Systematic experimental studies were performed to understand the role of two key degrees of freedom, flapping and pitching, in aerodynamic performance of a flapping wing, in both hover and forward flight. Required flapping kinematics is prescribed mechanically, and dynamic pitching/twisting
is obtained passively using inertial and aerodynamic forces. Forces produced by the wing are measured at the root using a six-component balance at different flapping frequencies, flapping/pitching amplitudes, and wind speeds. The results clearly show that maximum average thrust over a flap
cycle in hover can be achieved using symmetric, high amplitude passive pitching. However, in forward flight, optimum aerodynamic performance (lift and propulsive thrust) is obtained using asymmetric wing pitching with low pitching amplitudes. Furthermore, dynamic twisting (obtained using flexible
wings), instead of dynamic pitching, produces better performance in forward flight due to spanwise and temporal modulation of the wing pitch angle. Pure flapping (no pitching) of rigid wings in forward flight at high reduced frequencies and high pitch angles produces a threefold increase in
lift coefficient over static values. Maximum average propulsive thrust over a flap cycle in forward flight is obtained using symmetric pitching. To produce high values of both, average lift and thrust, an asymmetry in kinematics along with pitching is required in forward flight. This can be
achieved either through asymmetric pitching of rigid wings or dynamic twisting of torsionally flexible wings.
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