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P. Mukkavilli et al.

occur with a small time gap in contrast to the motion obtained through a conventional

linkage mechanism as in a flapping winged MAV. The angle of attack of the insect

wing is much higher than that of a normal aeroplane because of the cyclic motion

of the flapping flexible wings, which incidentally helps in the increased stall. Insects

use different methods to enhance lift such as wake capture, rotational circulation

(Magnus Effect) and delayed stall.

The aerodynamic forces on insect wings were predicted by using quasi-steady

approach.Inquasi-steadyapproach,themotionoftheinsectwingscanbedescribedin

terms of a series of steady-state aerodynamic problems. CL and CD are time-invariant

non-dimensional force coefficients and depend on lift-drag forces generated. In a

flying insect, the wings encounter at each stroke quasi-steady flow conditions similar

to those in the wind tunnel. Lift and drag can be estimated roughly throughout the

stroke cycle from aerodynamic studies. The elementary blade approach integrates

the force produced by each thin wing section, since wing velocity and wing chord

change from wing root to the tip of the wing. These studies provide data both on wing

velocity and angle of attack [7–9]. According to Ellington, quasi-steady assumption,

the instantaneous aerodynamic forces on a flapping wing are equal to the forces in a

steady motion, at an identical instantaneous velocity and angle of attack. This theory

needs a careful reconsideration.

In the flapping wing motion of insect flight, unsteady aerodynamics plays a crucial

role as it enhances both the lift and drag through wake capture, rotational effects

and delayed stall. Thus, there exists a discrepancy in measurements made from

direct force measurements as compared to quasi-steady state theory. However, time-

invariant drag and lift studies in steady state help in finding out certain mechanisms

of flapping flight to a preliminary level and they also help in estimating the unsteady

effects [10]. Most of the studies on insect flight have neglected the Wagner Effect and

focussed on unsteady effects [3, 4]. Flapping wings exhibit variable linear translatory

and rotational (flapping) movements. Inertial bending of the wing, due to decreasing

mass and small patches of resilin, adds to the complexity of wing kinematics. Our

present knowledge of the aeroelasticity of insect flight is fragmentary.

The Principle of Lift and Thrust Generation by Flapping

Wings

In the absence of energy input from the environment, the flapping wings of a flier in a

steady flight must provide the necessary lift and thrust so as to maintain the required

height and the forward speed. Therefore, it is essential to understand the complex

kinematics of the flapping wing. In order to provide the necessary weight support,

some symmetry has to be introduced into the pitching motion of the lifting surface.

The symmetry can be obtained by just maintaining a constant angle of attack for the

thrust generating wing. The easiest way for a flier is to tilt the whole body and the