26

P. Mukkavilli et al.

Fig. 3.1 Flow Structures in the Formation of LEVs along the Wing span (Modified and redrawn

from various sources)

number [21]. This leading edge vortex is a low-pressure region, and it increases

the lift by 80%.

Ellington et al. [22] observed that delayed stall of leading edge vortex (LEV) can

significantly enhance the lift associated with a normal flapping wing. The exact role

played by the LEV and its consequences on lift generation remain to be elucidated

fully. Leading Edge Vortex (LEV), Trailing Edge Vortex (TEV) and Tip Vortex (TP)

play an important role in effecting the delayed stall. It is interesting to note that

LEV generates a lower pressure area which in turn increases the suction force on the

upper surface of the wing. Formation of LEV is a general flow feature in flapping

wings having Re of 10⁴ or less. Changes in Re number reduce the frequency, and the

Strouhal number will affect delayed stall with an increase in frequency.

It is the airflow separation from flying aeroplane wing relatively at higher angles

of attack resulting in a sharp fall in lift and increase in drag leading to a serious

aircraft accident. Flow separation infers the separation of the boundary layer which

is in contact with the surface of an aeroplane. The stall angle limit in aeroplane ranges

from 15° to 16°. However, for insects, it is variable from 45° to 90° (Fig. 3.2). The

elastic nature and structural deformation of the wings and relatively high frequency

of the wingbeat help in enhancing the stall angle in insects. In biological fliers, the

pre-stall may continue for some time and this helps in quick recovery due to fast

feedback sensory systems prevalent in insects.

Aerodynamics of Insect Flight

The insects mostly depend on Leading Edge Vortices for lift generation. These

vortices create a spiralling motion of air along the leading edge. A flapping wing of

an insect moves through two basic half strokes. The downstroke starts up and slightly

backs so that the insect is plunged downward and moves forward. Immediately, the

wing is flipped over (supination: downstroke to upstroke) so as to make the leading

edge pointed backward. Sane [3, 4 emphasized the importance of the stability of