Flapping :



                A bird or an insect flaps its wings in a very complex manner - and (I think) the idea is not to completely baffle us - but to generate thrust - that overcomes the drag. The flapping motion of many birds, as well as insects, continue to intrigue researchers.

However, some basic ideas to remember are as follows:



1)         During flapping, the wing does not exhibit simply an up and down motion. The motion is more like



The animation of a flying dove highlights this - and some other characteristics discussed later.

2)        The trajectory of the wing tip can be extremely complicated - more so, for smaller flyers. However, a trajectory tracing out a “figure 8” is common in birds and insects.



3)        The time for upstroke is less than the time for downstroke.



4)        Most of the lift and thrust are generated during a downstroke.

The lift is perpendicular to the direction of the motion of the wing, and the drag is perpendicular to the lift, and parallel to the air flow. Hence, during downstroke, the resultant, R, of the lift (L) and drag (D), is tilted forward.

The upward component of R balances the weight, the forward component produces  thrust.                                                                                 



The forces at downstroke
(Adapted from http://bestanimations.com/)



5)        The angle of attack during upstroke is smaller than that during downstroke, to produce a smaller negative thrust!