Cruising Autopilot for a Flexible Aircraft with an Internal Loop of Model Following

A cruising autopilot is developed for a nonlinear flexible aircraft based on the model-following method. The autopilot controller, called the outer loop, is designed for a rigid body approximation of the flexible aircraft. The model-following control, named the inner loop, approximates the input-output behavior of the flexible aircraft to a rigid body. The autopilot is applied to the flexible aircraft with a model-following inner loop. This inner loop controls the flexible modes and the outer loop, which are the rigid body dynamics. The model-following control is a suboptimal linear quadratic formulation with output feedback. This technique can stabilize the flexible modes and keep their amplitudes acceptable. The autopilot is a speed and altitude tracker with a wing leveler. It is designed via a robust H∞ with static output feedback. An aircraft model with nine flexible modes is used to evaluate the method. It is a conceptual medium size jet, like an Embraer 190/190 or Boeing 737–200/300. Simulations are performed in the presence of a stochastic Dryden turbulence model combined with a 1-cosine (1-cos) shape wind gust. The model-following inner loop is validated via a comparison with an internal loop of flexible mode stabilization, designed via a standard linear quadratic regulator with output feedback.