AgilePod Brings Fiber Optic Sensing to Flight Testing

September 1, 2024

Air Force Test Pilot School
Edwards Air Force Base, CA
661-277-1110
edwards.af.mil

The Air Force Test Pilot School’s research division, in partnership with the 586th Flight Test Squadron at Holloman AFB and NASA’s Armstrong Flight Research Center, recently conducted a flight test campaign verifying the use of the new AgilePod. Dubbed “Have STRAINger Things,” the tests verified the AgilePod’s utility for rapid airborne experimentation, aimed at reducing lead times and costs that traditionally hinder flight-testing efforts.

NASA’s Armstrong Flight Research Center developed a unique Fiber Optic Sensing System, known as FOSS. While legacy copper straining has been used for flight test instrumentation since the 1930s, it requires extensive time and effort to modify the aircraft with resistive strain sensors. With FOSS, dedicated fiber optic strain sensors are utilized to report back critical data points.

With Air Force Test Pilot School and NASA AFRC located only minutes away at Edwards, the idea of testing the FOSS system airborne quickly became daily conversation. In conjunction, 586th FLTS, part of the Arnold Engineering and Development Complex in Tennessee, had recently made strides in their AgilePod Lab, which allows for quick-turn capability of carrying special payloads for data evaluation. A trilateral agreement quickly became the primary course of action that would benefit all sides of the test.

Strain sensors are typically applied in connection to loads, buffet, and fatigue testing of new aircraft, or in new store certification on existing aircraft. Stress and strain at critical aircraft and pylon/wing-mount structure points are measured to analyze the response of both the structure and responses to control deflections from either the pilot or the flight control system itself.

The first test articles of a new aircraft design are specially modified during the manufacturing process and commonly include thousands of resistive strain gauges, each requiring two electrical connections, signal conditioning, and data recording. The added weight of this test setup means other aircraft components, often mission systems components, are not installed which limits the utility of these test aircraft for other types of flight test.

In contrast, fiber optic strain sensors have previously demonstrated increased sensing bandwidth with the ability to provide electromagnetic interference compatibility, while providing capacity to have multiple sensors applied onto a single fiber.

Utilizing the AgilePod strapped onto the centerline of a T-38C, the campaign consisted of six test flights combining engineering flight test techniques with two primary experiments. The first was a direct comparison of the fiber optic strain sensors to legacy resistive strain sensors. Both types were installed with a simple cantilever beam that was weighted on one end. By pulling G in the aircraft, the test team was able to induce a controlled amount of deflection/ strain to the beam. Higher frequency vibrations were also introduced through slow flight in buffet as well as at the upper end of the airspeed range where high dynamic pressure also caused vibrations.

Not wanting to miss out on an opportunity to collect more data, the test team added additional loops of fiber optic sensors to the structure of the AgilePod and induced forces in the X, Y, and Z directions. Though there were no reference sensors to compare to, this experiment showed the utility of FOSS for future aeroacoustics and environmental tests for aircraft stores.

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