3

Aerodynamic Considerations

19

including insects, the wings can tolerate much higher angles of attack before stall,

implying higher stall margins. The cyclical motion of the flapping flexible wings

and fast feedback sensory systems delay the insect stall considerably through larger

angles of attack and help them to recover from eventual stalling by prolonging the

pre-stall period considerably as compared to an aircraft stall.

The biological fliers are quite different from the man-made aeroplanes, missiles

or helicopters. They exhibit the salient features of flapping wings in their flight. This

is nothing but the simultaneous moving of the wings in oscillating or flapping mode

along with a twisting motion of the wing. The extent of flapping with an added twist

is decided by the flier in-situ depending on the wing structure and the instantaneous

flight requirements. They have different modes of flight viz., Flapping, Hovering,

Bouncing, Gliding, Thermal Soaring as well as Passive flight. It may be mentioned

here that many insects have two sets of wings, with one pair partly overlapping the

other pair. However, in dragonflies, the individual wings can move independently.

The housefly, for example, possesses one pair of functional wings that develop all the

required aerodynamic forces. Another interesting feature existing in some insects is

the clap and fling mechanism during the flight which also enhances the lift. There

are six additional lift-developing mechanisms.

Certain asymmetry can be noticed in the downward and upward stroke or ‘beats’

of the wings. The downward stroke is a high-powered stroke that produces sufficient

lift forces for the insect to gain height or sustain its altitude position during hovering.

The upward stroke, on the other hand, is of relatively lighter intensity and is a recovery

stroke. The period of downward stroke is generally more than that of upward stroke.

In the case of T.j, downward and upward stroke periods are 14 and 6 ms and the total

stroke period is 20 ms. The downward and upward strokes put together constitute

one wingbeat cycle. The number of such beats or cycles per second is termed as

the wingbeat frequency and is expressed in cycles per second (cps) or Hertz (Hz).

In insects, the wingbeat frequency varies from 2 to 1000 cps. Based on the rate of

the number of wingbeats, they are grouped as Neuroegenic fliers (2–100 cps) and

Myogenic fliers (100–1000 cps), depending on neuronal impulses and subsequent

muscle oscillations (Tetanic). Myogenic muscles are more oxidative as compared to

neurogenic muscles.

One further characteristic feature of the biological fliers is the way they simul-

taneously generate the necessary lift and thrust by the typical stroke pattern of the

flapping flexible wings. The wings not only make an up and down oscillatory motion,

but they also make a partly twisting motion of the wings originating from the wing

base. In this process, a typical shape of ‘8’ is generated by the wing tip which can

be observed through a stroboscope by virtual freezing of the wing images. There is

a large variation in the size of the fliers and their wings as well as the body mass

which may extend from a few milligrams as in small insects up to a few grams (60

gm) as in larger beetles.