3

Aerodynamic Considerations

23

wings so that the beat plane describing the wing motion is inclined at an angle to the

flight path.

The wings sweep forward during the downstroke if the angle of attack is not zero

with respect to the wing root and backward during the upstroke. Therefore, the inci-

dent velocity and the geometric angle of attack are larger during the downstroke as

compared to the upstroke. Thus, the resultant force is greater than the force which is

produced during the upstroke. If the time taken for both the strokes is equal, the lift

forces act in opposite direction and may cancel out, and the net lift may be nearly zero.

In such scenarios, the flight may not be possible in biological and man-made fliers

with flapping wings. The kinematics of the flapping wing is more complex and its

proper understanding needs further study by using high-speed video for computing

kinematics and Digital Particles Velocimetry (DPV) studies in advancing our under-

standing of insect bio-aerodynamics. Hence, more experimental work is necessary

to understand the complexities of insect flight dynamics [3, 4].

Lift and Drag Coefficients

In the case of a body moving through a homogeneous fluid, the force may be

communicated by the fluid through two basic mechanisms as mentioned below:

1.

Pressure Distribution p(s) and

2.

Shear Stress Distribution τ(s) over the surface.

The resultant forces integrated over the whole surface are resolved into Lift

component (L) of Force (F) normal to the free stream U and Drag component (D)

of F parallel to Free Stream Velocity (U).

Dimensionless coefficients of lift and drag are denoted by CL and CD and can be

represented by

CL =

L

q∞S ⁼

L

1

2^{ρV 2S ,}

(3.1)

and

CD =

D

q∞S ⁼

D

1

2^{ρV 2S}

(3.2)

where

q∞

is the Dynamic Pressure.

S

is the Surface Area of the Wings.