2

Flight a Retrospect a Brief Review

13

problems, unsteady aerodynamics, wind gusts and vehicle control problems

have to be considered seriously.

Mukharjee and Omkar [48] stated that lift generation mechanisms have been

identified first experimentally and then confirmed by numerical simulations. An

understanding of the complete aeroelastic behaviour of wings is essential in order

to appreciate their aerodynamic performance. Shyy et al. [47] studied drag charac-

teristics of three aerofoils with different flexibilities in a sinusoidal oscillating free

stream. They concluded that modulating the flexibility can improve the aeroelastic

characteristics and thrust. The translational phase of wing rotation consists of phase

reversal in flapping cycles known as pronation and supination. The sweeping motion

of the wings coupled with pure pitching occurs during the rotary cum flapping motion

of the wings.

Flexiblewingstructures of simplifiedinsect-sizedflappingMAVs havebeeneluci-

dated based on the 1-DOF butterfly model, others on the 2-DOF Diptera model [49].

French researchers, Vanneste et al. [50] conceived a design of Flapping Wing for

Nano Air Vehicle (FWNAV). It is basically a scaled-down version of the flapping

wing with a wingspan below 7.5 cm. Lift generation involves a more complex fluid–

structure interaction in FWNAVs. Wing roots can be actuated on various degrees of

freedom. The authors conceived a method for wing optimization by playing with

mesh size and the number of time steps in the mean lift computation.

Ho et al. [51] discussed in detail unsteady aerodynamics and flow control and use

of MEMS as applicable to flapping wings of MAVs. Sibilski et al. [52] indicated

that a good model of MAV needs rotation of the wings in addition to flapping during

downstroke and upstroke for producing sustained aerodynamic forces.

Curet et al. [53] studied the wings for MAVs and suggested that when a thin wing

is stationary, it spontaneously flaps at a critical wind speed. Lift is enhanced and

it also increases drag. Enhanced lift is mainly due to strong Leading Edge Vortices

(LEVs).

In the case of insects and other avian fliers, however, the evolution from

parachuting to gliding to the powered flapping occurred due to the compliant wings

with positive camber and necessary structural deformations for the development of

required aerodynamic forces. A suitable note may be taken of these features for the

design of practical Insect Bio-mimicking vehicles.

Summary

This chapter gives a brief review of the general introduction to insect biological

features, morphological characteristics, wings, respiration, various anatomical and

flight considerations to establish a base for the engineers to understand differential

aspects involved in the design of a bio-mimicking MAV based on the above insect

features.

Thereview of literature can broadly be divided into four subdivisions: