A x flapping amplitude motion in x direction (m) A y flapping amplitude motion in y direction (m) c chord (m) c fw fore-wing chord (m) c hw hind-wing chord (m) C H — mean horizontal force coefficient C V — mean vertical force coefficient f wing beat frequency (Hz) F H horizontal force (N/m) F V vertical force (N/m) FWMAVs flapping wing micro aerial vehicles k ij stiffness constant between the centre node and its neighbours II matrix given by Equation (11) LE leading edge LEV leading-edge vortex LEVS leading-edge vortex shedding LS lower surface m arbitrary instant of grid deformation MAVs micro air vehicles n number of iterations N number of time steps n i number of the surrounding nodes in interconnection with the node i PIV particle image velocimetry Re Reynolds number S wing surface area (m 2 ) ABSTRACT This paper studies hovering capability of flapping two-dimensional tandem wing sections inspired by a real dragonfly wing configu- ration and kinematics. Based on unsteady numerical simulations, the dragonfly corrugated wings have been benchmarked against a flat wing in terms of the aerodynamic forces and flow structures generated during a flapping cycle. The timing of rotation at each stroke is studied by pitch rotation at three different rates, i.e., 80%, 60% and 40% of a flapping period. The results suggest that the longer time pitch rotation with the period of 80% of the overall flapping period is closer to the force calculations obtained of a balanced flight, that is, the mean vertical force C V — = 1 . 076 supports the dragonfly weight of 0 . 754 g with a small difference of 0 . 92% and the mean horizontal force C H — = 0 . 051 indicates negligible thrust. However, the corrugated wing performs aerodynamically differently from the flat plate with differences in C H — by ±4 . 32% and in C V — by ±2 . 06% for the corrugated shape. The vorticity flow field for both wings have been recorded at some instants of flapping motions which give more explanation of such dissimilarity. NOMENCLATURE 2D two dimensional 3D three dimensional A flapping amplitude (m) AOA angle-of-attack (degree) A r pitching amplitude (degree) THE AERONAUTICAL JOURNAL NOVEMBER 2010 VOL 114 NO 1161 699 Paper No. 3448. Manuscript received 31 March 2009, revised 6 November 2009, accepted 9 May 2010. Effects of pitching rotation on aerodynamics of tandem flapping wing sections of a hovering dragonfly E. M. Elarbi N. Qin n.qin@sheffield.ac.uk Department of Mechanical Engineering, University of Sheffield Sheffield, UK