Can a fighter jet or airliner suffer major damage and stay in flight? Work being done at NASA's Dryden Flight Research Center continues to look into ways to do just that. This
is a report from NASA’s technology transfer magazine, “Spinoff.” NASA does a
lot for the American people and I am glad to reprint this article about work
done at my former workplace, NASA’s Dryden Flight Research Center. Thanks to my former colleague at NASA
Dryden, Yvonne Gibbs Kellogg for permission to post this.
Integrated
Resilient Aircraft Control Flight Test
Adaptive flight control technology is being developed
under NASA’s Aviation Safety Program to enable control of aircraft in all
conditions, including those where the aircraft is degraded, where ice disrupts
its controllability, or where the aircraft is upset by turbulence. The
project’s primary goal is to develop adaptive and fault-tolerant flight control
systems, tackling the primary cause of accidents in commercial jet aviation. In
the summer of 2008, a series of flights were flown on NASA’s NF-15B Intelligent
Flight Control System (IFCS) airplane to provide
validation of two specific adaptive controls designs. The information obtained
from these flight tests provides validation of the concepts and points out
aspects needing further improvement. This is an ongoing effort with a long-term
goal of promoting adaptive control technology as a viable design solution for
increased aircraft safety and resilience.
Current control system technology is
based on a model of how an undamaged, unperturbed aircraft will behave in
normal flight conditions and thus may not be able to control the aircraft well
in abnormal conditions. The adaptive systems “learn” about the new flight condition as it is experienced, and thus can account for
much larger and more drastic changes. These new state-of-the-art control
algorithms can correctly identify and respond to changes in aircraft stability
and control characteristics, enabling the system to adjust immediately in order
to maintain the best possible flight performance in a wide range of abnormal
conditions. While benefits have been shown in simulations, NASA’s flight tests
provide unique validation of these benefits in a more realistic environment.
NASA’s NF-15B IFCS airplane provided
a research test bed platform uniquely suited to test the adaptive control, with
an unprecedented combination of control power
and onboard computational capability. The vehicle started as a robust F-15
fighter aircraft with high structural strength in case the experimental control
algorithm does not respond as predicted. Large canard control surfaces were
added to the vehicle. The excess control capability allows for in-flight
simulation of the effects of significant vehicle damage. The flight control computer was augmented with additional
processors that allow for testing of very complex adaptive control schemes.
During the flights in the summer of
2008, the adaptive systems were subjected to a destabilizing flight condition in which, without the adaptive system engaged,
the vehicle was almost impossible to pilot. When the adaptive system was
engaged, most of the good flying characteristics
of the vehicle were restored. In another test, a flight control surface was
“stuck.” Unlike normal flight conditions, in which some control surfaces
control pitch and others control roll, in this condition, the remaining pitch
control surfaces created an asymmetric command that also created roll, and thus
the control of pitch and roll was cross-coupled. With normal control systems,
this type of behavior can be very disorienting to a pilot, but the adaptive
control system regained control, provided maneuvering capability, and
eventually provided the ability to land
the damaged vehicle safely.
The flight results demonstrated the
potential of control systems to adapt to failures
and adverse conditions. The information collected from these flights will lead
to improvements in future adaptive systems and smooth their implementation in
many vehicle classes, including commercial, fighter, transport aircraft,
unmanned air vehicles, and spacecraft.
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