By Hank Hogan
If knowledge is power, then aircraft are about to become exponentially empowered. Consider the deployment of new types of sensors that can provide detailed information about the health of an aircraft’s components. As part of this, such sensors can monitor structural integrity, looking for problems caused by projectiles or collisions. They also can capture data critical to active surface control, in order to maximize the effect of aviation surfaces flexing and bending on airflow, fuel efficiency, and other aspects of aircraft performance.
The National Aeronautics and Space Administration’s (NASA’s) Armstrong Flight Research Center in Edwards, California, has demonstrated a new technology which promises to do just that, by enabling up to 2,000 strain sensors on a single 40-foot strand of optical fiber. Because this configuration is based on optical technology, it is lightweight and capable of being placed virtually anywhere on an aircraft.
Key to the Armstrong innovation is how the data is measured. According to Lance Richards, a structures engineer for aircraft structures and strain measurement, “The type of interrogation that we use is called optical frequency domain reflectometry (OFDR). That allows us to have a sensor every quarter inch along the length of the fiber. Each one of those sensors is the equivalent of a strain gauge.”
Contrast that wealth of information, obtained at the cost of a strand of optical fiber and a measurement box, with what would be required using electrical techniques. For a traditional electrical setup, each strain sensor would then need three copper wires, leading to a bulky and heavy wiring harness. Added to that would be the size and weight of the strain gauges themselves.
The ratio between electrical– and fiber optic–based approaches is 100 to 1000 to 1 in terms of number of sensors, with that figure being the opposite with respect to weight, per an Armstrong study. That is, the new methodology allows the use of up to 1,000 times the sensors at 1/1000th the weight.
Frank Peña, structures engineer for mechanical design and development, as well as structural simulation and testing, explains that a final drawback to the traditional technique is the fact that the strain gauges can only be attached to interior surfaces due to the wiring requirements. In theory, fiber optics can be attached to any type of surface, inside or outside of the aircraft.
To date, the Armstrong technology has been demonstrated on flights aboard various vehicles, ranging from the very small to a 27-foot long Predator. A bolt-on to existing aircraft, the sensor technology has been proven to not impact aerodynamic-related performance, such as time in the air of long-endurance platforms such as the Predator. One of the reasons that the fiber used does not have any real effect on flight performance is that it is not only lightweight but also incredibly thin, about 65 microns in diameter, or roughly the size of a human hair.
Other good news is that creating a dense, custom sensor array need not take much time, Richards says. “We can lay down dozens of feet of these sensors, basically hundreds of sensors, in a single day.”
The OFDR data collection technique does not gather data as quickly as another approach typically deployed, which depends on sensing wavelength changes. But OFDR does enable 32,000 sensors to be updated 100 times a second, thereby gathering data from a much denser array of sensors than is possible with other techniques. Updating strain and stress measurements in 1/100th of a second is enough for most aviation and aerospace applications.
The approach demonstrated at Armstrong also offers other advantages when it comes to sensing, Peña notes. “We adhere directly to the structure of interest. We are essentially monitoring the structure directly.” That and the sensor density make it possible to use the technology to look for micrometeorite strikes on a spacecraft or small projectile impact on a military aircraft.
This capability could be important in other applications as well, such as determining when a component is showing signs of reaching end-of-life. Discovering that a part needs to be replaced before it fails and something catastrophic happens is highly desirable. This is one of main reasons the technology has attracted so much interest from airframe makers and others.
While existing aircraft can benefit from more sensors, taking full advantage of these and other sensor advances will likely require designing and building new aircraft. At the same time, there is a direct benefit to getting highly specific information from a dense sensor array from new model design through flight testing. Having more data about strain could allow design margins to be tightened, thereby reducing the overall weight of the final aircraft, while making such design adjustments without such information could lead to a lighter structure flexing and bending to the point of failure.
Richards also points out that the same sensor technology can be used to measure cryogenic liquid levels in a tank, something for which there currently are no good techniques. For spacecraft, that use could be particularly beneficial in reducing launch weight.
Finally, more data can lead to new ways to fly. For example, additional surfaces could be designed to more dynamically bend and flex, with a sensor array providing information for a control loop. The result could be adaptive structures configured into optimized shapes for each distinct phase of a flight path. At Armstrong, NASA engineers demonstrated this concept with a 14-foot aircraft, giving us a glimpse of what might be accomplished with better sensor technology.
“That’s got a lot of potential if you’re talking about tuning structures and being able to increase performance and reduce drag,” Richards concludes.
Image #1 – Courtesy of NASA. Photo by Tony Landis
Image #2 - The metallic panel has both conventional and fiber optics sensors. The white bundle of cables contains 100 conventional strain gauges, the single yellow cable about 350. Armstrong aircraft have been used in validation of fiber optic sensors. Fiber optic sensors may also have applications in a number of different fields. Photo by Tony Landis, courtesy of NASA.
Image #3 – Anthony (Nino) Piazza inspects fiber runs on the top surface of the left wing of NASA’s Ikhana aircraft. The fiber can contain thousands of embedded strain sensors in a 40-foot span, making it possible to monitor structural surfaces and actively control them. Photo courtesy pf NASA.