Digital particle image velocimetry (DPIV) is employed to quantify instantaneous flow around the wings and deduce corresponding aerodynamic properties such as lift and drag coefficients.

Effects of perturbations on performance of flapping wings

Natural and artificial miniature flyers will need to contend with freestream perturbations that are ubiquitous in outdoor environments. Previous studies on flapping wings have predominantly been in quiescent conditions, and our understanding of effects of freestream disturbances on their performance remains limited. One of the motivations of our study is asymmetrical response of insects to upward and downward oriented disturbances. This raises important questions of how biologically inspired micro aerial vehicles (MAVs), which essentially implement flapping wings, will respond to vertical perturbations.

Here, we experimentally investigate the effects of uniform vertical perturbations oriented upward and downward with respect to the stroke plane of flapping wings over a wide range of perturbation magnitudes. Force measurements are taken through a six-axis force/torque (F/T) sensor to gain an understanding of the effect of the vertical flow on the mean and transient aerodynamic performance of the wing. Flow behaviour is also captured through digital particle image velocimetry (DPIV) to identify key flow features.

Underactuated flapping wings

Achieving the desired kinematics with the minimum weight and complexity of wings and actuating structures has led to a strong interest in adopting passively pitching flapping wings for small-scaled flying vehicles. The pitching angle of this kind of flapping wing can be passively modulated by an elastic hinge acting as a torsional spring which balances the moments resulting from fluid mechanic and inertial forces. The successful implementation of such underactuated flapping wings strongly depends on their ability to operate efficiently in wind disturbances. This raises important questions of how the time-history of the wing pitching motion, and consequently unsteady aerodynamic effects are altered by freestream disturbances compared to quiescent hovering.

Here we experimentally explore the interaction between a uniform vertical inflow perturbation and a passive-pitching flapping wing using a Reynolds-scaled apparatus operating in water. A parametric study is performed by systematically varying stiffness of the elastic hinge and magnitude of perturbation.

The passive-pitching flapping wing in quiescent hovering.