3

Aerodynamic Considerations

25

3.

Passive Pitching Mechanism: Three types of passive pitching modes have been

recognized. 2-D flows and LEVs may help in additional attachment to flexible

airfoils [15].

4.

Tip Vortex (TV) Formation: In fixed finite wings, the wing tip vortices gener-

ally increase drag [16]. Once it was believed that wing tip vortices contribute

to wastage of energy. However, in a flapping wing, the wing tip vortices may

influence the total force exerted on the wing by creating a low-pressure area

near the wing tip. In a flapping motion, the impact on aerodynamic forces by

tip vortices is relatively less for wings having an aspect ratio of 4, adapted for

delayed rotation [17]. It is quite possible that the wing tip vortices in a flapping

wing do influence lift and thrust.

The vortices associated with fixed finite wing appear to decrease lift and drag.

It may stabilize the shed vortices and this involves non-linear interactions.

However, for a low aspect ratio flapping wing, the impact of tip vortex may

not be significant [18].

5.

Rapid Pitching Rotation: In a flapping cycle, an insect wing carries out both

translational and rotational motions. The Kramer effect states that the coupling

of wing rotation with translational motion has aerodynamic advantages. An

increase in the angle of attack during translation may increase lift above steady-

state values and is closely related to delayed stall at higher angles of attack.

The rotational forces are caused by the flapping wing and the same fluid dynamic

mechanism happens during wing translation [19]. Magnus effect is rather appli-

cable to cylinders and spheres. However, biological wings have a sinusoidal

action. Hence, the significance of the Magnus effect cannot be ruled out. The

Kramer effect describes the rotational forces [3, 4. Therefore, LEV is the only

force generating the aerodynamic phenomenon in the flapping cycle and the

rotational effect may not contribute much to it [20]. There is also an increase in

the vorticity pattern around the wing due to rapid pitch-up rotation. This helps

in increasing the lift [17].

6.

LEV and Delayed Stall: One of the significant characteristics of an insect wing

is that in real time flow insect wing produces more lift as compared to a wing in a

wind tunnel [10]. Generally, the wings of an aeroplane stall and lose lift rapidly

beyond an angle of attack of 15° depending on the type of aerofoil. In contrast,

insect wings can sustain a maximum angle of attack of 45° in a flapping cycle.

The flow does not follow the contour of the wing and leading edge vortices are

developed, which contribute to the lift (Fig. 2.1). This development of lift force

is due to the presence of smaller viscous forces than pressure forces which

are associated with fluid velocity. The flow separation takes place from the

upper surface. This LEV forms a low-pressure area above the wing resulting in

enhanced lift. The LEVs sustain a balance between the pressure gradient, the

centripetal force and the Coriolis force 17. Diagrammatic representation of LEV

has been shown from the source for a better understanding (Fig. 3.1). A leading

edge vortex is larger than a stable separation bubble which remains attached to

the upper surface of the wing at a higher angle of attack and at a low Reynolds