According to foreign media reports, whether it is in the wake of an airplane taking off or in seemingly calm air, there is no problem that is more likely to cause interference to flight than turbulence. Not only would these "horizontal tornadoes" make air travel uncomfortable and potentially dangerous, but trying to avoid them would also consume a lot of fuel. NASA researchers have developed techniques to find these areas, and with some engineering ingenuity, they could revolutionize flight planning and aeronautical research.
Atypical microphones
Everything in the atmosphere makes a sound. But just like infrared is made up of frequencies that are invisible to the naked eye, there is a type of audio that is similar to infrasound. Infrasound is sound with a frequency below 20Hz, which is inaudible to the human ear, and ranges between 0.001 and 20Hz.
The sudden turbulence sometimes experienced while flying is called clear-air turbulence, so named because there are no visible clouds or atmospheric features to warn of this disturbance. Invisible turbulent air can appear suddenly and wreak havoc on an aircraft. Although visually imperceptible, clear-air turbulence has a clear infrasound signature. Researchers Qamar Shams and Allan Zuckerwar at NASA's Langley Research Center realized that if air traffic controllers or pilots could listen for these turbulences before planes encountered them, they could plan an alternate route.
Their experiments began in 2007, but as expected, initial tests showed that they couldn't just grab an off-the-shelf microphone and expect it to work on infrasound. Longwave frequencies tend to be covered by higher frequency sounds, causing interference.
"We found that the sensors would saturate and they wouldn't perform well," Shams said. "We thought, 'With our combined expertise in instrumentation, why not design a microphone ourselves?'" Shams and Zuckerwar began developing something that could listen to these low frequencies with high fidelity. Microphones use a moving diaphragm to pick up audio, and sound waves cause the surface to vibrate. The researchers used a low-tension diaphragm with a wide radius and a large sealed air chamber behind it to enable the microphone to hear these ultra-low sound waves that travel great distances. The infrasound microphones are manufactured by PCB Piezotronics of Depew, New York, under contract with Langley. Once the sensor is complete, testing begins. When the microphones were placed in an equidistant triangular pattern around Langley's runway, they were able to pick up and locate atmospheric turbulence over Pennsylvania, more than 300 miles away.
Into the Blue Skies
By 2017, Shams and Zuckerwar's technology had won NASA's Commercial Invention of the Year Award, was undergoing ground testing for the Department of Defense, and was being developed at Sandia National Laboratories research to verify its performance, but it has not flown on any aircraft yet. Interest in turbulence detection from Stratodynamics, a company in Lewes, Del., will soon change that.
The company’s founders were participants in the 2016 Space Race Challenge, which was led by the Center for Propulsion Innovation in partnership with NASA. Space Race is a global competition that awards licenses to groups that can demonstrate applications and business cases for various technologies. Stratodynamics competed in several categories, including unmanned aerial vehicle (UAV) control systems. After the company won first prize in an unmanned aerial vehicle competition, the team was invited to visit Langley and meet the researchers behind the patent.
"While we were there, Shams was championing infrasound technology," said Nick Craine, head of business development at Stratodynamics.
Stratodynamics recognized that the microphone system had great potential as an in-flight turbulence detection sensor and sought opportunities to test the technology. After licensing the patent from NASA, and with Shams' assistance, the company began implementing the sensor on an unmanned stratospheric glider called HiDRON designed by its Canadian subsidiary Stratodynamics Aviation Inc.
Stratodynamics has lifted their balloon-launched HiDRON glider to an altitude of over 100,000 feet, from where it slowly returns to Earth. Aided by infrasound microphones and wind probes, the drone measures the intensity of turbulence in its path at a distance and may detect heat plumes to allow the aircraft to sail longer. More work is underway to design the algorithms needed to understand the intensity and extent of turbulence features.
In recent preliminary tests, the microphone performed well. The team was able to isolate the low frequencies from ambient conditions, even as strong winds roared past the drone. Stratodynamics will conduct additional flight tests to further advance the technology. Not only will the company evaluate the NASA-developed microphone, it will also serve as a flight supplier to the University of Kentucky for a complementary turbulence detection technology that is supported by NASA's Flight Opportunities Program. Pending the results of these tests, infrasound microphones will become a standard technology option for Stratodynamics customers.
Stratodynamics Aviation is currently working with the Canadian Space Agency and the University of Waterloo in Ontario to develop a new version of the glider. The HiDRON suborbital space aircraft will have greater payload capacity and will be specifically designed for optimal performance in the stratosphere.
The team hopes that the data provided by infrasound microphones will become ubiquitous in detecting and predicting turbulence, air traffic control decision-making and aviation route planning. By making every aspect of flight easier to avoid turbulence, less fuel is wasted sailing around turbulent air and less carbon dioxide is released into the atmosphere.
“As infrasound sounding continues to prove its value as a turbulence mitigation technology, its potential to forever change the landscape of aviation becomes stronger with every flight,” Craine said.
NASA has a long history of transferring technology to the private sector. The agency's Spinoff publications profile NASA technologies that have been translated into commercial products and services, demonstrating the broader benefits of U.S. investments in its space program. Spinoff is a publication of the Technology Transfer Program of NASA's Space Technology Mission Directorate.