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Composite Structures
November 19, 2009

A commercial airline takes off from an airportA commercial airline takes off from an airport. Future composite structure aircraft, including commercial airliners, could benefit from research ongoing at Dryden. Research Focuses On Monitoring The Health Of Composite Airframes, Which Could Improve Safety And Maintenace For Commercial, Military And Business Aircraft

In an era of rapidly changing technologies to build and operate aircraft, it is becoming more and more important to understand the limits of those technologies to ensure safety and reduce maintenance costs and aircraft downtime.

Dryden researcher Sunil Kukreja, who holds a doctorate in nonlinear system identification, is leading an Innovative Partnerships Program Seed Fund project to do exactly that with aircraft made of composite materials: create a way to monitor the aircraft's health.

"If we can develop the criteria for health monitoring of composite aircraft in flight, we can help the aerospace industry improve safety. Also, airlines want their airplanes flying as much as possible because when they are on the ground for maintenance, airlines are losing money. This could greatly reduce downtime and refine maintenance schedules," he explained.

Metallic structures that have defined most of modern aviation are well understood in what they require in maintenance and when they will need attention. However, not as much is known about composite material that was first used in the construction of military aircraft to reduce weight and radar signature while increasing the structure's strength, Kukreja said.

As an increasing number of commercial airliners are moving in the same direction to reduce weight and add durability, the need is becoming greater to understand composite materials, he said. To those ends Kukreja formed a partnership with long-time colleague Dennis Bernstein of the University of Michigan in Ann Arbor. Bernstein has a doctorate in control engineering.

Additional Dryden partners include co-principal investigator Marty Brenner and aerospace engineer Shaun McWherter. Kukreja's team developed a plan that included facilities, people and in-kind services of about $975,000 and a strong proposal that earned $250,000 in IPP funds through an agreement, he said.

"It is not well understood how composite materials age, or how those materials behave," he said. "What we are trying to determine is if we can use system identification - mathematical modeling techniques - for determining the health of composite aircraft."

Researchers are approaching the challenge by developing a sensor-only, fault-detection approach using pseudo-transfer function identification, Kukreja said. His team's goal is to identify a pseudo-transfer function - a type of mathematical representation - between two sensors in the presence of variations from the baseline operation and external factors.

If the fault-detection architecture is validated, it could eliminate the need for ground testing or building onboard equipment to monitor aircraft health, he said.

"Using this fault detection architecture, we hypothesize that the pseudo-transfer function for the nominal system or aircraft, which can be determined at time of manufacture, should be significantly different when compared to a potentially faulty system," Kukreja said.

If his hypothesis is proven, then the next step is to determine what parameters to monitor for estimating its health, he said. That information could be used to develop criteria for monitoring composite structures, Kukreja explained.

Once the mathematical analysis and algorithmic developments are complete, the algorithm will be validated through simulation of aircraft models such as the F/A-18 or F-15B and eventually compared with flight-test data from different flight conditions to judge how well it monitors the health of the composite elements of the aircraft, he said.

An analysis of flight-test data offers verification of the algorithm or points the way to a revision of the theory and assumption behind them. With further developments, Kukreja said the theoretical work could lead to breakthroughs in safety - and possibly economy - for future air travelers.

One key benefit to developing a method for monitoring a composite structure's health could lead to a technician on the ground downloading computer data that will signal if something requires attention, as opposed to requiring a scheduled maintenance regime, Kukreja said.

A possibility as the project progresses is to demonstrate and verify the theoretical and simulation studies, as well as analysis of flight-test data, by applying this algorithm on the Aeroelastic Test Wing 2, a scale version of a composite aircraft wing, he said.

The ATW2 is attached to a test fixture under the Dryden F-15B flight research test bed and can be flown to conditions that induce stress and fatigue for the ATW2, yet retain operational conditions for the F-15B, allowing for in-flight demonstration of this advanced health-monitoring approach, he explained.

"While one flight test demonstration does not make the experiment globally valid, it would be a solid step toward establishing the applicability of this approach for in-flight health-monitoring systems," he added.

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