Since the advent of powered-flight, aircraft designers have sought to escape the bondage of heavy, rigid structures, and move to lighter, bird-like, adaptive shaping and sensing technology.
Such advances potentially offer dramatic improvements in performance and safety, allowing designers to reduce load margins and enabling the aircraft to react swiftly to changing conditions. The same adaptability could also improve damage tolerance, enabling aircraft to survive catastrophic failures or, alternatively, could be used to modify shaping in high-speed aircraft to reduce sonic boom.
While the theoretical benefits of adaptive aircraft have long been known, and dramatic damage tolerance demonstrations using sub-scale F-18 models have proven the viability of the concept, more needs to be done to make it a practical prospect. With this in mind, NASA is planning a lightweight, flexible structures research program which could build on previous basic research efforts to demonstrate ‘fly-by-feel’ sensing and control technology on a modified F-18. Researchers believe the hoped-for demonstration will boost technology readiness levels towards real-world applications of lighter commercial and military designs.
As part of continuing development of lightweight structures, the U.S. Air Force Research Laboratory is also discussing joining forces on the initiative. “We’ve been talking to them and they’re very interested in trying to pull together some collaborative effort in the future,” says Mark Dickerson, program manager for Model Reference Adaptive Control (MRAC) at NASA Dryden Flight Research Center, Calif.
The MRAC F-18 is fitted with a simplified adaptive controller which compensates for simulated failures of flight control surfaces. Using MRAC as a springboard, NASA believes it could take the concept much further. “The ultimate idea is to put a system in service on an aircraft that can sense the structure and things that are happening to the aircraft, and use that information and force it into a shape that it would like it to be,” says Dickerson. “The side benefit is that if it can sense shape it can also control in case of a failure. Load sensing can help you cut back on load margins, so you can have a lightweight structure that’s active.”Fly-by-feel flight tests would use the F-18 flown in the MRAC test effort (Guy Norris)
MRAC builds on several previous efforts including the F-15 Intelligent Flight Control System (IFCS), and the follow-on F-18 Full Scale Advanced Systems Technology (FAST). “The current adaptive control is comprised of less complex systems and algorithms that could be more easily certificated for commercial use in the future, when new technologies are fully validated and verified,” says Dickerson.
“We’re using much simpler algorithms than in the IFCS. We sort of dumbed it down if you will,” says Jim Lee, MRAC project chief engineer. “The system does not have all the feedbacks in it this time. It is simplified to something that will enable conventional processors to be used, and that can be shown to be safe in a commercial application.”
Earlier projects included a Rockwell Collins and Defense Advanced Research Projects Agency system which proved that a catastrophically damaged unmanned F-18 subscale model air vehicle with an adaptive control system could safely land. “That program showed you can design the algorithms to control a severely damaged aircraft, but the problem is getting it certified in the real world. We feel like NASA has the dollars to work on a full-scale vehicle to help this process,” says former IFCS chief engineer John Bosworth.
Following the end of MRAC flights and a maintenance layup, the F-18 will be fitted with fiber optic wing shape sensors to provide real-time measurement and feedback to the flight control system. This comprises the baseline F-18 system, a quad-redundant research flight control system developed for the FAST effort, as well as a dual-redundant airborne research test system, ARTS IV, that provides a way of rapidly testing new concepts. In addition, the system will be linked to 200 strain gages which remain on the flight control surfaces from the Active Aeroelastic Wing research program, which tested the concept of using lighter weight wings and wing twist for enhanced aircraft roll control.
Although fiber optic sensors have flown on composite-built unmanned air vehicles, none have so far been tested in a rigid, conventional airframe. “The F-18 has a stiff wing, with a complex structure. This is a ‘real’ aircraft with lots of panels,” adds leads controls engineer Ryan Dibley. Initial flight tests of the modified F-18 will begin in December 2011.