USAFTPS flight tests L1 adaptive flight control laws

“An adaptive flight controller that could help pilots save a critically damaged or out-of-control aircraft is being proposed for possible commercial development following a rigorous evaluation by U.S. Air Force Test Pilot School (TPS) students, using Calspan’s variable-stability Learjet 25 test aircraft.

The L1 controller is designed to automatically intervene in the case of control problems, immediately reconfiguring the flight control system to compensate for degraded flying qualities from mechanical failure or battle damage to a control surface, or even the unintended result of shifting center-of-gravity inflight for better cruise performance. Acting as a backup to the standard flight control system, the L1 is designed to provide safe, predictable, reliable and repeatable responses that would free up pilots to deal with the emergency and further compensate for reduced performance.

In development for more than a decade by researchers at the University of Illinois at Urbana-Champaign, the L1 controller architecture differs from most previous approaches to adaptive control systems. Until now, the standard has been gain-scheduled control systems in which the flight control computer selects the appropriate preprogrammed gains to suit current flight conditions and vehicle configuration. However, the L1 works in real-time to predict transient behavior and estimates lumped uncertainties, rather than every individual parameter that can affect system dynamics; it compensates for them within the bandwidth of a control channel.

The L1 controller comprises three blocks: a state predictor, a fast estimation scheme and a control law. The fast estimation scheme includes a state predictor and a fast estimation law which together approximate the dynamics of the aircraft to generate estimates of the uncertainties. These estimates are provided as input to a bandwidth-limited filter which generates a control signal to the flight control system.

L1 development from theory to flight test has been led by Naira Hovakimyan, a professor of mechanical science and engineering at Illinois, who worked on the concept under Air Force funding between 2004 and 2008 at Virginia Tech with postdoctoral fellow Chengyu Cao (now at the University of Connecticut).”

Link to full article.

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