Bats achieve lift at fast speeds by increasing the vertical (top to bottom) length of each wing flap. However, at low speeds, or while hovering to drink nectar, achieving lift is not as easy. Increasing flapping frequency can help, but the Pallas’s long-tongued bat compensates for the lack of lift in one very special way: by flipping its wing inside-out on every upstroke.
Flipping it’s wings keeps the bat airborne by creating pressure differences: above and below the wing, as well as along it. This pressure gradient creates vortices by causing air to move passively from the higher pressure area to the lower pressure area, which stirs up the air. As the wing flips between upside out and inside out during flight, pressure gradients are created that and generate vortices, which counteract air resistance and keep the bat in the air.
These vortices are created at different points along the wing, from the armpit to the wingtip, as well as at the leading (front) and trailing (back) edges of the wing. At the armpit, weaker vortices are overcome by stronger tip vortices, causing air to circulate along the bottom of the wings and back to the bat’s body. This air circulation creates the lift needed to counteract air resistance acting on the bat.
The vortex created at the trailing edge is weaker than at the leading edge, creating another pressure gradient. This creates additional vortices that provide their own lift and reinforce the effects of the circulating vortices created by the pressure differences under the wingtip and armpit. In this way, a bat’s wing acts like a flag in the wind. Unlike rigid tree branches or bird wings, flags bend, churning and spinning the air as it passes. Just as a flag flaps and curls on itself in the wind, the wings of Pallas’s long-tongued bats create similar vortices that generate lift.
This summary was contributed by Thomas McAuley-Biasi.Edit Summary
“In inviscid vortex dynamics, a line vortex must terminate either as a closed loop or at a solid surface, and so the start vortex connects to two tip and two root vortices, which grow in length during the downstroke.” (Muijres et al., 2008: 1252)