3

Aerodynamic Considerations

21

[3, 4]. The forward flight consists of a series of repeated downstrokes and

upstrokes. The downstroke is a powerful stroke and the upstroke is more or

less a recovery stroke.

(iii)

Gliding Flight: Occasionally, insects are seen to glide with the wings

outstretched. This is common in Odonata (Dragonflies), Orthoptera (Locusts)

and Lepidoptera (Butterflies and Moths) which possess large wings. The

ability to glide depends on the high Lift/Drag (L/D) ratio. L/D ratio changes

with the angle of attack. This ratio is at a maximum for Lepidoptera with an

angle of attack ranging from 5 to 15°. The wings of Lepidoptera are covered

by scales that help in the development of lift but do not affect the drag because

of their overlapping arrangement. Butterflies can glide for a longer duration

because of the large wing area. During gliding, very little energy is spent.

Locusts also practice long-distance gliding if the winds are favourable. Insects

have been reported to be making effective use of thermal currents, more so

in tropical areas and deserts. In the strict sense, it differs from true thermal

gliding or soaring of birds as the insects are only carried passively by thermal

currents during migration.

(iv)

Manoeuvring Flight: Manoeuvrable flight is a self-correcting flight that

enables the fliers in course correction and to overcome the obstacles coming

in the flight path. Wings with a high aspect ratio, above 4, are more manoeu-

vrable. MAVs are usually designed with a relatively low aspect ratio of around

3, and hence, they are less manoeuvrable.

(v)

Passive Mode of Flight: In addition to the above four modes of flight, a

Passive mode of flight is possible for insects having relatively larger wings

and with less powerful flying muscles. In the early stages of insect flight

evolution, parachuting, gliding and passive flight might have played a greater

role in their survival and small-distance passive migration when chased by

a predator. The passive mode of flight is analogous to the falling of a leaf

from the treetop. The passive flight may disturb the original flight plan of the

flier including landing and take-off, particularly if there is any wind gust in

the ambient. In passive flights, landing is always delayed depending on wind

velocity.

Various researchers observed different mechanisms in the flapping motion of

insects. One interesting kinematic mechanism has been occasionally a clap and fling

type. Clap and fling is a flapping mechanism in which the wings come close together

at the end of the upstroke above the body of the insect and meet dorsally. The

downstroke motion starts with the wings moving away from each other and the clap

is repeated at the end of each half stroke [6]. He suggested that the clap and fling

mechanism helps in lift augmentation and it is based on kinematics of wing motion.

The examples for clap and fling mechanisms are Encarsia and Tessaratoma Javanica.

Steady-state aerodynamic studies usually apply to conventional aircrafts with fixed

wings and a propelling mechanism.

Insect flapping flight is a highly complex one. The upward and downward flapping

movements of the insect wings are controlled by different sets of flight muscles and