Lecture 11 - Unsteady nature of flapping flight

Question 1

Describe the two parts of a flapping wing.

Answer 1

The outer wing or hand and the inner wing (arm). Produces forward momentum. It is more flexible than inner wing.

The inner wing, acts like an aircraft wing – lift developing part of the
wing.

Question 2

What is the downstroke?

Answer 2

the wing beats down and
forwards producing lift and
propulsion

Question 3

What is the upstroke?

Answer 3

the wing flexes in towards the body (largely due to a rotation) and than rises and extends ready for the next downstroke. Passive aerodynamic lift provides the restoring force for the upstroke.

Question 4

What are the three main aerodynamics mechanisms of unsteady flapping flight?

Answer 4

Delayed stall

Rotational lift

Wake capture

Question 5

What is delayed stall?

Answer 5

In flapping wings the leading edge vortex has
been observed to remain attached to the wing during the whole
wing stroke. This produces very high lift and prevents stalling

Question 6

What is rotational lift?

Answer 6

known as “Kramer effect” results from the
interaction of translational and rotational velocities about span
axis of the wing at the end of the two half-strokes, when the
wing decelerates and rotates. Depending on the direction of the
rotation the rotational force either add or subtract from the net
force due to translation

Question 7

What is wake capture?

Answer 7

results of interaction of the wing with the fluid
wake generated in the previous stroke. Inertial effects play
important role

Question 8

What is flutter?

Answer 8

A self-excited oscillation of the body.

Question 9

What is the strouhal number?

Answer 9

k = omega*L / V

k characterizes the way a disturbance at one
point is felt at other point in the flow field.