomes a single asymmetric pendulum. Unpowered (foil) gliding follows. For descent, the wings are shaped, untwisted parafoils and, just prior to touchdown, travelling waves are superimposed, producing, in contrast to taxiing, an impressively smooth small-angle-of-attack tail touchdown on water without any nose-down. The spiked crowns of Richtmyer-Meshkov interface instability are visible on the ocean surface during leaping but not during landing. Trailing hydraulic jumps are observable during landing but not during leaping. The leap is a high-acceleration and Weber number dominated (inertia/capillary forces) phenomenon, but the landing involves little impact force and is dominated by Froude number forces (inertia/gravity forces). The evidence suggests that, prior to leaping and while still underwater, the FF reads the surface wind direction to align the flight path.