9

Aeroelasticity

129

ha

is the flapping amplitude,

Uref

is the free stream velocity or forward velocity of the body.

The Strouhal number describes the wing kinematics and defines the ratio of local

inertial forces that are due to unsteadiness of the flow to convective inertial forces

that are due to the changes in velocity from point to point in the flow field. St is an

important parameter in the study of unsteady oscillating flows.

Salient Features of Aeroelastic Phenomena in Insect Flight

• Insect wings are anisotropic due to their membranous structure and longitudinal

vein configurations. Major veins run along the wing span. The spanwise bending

stiffness is 1 to 2 orders larger than the chordwise bending stiffness and spanwise

stiffness is proportional to (where C³ is chord length) whereas the chordwise

stiffness is proportional to C².

• All insect wings have a reinforced leading edge. Some insects have (ex. Dragon

Fly) corrugated planforms at the leading edge. This helps in preventing fatigue

fracture and enhances the flexibility and warping rigidity.

• The

process

of

achieving

flight

stability

and

the

influences

of

non-

linearities/anisotropy on wing aerodynamics are still not clear and hence need

further investigations.

• The insect flight in the low Re regime (10¹ to 10⁴) with respect to flow physics

remains to be elucidated fully.

• Pronation and Supination: At the end of the upstroke, the wings pitch up. This

is Pronation. Supination is at the end of downstroke when the wings pitch down.

These are more or less twisting/ untwisting movements at the wing base and have

aeroelastic implications. The elastic nature of resilin has been discussed in detail

in Chap. 7.

• In general, it is observed that in a flapping wing, the structural elastic forces are

predominant as compared to the aerodynamic forces.

• The elastic camber generated by the wing flexibility enhances the lift and thrust

forces.

Rigid, Flexible and Very Flexible Wings

Experimental work on wings made of steel sheets has been explained. The sheet

thicknesseswere3.81mm,0.127mmand0.05mm,respectively,fortherigid,flexible

and very flexible wings. The flexible wings contribute to the enhanced lift [5].

• 2-D versus 3-D, CFD studies: To reduce the computation time, quite often 2-D

studies were done. But in some cases, there can be a marked difference between

the 2-D and 3-D analyses.