  We experimentally study the aeroelastic instability boundaries and
three-dimensional vortex dynamics of pitching swept wings, with the sweep angle
ranging from 0 to 25 degrees. The structural dynamics of the wings are
simulated using a cyber-physical control system. With a constant flow speed, a
prescribed high inertia and a small structural damping, we show that the system
undergoes a subcritical Hopf bifurcation to large-amplitude limit-cycle
oscillations (LCOs) for all the sweep angles. The onset of LCOs depends largely
on the static characteristics of the wing. The saddle-node point is found to
change non-monotonically with the sweep angle, which we attribute to the
non-monotonic power transfer between the ambient fluid and the elastic mount.
An optimal sweep angle is observed to enhance the power extraction performance
and thus promote LCOs and destabilize the aeroelastic system. The frequency
response of the system reveals a structural-hydrodynamic oscillation mode for
wings with relatively high sweep angles. Force, moment, and three-dimensional
flow structures measured using multi-layer stereoscopic particle image
velocimetry are analyzed to explain the differences in power extraction for
different swept wings. Finally, we employ a physics-based Force and Moment
Partitioning Method (FMPM) to quantitatively correlate the three-dimensional
vortex dynamics with the resultant unsteady aerodynamic moment.
