Digital adaptive control laws for VTOL aircraft

The VTOL Approach and Landing Technology (VALT) Program of the National Aeronautics and Space Administration is conducting research in navigation, guidance, control display, and flight management for future VTOL aircraft. As part of the research in advanced control technology Honeywell designed a digital self-adaptive flight control system for flight test in the VALT Research Aircraft -- a tandem rotor medium transport helicopter (a modified CH-47). The control laws accept commands from an automatic on-board guidance system. The primary objective of the control laws is to provide good command following with a minimum cross-axis response. Three attitudes and vertical velocity are separately commanded. Adaptation of the control laws is based on information from rate and attitude gyros and a vertical velocity measurement. The final design resulted from a comparison of two different adaptive concepts -- one based on explicit parameter estimates from a real-time Maximum Likelihood Estimation algorithm, the other based on an implicit Model Reference adaptive system. The two designs were compared on the basis of performance and complexity.