Development of a Model Following Control Law for Inflight Simulation Using Analytical and Identified Models

A proposed model following control law for the U.S. Army and NASA Rotorcraft Aircrew Systems Concepts Airborne Laboratory (RASCAL) is presented. The law was developed to operate the vehicle as an inflight simulator, and to support basic handling qualities, flight controls, guidance, and displays research. The design approach was based on using a combination of higher-order analytically derived models and an identified model. The analytically derived models were used to develop the control law structure, while the identification model was used to adjust the gains to compensate for analytical model deficiencies. This new identification model includes an aerodynamic phase lag that accounts for dynamic wake effects on the off-axis responses, thereby improving the off-axis correlation. Closed-loop analysis with the identification model showed that on-axis gain adjustments were necessary to restore stability margins, while off-axis decoupling gain adjustments were necessary to restore the cross-coupling performance.