A series of flight test experiments of new adaptive flight controllers has been successfully demonstrated at NASA Dryden onboard an F-18 aircraft (TN 853), also referred to as Full-scale Advanced Systems Testbed (FAST). This flight test program is conducted under the technical direction of the NASA Integrated Resilient Aircraft Control (IRAC) project (PI: Kalmanje Krishnakumar). One of the goals of the IRAC project is to advance the state of the arts in adaptive control as a critical flight control technology for flight safety improvements.
The adaptive flight controllers were developed by NASA Dryden in collaboration with NASA Ames. One of these controllers is based on the optimal control modification (OCM) adaptive law, which was originally developed NASA Ames. In addition, a normalization approach is used to prevent excessive adaptation due to large persistently exciting inputs. This adaptive flight controller is referred to as “onMRAC” (OCM + normalization model-reference adaptive control). A variation of this adaptive flight controller, called “onMRAC+,” includes an additional adaptive law that deals with failures that result in unmatched uncertainty (uncertainty which cannot be completely cancelled out through feedback action). A simplified version of the adaptive flight controller without the OCM term, called “sMRAC,” is also implemented in the flight control software.
BACKGROUND: The FAST is an F-18 aircraft at NASA Dryden (TN 853) which previously was used in the Active Aeroelastic Wing (AAW) program. The 853 aircraft recently underwent extensive wing structural modifications and an avionics upgrade that includes a new Airborne Research Test System flight computer system. Failure emulations can be introduced via parameter changes in the flight control software to intentionally degrade aircraft stability, and through hardware-emulated faults (frozen stabilator and cross-coupled pilot stick inputs) that cause a cross-coupling in both the pitch and roll axes.
Initial flight test results have demonstrated an obvious improvement in flight performance with adaptive control. The “onMRAC” is noted to be working quite well and appears to adapt more quickly than “sMRAC” for the reduced pitch damping failure. When the adaptive law for unmatched uncertainty is used alone, the adaptation results in significant pitch oscillations. These undesired pitch oscillations were effectively suppressed with the “onMRAC” activated.
Flight testing of the adaptive flight controllers will be continued to the next phase that will evaluate handling quality ratings with several pilots flying the aircraft through various maneuvers. The results of the flight test will be disseminated to the public through NASA publication channels.
NASA PROGRAM FUNDING: NASA ARMD Aviation Safety, Integrated Resilient Aircraft Control Project
Contacts: Nhan Nguyen (NASA Ames); Curtis Hanson (NASA Dryden)