In the emergency medical services field, experts speak of the "golden hour", the time within which a seriously wounded patient has a better chance of survival if rushed to an emergency room.
In a transport aircraft that has been upset, there is an equivalent concept when it comes to the potential for recovering control and saving many lives, except that the "hour" is actually "seconds". Researchers at the University of Illinois at Urbana-Champaign and University of Connecticut have developed an adaptive flight-control algorithm, dubbed L1, that could do just that - give the pilot of an otherwise uncontrollable aircraft just a few extra seconds of controllability, enough to give him or her time to save the vehicle from a loss-of-control accident.
NASA Langley in early June flew the L1 adaptive controller as part of the agency's Airstar (airborne subscale transport aircraft research) testbed in rural Virginia, putting the algorithm to the test during post-stall high angle-of-attack and other scenarios. Adaptive controls take advantage of every available control surface to follow the pilot's commands despite the state of the aircraft. Photo credit: NASA
Langley senior researcher Irene Gregory explains that historically, adaptive control algorithms could guarantee stability in steady-state operations only, not during transients.
A new version of adaptive control called "L1", however, can predict transient behaviour. "That's a really big deal," says Gregory, "to know what will happen in the first seconds after a transient."
NASA worked with researchers to test the new algorithm on its subscale model to help research pilots maintain control of the aircraft when it departs the nominal operating envelope, a realm the device is intended to probe often. It is not too far of a leap to imagine such algorithms being used on commercial transport aircraft for the same purpose. Photo credit: NASA.
Below is a video showing the basic features of the AirSTAR testbed, courtesy of NASA.
During testing of the T2 subscale generic transport model from 2-4 June at Fort Pickett airfield in Virginia, the algorithm proved its worth. Gregory says one of the test cards called for the research pilot put the aircraft at a post-stall angle-of-attack without and then with the L1 controller. She explains that the research pilot's job was to put in step input of positive elevator at the post-stall AoA and attempt to hold the aircraft there. Without L1, "he couldn't hold the aircraft", she says. "It was rolling and slicing, exceeding 45° bank."
The NASA video below shows a stall manueuver for which pilots did not use one of several adaptive controllers being tested at Fort Pickett, including L1.
Using the L1 control law in the same manoeuvre however, she says the pilot was able to hold the nose-high attitude for 3-4s and bank angles never exceeding 20°. "It bought him time to make a proper recovery," she says.
The NASA video below shows a similar maneuver in which the L1 controller made a normally uncontrollable roll divergence controllable and made post-stall behavior very controllable.Thanks to NASA Langley PR guru Kathy Barnstorff and all the researchers at NASA for letting me see this amazing research "facility" in action.