  Stretching and retracting wingspan has been widely observed in the flight of
birds and bats, and its effects on the aerodynamic performance particularly
lift generation are intriguing. The rectangular flat-plate flapping wing with a
sinusoidally stretching and retracting wingspan is proposed as a simple model
of biologically-inspired dynamic morphing wings. Direct numerical simulations
of the low-Reynolds-number flows around the flapping morphing wing in a
parametric space are conducted by using immersed boundary method. It is found
that the instantaneous and time-averaged lift coefficients of the wing can be
significantly enhanced by dynamically changing wingspan in a flapping cycle.
The lift enhancement is caused not only by changing the lifting surface area,
but also manipulating the flow structures that are responsible to the
generation of the vortex lift. The physical mechanisms behind the lift
enhancement are explored by examining the three-dimensional flow structures
around the flapping wing.
