A model-following digital control law for an EBF-STOL aircraft

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Virginia Polytechnic Institute and State University

A method is presented for augmenting the control of an EBF-STOL aircraft using a real model-following technique. The method is based on the solution of the discrete matrix Riccati equation to generate the required feedback and feedforward gains. The feedback gains provide a means of closing the control loop around each of the plant states while the feedforward gains multiply the states of the inputs to some desirable reference model, thus making it a prefilter to the plant. The model chosen for this research is that of a B-26 aircraft.

The longitudinal equations of motion of the STOL aircraft are linearized about a preselected point of the flight regime and written as linear state equations. A new system state vector is then constructed which includes the difference between plant and model states. The problem is formulated as a discrete linear regulator with the inputs to the model simulated as state variables.

In solving the matrix Riccati equation, a quadratic cost function which includes the system state vector is minimized, thus making the plant states follow those of the model.

The type of STOL aircraft used in this study has three possible fixed flap positions. Results are obtained for the half-flap configuration with step inputs to the elevator and thrust coefficient. Results are also presented for a filtered gaussian input to each of these control variables.