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Bio-mimicking MAVs Based on Insect Flight Studies

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landing of MAVs as compared to the biological fliers. Biological fliers could over-

come these problems through a continuous process of evolution lasting over millions

of years, which involved both genetic and ecological adaptations. Therefore multidis-

ciplinary research and close coordination is absolutely necessary between biologists,

physicists and robotic engineers for developing successful models of MAVs based

on bio-mimicking principles of flight and landing techniques.

Insect flight is considered as an evolutionary and adaptive model for miniature

flight design. Flight of an insect is still a mystery. The forces acting on flapping wings

are quasi-steady and unsteady, which may lead to higher lift forces as compared to

the conventional lift generating devices. The wingbeat cycle is usually expressed as

cycles per second (cps) or Hertz (Hz). However, a single flapping wingbeat cycle

consists of a powerful downstroke and a recovery upstroke each lasting only a few

milliseconds. The wingbeat basically leads to both translatory and rotational move-

ments. These strokes are usually described in terms of three basic angles viz., sweep

angle, roll angle and angle of attack. In a flapping cycle, there are two rotational

stages: one is supination and the other one is pronation. During supination, the wing

begins to flap from behind the body. During pronation the wing rotates at the fulcrum

and also changes its direction where the leading edge is ahead as explained by Wilkins

and Knowles [2].

Reynolds Number and MAVs

Reynolds number (Re) is defined as a ratio of inertial forces to viscous forces, usually

in terms of fluid flow and geometrical parameters of the flier. It can be expressed as

follows:

Re = ^{(inertial f orces)}

(viscous f orces) ⁼

(total momemtum trans f er)

(molecular momemtum trans f er)

In biological flight viscous forces are more predominant as compared to inertial

forces and the Reynolds numbers are relatively low. It may be noted that Re is a non-

dimensional parameter. Acknowledgement of Re is important in bio-aerodynamic

studies because the lift and drag development depend on Re value. At low Reynolds

number, it is difficult to keep boundary layer attached to the surface of the flier. It

has to be noted that Reynolds number values for insects vary from 10² to 10⁴ while

for homoeotherm fliers it is in the range of 10⁴–10⁶. However, for an aircraft it is

usually above 10⁷. For small insects like thrips, Re is very low and viscous forces

are dominant. Re also helps in understanding the flows such as laminar, transitional

and turbulent. Re values are substantially low for sperms, algae, protozoa and fungi

(see Fig. 12.1). As flyers size reduces Re also becomes lower.