  Air vehicles of all scales are susceptible to large-amplitude gusts that may
lead to vehicle damage and instability. Therefore, effective physics-based
control strategies and an understanding of the dominant unsteady flow physics
underpinning gust encounters are desired to improve safety and reliability of
flight in these conditions. To this end, this paper develops and experimentally
demonstrates a proportional output-feedback lift regulation strategy based on
the classical unsteady aerodynamic theories of Wagner and Kussner for wings
encountering large-amplitude transverse gusts without a priori knowledge of
gust strength or onset time. The tested vertical gust velocities ranged between
25% and 71% of the freestream speed. This strategy is found to successfully
generalize to gusts of different strengths and directions, as well as wings at
pre- and post-stall angles of attack. In addition, this paper applies dynamical
systems analysis to Wagner's aerodynamic model to reveal the effect of the
pitch-axis location, pitch input, and closed-loop feedback gains on the
stability and robustness of the system along with the flow physics responsible.
The real-time output feedback control strategy is experimentally tested in a
water tow tank facility equipped with a transverse gust generator.
Time-resolved force and flow-field measurements are used to discover the
salient flow physics during these encounters and illustrate how closed-loop
actuation mitigates their lift transients.

Fluid Dynamics