Experimental investigation of dynamic stall on a flapping wing using pressure measurements

The mechanism of dynamic stall has been well investigated at higher Reynolds numbers in the context of helicopter and wind turbine blades. Recent interest in flapping wing micro air vehicles (MAVs) has necessitated further study of dynamic stall at low Reynolds numbers. The study presented here is part of a larger research program aiming to investigate the effect of turbulence, which is often encountered in the low-level flight envelope of MAVs, on the aerodynamics of flapping wings. We present measurements, under nominally smooth flow conditions, of the time-varying pressures on the surface of a flapping wing model in pure root-flapping motion. The measurements were taken at a single spanwise position approximately half way between root and tip, from a chordwise row of 35 pressure taps. This allowed estimation of section lift coefficients. Experiments were performed at a Reynolds number of 47,800 across a range of reduced frequencies 0-0.24. Departure from quasi-steady behaviour was observed even at low reduced frequencies, when leading edge separation took place during the flapping motion, which for the thin airfoil being studied occurred at an angle of attack arotmd 5°. For the highest reduced frequency, a low pressure peak on the top surface was observed to move backwards during the early downstroke, consistent with a leading edge vortex being advected over the wing.