The behavior of a typical autopilot structure is studied for flight under icing conditions. The study is based on a Twin Otter aircraft model and focuses on the pitch attitude behavior. A quadratic stability analysis using linear matrix inequalities is conducted to show that the time-varying closed-loop system maintains quadratic stability under icing conditions. In addition, the problem of envelope protection in the presence of icing is considered; in particular, how to maintain the angle-of-attack within the stall limits which are time varying. Based on steady-state behavior, a practical envelope protection scheme is developed and tested via simulation.
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