New Tool Could Accurately Gauge How An Aircraft Will Fly When A Flight Control Surface Is Damaged And Adaptive Flight Control Software Compensates For The Loss
The Art of Prediction
Research and software being developed at Dryden could provide future aircraft designers with a better prediction tool that will accurately gauge how an aircraft would fly when a flight control surface is damaged and adaptive, or "intelligent," flight control software is used to compensate for the loss.
Ample handing-quality metrics are available for standard areas of the flight envelope. However, damage to a flight control surface may require the aircraft to operate in regimes for which there currently are no handling-quality scales or tools that can predict how the aircraft will fly or if it is flyable, explained Bruce Cogan, a Dryden researcher and lead on the project.
One example is cross coupling, which causes an unexpected response to the aircraft when the pilot moves the control stick. If the pilot pulls back on the control stick to climb the aircraft may climb, but it will also roll to the left or right. That left or right motion could make the aircraft hard to control. This situation is generally not a concern, except in a case where the aircraft has been damaged.
This research is intended to provide a metric for use by future designers in measuring cross coupling and building control laws to eliminate it, Cogan said. The tool could eventually be part of standard engineering software that aircraft designers use. Somewhat ironically, research with one of the few fixed-wing aircraft where cross coupling applied also was done at Edwards Air Force Base. The AD-1 research aircraft, which was flight tested at Dryden, exhibited heavy cross coupling due to the ability to change the angle of the wing in flight.
The concepts were developed initially through Small Business Innovative Research work with Hoh Aeronautics Inc. of Lomita, Calif., and are based on a metric developed for helicopters. Unlike fixed-wing aircraft, helicopters exhibit cross coupling in routine flight. The overall concept is to apply helicopter cross-coupling metrics to a damaged fixed-wing aircraft.
The end result is software that predicts how an aircraft would fly when damage occurs in flight and how well an adaptive controller compensates for cross coupling. That software recently took flight in a U.S. Air Force Test Pilot School simulator and on the F-16 variable-stability in-flight simulator test aircraft, or VISTA.
"The VISTA is a good test bed for the research because the aircraft software is easy to modify and the aircraft is able to 'fly' like different aircraft," Cogan explained. "In this case, the software was modified to fly like an F-16 and simulate effects of various degrees of simulated cross-coupling damage to the flight control surfaces."
For the flight tests, the aircraft took off under student control and in the air the instructor pilot would switch to one of 25 pre-programmed configurations, changing it to enable the student to fly the new configurations, Cogan said. Also, safety trips were in place to ensure that flight controls reverted to those of a standard F-16 in case the aircraft approached dangerous conditions. Then, TPS students flew and rated the controllability of the aircraft in about 230 flight scenarios during nine research flights.
As a whole, the software predicts cross coupling and the controllability of the aircraft, which were then compared to the students' experiences. Students used the Cooper-Harper scale, a recognized pilot rating system for flying qualities, to judge the aircraft performance. A tracking task was designed by the students to evaluate the aircraft. Early results showed a good comparison between predicted and actual handling qualities for varying degrees of cross coupling.
The enthusiasm and contributions of the student pilots added to the positive results, Cogan said.
"We listened to their comments, and they thought of things we didn't about introducing cross-coupling challenges," Cogan said. "They wanted to get good results for us on what was a very tight, compact schedule. It was a win-win."
The Air Force Test Pilot School team included project pilots Maj. Dail Fields, Maj. David Marten and Italian Air Force Capt. Gianmarco Di Loreto. Project engineers included Robert Koo, Jade Lemery and Katherine Ryan. Bill Gray was staff advisor for the project, which the Test Pilot School called Project Icarus: Limited Handling Qualities Evaluation of Cross Coupling.
The work wasn't accomplished without a lot of perseverance and patience, as the concept originated in 2004.
Proposals originally were written for testing the ideas on the F-15 Intelligent Fight Control Systems aircraft. Dryden researcher Peggy Hayes first saw the need for this information when she was working on intelligent flight control systems on the F-15 and began looking for a solution to the problem.
The ideas were developed with help from Dave Mitchell through the Dryden SBIR agreement with Hoh Aeronautics. Mitchell was the principal investigator and used his experience in handling qualities and helicopters to make the program a success. Early simulation tests proved successful and led to the current effort. When Hayes moved on to another project, Cogan took on the work. Cogan hopes to analyze the information and refine it for potential use on Dryden test aircraft, such as the F/A-18 no. 853.
Now that the concept is proven through flight the development process will continue, leading to validation for its use in helping make aircraft safer.