Apparently the second earliest observation of the endangered humphead wrasse (Cheilinus undulatus) uploaded to iNat, this male off the Sinai desert in 1988 was longer than a diver, and probably twice as heavy. Ever curious, the only way to weave tightly among us was to bank its body. As this Wikipedia article points out, any turn involves an inward acceleration, which itself requires a centripetal force. (We often refer to the perception of its opposite on our body as ‘centrifugal force’ as if we were being pulled outwards). On a flat surface, the force is provided by friction. On a suitably banked surface, gravity (falling inwards) helps the centripetal force and the result acting through our body feels like an increase in weight (often expressed in G in exciting space films). In a fluid, it gets a bit more tricky as friction is limited, and ‘understeer’ ever-beckoning.
One way for animals, like this wrasse, to improve manoeuvrability, is to have a tall profile (or a collapsible ‘keel’ made from dorsal or anal fins) so it can act like a foil to reduce side-slip. Another is to have its paired fins above each other, near its centre of mass, to increase its ability to pivot about a near-vertical set of fin-brakes (on one side or the other). But in a fluid, one of the most effective ways to turn, without slipping or toppling over sideays, is to roll into the bend, and then use planes (fins or wings) to ‘pull up’ by deflecting oncoming fluid downwards (which will actually effect a lateral turn because of the pre-existing roll).
A fish like our wrasse is laterally flexible but vertically stiff, so it relies on large-area pectoral fins and its own foil-like body shape. Vertically flattened fish and animals in low-viscosity fluids such as most birds, can’t rely on a keel shape, so they will often have to use a huge bank angle, but usually not perpendicular or they may side-slip downwards. This fascinating article discusses the issue from the perspective of batoid rays, who often reach bank angles of 70°. But strangely, blue whales have their own, perhaps unbelievable, superpower: flexibility. As this article points out, they can flex their backs vertically to such a degree (and being neutrally buoyant are immune to the effects of gravity), that they can afford inward bank angles as high as 90°, and can even do the apparently impossible: turn with self-induced negative cambers!
- Do you have any examples of banked turning animals, maybe birds or land animals?