bat flight


above: the elastic wing membrane (patagium) is supported by the outstretched limbs, body, and tail. The membrane is very thin and consists of two layers of skin with small muscles between them, as shown on the left half of the diagram. On the right are shown the networks of elastic fibres and blood vessels within the membrane. (Diagram modified from Yalden and Morris) The intrinsic muscles and elastic fibres help to keep the membrane taut during changes of wing geometry. The bat can reduce its wing span by up to 20% while keeping the membrane taut. Birds have the advantage here: since the feathers can overlap to differing degrees, birds' wings can maintain an effective aerofoil over a greater range of shape changes.
flight muscles The bat has 4 downstroke muscles: pectoralis major, subscapularis, part of serratus anterior, and part of deltoid. (This compares with the single downstroke muscle in birds: pectoralis major.) Together, these muscles constitute about 12% of the bat's body weight. The upstroke is powered when required by the remainder of deltoid, trapezius, the rhomboids, infraspinatus, and supraspinatus. This compares with the sole upstroke muscle in birds: supracoracoideus. In bats, the scapula provides attachment for many of the flight muscles and is mobile across the back of the rib cage during the wing beat cycle. The pectoralis major attaches to the sternum, as it does in birds, but bats do not generally have such a well developed ridge (carina) that is seen on the sternum of birds. In the forearm region of the bat wing, the radius is very slender compared with the ulna. The first digit of the bat's wing is small and clawed, and is used during climbing and walking. The muscle extensor carpi radialis muscle inserts on the base of the metacarpal of the second digit - this muscle pulls forward on the metacarpal and a ligament between digits 2 and 3 transfers tension to the third digit, thus keeping the outer part of the wing taut and extended Digits 4 and 5 extend across the chord of the wing. The muscle abductor digiti minimi along the ventral surface of the 5th digit can bring about changes in the camber of the aerofoil. As the wing is an oscillating structure, it is advantageous to keep weight at a minimum further out towards the tip, while heavier components are positioned inboard. The muscles responsible for extending the wing attach distally via long tendons.	above: during the downstroke, the scapula (s) and clavicle (c) move laterally and ventrally, as indicated by the two smaller arrows. This enables a wider arc of rotation for the humerus (h). During the upstroke the scapula slides back to its more dorsal position.
	above: section through outer part of bat's wing to show cambered aerofoil surface supported by bony digits. The ridges produced by the skeletal elements help to maintain boundary flow across the wing at low speeds.

flapping flight There is a similar pattern of wing beat between bats and birds. During the downstroke, the wings are fully extended and sweep downwards and forwards until the tips are ahead of the bat’s nose. The leading edge is tilted down during this phase, particularly towards the tip. The downstroke produces lift and forward propulsion. Since the aerodynamic centre of pressure lies behind the skeleton, the bones experience considerable twisting forces (Swartz, 1992). During the upstroke, the elbows and wrist flex so the wing partially folds, and the leading edge tilts upwards. The lift-drag ratio of a typical bat’s wing is about 6.8, comparable to that of a pigeon.

References Swartz, S.M. (1992) Wing bone stresses in free flying bats and the evolution of skeletal design for flight. Nature, 359, 726-729. Yalden, D.W., and Morris, P.A. ( ) The lives of bats. Newton Abbot: David & Charles.