3

Aerodynamic Considerations

37

simulated the wing by properly placing vortex filament and called this as a bound

vortex or vortex lifting line. This can also be termed as bound vortex as it is espe-

cially bound to the inside of the wing. The bound vortex with its infinite velocity

does not exist in the fluid. The fluid surrounding the wing behaves as if the vortex is

really there. Prandtl further realized that the Helmholtz vortex laws were applied as

if the bound vortex really existed. With reference to the theorem (1) of Helmholtz,

the bound vortex would not disappear when the lift drops down to zero at the wing

tips. With reference to theorem (2), the free vortices could have the same strength as

the bound vortex. From the theorem number (3), the vorticity would remain attached

to the same air particles initially present at the wing tip and, therefore, would trail

behind the flying object [26].

From the above considerations, these vortices are termed as trailing vortices and

in a steady motion would trail downstream to infinity. This pattern of trailing vortices

was named as horseshoe-shaped vortices by Prandtl. From the direction of the circu-

latory flows as required for producing lift, it can be summarized that the effect of

trailing vortices is to produce the downward flow of air behind the wings. This flow is

called as ‘downwash’. The trailing vortices and the flow field they create, particularly

the downwash, have profound effects on the flight performance and stability of the

flier. This is also called Prandtl’s Horseshoe Vortex System. A biological wing could

be represented by a vortex system for understanding the flow patterns.

Summary

From the literature survey, it can be seen that many aspects like lift production, the

effect of the body and the planform shape influence the aerodynamic performance of

aninsectflight.Insectflightaerodynamicsinvolvesacombinationofquasi-steadyand

unsteady aerodynamic phenomena which play an important role in the manifestation

oftheinsectflight.Moreexperimentalandtheoreticalstudieswithsuitablesimulation

experiments are necessary to understand the complexities of insect flight. These

studies will also help in the design of the wings for the man-made MAVs.

The different aspects of Insect wing aerodynamics covered in this chapter can

briefly be summarized as follows.

1.

Insects are among the first class of invertebrate fliers which differ phylogenet-

ically, morphologically and structurally from vertebrate fliers (birds and bats).

However, all these fliers develop similar aerodynamic forces needed for flight by

moving wings during flight and these diversified wings are analogous structures.

2.

Insects exhibit their flight style involving complex wing movements such as

flapping, twisting and to-and-fro oscillatory motion. They can perform hovering,

gliding, forward flight (flapping), manoeuvring and passive flight as per their

biological needs dictated by various environmental factors.