The wing of Pallas's long-tongued bat generates lift by flipping the outer edge upside down and quickly back up for the upstroke.

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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.

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References

“At the end of the downstroke the wing goes through a pitch-up motion, flipping the wing upside down, which results in the shedding of a distinct combined start/stop vortex for the down- and upstroke.” (Johansson et al., 2008: 2909)
 
“At the end of the upstroke the wing goes through a pitch-down motion, shedding a combined stop/start vortex for the up- and downstroke.” (Johansson et al., 2008: 2909)
 
“At the transition from downstroke to upstroke, the wing goes through a large supination (pitch-up rotation), so that the wing is flipped upside down.” (Hedenström et al., 2007: 894)
 
“The vorticity field and velocity vectors around the bat wing show that the flow separates at the leading edge, generating a patch of high negative vorticity (clockwise spin).  But, remarkably, behind this patch of vorticity the airflow reattaches, resulting in attached and laminar flow at the trailing edge.” (Muijres et al., 2008: 1251)
 
“The vorticity field and velocity vectors show the presence of a tip vortex with negative vorticity (clockwise spin) and a weaker vortex near the wing root (root vortex) with positive vorticity (counterclockwise spin).” (Muijres et al., 2008: 1252)

“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)
 
“Because the LEV [Leading Edge Vortex] circulation strength is similar to the root-vortex circulation, these are probably connected, hence the absence of a LEV across the body.  The near wake of slow-flying bats did not show a separately shed LEV, suggesting that the LEV stays attached throughout the downstroke and merges with the stop vortex.” (Muijres et al., 2008: 1252)

Journal article
Bat Flight Generates Complex Aerodynamic TracksScienceMay 10, 2007
A. Hedenstrom, L. C. Johansson, M. Wolf, R. von Busse, Y. Winter, G. R. Spedding

Journal article
The near and far wake of Pallas' long tongued bat (Glossophaga soricina)Journal of Experimental BiologySeptember 5, 2008
L. C. Johansson, M. Wolf, R. von Busse, Y. Winter, G. R. Spedding, A. Hedenstrom

Journal article
Leading-Edge Vortex Improves Lift in Slow-Flying BatsScienceFebruary 28, 2008
F. T. Muijres, L. C. Johansson, R. Barfield, M. Wolf, G. R. Spedding, A. Hedenstrom

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Organism
Pallas' Long-tongued BatGlossophaga soricinaSpecies


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