Bat wing structures important for aerodynamics and rigidity (Mammalia, chiroptera)
The anterior part of the wing, formed by the membrane parts anterior to the arm and third digit, is proportionally broad in megachiropteran bats as well as in many broad-winged microchiropteran bats. These parts can be lowered by the thumb and by pronation of the manus, and may together function as a leading edge flap. Leading edge flaps of aeroplanes permit, when lowered, higher angles of attack without separation, and thus higher lift coefficients. The leading edge in bats is very sharp, which increases the effectiveness of the leading edge flap.
The Reynolds number of bat wings lies in an interesting range, where the lift coefficient can be improved by induced turbulence of the boundary layer. The arm and digits, projecting markedly over the dorsal surface of the wing, and hair may function as turbulence generators.
The tension forces of the membrane on the digits have different effects upon the different digits, depending on the tautness of the surrounding membrane parts. The second digit and distal phalanx of the third digit are exposed mostly to bending in the membrane plane. The phalanges of the fourth and fifth digits are exposed to large dorsoventral bending.
Two arrangements add to relieving the distal part of the wing of large tension forces, thereby reducing the demand for a powerful extensor muscle of the distal phalanx (-ges) of the third digit: 1. The fourth and fifth digits act to alter the direction of tension. 2. By splitting the wing membrane in several parts by the digits, the second and third phalanges (the second in fruit-bats) of the third digit, which constitute the distal part of the wing's leading edge, are exposed to tension forces transformed from forces only from the nearest patagium. If the wing membrane would be outstretched only by one digit, as was the case in pterosaurs, the leading edge digit would have to resist the tension forces transformed from forces from the entire membrane posterior to the arm.
The fourth digit is angled in such a way that the proximal part of the membrane between the third and fourth digits is kept very taut, and the fourth metacarpophalangeal and interphalangeal joints are held very steady without any need of large muscular forces.
KeywordsTension Force Lift Coefficient Distal Phalanx High Lift Wing Membrane
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